# Preparation and Mechanical Properties of High Silicon Molybdenum Cast Iron Materials: Based on Deep Learning Model

Authors: Fuyuan Dong; Mingxu Zhang; Fan Peng; Jianglong Zhang
Journal: Computational Intelligence and Neuroscience
 (2022)

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## Abstract

In the production and life of machines, wear is the main form of material failure. Many workpieces and equipment will produce friction and wear during use, causing energy and material consumption, resulting in huge economic losses. In the process of using engineering equipment, in order to improve the service life of the workpiece and improve the wear-resistant material, the research of the material itself must be closely linked with the wear mechanism. Among the antiwear materials, the application of high-chromium cast iron is the most common. The reason is that high-chromium cast iron has a series of advantages in performance, manufacturing technology, and price. However, high-chromium cast iron itself has high C content and high brittleness, resulting in defects in toughness and poor corrosion resistance. When working in harsh environments such as mechanical extrusion, high-strength wear, and chemical corrosion, cracking and deformation are prone to occur, corrosion and peeling, and another damage failure. High chromium cast iron must have a relatively stable structure, good strength and toughness, excellent wear resistance, and corrosion resistance. In this study, the static corrosion performance of high-chromium cast iron was tested to study the effects of Mo and V elements addition and content changes on corrosion resistance and vanadium, molybdenum, and subcritical treatment processes on the microstructure and properties of high-chromium cast iron, in order to obtain better performance. Lower cost high chromium cast iron castings. As a wear-resistant part, high-chromium cast iron castings are widely used in wear and can cope with harsh working environments, but their excellent hardness conditions bring certain defects in toughness and at the same time lack good corrosion resistance, in mechanical extrusion., high-strength wear, chemical corrosion, and other harsh environments are prone to damage and failure. Therefore, high-chromium cast iron should have dual properties, not only high hardness but also suitable for mud, sand, chemicals, and other media corrosion and wear resistance.


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## Body

## 1. Introduction

In recent years, with the development and application of some new technologies, some research results have gradually been transformed into actual production technologies, which has made the development of my country’s manufacturing industry reach a new height, and the related automobile industry has developed by leaps and bounds. Energy and environmental issues are becoming increasingly prominent, and the automotive industry is also facing challenges. Among them, increasing the combustion temperature of the fuel is one of the important means to deal with energy and environmental problems. The increase in fuel combustion temperature corresponds to strict requirements for engine materials, including exhaust system materials. It is of great significance to increase the temperature of exhaust manifold materials to increase the combustion temperature of engine fuel.Si Mo nodular cast iron has good thermal fatigue, high-temperature mechanical properties, good high-temperature creep resistance, excellent oxidation resistance, and growth performance. It is commonly used to manufacture exhaust manifolds of diesel engines with high heat load (≥600°C). The main functions of silicon in improving the oxidation resistance and heat resistance of cast iron for exhaust manifolds are as follows: (1) silicon is a graphitized element. Adding 4%–6% silicon can promote the carbon in the structure to exist in the form of graphite, and finally, form a single ferrite matrix. Because there is a very small amount or no pearlite in the cast iron matrix, the volume change caused by the decomposition of pearlite at high temperature is avoided; (2) silicon can increase the eutectoid transformation temperature. When the silicon content is 4.88%, A1 will increase to 900°C. When the service temperature is below the eutectoid transformation temperature, the volume will not change due to the phase transformation stress, thereby increasing the heat-resistant temperature of the material; and (3) at high temperature, silicon can form a relatively dense oxide film with silicon oxide as the main body on the table surface of the casting, which can prevent the diffusion of oxygen ions to the interior of the casting, thereby improving the high-temperature oxidation resistance of the casting. However, if the silicon content is too high, the viscosity of molten iron will increase and the fluidity will deteriorate, resulting in insufficient mold filling in the thin-walled area of the exhaust manifold, forming casting defects such as insufficient pouring and cold shut. At the same time, higher silicon content will reduce the plasticity of cast iron. If the silicon content is too high, it is easy to deteriorate the mechanical properties of castings, and it is easy to promote the precipitation of fragmented or flaky graphite. Therefore, the silicon content is usually controlled below 6%. Molybdenum is a strong carbide forming element, forming a special fe3mo3c type stable carbide, strengthening the ferrite matrix, slowing down the Softening Trend of ductile iron at high temperature, and then improving the creep resistance at high temperature. In addition, molybdenum often forms MoSi2 intermetallic compound with silicon, which can improve the high-temperature corrosion resistance, oxidation resistance, and wear resistance of materials, but the processability at room temperature will be reduced. When the molybdenum content is less than 0.5%, the influence of molybdenum on the strength at room temperature and high temperature is not obvious. Si Mo ductile iron is widely used in automotive exhaust manifolds at home and abroad. In the 1990s, wescast industries of Canada used Si Mo ductile iron to manufacture exhaust manifolds; there are two kinds of materials for the exhaust manifold manufactured by Citroen, one is ordinary ferrite ductile iron, and the other is silicon molybdenum ductile iron. At the same time, silicon molybdenum ductile iron must be used when the working temperature is above 750°C; The exhaust manifold of Deutz engine introduced by FAW Group is also made of silicon molybdenum ductile iron, and the service temperature can reach 800°C. Due to the wide application of silicon molybdenum ductile iron in automotive exhaust manifolds, qtrsi4mo, and qtrsi4mo1 have been incorporated into the national standard GB/t 26653-2011. [1–8] This study takes high-silicon-molybdenum ductile iron as the research object, and optimizes the production process of high-silicon molybdenum ductile iron by changing the types of nodularizers, the amount of inoculant added, and the unpacking time and other process factors. The microstructure of high silicon molybdenum ductile iron was analyzed by metallographic technology, and the variation law of microstructure with process factors was analyzed. The segregation behavior of alloy elements in ductile iron was studied by means of the coloring corrosion method combined with microenergy spectrum analysis method, and the relationship between process parameters and ductile iron structure was further explored correlation between performance.


## 2. Related Work

At present, high-silicon molybdenum ductile iron has become one of the preferred materials for engine exhaust pipes, but there are also a series of problems. The casting performance of high silicon molybdenum ductile iron is also one of the important parameters of its production. After the ductile iron liquid iron is spheroidized, it is beneficial to improve the fluidity due to desulfurization, degassing and removal of some metal inclusions, and purification of the iron liquid. Therefore, compared with other cast irons, ductile iron has better fluidity. However, after the molten iron is spheroidized and inoculated, the temperature decreases a lot, so that the actual pouring temperature is low, and the residual magnesium in the molten iron will increase the table tension of the liquid, resulting in a higher fluidity in the actual production. Poor, the pouring temperature of ductile iron should be appropriately increased to improve this situation.Chromium-based wear-resistant cast iron is a wear-resistant material with excellent performance. It has better strength, stronger wear resistance, and higher toughness; compared with alloy steel, it has good high-temperature resistance and corrosion resistance, so it is regarded as one of the most excellent antiwear materials in the contemporary era. High-silicon molybdenum ductile iron, as an emerging exhaust manifold material, has been gradually recognized by major automobile manufacturers for its excellent medium and high-temperature comprehensive performance and suitable price. Because the working temperature of the exhaust manifold is usually cyclical, therefore, it is of great significance to study the high-temperature oxidation resistance, growth resistance, and thermal fatigue properties of the material, and to analyze and explore its failure mechanism. In the research on the high-temperature properties of high Si–Mo ductile iron, Li Xiaoqin found that high Si–Mo ductile iron has good antioxidation and antigrowth properties under high-temperature conditions of high Si–Mo ductile iron and other cast iron materials. High-temperature resistance, oxidation resistance, and growth resistance increase with the decrease of carbon content and the increase of silicon content. Chen Huiren analyzed and studied the failed manifold, compared it with the uncracked exhaust manifold, and conducted thermal simulation tests. The study found that high-temperature oxidation and corrosion fatigue are the main reasons for the cracking of the exhaust manifold thermal cycle test. At the same time, the existence of irregular graphite in the material and the decomposition of pearlite and microstructure defects will reduce its thermal fatigue performance. Increasing the content of Si and Mo in the ductile iron can improve the high-temperature comprehensive performance of the exhaust manifold. Cao Zhanyi found that thermal fatigue cracks always appear at grain boundaries and near carbides in the thermal fatigue study of ductile iron for automobile exhaust manifolds. Coarse carbides are easy to promote cracks, and oxidation at the cracks will exacerbate cracks extension. At the same time, it was found that thermal fatigue cracks always appeared on the table of the sample, and the higher the heating temperature, the more prone to thermal fatigue cracks. Xie Chunsheng conducted a comprehensive analysis of the cracking of the exhaust manifold of the high-silicon molybdenum ductile iron by means of electron microscopy, metallography, and energy spectrum analysis. It is found that under high-temperature conditions, the oxidation proceeds from the outside to the inside, the stress intensity distribution is uneven, cracks are generated in the oxidized area, and the rupture of the oxide layer causes the oxidation to spread inward; at the same time, in the local area of the matrix, the graphite presents a nonspherical shape and cracks produced near graphite and expanded rapidly. It is concluded that the cracking of the exhaust manifold is caused by oxidation corrosion. It is recommended to strengthen the control of the spheroidization rate and the effectiveness of table shot peening; meanwhile, pay attention to reducing the stress intensity at the root of the exhaust manifold. In the research on the oxidation resistance of heat-resistant ductile iron for automobile exhaust manifolds, Yu Haixia studied the quality change and oxide layer thickness change of the developed high-silicon molybdenum ductile iron and high-nickel ductile iron for automobile exhaust manifolds. The results show that the quality and thickness of the oxide layer of high-silicon molybdenum ductile iron and high-nickel ductile iron increase with working time, and the increase of high-silicon-molybdenum ductile iron is greater than that of high-nickel ductile iron. After understanding the change of table composition, it is finally determined that silicon mainly exists in the form of Fe2SiO4 [9–15]. The content of silicon and molybdenum in the high-silicon molybdenum ductile iron has a great influence on its performance. During the production process, due to the spheroidization inoculation treatment, the spheroidization rate, the number and shape of graphite nodules, and the proportion of each organization. The ratio determines the mechanical properties of the high-silicon molybdenum ductile iron; therefore, the research and analysis of the formation mechanism of its microstructure are of great significance.


## 3. Test Conditions and Methods

### 3.1. Experimental Design

#### 3.1.1. Determination of Basic Chemical Composition

(1) Carbon (C). Carbon is one of the important elements in ductile iron. The content of carbon not only determines the quantity, distribution, and type of carbides but also has a great influence on shrinkage and fluidity during molding. In order to improve the casting process, the upper limit of the carbon content is the upper limit of the allowable range of the technical acceptance criteria, and the mass fraction of carbon is preferably controlled at 2.3%–3.5%. The range selected for this experiment was 3.0%–3.2%.(2) Silicon (Si). Silicon is the most common element that increases the phase transition temperature, improves oxidation resistance, and promotes graphitization, and is one of the important elements in cast iron. At high temperatures, silicon elements can form a SiO2 oxide film with a good protective effect on the table of cast iron. The oxide film has good oxidation resistance in various atmospheres and has a certain self-healing ability. According to research, the oxidation resistance and thermal fatigue properties of cast iron can be improved only when the content of silicon elements reaches 4% or more. However, the amount of silicon is also limited by the saturation number. If the silicon content is too high, it is easy to deteriorate the mechanical properties of the casting, and it is easy to promote the precipitation of fragmented or flake graphite. For this reason, the silicon content can take the lower limit of the technical standard, which is generally controlled at 4.0 %–5.0%. The range selected for this experiment was 4.2%–4.5%.(3) Molybdenum (Mo). Molybdenum is a strong carbide forming element, which forms a special Fe3Mo3C type stable carbide, strengthens the ferrite matrix, slows down the softening trend of ductile iron at high temperature, and improves the ability to resist creep at high temperature. In addition, molybdenum often forms MoSi2 intermetallic compounds with silicon, which can improve the high-temperature corrosion resistance, oxidation resistance, and wear resistance of the material, but the workability at room temperature will be reduced. When the molybdenum content is lower than 0.5%, the effect of molybdenum on the normal and high-temperature strength of the material is not obvious. Therefore, the upper limit of the process of molybdenum is determined to be 1.0%∼1.3% in this experiment.(4) Manganese (Mn). Manganese can exist independently as an alloying element in ductile iron because sulfur and oxygen have been removed during the spheroidization reaction, or combined into a stable compound. Due to the severe positive segregation tendency of manganese, it is usually enriched at the grain boundaries of the eutectic group. Therefore, in the production of ferritic ductile iron, attention should be paid to controlling the content of manganese, so this test controls Mn ≤ 0.2%.(5) Phosphorus of other elements (P, S, etc.). Phosphorus has a serious segregation tendency in ductile iron, and it is easy to form phosphorus eutectic at the boundary, which reduces the toughness of ductile iron. At the same time, phosphorus also increases the shrinkage tendency of ductile iron. However, due to the restriction of raw materials, it is impossible to completely eliminate phosphorus elements, so control wp <0.05%. Sulfur is an antigraphite spheroidizing element and is a harmful impurity. On the one hand, sulfur has a strong binding ability with magnesium and rare Earth elements in nodularizing agent, which will generate harmful impurities; Therefore, under the premise of ensuring the spheroidization rate, control ws <0.015%. [16].


#### 3.1.2. Determination of the Content Range of Alloying Elements

Schemical composition has a great influence on the properties of as-cast ferritic ductile iron, and the properties of ductile iron will be different depending on the content and proportion of added alloying elements. The effects of chromium and vanadium elements added in this experiment on their microstructure and properties and the amount of addition are briefly described as follows: [17](a). Chromium (Cr) After chromium is dissolved in austenite, the solid solution strengthening effect on the structure is relatively large, which can improve the hardness, strength, and wear resistance of the material. However, due to the solid solution strengthening the structure by chromium, it will promote the formation of lattice. Distortion, reduces the plasticity and toughness of the material. Chromium is a strong carbide-forming element. The higher the chromium content, the more carbides are precipitated, the amount of graphite precipitation is relatively reduced, and the self-feeding effect caused by the volume expansion of graphite precipitation will also decrease, resulting in an increase in the shrinkage porosity of the casting. In addition, the carbide formed by chromium is in the form of a grid, which reduces the elongation of the casting. Therefore, the lower limit of chromium is generally selected in the process, and the chromium element in this test is controlled at 0.40%∼0.60%. (b). Vanadium (V) is a strong carbide-forming element. Regardless of the carbon content in cast iron, it forms agglomerated carbides with high strength, high hardness, and high melting point, and it is the first to precipitate from the matrix, which makes the ductile iron perform at room temperature and high temperature. both increased. In ductile iron, the strength increases by 30∼40 MPa for every 0.1% V increase. However, when vanadium is higher than 0.5%, the precipitation of carbides is too much, and the elongation is reduced. Therefore, the range of vanadium addition in this test is selected from 0.30% to 0.50%.


#### 3.1.3. Experiment Sequence

This test mainly studies the effects of adding chromium and vanadium elements alone and compounding chromium and vanadium elements on high-silicon molybdenum ductile iron. Based on this idea, the research sequence of this test is formulated. According to the selected range of the abovementioned elements and the research objectives, four groups of experimental protocols as shown in Table1 were formulated.Table 1
The chemical composition of the test material.
Chemical composition (mass fraction %)Serial numberCSiMoMnPSVCrOther13.0–3.24.2–4.51.0–1.3———23.0–3.24.2–4.51.0–1.3≤0.2≤0.05≤0.015—0.40–0.60—33.0–3.24.2–4.51.0–1.3.30–0.50——43.0–3.24.2–4.51.0–1.30.30–0.500.40–0.60—


### 3.2. Preparation of Alloy Samples

The metal charge used in the experiment mainly includes Q10 primary iron, high-quality low-carbon scrap steel, 75 ferrosilicons, 60 ferromolybdenum, 55 ferrochromium, etc. The molten iron is melted in a 500 kg high-frequency induction furnace. The pig iron and scrap steel are sequentially added to the electric furnace for smelting. After the iron is liquefied and cleared, ferrosilicon, ferromolybdenum, ferrochromium, and ferrovanadium are added, respectively, and the temperature is kept at 1550∼ 1560°C for 2 ∼ 5 min, and then cooled to 1400 ∼ 1450°C sampling for chemical composition analysis, after adjusting the chemical composition to meet the requirements, the temperature is quickly raised to about 1650°C and the furnace is released. After the spheroidization is completed and the slag is removed, the molten iron is poured into the pouring ladle and inoculated at the same time, and then the molten iron is poured into the sand mold and simultaneously inoculated with the flow, in order to obtain a ductile iron sample with qualified quality. The pouring temperature should be strictly controlled during the pouring process, and the pouring should be stopped when the molten iron temperature is lower than 1360 °C. In order to avoid the influence caused by casting defects, 8 Y-shaped test blocks were cast for each component, and the structure and size of the test blocks are shown in Figure 1. After the pouring is completed, the sanding and cleaning are carried out in sequence [18–20].Figure 1
Schematic diagram of the size of the Y -type test block.The spheroidizing agent with low magnesium and rare Earth is selected. The spheroidizing effect, reaction intensity, and magnesium absorption rate of this type of spheroidizing agent are better than those of ordinary spheroidizing agents. The spheroidizing agent used in this test is MgRE2, and the addition amount is 1.4%. Using 75Fe–Si for inoculation treatment and composition adjustment, the use of the large dose multiple inoculation method can increase the number of graphite nuclei, increase the number of graphite balls, and ensure the spheroidization rate at different wall thicknesses. In this experiment, the breeding method was one-time breeding + concurrent breeding, and the addition amount was 1.0% + 0.1%.


## 3.1. Experimental Design

### 3.1.1. Determination of Basic Chemical Composition

(1) Carbon (C). Carbon is one of the important elements in ductile iron. The content of carbon not only determines the quantity, distribution, and type of carbides but also has a great influence on shrinkage and fluidity during molding. In order to improve the casting process, the upper limit of the carbon content is the upper limit of the allowable range of the technical acceptance criteria, and the mass fraction of carbon is preferably controlled at 2.3%–3.5%. The range selected for this experiment was 3.0%–3.2%.(2) Silicon (Si). Silicon is the most common element that increases the phase transition temperature, improves oxidation resistance, and promotes graphitization, and is one of the important elements in cast iron. At high temperatures, silicon elements can form a SiO2 oxide film with a good protective effect on the table of cast iron. The oxide film has good oxidation resistance in various atmospheres and has a certain self-healing ability. According to research, the oxidation resistance and thermal fatigue properties of cast iron can be improved only when the content of silicon elements reaches 4% or more. However, the amount of silicon is also limited by the saturation number. If the silicon content is too high, it is easy to deteriorate the mechanical properties of the casting, and it is easy to promote the precipitation of fragmented or flake graphite. For this reason, the silicon content can take the lower limit of the technical standard, which is generally controlled at 4.0 %–5.0%. The range selected for this experiment was 4.2%–4.5%.(3) Molybdenum (Mo). Molybdenum is a strong carbide forming element, which forms a special Fe3Mo3C type stable carbide, strengthens the ferrite matrix, slows down the softening trend of ductile iron at high temperature, and improves the ability to resist creep at high temperature. In addition, molybdenum often forms MoSi2 intermetallic compounds with silicon, which can improve the high-temperature corrosion resistance, oxidation resistance, and wear resistance of the material, but the workability at room temperature will be reduced. When the molybdenum content is lower than 0.5%, the effect of molybdenum on the normal and high-temperature strength of the material is not obvious. Therefore, the upper limit of the process of molybdenum is determined to be 1.0%∼1.3% in this experiment.(4) Manganese (Mn). Manganese can exist independently as an alloying element in ductile iron because sulfur and oxygen have been removed during the spheroidization reaction, or combined into a stable compound. Due to the severe positive segregation tendency of manganese, it is usually enriched at the grain boundaries of the eutectic group. Therefore, in the production of ferritic ductile iron, attention should be paid to controlling the content of manganese, so this test controls Mn ≤ 0.2%.(5) Phosphorus of other elements (P, S, etc.). Phosphorus has a serious segregation tendency in ductile iron, and it is easy to form phosphorus eutectic at the boundary, which reduces the toughness of ductile iron. At the same time, phosphorus also increases the shrinkage tendency of ductile iron. However, due to the restriction of raw materials, it is impossible to completely eliminate phosphorus elements, so control wp <0.05%. Sulfur is an antigraphite spheroidizing element and is a harmful impurity. On the one hand, sulfur has a strong binding ability with magnesium and rare Earth elements in nodularizing agent, which will generate harmful impurities; Therefore, under the premise of ensuring the spheroidization rate, control ws <0.015%. [16].


### 3.1.2. Determination of the Content Range of Alloying Elements

Schemical composition has a great influence on the properties of as-cast ferritic ductile iron, and the properties of ductile iron will be different depending on the content and proportion of added alloying elements. The effects of chromium and vanadium elements added in this experiment on their microstructure and properties and the amount of addition are briefly described as follows: [17](a). Chromium (Cr) After chromium is dissolved in austenite, the solid solution strengthening effect on the structure is relatively large, which can improve the hardness, strength, and wear resistance of the material. However, due to the solid solution strengthening the structure by chromium, it will promote the formation of lattice. Distortion, reduces the plasticity and toughness of the material. Chromium is a strong carbide-forming element. The higher the chromium content, the more carbides are precipitated, the amount of graphite precipitation is relatively reduced, and the self-feeding effect caused by the volume expansion of graphite precipitation will also decrease, resulting in an increase in the shrinkage porosity of the casting. In addition, the carbide formed by chromium is in the form of a grid, which reduces the elongation of the casting. Therefore, the lower limit of chromium is generally selected in the process, and the chromium element in this test is controlled at 0.40%∼0.60%. (b). Vanadium (V) is a strong carbide-forming element. Regardless of the carbon content in cast iron, it forms agglomerated carbides with high strength, high hardness, and high melting point, and it is the first to precipitate from the matrix, which makes the ductile iron perform at room temperature and high temperature. both increased. In ductile iron, the strength increases by 30∼40 MPa for every 0.1% V increase. However, when vanadium is higher than 0.5%, the precipitation of carbides is too much, and the elongation is reduced. Therefore, the range of vanadium addition in this test is selected from 0.30% to 0.50%.


### 3.1.3. Experiment Sequence

This test mainly studies the effects of adding chromium and vanadium elements alone and compounding chromium and vanadium elements on high-silicon molybdenum ductile iron. Based on this idea, the research sequence of this test is formulated. According to the selected range of the abovementioned elements and the research objectives, four groups of experimental protocols as shown in Table1 were formulated.Table 1
The chemical composition of the test material.
Chemical composition (mass fraction %)Serial numberCSiMoMnPSVCrOther13.0–3.24.2–4.51.0–1.3———23.0–3.24.2–4.51.0–1.3≤0.2≤0.05≤0.015—0.40–0.60—33.0–3.24.2–4.51.0–1.3.30–0.50——43.0–3.24.2–4.51.0–1.30.30–0.500.40–0.60—


## 3.1.1. Determination of Basic Chemical Composition

(1) Carbon (C). Carbon is one of the important elements in ductile iron. The content of carbon not only determines the quantity, distribution, and type of carbides but also has a great influence on shrinkage and fluidity during molding. In order to improve the casting process, the upper limit of the carbon content is the upper limit of the allowable range of the technical acceptance criteria, and the mass fraction of carbon is preferably controlled at 2.3%–3.5%. The range selected for this experiment was 3.0%–3.2%.(2) Silicon (Si). Silicon is the most common element that increases the phase transition temperature, improves oxidation resistance, and promotes graphitization, and is one of the important elements in cast iron. At high temperatures, silicon elements can form a SiO2 oxide film with a good protective effect on the table of cast iron. The oxide film has good oxidation resistance in various atmospheres and has a certain self-healing ability. According to research, the oxidation resistance and thermal fatigue properties of cast iron can be improved only when the content of silicon elements reaches 4% or more. However, the amount of silicon is also limited by the saturation number. If the silicon content is too high, it is easy to deteriorate the mechanical properties of the casting, and it is easy to promote the precipitation of fragmented or flake graphite. For this reason, the silicon content can take the lower limit of the technical standard, which is generally controlled at 4.0 %–5.0%. The range selected for this experiment was 4.2%–4.5%.(3) Molybdenum (Mo). Molybdenum is a strong carbide forming element, which forms a special Fe3Mo3C type stable carbide, strengthens the ferrite matrix, slows down the softening trend of ductile iron at high temperature, and improves the ability to resist creep at high temperature. In addition, molybdenum often forms MoSi2 intermetallic compounds with silicon, which can improve the high-temperature corrosion resistance, oxidation resistance, and wear resistance of the material, but the workability at room temperature will be reduced. When the molybdenum content is lower than 0.5%, the effect of molybdenum on the normal and high-temperature strength of the material is not obvious. Therefore, the upper limit of the process of molybdenum is determined to be 1.0%∼1.3% in this experiment.(4) Manganese (Mn). Manganese can exist independently as an alloying element in ductile iron because sulfur and oxygen have been removed during the spheroidization reaction, or combined into a stable compound. Due to the severe positive segregation tendency of manganese, it is usually enriched at the grain boundaries of the eutectic group. Therefore, in the production of ferritic ductile iron, attention should be paid to controlling the content of manganese, so this test controls Mn ≤ 0.2%.(5) Phosphorus of other elements (P, S, etc.). Phosphorus has a serious segregation tendency in ductile iron, and it is easy to form phosphorus eutectic at the boundary, which reduces the toughness of ductile iron. At the same time, phosphorus also increases the shrinkage tendency of ductile iron. However, due to the restriction of raw materials, it is impossible to completely eliminate phosphorus elements, so control wp <0.05%. Sulfur is an antigraphite spheroidizing element and is a harmful impurity. On the one hand, sulfur has a strong binding ability with magnesium and rare Earth elements in nodularizing agent, which will generate harmful impurities; Therefore, under the premise of ensuring the spheroidization rate, control ws <0.015%. [16].


## 3.1.2. Determination of the Content Range of Alloying Elements

Schemical composition has a great influence on the properties of as-cast ferritic ductile iron, and the properties of ductile iron will be different depending on the content and proportion of added alloying elements. The effects of chromium and vanadium elements added in this experiment on their microstructure and properties and the amount of addition are briefly described as follows: [17](a). Chromium (Cr) After chromium is dissolved in austenite, the solid solution strengthening effect on the structure is relatively large, which can improve the hardness, strength, and wear resistance of the material. However, due to the solid solution strengthening the structure by chromium, it will promote the formation of lattice. Distortion, reduces the plasticity and toughness of the material. Chromium is a strong carbide-forming element. The higher the chromium content, the more carbides are precipitated, the amount of graphite precipitation is relatively reduced, and the self-feeding effect caused by the volume expansion of graphite precipitation will also decrease, resulting in an increase in the shrinkage porosity of the casting. In addition, the carbide formed by chromium is in the form of a grid, which reduces the elongation of the casting. Therefore, the lower limit of chromium is generally selected in the process, and the chromium element in this test is controlled at 0.40%∼0.60%. (b). Vanadium (V) is a strong carbide-forming element. Regardless of the carbon content in cast iron, it forms agglomerated carbides with high strength, high hardness, and high melting point, and it is the first to precipitate from the matrix, which makes the ductile iron perform at room temperature and high temperature. both increased. In ductile iron, the strength increases by 30∼40 MPa for every 0.1% V increase. However, when vanadium is higher than 0.5%, the precipitation of carbides is too much, and the elongation is reduced. Therefore, the range of vanadium addition in this test is selected from 0.30% to 0.50%.


## 3.1.3. Experiment Sequence

This test mainly studies the effects of adding chromium and vanadium elements alone and compounding chromium and vanadium elements on high-silicon molybdenum ductile iron. Based on this idea, the research sequence of this test is formulated. According to the selected range of the abovementioned elements and the research objectives, four groups of experimental protocols as shown in Table1 were formulated.Table 1
The chemical composition of the test material.
Chemical composition (mass fraction %)Serial numberCSiMoMnPSVCrOther13.0–3.24.2–4.51.0–1.3———23.0–3.24.2–4.51.0–1.3≤0.2≤0.05≤0.015—0.40–0.60—33.0–3.24.2–4.51.0–1.3.30–0.50——43.0–3.24.2–4.51.0–1.30.30–0.500.40–0.60—


## 3.2. Preparation of Alloy Samples

The metal charge used in the experiment mainly includes Q10 primary iron, high-quality low-carbon scrap steel, 75 ferrosilicons, 60 ferromolybdenum, 55 ferrochromium, etc. The molten iron is melted in a 500 kg high-frequency induction furnace. The pig iron and scrap steel are sequentially added to the electric furnace for smelting. After the iron is liquefied and cleared, ferrosilicon, ferromolybdenum, ferrochromium, and ferrovanadium are added, respectively, and the temperature is kept at 1550∼ 1560°C for 2 ∼ 5 min, and then cooled to 1400 ∼ 1450°C sampling for chemical composition analysis, after adjusting the chemical composition to meet the requirements, the temperature is quickly raised to about 1650°C and the furnace is released. After the spheroidization is completed and the slag is removed, the molten iron is poured into the pouring ladle and inoculated at the same time, and then the molten iron is poured into the sand mold and simultaneously inoculated with the flow, in order to obtain a ductile iron sample with qualified quality. The pouring temperature should be strictly controlled during the pouring process, and the pouring should be stopped when the molten iron temperature is lower than 1360 °C. In order to avoid the influence caused by casting defects, 8 Y-shaped test blocks were cast for each component, and the structure and size of the test blocks are shown in Figure 1. After the pouring is completed, the sanding and cleaning are carried out in sequence [18–20].Figure 1
Schematic diagram of the size of the Y -type test block.The spheroidizing agent with low magnesium and rare Earth is selected. The spheroidizing effect, reaction intensity, and magnesium absorption rate of this type of spheroidizing agent are better than those of ordinary spheroidizing agents. The spheroidizing agent used in this test is MgRE2, and the addition amount is 1.4%. Using 75Fe–Si for inoculation treatment and composition adjustment, the use of the large dose multiple inoculation method can increase the number of graphite nuclei, increase the number of graphite balls, and ensure the spheroidization rate at different wall thicknesses. In this experiment, the breeding method was one-time breeding + concurrent breeding, and the addition amount was 1.0% + 0.1%.


## 4. Research on Microstructure and Properties of High-Silicon-Molybdenum Cast Iron Materials

High-chromium cast iron is widely used in machine wear parts as wear parts, and its excellent wear resistance can cope with harsh working environments. However, due to the lack of wear resistance and toughness of ordinary as-cast high-chromium cast iron, it is prone to damage and failure when working in harsh environments such as mechanical extrusion and high-strength wear. Machines lose a lot of time and cost when replacing damaged high-chromium cast iron parts, directly affecting the production efficiency of the factory. This chapter mainly takes four as-cast high-chromium cast irons with different contents of vanadium and molybdenum as the research objects and conducts experimental analysis on the as-cast microstructure and related properties of high-chromium cast iron under professional experimental conditions. The influence laws of microstructure, hardness, toughness, and corrosion resistance provide a relevant reference for optimizing the structure of high-chromium cast iron, improving the comprehensive performance of high-chromium cast iron, and developing high-performance high-chromium cast iron at low cost.


### 4.1. X-Ray Diffraction Phase Analysis

The X-ray diffraction patterns of four high-chromium cast iron samples were qualitatively analyzed by using jade software. From the analysis results, the microstructure of as-cast high-chromium cast iron was mainly composed of austenite, (Cr, Fe)7C3 carbide, and martensitic structure. In the composition of three phases in the body, the addition of alloying elements has little effect on the types of main phases of high-chromium cast iron. Image-pro-plus software was used to determine the area percentage of carbides on the metallographic grinding table, and then combined with statistical analysis and X-ray spectrum to approximate the carbide content Vc = 23.07%, and then the direct comparison method proposed by F. Martray (Equation (1)) calculates the content of the remaining two phases [21, 22].(1)Vγ=100−Vc1+GIa/Iγ×100%.High-chromium cast iron samples were calculated based on the X-ray diffraction pattern and the results are shown in Table2:Table 2
High-chromium cast iron samples.
PhaseAs-cast high chromium cast iron1# (%)2#3#4# (%)Austenite30.6232.68%37.71%55.44Martensite46.3144.25%39.22%21.49Combination as shown in Table2 shows that after adding Mo and V elements, the intensity of austenite diffraction peaks of high chromium cast iron increases, the intensity of martensite diffraction peaks decreases, and the intensity of carbide (M7C3) diffraction peaks does not change significantly. It shows that Mo and V can change the composition distribution of the as-cast phase of high-chromium cast iron. Adding Mo and V to high-chromium cast iron is beneficial to increase the content of austenite, while the content of martensite decreases accordingly.Cast structure of sample 4# is greater than that of samples 1#, 2#, and 3#, and the content of retained austenite can reach 55.4%. This is mainly because the sample contains more Mo, which is a strong carbide-forming element, and the strong mutual attraction between Mo and C element can hinder the diffusion of C atoms from austenite to the liquid phase during solidification and cooling, thus making the There is a high carbon content in austenite, which reduces the Ms point of austenite and improves the stability of austenite. At the same time, with the addition of V, the austenite content of high-chromium cast iron in as-cast conditions increases, but the increase is not obvious, this is because V is also austenite stabilizing element, but retains residual austenite is less effective than Mo.


### 4.2. Microstructure Analysis of as-Cast High Chromium Cast Iron

Figure2 shows the metallographic structure (left figure) and scanning electron microscope (right figure) figures of four as-cast high-chromium cast irons with different contents of vanadium and molybdenum elements.Figure 2
Microstructure of high chromium cast iron.
(a)(b)(c)(d)by Figure2 It can be seen that the microstructures of the four as-cast high-chromium cast irons with different contents of vanadium and molybdenum elements are typical high-carbon hypoeutectic high-chromium cast iron microstructures, and the structure types are similar, mainly proeutectic dendrites (Austria), tensite (some martensite), and eutectic structure (dendritic solid solution and eutectic carbide). Combined with XRD pattern analysis, the main microstructures of high chromium cast iron are austenite, (Cr, Fe)7C3 carbide, and martensite. The analysis shows that in the solidification process, the dendritic primary austenite structure is first precipitated in the liquid phase, and the size and distribution of the dendrites are affected by the solidification speed. Continue to cool, C, Cr, Mo, and V elements in austenite have no time to diffuse outward, and there is less segregation, while Cr, Mo, and V elements will inhibit the diffusion of carbon atoms in austenite and make primary austenite stable increased, retaining a certain amount of primary austenite. The eutectic structure (dendritic austenite + eutectic carbide M7C3) is precipitated in the remaining liquid phase, and the size of Cr atoms is close to that of Fe atoms, which is beneficial to replacing Fe atoms and form (Cr, Fe)7C3 carbides and are relatively stable. When the carbides are precipitated, the C and Cr atoms at the edge of the austenite diffuse rapidly toward the eutectic carbides, resulting in a decrease in the content of alloying elements around the eutectic carbides, forming a Cr-depleted and C-depleted area, which will increase the Ms point in this area. high, where the austenite is less stable and will transform into martensite.Comparing samples 1#, 2#, 3#, and 4#, it can be seen that with the addition of V and Mo, the eutectic carbides change from the original continuous network and strip shape to the distribution of isolated small clusters, and the carbides appear obvious. Refinement, finer, more uniform distribution, and the matrix also has an obvious refinement effect. This phenomenon is reflected in the obvious carbide and matrix refinement effect of sample 4#. The analysis believes that this is due to the fact that V and Mo reduce the activity of carbon so that fine secondary carbides are dispersed around the grain boundaries of dendritic austenite grains, and the grain growth is hindered. These carbides can also be As the core of the new phase, it hinders the growth of grains, so it plays the role of refining the austenite matrix structure. At the same time, during the solidification process of high-chromium cast iron, V can reduce the liquid space for eutectic transformation so that the liquid between dendrites is divided into smaller spaces, and the space for eutectic carbide growth is reduced. VC and Mo2C can also form a crystalline core to accelerate the refinement and hinder the growth of other carbides, so the combined action of the two promotes the refinement of eutectic carbides and makes the eutectic carbides finer. Figure 3 shows the SEM morphology and point scanning position map results of sample 4#.Figure 3
SEM photo of sample 4#.From the content of Mo and V elements at point A in Table3, it can be seen that some Mo and V elements are solid-dissolved in the austenite matrix, which can play a role in improving the hardenability of the austenite matrix. The content shows that some Mo elements are solid-dissolved in carbides, which can form secondary solid solutions, while V elements are mainly solid-dissolved in eutectic carbides, which can refine eutectic carbides and form V carbides to improve wear resistance performance. The Mo element is distributed in the austenite matrix and carbides in the microstructure of high-chromium cast iron and is mostly distributed near the eutectic carbides. The analysis shows that the Mo element can partially form with the C element under these conditions. A small amount of Mo2C carbides play a role in refining the structure.Table 3
EDS analysis results of two points A and B (%).
ElementAtomic fractionQuality scoreAtomic fractionQuality scoreABC0.600.132.730.64Cr16.9416.1354.8554.53V0.320.311.481.44Mo0.570.760.350.65Fe79.6181.4239.0741.72


### 4.3. TEM Microstructure Analysis of as-Cast High Chromium Cast Iron

TEM high-resolution microstructure and morphology observation was performed on sample 4#, and selected area electron diffraction spot calibration was performed on sample 4#. The measurement results are shown in Figure4.Figure 4
Sample 4 #TEM microstructure analysis. (a) Austenite. (b) Martensite. (c) (Cr, Fe)7C3.
(a)(b)(c)From Figure4 that the matrix in the microstructure of the as-cast high-chromium cast iron in Figure (a) is mainly austenite, and there are a few dislocations. Figure (b) A small amount of black stripes in the matrix are martensite. Figure (c) In the eutectic structure, the white transparent coarse structure is (Cr, Fe)7C3 carbide, there are some stacking faults inside, and a certain amount of martensite structure exists at the boundary. The microstructure of as-cast high chromium cast iron is mainly composed of proeutectoid dendritic solid solution and eutectic structure (dendritic solid solution + eutectic carbide). Among them, the proeutectoid dendrite is a primary austenite structure + a small amount of martensite, and martensite appears at the edge of the matrix, close to the eutectic carbide, which is mainly due to the low content of C element in the edge region of the matrix, and it is relatively It is stably preserved in the as-cast structure. The eutectic structure is mainly composed of carbides and martensite. The carbide type is mainly (Cr, Fe)7C3 type carbide, and the martensite structure mainly exists in the surrounding area of the carbide., the content of alloying elements in these regions is low, and the transformation from austenite to martensite is easy to occur.


### 4.4. Electrochemical Corrosion Analysis of as-Cast High Chromium Cast Iron

High-chromium cast iron workpieces sometimes need to work in an environment with cement mortar as raw material and will be corroded to varying degrees. Therefore, it is necessary to analyze the corrosion resistance of high-chromium cast iron in order to analyze its failure mechanism. In the test, the Tafel curve was measured for four kinds of high-chromium cast iron in the corrosive medium under acidic, alkaline, and neutral conditions, to analyze the corrosion resistance under static conditions. The test results are shown in Table4.Table 4
Self-corrosion potential and current density of high chromium cast iron under different corrosion conditions.
NumberingAcid (PH = 2)Neutral (PH = 7)Alkaline (PH = 11)Self-corrosion potential ecorr (V)Corrosion current density icorr (A·cm-2)Self-corrosion potential ecorr (V)Corrosion current density icorr (A·cm-2)Self-corrosion potential ecorr (V)Corrosion current density icorr (A·cm-2)1#−0.5713.97 ×10-5−0.6824.14 ×10-5−0.5010.912 ×10-62#−0.5874.09 ×10-5−0.6203.86 ×10-5−0.4890.844 ×10-63#−0.5613.73 ×10-5−0.6423.93 ×10-5−0.4940.854 ×10-64#−0.5233.38 ×10-5−0.5913.46 ×10-5−0.4650.795 ×10-6The amount of substances precipitated or dissolved on the electrode is proportional to the amount of electricity passed through the electrochemical corrosion system, so the corrosion current density reflects the corrosion resistance of the material. Therefore, in the electrochemical corrosion system, the smaller the corrosion current density, the better the corrosion resistance of the material, and when the corrosion current density is close, the more positive the self-corrosion potential, the better the corrosion resistance. Under acidic conditions, the comparison table It can be seen from samples 1# and 2# in 3 that when a certain amount of V element is added, the self-corrosion potential hardly changes, the corrosion current density even increases slightly, and the corrosion resistance does not change much. Comparing samples 2#, 3#, and 4#, it can be seen that with the addition of a small amount of Mo element, the self-corrosion potential increases slightly, the corrosion current density begins to decrease, and the corrosion resistance also begins to increase. The current density was reduced by 15.1%, and the corrosion resistance was significantly improved. Under neutral and alkaline conditions, the corrosion resistance of high-chromium cast iron is similar to that under acidic conditions. The difference is that the comparison of samples 2#, 3#, and 4# found that adding a small amount of Mo element did not cause corrosion. The decrease of current density, but the need to add more Mo can significantly improve the corrosion resistance of high chromium cast iron. Under alkaline conditions, the corrosion current density is the smallest and the self-corrosion potential is positive, indicating that high-chromium cast iron has better corrosion resistance under alkaline conditions. Analysis believes that the mechanism of electrochemical corrosion is a chemical reaction generated by electron migration, which makes the metal ions in the material enter the corrosive solution and cause material damage. During the corrosion process, the substrate corrodes first, and the potential difference between the carbide and the substrate constitutes battery corrosion. In addition, intergranular corrosion easily occurs at the interface between the substrate and the carbide. In acidic and neutral solutions, the substrate with lower electrode potential chemically reacts with Na+ and H+, and its electrochemical corrosion rate is faster than that of the passivation film produced during the period, so the electrochemical corrosion is more serious. In the alkaline solution, the Fe and O2 in the solution undergo oxygen absorption corrosion, and the formed corrosion accumulation layer will adhere to the table of the high-chromium cast iron, reducing the corrosion rate, and the potential difference between the carbide and the substrate in the alkaline solution is small, so it has strong corrosion resistance under alkaline conditions. The passivation film formed by the austenite matrix of high chromium cast iron is relatively stable and shows good corrosion resistance in corrosive media. When Mo element is added to high-chromium cast iron, the austenite region is enlarged, the austenite structure is stabilized, the austenite content is increased, and the high-chromium cast iron is more easily passivated, the matrix potential is increased, and its corrosion resistance is enhanced, when the Mo content increases, the corrosion resistance is further improved. The Mo element refines the grains in the structure, exposes more carbides to the corrosion solution, reduces the probability of corrosion of the matrix, and the generated corrosion products can be uniformly attached to the table of the austenite matrix, avoiding the occurrence of crystallites. At the same time, the Mo element promotes the enrichment of alloy elements in the passivation film, which plays an important role in the formation of passivation film and the improvement of corrosion resistance. The V element can also stabilize the austenite structure, but the effect is not obvious, and the effect of improving the corrosion resistance is small. At the same time, because the V element is mainly dissolved in the eutectic carbide, it increases the potential of the carbide and offsets part of Mo. The effect of high chromium cast iron requires more Mo elements to significantly improve the corrosion resistance.


### 4.5. Toughness Analysis of as-Cast High Chromium Cast Iron

High-chromium cast iron has good hardness conditions, but too high hardness often brings the defect of material toughness, so that high-chromium cast iron cannot give full play to its excellent wear resistance under specific wear conditions. The elastic modulus E∗ reflects the strength of the bonding force between the atoms of the metal material, and is an important indicator to characterize the fracture toughness of the material. The relationship between the microhardness H and the elastic modulus E∗ H3/E∗2 is inversely proportional to the toughness of the material, that is, the smaller the value of H3/E∗2, the better the toughness of the material. By controlling the load and displacement, the nanoindenter can obtain the continuous curve of the load and displacement of the indenter during the loading and unloading process, and the elastic modulus E∗ of the material can be obtained by analyzing the loading and unloading curve. Nanoindentation tests were performed on four as-cast high-chromium cast irons with different vanadium and molybdenum contents, and the obtained loading-unloading curves are shown in Figure5.Figure 5
Nanoindentation loading-unloading curves of high chromium cast iron.By Figure5 It can be seen that for sample 4#, when the load reaches the maximum value of 200 mN, the indentation depth is 1372.5 nm, and the indentation depth is the deepest. Under the same load, the indentation depths of samples 1# and 3# are shallower, 1191.8 nm and 1217.4 nm, respectively. The indentation depth of sample 2# is 1013.4 nm, and the indentation depth is the shallowest. The elastic moduli E∗ of samples 1#, 2#, 3#, and 4# derived from the nanoindentation test are 231.6 GPa, 249.5 GPa, 230.5 GPa, and 220.9 GPa, respectively.In order to further explore the toughness of high chromium cast iron materials, microhardness tests were carried out on samples 1#, 2#, 3#, and 4#. Combined with the measured elastic modulus E∗, the calculated results of H3/E∗2 are as follows: shown in Table5.Table 5
Nanoindentation properties of high chromium cast iron.
NumberingIndentation depthλ, nmHardness H, HVElastic modulus E∗, GPaH3/E∗21#1191.8400.5231.61197.62#1013.4432.1249.51296.03#1217.4398.4230.51190.14#1372.5379.8220.91122.7By Table4 It can be seen that the microhardness of samples 1#, 2#, 3#, and 4# are 400.5 HV, 432.1 HV, 398.4 HV, and 379.8 HV, respectively. The microhardness of sample 2# is the largest, which is mainly related to the solid solution strengthening of V element in a large amount of solid solution in eutectic carbides, and the fine-grain strengthening of V and the dispersion of VC carbides that are easy to generate high hardness. Strengthening increases the microhardness of high-chromium cast iron. However, the high hardness of the microstructure makes it more brittle, so the value of H3/E∗2 is significantly higher than that of other components. Combined with Figure 6, the pit is caused by extrusion around the hardness notch of sample 2#. There are more sink cracks, indicating that the toughness of the material is reduced. Comparing samples 2#, 3#, and 4#, it can be seen that with the addition of Mo element, the hardness of the sample decreases slightly. When its content increases more, the hardness of high chromium cast iron decreases significantly, mainly because Mo element is strong. Carbide-forming elements can effectively expand the austenite phase region, enhance the stability of austenite, and retain a certain amount of austenite, while the hardness of the austenite matrix is small, which makes the overall microscopic microstructure of the high-chromium cast iron sample. Similarly, due to the high toughness of austenite itself and the refining effect of V and Mo elements on grains and carbides, the H3/E∗2 value of high-chromium cast iron decreases. In Figure 6, there are fewer pits and cracks around the hardness notch of sample 4# due to extrusion, indicating that the material has good toughness and plasticity. In the process of friction and wear, the metal table will undergo plastic deformation under the action of normal load, and the shear force will also cause the material to migrate. The toughness of the material can also play a role in the wear process. When considering the wear resistance of high-chromium cast iron materials, not only the hardness of the material itself, but also its toughness should be taken into account, and the wear resistance should be comprehensively evaluated in combination with the microstructure.Figure 6
Scanning image of the microhardness test table of high chromium cast iron.
(a)(b)(c)(d)


### 4.6. Rockwell Hardness Analysis of as-Cast High Chromium Cast Iron

Hardness is one of the most basic properties of materials and is a direct reaction of microstructure. In this test, Rockwell hardness is selected as the hardness characterization method for high-chromium cast iron. The Rockwell hardness values of four as-cast high-chromium cast irons with different contents of vanadium and molybdenum are shown in Table6.Table 6
Rockwell hardness of high chromium cast iron.
Numbering123456789Mean1#38.937.038.339.237.336.538.040.137.838.12#40.340.540.641.040.141.239.941.341.240.63#37.040.137.440.036.138.239.339.639.138.54#34.337.437.035.637.638.136.538.337.136.9By Table5 It can be seen that the Rockwell hardness of the four as-cast high-chromium cast irons varies in the range of 36.9 to 40.6 HRC. The hardness distribution of each as-cast high-chromium cast iron is relatively uniform, and the difference between the Rockwell hardness values is small, indicating that the as-cast high-chromium cast iron has a small difference. The organization is relatively uniform and the performance is relatively stable. Comparing samples 1# and 2#, it can be seen that the addition of V can greatly improve the hardness of as-cast high-chromium cast iron. The overall hardness of crystalline carbides is related. At the same time, the fine-grain strengthening of V acting on the matrix and the strengthening effect of the dispersion of VC carbides that are easy to generate high hardness in the grain boundaries increase the hardness of high-chromium cast iron. Comparing samples 2#, 3#, and 4#, it can be seen that with the addition of Mo element, the hardness of the sample decreases slightly. When its content increases more, the hardness of high-chromium cast iron decreases significantly. This is due to the strong carbide of Mo. The characteristics of forming elements, when solidified, combine with C atoms and dissolve in austenite, which hinders the diffusion of C atoms, increases the carbon content in eutectic austenite, and reduces the amount of precipitated eutectic carbides, and the dendrites are short and present. Dispersed block or granular, the stability of austenite is enhanced, a large amount of austenite remains in the as-cast state, and the hardness of the austenite matrix is small, which makes the overall hardness of the high-chromium cast iron sample lower. However, due to the solid solution strengthening of V and Mo elements in the structure and the dispersion strengthening effect of their carbides VC and Mo2C, some of the hardness defects caused by a large amount of retained austenite are offset, making the as-cast state containing Mo and V. High chromium cast iron still has good hardness conditions.


## 4.1. X-Ray Diffraction Phase Analysis

The X-ray diffraction patterns of four high-chromium cast iron samples were qualitatively analyzed by using jade software. From the analysis results, the microstructure of as-cast high-chromium cast iron was mainly composed of austenite, (Cr, Fe)7C3 carbide, and martensitic structure. In the composition of three phases in the body, the addition of alloying elements has little effect on the types of main phases of high-chromium cast iron. Image-pro-plus software was used to determine the area percentage of carbides on the metallographic grinding table, and then combined with statistical analysis and X-ray spectrum to approximate the carbide content Vc = 23.07%, and then the direct comparison method proposed by F. Martray (Equation (1)) calculates the content of the remaining two phases [21, 22].(1)Vγ=100−Vc1+GIa/Iγ×100%.High-chromium cast iron samples were calculated based on the X-ray diffraction pattern and the results are shown in Table2:Table 2
High-chromium cast iron samples.
PhaseAs-cast high chromium cast iron1# (%)2#3#4# (%)Austenite30.6232.68%37.71%55.44Martensite46.3144.25%39.22%21.49Combination as shown in Table2 shows that after adding Mo and V elements, the intensity of austenite diffraction peaks of high chromium cast iron increases, the intensity of martensite diffraction peaks decreases, and the intensity of carbide (M7C3) diffraction peaks does not change significantly. It shows that Mo and V can change the composition distribution of the as-cast phase of high-chromium cast iron. Adding Mo and V to high-chromium cast iron is beneficial to increase the content of austenite, while the content of martensite decreases accordingly.Cast structure of sample 4# is greater than that of samples 1#, 2#, and 3#, and the content of retained austenite can reach 55.4%. This is mainly because the sample contains more Mo, which is a strong carbide-forming element, and the strong mutual attraction between Mo and C element can hinder the diffusion of C atoms from austenite to the liquid phase during solidification and cooling, thus making the There is a high carbon content in austenite, which reduces the Ms point of austenite and improves the stability of austenite. At the same time, with the addition of V, the austenite content of high-chromium cast iron in as-cast conditions increases, but the increase is not obvious, this is because V is also austenite stabilizing element, but retains residual austenite is less effective than Mo.


## 4.2. Microstructure Analysis of as-Cast High Chromium Cast Iron

Figure2 shows the metallographic structure (left figure) and scanning electron microscope (right figure) figures of four as-cast high-chromium cast irons with different contents of vanadium and molybdenum elements.Figure 2
Microstructure of high chromium cast iron.
(a)(b)(c)(d)by Figure2 It can be seen that the microstructures of the four as-cast high-chromium cast irons with different contents of vanadium and molybdenum elements are typical high-carbon hypoeutectic high-chromium cast iron microstructures, and the structure types are similar, mainly proeutectic dendrites (Austria), tensite (some martensite), and eutectic structure (dendritic solid solution and eutectic carbide). Combined with XRD pattern analysis, the main microstructures of high chromium cast iron are austenite, (Cr, Fe)7C3 carbide, and martensite. The analysis shows that in the solidification process, the dendritic primary austenite structure is first precipitated in the liquid phase, and the size and distribution of the dendrites are affected by the solidification speed. Continue to cool, C, Cr, Mo, and V elements in austenite have no time to diffuse outward, and there is less segregation, while Cr, Mo, and V elements will inhibit the diffusion of carbon atoms in austenite and make primary austenite stable increased, retaining a certain amount of primary austenite. The eutectic structure (dendritic austenite + eutectic carbide M7C3) is precipitated in the remaining liquid phase, and the size of Cr atoms is close to that of Fe atoms, which is beneficial to replacing Fe atoms and form (Cr, Fe)7C3 carbides and are relatively stable. When the carbides are precipitated, the C and Cr atoms at the edge of the austenite diffuse rapidly toward the eutectic carbides, resulting in a decrease in the content of alloying elements around the eutectic carbides, forming a Cr-depleted and C-depleted area, which will increase the Ms point in this area. high, where the austenite is less stable and will transform into martensite.Comparing samples 1#, 2#, 3#, and 4#, it can be seen that with the addition of V and Mo, the eutectic carbides change from the original continuous network and strip shape to the distribution of isolated small clusters, and the carbides appear obvious. Refinement, finer, more uniform distribution, and the matrix also has an obvious refinement effect. This phenomenon is reflected in the obvious carbide and matrix refinement effect of sample 4#. The analysis believes that this is due to the fact that V and Mo reduce the activity of carbon so that fine secondary carbides are dispersed around the grain boundaries of dendritic austenite grains, and the grain growth is hindered. These carbides can also be As the core of the new phase, it hinders the growth of grains, so it plays the role of refining the austenite matrix structure. At the same time, during the solidification process of high-chromium cast iron, V can reduce the liquid space for eutectic transformation so that the liquid between dendrites is divided into smaller spaces, and the space for eutectic carbide growth is reduced. VC and Mo2C can also form a crystalline core to accelerate the refinement and hinder the growth of other carbides, so the combined action of the two promotes the refinement of eutectic carbides and makes the eutectic carbides finer. Figure 3 shows the SEM morphology and point scanning position map results of sample 4#.Figure 3
SEM photo of sample 4#.From the content of Mo and V elements at point A in Table3, it can be seen that some Mo and V elements are solid-dissolved in the austenite matrix, which can play a role in improving the hardenability of the austenite matrix. The content shows that some Mo elements are solid-dissolved in carbides, which can form secondary solid solutions, while V elements are mainly solid-dissolved in eutectic carbides, which can refine eutectic carbides and form V carbides to improve wear resistance performance. The Mo element is distributed in the austenite matrix and carbides in the microstructure of high-chromium cast iron and is mostly distributed near the eutectic carbides. The analysis shows that the Mo element can partially form with the C element under these conditions. A small amount of Mo2C carbides play a role in refining the structure.Table 3
EDS analysis results of two points A and B (%).
ElementAtomic fractionQuality scoreAtomic fractionQuality scoreABC0.600.132.730.64Cr16.9416.1354.8554.53V0.320.311.481.44Mo0.570.760.350.65Fe79.6181.4239.0741.72


## 4.3. TEM Microstructure Analysis of as-Cast High Chromium Cast Iron

TEM high-resolution microstructure and morphology observation was performed on sample 4#, and selected area electron diffraction spot calibration was performed on sample 4#. The measurement results are shown in Figure4.Figure 4
Sample 4 #TEM microstructure analysis. (a) Austenite. (b) Martensite. (c) (Cr, Fe)7C3.
(a)(b)(c)From Figure4 that the matrix in the microstructure of the as-cast high-chromium cast iron in Figure (a) is mainly austenite, and there are a few dislocations. Figure (b) A small amount of black stripes in the matrix are martensite. Figure (c) In the eutectic structure, the white transparent coarse structure is (Cr, Fe)7C3 carbide, there are some stacking faults inside, and a certain amount of martensite structure exists at the boundary. The microstructure of as-cast high chromium cast iron is mainly composed of proeutectoid dendritic solid solution and eutectic structure (dendritic solid solution + eutectic carbide). Among them, the proeutectoid dendrite is a primary austenite structure + a small amount of martensite, and martensite appears at the edge of the matrix, close to the eutectic carbide, which is mainly due to the low content of C element in the edge region of the matrix, and it is relatively It is stably preserved in the as-cast structure. The eutectic structure is mainly composed of carbides and martensite. The carbide type is mainly (Cr, Fe)7C3 type carbide, and the martensite structure mainly exists in the surrounding area of the carbide., the content of alloying elements in these regions is low, and the transformation from austenite to martensite is easy to occur.


## 4.4. Electrochemical Corrosion Analysis of as-Cast High Chromium Cast Iron

High-chromium cast iron workpieces sometimes need to work in an environment with cement mortar as raw material and will be corroded to varying degrees. Therefore, it is necessary to analyze the corrosion resistance of high-chromium cast iron in order to analyze its failure mechanism. In the test, the Tafel curve was measured for four kinds of high-chromium cast iron in the corrosive medium under acidic, alkaline, and neutral conditions, to analyze the corrosion resistance under static conditions. The test results are shown in Table4.Table 4
Self-corrosion potential and current density of high chromium cast iron under different corrosion conditions.
NumberingAcid (PH = 2)Neutral (PH = 7)Alkaline (PH = 11)Self-corrosion potential ecorr (V)Corrosion current density icorr (A·cm-2)Self-corrosion potential ecorr (V)Corrosion current density icorr (A·cm-2)Self-corrosion potential ecorr (V)Corrosion current density icorr (A·cm-2)1#−0.5713.97 ×10-5−0.6824.14 ×10-5−0.5010.912 ×10-62#−0.5874.09 ×10-5−0.6203.86 ×10-5−0.4890.844 ×10-63#−0.5613.73 ×10-5−0.6423.93 ×10-5−0.4940.854 ×10-64#−0.5233.38 ×10-5−0.5913.46 ×10-5−0.4650.795 ×10-6The amount of substances precipitated or dissolved on the electrode is proportional to the amount of electricity passed through the electrochemical corrosion system, so the corrosion current density reflects the corrosion resistance of the material. Therefore, in the electrochemical corrosion system, the smaller the corrosion current density, the better the corrosion resistance of the material, and when the corrosion current density is close, the more positive the self-corrosion potential, the better the corrosion resistance. Under acidic conditions, the comparison table It can be seen from samples 1# and 2# in 3 that when a certain amount of V element is added, the self-corrosion potential hardly changes, the corrosion current density even increases slightly, and the corrosion resistance does not change much. Comparing samples 2#, 3#, and 4#, it can be seen that with the addition of a small amount of Mo element, the self-corrosion potential increases slightly, the corrosion current density begins to decrease, and the corrosion resistance also begins to increase. The current density was reduced by 15.1%, and the corrosion resistance was significantly improved. Under neutral and alkaline conditions, the corrosion resistance of high-chromium cast iron is similar to that under acidic conditions. The difference is that the comparison of samples 2#, 3#, and 4# found that adding a small amount of Mo element did not cause corrosion. The decrease of current density, but the need to add more Mo can significantly improve the corrosion resistance of high chromium cast iron. Under alkaline conditions, the corrosion current density is the smallest and the self-corrosion potential is positive, indicating that high-chromium cast iron has better corrosion resistance under alkaline conditions. Analysis believes that the mechanism of electrochemical corrosion is a chemical reaction generated by electron migration, which makes the metal ions in the material enter the corrosive solution and cause material damage. During the corrosion process, the substrate corrodes first, and the potential difference between the carbide and the substrate constitutes battery corrosion. In addition, intergranular corrosion easily occurs at the interface between the substrate and the carbide. In acidic and neutral solutions, the substrate with lower electrode potential chemically reacts with Na+ and H+, and its electrochemical corrosion rate is faster than that of the passivation film produced during the period, so the electrochemical corrosion is more serious. In the alkaline solution, the Fe and O2 in the solution undergo oxygen absorption corrosion, and the formed corrosion accumulation layer will adhere to the table of the high-chromium cast iron, reducing the corrosion rate, and the potential difference between the carbide and the substrate in the alkaline solution is small, so it has strong corrosion resistance under alkaline conditions. The passivation film formed by the austenite matrix of high chromium cast iron is relatively stable and shows good corrosion resistance in corrosive media. When Mo element is added to high-chromium cast iron, the austenite region is enlarged, the austenite structure is stabilized, the austenite content is increased, and the high-chromium cast iron is more easily passivated, the matrix potential is increased, and its corrosion resistance is enhanced, when the Mo content increases, the corrosion resistance is further improved. The Mo element refines the grains in the structure, exposes more carbides to the corrosion solution, reduces the probability of corrosion of the matrix, and the generated corrosion products can be uniformly attached to the table of the austenite matrix, avoiding the occurrence of crystallites. At the same time, the Mo element promotes the enrichment of alloy elements in the passivation film, which plays an important role in the formation of passivation film and the improvement of corrosion resistance. The V element can also stabilize the austenite structure, but the effect is not obvious, and the effect of improving the corrosion resistance is small. At the same time, because the V element is mainly dissolved in the eutectic carbide, it increases the potential of the carbide and offsets part of Mo. The effect of high chromium cast iron requires more Mo elements to significantly improve the corrosion resistance.


## 4.5. Toughness Analysis of as-Cast High Chromium Cast Iron

High-chromium cast iron has good hardness conditions, but too high hardness often brings the defect of material toughness, so that high-chromium cast iron cannot give full play to its excellent wear resistance under specific wear conditions. The elastic modulus E∗ reflects the strength of the bonding force between the atoms of the metal material, and is an important indicator to characterize the fracture toughness of the material. The relationship between the microhardness H and the elastic modulus E∗ H3/E∗2 is inversely proportional to the toughness of the material, that is, the smaller the value of H3/E∗2, the better the toughness of the material. By controlling the load and displacement, the nanoindenter can obtain the continuous curve of the load and displacement of the indenter during the loading and unloading process, and the elastic modulus E∗ of the material can be obtained by analyzing the loading and unloading curve. Nanoindentation tests were performed on four as-cast high-chromium cast irons with different vanadium and molybdenum contents, and the obtained loading-unloading curves are shown in Figure5.Figure 5
Nanoindentation loading-unloading curves of high chromium cast iron.By Figure5 It can be seen that for sample 4#, when the load reaches the maximum value of 200 mN, the indentation depth is 1372.5 nm, and the indentation depth is the deepest. Under the same load, the indentation depths of samples 1# and 3# are shallower, 1191.8 nm and 1217.4 nm, respectively. The indentation depth of sample 2# is 1013.4 nm, and the indentation depth is the shallowest. The elastic moduli E∗ of samples 1#, 2#, 3#, and 4# derived from the nanoindentation test are 231.6 GPa, 249.5 GPa, 230.5 GPa, and 220.9 GPa, respectively.In order to further explore the toughness of high chromium cast iron materials, microhardness tests were carried out on samples 1#, 2#, 3#, and 4#. Combined with the measured elastic modulus E∗, the calculated results of H3/E∗2 are as follows: shown in Table5.Table 5
Nanoindentation properties of high chromium cast iron.
NumberingIndentation depthλ, nmHardness H, HVElastic modulus E∗, GPaH3/E∗21#1191.8400.5231.61197.62#1013.4432.1249.51296.03#1217.4398.4230.51190.14#1372.5379.8220.91122.7By Table4 It can be seen that the microhardness of samples 1#, 2#, 3#, and 4# are 400.5 HV, 432.1 HV, 398.4 HV, and 379.8 HV, respectively. The microhardness of sample 2# is the largest, which is mainly related to the solid solution strengthening of V element in a large amount of solid solution in eutectic carbides, and the fine-grain strengthening of V and the dispersion of VC carbides that are easy to generate high hardness. Strengthening increases the microhardness of high-chromium cast iron. However, the high hardness of the microstructure makes it more brittle, so the value of H3/E∗2 is significantly higher than that of other components. Combined with Figure 6, the pit is caused by extrusion around the hardness notch of sample 2#. There are more sink cracks, indicating that the toughness of the material is reduced. Comparing samples 2#, 3#, and 4#, it can be seen that with the addition of Mo element, the hardness of the sample decreases slightly. When its content increases more, the hardness of high chromium cast iron decreases significantly, mainly because Mo element is strong. Carbide-forming elements can effectively expand the austenite phase region, enhance the stability of austenite, and retain a certain amount of austenite, while the hardness of the austenite matrix is small, which makes the overall microscopic microstructure of the high-chromium cast iron sample. Similarly, due to the high toughness of austenite itself and the refining effect of V and Mo elements on grains and carbides, the H3/E∗2 value of high-chromium cast iron decreases. In Figure 6, there are fewer pits and cracks around the hardness notch of sample 4# due to extrusion, indicating that the material has good toughness and plasticity. In the process of friction and wear, the metal table will undergo plastic deformation under the action of normal load, and the shear force will also cause the material to migrate. The toughness of the material can also play a role in the wear process. When considering the wear resistance of high-chromium cast iron materials, not only the hardness of the material itself, but also its toughness should be taken into account, and the wear resistance should be comprehensively evaluated in combination with the microstructure.Figure 6
Scanning image of the microhardness test table of high chromium cast iron.
(a)(b)(c)(d)


## 4.6. Rockwell Hardness Analysis of as-Cast High Chromium Cast Iron

Hardness is one of the most basic properties of materials and is a direct reaction of microstructure. In this test, Rockwell hardness is selected as the hardness characterization method for high-chromium cast iron. The Rockwell hardness values of four as-cast high-chromium cast irons with different contents of vanadium and molybdenum are shown in Table6.Table 6
Rockwell hardness of high chromium cast iron.
Numbering123456789Mean1#38.937.038.339.237.336.538.040.137.838.12#40.340.540.641.040.141.239.941.341.240.63#37.040.137.440.036.138.239.339.639.138.54#34.337.437.035.637.638.136.538.337.136.9By Table5 It can be seen that the Rockwell hardness of the four as-cast high-chromium cast irons varies in the range of 36.9 to 40.6 HRC. The hardness distribution of each as-cast high-chromium cast iron is relatively uniform, and the difference between the Rockwell hardness values is small, indicating that the as-cast high-chromium cast iron has a small difference. The organization is relatively uniform and the performance is relatively stable. Comparing samples 1# and 2#, it can be seen that the addition of V can greatly improve the hardness of as-cast high-chromium cast iron. The overall hardness of crystalline carbides is related. At the same time, the fine-grain strengthening of V acting on the matrix and the strengthening effect of the dispersion of VC carbides that are easy to generate high hardness in the grain boundaries increase the hardness of high-chromium cast iron. Comparing samples 2#, 3#, and 4#, it can be seen that with the addition of Mo element, the hardness of the sample decreases slightly. When its content increases more, the hardness of high-chromium cast iron decreases significantly. This is due to the strong carbide of Mo. The characteristics of forming elements, when solidified, combine with C atoms and dissolve in austenite, which hinders the diffusion of C atoms, increases the carbon content in eutectic austenite, and reduces the amount of precipitated eutectic carbides, and the dendrites are short and present. Dispersed block or granular, the stability of austenite is enhanced, a large amount of austenite remains in the as-cast state, and the hardness of the austenite matrix is small, which makes the overall hardness of the high-chromium cast iron sample lower. However, due to the solid solution strengthening of V and Mo elements in the structure and the dispersion strengthening effect of their carbides VC and Mo2C, some of the hardness defects caused by a large amount of retained austenite are offset, making the as-cast state containing Mo and V. High chromium cast iron still has good hardness conditions.


## 5. Conclusion

As a key wear-resistant material for construction machineries such as cement machines, crushers, and trolley grate bars, high-chromium cast iron is widely used in the face of harsh working environments such as corrosion and wear, and its performance determines the service life of components and machinery. production efficiency. However, as-cast high-chromium cast iron often has shortcomings such as poor toughness, insufficient corrosion resistance and high-temperature performance, and poor wear resistance under certain wear conditions, making it difficult to adapt to some harsh conditions of corrosion and wear. In order to further improve the comprehensive properties of high-chromium cast iron to meet the needs of more severe working environment conditions, alloying and heat treatment are often performed on the as-cast state. In this paper, on the basis of the composition of ordinary high-chromium cast iron, four alloyed high-chromium cast irons with V and (or) Mo are added as the research objects, and the effects of adding V and Mo on the as-cast microstructure and properties of high-chromium cast iron are studied, mainly as follows:(1)
The main constituent phases of the four as-cast high-chromium cast irons with different contents of vanadium and molybdenum are (Cr, Fe)7C3 carbide, austenite and martensite, and the microstructure is mainly proeutectic dendrite and eutectic composition. In this experiment, the addition of Mo and V elements has a great influence on the microstructure. The appropriate addition of Mo and V elements is conducive to stabilizing the austenite phase region and increasing the austenite content. At the same time, adding Mo and V elements is beneficial to the eutectic. The refinement of carbide and dendrite structure can improve the structure and distribution.(2)
With the change of the added content of vanadium and molybdenum elements, the corrosion resistance of as-cast high-chromium cast iron changes, among which Mo element has obvious influence. When the Mo content is added to about 0.7%, it can effectively improve the as-cast high-chromium cast iron. The corrosion resistance of cast iron improves the corrosion resistance of high chromium cast iron in acidic, neutral, and alkaline environments.(3)
With the change of the added content of vanadium and molybdenum elements, the hardness of as-cast high-chromium cast iron changes in the range of 36.9∼40.6 HRC. When V element is added, the hardness increases obviously, but the brittleness becomes larger, and the toughness of the material increases. reduce. With the addition of Mo element, the content of austenite with lower hardness will increase accordingly, and the hardness will begin to decrease, but the strengthening effect of V and Mo elements themselves will not make the hardness of high chromium cast iron too low, and because of the austenite With a large amount of retention and the refining effect of V and Mo elements, the brittleness of high-chromium cast iron is reduced, and the toughness and plastic properties of the material are improved.



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*Source: 4825356-2022-09-26.xml*