TechnologyArticle by: Hsien-Ming, Chang / Copyright owned by Fastener World 著作權所有：惠達雜誌Technical
Innovation of
Stainless Steel
Wire Rods forCold Forging
in China“冷頂鍛用不銹鋼盤條“技術創新內容
1. Overview
Bolt connection is one of the most common connection methods inautomobiles. As the basic component of a connection node, a bolt is usuallysubjected to damaging loads such as acceleration, speed change and alternatingload, which is crucial to the load-bearing safety of the structure. For this reason,a technical overview of the main applications of stainless steel materials inautomobiles is given, and the crafting characteristics and technical elements ofthe four most widely used stainless steel materials, namely austenitic, ferritic,martensitic and austenitic-ferritic duplex stainless steel, are analyzed andexplained, with the aim of providing a strong reference and theoretical basis forthe selection of stainless steel materials for applications in automobiles and theformulation of the best connection.

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With the rapid development of modern equipment industry, the demand forhigh strength and high performance is forcing equipment parts to go lightweightand high-strength. As a connection component, a bolt plays a pivotal role inthe safe operation of equipment, and the performance level will become higher and higher with the improvement of the overall
performance of equipment. However, stainless steel bolts can be made from various stainless alloys, and their ultimate strength canreach 480 to 1500 MPa. Although high-strength carbon steel and alloy steel bolts are widely used for their higher strengthand new design methods, their performance decreases faster in specific occasions, and the sensitivity of high-strength bolts
to stress corrosion increases, and failure as well as rupture occur from time to time, bringing safety risks to the operation ofthe equipment. For this reason, the performance of stainless steel bolts is gradually improved, which has attracted attention.

2. Stainless Steel Coil for Cold Top Forging
Stainless steel wire rods for cold forging are used to manufacture various kinds of cold forged products like bolts, screws, nuts,rivets, pins, etc. Most of China’s current low-end cold forged products use Chinese materials, and high-end cold forged products
still rely on imported materials. Therefore, it is necessary to vigorously promote the research and development of stainless steel
wire rods for cold forging, formulate industry standards accordingly, take the lead in technology, improve the quality of Chinese
cold forged products, provide reference and guidance for the development of Chinese cold forged products, and realize total
domestication of high-end cold forged products in China.

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The industry standard of these wire rods mainly refers
to the following standards: Steel wire standard: GB/T 4232
stainless steel wires for cold forging, ASTM A493 stainless andheat resistant steel wires for cold heading and cold forging, JISG4315 stainless steel wires for cold heading and cold forging.
Standard for wire rods: GB/T 4356 Stainless steel wire rods.
T he pu r pose of t he st a nd a rd is to for m t he te ch n ica lrequirements of stainless steel wire rods for cold forging and toprovide direction and basis for the technical and quality progressof the forging stainless steel manufacturing firms in China. At
the same time, in order to enable customers to purchase materials
based on this standard, they can produce stainless steel wires forcold forging that meet the requirements of different standards athome and abroad. The standard is determined mainly according
to GB/T 4232, ASTM A493, JIS G4315, GB/T 4356, GB/T 4356,
GB/T 4232, GB/T 4232, GB/T 4232, GB/T 4232, GB/T 4232, GB/T 4232, GB/T 4232, ASTM A493, JIS G4315, GB/T 4356, GB/T 4237 and other relevant domestic and international standards,relevant sample analysis data, various performance data during
the development process, and laboratory testing data on productrelated indicators.

3. Introduction of Technical Innovation
In the standard of stainless steel wire rods for cold forging,
the range of steel types and chemical composition covers thetechnical requirements for these wire rods specified in various
standards, and the relevant requirements for stainless steel wirerods in China’s standard are referenced.

3.1 Chemical Composition
According to the application of stainless steel wire rods forcold forging at home and abroad and with reference to the Chinese
standard for these wire rods, 24 steel grades are identified to fully
cover the requirements on austenitic, ferritic, martensitic andaustenitic-ferritic duplex stainless steel series.
These wire rods are recommended as an industry standard.
The technical indexes of this standard are more stringent thanthe American standard ASTM A493 for "Cold heading, forging
and heat resistant stainless steel wire rods" as well as the Chinese
standard GB/T 4356 for "Stainless steel wire rods". The harmful
sulfur content is reduced by more than 0.005%, which means
this technical standard is on par with international advanced
standards.
Carbon is a strong austenite forming element, which cansignificantly improve the strength of steel, but the higher
the carbon content in stainless steel, the worse the plasticity,and carbon also has a negative effect on corrosion resistance.
Ch romium is the main fer rite for ming element, a nd the

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At present, there is no standard for stainless steel wire rods
for cold forging at home and abroad. Meanwhile, because thequality of stainless steel wire rods for cold forging in China
is still unstable, these wire rods usually use the American orJapanese standards. Based on the current state of the industry,
the standard is compatible with the existing standards ofChina, Japan and the United States for stainless steel wire rods
for cold forging, so that the standard is operable.

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combination of chromium and oxygen can produce thecorrosion-resistant Cr203 passivated film, which is oneof the basic elements to maintain the corrosion resistanceof stainless steel. Nickel is the main austenite-forming
element, which can reduce the corrosion phenomenonof steel and the growth of grains when heated. The
application of nickel in stainless steel is mainly in
combination with chromium in order to play a better roleand change the structure of stainless steel, thus improving
the mechanical properties, processing properties andcorrosion resistance of stainless steel in certain corrosion
media. In contrast, the tensile test reflects the overallmacroscopic mechanical properties of wire rods, so thepresence of decarburization and sulfide causes the actual
strength of Chinese stainless steel wire rods to be lower
than the theoretical strength, which must be noted.
In ferritic stainless steels, the increase of Si and Mnelements improves the solid solution strengtheningof ferrites, while C and N have a great influence on
toughness. Impurities in ferritic stainless steels, including
oxygen, aluminum, phosphorus, silicon and manganese,
reduce toughness, but their effect is small compared to thatof carbon and nitrogen, and sulfur has almost no effect.
Niobium and titanium are the main stabilizing elements
in ferritic steels, and Niobium and titanium have opposite
effects on toughness and plasticity, so the metallurgicaldesign can be adjusted to the desired product shape.

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I n ma r tensitic st ai n less steels, t he mecha n ica lproperties are strongly inf luenced by the chemicalcomposition; higher Cr content and lower Ni content result

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in higher ferritic content and lower strength properties in a quenched
and tempered condition. On the contrary, when the Cr content is lower
and the Ni content is higher, the ferrite content in the steel is lower andthe strength of the steel is higher.
The results of austenitic, ferritic, martensitic and austenitic-ferritic
duplex stainless steel bolts were studied in the temperature range of 20600°C. At temperatures ≤450°C, the austenitic and martensitic stainlesssteel bolts can maintain the elastic modulus, yield strength and ultimate
strength better than carbon steel and alloy structural steel bolts.

3.2 Surface Defects
Surface defects is an important factor that causes cold forging
cracking. According to statistics on customer use, each year scrapped
cold forging parts take up about 50% of all the scraps. Stainless steel
wire rods used for cold forging must first go through cold-drawing. Ifsurface defect is not eliminated after cold-drawing, there will likely
be head cracking in the subsequent cold forging, resulting in scrapped
parts. Therefore, it is necessary to restrict the allowable depth of defecton the surface of wire rods, and control the depth while comparing to
the said standard and China’s national standard GB / T 4356, where
when the allowable wire rod diameter is ≥ 4.5 ~ 14mm, the allowabledefects depth is from ≤ 0.15mm to ≤ 0.08mm; when the wire rod
diameter is > 14 ~ 20.0mm, the allowable defects depth is from ≤ 0.20mm
to ≤ 0.15mm; when the wire rod diameter is >20∼40mm, allow thedepth of defects according to mutual agreement.

3.3 Process and Mechanical Properties
The hardness of the ferritic and martensitic stainless steel wire rods
for cold forging affects customers success rate in the cold forging.
Ferritic stainless steel wire rods are usually annealed to improve

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Stainless steel wire rods need to undergo limited drawing
deformation before going through cold forging. To ensure asmooth wire drawing process, the wire rods need to have acertain degree of plasticity. The original strength of the wirerods should not be too high, so the mechanical properties of
the austenitic stainless steel wire rods for cold forging arelimited. Austenitic-ferritic duplex stainless steel is heat treatedand solution treated at 950-1120°C. The reason why hightemperature solution can be used is that the carbon content
of duplex stainless steel is much lower than that of ferritic
stainless steel. Considering the effect of solution temperatureon microstructure, mechanical properties and corrosion
resistance, the solution heating temperature of duplex stainlesssteel generally takes the middle limit value in the specification.
To prevent the precipitation of σ phase, it should be cooled
rapidly after the solution temperature is kept warm. The σphase is distributed along the grain boundary. The plasticityof the steel is significantly reduced, and the dispersion is lesslikely to affect toughness, and has a certain strengthening effect. Theσ phase increases the notch sensitivity of the steel, has little effecton strength and hardness, and has a significant effect on toughness.
The σ phase significantly reduces the plasticity, toughness,oxidation resistance, and intergranular corrosion resistance of steel,and increases the occurrence of thermal fatigue. After the σ phase
is formed, the matrix is depleted of chromium (or molybdenum,tungsten), thus reducing the corrosion resistance of the matrix andweakening the effect of solid solution strengthening. In short, the σphase is more harmful, and the appearance of this phase should be
avoided as much as possible.

4. Metallographic Examination of
Stainless Steel
4.1 Preparation of Samples
The preparation process for the metallographic samples ofstainless steel is basically the same as that of high alloy steel. Amongthem, the matrix structure of austenitic stainless steel is relatively
soft, the toughness is high, and it is easy to cause work hardening.
It is difficult to prepare the sample, and it is easy to produce
mechanical slippage and disturb the metal layer structure during
the polishing process, which will affect the normal metallography.
analysis and inspection. Improper sample preparation of semimartensitic steel will cause austenite to transform into martensite, so
the preparation of the sample should not cause the sample to generatehigh heat. The polishing force should not be too large, and thepolishing time should not be too long.

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plasticity and toughness, ensure corrosion resistance andeliminate stress. Generally, the annealing temperature of ferritic
stainless steel is 750-850°C, and the annealing temperature ofmartensitic stainless steel is 800-900°C. After sufficient heat
preservation, a homogeneous ferrite and carbide equilibrium
state is obtained. After a small amount of drawing and cold
forging, if the original hardness of the wire riods is too high,the hardness will be too high after drawing, and the cold forged
portion will be likely to crack up. For ferritic and martensitic
stainless steel wire rods for cold forging, the maximum
hardness limit is specified.

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4.2 Chemical Corrosion
Stainless steel has high corrosion resistance, so the etchantthat shows its microstructure must be strongly erosive to makethe structure clearly visible. The appropriate etchant should beselected according to the steel composition and heat treatment
state. Commonly used etchants are:1. Ferric chloride 5g + hydrochloric acid 50ml + water 10ml
(applicable to A stainless steel and F-A stainless steel);
2. Hydrochloric acid 10ml + nitric acid 10ml + alcohol 100ml;
3. Picric acid 4g + hydrochloric acid 5ml + alcohol 100ml (harderto corrode samples can be heated in a water bath);
4. Hydrofluoric acid (5%) 1ml + nitric acid (5%) 4ml + water 45ml.
In addition, ferrite, austenite, carbide, δ ferrite, and σ mayalso appear in stainless steel at the same time, which can bedistinguished by chemical erosion. In terms of morphology,
austenite has twin grains, and ferrite is often band-like ordendritic; the ferrite becomes rose-colored after being corroded
with red salt potassium hydroxide solution (red salt 10g-15g+NaOH
10g-30g+water 100mL, boiled at 60-90°C). The austenite bodyis bright, and after being corroded with alkaline potassium
permanganate, the carbide is light brown, and the σ phase isorange-red.

4.3 Examples of Metallographic Structure
For ferritic stainless steel annealing, the metallographic
structure is ferrite + M7C3 (carbide). If heated above 900 °C, thegrains will expand and cannot be refined, so the temperature needs
to be controlled. The structure of 1Cr17 steel heated to 850°C
and air-cooled is ferrite + carbides distributed along the rolling
direction, as shown in Figure 1.

Fig. 1
10Cr17 steel structure
is ferrite + carbide

Fig. 2
30Cr13:
pearlite + carbide

After the annealing of martensitic stainless steel is fully
maintained, a uniform ferrite and equilibrium structureof carbide is obtained. Figure 2 shows the annealed stateof 30Cr13 steel raw materials, corroded by ferric chloridehydrochloric acid aqueous solution; the structure is pointlike and spherical pearlite, as well as secondary carbides
distributed intermittently along the grain boundaries.
Solution treatment of austenitic stainless steel, heats up
to 1000 ℃ - 1100 ℃, so that all carbides are dissolved inaustenite, and then quickly cooled to room temperature toobtain a uniform austenite structure. Figure 3 shows that thestructure of 06Cr17Mn7Ni5CuN steel is austenite; Figure 4
shows that the structure of 0Cr19Ni10 steel is austenite + asmall amount of ferrite.
In austenitic-ferritic duplex stainless steel heat treatment
solution treatment (950℃ ~ 1140℃), the metallographic
structure is ferrite + austenite, corroded by potassium
ferricyanide and potassium hydroxide aqueous solution; thestructure is white austenite and gray-black (brown) ferrite. δphase (δ ferrite) is a phase formed in the high temperature
region, generally called high-temperature ferrite, to distinguishfrom low-temperature α-ferrite. δ ferrite is a body-centeredcubic lattice, but the lattice constant is different from thatof α ferrite, and exhibits higher brittleness. The ferritecontent should be inspected according to GB/T 13305-2008"Metallographic Determination of α-phase Area Content in
Stainless Steel". After negotiation between the supplier and thebuyer, other inspection methods can also be used. The ferrite
content must be controlled at 40% ~60%. Figure 5 shows thatthe structure of 022Cr23Ni5Mo3N steel after solid solution isaustenite + ferrite.

Fig. 3
06Cr17Mn7Ni5CuN
structure is austenite

Fig. 4
0Cr19Ni10 structureis austenite + a small
amount of ferrite

Fig. 5022Cr23Ni5Mo3Nstructure is austenite+ ferrite

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5. Conclusion
In recent years, with the continuous improvement of automotive lightweighting, long service life and user-perceived quality
requirements, coupled with the current stricter requirements for automobile emission regulations, stainless steel materials
have been developed and applied relatively rapidly in China’s automobile industry. In order to track the rapid development andapplication of high-performance stainless steel materials in automobiles, it is necessary and urgent to increase the development
and accumulation of advanced stainless steel material connection technologies for automobiles in the next step.

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