A METHOD FOR SURFACE HARDENING A FERROUS-ALLOY ARTICLE AND THE RESULTING PRODUCT

Abstract: A method of providing the surface of a ferrous alloy arti­ cle containing at least 0.2% by weight of carbon with a surface layer composed of the nitrocarbide of an element of Group Va of the periodic table and an inner ferrous alloy layer contain­ ing nitrogen located immediately below the surface layer. The layers are formed by (a) nitriding the article surface and (b) a thermal diffusion treatment thereof with a treating agent con­ taining the Group Va element. The formation of the layers can be accomplished easily and rapidly without presenting any problem when the article is later hardened.

Description: A METHOD FOR SURFACE HARDENING A FERROUS-ALLOY ARTICLE AND THE RESULTING PRODUCT Background of the Invention Field of the Invention This invention relates to a method for surface hardening a ferrous alloy article which forms a nitrocarbide layer, particularly a. layer of nitrocarbide of a Group Va element, on the surface of the article and to the ferrous alloy article obtained thereby. Description of the Prior Art: The inventors of this invention have previously developed a method for surface hardening of a ferrous alloy article containing at least 0.2t by weight of carbon. A layer of carbide of an element of Group Va of the periodic table is formed on the article surface by treatment thereof with, for example, a molten salt, powder or gas containing the Group Va element. This method is widely used to prolong the life of, for. example, a tool or mold. The carbide layer is formed by thermal diffusion and combination of a Group Va element, i.e. vanadium, niobium or tantalum, and carbon in the ferrous alloy article. The amount of carbon in the article immediately below the carbide layer is reduced as it is consumed for the formation of the carbide layer. As a result, when the article is hardened, its portion immediately below the carbide layer is likely to obtain a lower degree of hardness than the inner portion of the article. If this article is used for an application in which it is subjected to a high stress, it is likely that the portion immediately below the carbide layer may be deformed to cause the cracking or peeling of the carbide layer. If the article is a bar having a diameter of no more- than 1 mm or has a small thickness lifie the cutting edge of a sharp knife, the prior method for surface treatment reduces the amount of carbon throughout the article and thereby makes it difficult to harden the article satisfactorily. A layer of a carbide containing nitrogen or a nitrocarbide has recently been found to be higher in toughness than a carbide layer and has come to replace the carbide layer for the purpose of coating the surface of an article. This nitrocarbide coating treatment is presently carried out by a method employing a gas, such as titanium tetrachloride. This method not only requires substantial caution for protecting the health of workers, but also is complicated and necessitates expensive equipment. Summary of the Invention An object of this invention is to improve a previously-developed method for a carbide layer and provide a method which effectively forms (on the surface of a ferrous-alloy article) a surface layer composed of the nitrocarbide of a Group Va element and an inner layer composed of a ferrous alloy containing a solid solution of nitrogen. Another object of this invention is to provide a method which easily and quickly forms a nitrocarbide layer. A further object of this invention is to provide a ferrous alloy article which can be satisfactorily hardened. According t6 the method of this invention, a layer of nitrocarbide of a Group Va element and a layer of a ferrous alloy containing a solid solution of nitrogen are formed on the surface of a ferrous alloy article by nitriding the article surface, the thermal diffusion treatment of the nitrided article with a treating agent containing the Group Va element and the combination of the Group Va element and the carbon and nitrogen in the article by virtue of their mutual chemical affinity at the treating temperature. This invention is applicable to a ferrous alloy article containing at least 0.2% by weight of carbon. The presence of at least 0.2% by weight of carbon in the ferrous alloy is essential to form a nitrocarbide layer of good quality. In the event a ferrous alloy article contains only less than 0.2% by weight of carbon, it is carburized before, during or after nitriding so that its surface portion may contain at least 0.28 by weight of carbon prior to its treatment for the formation of a nitrocarbide thereon. The ferrous alloy article produced by the method of this invention has (on its surface) a surface layer composed of the nitrocarbide of a Group Va element and an inner layer composed of a ferrous alloy containing a solid solution of nitrogen immediately below the surface layer of the article, and can be hardened and quenched satisfactorily. These and other objects, features and advantages of this invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the invention is shown by way of illustrative example. Brief Description of the Drawings -Figures 1 and 4 are graphs showing the hardness of the base material in the ferrous alloy article treated in accordance with this invention in Examples 1 and 3, respectively. Figures 2, 3 and 6 to 11 are graphs showing the results of X-ray microanalysis of the surface portion of the ferrous alloy article treated in accoraance with this invention in Examples 1, 3 and 5 to 10; and Figure 5 is a microphotograph of 400 magnifications showing the cross section of the surface layer formed in accordance with this invention in Example 4. Detailed Description The surface hardening treatment of this invention includes nitriding. It is possible to employ any method, such as gas nitriding, gas soft nitriding, salt bath soft nitriding or glow discharge nitriding. Each of these methods is conventional and well known. It is preferable to form a nitrided layer having a high nitrogen content and a large depth. In the event carburization treatment follows nitriding, it is particularly important to form a nitrided layer having a sufficiently large depth and a sufficiently high nitrogen content, since diffusion of nitrogen into the inner or center portion of an article during carburization leads to a reduction in the amount of nitrogen in its surface portion. The carburization must be effected in a short time. Suck carburization is well known and is readily effected by any skilled artisan. The formation of a nitrocarbide layer can be effected by various processes, including molten salt dipping, molten salt electrolysis, powder packing, slurry process and fluidized furnace process. These processes and required compositions are, per se,- known, but not in the context of the present invention This treatment combines a Group Va element in a treating agent and the carbon and nitrogen in the ferrous alloy article. This treatment is carried out at a-temperature of at least about 600"C and lower than the melting point of the material to be treated. An appropriate temperature range is from 800"C to 1200bC. The period of time required for the treatment is in the range of from 30 minutes to 24 hours. When an electrolysis process is employed, it is appropriate to apply a cathode current density of from 0.01 to 3 A/cm2. The treatment provides a surface layer having an increased thickness when the temperature and period of time are increased. An increase in the thickness of the layer., however, tends to develop fine pores in the layer and to lower its pitting and wear resistance. Although the critical thickness beyond which those defects are intolerable depends on the Group Va element employed, nitriding conditions, the kind of ferrous alloy article and treatment conditions for the nitrocarbide layer, there is usually no appreciable problem when the thickness does not exceed 10 pm. The treatment of this invention forms on the surface of a ferrous alloy article a surface layer composed of the nitrocarbide of a Group Va element; the surface layer is outstanding in wear resistance and toughness. As the formation of the layer according to this invention is effected by the combination of not only carbon, but also nitrogen, in the article with the Group Va element, it obtains a predetermined thickness within a shorter period of time than a carbide layer does. The treatment of this invention also forms a ferrous alloy layer containing a solid solution of nitrogen immediately below the nitrocarbide layer. This makes it possible to obtain a sufficiently high degree of hardness in the area immediately below the surface layer. Therefore, it is possible to harden the article satisfactorily, even when the article has only a small thickness. EXAMPLE 1 An untreated sample and a nitrided sample, each having a diameter of 20 mm, were formed from alloy tool steel for hot forming use, AISI H13 (equivalent to Japanese Industrial Standard (JIS) SKD61). The nitrided sample was prepared by two hours of immersion in a salt bath containing cyanide and having a temperature of 5600C. These samples were subjected to molten salt dipping, i.e. immersed in a bath of molten borax containing 20% by weight of vanadium oxide (V205) and 108 by weight of boron carbide (BqC) and having a temperature of 10000C for from 30 minutes to 16 hours, and oil quenched. After the molten borax adhering to the samples had been removed by washing, each sample was cut in cross section and the cut section was polished; the thickness of the exposed surface layer was then measured. A layer thickness of 8 or 6 pm was obtained on the nitrided samples by immersing the same for eight or four hours, respectively, in the molten salt bath for forming the carbide layer but twice as much time, i.e. 16 or 8 hours, was required to obtain the same layer thickness on the untreated samples. The cross-sectional hardness distribution of the article was examined, and the results are shown in Figure 1. Curve B indicates that the unnitrided sample suffered from a reduction in hardness immediately below the surface layer (of vanadium carbide), but, as is obvious from curve A, the nitrided sample did not substantially show any such. hardness reduction. The X-ray microanalysis of the cross section of the nitrided sample, which had been treated with molten borax for an hour, revealed the presence of C and N, as well as V, in the layer as shown in Figure 2. The lattice constant obtained by X-ray diffraction confirmed that the layer was one of vanadium nitrocarbide expressed as V(C, N). The layer was found to contain 10% by weight of nitrogen. A ferrous alloy layer containing a solid solution of nitrogen was found to have been formed immediately below the nitrocarbide layer. It coincided with the area in which no substantial hardness reduction took place as shown by curve A in Figure 1. EXAMPLE 2 A nitrided sample and an unnitrided sample were prepared from carbon steel for machine structural use, AISI H13 (equivalent to JIS 54 SC). The nitrided sample was prepared by three hours of glow discharge nitriding (ion nitriding) at 555"C. The samples were each 7 mm in diameter. They were immersed in a bath of molten borax containing 20t by weight of Fe-V powder and having a temperature of 9000C for from 30 minutes to 16 hours. The cross-sectional examination of the samples for layer thickness indicated that a surface layer grew at a faster rate (about 1.5 times faster) on the nitrided sample than on the unnitrided sample. A surface layer of V(C, N) and an inner layer composed of a ferrous alloy containing nitrogen and located immediately below the surface layer were formed in the nitrided sample. EXAMPLE 3 The procedures of EXAMPLE 1 were repeated for the preparation of unnitrided and nitrided samples made of alloy tool steel for hot forming use, AISI H13 (equivalent to JIS SKD6l). They were immersed in a bath of molten borax containing 20% by weight of iron-niobium-tantalum (Fe-Nb-Ta) powder for 30 minutes to four hours. The formation of 4 and 5 Fm thick layers required one and two hours,of immersion for the carbide layer formation, respectively, on the nitrided sample, and two and four hours, respectively, on the unnitrided sample. The nitrided sample which had been subjected to 30 minutes of thermal diffusion treatment in the molten borax bath was found by X-ray microanalysis to contain N, as well as C, in its surface layer, as shown in Figure 3. As Ta and Nb also showed a similar change in concentration, the results of the X-ray diffraction could be interpreted to conclude that there had beenformed a surface layer of the nitrocarbide expressed as Nb, Ta(C, N) and an inner layer composed of a ferrous alloy containing a solid solution of nitrogen. As is obvious. from Figure 4, the unnitrided sample (curve B) showed a substantial reduction in hardness immediately below the surface layer, but the nitrided sample (curve A) did not show any such phenomenon. EXULE 4 A nitrided sample made of alloy tool steel for hot forming use, AISI H13 (equivalent to JIS Sky61) was prepared as described in Example 1, and subjected to molten salt electrol ysis. Namely, it was immersed in a bath of molten borax containing 10% by weight of niobium oxide (nub205) and having a temperature of 1000 C. The sample was employed as a cathode and subjected to two hours of electrolysis at a cathode current density of 0.05 A/cm2 in a graphite vessel employed as an anode. A layer having a smooth surface (as shown by the microphotograph of Figure 5 was formed, and it was found to be composed of a nitrocarbide Nb(C, N). An inner layer (composed of a ferrous alloy containing nitrogen) was found to exist immediately below the surface nitrocarbide-layer, as was also the case in the following examples. EXAMPLE 5 A sample made of carbon steel: for machine structural use, AISI 1045 (equivalent to JIS S45C) was subjected to salt bath nitriding at 5700C for 90 minutes and then immersed for the carbide layer formation for 30 minutes in a nitrogen-protected bath of barium chloride (BaC12) containing 108 by weight of Fe-V powder and having a temperature of I0000C. A layer having a thickness of 3 um was thereby formed. The X-ray microanalysis of the layer indicated that it was a layer of the nitrocarbide expressed as V(C, N) , as shown in Figure 6. Moreover, the microscopic examination of the sample revealed the presence of a structural containing nitrogen in the area of the article adjoining the nitrocarbide layer. EXAMPLE 6 A sample made of alloy tool steel for cold forming use, AISI D2 (equivalent to JIS S in the shape of-a round bar was subjected to gas soft nitriding at 570CC for 150 minutes, and was subjected to powder packing, i.e. buried in a powder mixture consisting of Fe-V powder and 10% by weight of potassium borofluoride (KBF4) powder, having a particle size of -lCO-mesh and placed in a stainless steel vessel. The vessel was heated at 6000C for 16 hours in an atmospheric furnace. After the vessel had been taken out of te furnace and air cooled, the sample was taken out of the powder. The sample was found by X-ray microanalysis to have a layer composed of V, N and C as shown in Figure 7, i.e., a layer of the nitrocarbide expressed as V(C, N). EXAMPLE 7 A round bar made of carbon steel for machine structural use, AISI 1045 (equivalent to JIS S45C) was subjected to gas nitriding at 5000C for 60 hours, and the procedures of EXAMPLE 6 were repeated for treating the sample, except that a heating temperature of 650"C was employed. A layer composed of V, N and C was found on this sample, too, as shown in Figure 8, and this V(C, N) layer was formed on the outermost surface of the sample. EXAMPLE 8 A round bar made of carbon tool steel, AISI W 1-9 (equivalent to JIS SK4) was gas nitrided as described in EXAMPLE 7, and treated at 1000 OC for five hours with a powder mixture consisting mainly of Fe-V and containing 5% by weight of ammonium chloride (NH4C1). A layer composed of V, N and C was formed as shown in Figure 9. The X-ray diffraction of this surface layer revealed diffraction lines indicating VC and VN, and thereby confirmed that the surface layer was composed of the nitrocarbide expressed as v(C, N). EXAMPLE 9 A bar made of carbon tool steel, AISI W 1-9 (equivalent to JIS SK4) was nitrided ab described in EXAMPLE 6, and treated at 10000C for five hours with a powder mixture consisting of Fe-Nb-Ta and 10% by weight of KBF4 and having a particle size of -100 mesh. A surface layer composed of Nb, Ta, C and N (as shown in Figure 10) was formed. The layer was found to be of the nitrocarbide Nb, Ta(C, N). EXAMPLE 10 A slurry was formed from a powder mixture consisting of 40E by weight of alumina (A1203), 55% by weight of Fe-V and 58 by weight of ammonium chloride (NH4Cl) by using a solvent prepared by dissolving ethyl cellulose in ethyl alcohol. The slurry was applied in a thickness of 3 to 5 mm onto a sample made of carbon tool steel, AISI W 1-9 (equivalent to JIS SK4) which had been gas nitrided as described in EXAMPLE 7. The sample was placed in a stainless steel vessel, and heated at 10000C for five hours in an argon gas atmosphere. The surface layer thereby formed was found by X-ray microanalysis to be composed of V(C, N). EXAMPLE 11 A powder mixture consisting of 60% by weight of alumina (A1203) r 38.8% by weight of Fe-V and 1.2% by weight of NH4Cl was placed in a fluidizing furnace1 and fluidized by the argon gas introduced into the furnace through the bottom thereof. A bar made of alloy tool steel for hot forming use, AISI H13 (equivalent to JIS So61), which had been salt-bath nitrided as described in EXAMPLE 1, was placed in the furnace, taken out after eight hours, and air hardened. A layer of the nitrocarbide V(C; N) was formed on the bar, as shown in Figure 11. EXAMP-TE 12 A standard thread-cutting tap made of high speed tool steel, AISI M2 (equivalent to JIS SKH9) and having a pitch diameter of 8 mm was dipped in a bath of molten borax containing 30% by Weight of V2O5 and 15% by weight of BqC, and treated at 10250C for an hour, whereby a vanadium carbide layer was formed on the surface of the tap. The tap was then heated at 1190 OC for 30 minutes in a vacuum furnace and thereafter gas hardened. The tap was subjected to salt bath soft nitriding at 5600C for 20 minutes, and treated in a bath of molten borax as hereinabove described, whereby a vanadium nitrocarbide layer was formed on the surface of the tap. The tap was then hardened. The tap treated in accordance with this invention and a commercially-available nitrided tap were tested for thread cutting in carbon steel material, AISI 1045 (equivalent to JIS S45C), and the life of each such tap was examined. About 1500 holes could be cut by the commercially-available tap, about 2500 holes by the tap on which the carbide layer had been formed, and about 3000 holes by the tap on which the nitrocarbide layer had been formed. This example also established that a ferrous alloy layer containing nitrogen could be formed immediately below the surface layer in accordance with this invention and enabled the satisfactory hardening and quenching of a ferrous alloy article. As is obvious from the foregoing description, this invention makes it possible to form a surface layer composed of the nitrocarbide of a Group Va element and an inner layer of a ferrous alloy containing nitrogen in a ferrous alloy article by nitriding it and introducing the Group Va element to combine it by thermal diffusion with the nitrogen and carbon in the ferrous alloy. The nitrocarbide layer defines a surface having good properties, and the inner layer enables the material immediately below the surface layer to be hardened and quenched satisfactorily. The treatment of this invention can be carried out very quickly. This invention is particularly useful if applied to cutting tools, as it can greatly prolong. their life.

Claims: WHAT IS CLAIMED IS: 1. A method for the surface hardening treatment of a ferrous alloy article, comprising the steps of: nitriding the surface of a ferrous alloy article containing at least 0.28 by weight of carbon, and subjecting the surface of said article to thermal diffusion treatment with a treating agent containing an element of Group Va of the periodic table to provide the surface of said article with a surface layer composed of a nitrocarbide and an inner ferrous alloy layer containing a solid solution of nitrogen and located immediately below said surface layer. 2. A method as set forth in claim 1, wherein said nitriding is selected from gas nitriding, gas soft nitriding, salt bath soft striding and glow discharge nitriding. 3. A method as set forth in daim 1 or 2,wherein said thermal diffusion treatment is carried out by a process selected from molten salt dipping, molten salt electrolysis, powder packing, slurry treatment and powder fluidization. 4. A method as set forth in claim 1 or 2, Wherein said thermal diffusion treatment is carried out in a molten salt bath containing said Group Va element and a borage 5. A method as set forth in claim 4, wherein said article is dipped in said molten salt bath. 6. A method as set forth in claim 4, wherein said article is employed as a cathode, and subjected to electrolysis in said molten salt bath at a cathode current density of from 0.01 to 3 A/cm2. 7. A method as set forth in claim 1 or 2., wherein said thermal diffusion treatment is carried out by burying said article in powder of said treating agent. 8. A method as set forth in claim 1 or 2, wherein said thermal diffusion treatment is carried out by coating said article with a slurry of said treating agent. 9. A method as set forth in claim 1 or 2, wherein said thermal diffusion treatment is carried out by fluidizing powder of said treating agent and placing said article in said fluidized powder. 10. A method as set forth in any of claims 1 to 9, wherein said treating agent is an alloy of at least one of vanadium oxide, niobium oxide and tantalum oxide. 11. A method as set forth in any of claims 1 to 9, wherein said treating agent is at least one alloy selected from the group consisting of ferrovanadium, ferroniobium and ferrotantalum. 12. A method as set forth in any of claims 1 to 11, wherein said thermal diffusion treatment is carried out at a temperature of at least 6000C, but lower than the melting point of said article. 13. A method as set forth in claim 12, wherein said thermal diffusion treatment is carried out at a temperature of from 8000C to 12000C. 14. A method as set forth in claim 12, wherein said thermal diffusion treatment is continued for from 30 minutes to 24 hours. 15. A method as set forth in claim 5, wherein said nitriding is carried out in a salt bath. 16. A method as set forth in claim 7, wherein said nitriding is gas soft nitriding. 17. A ferrous alloy article having a surface layer composed of the nitrocarbide of an element of Group Va of the periodic table and an inner ferrous alloy layer containing a solid solution of nitrogen and located immediately below said surface layer. 18. A hardened ferrous alloy article wherein the article is one as set forth in claim 17.

Inventors: ARAI, TOHRU, ENDO, JUNJI, SUGIMOTO, YOSHIHIKO
Applicants: KABUSHIKI KAISHA TOYOTA CHUO KENKYUSHO
Application year: 1984
Main classification: C23C8
IPC codes: C23C28, C23C10, C23C12