Surface hardening treatment method

Abstract

This process reliably improves the wear resistance, seizure resistance, fatigue strength, and heat check resistance of the workpiece W without requiring prolonged gas nitriding, blasting, or pickling. [Solution] The present invention is a surface hardening treatment method for a workpiece W made of alloy tool steel or high-speed tool steel. In the surface hardening treatment method of the present invention, the workpiece W is subjected to a preliminary treatment in which it is heated and held in an atmosphere of heat treatment gas, and then the workpiece W is subjected to a gas nitriding treatment.

Description

Technical Field

[0001] The present invention relates to a surface hardening treatment method applied to a workpiece made of alloy tool steel or high-speed tool steel.

Background Art

[0002] In recent years, with the near-net shaping of various parts in hot and cold applications, an increase in the load on workpieces such as molds, that is, the workpieces, has been regarded as a problem. In this regard, as means for improving characteristics such as wear resistance, seizure resistance, and heat check resistance of workpieces, surface hardening treatments such as nitriding treatment and carburizing treatment have been known for a long time. Among them, gas soft nitriding treatment and gas nitriding treatment have the advantage that the treatment temperature is about 500 to 580 ° C, which is relatively low compared to the quenching temperature and the like, so that the workpiece is less likely to be distorted, that is, the dimensional change is small (see, for example, Patent Documents 1 and 2).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Disclosure of the Invention

Problems to be Solved by the Invention

[0004] However, in a workpiece containing an element such as chromium that has a strong affinity for oxygen, such as alloy tool steel or high-speed tool steel, the passive film (chromium sesquioxide: Cr2O3) formed on the surface is strong and exhibits high stability. Therefore, in the conventional technology, due to the influence of the passive film on the workpiece surface, non-uniform nitriding is likely to occur, and there is also a problem that a long-time treatment is required to obtain a deep surface hardened layer, resulting in high costs.

[0005] To eliminate the effects of the passivation film, one might consider, for example, performing blasting or pickling treatments prior to gas nitriding, and this is actually done in some cases. However, despite adding an extra step, there is a possibility that the workpiece surface will be re-oxidized (the passivation film will be reformed) due to the oxygen content in the atmosphere during heating or gas nitriding, so it is not necessarily considered an effective method. [Means for solving the problem]

[0006] The inventors of this application have diligently conducted research on practical processing techniques that can reliably improve the properties of a workpiece, such as wear resistance, seizure resistance, fatigue strength, and heat check resistance, without the need for prolonged gas nitriding, blasting, or pickling, and have established the surface hardening method of the present invention.

[0007] The present invention relates to a surface hardening treatment method for a workpiece made of alloy tool steel or high-speed tool steel, wherein the workpiece is subjected to a preliminary treatment in which it is heated and held in an atmosphere of heat treatment gas, and then subjected to a gas nitriding treatment.

[0008] In the surface hardening treatment method of the present invention, the heat treatment gas may be an oxynitriding gas obtained by mixing ammonia gas and an oxidizing gas.

[0009] In the surface hardening treatment method of the present invention, the nitriding treatment with the gas is performed in the same heat treatment furnace as the preliminary treatment, and the transition from the preliminary treatment to the nitriding treatment with the gas may be made continuous by switching the atmospheric gas supplied into the heat treatment furnace from the oxynitriding gas to a nitriding gas containing ammonia gas.

[0010] Furthermore, in the surface hardening treatment method of the present invention, the treatment temperature in the preliminary treatment and the treatment temperature in the gas soft nitriding treatment or the gas nitriding treatment may be set to be the same.

[0011] Furthermore, in the surface hardening treatment method of the present invention, the treatment time for which the preliminary treatment is performed may be set to be half or longer than the treatment time for which the nitriding treatment with gas is performed. [Effects of the Invention]

[0012] According to the present invention, by performing a preliminary treatment as a pretreatment before gas nitriding, defects in the passive film on the workpiece surface are increased or destroyed, thereby reducing the nitriding resistance of the workpiece surface, and thus activating the workpiece surface. Then, since gas nitriding is performed thereafter, nitrogen penetrates and diffuses more easily into the workpiece surface during this treatment, and as a result, a workpiece with high hardness throughout is obtained. Therefore, the present invention can obtain good surface quality of the workpiece while suppressing processing costs by using different heat treatment gases (atmosphere gases) without requiring long gas nitriding treatments or blasting or pickling treatments. As a result, properties such as wear resistance, seizure resistance, fatigue strength, and heat check resistance of the workpiece can be improved. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram of a heat treatment facility for implementing the present invention. [Figure 2] This is a process diagram showing an example of a gas soft nitriding or gas nitriding treatment. [Figure 3] This graph shows the results of hardness measurements using SKD11, comparing cases where pretreatment was performed before gas soft nitriding and cases where no pretreatment was performed. [Figure 4] This graph shows the results of hardness measurements using SKD61, comparing cases where pretreatment was performed before gas soft nitriding and cases where no pretreatment was performed. [Figure 5] This graph shows the results of hardness measurements using SKH51, comparing cases where pretreatment was performed before gas nitriding and cases where no pretreatment was performed. [Figure 6]A graph showing the results of measuring the hardness of SKD61 that was surface hardened by setting the pretreatment time and the gas soft nitriding treatment time. [Figure 7] (A) is the workpiece surface image when the surface hardening treatment (pretreatment + gas soft nitriding treatment) of the present invention is performed, and (B) is the workpiece surface image when only the pretreatment (acid nitriding treatment) is performed. [Figure 8] A table summarizing the evaluation results of Examples 1 to 6 and Comparative Examples 1 to 4.

Mode for Carrying Out the Invention

[0014] Hereinafter, embodiments embodying the present invention will be described based on the drawings. Although the drawings show preferred embodiments, it can be implemented in many different forms and is not limited to the embodiments described in this specification.

[0015] (1) Implementation Equipment First, referring to FIG. 1, the heat treatment equipment for implementing the present invention will be described. The heat treatment equipment is the same as the conventional one and includes a heat treatment furnace 1 having a heating heater 2. The heat treatment furnace 1 is provided with a gas supply port 3 for supplying an atmosphere gas such as nitrogen gas, oxidizing gas or carburizing gas, a stirring fan 6 for stirring the atmosphere gas introduced into the heat treatment furnace 1, a fan motor 5 for operating the stirring fan 6, and a gas discharge port 4 for discharging exhaust gas.

[0016] The gas supply port 3 is connected to a gas conversion device 11 for generating a carburizing gas such as RX gas, an ammonia gas cylinder 14, a nitrogen gas cylinder 13, and an air supply blower 12. Each of them including the gas conversion device 11 is configured to supply a carburizing gas, ammonia gas, nitrogen gas, and air as the atmosphere gas. The gas conversion device 11 is connected to a propane gas cylinder 16 for supplying propane gas as the enriching gas.

[0017] In the supply path from the gas conversion device 11 towards the gas supply port 3, a pneumatic operated valve 8 and a gas flow meter 10 are provided. In the supply paths from each gas cylinder 14, 13 towards the gas supply port 3, a pneumatic operated valve 8, a pressure reducing valve 9 and a gas flow meter 10 are provided. In the supply path from the air supply blower 12 towards the gas supply port 3, a pneumatic operated valve 8, a pressure reducing valve 9 and a gas flow meter 10 are provided. A gas supply source valve 7 is provided on the downstream side of the confluence path where each supply path converges. In the discharge path extending from the gas discharge port, an exhaust gas detoxifying device 17 is provided.

[0018] The workpiece W, which is the object to be surface hardened in the present invention, is made of alloy tool steel or high speed tool steel. Suitable materials for the workpiece W include, in the case of alloy tool steel, hot die steel and cold die steel according to JIS standard (JISG4404:2015), and in the case of high speed tool steel, molybdenum series, tungsten series and powder high speed steel according to JIS standard (JISG4403:2015). That is, it is preferable to use tool steel with a chromium content of 13 mass% or less, which is less than that of stainless steel, as the material of the workpiece W. More preferably, it is tool steel with a chromium content of 6 mass% or less.

[0019] Specific alloy tool steels include SKD61, SKD62, SKD4, SKD5, SKD6, SKD7, SKD8, SKT3, SKT4, SKT6, SKS3, SKS31, SKS93, SKS94, SKS95, SKD1, SKD2, SKD10, SKD11, SKD12 according to JIS standard, and those corresponding thereto.

[0020] Also, specific high speed tool steels include SKH50, SKH51, SKH52, SKH53, SKH54, SKH55, SKH56, SKH57, SKH58, SKH59, SKH2, SKH3, SKH4, SKH10, SKH40.

[0021] (2) Surface hardening treatment As shown in Figure 2, the surface hardening treatment for the workpiece W proceeds in the following order: a heating step using a heat treatment furnace 1, a heat retention step, a pretreatment step using an oxynitriding gas mixed with ammonia gas and an oxidizing gas, a nitriding treatment step using gas soft nitriding or gas nitriding gas, and a cooling step.

[0022] In the heating step, the degreased and cleaned workpiece W is placed in the heat treatment furnace 1, the furnace atmosphere is thoroughly replaced with an inert gas (in this case, nitrogen gas), and then heated to a set temperature Ts (for example, 500-580°C). At this time, the nitrogen gas heated to a high temperature in the heat treatment furnace 1 is stirred by a stirring fan 6 to generate convection, which causes the inside of the heat treatment furnace 1 to heat up quickly. Since the inside of the heat treatment furnace 1 is filled with nitrogen, an inert gas, and oxygen is absent, surface oxidation of the workpiece W during the heating step does not proceed or is significantly suppressed. Although argon gas and other gases can also be used as inert gases, nitrogen gas is preferable from a cost perspective.

[0023] Once the heat treatment furnace 1 is heated to the set temperature Ts, the process moves to a holding stage where the workpiece W is heated continuously for a predetermined holding time tb2 while maintaining the set temperature Ts, thereby achieving uniform heating at the set temperature. The set temperature Ts can be appropriately determined according to the material and surface processing state of the workpiece W. Needless to say, the holding time tb2 will vary greatly depending on the thickness and shape of the workpiece W, the processing volume of one batch, etc.

[0024] Once the workpiece W in the heat treatment furnace 1 is uniformly heated and the heat retention process is complete, the process moves to the pretreatment process, i.e., the oxynitriding process. In the pretreatment process, a heat treatment gas mainly composed of ammonia gas is introduced into the heat treatment furnace 1. The heat treatment gas is an oxynitriding gas, which is a mixture of ammonia gas and an oxidizing gas (in this case, air). The atmosphere inside the furnace is switched from an inert gas, nitrogen gas, to an oxynitriding gas atmosphere. Then, while maintaining the set temperature Ts in the oxynitriding gas atmosphere, the workpiece W is continuously heated and held for a predetermined processing time t1. Note that the oxidizing gas is not limited to air; oxygen gas or water vapor may also be used. The oxygen concentration of the oxidizing gas is preferably set in the range of 1 to 5 volume percent. This corresponds to approximately 5 to 25 volume percent when converted to air. If the oxygen concentration is less than 1 volume percent, the pretreatment effect is difficult to obtain, and if it exceeds 5 volume percent, there is a possibility of excessive oxidation and the formation of an oxide film that inhibits nitriding. The preferred ammonia gas concentration at this time is 30 volume percent or more. Furthermore, when adding atmospheric gases other than ammonia gas and oxidizing gases, it is desirable to add inert gases such as nitrogen gas, as these gases should not inhibit the surface activation reaction.

[0025] During the preliminary treatment, the oxygen contained in the air of the oxynitriding gas promotes the decomposition of ammonia. The radical (active) hydrogen (H) generated by the decomposition of ammonia and present near the workpiece surface W reduces oxygen (O) from the passive film (chromium trioxide: Cr2O3) on the workpiece W surface, thereby increasing or destroying defects in the passive film and reducing the nitriding resistance of the workpiece W surface, in other words, activating the workpiece W surface.

[0026] Of course, even during the pretreatment process, it is understood that radical nitrogen (N) generated by the decomposition of ammonia and other processes and present near the workpiece surface W penetrates and diffuses into the workpiece W.

[0027] Once the preliminary processing is complete, the process moves on to gas nitriding. Examples of gas nitriding include gas soft nitriding and gas nitriding.

[0028] In the gas soft nitriding process, ammonia gas and a carburizing gas (in this case, RX gas) are introduced into the heat treatment furnace 1, and the furnace atmosphere is switched to an atmosphere of soft nitriding gas, which is a mixture of ammonia gas and RX gas. In this case, the supply of ammonia gas to the heat treatment furnace 1 is continued, the supply of oxidizing gas (air) is stopped, and the carburizing gas (RX gas) is supplied in place of the oxidizing gas. Then, the workpiece W is heated and held for a predetermined time (processing time t2) while maintaining the set temperature Ts in the soft nitriding gas atmosphere. Note that the carburizing gas is not limited to RX gas; methanol decomposition modified gas or a mixture of nitrogen gas and carbon dioxide gas may also be used.

[0029] In the gas nitriding process, ammonia gas is introduced into the heat treatment furnace 1, and the furnace atmosphere is switched to an ammonia gas atmosphere. In this case, the supply of ammonia gas to the heat treatment furnace 1 is continued, and the supply of oxidizing gas (air) is stopped. Then, the workpiece W is continuously heated and held for a predetermined time (processing time t2) while maintaining the set temperature Ts in the ammonia gas atmosphere. At this time, an inert gas such as nitrogen gas may be added in addition to ammonia gas.

[0030] Once the gas soft nitriding process or gas nitriding process is completed, the process moves to the cooling process. In the cooling process, the atmospheric gas is switched to nitrogen gas, an inert gas, heating in the heat treatment furnace 1 is terminated, and the workpiece W is cooled in the heat treatment furnace 1. Once the workpiece W has cooled sufficiently, it is removed from the heat treatment furnace 1, and the series of surface hardening processes is completed.

[0031] In the preliminary treatment process performed in the preceding stage, the nitriding resistance of the workpiece surface W is reduced by increasing or destroying defects in the passive film of the workpiece surface W, that is, the workpiece surface W is activated. Therefore, in both the gas soft nitriding process and the gas nitriding process, ammonia passes through the defects and destroyed areas of the passive film, adsorbs onto the iron (Fe) on the workpiece surface W, and decomposes, and the radical nitrogen (N) produced by the decomposition penetrates and diffuses into the workpiece W.

[0032] In the gas soft nitriding process, carbon monoxide (CO) in the RX gas of the soft nitriding gas generates carbon (C) through the Boudouar reaction, and carbon monoxide produced due to carbon dioxide (CO2) via the Boudouar reaction generates carbon again through the Boudouar reaction. Furthermore, radical carbon present near the workpiece surface W penetrates and diffuses into the workpiece W, just like radical nitrogen.

[0033] As a result of the above, the surface (surface layer) of the workpiece W after gas soft nitriding treatment contains ε(Fe), which is mainly composed of Fe. 2-3 A compound layer is formed containing a mixed phase of iron oxide, mainly Fe3C and trace amounts of Fe3O4, and a diffusion layer is further formed inside. Near the surface (surface layer) of the workpiece W after gas nitriding treatment, ε(Fe) is formed, mainly composed of Fe. 2-3 A compound layer is formed containing the N) phase and a mixed phase of iron oxide mainly composed of Fe3O4 in small amounts, and a diffusion layer is further formed internally. In either treatment, the hardness of the surface layer of the workpiece W is increased.

[0034] As mentioned above, the preliminary treatment process performed in the preceding stage is thought to reduce the nitriding resistance of the workpiece surface W by increasing or destroying defects in the passive film on the workpiece surface W, that is, by activating the workpiece surface W. Therefore, in the subsequent gas soft nitriding treatment, a compound layer of iron nitride or carbonitride can be well formed on the workpiece surface W, and in the subsequent gas nitriding treatment, a compound layer of iron nitride can be well formed on the workpiece surface W, thereby increasing the surface hardness of the workpiece W and improving its wear resistance and seizure resistance. In addition, nitrogen can be penetrated and diffused into the interior of the workpiece W to form a well-formed diffusion layer, improving the fatigue strength and heat check resistance of the workpiece W. In particular, if the workpiece W is made of steel for hot work dies, the effect of reducing erosion by molten metal and improving heat check resistance is high.

[0035] Furthermore, as mentioned above, since the pretreatment and gas nitriding treatment are carried out in the same heat treatment furnace 1, the workpiece surface W, which has increased or destroyed defects in the passive film, that is, has reduced or activated nitriding resistance, can be reliably maintained from the pretreatment to the gas nitriding treatment without re-oxidation, and nitrogen penetration and diffusion into the workpiece W can proceed easily. Moreover, since the transition from pretreatment to gas nitriding treatment is carried out simply by switching the atmospheric gas, existing equipment can be effectively utilized without adding new equipment, and the equipment costs associated with adding a pretreatment process can be significantly reduced.

[0036] Furthermore, the processing temperature in the heat treatment furnace 1 during gas nitriding can be set to the same temperature Ts as the processing temperature during the preliminary treatment. While it is possible to use different processing temperatures for the preliminary treatment and the gas nitriding treatment, using the same temperature Ts is preferable as it helps to reduce labor costs and other operational expenses. The processing temperatures for both the preliminary treatment and the gas nitriding treatment are preferably within the temperature range of 500 to 580°C. Processing within this temperature range allows for smooth nitrogen penetration and diffusion while minimizing distortion of the workpiece W, i.e., resulting in minimal dimensional changes. This is because temperatures below 500°C hinder the subsequent nitriding reaction, and temperatures above 580°C reduce the hardness of the workpiece W itself.

[0037] In the pretreatment process, it is understood that defects in the passive film on the surface of the workpiece W are increased or destroyed. Therefore, the treatment time t1 for the pretreatment shown in Figure 2 must be sufficiently long, and it is preferable that it be set to more than half of the treatment time t2 for the gas nitriding treatment (t1≧0.5t2). More preferably, the treatment time t1 is set to be the same as or longer than the treatment time t2 (t1≧t2). In this way, the overall treatment time for the surface hardening process can be shortened, and the treatment cost can be reduced. On the other hand, since a certain percentage of the nitriding treatment time is also required in addition to the pretreatment, it is preferable that the pretreatment time t1 is in the range of 0.5 to 5 times the nitriding treatment time t2. If the pretreatment time t1 is less than 0.5 times the nitriding treatment time t2, the effect of increasing or destroying defects in the passive film is weak, and the penetration and diffusion of nitrogen into the workpiece W is insufficient. If the pretreatment time t1 exceeds 5 times the nitriding treatment time t2, the activation effect on the surface of the workpiece W becomes too high, making it prone to embrittlement. The nitriding treatment time t2 can be appropriately determined according to the material of the workpiece W, the surface processing state, and the required quality, but it is preferable to set it to 1 hour or more from the viewpoint of preventing embrittlement. More preferably, it is 2 hours or more.

[0038] As explained above, it is presumed that by performing a preliminary treatment as a pretreatment before gas nitriding, defects in the passive film on the surface of the workpiece W can be increased or destroyed. Then, since gas nitriding is performed afterward, nitrogen can penetrate and diffuse more easily into the surface of the workpiece W during this treatment, and as a result, a workpiece W with high hardness throughout is obtained. Therefore, according to the present invention, good surface quality of the workpiece W can be obtained while suppressing processing costs by using different heat treatment gases (atmosphere gases) without spending a long time on gas nitriding or performing blasting or pickling treatments. As a result, properties such as wear resistance, seizure resistance, fatigue strength, and heat check resistance of the workpiece W can be improved.

[0039] Figure 8 is a table summarizing the evaluation results of Examples 1-6 and Comparative Examples 1-4, which are explained using Figures 3 and later. In Figure 8, the Hv1000 depth is the depth at which a line connecting hardness measurements at 20 μm intervals intersects with Hv1000, and the value is rounded down to the nearest 10 μm. In the overall evaluation column of Figure 8, an Hv1000 depth of 70 μm or more with no cracks is evaluated as "◎", and an Hv1000 depth of 50 μm or more with no cracks, or an Hv1000 depth of 70 μm or more with minor cracks is evaluated as "〇". Furthermore, an Hv1000 depth of 20 μm or more with no cracks is evaluated as "△", and an Hv1000 depth of less than 20 μm or with cracks is evaluated as "×".

[0040] (3) Hardness measurement results Figures 3 to 5 show the hardness measurement results for Examples 1 to 3, which underwent pretreatment before gas nitriding, and Comparative Examples 1 to 3, which did not undergo pretreatment. Here, Example 1 is SKD11, a steel for cold work dies, which underwent pretreatment with 80 vol% ammonia gas and 20 vol% air, while Comparative Example 1 is SKD11 without pretreatment, and the nitriding treatment was gas soft nitriding. Example 2 is SKD61, a steel for hot work dies, which underwent pretreatment with 50 vol% ammonia gas, 10 vol% air, and 40 vol% nitrogen gas, while Comparative Example 2 is SKD61 without pretreatment, and the nitriding treatment was gas soft nitriding. Example 3 is SKH51, a molybdenum-based high-speed tool steel, which underwent pretreatment with 30 vol% ammonia gas, 6 vol% air, and 64 vol% nitrogen gas, while Comparative Example 3 is SKH51 without pretreatment, and the nitriding treatment was gas nitriding. The gas soft nitriding treatment was performed using 70% ammonia gas and 30% RX gas, while the gas nitriding treatment was performed using 70% ammonia gas and 30% nitrogen gas. Figure 3 is a graph comparing the hardness of SKD11, Figure 4 is a graph comparing the hardness of SKD61, and Figure 5 is a graph comparing the hardness of SKH51.

[0041] As is clear from Figures 3 to 5, Examples 1 to 3 of the present invention, regardless of whether the nitriding treatment was gas soft nitriding or gas nitriding, showed higher hardness at the same depth than Comparative Examples 1 to 3, and the depth at which hardness decreased also decreased from a deeper position. In other words, in Examples 1 to 3, nitrogen penetration and diffusion into the workpiece W proceeded more easily than in Comparative Examples 1 to 3, resulting in a workpiece W with high hardness throughout. In Comparative Examples 1 to 3, where no pretreatment was performed, nitrogen penetration and diffusion into the workpiece W did not proceed well even over a long period of time (e.g., 12 hours or more), and as a result, sufficient surface hardness could not be obtained on the surface of the workpiece W.

[0042] Figure 6 shows the hardness measurements of Examples 2, 4, 5, and 6 of SKD61, in which the pretreatment time t1 and gas soft nitriding treatment time t2 were varied, and the total time of t1 and t2 was kept constant. Example 2 is the one described above, where the pretreatment time t1 is 1.3 times the gas soft nitriding treatment time t2 (t1=1.3t2). Example 4 is where the pretreatment time t1 is 0.5 times the gas soft nitriding treatment time t2 (t1=0.5t2). Example 5 is where the pretreatment time t1 is 5.0 times the gas soft nitriding treatment time t2 (t1=5.0t2). Example 6 is where the pretreatment time t1 is 0.1 times the gas soft nitriding treatment time t2 (t1=0.1t2).

[0043] As shown in Figure 6, in Examples 2, 4, and 5 of the present invention, where the pretreatment time t1 is set to be more than half the length of the gas soft nitriding treatment time t2, the depth at which hardness decreases is clearly deeper than in Comparative Example 2, despite the shorter treatment time. On the other hand, Example 6 has a shallower Hv1000 depth compared to Examples 2, 4, and 5, indicating that the effect of the pretreatment is recognized, but it is desirable to secure a pretreatment time of a certain proportion or more to obtain a greater effect. Furthermore, considering the results of the brittleness evaluation test using a Rockwell hardness tester described later, embrittlement is confirmed when the proportion of the nitriding treatment time t2 is too short. Therefore, it is understood that when the nitriding treatment time t2 is set in the range of 0.1 to 5 times the pretreatment time t1, sufficient hardness can be obtained on the surface of the workpiece W while suppressing embrittlement.

[0044] Figures 7(A) and 7(B) show the results of observing the surface of workpiece W after a brittleness evaluation test, using SKD61, a steel for hot-working dies, as a sample, when the surface hardening treatment of Example 2 of the present invention was performed, and when only the preliminary treatment (oxynitriding treatment) of Comparative Example 4 was performed. Comparative Example 4 was performed using the same gas composition as the preliminary treatment of Example 2: 50 vol% ammonia gas, 10 vol% air, and 40 vol% nitrogen gas. The brittleness evaluation test was performed by making an indentation on the surface using a Rockwell hardness tester on the A scale and checking whether a clear crack occurred around the indentation. Figure 7(A) is an image of the workpiece surface when the surface hardening treatment (preliminary treatment + gas soft nitriding treatment) of Example 2 of the present invention was performed, and Figure 7(B) is an image of the workpiece surface when only the preliminary treatment (oxynitriding treatment) of Comparative Example 4 was performed.

[0045] As is clear from Figures 7(A) and 7(B), the surface of workpiece W that has only undergone pretreatment (oxynitriding) shows numerous cracks around the formed indentations, unlike the surface of workpiece W that has undergone the surface hardening treatment of the present invention (pretreatment + gas soft nitriding). In other words, it was found that pretreatment (oxynitriding) alone is not sufficient to ensure the toughness of the workpiece W surface (hardened layer), and that applying the surface hardening treatment of the present invention (pretreatment + gas soft nitriding) can significantly suppress the decrease in brittleness of the workpiece W surface (hardened layer).

[0046] (4) Others The configuration of each part in the present invention is not limited to the illustrated embodiment, and various modifications are possible without departing from the spirit of the present invention. [Explanation of symbols]

[0047] Double job 1. Heat treatment furnace 11. Gas Transformer 12. Intake blower 13 Nitrogen gas cylinder 14 Ammonia gas cylinder 16 Propane gas cylinders

Claims

1. A surface hardening treatment method for a workpiece made of alloy tool steel or high-speed tool steel, After performing a preliminary treatment in which the workpiece is heated and held in a heat treatment gas atmosphere, the workpiece is subjected to a gas nitriding treatment. Surface hardening treatment method.

2. The heat treatment gas is an oxynitriding gas obtained by mixing ammonia gas and an oxidizing gas. The surface hardening treatment method described in claim 1.

3. The aforementioned gas nitriding treatment is carried out in the same heat treatment furnace as the aforementioned preliminary treatment. By switching the atmospheric gas supplied to the heat treatment furnace from the oxynitriding gas to a nitriding gas containing ammonia gas, the process is continuously transitioned from the preliminary treatment to the nitriding treatment using the gas. The surface hardening treatment method described in claim 2.

4. The ratio of the processing time of the preliminary treatment to the processing time of the nitriding treatment with the gas is 0.5 or more. The surface hardening treatment method described in claim 3.

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