JP2021066907A - Surface hardening method of stainless steel and surface hardening apparatus - Google Patents

Abstract

To provide a surface hardening technology to further increase a surface hardness in a short time without reducing corrosion resistance of a stainless steel.SOLUTION: A surface hardening method evacuates a heat treatment furnace in which a work made of stainless steel is accommodated, heats the work to reach a maximum temperature of 950°C or more and keep for a predetermined time. Then, after a pressure inside the heat treatment furnace is restored to an atmospheric pressure by supplying nitrogen gas into the heat treatment furnace, nitrogen gas and ammonia gas are supplied with the work kept at 950°C or more to form a solid solution of nitrogen in the work. Then, the work is cooled while nitrogen gas is supplied into the heat treatment furnace or nitrogen gas is circulated in the heat treatment furnace.SELECTED DRAWING: Figure 2

Description

The present invention relates to a surface hardening treatment method and a surface hardening treatment apparatus for hardening the surface by invading the surface layer of stainless steel with nitrogen.

Conventionally, nitriding treatment is performed in order to increase the surface hardness of stainless steel. There are various methods for this nitriding treatment, such as salt bath nitriding, ion nitriding, and gas nitriding. Gas nitriding is a method of infiltrating nitrogen (N) into the surface layer of stainless steel to harden the surface by low-temperature treatment using ammonia gas or high-temperature treatment using nitrogen gas.

As one of gas nitriding, while arranging stainless steel in a vacuum environment, it is held at 900 ° C. or higher (A3 transformation point or higher) for a certain period of time, and then held at 900 ° C. or higher in a nitrogen gas atmosphere to produce nitrogen (). There is a method of solidifying N) on the surface and then cooling it (see Patent Document 1).

When heated to 900 ° C. or higher in a vacuum environment, the dense passivation film formed on the surface of stainless steel is removed and the surface is activated. As a result, the dissociated nitrogen of the nitrogen gas added thereafter can be dissolved on the surface of the stainless steel.

Japanese Unexamined Patent Publication No. 60-165370

However, according to an unknown study by the present inventors, there is a problem that the surface hardness can be increased only up to 600 HV by the conventional gas nitriding method. On the other hand, if the holding time of the stainless steel in a nitrogen atmosphere is lengthened for the purpose of increasing the surface hardness, chromium nitride crystals are precipitated in the nitrided layer on the surface, in order to improve the original corrosion resistance of the stainless steel. There is a problem that the chromium concentration of stainless steel is significantly reduced, resulting in deterioration of corrosion resistance.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a surface hardening treatment method capable of further increasing the surface hardness in a short time without causing a decrease in the corrosion resistance of stainless steel. Is.

The present invention that achieves the above object is a vacuuming process for vacuuming the inside of a heat treatment furnace containing a work made of stainless steel, and setting the final reaching temperature of the work to 950 ° C. or higher while maintaining the vacuum state. After the heating process, the standby step of maintaining the work at the final reaching temperature of 950 ° C. or higher and the vacuum state for a predetermined time, and the standby step, by supplying nitrogen gas into the heat treatment furnace, the inside of the heat treatment furnace In the repressurization step of repressurizing the work to a pressure higher than the vacuum state, and in a state where the work is maintained at 950 ° C. or higher, nitrogen gas and ammonia gas are supplied into the heat treatment furnace, and nitrogen is solid-dissolved in the work. A nitrogen gas cooling step executed after the ammonia gas introduction step and cooling the work while supplying nitrogen gas into the heat treatment furnace or circulating the nitrogen gas in the heat treatment furnace. It is a surface hardening treatment method characterized by having.

In relation to the surface hardening treatment method, in the ammonia gas introduction step, the supply amount of the nitrogen gas is larger than the supply amount of the ammonia gas.

In relation to the surface hardening treatment method, in the nitrogen gas cooling step, the time for the work to pass through the temperature range of 750 ° C. to 950 ° C. is set to 40 minutes or less.

In relation to the surface hardening treatment method, the work is characterized in that the time for the work to reach 500 ° C. or higher is 30 minutes or longer in the heating process.

In relation to the surface hardening treatment method, the time of the ammonia gas introduction step is set to 60 minutes or less.

In relation to the surface hardening treatment method, the surface hardness of the work is set to 700 Hv or more.

The nitrogen gas cooling step is characterized in that the nitrogen gas newly supplied into the heat treatment furnace or the nitrogen gas staying in the heat treatment furnace is cooled by a heat exchanger.

The present invention that achieves the above object includes a heat treatment furnace that houses a work made of stainless steel, an exhaust device that creates a vacuum inside the heat treatment furnace, and a heating device that is provided in the heat treatment furnace to heat the work. An ammonia gas supply system that supplies ammonia gas to the heat treatment furnace, a nitrogen gas supply system that supplies nitrogen gas to the heat treatment furnace, the exhaust device, the heating device, the ammonia gas supply system, and the nitrogen gas supply system. The control device includes a vacuum processing unit that controls the exhaust device to bring the heat treatment furnace into a vacuum state, and controls the heating device while maintaining the vacuum state. Controlled by a temperature raising processing unit that raises the final reaching temperature of the work to 950 ° C. or higher, a maintenance processing unit that maintains the work at a final reaching temperature of 950 ° C. or higher and the work in the vacuum state for a predetermined time, and a maintenance processing unit. After that, by controlling the nitrogen gas supply system and supplying nitrogen gas into the heat treatment furnace, the pressure recovery unit for restoring the pressure inside the heat treatment furnace to a pressure higher than the vacuum state, and the pressure recovery unit. After the control by the processing unit, the nitrogen gas supply system and the ammonia gas supply system are controlled to supply the nitrogen gas and the ammonia gas into the heat treatment furnace while the work is maintained at 950 ° C. or higher. After control by the ammonia gas introduction processing unit that dissolves nitrogen in the work and the ammonia gas introduction processing unit, the nitrogen gas supply system is controlled to supply nitrogen gas into the heat treatment furnace or in the heat treatment furnace. It is a surface hardening treatment apparatus characterized by having a nitrogen gas cooling treatment unit for cooling the work while circulating the nitrogen gas of the above.

The surface hardening treatment apparatus includes a cooling apparatus for cooling nitrogen gas newly supplied to the heat treatment furnace or nitrogen gas staying in the heat treatment furnace by a heat exchanger, and the nitrogen gas cooling treatment unit is described. It is characterized in that the work is cooled by controlling the cooling device.

According to the surface hardening treatment method and the surface hardening treatment apparatus according to the present invention, it is possible to obtain an excellent effect that the surface of stainless steel can be hardened in a short time.

It is the schematic which showed the structure of the surface hardening treatment apparatus which concerns on embodiment of this invention. It is a figure which showed an example of the time chart of the surface hardening treatment method. It is a graph which showed the result of the hardness test by an Example. It is a graph which showed the result of the hardness test by a comparative example. It is a graph which showed the sensitization property of stainless steel.

Hereinafter, the surface hardening treatment apparatus and the surface hardening treatment method according to the embodiment of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a schematic view showing the configuration of the surface hardening treatment apparatus 1 according to the present embodiment. The surface hardening treatment apparatus 1 of the present embodiment is configured to perform a treatment of solid-solving nitrogen in stainless steel. As shown in FIG. 1, the surface hardening treatment device 1 includes a heat treatment furnace 10, a gas supply device 20 for supplying various gases to the heat treatment furnace 10, and an exhaust device 30 for discharging atmospheric gas in the heat treatment furnace 10. A cooling device 60 for cooling the atmospheric gas in the heat treatment furnace 10 and a control device 40 for controlling each part of the surface hardening treatment device 1 are provided.

The heat treatment furnace 10 accommodates and heats the work 100, which is a stainless steel material, and nitrides with the atmospheric gas in the furnace 10a. The heat treatment furnace 10 is configured so that the inside of the furnace 10a is kept in an airtight state from the outside and is kept warm by an appropriate heat insulating material. Further, the inside of the furnace 10a is configured to be able to accommodate a plurality of works 100 in a state of being placed on an appropriate jig 11 or the like.

Stainless steel is used as the base material of the work 100. Stainless steel is defined in the ISO standard as a steel having a carbon content of 1.2% (mass percent concentration) or less and a chromium content of 10.5% (mass percent concentration) or more. The type of stainless steel to which the surface hardening treatment method is applied is not particularly limited, and is applied to martensitic stainless steel, ferritic stainless steel, austenite stainless steel, austenite / ferritic stainless steel, precipitation hardening stainless steel, etc. it can.

The surface hardening treatment method and the like are particularly preferably applied to martensitic stainless steel. The chromium content of martensitic stainless steel ranges from approximately 11% to 18% (mass percent concentration), with a relatively low chromium content and a relatively high carbon content, quenching and / or quenching. It is returned and used. Martensitic stainless steel is used when hardness is required, but further hardness may be required depending on the purpose. Therefore, it is preferable to combine the surface effect processing apparatus 1 of the present embodiment. Specifically, in the steel grades of martensitic stainless steel according to the Japanese Industrial Standards (JIS), SUS410 and SUS410, which have chromium 11.50 to 13.000% (mass percent concentration) and carbon 0.15% (mass percent concentration) or less. , SUS420J2 containing 12.00 to 14.00% (mass percent concentration) of chromium and 0.26% to 0.40% (mass percent concentration) of carbon are typical examples. Since this martensitic stainless steel has a higher carbon concentration than the austenitic stainless steel, it has a characteristic that chromium nitride is easily precipitated when nitrogen, which will be described later, is dissolved in a solid solution.

The heat treatment furnace 10 includes a heating device (for example, a heating wire heater) 12 for heating the inside of the furnace 10a, a thermometer (for example, a thermocouple) 13 for measuring the temperature of the inside of the furnace 10a, and a pressure for measuring the pressure of the inside of the furnace 10a. A total of 14 and a stirring device (for example, a fan) 16 for stirring the atmospheric gas in the furnace 10a are provided.

The gas supply device 20 supplies the ammonia gas supply system 21 that supplies ammonia (NH3) gas to the inside of the furnace 10a, the nitrogen gas supply system 22 that supplies nitrogen (N2) gas to the inside of the furnace 10a, and the nitrogen (N2) gas. It has a pressure recovery system 23 that supplies the inside of the furnace 10a and returns the inside of the furnace 10a in a vacuum state to atmospheric pressure in a short time. Ammonia gas plays a role of supplying nitrogen (N) to enter the work 100. The nitrogen gas supplies nitrogen (N) that penetrates into the work 100, and at the same time, plays a role of cooling the work 100 by convection.

The ammonia gas supply system 21 supplies ammonia gas to the furnace 10a at a predetermined supply flow rate. The ammonia gas supply system 21 includes an ammonia gas supply source 21a made of a gas cylinder or the like containing ammonia, and a supply pipe 21b connecting the ammonia gas supply source 21a and the inside of the furnace 10a. Then, in the middle of the supply pipe 21b, a flow control valve 21c for adjusting the flow rate of ammonia gas, a solenoid valve 21d controlled by the control device 40 to open and close, and a flow meter 21e for measuring the flow rate of ammonia gas. Is provided.

The nitrogen gas supply system 22 supplies nitrogen gas to the furnace 10a at a predetermined supply flow rate. The nitrogen gas supply system 22 includes a nitrogen gas supply source 22a including a gas cylinder containing nitrogen gas, a nitrogen gas generator that separates nitrogen from the air by a pressure swing adsorption method (PSA), and the like, and nitrogen gas. It is provided with a supply pipe 22b that connects the supply source 22a and the inside of the furnace 10a. Further, in the middle of the supply pipe 22b, a flow control valve 22c for adjusting the flow rate of nitrogen gas, a solenoid valve 22d controlled by the control device 40 to open and close, and a flow meter 22e for measuring the flow rate of nitrogen gas. Is provided.

The decompression system 23 supplies nitrogen gas to the inside of the furnace 10a at a predetermined supply flow rate to restore the vacuum state of the inside of the furnace 10a to atmospheric pressure in a short time. Specifically, the decompression system 23 recompresses the furnace 10a within 60 seconds, preferably within 20 seconds, by supplying a large amount of nitrogen gas in a short time. The pressure-reducing system 23 includes a nitrogen gas supply source 23a including a gas cylinder containing nitrogen gas, a nitrogen gas generator that separates nitrogen from the air by a pressure swing adsorption method (PSA), and the like, and nitrogen gas. It is provided with a supply pipe 23b that connects the supply source 23a and the inside of the furnace 10a. Further, in the middle of the supply pipe 23b, a flow control valve 23c for adjusting the flow rate of nitrogen gas and a solenoid valve 23d controlled by the control device 40 to open and close are provided.

The exhaust device 30 adjusts the pressure in the furnace 10a by discharging the atmospheric gas in the furnace 10a. Specifically, the exhaust system 30 is a first exhaust system 31 that discharges atmospheric gas during supply of each gas from the gas supply device 20 and holds the inside of the furnace 10a at a predetermined pressure of atmospheric pressure or higher or around atmospheric pressure. From the second exhaust system 32 and the first exhaust system 31 or the second exhaust system 32, which sucks the atmospheric gas to reduce the pressure in the furnace 10a to a predetermined pressure of atmospheric pressure or less during the vacuum treatment. It includes a decomposition furnace 33 that decomposes ammonia remaining in the discharged atmospheric gas, and a combustion tower 34 that burns the combustible gas discharged from the decomposition furnace 33.

The first exhaust system 31 includes an exhaust pipe 31a that connects the inside of the furnace 10a and the decomposition furnace 33, and an electromagnetic valve 31b that is controlled by the control device 40 to open and close is provided in the middle of the exhaust pipe 31a. .. The second exhaust system 32 includes an exhaust pipe 32a that connects the inside of the furnace 10a and the decomposition furnace 33, and an electromagnetic valve 32b that is controlled by the control device 40 to open and close is provided in the middle of the exhaust pipe 32a. .. Further, a vacuum pump 32c that is controlled by the control device 40 and operates is provided in the exhaust pipe 32a between the solenoid valve 32b and the decomposition furnace 33. The second exhaust system 32 controls the inside of the furnace 10a to a vacuum state (low vacuum state, preferably medium vacuum state, more preferably high vacuum state). More specifically, in the case of a low vacuum state, for example, it is set to 10,000 Pa (about 75 Torr) or less, preferably 1000 Pa (about 7.5 Torr) or less. Further, in the case of a medium vacuum state, the control is controlled to 100 Pa (about 0.75 Torr) or less, preferably 30 Pa (about 0.23 Torr) or less, and more preferably 15 Pa (0.11 Torr) or less. Of course, it may be controlled to a high vacuum state (0.1 Pa or less).

The cooling device 60 lowers the temperature of the furnace 10a by cooling the circulating gas while circulating the atmospheric gas in the furnace 10a. Specifically, the cooling device 60 is provided with a circulation pipe 62 for leading out the atmospheric gas in the furnace 10a to the outside and returning the derived gas (circulating gas) to the inside of the furnace 10a, and in the middle of the circulation pipe 62. It has a blower 64 for forcibly circulating the circulating gas, and a heat exchanger 66 provided in the middle of the circulation pipe 62 to cool the circulating gas. Further, in the middle of the circulation pipe 62, solenoid valves 68a and 68b that are controlled by the control device 40 to open and close are provided. As a result, the temperature of the furnace 10a drops in a short time, so that the work 100 is also cooled. Here, a case where the cooling device 60 draws out and circulates the atmospheric gas to the outside of the furnace to cool the circulating gas is illustrated, but the present invention is not limited to this. For example, the cooling device 60 positively cools the nitrogen gas newly supplied by the nitrogen gas supply system 22 and then supplies the nitrogen gas to the inside of the furnace 10a to lower the temperature of the inside of the furnace 10a in a short time. You can also.

The control device 40 is composed of an appropriate microcomputer, PC, or the like, and controls the entire surface hardening treatment device 1. The control device 40 controls the heating device 12 based on the signal output of the thermometer 13, raises the temperature inside the heat treatment furnace 10 to a preset processing temperature T, and holds the temperature inside the heat treatment furnace 10. Further, the control device 40 controls the solenoid valve 23d of the gas supply device 20, supplies a large amount of nitrogen gas to the furnace 10a in a short time, and restores the vacuum state furnace 10a to atmospheric pressure in a short time. The control device 40 also controls the solenoid valves 21d and 22d of the gas supply device 20 to start or stop the supply of ammonia gas and nitrogen gas to the furnace 10a at a predetermined timing. Further, the control device 40 controls the cooling device 60 (solenoid valves 68a, 68b, blower 64, etc.) based on the signal output of the thermometer 13 to cool the inside of the heat treatment furnace 10 to a preset temperature.

The control device 40 controls the solenoid valve 31b of the first exhaust system 31 based on the signal output of the pressure gauge 14, and holds the inside of the furnace 10a at a preset pressure. The control device 40 also controls the solenoid valve 32b and the vacuum pump 32c of the second exhaust system 32 based on the signal output of the pressure gauge 14, and keeps the inside of the furnace 10a in a vacuum state.

Next, a specific procedure of the surface hardening treatment method of the present embodiment will be described. This surface hardening treatment method is realized by executing each processing unit of the surface hardening treatment program in the control device 40 by a microcomputer.

FIG. 2 is a diagram showing an example of a time chart of the surface hardening treatment method of the present embodiment.

The opening degrees of the flow control valves 21c and 22c are adjusted in advance, and the supply flow rates of ammonia gas and nitrogen gas are set. The supply flow rate of each gas is not particularly limited, and may be appropriately set according to the volume of the furnace 10a, the surface area of the work 100, the required thickness of the nitrided layer, and the like. On the other hand, in the present embodiment, it is preferable that the flow rate of nitrogen gas is larger than the flow rate of ammonia gas. If the supply flow rate of each gas is set appropriately, the nitriding process is started.

<Work placement process>

The work 100 is placed in the furnace 10a. The work 100 is arranged by a transfer device (not shown).

<Vacuum process>

Next, a vacuuming step is performed. In the vacuuming step, the control device 40 closes the solenoid valve 31b of the first exhaust system 31 and the solenoid valve of the second exhaust system 32 in a state where the supply of ammonia gas and nitrogen gas to the furnace 10a is stopped. 32b is opened, the vacuum pump 32c is started, and the atmospheric gas in the furnace 10a is sucked. For example, in about 10 to 30 minutes, the pressure in the furnace 10a becomes a vacuum state.

<Heating process and standby process>

After the inside of the furnace 10a is evacuated by the vacuuming step (or during the vacuuming process), the control device 40 controls the heating device 12 and presets the _{temperature rise times t A} , t _B , and t _C. The temperature of 10a in the furnace is raised to _{the processing temperatures T 1} , T ₂ , and T ₃ in a plurality of stages (here, three stages). Further, at each of the temperatures T ₁ , T ₂ , and T ₃ , the temperature is maintained over the preset standby times t ₁ , t ₂ , and t _3. It is preferable that the temperature rise times t _A , t _B , and t _C are as short as possible to quickly raise the temperature to the processing temperature.

Specifically, a part of the treatment temperatures of the plurality of stages is the treatment temperature on the low temperature side (T ₁ , T ₂ ), and the other part is the treatment temperature on the high temperature side (T ₃ ). The processing temperature (T ₁ , T ₂ ) on the low temperature side is preferably set to 500 ° C. or higher and 880 ° C. or lower. The processing temperature (T ₃ ) on the high temperature side is preferably set to 950 ° C. or higher. In this way, by avoiding the range of 880 ° C to 950 ° C at the processing temperatures T ₁ , T ₂ , and T ₃ , the risk is low because of the vacuum state, but chromium nitride is contained in these temperature ranges. Avoid it as it tends to precipitate.

In particular, the final processing temperature (T ₃ ) on the high temperature side is preferably 980 ° C. or higher, more preferably 1000 ° C. or higher. As a result, the passivation film on the surface of the work 100 can be completely removed and activated, and at the same time, the stainless steel of the work 100 is transformed into an austenite structure, so that nitrogen is easily dissolved. On the other hand, the final processing temperature (T ₃ ) on the high temperature side is preferably 1200 ° C. or lower, more preferably 1150 ° C. or lower. If the temperature is too high, the mechanical properties of stainless steel will change.

The standby times t ₁ , t ₂ , and t _{3 at the processing temperatures T 1} , T ₂ , and T ₃ exceeding 500 ° C. are preferably 10 minutes or more, and preferably 20 minutes or more, respectively. _{The total waiting time t 1} , t ₂ , and t ₃ exposed to the vacuum state is preferably 30 minutes or more, and preferably 1 hour or more. On the other hand, _{the total of the waiting times t 1} , t ₂ , and t ₃ is preferably 6 hours or less, and more preferably 3 hours or less. As a result, the surface of the work 100 can be activated efficiently.

The standby step in the present embodiment means a time zone in which the vacuum state and the processing temperature are 950 ° C. or higher. The time of this standby step is, for example, preferably 5 minutes or more, preferably 10 minutes or more, and more preferably 20 minutes or more.

<Recompression process with nitrogen gas>

After the heating stroke and the standby stroke are completed, the control device 40 closes the solenoid valve 32b of the second exhaust system 32 to stop the vacuum pump 32c, and then executes the repressurization step. In the pressure recovery step, the control device 40 opens both the solenoid valve 23d of the pressure recovery system 23 and the solenoid valve 22d of the nitrogen gas supply system 22 to supply a large amount of nitrogen gas to the furnace 10a in a short time. As a result, the pressure inside the furnace 10a is restored to the atmospheric pressure in 10 to 20 seconds. After the pressure recovery, the solenoid valve 23d of the pressure recovery system 23 is closed, and the control device 40 controls the pressure inside the furnace 10a by the first exhaust system 31, so that the nitrogen gas from the nitrogen gas supply system 22 is used to control the furnace. The inner 10a is maintained in an atmospheric pressure state (pressure higher than vacuum). When the nitrogen gas supply system 22 is provided with a flow rate control valve whose flow rate can be controlled by the control device 40, the nitrogen gas supply system 22 can also serve as the pressure recovery system 23. In other words, the decompression system 23 can be defined as a part of the nitrogen gas supply system 22.

<Ammonia gas introduction process>

When the depressurization step is completed and the inside of the furnace 10a is maintained at atmospheric pressure (pressure higher than vacuum) by the nitrogen gas of the nitrogen gas supply system 22, the control device 40 uses the electromagnetic valve 21d of the ammonia gas supply system 21. Is opened to supply ammonia gas to the inside of the furnace 10a. At this time as well, the control device 40 controls the pressure inside the furnace 10a by the first exhaust system 31 to maintain the inside 10a in an atmospheric pressure state (higher pressure than vacuum), but the partial pressure of ammonia gas is About 3 to 5 kPa is preferable. Here, the case where ammonia gas and nitrogen gas are also supplied at the same time is illustrated, but even when nitrogen gas and nitrogen gas are alternately supplied, the mixed state of ammonia gas and nitrogen gas is maintained in the furnace 10a. As long as it is, it is within the scope of the present invention.

Further, in this ammonia gas introduction step, the control device 40 controls the heating device 12 to maintain the _{preset processing temperature TN.} It is preferable that the processing temperature _{TN matches the processing temperature (T 3} ) on the high temperature side in the temperature raising process. As a result, the treatment temperature _TN is preferably 950 ° C. or higher, preferably 1000 ° C. or higher. When the temperature is lower than 950 ° C., chromium nitride (CrN) is precipitated on the surface, and the Cr concentration in the base metal is reduced. The treatment temperature _TN is preferably 1200 ° C. or lower, preferably 1100 ° C. or lower. Above these temperatures, the mechanical properties of the base metal are likely to change. As a result, it is most preferable to control the temperature to 1000 ° C. to 1050 ° C.

The ammonia gas introduction step is continued for a _{preset ammonia gas introduction time t N.} The ammonia gas introduction time t _N is preferably 5 minutes or more, preferably 10 minutes or more, and more preferably 15 minutes or more. Thereby, the solid solution amount of nitrogen can be increased. The ammonia gas introduction time t _N is preferably 60 minutes or less, preferably 40 minutes or less, and more preferably 30 minutes or less. If it is too long, chromium nitride (CrN) is precipitated and the corrosion resistance tends to be lowered.

The amount of ammonia gas supplied depends somewhat on the capacity of the furnace 10a, but is preferably about 5 Nl / min to 30 Nl / min, and preferably 5 Nl / min to 20 Nl / min, for example. The amount of nitrogen gas supplied depends somewhat on the capacity of the furnace 10a, but is preferably about 15 Nl / min to 90 Nl / min, and preferably 15 Nl / min to 60 Nl / min, for example.

It is preferable that the supply amount of nitrogen gas is larger than the supply amount of ammonia gas. While ensuring the minimum amount of solid solution with low-cost nitrogen gas, by supplementing the amount of solid solution exceeding that amount with high-cost ammonia gas, both improvement of surface hardness and low-cost operation can be reasonably achieved. Let me.

<Nitrogen gas cooling process>

When the ammonia gas introduction step is completed, the control device 40 closes the electromagnetic valve 21d of the ammonia gas supply system 21, stops the supply of ammonia gas, opens the electromagnetic valve 22d of the nitrogen gas supply system 22, and enters the furnace 10a. Supply only nitrogen gas. At this time, the control device 40 controls the pressure inside the furnace 10a by the first exhaust system 31 to maintain the inside 10a in an atmospheric pressure state (higher pressure than vacuum). As a result, the ammonia gas in the furnace 10a is replaced with the nitrogen gas. Further, in this nitrogen gas cooling step, after the concentration of ammonia gas in the furnace 10a decreases, the heating device 12 is turned off and the work 100 is heated to 70 ° C. or lower, preferably 60 ° C. by utilizing the convection of nitrogen gas. Hereinafter, more preferably, the mixture is rapidly cooled to room temperature. As a guideline for this quenching speed, the time for the work 100 to pass through the temperature range of 750 ° C. to 950 ° C. is preferably 40 minutes or less, preferably 30 minutes or less, and more preferably 20 minutes or less. More preferably, it is 10 minutes or less. By doing so, it is possible to suppress the reaction of nitrogen gas with chromium Cr in the base metal to precipitate chromium nitride (so-called sensitization) during the cooling process. If cooling is performed with a large amount of ammonia gas remaining, sensitization tends to occur, the precipitation rate of chromium nitride increases, and corrosion resistance tends to decrease.

A nitriding test of the work 100 was performed using the surface effect processing apparatus 1 according to the present embodiment. Martensitic stainless steel SUS420J2 was used as the base material for the work 100.

The vacuum state in the vacuuming step was controlled to 1 Pa in the furnace 10a. While maintaining this vacuum state, the temperature was raised to three stages of processing temperature T ₁ = 650 ° C, T ₂ = 850 ° C, and T ₃ = 1030 ° C, and the standby time at each processing temperature was set to t ₁ = 30 minutes. It _{was set to t 2} = 30 minutes and t ₂ = 30 minutes.

In the decompression step, a large amount of nitrogen gas for decompression was supplied to the furnace 10a in a short time, and the pressure was restored to atmospheric pressure in 10 to 20 seconds. After the pressure was restored, the flow rate of nitrogen gas was reduced to about 16 Nl / min to maintain the inside of the furnace at atmospheric pressure.

In the ammonia gas introduction process, nitrogen gas was continuously supplied at about 16 Nl / min to maintain the inside of the furnace at atmospheric pressure, and ammonia gas was supplied at about 5 Nl / min. The treatment temperature _TN was set to 1030 ° C., and the ammonia gas introduction time t _N was set to 20 minutes.

In the nitrogen gas cooling step, the atmosphere gas was cooled by the cooling device 60 while maintaining the inside of the furnace 10a in an atmospheric pressure state, and the work 100 was cooled to room temperature. Then, the hardness of the work 100 was measured using a Micro Vickers hardness tester.

Comparative example

As a comparative example, in the process of introducing ammonia gas, the inside of the furnace 10a was maintained at atmospheric pressure only with nitrogen gas without supplying ammonia gas. All other conditions were consistent with the Examples.

The hardness measurement result of the work 100 of the example is shown in FIG. The hardness of the outermost surface was 811 Hv, and a hardness exceeding 700 Hv (preferably 750 Hv) was obtained. Further, the distance from the outermost surface when the hardness was 550 Hv was about 0.23 mm. That is, it was clarified that a nitrogen solid solution cured layer having a hardness of more than 550 Hv can be obtained in an amount of 0.2 mm or more. Further, it was clarified that a nitrogen solid solution cured layer having a hardness higher than that of the base material was formed up to a distance of about 0.35 mm from the outermost surface.

The hardness measurement result of the work 100 of the comparative example is shown in FIG. The hardness of the outermost surface was 618 Hv, which was less than 650 Hv. Further, the distance from the outermost surface when the hardness was 550 Hv was about 0.08 mm. That is, it was clarified that the nitrogen solid solution cured layer having a hardness exceeding 550 Hv was less than 0.10 mm. Further, it was clarified that a nitrogen solid solution cured layer having a hardness higher than that of the base material was formed up to a distance of about 0.20 mm from the outermost surface.

As can be seen by comparing the examples and the comparative examples, the surface hardness of the examples is about 200 Hv higher than that of the comparative examples, and the hardness is improved by about 30% based on the comparative examples. Since there is an error in the hardness of the base material (core) of about 40 Hv, when this is taken into consideration, the surface hardness of the example is about 160 Hv larger than that of the comparative example, which is based on the comparative example. Then, the hardness is improved by about 26%. Further, the thickness of the cured layer based on the hardness of 550 Hv is also 0.1 mm or more larger in the examples than in the comparative examples. The overall thickness of the cured layer is also about 0.15 mm larger in the examples than in the comparative examples.

As described above, the surface hardening treatment method and the like of the present embodiment include a vacuuming process for making the inside of the heat treatment furnace in a vacuum state, a heating process for raising the final reaching temperature to 950 ° C. or higher while maintaining the vacuum state, and the state thereof. A standby process for maintaining the temperature for a predetermined time, a repressurization step for repressurizing the inside of the heat treatment furnace with nitrogen gas after the standby process, and an ammonia gas and nitrogen gas after the decompression are supplied to dissolve nitrogen in the work. After the ammonia gas introduction step and the ammonia gas introduction step, there is a nitrogen gas cooling step of cooling the work while supplying the nitrogen gas into the heat treatment furnace or circulating the nitrogen gas in the heat treatment furnace.

According to the surface hardening treatment method, etc., the surface of the stainless steel to be the work is activated by the heating process in the vacuum state, and subsequently, nitrogen gas and ammonia are subjected to the ammonia gas introduction step in a high temperature environment of 950 ° C. or higher. By supplying gas, ammonia gas can _{be rapidly thermally decomposed by the relational expression of 2NH 3} → 2N + 3H ₂ , and a large amount of nitrogen atoms can be supplied to the surface of the work in a short time. As a result, a large amount of nitrogen is dissolved in the surface layer of the work, and the surface hardness can be dramatically increased. By the way, even when nitrogen gas is supplied, thermal dissociation occurs at a temperature of 950 ° C or higher, preferably 1000 ° C or higher, and nitrogen atoms can be supplied to the surface of the work, but ammonia gas is faster to supply in a short time. Is realized.

Further, when the work is cooled, nitrogen gas is used instead of ammonia gas to cool the work, so that the waste of ammonia can be suppressed and the precipitation of chromium nitride on the surface of the work can be suppressed.

In particular, in the ammonia gas introduction step of the surface hardening treatment method of the present embodiment, nitrogen gas is supplied in addition to the ammonia gas. By mixing inexpensive nitrogen gas and expensive ammonia gas, it is possible to suppress an increase in running cost. In particular, it is preferable that the supply amount of nitrogen gas is larger than the supply amount of ammonia gas.

Further, in the nitrogen gas cooling step such as the surface hardening treatment method of the present embodiment, the time for the work to pass through the temperature range of 750 ° C. to 950 ° C. is set to 40 minutes or less. In this way, it is possible to suppress the generation of chromium nitride precipitates. In addition, FIG. 5 shows the precipitation start boundary of chromium nitride in stainless steels (SUS304, SUS316). This boundary line means the temperature and time (sensitization start temperature, sensitization start time) at which the precipitation of chromium nitride is started. Therefore, the range on the right side of this boundary line is a range in which chromium nitride is likely to precipitate, and the range on the left side is a range in which chromium nitride is difficult to precipitate. Therefore, it can be seen that stainless steel tends to be sensitized in the range of 750 ° C to 950 ° C. Therefore, in the present embodiment, the time for passing through this temperature range is set to 40 minutes or less by cooling with nitrogen gas.

Furthermore, in the temperature raising process of the surface hardening treatment method of the present embodiment, the time for the work to reach 500 ° C. or higher in a vacuum state is set to 30 minutes or longer. In this way, the surface of the work is further activated, and nitrogen is easily dissolved in a solid solution. In particular, in the standby step, if the vacuum state is maintained for a predetermined time in the range of 950 ° C. or higher where sensitization is unlikely to occur, the surface of the work can be further activated.

Furthermore, the time of the ammonia gas introduction step of the surface hardening treatment method of the present embodiment is set to 60 minutes or less. According to the verification by the present inventors, the surface hardness can be sufficiently improved even if the supply time of the ammonia gas is shortened in this way.

The present invention is not limited to the above embodiment, and can be appropriately modified without departing from the spirit of the present invention.

1 Surface hardening treatment device 10 Heat treatment furnace 22 Ammonia gas supply system 23 Nitrogen gas supply system 30 Exhaust device 100 Work

Claims (9)

The vacuuming process to create a vacuum inside the heat treatment furnace containing the work made of stainless steel,
While maintaining the vacuum state, a temperature raising process for raising the final temperature of the work to 950 ° C. or higher, and
A standby step of maintaining the work for a predetermined time in the vacuum state with a final reaching temperature of 950 ° C. or higher.
After the standby step, a pressure recovery step of supplying nitrogen gas into the heat treatment furnace to restore the pressure inside the heat treatment furnace to a pressure higher than that in the vacuum state.
An ammonia gas introduction step of supplying nitrogen gas and ammonia gas into the heat treatment furnace to dissolve nitrogen in the work while maintaining the work at 950 ° C. or higher.
It is characterized by having a nitrogen gas cooling step which is executed after the ammonia gas introduction step and cools the work while supplying nitrogen gas into the heat treatment furnace or circulating nitrogen gas in the heat treatment furnace. To
Surface hardening treatment method. In the ammonia gas introduction step, the supply amount of the nitrogen gas is larger than the supply amount of the ammonia gas.
The surface hardening treatment method according to claim 1. In the nitrogen gas cooling step, the time for the work to pass through the temperature range of 750 ° C. to 950 ° C. is 40 minutes or less.
The surface hardening treatment method according to claim 1 or 2. In the heating process, the time for the work to reach 500 ° C. or higher is 30 minutes or longer.
The surface hardening treatment method according to any one of claims 1 to 3. The time of the ammonia gas introduction step is 60 minutes or less.
The surface hardening treatment method according to any one of claims 1 to 4. The surface hardness of the work is 700 Hv or more.
The surface hardening treatment method according to any one of claims 1 to 5. The nitrogen gas cooling step is characterized in that the nitrogen gas newly supplied into the heat treatment furnace or the nitrogen gas staying in the heat treatment furnace is cooled by a heat exchanger.
The surface hardening treatment method according to any one of claims 1 to 6. A heat treatment furnace that houses a work made of stainless steel,
An exhaust device that creates a vacuum inside the heat treatment furnace,
A heating device provided in the heat treatment furnace to heat the work,
An ammonia gas supply system that supplies ammonia gas to the heat treatment furnace,
A nitrogen gas supply system that supplies nitrogen gas to the heat treatment furnace,
The exhaust device, the heating device, the ammonia gas supply system, and the control device for controlling the nitrogen gas supply system are provided.
The control device is
A vacuum processing unit that controls the exhaust device to put the heat treatment furnace into a vacuum state,
While maintaining the vacuum state, the heating device is controlled to raise the final temperature of the work to 950 ° C. or higher.
A maintenance processing unit that maintains the work for a predetermined time in the vacuum state at a final temperature of 950 ° C. or higher.
After the control by the maintenance processing unit, the nitrogen gas supply system is controlled to supply nitrogen gas into the heat treatment furnace to restore the pressure inside the heat treatment furnace to a pressure higher than that in the vacuum state. Processing unit and
After the control by the decompression treatment unit, the nitrogen gas supply system and the ammonia gas supply system are controlled to supply nitrogen gas and ammonia gas into the heat treatment furnace while the work is maintained at 950 ° C. or higher. Ammonia gas introduction processing unit that dissolves nitrogen in the work,
After the control by the ammonia gas introduction processing unit, the nitrogen gas supply system is controlled to supply the nitrogen gas into the heat treatment furnace, or the nitrogen gas in the heat treatment furnace is circulated to cool the work. It is characterized by having a gas cooling processing unit.
Surface hardening treatment equipment. A cooling device for cooling the nitrogen gas newly supplied to the heat treatment furnace or the nitrogen gas staying in the heat treatment furnace by a heat exchanger is provided.
The nitrogen gas cooling treatment unit is characterized in that the work is cooled by controlling the cooling device.
The surface hardening treatment apparatus according to claim 8.

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