Method for reducing the hardness of 420w material by sintering in combination with heat treatment annealing

Abstract

The present invention discloses a method for reducing the hardness of a 420w material by combining a sintering method with heat treatment annealing. The method adjusts the rate of the cooling stage of the sintering process, that is, first keeping the temperature at 800°C, then keeping the temperature at a cooling rate of 50°C / min from 800°C to 500°C, and then cooling from 500°C to room temperature with the furnace. The design of the present invention is reasonable, and the heat treatment annealing process and the sintering cooling stage of the 420w stainless steel material are combined. The sintering hardness of the 420w stainless steel material can be reduced by adjusting the parameters of the cooling stage of the sintering process, which is convenient for subsequent shaping, tapping, polishing and other processes, and there is no need to use a special heat treatment furnace for annealing, which reduces the heating and cooling time of the special heat treatment furnace, and reduces the material transfer time, saving labor costs, which is equivalent to reducing the cost of heat treatment, and the size and hardness stability of the sintered product are more suitable for mass production.

Description

Technical Field

[0001] The present invention relates to the technical field of stainless steel material processing, and more specifically, to a method for reducing the hardness of a 420w material by sintering in combination with heat treatment annealing. Background Art

[0002] MIM-420w stainless steel is 420 "cutting tool grade" martensitic steel, similar to Brinell high chromium steel, the earliest stainless steel. It has certain wear resistance and corrosion resistance, high hardness, and its price is relatively low among stainless steels. It is suitable for working environments that require hardness of stainless steel.

[0003] The Rockwell hardness of 420W stainless steel material after ordinary sintering is 34-40HRC. Under such high hardness, if the product with large sintering deformation is to be corrected by shaping, the loss of shaping mold is very high and the shaping effect cannot achieve the ideal effect. At the same time, tapping cannot be completed under high hardness, and the tap is damaged before tapping is completed. When the hardness exceeds 30HRC, it also has a greater impact on the polishing process, and the production capacity and yield of polishing are greatly reduced. In summary, for products with shaping, tapping and surface polishing, the hardness of ordinary sintered stainless steel 420w material is relatively high, and it will be very difficult to make a prototype sample, and it is not suitable for mass production. It is necessary to increase the annealing process to reduce the hardness first, to a hardness range of about 120HV similar to 316L or about 280HV of 17-4ph, and then carry out shaping, tapping, polishing and other processes.

[0004] In view of the annealing process requirements of 420W, a separate furnace heat treatment annealing method can be used. The product needs to be heated from room temperature to 800-850℃, kept warm for 1-2 hours, and then slowly cooled by 500℃ at 30℃ / min. After 500℃, it will be cooled to room temperature with the furnace. The advantage of this method is that the hardness can be reduced to about 160-200HV without vacuuming the furnace, which is more conducive to shaping and polishing processes. However, the disadvantage of this process is that after the sintering furnace is taken out of the furnace, it is necessary to re-enter the heat treatment furnace for annealing, which increases the transfer time of the material in and out of the furnace, and increases the heating and cooling process of the heat treatment furnace. The whole time in the furnace alone takes 17-18 hours. At the same time, the appearance is brown, and it needs to be hardened by heat treatment quenching + tempering to restore its original color. Summary of the invention

[0005] The purpose of the present invention is to overcome the above-mentioned defects in the prior art and provide a method for reducing the hardness of 420w material by combining sintering with heat treatment annealing, which takes into account the hardness requirements after annealing, facilitates subsequent shaping, tapping, polishing and other processes, and does not require additional annealing in a special heat treatment furnace, reduces the heating and cooling time of the special heat treatment furnace, and reduces the material transfer time.

[0006] To achieve the above object, the present invention provides a method for reducing the hardness of a 420w material by sintering in combination with heat treatment annealing, comprising the following steps:

[0007] Step 1: Place the 420w stainless steel blank to be sintered in the sintering furnace, heat the furnace temperature from room temperature to 300℃, and then keep it warm for 25-35 minutes. Set the furnace pressure to 0Pa and the flow rate of nitrogen to 30L / min.

[0008] Step 2: After the 300°C insulation is completed, continue to heat up to 450°C and keep it for another 85-95 minutes. The flow rate of nitrogen is set to 40L / min;

[0009] Step 3: Keep the nitrogen flow rate and the pressure in the furnace unchanged, continue to raise the temperature to 600°C, and keep it warm for 55-65 minutes;

[0010] Step 4: Continue to maintain the nitrogen flow rate and the pressure in the furnace unchanged, raise the temperature to 850°C, and then keep it warm for 25-35 minutes under vacuum;

[0011] Step 5: In the vacuum firing state, raise the temperature to 1050°C and keep it warm for 55-65 minutes;

[0012] Step 6: Set the pressure in the furnace from 0Pa to 10Pa, and continue to raise the temperature to 1200°C while introducing argon gas at a flow rate of 15L / min, and then keep it warm for 15-25 minutes;

[0013] Step 7: Keep the argon flow rate and the pressure in the furnace unchanged, continue to raise the temperature to 1350°C, and keep it warm for 175-185 minutes;

[0014] Step 8: Keep the argon flow rate and the pressure in the furnace unchanged and cool down to 800°C;

[0015] Step 9: Change to vacuum burning state, set the furnace pressure to 0Pa, set the argon flow rate to 0L / min, and keep warm at 800℃ for 55-65 minutes;

[0016] Step 10: Under the conditions of the furnace pressure of 0Pa and the argon gas flow rate of 0L / min, change to the forced cooling state and cool down to 500°C at a cooling rate of 50°C / min;

[0017] Step 11: Set the pressure in the furnace to 86Pa, and under the condition of argon flow rate of 0L / min, cool the furnace to room temperature and take it out of the furnace to obtain 420w stainless steel semi-finished product (i.e. semi-finished blank to be shaped).

[0018] Preferably, in step 1, the time for heating the temperature in the furnace from room temperature to 300° C. is 55-65 minutes.

[0019] Preferably, in step 2, the time for heating to 450° C. is 55-65 minutes.

[0020] Preferably, in step three, the time for heating to 600° C. is 75-85 minutes.

[0021] Preferably, in step 4, the time for heating to 850° C. is 45-55 minutes.

[0022] Preferably, in step five, the time for heating to 1050° C. is 55-65 minutes.

[0023] Preferably, in step six, the time for heating to 1200° C. is 60-70 minutes.

[0024] Preferably, in step seven, the time for heating to 1350° C. is 35-45 minutes.

[0025] Preferably, in step eight, the time for cooling to 800° C. is 85-95 minutes.

[0026] Preferably, in step ten, the time for cooling to 500° C. is 355-365 minutes.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The invention has a reasonable design and a simple method. The heat treatment annealing process and the sintering cooling stage of the 420W stainless steel material are combined. Only the parameters of the cooling stage of the sintering process need to be adjusted to reduce the sintering hardness of the 420W stainless steel material, which is convenient for subsequent shaping, tapping, polishing and other processes. There is no need to additionally use a special heat treatment furnace for annealing, which reduces the heating and cooling time of the special heat treatment furnace, as well as the material transfer time (i.e., the time for switching furnaces), saving labor costs, which is equivalent to reducing the cost of heat treatment. In addition, the size and hardness stability of the sintered product are more suitable for mass production. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] An embodiment of the present invention provides a method for reducing the hardness of a 420w material by sintering in combination with heat treatment annealing, comprising the following steps:

[0031] Step 1: After placing the 420w stainless steel blank to be sintered in the sintering furnace, heat the furnace temperature from room temperature to 300℃ for 55-65 minutes, and then keep warm for 25-35 minutes. Set the furnace pressure to 0Pa and the flow rate of nitrogen to 30L / min.

[0032] Step 2: After the 300°C insulation is completed, the temperature is continued to rise to 450°C for 55-65 minutes, and then the temperature is maintained for 85-95 minutes. The flow rate of nitrogen is set to 40L / min;

[0033] Step 3: Keep the nitrogen flow rate and the pressure in the furnace unchanged, continue to raise the temperature to 600°C for 75-85 minutes, and then keep warm for 55-65 minutes;

[0034] Step 4: Continue to maintain the nitrogen flow rate and the pressure in the furnace unchanged, raise the temperature to 850°C for 45-55 minutes, and then keep it warm for 25-35 minutes under vacuum;

[0035] Step 5: In the vacuum firing state, heat up to 1050°C for 55-65 minutes, and then keep warm for 55-65 minutes;

[0036] Step 6: Set the pressure in the furnace from 0Pa to 10Pa, and continue to raise the temperature to 1200°C for 60-70 minutes while introducing argon gas at a flow rate of 15L / min, and then keep warm for 15-25 minutes;

[0037] Step 7: Keep the argon flow rate and the pressure in the furnace unchanged, continue to raise the temperature to 1350°C for 35-45 minutes, and then keep warm for 175-185 minutes;

[0038] Step 8: Keep the argon flow rate and the pressure in the furnace unchanged, and cool down to 800°C, which takes 85-95 minutes;

[0039] Step 9: Change to vacuum burning state, set the furnace pressure to 0Pa, set the argon flow rate to 0L / min, and keep warm at 800℃ for 55-65 minutes;

[0040] Step 10: Under the conditions of the furnace pressure of 0Pa and the argon gas flow rate of 0L / min, the furnace is changed to a forced cooling state, and the temperature is lowered to 500°C at a cooling rate of 50°C / min, which takes 355-365 minutes;

[0041] Step 11: Set the pressure in the furnace to 86Pa, and under the condition of argon flow rate of 0L / min, cool the furnace to room temperature and take it out of the furnace to obtain 420w stainless steel semi-finished product (i.e. semi-finished blank to be shaped).

[0042] Comparative example: The parameters of the 420W single sintering process (excluding annealing process) are as follows:

[0043]

[0044]

[0045] Without annealing process, the total sintering time of the comparative example is about 22 to 24 hours, which is related to the number of sintered products. The cooling time will be longer when the furnace is full.

[0046] In this embodiment (preferred example), the sintering process combined with heat treatment annealing is as follows:

[0047]

[0048]

[0049] The total duration of the sintering curve of this embodiment is about 28 to 31 hours, which is related to the number of sintered products. The cooling time will be longer when the furnace is full.

[0050] Comparison of the total duration of the above two processes: the sintering process of the comparative example takes 22 to 24 hours, and the sintering process combined with heat treatment annealing in this embodiment takes 28 to 31 hours. Although the sintering time is increased relative to the comparative example, the hardness of the product sintered in the comparative example is relatively high, and it is necessary to subsequently anneal in a special heat treatment furnace to reduce the hardness. The annealing process in the special heat treatment furnace needs to heat up from room temperature to a preset temperature, then keep warm, and then cool down. It takes a total of 17 to 18 hours from entering the furnace to exiting the furnace. The above does not include the turnover time of the materials entering and exiting the furnace. The total time of the materials in the sintering furnace and the heat treatment furnace is only 39 to 42 hours.

[0051] After multiple verifications and comparisons, the sintering method combined with heat treatment annealing in this embodiment can reduce the cycle of one entry and one exit from the furnace compared to the comparative example, that is, it reduces the material transfer time, reduces the labor cost, and reduces the heating stage from room temperature to the preset temperature in the heat treatment furnace, that is, it reduces the heat treatment time in the furnace. Considering the waiting time for material turnover, this embodiment generally reduces the time by at least 11 to 12 hours compared with the ordinary sintering + heat treatment furnace annealing method.

[0052] After the sintering process of combined heat treatment and annealing in this embodiment, the hardness range of the product can be 200-240HV, with an average of about 220HV. This hardness range is about 15% lower than the hardness of 17-4ph material after sintering. Although it is slightly higher than the hardness of a single furnace heat treatment, it can meet the needs of large-scale shaping, tapping, and polishing processes, and the size and hardness uniformity and stability are relatively good.

[0053] In this embodiment, the sintering hardness of the 420w stainless steel material can be reduced by adjusting the parameters of the cooling stage of the sintering process, that is, the Vickers hardness of the blank after sintering can be reduced, which is convenient for subsequent shaping, tapping, polishing and other processes. After the shaping, tapping, polishing and other processes are completed, the hardness of the 420w stainless steel material can be further increased through heat treatment quenching + tempering to meet customer needs.

[0054] It is a preferred embodiment of the present invention, but the embodiment of the present invention is not limited to the above-mentioned embodiment. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for reducing the hardness of 420w material by sintering in combination with heat treatment annealing, characterized in that: The following steps are involved: Step 1: Place the 420w stainless steel blank to be sintered in the sintering furnace, heat the furnace temperature from room temperature to 300℃, and then keep it warm for 25-35 minutes. Set the furnace pressure to 0Pa and the flow rate of nitrogen to 30L / min. Step 2: After the 300°C insulation is completed, continue to heat up to 450°C and keep it for another 85-95 minutes. The flow rate of nitrogen is set to 40L / min; Step 3: Keep the nitrogen flow rate and the pressure in the furnace unchanged, continue to raise the temperature to 600°C, and keep it warm for 55-65 minutes; Step 4: Continue to maintain the nitrogen flow rate and the pressure in the furnace unchanged, raise the temperature to 850°C, and then keep it warm for 25-35 minutes under vacuum; Step 5: In the vacuum firing state, raise the temperature to 1050°C and keep it warm for 55-65 minutes; Step 6: Set the pressure in the furnace from 0Pa to 10Pa, and continue to raise the temperature to 1200°C while introducing argon gas at a flow rate of 15L / min, and then keep it warm for 15-25 minutes; Step 7: Keep the argon flow rate and the pressure in the furnace unchanged, continue to raise the temperature to 1350°C, and keep it warm for 175-185 minutes; Step 8: Keep the argon flow rate and the pressure in the furnace unchanged and cool down to 800°C; Step 9: Change to vacuum burning state, set the furnace pressure to 0Pa, set the argon flow rate to 0L / min, and keep warm at 800℃ for 55-65 minutes; Step 10: Under the conditions of the furnace pressure of 0Pa and the argon gas flow rate of 0L / min, change to the forced cooling state and cool down to 500°C at a cooling rate of 50°C / min; Step 11: Set the pressure in the furnace to 86Pa, and under the condition that the flow rate of argon gas is 0L / min, cool the furnace to room temperature and take it out of the furnace, so as to obtain a 420w stainless steel semi-finished product; In step 1, the time for heating the temperature in the furnace from room temperature to 300°C is 55-65 minutes; In step 2, the time for heating to 450°C is 55-65 minutes; In step 3, the time for heating to 600°C is 75-85 minutes; In step 4, the time of heating to 850° C. is 45-55 minutes; In step 5, the time of heating to 1050° C. is 55-65 minutes; In step six, the time for heating to 1200° C. is 60-70 minutes; In step 7, the time of heating to 1350° C. is 35-45 minutes; In step eight, the time for cooling to 800° C. is 85-95 minutes; In step ten, the time for cooling to 500° C. is 355-365 minutes.

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