A heat treatment method for obtaining low retained austenite high hardenability steel parts

Abstract

The present invention relates to a carburizing method for high hardenability steel parts, specifically to a heat treatment method for obtaining high hardenability steel parts with low retained austenite, which is used to solve the problems that cryogenic treatment is not suitable for mass production and there is a risk of part cracking, as well as the deficiencies that carburizing normalizing + secondary heating quenching consumes electric energy and the process is relatively complex. In the heat treatment method for obtaining high hardenability steel parts with low retained austenite, the carbon potential in the fourth stage is 0.45%, and the carbon potential remains unchanged during the cooling in the fifth stage. Before the final quenching, the carbon potential is increased to 0.60 - 0.70% in the sixth stage to avoid decarburization of the parts due to low surface carbon potential, so that the volume percentage of retained austenite of high hardenability steel parts such as 17NiCrMo6-4 and 20MnCr5HHA can be maintained at ≤15% after one-time heating quenching.

Description

Technical Field

[0001] The present invention relates to a carburizing method for high hardenability steel parts, and particularly to a heat treatment method for obtaining high hardenability steel parts with low retained austenite. Background Art

[0002] High hardenability steel refers to low-carbon alloy steels such as 17NiCrMo6-4 and 20MnCr5HHA that contain relatively more alloying elements. Parts made of it are prone to coarse metallographic structures after carburizing quenching + low-temperature tempering. For low-carbon alloy steel parts, the less retained austenite, the more wear-resistant the parts and the more stable their dimensions. According to the technical requirements of heat treatment, retained austenite can generally be rated into 7 grades: in ascending order of retained austenite volume percentage (approximate martensite size), they are 10%-A, 15%-B, 20%-C, 25%-D, 30%-E, 35%-F, and 40%-G. Generally, for the metallographic structure of low-carbon alloy steel parts after carburizing quenching + low-temperature tempering ≤ Grade E and the retained austenite volume percentage ≤ 30% are judged to be qualified.

[0003] To stably obtain high hardenability steel parts with a retained austenite volume percentage ≤ 15%, cryogenic treatment or carburizing normalizing + secondary heating quenching process is usually adopted.

[0004] Chinese Patent CN103993154A discloses a heat treatment method for alloy steel gears, which heat-treats alloy steel gears made of 17NiCrMo6-4 and 20MnCr5HHA by carbonitriding under controlled atmosphere conditions, including carbonitriding, quenching, and tempering processes. After the alloy steel gears are cleaned and dried, they are placed in a controlled atmosphere device and processed as follows: 1) exhaust heating and temperature rise, 2) strong carburizing treatment, 3) diffusion stage, 4) temperature reduction treatment, 5) temperature equalization treatment, 6) oil quenching treatment, 7) tempering heating treatment, and 8) finally air-cooled to room temperature to obtain the product. This heat treatment method belongs to cryogenic treatment, which places the parts after carburizing quenching in liquid nitrogen, and the low temperature makes the retained austenite further transform into martensite, and then low-temperature tempering is carried out. However, cryogenic treatment is not suitable for batch production, and there is a risk of part cracking. In addition, carburizing normalizing + secondary heating quenching has the effect of refining grains, but secondary heating quenching consumes too much electric energy and requires two processes to be called, and the process is relatively complex. Summary of the Invention

[0005] The purpose of the present invention is to solve the deficiencies that cryogenic treatment is not suitable for batch production and there is a risk of part cracking, and that carburizing normalizing + secondary heating quenching consumes electric energy and the process is relatively complex, and to provide a heat treatment method for obtaining high hardenability steel parts with low retained austenite.

[0006] To solve the deficiencies of the above-mentioned existing technologies, the present invention provides the following technical solutions:

[0007] A heat treatment method for obtaining low retained austenite and high hardenability steel parts, which is characterized in that it includes the following steps:

[0008] Step 1: After cleaning the parts to be processed, place them flat or hang them on heat-resistant steel workpieces;

[0009] Step 2: Set the process parameters of the carburizing equipment, and put the parts to be processed and the heat-resistant steel workpieces into the carburizing equipment for quenching;

[0010] Step 2.1: At the start of the carburizing process, the temperature is 915 - 935 °C and the carbon potential is 0.60 - 0.70%;

[0011] Step 2.2: Carry out the first stage, which takes 10 - 30 minutes, with the temperature remaining unchanged and the carbon potential rising to 0.95 - 1.05%;

[0012] Step 2.3: Carry out the second stage, which takes 270 - 290 minutes, with the temperature and carbon potential remaining unchanged;

[0013] Step 2.4: Carry out the third stage, which takes 20 - 40 minutes, causing the temperature to drop by 5 - 10 °C and the carbon potential to drop to 0.45%;

[0014] Step 2.5: Carry out the fourth stage, which takes 230 - 250 minutes, with the temperature and carbon potential remaining unchanged;

[0015] Step 2.6: Carry out the fifth stage, which takes 50 - 70 minutes, with the temperature dropping to 860 - 880 °C and the carbon potential remaining unchanged;

[0016] Step 2.7: Carry out the sixth stage, which takes 10 - 30 minutes, with the temperature dropping to 820 - 840 °C and the carbon potential rising to 0.60 - 0.70%;

[0017] Step 2.8: Carry out the seventh stage, which takes 50 - 70 minutes, with the temperature and carbon potential remaining unchanged;

[0018] Step 3: Quench the parts, and after quenching, use a cleaning machine to clean the quenching oil on the parts;

[0019] Step 4: After cleaning, carry out low-temperature tempering on the parts, with the temperature being 155 - 175 °C and the time being 170 - 190 minutes;

[0020] Step 5: After tempering, place the parts in a room-temperature environment for natural cooling.

[0021] Further, the specific content of Step 2.2 is as follows:

[0022] Step 2.2.1: Raise the carbon potential to 0.85 - 0.94%;

[0023] Step 2.2.2: Raise the carbon potential to 0.95 - 1.05%;

[0024] The time for Step 2.2.1 is equal to that for Step 2.2.2;

[0025] The purpose is to avoid excessive carbon black caused by too much carburizing agent.

[0026] Furthermore, Step 2 is specifically as follows:

[0027] Step 2.1: At the beginning, the temperature is 925°C and the carbon potential is 0.65%;

[0028] Step 2.2: Conduct the first stage, which takes 20 minutes. The temperature remains unchanged and the carbon potential rises to 1.00%;

[0029] Step 2.3: Conduct the second stage, which takes 280 minutes. The temperature and carbon potential remain unchanged;

[0030] Step 2.4: Conduct the third stage, which takes 30 minutes. The temperature drops to 920°C and the carbon potential drops to 0.45%;

[0031] Step 2.5: Conduct the fourth stage, which takes 240 minutes. The temperature and carbon potential remain unchanged;

[0032] Step 2.6: Conduct the fifth stage, which takes 60 minutes. The temperature drops to 870°C and the carbon potential remains unchanged;

[0033] Step 2.7: Conduct the sixth stage, which takes 20 minutes. The temperature drops to 830°C and the carbon potential rises to 0.65%;

[0034] Step 2.8: Conduct the seventh stage, which takes 60 minutes. The temperature and carbon potential remain unchanged.

[0035] Furthermore, in Step 4, after cleaning, the parts are subjected to low-temperature tempering at 165°C for 180 minutes.

[0036] Furthermore, in Step 2, the carburizing equipment is a box-type multi-purpose furnace, and the auxiliary materials used are nitrogen and methanol, and the carburizing agent is acetone.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] In the heat treatment method for obtaining low retained austenite and high hardenability steel parts of the present invention, the carbon potential in the fourth stage (high-temperature diffusion stage) is 0.45%, and the carbon potential remains unchanged during the cooling in the fifth stage. In the sixth stage before the final quenching, the carbon potential is increased to 0.60 - 0.70% to avoid decarburization caused by low surface carbon potential of the parts, so that the retained austenite volume percentage of high hardenability steel parts such as 17NiCrMo6-4 and 20MnCr5HHA can be maintained at ≤15% after one-time heating quenching. Brief Description of the Drawings

[0039] Figure 1 It is a magnified metallographic structure diagram of the parts obtained in the first embodiment of the heat treatment method for obtaining low retained austenite and high hardenability steel parts of the present invention. Detailed Description of the Invention

[0040] The present invention will be further described below in conjunction with the drawings and exemplary embodiments.

[0041] Embodiment 1

[0042] A heat treatment method for obtaining low retained austenite and high hardenability steel parts includes the following steps:

[0043] Step 1: After cleaning the parts to be treated, place them flat or hang them on a heat-resistant steel tooling.

[0044] Step 2: Set the process parameters of the carburizing equipment, and put the parts to be treated and the heat-resistant steel tooling into the carburizing equipment for quenching; the carburizing equipment is a box-type multi-purpose furnace, and the auxiliary materials used are nitrogen and methanol, and the carburizing agent is acetone.

[0045] Step 2.1: At the beginning, the temperature is 925°C and the carbon potential is 0.65%.

[0046] Step 2.2: Conduct the first stage, which takes 20 minutes and the temperature remains unchanged.

[0047] Step 2.2.1: Raise the carbon potential to 0.90%, which takes 10 minutes.

[0048] Step 2.2.2: Raise the carbon potential to 1.00%, which takes 10 minutes.

[0049] Step 2.3: Conduct the second stage, which takes 280 minutes and the temperature and carbon potential remain unchanged.

[0050] Step 2.4: Conduct the third stage, which takes 30 minutes, the temperature drops to 920°C, and the carbon potential drops to 0.45%.

[0051] Step 2.5: Conduct the fourth stage, which takes 240 minutes and the temperature and carbon potential remain unchanged.

[0052] Step 2.6: Conduct the fifth stage, which takes 60 minutes, with the temperature dropping to 870°C and the carbon potential remaining unchanged;

[0053] Step 2.7: Conduct the sixth stage, which takes 20 minutes, with the temperature dropping to 830°C and the carbon potential rising to 0.65%;

[0054] Step 2.8: Conduct the seventh stage, which takes 60 minutes, with the temperature and carbon potential remaining unchanged;

[0055] Step 3: Quench the parts, and after quenching, use a cleaning machine to clean the quenching oil on the parts;

[0056] Step 4: After cleaning, conduct low-temperature tempering on the parts at a temperature of 165°C for 180 minutes;

[0057] Step 5: After tempering, place the parts in a room-temperature environment and let them cool naturally.

[0058] In this embodiment, the parts to be processed are gears.

[0059] Slice the parts obtained in Step 5 of this embodiment. After magnifying the metallographic structure at the tooth profile position by 500 times, as Figure 1 shown, the volume percentage of retained austenite in the metallographic structure at the tooth profile position is detected to be about 10%, and other heat treatment indexes such as case depth and surface hardness also meet the technical requirements.

[0060] Embodiment 2

[0061] A heat treatment method for obtaining low-retained-austenite and high-hardening-ability steel parts, comprising the following steps:

[0062] Step 1: After cleaning the parts to be processed, place them flat or hanging on a heat-resistant steel tooling;

[0063] Step 2: Set the process parameters of the carburizing equipment, and put the parts to be processed and the heat-resistant steel tooling into the carburizing equipment for quenching;

[0064] Step 2.1: At the beginning, the temperature is 915°C and the carbon potential is 0.60%;

[0065] Step 2.2: Conduct the first stage, which takes 10 minutes, with the temperature remaining unchanged;

[0066] Step 2.2.1: Raise the carbon potential to 0.85%, which takes 5 minutes;

[0067] Step 2.2.2: Raise the carbon potential to 0.95%, which takes 5 minutes;

[0068] Step 2.3: Conduct the second stage, which takes 270 minutes, with the temperature and carbon potential remaining unchanged;

[0069] Step 2.4: Conduct the third stage, which takes 20 minutes, with the temperature dropping to 910 °C and the carbon potential dropping to 0.45%.

[0070] Step 2.5: Conduct the fourth stage, which takes 230 minutes, with the temperature and carbon potential remaining unchanged.

[0071] Step 2.6: Conduct the fifth stage, which takes 50 minutes, with the temperature dropping to 860 °C and the carbon potential remaining unchanged.

[0072] Step 2.7: Conduct the sixth stage, which takes 10 minutes, with the temperature dropping to 820 °C and the carbon potential rising to 0.60%.

[0073] Step 2.8: Conduct the seventh stage, which takes 50 minutes, with the temperature and carbon potential remaining unchanged.

[0074] Step 3: Quench the parts, and after quenching, use a cleaning machine to clean the quenching oil on the parts.

[0075] Step 4: After cleaning, conduct low-temperature tempering on the parts at a temperature of 155 °C for 170 minutes.

[0076] Step 5: After tempering, place the parts in a room-temperature environment for natural cooling.

[0077] Slice the parts obtained in Step 5 of this embodiment, and detect that the volume percentage of retained austenite in the metallographic structure at the tooth profile position is not more than 15%.

[0078] Embodiment Three

[0079] A heat treatment method for obtaining low retained austenite and high hardenability steel parts, comprising the following steps:

[0080] Step 1: After cleaning the parts to be processed, place them flat or hanging on a heat-resistant steel tooling.

[0081] Step 2: Set the process parameters of the carburizing equipment, and put the parts to be processed and the heat-resistant steel tooling into the carburizing equipment for quenching.

[0082] Step 2.1: At the beginning, the temperature is 935 °C and the carbon potential is 0.70%.

[0083] Step 2.2: Conduct the first stage, which takes 30 minutes, with the temperature remaining unchanged.

[0084] Step 2.2.1: Raise the carbon potential to 0.94%, which takes 15 minutes.

[0085] Step 2.2.2: Raise the carbon potential to 1.05%, which takes 15 minutes.

[0086] Step 2.3: Conduct the second stage, which takes 290 minutes, with the temperature and carbon potential remaining unchanged.

[0087] Step 2.4: Conduct the third stage, which takes 40 minutes, with the temperature dropping to 925 °C and the carbon potential dropping to 0.45%.

[0088] Step 2.5: Conduct the fourth stage, which takes 250 minutes, with the temperature and carbon potential remaining unchanged.

[0089] Step 2.6: Conduct the fifth stage, which takes 70 minutes, with the temperature dropping to 880 °C and the carbon potential remaining unchanged.

[0090] Step 2.7: Conduct the sixth stage, which takes 30 minutes, with the temperature dropping to 840 °C and the carbon potential rising to 0.70%.

[0091] Step 2.8: Conduct the seventh stage, which takes 70 minutes, with the temperature and carbon potential remaining unchanged.

[0092] Step 3: Quench the parts, and after quenching, use a cleaning machine to clean the quenching oil on the parts.

[0093] Step 4: After cleaning, perform low-temperature tempering on the parts at a temperature of 175 °C for 190 minutes.

[0094] Step 5: After tempering, place the parts in a room-temperature environment for natural cooling.

[0095] Slice the parts obtained in Step 5 of this embodiment, and detect that the volume percentage of retained austenite in the metallographic structure at the tooth profile position is not more than 15%.

[0096] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. For those of ordinary professional skills in the art, the specific technical solutions recorded in the foregoing embodiments can be modified, or some of the technical features can be equivalently replaced. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions protected by the present invention.

Claims

1. A heat treatment method for obtaining low retained austenite and high hardenability steel parts, characterized in that, It includes the following steps: Step 1: After cleaning the parts to be processed, lay them flat or hang them on heat-resistant steel tooling; Step 2: Set the process parameters of the carburizing equipment, and put the parts to be processed and the heat-resistant steel tooling into the carburizing equipment for quenching; Step 2.1: At the start of the carburizing process, the temperature is 915 - 935 °C and the carbon potential is 0.60 - 0.70%; Step 2.2: Conduct the first stage, which takes 10 - 30 minutes, with the temperature remaining unchanged and the carbon potential rising to 0.95 - 1.05%; Step 2.3: Conduct the second stage, which takes 270 - 290 minutes, with the temperature and carbon potential remaining unchanged; Step 2.4: Conduct the third stage, which takes 20 - 40 minutes, causing the temperature to drop by 5 - 10 °C and the carbon potential to drop to 0.45%; Step 2.5: Conduct the fourth stage, which takes 230 - 250 minutes, with the temperature and carbon potential remaining unchanged; Step 2.6: Conduct the fifth stage, which takes 50 - 70 minutes, with the temperature dropping to 860 - 880 °C and the carbon potential remaining unchanged; Step 2.7: Conduct the sixth stage, which takes 10 - 30 minutes, with the temperature dropping to 820 - 840 °C and the carbon potential rising to 0.60 - 0.70%; Step 2.8: Conduct the seventh stage, which takes 50 - 70 minutes, with the temperature and carbon potential remaining unchanged; Step 3: Quench the parts, and after quenching, use a cleaning machine to clean the quenching oil on the parts; Step 4: After cleaning, conduct low-temperature tempering on the parts, with the temperature being 155 - 175 °C and taking 170 - 190 minutes; Step 5: After tempering, place the parts in a room-temperature environment for natural cooling.

2. The heat treatment method for obtaining a low retained austenite and high hardenability steel part according to claim 1, characterized in that, Specifically, Step 2.2 is as follows: Step 2.2.1: Raise the carbon potential to 0.85 - 0.94%; Step 2.2.2: Raise the carbon potential to 0.95 - 1.05%; The time of Step 2.2.1 is equal to that of Step 2.2.

2.

3. A heat treatment method for obtaining a low retained austenite and high hardenability steel part according to claim 1 or 2, characterized in that, Specifically, Step 2 is as follows: Step 2.1: At the start, the temperature is 925 °C and the carbon potential is 0.65%; Step 2.2: Conduct the first stage, which takes 20 minutes, with the temperature remaining unchanged and the carbon potential rising to 1.00%; Step 2.3: Conduct the second stage, which takes 280 minutes, with the temperature and carbon potential remaining unchanged; Step 2.4: Conduct the third stage, which takes 30 minutes, with the temperature dropping to 920 °C and the carbon potential dropping to 0.45%; Step 2.5: Conduct the fourth stage, which takes 240 minutes, with the temperature and carbon potential remaining unchanged; Step 2.6: Conduct the fifth stage, which takes 60 minutes, with the temperature dropping to 870 °C and the carbon potential remaining unchanged; Step 2.7: Conduct the sixth stage, which takes 20 minutes, with the temperature dropping to 830 °C and the carbon potential rising to 0.65%; Step 2.8: Conduct the seventh stage, which takes 60 minutes, with the temperature and carbon potential remaining unchanged.

4. A heat treatment method for obtaining a low retained austenite and high hardenability steel part according to claim 3, characterized in that: Step 4: After cleaning, conduct low-temperature tempering on the parts, with the temperature being 165 °C and taking 180 minutes.

5. A heat treatment method for obtaining a low retained austenite and high hardenability steel part according to claim 4, characterized in that: In Step 2, the carburizing equipment is a box-type multi-purpose furnace, the auxiliary materials used are nitrogen and methanol, and the carburizing agent is acetone.

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