A short-process nitriding process for ASTM 4140 steel

Abstract

The present application relates to a kind of short process nitriding process of ASTM 4140 steel, on the basis of considering multiple nitriding layer thickness main influencing factors in nitriding process, theoretical thickness function of pre-nitriding layer is built, as the basis of nitriding parameter regulation in pre-nitriding process, the accurate control of nitriding layer thickness in pre-nitriding stage can be realized.In addition, the relationship between the theoretical thickness of pre-nitriding layer, the theoretical thickness of secondary nitriding layer and the target nitriding layer thickness is determined through experimental research, thereby providing the basic principles for accurate control in the nitriding process.Using the short process nitriding process of the present application, the pre-nitriding in the high temperature tempering process is completed simultaneously, so it does not bring additional process time increase, and due to the presence of pre-nitriding treatment, the subsequent conventional secondary nitriding treatment is laid, so that the process time of secondary nitriding treatment is shortened by more than 10h, and the process efficiency is improved.

Description

Technical Field

[0001] This invention relates to a short-process nitriding process for ASTM 4140 steel. By performing pre-nitriding treatment near the end of the high-temperature tempering process of ASTM 4140 steel, and simultaneously performing short-time nitriding during the high-temperature tempering, it is possible to achieve rapid nitriding while avoiding excessively large nitride sizes. This lays the foundation for subsequent nitriding treatments and shortens the time of subsequent nitriding processes. Background Technology

[0002] ASTM 4140 steel is typically nitrided after quenching and tempering (quenching + high-temperature tempering). The quenching and tempering process gives the steel a good balance of strength, plasticity, and toughness, while the nitriding treatment effectively improves surface hardness, wear resistance, and fatigue resistance. The required thickness of the nitrided layer varies depending on the application, but is usually controlled to be around 0.2-0.65 mm.

[0003] Nitriding temperature is usually between 760K and 810K. At this temperature, the diffusion rate of nitrogen atoms is very slow, which results in a long nitriding time. For example, a nitriding thickness of 0.3mm requires about 30 hours (this is just an estimate, and it is also related to the content of alloying elements and the ammonia decomposition rate). The time and energy costs of the entire process are very high, which is not conducive to improving production efficiency and reducing production costs.

[0004] High-temperature tempering during quenching and tempering is a pretreatment process for nitriding. Its temperature is 850K-890K, which is much higher than the conventional nitriding temperature. At this temperature, the diffusion rate of nitrogen atoms increases significantly. However, the size of nitrides also becomes larger, which means that the wear resistance of the steel surface cannot be improved by nitriding. Therefore, the traditional view is that it is not feasible to carry out nitriding at the same time as the high-temperature tempering stage of quenching and tempering. Summary of the Invention

[0005] This invention provides a short-process nitriding method according to ASTM 4140, which adopts a combination of high-temperature short-time rapid pre-nitriding at the end of high-temperature tempering and conventional secondary nitriding. It effectively utilizes the characteristics of high temperature and fast nitrogen atom penetration rate in the high-temperature tempering process. The short-time pre-nitriding process can achieve rapid nitriding while avoiding excessively large nitride sizes, thus laying the foundation for subsequent conventional secondary nitriding treatment, shortening the processing time of conventional secondary nitriding, shortening the nitriding process flow, improving process efficiency, and reducing production costs.

[0006] The objective of this invention is achieved as follows.

[0007] A short-process nitriding process for ASTM 4140 steel, specifically including the following steps: 1) Perform composition analysis on ASTM 4140 steel to obtain the mass percentage content of Cr and Mo; 2) Determine the target nitrided layer thickness d0 for ASTM 4140 steel, in mm; 3) Quenching and tempering + pre-nitriding treatment: ASTM 4140 steel is quenched and then tempered at a high temperature of T1. Ammonia gas is introduced into the high-temperature tempering furnace at time t1 before the end of the tempering process, controlling the ammonia decomposition rate to α. A1 Pre-nitriding treatment was performed on ASTM 4140 steel to obtain a theoretical pre-nitrided layer with a thickness of d1 in mm, and 0.09 mm ≤ d1 ≤ 0.12 mm. 4) The ASTM 4140 steel obtained in step 3) is subjected to a process at temperature T2, time t2, and ammonia decomposition rate α. A2 The secondary nitriding treatment yields a theoretical secondary nitrided layer with a thickness of d2, in mm. in: d1=[ ·exp(-896.47 / T1)·ln(1+[α A1 ] / 10.6)] / (1+0.57[Cr]+0.32[Mo]); d2=[ ·exp(-1757.69 / T2)·ln(1+[α A2 ] / 20)] / (1+0.8[Cr]+0.5[Mo]); d0 = 0.86d1 + d2; In the formula: t1 and t2 are in units of h; T1 and T2 are in units of K; [α] A1 ]、[α A2 [] represents the ammonia decomposition rate during nitriding, the value before %; [Cr] and [Mo] are the mass percentages of Cr and Mo in Cr-Mo alloy steel, respectively, the values ​​before %.

[0008] This invention comprehensively considers the main factors affecting the thickness of the nitriding layer during the nitriding process. Through repeated experiments, the weight of each influencing factor and its impact on the thickness of the nitriding layer are analyzed, summarized, and statistically calculated. Based on different nitriding temperatures, the theoretical thickness d2 of the secondary nitriding layer and the theoretical thickness d1 of the pre-nitriding layer are determined.

[0009] It should be noted that the theoretical thickness d2 of the secondary nitriding layer was calculated and statistically obtained without pre-nitriding. Therefore, it is necessary to fit the theoretical thickness d1 of the pre-nitriding layer with the theoretical thickness d2 of the secondary nitriding layer to obtain the actual thickness d0 of the nitriding layer.

[0010] The following is a brief analysis of the influence of the parameters involved in d1 and d2 on the thickness of the surface nitriding layer.

[0011] Firstly, regarding the elements, Cr and Mo in ASTM 4140 are nitride-forming elements during the nitriding process of Cr-Mo alloy steel. Both have a high affinity for nitrogen atoms and readily form stable nitride phases, such as CrN and Mo2N, on or near the steel surface. The formation of these nitrides consumes nitrogen atoms that would otherwise be used to penetrate the matrix. Nitrogen atoms are fixed in compounds like CrN or Mo2N, reducing the amount of nitrogen atoms available for forming the diffusion layer (diffusion zone). Furthermore, CrN and Mo2N nitrides are typically dense and stable; the resulting dense compound layer (bright white layer) hinders further diffusion of nitrogen atoms into the matrix, thus reducing the thickness of the diffusion layer. Higher Cr and Mo contents significantly impede nitrogen atom diffusion, reducing the number of active nitrogen atoms and limiting the thickness of the nitrided layer.

[0012] Secondly, regarding parameter control in the nitriding process, increasing nitriding time, raising nitriding temperature, and increasing ammonia decomposition rate during nitriding all lead to a greater thickness of the nitrided layer. Therefore, it is necessary to regulate these parameters. In particular, the pre-nitriding of this invention is carried out at the end of high-temperature tempering, and its nitriding temperature is higher than that of conventional nitriding. Statistical analysis shows that the influence of tempering temperature on the thickness of the nitrided layer is much greater than the influence of conventional temperature on the thickness of the nitrided layer. Therefore, two formulas, d1 and d2, are formulated for conventional secondary nitriding and high-temperature pre-nitriding, respectively.

[0013] This invention considers both the influence of alloy composition and the influence of nitriding parameters during the nitriding process when constructing the theoretical nitriding layer thickness prediction function. By coupling composition and process parameters, it comprehensively integrates the main parameters affecting the nitriding layer thickness. Nitriding process parameters are adjusted based on the theoretical nitriding layer thickness prediction function, resulting in an extremely low error between the actual nitriding layer thickness and the target nitriding layer thickness.

[0014] As mentioned earlier, the inventors also discovered during the experiment that when using the two-stage nitriding process of pre-nitriding followed by conventional secondary nitriding at the high-temperature tempering end proposed in this invention, the final thickness of the nitrided layer is not equal to the sum of the theoretical thicknesses of the two nitriding layers. This is because: during the pre-nitriding process, the nitride size grows very rapidly at the high-temperature tempering temperature. Therefore, the pre-nitriding time needs to be controlled very short. As a result, although most of the nitrides are generated in the pre-nitrided layer, the nitriding is not complete. During the secondary nitriding process, some nitrogen atoms continue to diffuse into the pre-nitrided layer. After statistical analysis and summarization, the actual thickness of the secondary nitrided layer after secondary nitriding is equal to d2-0.14d1. By fitting the thicknesses of the two nitriding processes, the final actual thickness of the nitrided layer (target thickness of the nitrided layer) is d0=d1+d2-0.14d1, or d0=0.86d1+d2.

[0015] The short-process nitriding process of this invention first determines the steel composition, and then rationally sets the target nitriding layer thickness d0 according to the specific application scenario of the steel (components). Next, a tempering treatment is performed, namely quenching + high-temperature tempering. Near the end of the high-temperature tempering, ammonia gas is introduced into the high-temperature tempering equipment, and the ammonia decomposition rate is controlled. Based on the specific steel composition and the specific high-temperature tempering temperature, with d1 = 0.09mm-0.12mm as the target, the ammonia decomposition rate and nitriding time are adjusted to achieve pre-nitriding. Then, d2 is calculated according to d0 = 0.86d1 + d2. Then, based on the given steel composition, with d2 as the target, the temperature, ammonia decomposition rate, and nitriding time of the secondary nitriding are adjusted to finally obtain the final nitriding layer thickness d0. This effectively utilizes the high temperature advantage of the high-temperature tempering stage for short-time and rapid nitriding, thus laying the foundation for subsequent conventional secondary nitriding, shortening the nitriding time, and achieving short-process nitriding.

[0016] In summary, based on the theoretical thickness d1 of the pre-nitriding layer being 0.1 mm, the time of the conventional secondary nitriding process can be shortened by more than 10 hours. Since the pre-nitriding is carried out simultaneously with the high-temperature tempering, it does not increase the process time. Therefore, the nitriding process of the present invention can effectively improve the production efficiency of the nitriding process and reduce the process cost.

[0017] As a further limitation, the high-temperature tempering temperature T1 of the present invention is controlled at 850K-890K, the time t1 before the end of high-temperature tempering (i.e., pre-nitriding time) when ammonia gas is introduced is 0.08h-0.16h, and the ammonia decomposition rate α during the pre-nitriding process is... A1 The temperature range is 8%-12%. Since this process is carried out simultaneously with the high-temperature tempering process, the temperature can only be determined based on the high-temperature tempering temperature. Therefore, this invention needs to strictly limit and constrain the nitriding time and ammonia decomposition rate of the pre-nitriding process. If the pre-nitriding time is too long or the ammonia decomposition rate is too high, the nitride growth in the nitrided layer will be significant, resulting in the surface hardness and wear resistance of the workpiece not meeting the requirements of the invention. If the pre-nitriding time is too short or the ammonia decomposition rate is too low, the depth of the nitrided layer will be insufficient, and the effect of shortening the time of the subsequent secondary conventional nitriding process will not be significant.

[0018] As a further description, in the secondary nitriding process of the present invention, the time t2 is controlled between 8h and 40h, the nitriding temperature T2 is controlled between 760K and 810K, and the ammonia decomposition rate α during the nitriding process is... A2 Keep it between 30% and 50%.

[0019] The present invention also relates to an ASTM 4140 steel prepared by the aforementioned short-process nitriding process for ASTM 4140 steel.

[0020] Furthermore, the ASTM 4140 steel is used in planetary gear carriers, automotive drive shafts, gearbox gears, transmission gears, and other components.

[0021] The present invention further provides a planetary carrier for a planetary gear, which is prepared by the aforementioned short-process nitriding process of ASTM 4140 steel, or is prepared from the aforementioned ASTM 4140 steel.

[0022] The short-process nitriding process for ASTM 4140 steel of this invention overcomes the technical prejudice that nitriding cannot be performed during the high-temperature tempering stage. It adopts a technical concept of pre-nitriding during the high-temperature tempering stage followed by secondary nitriding. During the pre-nitriding stage, taking advantage of the rapid penetration of nitrogen atoms, the nitriding time and ammonia decomposition rate are strictly controlled, keeping the pre-nitrided layer thickness between 0.09mm and 0.12mm. This also avoids the problem of coarsening of nitride size in the pre-nitrided layer under high-temperature nitriding, laying the foundation for subsequent conventional secondary nitriding. This effectively shortens the time required for conventional secondary nitriding, reduces production costs, and achieves the same hardness as a conventional nitrided layer.

[0023] Based on a comprehensive consideration of multiple key factors influencing the thickness of the nitriding layer during the nitriding process, a theoretical thickness function for the pre-nitriding layer was constructed. This function serves as the basis for adjusting nitriding parameters during the pre-nitriding process, enabling precise control of the nitriding layer thickness in the pre-nitriding stage. Furthermore, experimental studies determined the relationship between the theoretical thickness of the pre-nitriding layer, the theoretical thickness of the secondary nitriding layer, and the target nitriding layer thickness, thus providing fundamental principles for precise control during the nitriding process.

[0024] The short-process nitriding process of this invention is completed simultaneously during the high-temperature tempering process, thus without increasing the time of the additional process. The presence of pre-nitriding treatment lays the foundation for the subsequent conventional secondary nitriding treatment, which shortens the process time of the secondary nitriding treatment by more than 10 hours and improves the process efficiency. Detailed Implementation

[0025] To enable those skilled in the art to fully understand the technical solution and beneficial effects of the present invention, the following description is provided in conjunction with specific experiments. Example 1

[0026] ASTM 4140 steel was used for nitriding tests. The composition (mass percentage) was as follows: C: 0.39%, Si: 0.25%, Mn: 0.91%, Cr: 1.02%, Mo: 0.19%, P: 0.019%, S: 0.019%, with the balance being Fe. Three specimens (20cm × 20cm × 2cm) were prepared and nitrided, and labeled 1-1, 1-2, and 1-3 respectively.

[0027] The target nitrided layer thickness was set to d0 of 0.25 mm. Test 1-1 adopted the short-process nitriding process of this invention, test 1-2 only underwent pre-nitriding treatment during the high-temperature tempering process, and test 1-3 adopted the conventional nitriding process. The three samples were first held at 850℃ (1123K) for 1 h and then water-quenched to room temperature. Then, they were tempered at 590℃ (863K) for 3 h and air-cooled to room temperature. Using d1 of this invention as a constraint condition and the tempering temperature as a reference, the pre-nitriding time and ammonia decomposition rate of samples 1-1 and 1-2 were adjusted. When the tempering holding time was 171 min (i.e., 9 min (0.15 h) before the end of tempering), ammonia gas was introduced at an ammonia decomposition rate of 10% for samples 1-1 and 1-2. They were immediately air-cooled after the tempering holding time was 3 h. According to the function calculation of this invention, the theoretical thickness of the pre-nitriding layer during the pre-nitriding process is d1=0.1057mm. After the pre-nitriding is completed, the actual thickness of the pre-nitriding layer tested according to the metallographic method test serial numbers 1-2 specified in GB / T 11354-2005 "Determination of Nitriding Layer Depth and Metallographic Structure Inspection of Steel Parts" is 0.106mm, which confirms that it matches the theoretical thickness of the pre-nitriding layer well.

[0028] Next, conventional secondary nitriding treatment was performed on test number 1-1 and test number 1-3 under the same nitriding temperature and ammonia decomposition rate. Based on the formula derived from this invention, the actual nitrided layer thickness d0 = 0.86d1 + d2, the theoretical thickness of the secondary nitrided layer after conventional secondary nitriding treatment of test number 1-1 is d2 = 0.159 mm, while the theoretical thickness of the nitrided layer after conventional nitriding treatment of test number 1-3 is d2 = 0.25 mm. Using d2 = [ ·exp(-1757.69 / T2)·ln(1+[α A2 The constraints are set as follows: 1 + 0.8[Cr] + 0.5[Mo]), nitriding temperature T2 = 783 K, and ammonia decomposition rate α. A2 Based on a 35% calculation, test sequence 1-1, after pre-nitriding treatment, requires approximately 8 hours to achieve a secondary nitriding layer thickness of 0.159 mm (actual nitriding layer thickness of 0.25 mm), while test sequence 1-3 requires approximately 19.9 hours to achieve an actual nitriding layer thickness of 0.25 mm. In terms of the process time required to achieve the target nitriding layer thickness, test sequence 1-1, after pre-nitriding treatment, can save 11.9 hours of process time.

[0029] After nitriding, the actual nitrided layer thicknesses of test serial numbers 1-1 and 1-3, as specified in GB / T 11354-2005 "Determination of Nitrided Layer Depth and Metallographic Structure Inspection of Steel Parts", were 0.248 mm and 0.252 mm, respectively, which confirmed that they were in good agreement with the target nitrided layer thickness of 0.25 mm.

[0030] The Vickers hardness of the nitrided surface was determined according to the GB / T 4340 series standards. Using a load of 0.3 kgf, the Vickers hardness of test serial number 1-1 reached HV613, and the Vickers hardness of test serial number 1-2 reached HV619. The hardness levels of the two are comparable. Example 2

[0031] ASTM 4140 steel was used for nitriding tests. The composition (mass percentage) was as follows: C: 0.42%, Si: 0.27%, Mn: 0.84%, Cr: 0.95%, Mo: 0.21%, P: 0.021%, S: 0.015%, with the balance being Fe. Three specimens (20cm × 20cm × 2cm) were prepared and nitrided, designated as 2-1, 2-2, and 2-3.

[0032] The target nitrided layer thickness was set to d0 of 0.45 mm. Test No. 2-1 adopted the short-process nitriding process of this invention, Test No. 2-2 only underwent pre-nitriding treatment during the high-temperature tempering process, and Test No. 2-3 adopted the conventional nitriding process. The three samples were first held at 865℃ (1138K) for 1 hour and then water-quenched to room temperature. Then, they were tempered at 600℃ (873K) for 2.5 hours and air-cooled to room temperature. Using d1 of this invention as a constraint condition and the tempering temperature as a reference, the pre-nitriding time and ammonia decomposition rate of samples No. 2-1 and 2-2 were adjusted. When the tempering holding time was 144 minutes (i.e., 6 minutes (0.1 hours) before the end of tempering), ammonia gas was introduced at an ammonia decomposition rate of 12% for pre-nitriding of samples No. 2-1 and 2-2. They were immediately air-cooled after the tempering holding time was 2.5 hours. According to the function calculation of this invention, the theoretical thickness of the pre-nitriding layer during the pre-nitriding process is d1=0.1166mm. After the pre-nitriding is completed, the actual thickness of the pre-nitriding layer tested according to the metallographic method test serial number 2-2 specified in GB / T 11354-2005 "Determination of Nitriding Layer Depth and Metallographic Structure Inspection of Steel Parts" is 0.115mm, which confirms that it matches the theoretical thickness of the pre-nitriding layer well.

[0033] Next, conventional secondary nitriding treatment was performed on test number 2-1 and test number 2-3 under the same nitriding temperature and ammonia decomposition rate. Based on the formula derived from this invention, the actual nitrided layer thickness d0 = 0.86d1 + d2, the theoretical thickness of the secondary nitrided layer after conventional secondary nitriding treatment in test number 2-1 is d2 = 0.350 mm, while the theoretical thickness of the nitrided layer after conventional secondary nitriding treatment in test number 2-3 is d2 = 0.45 mm. Using d2 = [ ·exp(-1757.69 / T2)·ln(1+[α A2 The constraints are set as follows: 1 + 0.8[Cr] + 0.5[Mo]), nitriding temperature T2 = 773 K, and ammonia decomposition rate α. A2 Based on a 40% calculation, test number 2-1, after pre-nitriding treatment, requires approximately 33.4 hours to achieve an actual nitrided layer thickness of 0.45 mm, while test number 2-3 requires approximately 55 hours. In terms of the process time required to achieve the target nitrided layer thickness, test number 2-1, after pre-nitriding treatment, can save 21.6 hours of process time.

[0034] After nitriding, the actual nitrided layer thicknesses of test serial numbers 2-1 and 2-3, as specified in GB / T 11354-2005 "Determination of Nitrided Layer Depth and Metallographic Structure Inspection of Steel Parts", were 0.454 mm and 0.451 mm, respectively, which confirmed that they matched the target nitrided layer thickness of 0.45 mm well.

[0035] The Vickers hardness of the nitrided surface was determined according to the GB / T 4340 series standards. Using a load of 0.3 kgf, the Vickers hardness of test number 2-1 reached HV643, and the Vickers hardness of test number 2-2 reached HV640. The hardness levels of the two are comparable. Example 3

[0036] ASTM 4140 steel was used for nitriding tests. The composition (mass percentage) was as follows: C: 0.40%, Si: 0.21%, Mn: 0.92%, Cr: 0.82%, Mo: 0.17%, P: 0.017%, S: 0.017%, with the balance being Fe. Three specimens (20cm × 20cm × 2cm) were prepared and nitrided, designated as 3-1, 3-2, and 3-3.

[0037] The target nitrided layer thickness was set to d0 of 0.60 mm. Test 3-1 adopted the short-process nitriding process of this invention, test 3-2 only underwent pre-nitriding treatment during the high-temperature tempering process, and test 3-3 adopted the conventional nitriding process. All three samples were first held at 880℃ (1153K) for 1 hour, then water-quenched to room temperature, and then tempered at 610℃ (883K) for 2 hours before being air-cooled to room temperature. Using d1 of this invention as a constraint and the tempering temperature as a reference, the pre-nitriding time and ammonia decomposition rate of samples 3-1 and 3-2 were adjusted. For samples 3-1 and 3-2, ammonia gas was introduced at an ammonia decomposition rate of 8% at 111 min of tempering (9 minutes (0.15 hours) remaining before the end of tempering), and air-cooling was performed immediately after the 2-hour tempering period. According to the function calculation of this invention, the theoretical thickness of the pre-nitriding layer during the pre-nitriding process is d1=0.0988mm. After the pre-nitriding is completed, the actual thickness of the pre-nitriding layer tested according to the metallographic method test serial number 3-2 specified in GB / T 11354-2005 "Determination of Nitriding Layer Depth and Metallographic Structure Inspection of Steel Parts" is 0.097mm, which confirms that it matches the theoretical thickness of the pre-nitriding layer well.

[0038] Next, conventional secondary nitriding treatment was performed on test number 3-1 and test number 3-3 using the same nitriding temperature and ammonia decomposition rate. Based on the formula derived from this invention, the actual nitrided layer thickness d0 = 0.86d1 + d2, the theoretical thickness d2 of the secondary nitrided layer after conventional secondary nitriding treatment of test number 3-1 is 0.515 mm, while the theoretical thickness d2 of the nitrided layer after conventional nitriding treatment of test number 3-3 is 0.60 mm. Using d2 = [ ·exp(-1757.69 / T2)·ln(1+[α A2 The constraints are 1 + 0.8[Cr] + 0.5[Mo]), with the nitriding temperature T2 = 808 K and the ammonia decomposition rate α. A2 Based on a 50% calculation, test number 3-1, after pre-nitriding treatment, requires approximately 39.7 hours to achieve an actual nitrided layer thickness of 0.60 mm, while test number 3-3 requires approximately 54 hours. In terms of the process time required to achieve the target nitrided layer thickness, test number 3-1, after pre-nitriding treatment, can save 14.3 hours of process time.

[0039] After nitriding, the actual nitrided layer thicknesses of test serial numbers 3-1 and 3-3, as specified in GB / T 11354-2005 "Determination of Nitrided Layer Depth and Metallographic Structure Inspection of Steel Parts", were 0.598 mm and 0.602 mm, respectively, which confirmed that they were in good agreement with the target nitrided layer thickness of 0.60 mm.

[0040] The Vickers hardness of the nitrided surface was determined according to the GB / T 4340 series standards. Using a load of 0.3 kgf, the Vickers hardness of test number 3-1 reached HV688, and the Vickers hardness of test number 3-3 reached HV685. The hardness levels of the two are comparable.

[0041] As verified by the above Examples 1-3, according to the method proposed in this invention, pre-nitriding treatment using the high-temperature range of high-temperature tempering can achieve rapid nitriding. By precisely controlling the nitriding time and ammonia decomposition rate, the thickness of the pre-nitrided layer can be controlled at around 0.1 mm, while avoiding the problem of coarsening of nitride size in the pre-nitrided layer under high-temperature nitriding. This lays the foundation for subsequent conventional secondary nitriding treatment, effectively shortening the conventional secondary nitriding process by more than 10 hours, reducing production costs. Furthermore, the hardness of the nitrided layer obtained by the two-stage nitriding process of this invention is comparable to that of the conventional nitrided layer.

[0042] Furthermore, according to the nitriding layer thickness function proposed in this invention, the parameters of the nitriding process can be adjusted with the target thickness as a constraint, thereby achieving precise control of the nitriding layer thickness.

[0043] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0044] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A short-process nitriding process for ASTM 4140 steel, characterized in that, Includes the following steps: 1) Perform composition analysis on ASTM 4140 steel to obtain the mass percentage content of Cr and Mo; 2) Determine the target nitrided layer thickness d0 for ASTM 4140 steel, in mm; 3) Quenching and tempering + pre-nitriding treatment: ASTM 4140 steel is quenched and then tempered at a high temperature of T1. Ammonia gas is introduced into the high-temperature tempering furnace at time t1 before the end of the tempering process, controlling the ammonia decomposition rate to α. A1 Pre-nitriding treatment was performed on ASTM 4140 steel to obtain a theoretical pre-nitrided layer with a thickness of d1 in mm, and 0.09 mm ≤ d1 ≤ 0.12 mm. 4) The ASTM 4140 steel obtained in step 3) is subjected to a process at temperature T2, time t2, and ammonia decomposition rate α. A2 The secondary nitriding treatment yields a theoretical secondary nitrided layer with a thickness of d2, in mm. in: d1=[ ·exp(-896.47 / T1)·ln(1+[α A1 ] / 10.6)] / (1+0.57[Cr]+0.32[Mo]); d2=[ ·exp(-1757.69 / T2)·ln(1+[α A2 ] / 20)] / (1+0.8[Cr]+0.5[Mo]); d0 = 0.86d1 + d2; In the formula: t1 and t2 are in units of h; T1 and T2 are in units of K; [α] A1 ]、[α A2 [] represents the ammonia decomposition rate during nitriding, the value before %; [Cr] and [Mo] are the mass percentages of Cr and Mo in Cr-Mo alloy steel, respectively, the values ​​before %.

2. The short-process nitriding process for ASTM 4140 steel according to claim 1, characterized in that: The high-temperature tempering temperature T1 is 850K-890K.

3. The short-process nitriding process for ASTM 4140 steel according to claim 1, characterized in that: The time t1 before the end of high-temperature tempering is 0.08h-0.16h.

4. The short-process nitriding process for ASTM 4140 steel according to claim 1, characterized in that: The ammonia decomposition rate α during the pre-nitriding treatment process A1 It ranges from 8% to 12%.

5. The short-process nitriding process for ASTM 4140 steel according to claim 1, characterized in that: The time t2 during the secondary nitriding process is 8h-40h.

6. The short-process nitriding process for ASTM 4140 steel according to claim 1, characterized in that: The temperature T2 during the secondary nitriding process is 760K-810K.

7. The short-process nitriding process for ASTM 4140 steel according to claim 1, characterized in that: The ammonia decomposition rate α during the secondary nitriding process A2 It ranges from 30% to 50%.

8. An ASTM 4140 steel prepared by the short-process nitriding process of ASTM 4140 steel as described in any one of claims 1-7.

9. The ASTM 4140 steel according to claim 8, characterized in that, The ASTM 4140 steel is used in planetary gear carriers, automotive drive shafts, gearbox gears, transmission gears, and other components.

10. A planet carrier for a planetary gear, characterized in that, It is prepared by the short-process nitriding process of ASTM 4140 steel as described in any one of claims 1-7, or by ASTM 4140 steel as described in any one of claims 8-9.

Images