Heat treatment process for relieving large-size carbide of 17CrNiMo6 large-modulus gear

Abstract

The invention discloses a heat treatment process for relieving large-size carbides of a 17CrNiMo6 large-modulus gear. The heat treatment process comprises the following steps: a) first-time heating: heating a workpiece to a carburizing temperature in a gradient heating manner; b) heat-preservation carburizing: carburizing the workpiece by adopting multi-section pulse carbon potential; c) cooling after carburization: carrying out furnace gradient cooling on the workpiece to 660 + / -5 DEG C; d) heat preservation transformation: the workpiece is subjected to heat preservation at 660 + / -5 DEG C for a period of time, so that dispersed cementite precipitation and pearlite transformation are fully carried out; e) heating and preserving heat for the second time: heating the workpiece to 820 + / -5 DEG C, and preserving heat for austenite transformation; and f) quenching. Therefore, large-size carbides in a gear structure after heat treatment can be reduced, martensite needles in a quenched workpiece are effectively promoted to be smaller and thinner, the content of retained austenite is low, the metallographic structure of the workpiece is guaranteed, the use performance of parts is met, and the heat treatment method is particularly suitable for heat treatment of large-modulus gears.

Description

Technical Field

[0001] The invention relates to a heat treatment process for reducing large-size carbides of 17CrNiMo6 large-module gears. Background Art

[0002] Mining excavator gears are typical heavy-duty gears with large size, high transmission power, low speed, high impact, harsh working conditions and high safety requirements. They place high demands on their surface hardness, wear resistance and contact fatigue strength, and are usually strengthened by carburizing and quenching process.

[0003] Currently, the industry generally adopts Figure 1 The traditional carburizing and quenching process shown in the figure usually takes a long time for a strong carburizing and a diffusion to obtain a deeper carburized layer, and then quenching is performed. With the improvement of product quality requirements, this process gradually exposed two defects. First, due to the large modulus of heavy-duty gears, a deep hardened layer (greater than 4.0mm) is required, and the solid carburizing time is long. The strong carburizing and diffusion carburizing method will cause the sub-surface carbon concentration to be higher than the solid solubility of carbon in austenite due to the one-time strong carbon potential for a long time, and large pieces of carbides will precipitate from the austenite and remain after quenching (refer to the attached Figure 2 ), resulting in excessive gear structure and quality problems such as a steep drop in carbon concentration gradient in the transition layer, which will affect the performance of the parts; secondly, in order to obtain a better quenching structure, the carburizing temperature is lowered and then reheated for austenitization before quenching, resulting in low production efficiency. Summary of the invention

[0004] The purpose of the present invention is to provide a heat treatment process for reducing large-sized carbides in 17CrNiMo6 large-module gears, reduce large-sized carbides in the gear structure after heat treatment, and meet the performance requirements of the parts.

[0005] In order to achieve the above technical objectives, the present invention will adopt the following technical solutions:

[0006] A heat treatment process for reducing large-size carbides of 17CrNiMo6 large-module gears comprises the following steps:

[0007] a) First heating:

[0008] The workpiece is heated to the carburizing temperature by gradient heating;

[0009] b) Insulation carburizing:

[0010] The workpiece is carburized using multi-stage pulse carbon potential;

[0011] c) Cooling after carburizing:

[0012] The workpiece is cooled down to 660±5℃ along with the furnace gradient;

[0013] d) Insulation transformation:

[0014] Keep the workpiece at 660±5℃ for a period of time to fully carry out the precipitation of dispersed cementite and the transformation of pearlite;

[0015] e) Second heating and insulation:

[0016] The workpiece is heated to 820±5℃ and then kept warm for austenite transformation;

[0017] f) Quenching.

[0018] Preferably, in step a), the workpiece is heated to the carburizing temperature by gradient heating, specifically: firstly, the workpiece is heated to 650±5°C and kept at the same temperature for 2 hours, then heated to 880±5°C and kept at the same temperature for 2 hours, and finally heated to the carburizing temperature of 930±5°C.

[0019] Preferably, the multi-stage pulse carbon potential in step b) is specifically two-stage pulse carbon potential, corresponding to the first and second stage pulse carbon potentials;

[0020] In the first pulse carbon potential, the strong carburizing potential is 1.15% and the diffusion carbon potential is 0.70%;

[0021] In the second pulse carbon potential, the strong carburizing potential is 1.05% and the diffusion carbon potential is 0.75%.

[0022] Preferably, in step b), the process time of the two-stage pulse carbon potential exceeds 80 hours and is adjusted according to the module of the gear to be heat treated.

[0023] Preferably, in the first pulse carbon potential, the time of strong carburizing potential is 30 hours, and the time of diffusion carbon potential is 12 hours;

[0024] In the second pulse carbon potential, the time of strong carburizing potential is 26h and the time of diffusion carbon potential is 16h.

[0025] Preferably, in step c), the workpiece is cooled to 660±5°C along the furnace gradient, specifically: the workpiece is cooled to 820°C while maintaining a carbon potential of 0.8%, and then rapidly cooled to 660±5°C under nitrogen protection.

[0026] Preferably, in step d), the workpiece is kept at 660±5° C. for 4 hours.

[0027] Preferably, in step f), the quenching process is: oil-quenching the workpiece, the oil cooling time is controlled at 80-90 minutes, and the oil temperature is 60-80°C.

[0028] Preferably, the workpiece composition is: ω[C] 0.15%-0.20%, ω[Si] 0.15%-0.35%, ω[Mn] 0.60%-0.90%, ω[Cr] 1.50%-1.80%, ω[Ni] 1.40%-1.70%, ω[Mo] 0.25%-0.35%, ω[Al] 0.012%-0.015%, ω[O] 0.06%-0.15%, ω[S] 0.20%-0.50%, ω[Ti] <0.25%, ω[N] <0.60%, ω[H] <0.02%, ω

[0029] [Ca]<0.02%, ω[Mg]0.03%-0.06%, and the rest is Fe and residual elements.

[0030] Based on the above technical objectives, the present invention has the following advantages over the prior art:

[0031] 1. The heat treatment process for reducing large-sized carbides in 17CrNiMo6 large-module gears described in the present invention is to gradually cool the workpiece to 660±5°C after carburizing and keep it at a constant temperature for a period of time to fully precipitate dispersed cementite and transform pearlite. The existence of dispersed cementite is beneficial to the next process of austenite transformation, which promotes the reduction of carbon content in austenite and the reduction of stability of supercooled austenite, so that it can be transformed into martensite more fully and the residual austenite in the final organization is reduced; secondly, it promotes the formation of austenite grains, reduces the size of austenite grains, and finally reduces the size of martensite; thirdly, it has a pinning effect on the growth of martensite during quenching, and promotes the martensite needles to become smaller. In summary, this process has the effect of refining martensite needles and reducing residual austenite in the final organization.

[0032] 2. For large-module gears (usually gears with a module > 20), in order to obtain a deep hardened layer (greater than 4.0 mm), the solid carburizing time is long, and the conventional method is prone to cause the formation of large-sized carbides. For this reason, the present invention adopts a multi-stage pulse carburizing method to overcome the formation of large-sized carbides. Specifically, the present invention adopts a two-stage pulse carbon potential, which not only improves the carburizing speed, but also optimizes the carbide structure and the carbon concentration gradient.

[0033] In summary, the heat treatment process of the present invention adopts a multi-stage pulse carburizing method during carburizing treatment, optimizes the carbide structure and the carbon concentration gradient, and after carburizing treatment, the workpiece is cooled to 660±5°C with the furnace gradient and is evenly heated for a period of time to precipitate dispersed cementite and transform pearlite, effectively promoting the smaller and finer martensitic needles and less residual austenite content in the workpiece after quenching, ensuring the metallographic structure of the workpiece, and ensuring that the martensitic structure and carbide structure of the metallographic structure formed after quenching are within the 1-3 level. The heat treatment process of the present invention is particularly suitable for the heat treatment of large modulus gears. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a traditional carburizing and quenching process diagram;

[0035] Figure 2 It is the subsurface metallographic structure of the workpiece made by the traditional carburizing and quenching process;

[0036] Figure 3 This is a heat treatment process diagram for reducing large-size carbides of 17CrNiMo6 large-module gears according to the present invention;

[0037] Figure 4 The subsurface metallographic structure of a workpiece is manufactured by adopting the heat treatment process for reducing large-size carbides of 17CrNiMo6 large-module gears according to the present invention. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means any limitation to the present invention and its application or use. Based on the embodiments in 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. Unless otherwise specified, the relative arrangement, expressions and numerical values ​​of the components and steps described in these embodiments do not limit the scope of the present invention. The techniques, methods and equipment known to ordinary technicians in the relevant fields may not be discussed in detail, but in appropriate cases, the techniques, methods and equipment should be regarded as part of the specification. In all examples shown and discussed here, any specific value should be interpreted as merely exemplary, rather than as a limitation. Therefore, other examples of the exemplary embodiments may have different values.

[0039] The invention discloses a heat treatment process for reducing large-sized carbides of 17CrNiMo6 large-module gears. Considering that the traditional carburizing and quenching process is very likely to produce large-sized carbides when heat treating large-module gears, on the one hand, starting from optimizing the carburizing process, a multi-stage pulse carburizing method is adopted to overcome the formation of large-sized carbides, so as to achieve the purpose of optimizing the carbide structure. On the other hand, a temperature gradient of cooling-heat preservation-heating is added between the carburizing process and the quenching process, so that fine carbides are partially analyzed from the hypereutectoid components after carburizing and pearlite transformation occurs, so as to achieve the goal of promoting smaller and finer martensite needles and less residual austenite content after quenching through the dispersed cementite formed first.

[0040] Specifically, the heat treatment process for reducing large-size carbides of 17CrNiMo6 large-module gears of the present invention is carried out by a pit furnace, such as Figure 3 As shown, the following steps are included:

[0041] a) First heating:

[0042] The workpiece is heated to the carburizing temperature, 930±5℃, by gradient heating.

[0043] In the attached figure, the workpiece is first heated to 650±5°C and kept at an average temperature for 2 hours, then heated to 880±5°C and kept at an average temperature for 2 hours, and finally heated to the carburizing temperature of 930±5°C.

[0044] The composition of the workpiece is: ω[C] 0.15%-0.20%, ω[Si] 0.15%-0.35%, ω[Mn] 0.60%-0.90%, ω[Cr] 1.50%-1.80%, ω[Ni] 1.40%-1.70%, ω[Mo] 0.25%-0.35%, ω[Al] 0.012%-0.015%, ω[O] 0.06%-0.15%, ω[S] 0.20%-0.50%, ω[Ti] <0.25%, ω[N] <0.60%, ω[H] <0.02%, ω[Ca] <0.02%, ω[Mg] 0.03%-0.06%, and the rest is Fe and unavoidable impurity elements (residual elements).

[0045] b) Insulation carburizing:

[0046] The workpiece is carburized using a multi-stage pulse carbon potential; in this step, the multi-stage pulse carbon potential is specifically two-stage pulse carbon potential, corresponding to the first and second stage pulse carbon potentials; in the first stage pulse carbon potential, the strong carburizing potential is 1.15%, and the diffuse carbon potential is 0.70%; in the second stage pulse carbon potential, the strong carburizing potential is 1.05%, and the diffuse carbon potential is 0.75%.

[0047] It can be seen that in the first pulse carbon potential, the strong carburizing potential is 1.15%, which increases the carbon potential compared to the 1.05% strong carburizing potential of the traditional process, which can speed up the carburizing speed, while the diffusion carbon potential is 0.70%, which reduces the carbon potential compared to the 0.75% diffusion carbon potential of the traditional process, and promotes the full diffusion of high-concentration carbon content on the surface; in the second pulse carbon potential, the strong carburizing potential is 1.05%, and the diffusion carbon potential is 0.75%, which further avoids excessive subsurface carbon concentration and insufficient surface carbon content. In this way, the two pulse carbon potentials not only increase the carburizing speed, but also optimize the carbide structure and the carbon concentration gradient.

[0048] For the carburizing treatment of large modulus gears, in order to obtain a deep hardened layer of the target depth, the process time of the two-stage pulse carbon potential of the present invention exceeds 80 hours, and is adjusted according to the modulus of the gear to be heat treated. In the accompanying drawings, in the first stage of pulse carbon potential, the time of strong carburizing potential is 30 hours, and the time of diffusion carbon potential is 12 hours; in the second stage of pulse carbon potential, the time of strong carburizing potential is 26 hours, and the time of diffusion carbon potential is 16 hours.

[0049] c) Cooling after carburizing:

[0050] The workpiece is cooled down to 660±5℃ along with the furnace gradient;

[0051] Specifically, the temperature is furnace cooled to 820° C. while maintaining a carbon potential of 0.8%, and then rapidly cooled to 660±5° C. under nitrogen protection.

[0052] In this step, it is important to note that the temperature should not be lowered too low (e.g. below 500°C) during the cooling process to prevent the workpiece from undergoing bainite transformation (Bs point is about 500°C) during the cooling process. The reason is that bainite is easily left behind after quenching, increasing the content of non-martensitic structure. After the heat treatment process is completed, we need to grade the martensitic structure in the workpiece in accordance with GB / T25744 "Metallographic Inspection of Carburizing, Quenching and Tempering of Steel Parts". The qualified standard is that the level of the martensitic structure is at level 1-3.

[0053] In addition, during the cooling process, the hypereutectoid component of the workpiece will first precipitate dispersed fine carbides.

[0054] Furthermore, in this step, the carbon potential is maintained at 0.8% by furnace cooling to 820° C., and then rapidly cooled to 660±5° C. under nitrogen protection. Under the premise of meeting the process requirements, the process time is reduced and the energy cost is saved.

[0055] d) Insulation transformation:

[0056] Keep the workpiece at 660±5℃ for a period of time, which may be 4h, to fully carry out the precipitation of dispersed cementite and the transformation of pearlite.

[0057] e) Second heating and insulation:

[0058] The workpiece is heated to 820±5℃ and then kept at this temperature for austenite transformation.

[0059] In this step, during the cooling-insulation process after carburizing, the carbon element of the workpiece is precipitated from the lattice of the iron element to form dispersed cementite. On the one hand, it can promote the reduction of carbon content in the austenite formed in the next step, reduce the stability of supercooled austenite, so that it can transform into martensite more fully, and finally reduce the residual austenite in the organization; second, the dispersed cementite promotes the formation of austenite grains, so that the austenite grain size formed in the next step is reduced, and the final martensite size is reduced; third, the dispersed cementite plays a pinning and hindering role in the growth of martensite during quenching, and promotes the martensite needles to become smaller. In summary, this process plays a role in refining martensite needles and reducing residual austenite in the final organization. Compared with the original carburizing and quenching process, this process has a shortened process time and improved efficiency. Compared with the process of direct carburizing and cooling quenching without this process, the metallographic structure of the workpiece is guaranteed.

[0060] The pearlite transformed in step d) undergoes austenite transformation after the temperature is subsequently raised to A1 (660±5° C.), and the carbon content in the transformed austenite is lower than the carbon content of the austenite during carburization.

[0061] f) Quenching: Quench the workpiece with oil, the oil cooling time is controlled at 80-90 minutes, and the oil temperature is 60-80℃.

[0062] The mechanical properties of the workpiece after carburizing and quenching treatment are as follows: hardness of 58-62HRC, tensile strength ≥1080MPa, yield strength ≥850MPa, elongation after fracture ≥10%, and shrinkage ≥35%.

[0063] The heat treatment process of the present invention adopts a multi-stage pulse carburizing process of sufficient duration in step b), which, on the one hand, meets the needs of deep hardening layer (greater than 4.0 mm) of large modulus gears, and on the other hand, optimizes the carbon concentration gradient, avoids carburizing to form large blocks of carbides, and ensures that the dispersed cementite formed by the precipitation of carbon elements from the lattice of iron elements during cooling-insulation is small in size, and then completes the precipitation of dispersed cementite and pearlite transformation through cooling-insulation. The synergistic effect of these two steps ensures that the martensitic structure and carbide structure of the metallographic structure formed by the large modulus gear workpiece after quenching are both within the 1-3 level.

[0064] Example

[0065] The embodiments of the present invention will be described in detail below in conjunction with Table 1 below.

[0066] Table 1 Comparison of the organization of the conventional carburizing and quenching process and the process of the present invention

[0067]

[0068] As shown in Table 1, for the traditional carburizing and quenching process, the organizational level of the samples in the furnace for 3 heats (heat 1-heat 3) was counted; for the heat treatment process described in the present invention, the organizational level of the samples in the furnace for 3 heats (heat 4-heat 6) was also counted. The detection standard is in accordance with GB / T25744 "Metallographic Inspection of Carburizing, Quenching and Tempering of Steel Parts", and the martensite level 1-3 is martensite needle ≤8um, and the residual austenite content is ≤18%; the carbide level 1-3 is the maximum carbide size of fine-grained carbides plus coarser and smaller block carbides; the core organization level 1-3 is a small amount of free ferrite. According to the test results, using the traditional carburizing and quenching process, the carbide level of the samples in the furnace after treatment is 4, which does not meet the requirements. However, using the heat treatment process described in the present invention, the carbide level of the obtained samples in the furnace is less than 3, which meets the requirements.

Claims

1. A heat treatment process for reducing large-size carbides in 17CrNiMo6 large-module gears, characterized in that: The steps include: a) First heating: The workpiece is heated to the carburizing temperature by gradient heating; b) Insulation carburizing: The workpiece is carburized using multi-stage pulse carbon potential; c) Cooling after carburizing: The workpiece is cooled down to 660±5℃ along with the furnace gradient; d) Insulation transformation: Keep the workpiece at 660±5℃ for a period of time to fully carry out the precipitation of dispersed cementite and the transformation of pearlite; e) Second heating and insulation: The workpiece is heated to 820±5℃ and then kept warm for austenite transformation; f) Quenching.

2. The heat treatment process for reducing large-size carbides of 17CrNiMo6 large-module gears according to claim 1 is characterized in that: In step a), the workpiece is heated to the carburizing temperature by gradient heating, specifically: firstly, the workpiece is heated to 650±5°C and kept at the same temperature for 2 hours, then heated to 880±5°C and kept at the same temperature for 2 hours, and finally heated to the carburizing temperature of 930±5°C.

3. The heat treatment process for reducing large-size carbides of 17CrNiMo6 large-module gears according to claim 1 is characterized in that: The multi-stage pulse carbon potential in step b) is specifically two-stage pulse carbon potential, corresponding to the first and second stage pulse carbon potentials; In the first pulse carbon potential, the strong carburizing potential is 1.15% and the diffusion carbon potential is 0.70%; In the second pulse carbon potential, the strong carburizing potential is 1.05% and the diffusion carbon potential is 0.75%.

4. The heat treatment process for reducing large-size carbides of 17CrNiMo6 large-module gears according to claim 3 is characterized in that: In step b), the process time of the two-stage pulse carbon potential exceeds 80 hours and is adjusted according to the module of the gear to be heat treated.

5. The heat treatment process for reducing large-size carbides of 17CrNiMo6 large-module gears according to claim 3 is characterized in that: In the first pulse carbon potential, the time of strong carburizing potential is 30h, and the time of diffusion carbon potential is 12h; In the second pulse carbon potential, the time of strong carburizing potential is 26h and the time of diffusion carbon potential is 16h.

6. The heat treatment process for reducing large-size carbides of 17CrNiMo6 large-module gears according to claim 1 is characterized in that: In step c), the workpiece is cooled to 660±5° C. along the furnace gradient, specifically: the workpiece is cooled to 820° C. while maintaining a carbon potential of 0.8%, and then rapidly cooled to 660±5° C. under nitrogen protection.

7. The heat treatment process for reducing large-size carbides of 17CrNiMo6 large-module gears according to claim 1 is characterized in that: In step d), the workpiece is kept at 660±5° C. for 4 hours.

8. The heat treatment process for reducing large-size carbides of 17CrNiMo6 large-module gears according to claim 1 is characterized in that: In step f), the quenching process is: the workpiece is oil-quenched, the oil cooling time is controlled at 80-90 minutes, and the oil temperature is 60-80°C.

9. The heat treatment process for reducing large-size carbides of 17CrNiMo6 large-module gears according to claim 1 is characterized in that: The workpiece comprises: ω[C] 0.15%-0.20%, ω[Si] 0.15%-0.35%, ω[Mn] 0.60%-0.90%, ω[Cr] 1.50%-1.80%, ω[Ni] 1.40%-1.70%, ω[Mo] 0.25%-0.35%, ω[Al] 0.012%-0.015%, ω[O] 0.06%-0.15%, ω[S] 0.20%-0.50%, ω[Ti]<0.25%, ω[N]<0.60%, ω[H]<0.02%, ω[Ca]<0.02%, ω[Mg]0.03%-0.06%, and the rest are Fe and residual elements.

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