Surface medium-temperature strengthening method of heavy-duty gear and heavy-duty gear

Abstract

The invention relates to a surface medium-temperature strengthening method of a heavy-duty gear and the heavy-duty gear, and belongs to the technical field of metal material forming manufacturing and surface heat treatment strengthening. The surface medium-temperature strengthening method comprises the following steps: carrying out quenching and tempering heat treatment on a heavy-duty gear steel blank, and carrying out cold rough machining on an outer tooth-shaped part; carrying out medium-frequency induction heating on each outer tooth-shaped part, so that the surface layer of each outer tooth-shaped part is completely austenitized; the outer tooth-shaped part is cooled to be below the recrystallization temperature and above the martensite phase transformation temperature, and medium-temperature deformation treatment is conducted on the heated surface layer; performing water-cooling quenching on the outer tooth-shaped part subjected to the medium-temperature deformation treatment; and after all the outer tooth-shaped parts are quenched, low-temperature tempering treatment is conducted on the whole gear, and finally accurate grinding machining is conducted. On the basis of not influencing the traditional induction quenching production efficiency, the performance of the tooth surface hardened layer is greatly improved, and the surface hardness of the outer tooth-shaped part is HV700 or above.

Description

Technical Field

[0001] The invention relates to the technical field of metal material forming and manufacturing and surface heat treatment strengthening, and in particular to a medium-temperature surface strengthening method for a heavy-loaded gear and the heavy-loaded gear. Background Art

[0002] Heavy-duty gears are mostly used in the production and manufacturing of large equipment. Among them, the induction hardening process has been widely used in the surface strengthening of medium-carbon quenched and tempered gear steel due to its high production efficiency and obvious hardening. Increasing strength while maintaining plasticity and toughness is a common requirement for the mechanical properties of heavy-duty gear steel.

[0003] At present, the induction hardening layer of the tooth surface mainly relies on the solid solution strengthening of the quenched martensite, and combined with the appropriate carbon content, the surface hardness and strength can be improved. However, too high a carbon content is very likely to cause quenching cracks in the hardened layer, which directly leads to the scrapping of the gear. Therefore, improving the tooth surface performance of heavy-duty gears from the aspects of alloy composition regulation and induction hardening process optimization is extremely limited, and the plastic toughness of the tooth surface of heavy-duty gears is sacrificed.

[0004] With the development, medium-temperature deformation heat treatment can significantly improve the strength of metal materials without losing too much plasticity and toughness, which has been generally recognized in the steel manufacturing industry. In addition, the secondary strengthening of traditional gear tooth surface is mainly used in the terminal finishing link of gear processing, including surface high-energy shot peening, ultrasonic impact nano-processing, etc., and the surface strengthening is mainly applied to martensite. However, the hardness and strength of the outermost layer cannot meet the technical requirements of heavy-duty gears. Summary of the invention

[0005] In view of the above analysis, the present invention aims to provide a medium-temperature surface strengthening method for heavy-loaded gears and heavy-loaded gears, so as to improve the mechanical properties and wear resistance of the heavy-loaded gears.

[0006] In one aspect, the present invention provides a method for medium-temperature strengthening of the surface of a heavy-duty gear, comprising the following steps:

[0007] Step 1: Perform quenching and tempering heat treatment on the heavy-duty gear steel blank, and then perform cold rough processing on the outer tooth profile;

[0008] Step 2: Perform medium frequency induction heating on each external toothed portion to completely austenitize the surface layer of the external toothed portion;

[0009] Step 3: Cool the outer toothed portion to below the recrystallization temperature and above the martensitic phase transformation temperature, and perform medium-temperature deformation treatment on the heated surface layer;

[0010] Step 4: water-cooling quenching the outer toothed portion after the medium-temperature deformation treatment;

[0011] Step 5: After all external toothed parts have been quenched, the entire gear is subjected to low-temperature tempering and finally fine grinding.

[0012] Furthermore, the hardness after the quenching and tempering heat treatment is not higher than HB350.

[0013] Furthermore, the thickness margin of the tooth surface after the cold rough processing does not exceed 0.5 mm.

[0014] Furthermore, the carbon content of the heavy-duty gear is 0.35-0.55wt.%, and the total content of Cr, Ni, Mo, Mn and Si elements is not less than 3wt.%; the pitch of the heavy-duty gear is not less than 80mm.

[0015] Furthermore, in step 2, the temperature of the medium frequency induction heating is ≤950°C; and the depth of complete austenitization of the surface layer is not less than 5mm.

[0016] Furthermore, the waiting cooling of the outer toothed portion may be air cooling or water cooling, the waiting time may not exceed 40 seconds, and the waiting end temperature may not be higher than 700°C.

[0017] Furthermore, the medium-temperature deformation treatment is surface rolling, and the surface rolling is mold rolling or high-energy shot peening rolling.

[0018] Furthermore, the depth range of the maximum concave-convex defect caused by the surface rolling on the tooth surface does not exceed 0.5 mm; the number of cycles of the surface rolling is 1 to 4 times; and the total duration of the surface rolling does not exceed 1 minute.

[0019] Furthermore, the maximum hardness of the surface of the outer toothed portion is above HV700; and the surface removal depth of the fine grinding of the heavy-duty gear with medium-temperature surface strengthening is not greater than 0.5 mm.

[0020] On the other hand, the present invention provides a heavy-duty gear obtained by the surface medium-temperature strengthening method described in the present invention.

[0021] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0022] 1. The surface medium-temperature strengthening method of heavy-duty gears provided by the present invention, the cooling and medium-temperature deformation treatment immediately after induction heating, can promote the refinement of the austenite grains on the surface, and at the same time introduce a large number of high-density dislocations, twins and subgrain boundaries and other lattice defects into the supercooled austenite. Such defects are then inherited into the medium-carbon martensite of the phase transformation, and interact with the carbon atoms in the martensite at the microscopic lattice, thereby greatly improving the surface strength; while introducing defects into the supercooled austenite, the thermal stability of the austenite is also improved, and the martensitic phase transformation temperature is reduced. A small amount of untransformed residual austenite exists in the medium-carbon martensite structure after quenching and cooling to room temperature, which can effectively release the stress between the quenched martensite lamellae and avoid the initiation of quenching microcracks; at the same time, the presence of residual austenite increases the plastic toughness of the strengthening layer;

[0023] 2. The surface medium-temperature strengthening method for heavy-duty gears provided by the present invention is used for surface deformation treatment of metastable austenite in the medium-temperature zone, which has excellent ductility, hardness and strength in the medium-temperature state;

[0024] 3. Compared with the traditional induction hardening hardening method, the medium-temperature surface hardening method of the heavy-duty gear provided by the present invention only applies medium-temperature deformation to the surface after induction heating to achieve the purpose of medium-temperature hardening without changing the heating and cooling heat treatment process time of the traditional induction hardening. On the basis of not affecting the production efficiency of traditional induction hardening, the performance of the tooth surface hardening layer is greatly improved, the surface hardness of the external tooth shape is above HV700, and the tensile strength is above 2100MPa, thereby improving the wear resistance.

[0025] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0027] Figure 1 It is a schematic diagram of the implementation of the surface medium temperature strengthening method;

[0028] Figure 2 Comparison of cross-sectional hardness gradients after surface medium-temperature strengthening and traditional induction hardening;

[0029] Figure 3 Comparison of the mechanical properties of the surface after medium temperature strengthening and traditional induction quenching strengthening;

[0030] Figure 4 The surface microstructure characteristics of heavy-loaded gears after medium-temperature strengthening. DETAILED DESCRIPTION

[0031] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0032] Heavy-duty gears are mostly used in the production and manufacturing of large equipment. Increasing strength while maintaining plasticity and toughness are common requirements for the mechanical properties of heavy-duty gear steel.

[0033] In order to improve the surface properties of gears, the induction hardening process is mainly used, and combined with appropriate carbon content, the surface hardness and strength can be improved. However, too high carbon content is very likely to cause quenching cracks in the hardened layer, directly leading to the scrapping of the gear.

[0034] With the development, medium-temperature deformation heat treatment can significantly improve the strength of metal materials without losing too much plasticity and toughness, which has been generally recognized in the steel manufacturing industry. The secondary strengthening of traditional gear tooth surface is mainly used in the terminal finishing link of gear processing, including surface high-energy shot peening, ultrasonic impact nano-processing, etc. The surface strengthening is mainly applied to martensite. However, the hardness and strength of the outermost layer cannot meet the technical requirements of heavy-duty gears.

[0035] Therefore, the present invention provides a method for medium-temperature strengthening of the surface of a heavy-duty gear, comprising the following steps:

[0036] Step 1: Perform quenching and tempering heat treatment on the heavy-duty gear steel blank, and then perform cold rough processing on the outer tooth profile;

[0037] Step 2: Perform medium frequency induction heating on each external toothed portion to completely austenitize the surface layer of the external toothed portion;

[0038] Step 3: Cool the outer toothed portion to below the recrystallization temperature and above the martensitic phase transformation temperature, and perform medium-temperature deformation treatment on the heated surface layer;

[0039] Step 4: water-cooling quenching the outer toothed portion after the medium-temperature deformation treatment;

[0040] Step 5: After all external toothed parts have been quenched, the entire gear is subjected to low-temperature tempering and finally fine grinding.

[0041] Compared with the prior art, the medium-temperature surface strengthening method for heavy-duty gears provided by the present invention mainly includes the steps of quenching and tempering, medium-frequency induction heating, medium-temperature deformation treatment, water-cooled quenching and low-temperature tempering. Compared with the traditional induction quenching strengthening method, the present invention only applies medium-temperature deformation to the surface after induction heating to achieve the purpose of medium-temperature strengthening, without changing the heating and cooling heat treatment process time of traditional induction quenching. On the basis of not affecting the production efficiency of traditional induction quenching, the performance of the hardened layer on the tooth surface is greatly improved.

[0042] Specifically, the hardness after the quenching and tempering heat treatment is not higher than HB350.

[0043] It should be noted that the heavy-duty gear steel blank is first treated by quenching and tempering heat treatment to adjust the heavy-duty gear steel blank to ensure that the hardness of the heavy-duty gear steel blank is not higher than HB350. The quenching and tempering treatment adopts existing processes, such as quenching and high-temperature tempering.

[0044] Specifically, the thickness margin of the tooth surface after the cold roughing does not exceed 0.5 mm.

[0045] It should be noted that after the quenching and tempering treatment, the tooth shape of the heavy-duty gear steel blank needs to be cold roughed, such as by cold turning. After cold roughing, the tooth surface thickness allowance does not exceed 0.5mm. If the tooth surface allowance is too large, the part with the best surface performance after subsequent medium-temperature deformation will be removed by subsequent precision grinding, affecting the use of heavy-duty gears.

[0046] Specifically, the carbon content of the heavy-duty gear is 0.35-0.55wt.%, and the total content of Cr, Ni, Mo, Mn and Si elements is not less than 3wt.%.

[0047] It should be noted that the surface medium temperature strengthening method provided in the present invention can be applied to heavy-duty gears of various compositions, including elements such as C, Cr, Ni, Mo, Mn and Si. However, it is necessary to ensure that the carbon content of the heavy-duty gears is 0.35-0.55wt.%, and the total content of Cr, Ni, Mo, Mn and Si elements is not less than 3wt.%. For example, 40CrNi2Mo, 35Cr2Ni2Mo, 35CrNi4MoV.

[0048] Specifically, the gear pitch is not less than 80 mm.

[0049] It should be noted that the surface medium-temperature strengthening method provided in the present invention is also applicable to heavy-loaded gears of various specifications or models, but the pitch of the heavy-loaded gears is not less than 80 mm to ensure that there is sufficient implementation space during surface medium-temperature deformation, thereby ensuring the performance of the heavy-loaded gears.

[0050] Specifically, in step 2, the temperature of the medium frequency induction heating is ≤950° C.; and the depth of complete austenitization of the surface layer is not less than 5 mm.

[0051] It should be noted that, in the present invention, each external toothed portion needs to be subjected to medium frequency induction heating, and the heating temperature needs to be controlled to be less than or equal to 950°C, such as 940°C, 930°C or 920°C, but at the same time, it is necessary to ensure that the surface layer can be completely austenitized, and the depth is not less than 5mm. If the temperature of medium frequency induction heating is too high, it will lead to coarse grains, overheating, severe deformation and other problems, affecting the performance of heavy-loaded gears.

[0052] Specifically, the waiting cooling of the outer toothed portion may be air cooling or water cooling, the waiting time shall not exceed 40 seconds, and the waiting end temperature shall not be higher than 700°C.

[0053] It should be noted that, in the present invention, after the induction heating operation, it is necessary to perform temperature cooling, cooling to below the recrystallization temperature and above the martensitic phase transformation temperature, that is, the medium temperature deformation treatment temperature zone. The temperature end temperature is not higher than 700°C, and the temperature end temperature is preferably 600°C to 700°C. For example, 690°C, 680°C, 670°C, 660°C, 650°C, 640°C, 630°C, 620°C, 610°C or 600°C.

[0054] Specifically, the medium temperature deformation treatment is surface rolling, which is mold rolling or high energy shot peening rolling.

[0055] Preferably, the depth range of the maximum concave-convex defect caused by surface rolling on the tooth surface does not exceed 0.5 mm; the number of cycles of surface rolling is 1 to 4 times; and the total duration of surface rolling does not exceed 1 minute.

[0056] Specifically, the rolling end temperature is not lower than the martensitic transformation temperature, and the corresponding martensitic transformation temperature can be calculated from the alloy composition of the steel and is in the range of 200 to 400°C.

[0057] It should be noted that the medium-temperature surface strengthening method for heavy-duty gears provided by the present invention, the cooling and medium-temperature surface deformation treatment immediately performed after induction heating, can promote the refinement of the surface austenite grains, and at the same time introduce a large number of high-density dislocations, twins, subgrain boundaries and other lattice defects into the supercooled austenite. Such defects are subsequently inherited in the phase transformed medium-carbon martensite, and interact with the carbon atoms in the martensite at the microscopic lattice, thereby greatly improving the surface strength; while introducing defects into the supercooled austenite, the thermal stability of the austenite is also improved, and the martensitic phase transformation temperature is reduced. A small amount of untransformed residual austenite exists in the medium-carbon martensite structure after quenching and cooling to room temperature, which can effectively release the stress between the quenched martensite lamellae and avoid the initiation of quenching microcracks; at the same time, the presence of residual austenite increases the plasticity and toughness of the strengthening layer.

[0058] The surface medium-temperature strengthening method for heavy-duty gears provided by the present invention performs surface deformation treatment on metastable austenite in the medium-temperature zone, which has excellent ductility in the medium-temperature state. The induction heating and surface rolling of the external toothed portion can be performed one by one on the tooth surface on one side, or on the tooth surface on both sides at the same time; when the heating process and the surface rolling process are performed on both sides at the same time, the consistency of the heating process and the surface rolling process must be ensured.

[0059] Compared with the traditional induction hardening hardening method, the medium-temperature surface hardening method of the heavy-duty gear provided by the present invention only applies medium-temperature deformation to the surface after induction heating to achieve the purpose of medium-temperature hardening without changing the heating and cooling heat treatment process time of the traditional induction hardening. On the basis of not affecting the production efficiency of traditional induction hardening, the performance of the tooth surface hardening layer is greatly improved, the surface hardness of the external tooth shape is above HV700, and the tensile strength is above 2100MPa, thereby improving the wear resistance.

[0060] Specifically, in step 4, after the medium-temperature surface rolling treatment of the tooth-shaped portion, water spray quenching is immediately performed, and the surface temperature of the tooth-shaped portion after the water spraying stops is not higher than 50°C.

[0061] It should be noted that, in the present invention, water cooling quenching is performed immediately after the surface is deformed at medium temperature, and the surface temperature is kept below 50° C., in order to ensure complete martensitic transformation and avoid the presence of residual austenite structure.

[0062] Specifically, in step 5, the time interval from the completion of quenching of the last external toothed portion to the low-temperature tempering treatment of the entire gear is no more than 2 hours, and the low-temperature tempering temperature is 160-220°C.

[0063] It should be noted that in the present invention, the time interval of low-temperature tempering is controlled within 2 hours, which can effectively avoid quenching cracks caused by quenching stress on the surface. The tempering temperature cannot be higher than 220°C to avoid insufficient hardness of the surface layer; the tempering temperature is not lower than 160°C to ensure sufficient tempering.

[0064] Specifically, the maximum hardness of the surface of the external toothed portion is above HV700; the surface removal depth of the fine grinding of the heavy-duty gear with medium-temperature surface strengthening is not more than 0.5 mm, for example, 0.5 mm, 0.4 mm, 0.3 mm, 0.2 mm, 0.1 mm.

[0065] Specifically, the present invention provides a heavy-duty gear obtained by the surface medium-temperature strengthening method described in the present invention.

[0066] In order to more clearly describe the present invention, it is further illustrated by the following examples and comparative examples.

[0067] Example 1

[0068] This embodiment provides a medium-temperature surface strengthening method for heavy-duty gears, which is implemented on large-module gears, with a gear module of 30, a number of teeth of 17, a pitch of 94 mm, and a gear material of 40CrNi2Mo, wherein the C content is 0.4wt.%, the Cr content is 1.2wt.%, the Ni content is 2wt.%, the Mo content is 0.15wt.%, the Si content is 0.35wt.%, and the Mn content is 0.15wt.%; Figure 1 Shown is a schematic diagram of the preparation method;

[0069] The preparation method is as follows:

[0070] Step 1: Perform quenching and tempering heat treatment on the heavy-duty gear steel blank, and use the cold turning method to perform rough machining on the outer tooth profile; the hardness after quenching and tempering heat treatment is HB350; the tooth surface thickness allowance after cold rough machining is 0.5mm;

[0071] Step 2: Perform medium frequency induction heating on each external toothed part at a heating temperature of 920°C to completely austenitize the surface of the external tooth, with an effective austenitization depth of 5.5 mm;

[0072] Step 3: Cool the outer toothed part to 650°C by spraying water within 20 seconds, and perform medium-temperature deformation treatment on the heated surface layer by surface mold rolling. Reciprocating rolling is performed twice, and the total time is controlled within 40 seconds. The temperature at the end of rolling is 420°C.

[0073] Step 4: After removing the rolling die, the outer toothed portion after the medium temperature deformation treatment is water-cooled and quenched to room temperature; the maximum concave-convex defect caused by surface rolling on the tooth surface has a depth range of 0.2 mm;

[0074] Step 5: After all tooth-shaped parts have been quenched, the entire gear is subjected to low-temperature tempering treatment within 1 hour at a tempering temperature of 180°C; finally, fine grinding is performed with a surface removal depth of 0.2mm to meet dimensional accuracy requirements.

[0075] Example 2

[0076] The preparation process of Example 2 is substantially the same as that of Example 1, except that in Example 2, the gear module is 40, the number of teeth is 17, the pitch is 126 mm, and the gear material is 35Cr2Ni2Mo, wherein the carbon content is 0.35wt.%, the Cr content is 2wt.%, the Ni content is 2wt.%, the Mo content is 0.15wt.%, the Si content is 0.35wt.%, and the Mn content is 0.15wt.%;

[0077] Step 2: Perform medium frequency induction heating on each external toothed part, with the highest heating temperature being 940°C, so that the surface of the external tooth is completely austenitized, and the effective austenitization depth is 5 mm;

[0078] Step 3: Spray water to cool the outer toothed part to 630°C within 25 seconds, and perform medium-temperature deformation treatment on the heated surface layer by surface mold rolling. Reciprocating rolling is performed 3 times, and the total time is controlled within 50 seconds. The temperature at the end of rolling is 400°C.

[0079] Step 5: After all external toothed parts have been quenched, the entire gear is subjected to low-temperature tempering within 1 hour at a tempering temperature of 190°C; finally, fine grinding is performed with a surface removal depth of 0.2mm to meet dimensional accuracy requirements.

[0080] Example 3

[0081] The preparation process of Example 3 is substantially the same as that of Example 1, except that in Example 3, the gear module is 40, the number of teeth is 17, the pitch is 126 mm, and the gear material is 35CrNi4MoV, wherein the carbon content is 0.35wt.%, the Cr content is 1wt.%, the Ni content is 4wt.%, the Mo content is 0.15wt.%, the V content is 0.15wt.%, the Si content is 0.25wt.%, and the Mn content is 0.05wt.%;

[0082] Step 2: Perform medium frequency induction heating on each external toothed part at a heating temperature of 950°C to completely austenitize the surface of the external tooth, with an effective austenitization depth of 5 mm;

[0083] Step 3: Spray water to cool the toothed part to 600°C within 30 seconds, and use surface mold rolling to perform medium-temperature deformation treatment on the heated surface layer. Reciprocate rolling 4 times, the total time is controlled within 60 seconds, and the temperature at the end of rolling is 350°C.

[0084] Comparative Example 1

[0085] The preparation process of Comparative Example 1 is substantially the same as that of Example 1, except that the medium-temperature deformation treatment is not included in Comparative Example 1;

[0086] After removing the induction heating coil, the tooth-shaped part is water-cooled and quenched to room temperature;

[0087] After all tooth-shaped parts have been quenched, the entire gear is subjected to low-temperature tempering treatment within 1 hour at a tempering temperature of 200°C; finally, fine grinding is performed with a surface removal depth of 0.1mm to meet dimensional accuracy requirements.

[0088] Comparative Example 2

[0089] The preparation process of Comparative Example 2 is substantially the same as that of Example 1, except that, in step 3 of Comparative Example 2: the toothed portion is cooled to 800°C by spraying water within 10 seconds, and the heated surface layer is subjected to medium-temperature deformation treatment by surface mold rolling, with reciprocating rolling once, and the total time is controlled within 30 seconds, and the temperature at the end of rolling is 500°C.

[0090] Comparative Example 3

[0091] The preparation process of Comparative Example 3 is substantially the same as that of Example 1, except that in Comparative Example 3, the gear material is 20CrNi2Mo, wherein the C content is 0.2wt.%, the Cr content is 1.2wt.%, the Ni content is 2wt.%, the Mo content is 0.15wt.%, the Si content is 0.35wt.%, and the Mn content is 0.15wt.%.

[0092] Performance Testing

[0093] The above-mentioned Examples 1-3 and Comparative Examples 1-3 were subjected to performance tests, mainly including the depth of the hardened layer, the maximum surface hardness, and the tensile strength. The test results are shown in Table 1.

[0094] Table 1 Performance test results

[0095] Group Depth of hardened layer Maximum surface hardness tensile strength Example 1 5.5mm HV700 2100MPa Example 2 5mm HV720 2160MPa Example 3 5.5mm HV750 2250MPa Comparative Example 1 5.5mm HV650 1700MPa Comparative Example 2 5.5mm HV670 1800MPa Comparative Example 3 5.5mm HV550 1600MPa

[0096] Combined with Examples 1-3 and Comparative Examples 1-3 and referring to Table 1, Figure 2-4 It can be seen that the medium-temperature surface strengthening method for heavy-loaded gears proposed in the present invention can significantly improve the surface strength, hardness and wear resistance of heavy-loaded gear components; the maximum surface hardness is above HV700, and the tensile strength is above 2100MPa.

[0097] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for medium-temperature strengthening of the surface of a heavy-duty gear, characterized in that: The following steps are involved: Step 1: Perform quenching and tempering heat treatment on the heavy-duty gear steel blank, and then perform cold rough processing on the outer tooth profile; Step 2: Perform medium frequency induction heating on each external toothed portion to completely austenitize the surface layer of the external toothed portion; Step 3: Cool the outer toothed portion to below the recrystallization temperature and above the martensitic phase transformation temperature, and perform medium-temperature deformation treatment on the heated surface layer; Step 4: water-cooling quenching the outer toothed portion after the medium-temperature deformation treatment; Step 5: After all external toothed parts have been quenched, the entire gear is subjected to low-temperature tempering and finally fine grinding.

2. The method for medium temperature strengthening of the surface of heavy-duty gears according to claim 1, characterized in that: The hardness after the quenching and tempering heat treatment is not higher than HB350.

3. The method for medium temperature strengthening of the surface of heavy-duty gears according to claim 1, characterized in that: The thickness margin of the tooth surface after the cold rough working does not exceed 0.5 mm.

4. The method for medium-temperature strengthening of the surface of heavy-duty gears according to claim 1, characterized in that: The carbon content of the heavy-duty gear is 0.35-0.55wt.%, and the total content of Cr, Ni, Mo, Mn and Si elements is not less than 3wt.%; the pitch of the heavy-duty gear is not less than 80mm.

5. The method for medium-temperature strengthening of the surface of heavy-duty gears according to claim 1, characterized in that: In step 2, the temperature of the medium frequency induction heating is ≤950°C; and the depth of complete austenitization of the surface layer is not less than 5mm.

6. The method for medium-temperature strengthening of the surface of heavy-duty gears according to claim 1, characterized in that: The waiting cooling of the outer toothed portion can be air cooling or water cooling, the waiting time is not longer than 40 seconds, and the waiting end temperature is not higher than 700°C.

7. The medium temperature surface strengthening method for heavy-duty gears according to claim 1, characterized in that: The medium temperature deformation treatment is surface rolling, and the surface rolling is mold rolling or high energy shot peening rolling.

8. The method for medium-temperature strengthening of the surface of heavy-duty gears according to claim 7, characterized in that: The depth range of the maximum concave-convex defect caused by the surface rolling on the tooth surface does not exceed 0.5 mm; the number of cycles of the surface rolling is 1 to 4 times; and the total duration of the surface rolling does not exceed 1 minute.

9. The method for medium-temperature strengthening of the surface of heavy-duty gears according to claim 1, characterized in that: The maximum hardness of the surface of the outer toothed portion is above HV700; the surface removal depth of the fine grinding of the heavy-duty gear with medium-temperature surface strengthening is not more than 0.5 mm.

10. A heavy-duty gear, characterized in that: The method is obtained by the surface medium-temperature strengthening method according to any one of claims 1 to 9.

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