A normalizing method for 18CrNiMo7-6 precision forged gears

Abstract

This invention discloses a normalizing method for 18CrNiMo7-6 precision forged gears. The method includes heating the 18CrNiMo7-6 precision forged gear blank in a protective atmosphere to 930-950℃ and holding for 2-4 hours; then cooling the blank in the furnace to 820-840℃ and holding for 1-2 hours; followed by protective atmosphere air cooling to below 200℃; air cooling to room temperature after removal from the furnace; and finally, two high-temperature tempering processes in a nitrogen atmosphere at 680-700℃ for 4-6 hours, followed by water cooling to room temperature. This invention ensures a decarburized layer of <0.1mm on the surface of the precision forged gear, while also allowing adjustment of the pre-heat hardness, uniform microstructure, and carbide dispersion, resulting in consistent deformation patterns during subsequent carburizing and quenching heat treatment.

Description

Technical Field

[0001] This invention relates to a normalizing method for 18CrNiMo7-6 precision forged gears, belonging to the field of heat treatment technology for metallic materials. Background Technology

[0002] Currently, the grinding allowance for precision-forged gears is only 0.2-0.3mm. If ordinary normalizing is used, surface decarburization and oxide scale can easily exceed 0.5mm, causing decarburization of the teeth and affecting the grinding amount and diffusion layer structure after carburizing. To prevent decarburization after normalizing, gears without splines are generally not normalized after precision forging and are directly machined, carburized, and quenched. For precision-forged gears made of 18CrNiMo7-6 material, if normalizing is not performed after forging, the hardness will inevitably be high and uneven, making machining difficult and inefficient. At the same time, for some 18CrNiMo7-6 precision-forged gears with internal splines, without effective normalizing, the deformation of the spline area during heat treatment cannot be guaranteed, and deformation exceeding tolerance is very likely to occur, resulting in scrap. Therefore, there is an urgent need for a normalizing method that can both uniformly refine the grain structure and reduce the surface hardness of 18CrNiMo7-6 precision-forged gears to facilitate the machining efficiency of subsequent splines and other parts, while ensuring that the surface of the parts is free from oxidation and decarburization. Summary of the Invention

[0003] Purpose of the invention: The purpose of this invention is to provide a normalizing method for 18CrNiMo7-6 precision forged gears, which can refine grains, reduce and stabilize heat treatment deformation, reduce matrix hardness, and prevent surface oxidation and decarburization.

[0004] Technical solution: The present invention provides a normalizing method for 18CrNiMo7-6 precision forged gears, comprising the following steps:

[0005] (1) Heat treatment of 18CrNiMo7-6 precision forged gears;

[0006] (2) The 18CrNiMo7-6 precision forged gears after heat treatment in step (1) are subjected to furnace cooling treatment.

[0007] (3) Cool the 18CrNiMo7-6 precision forged gear after the furnace cooling treatment in step (2);

[0008] (4) The 18CrNiMo7-6 precision forged gear after the cooling treatment in step (3) is subjected to a first high-temperature tempering and water cooling.

[0009] (5) The 18CrNiMo7-6 precision forged gear after step (4) is subjected to a second high-temperature tempering and water cooling.

[0010] Furthermore, in step (1), the heating treatment is carried out under a nitrogen + methanol atmosphere.

[0011] Furthermore, in step (1), the temperature of the heat treatment exceeds the subsequent carburizing and quenching temperature of the gear in the finishing process by 10-20°C.

[0012] Furthermore, in step (1), the heat treatment time is at least 2 × the effective thickness min of the 18CrNiMo7-6 precision forged gear.

[0013] Furthermore, the heat treatment temperature is 930-950℃, and the heat treatment time is 2-4 hours. Preferably, the temperature is 930℃ and the time is 3 hours.

[0014] Further, in step (2), the furnace cooling treatment is carried out at 820-840°C for 1-2 hours. Preferably, the furnace is cooled to 820°C and held for 1 hour.

[0015] Furthermore, in step (3), the cooling process includes the following two stages:

[0016] Stage 1: Cool the 18CrNiMo7-6 precision forged gears to below 200℃;

[0017] Phase 2: Air-cool the 18CrNiMo7-6 precision forged gears to room temperature.

[0018] Furthermore, in stage 1, the cooling to below 200°C is carried out under a nitrogen + methanol atmosphere.

[0019] Furthermore, in step (4), the first high-temperature tempering is carried out under nitrogen protection, the temperature of the first high-temperature tempering is slightly lower than the Al eutectoid temperature, and the holding time is at least 3 × 18CrNiMo7-6 forged gear effective thickness min.

[0020] Furthermore, the temperature of the first high-temperature tempering is 680-700℃, and the holding time is 4-6 hours. The preferred temperature is 680℃, and the holding time is 4.5 hours.

[0021] Furthermore, in step (5), the second high-temperature tempering is carried out under nitrogen protection, the temperature of the first high-temperature tempering is slightly lower than the Al eutectoid temperature, and the holding time is at least 4 × 18CrNiMo7-6 forged gear effective thickness min.

[0022] Furthermore, the second high-temperature tempering is performed at a temperature of 680-700℃ for 5-8 hours. The preferred temperature is 680℃ for 6 hours.

[0023] Furthermore, in steps (1) and (2), the heating, heating, heat preservation and furnace cooling processes are all protected by a nitrogen + methanol atmosphere.

[0024] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0025] (1) The method of the present invention achieves a refined and uniform grain structure by heating and cooling in a protective atmosphere in a multi-purpose furnace, ensuring that there is no oxidation or decarburization on the surface of the gear during the entire high-temperature stage; the first stage of heat preservation temperature is selected to be 10-20℃ higher than the subsequent carburizing temperature to ensure that no new core structure grows during the carburizing heating process of the gear blank, which can reduce heating deformation; the second stage of heat preservation is the same as the quenching temperature of the subsequent carburizing process, and after heat preservation for 1 hour, the front chamber is air-cooled, which significantly optimizes the transformation efficiency, strength and density of the carburized layer structure and core structure in the subsequent carburizing quenching cooling process.

[0026] (2) The method of the present invention uses a multi-purpose furnace production line tempering furnace for high-temperature tempering. High-temperature tempering decomposes the bainite generated by normalizing air cooling into pearlite + ferrite, reducing the hardness of the gear, ensuring the uniformity of the structure, and ensuring the efficiency of subsequent processing. Nitrogen gas is introduced for protection during tempering heating and heat preservation, which further prevents oxidation and decarburization on the surface of the gear. Water cooling after tempering can ensure rapid cooling and improve production efficiency.

[0027] (3) The 18CrNiMo7-6 precision forged gears processed by the process of the present invention have an oxide decarburization layer of no more than 0.1 mm on the surface; more than 90% of them are fine grains of grade 8 or above, and there are no coarse grains of grade 4 or below; the structure is uniform, and after two high-temperature temperings, the matrix structure has high carbide dispersion and matrix hardness of 200-220HB, good machining efficiency, good inhibition of grain growth during carburizing heating, and consistent deformation pattern after heat treatment, and controllable deformation. Attached Figure Description

[0028] Figure 1 This is a normalizing process curve diagram of 18CrNiMo7 precision forged gear in Example 1;

[0029] Figure 2 The normalizing process curve of 18CrNiMo7-precision forged gear in Comparative Example 1;

[0030] Figure 3 The normalizing process curve of 18CrNiMo7-precision forged gear in Comparative Example 2;

[0031] Figure 4 The graph shows the process curves of carburizing and quenching in the same furnace for three types of 18CrNiMo7 precision forged gears with normalizing. Detailed Implementation

[0032] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0033] The normalizing method for 18CrNiMo7-6 precision forged gears according to the present invention includes the following steps:

[0034] (1) The 18CrNiMo7-6 precision forged gear is heated in a nitrogen-methanol atmosphere in the heating chamber of a multi-purpose furnace. The heating temperature is 930-950℃. The heating temperature should exceed the carburizing and quenching temperature of the subsequent precision gear by 10-20℃. The holding time is at least 2 × effective thickness min. Sufficient holding time ensures that the gear structure is completely austenitized.

[0035] (2) The 18CrNiMo7-6 precision forged gear treated in step (1) is cooled to 820-840℃ in the heating chamber of a multi-purpose furnace and kept at that temperature for 1 hour to ensure that the gear has a uniform internal and external temperature before air cooling.

[0036] (3) Cooling treatment is performed on the 18CrNiMo7-6 precision forged gears processed in step (2), including:

[0037] Phase 1: The 18CrNiMo7-6 precision forged gear is transferred to the front chamber of the heating furnace and air-cooled under the protection of nitrogen + methanol atmosphere until the gear temperature is below 200℃.

[0038] Phase 2: Transfer the 18CrNiMo7-6 precision forged gears at temperatures below 200℃ to a multi-purpose furnace air-cooling table and air-cool them to room temperature.

[0039] (4) Transfer the 18CrNiMo7-6 precision forged gear cooled to room temperature to a tempering furnace for the first high-temperature tempering. The tempering temperature is 680-700℃, which is slightly lower than the Al eutectoid temperature. The first tempering time is at least 3 × effective thickness min. Nitrogen gas is introduced for protection during the tempering heating and holding stages to prevent oxidation and decarburization of the workpiece surface. After tempering, water cooling is performed for rapid cooling. The purpose of the first high-temperature tempering is to decompose the granular bainite structure generated by rapid cooling into ferrite + pearlite.

[0040] (5) After the first high-temperature tempering and water cooling, the 18CrNiMo7-6 precision forged gear is transferred to a tempering furnace for a second high-temperature tempering. The tempering temperature is 680-700℃ and the tempering time is at least 4 × effective thickness min. Nitrogen gas is introduced for protection during the tempering heating and holding stages to prevent oxidation and decarburization of the workpiece surface. After tempering, water cooling is performed for rapid cooling. The purpose of the second high-temperature tempering is to disperse and strengthen the carbides, so that the cementite in the pearlite is dispersed and uniformly refined. During subsequent carburizing and quenching, the dispersed cementite needs a long time to gather and reintegrate into the austenite, which can effectively reduce the growth rate of grains during the carburizing and holding process. This patented method is of great help in controlling the deformation of carburized gears with deep carburized layers.

[0041] Example 1

[0042] 1. Prepare precision-forged gears, with the following requirements:

[0043] Material: 18CrNiMo7-6

[0044] Gear module: 6

[0045] Effective thickness: 90mm

[0046] Grinding allowance: 0.2mm

[0047] Single piece weight: 25kg

[0048] 2. Normalizing is performed using the method described above in this invention:

[0049] (1) The 18CrNiMo7-6 precision forged gear was heated in a nitrogen + methanol atmosphere in the heating chamber of a multi-purpose furnace at a temperature of 930℃ and a holding time of 2 times the effective thickness = 2*90 = 180min. The heating temperature was selected to be 10℃ higher than the subsequent carburizing temperature. The holding time was based on the effective thickness of the workpiece and was 2 times the effective thickness. Sufficient holding time is the key to complete austenitization of the microstructure.

[0050] (2) The 18CrNiMo7-6 precision forged gear treated in step (1) is cooled in a nitrogen + methanol atmosphere in the heating chamber of a multi-purpose furnace to 820°C. The holding temperature is consistent with the quenching temperature after carburizing. The holding time is 1 hour to ensure that the internal and external temperatures of the 18CrNiMo7-6 precision forged gear blank are uniform.

[0051] (3) Cooling treatment is performed on the 18CrNiMo7-6 precision forged gears processed in step (2), including:

[0052] Phase 1: The 18CrNiMo7-6 precision forged gear is transferred to the front chamber of the heating furnace and air-cooled under the protection of nitrogen + methanol atmosphere until the temperature of the 18CrNiMo7-6 precision forged gear is below 200℃; the gear is protected from oxidation during the rapid cooling process from 820℃ to 200℃ in the nitrogen and methanol atmosphere in the first phase, and the surface of the workpiece exiting the furnace at 200℃ will no longer show oxidation and decarburization.

[0053] Phase 2: Transfer the 18CrNiMo7-6 precision forged gears at temperatures below 200℃ to a multi-purpose furnace air-cooling table and air-cool them to room temperature.

[0054] (4) Transfer the 18CrNiMo7-6 precision forged gear cooled to room temperature to a tempering furnace for the first high-temperature tempering. The tempering temperature is 680℃ (slightly lower than the Al eutectoid temperature of 690℃), and the tempering time is 3 times the effective thickness, which is 3*90=270min. Nitrogen gas is introduced for protection during the tempering heating and holding stages to prevent oxidation and decarburization of the workpiece surface. After tempering, water cooling is performed for rapid cooling. The first high-temperature tempering decomposes the bainite structure generated by normalizing air cooling into pearlite + ferrite, making the matrix structure a uniform pearlite + ferrite normalized structure.

[0055] (5) After the first high-temperature tempering and water cooling, the 18CrNiMo7-6 precision forged gear is transferred to a tempering furnace for a second high-temperature tempering at 680℃ for 4 times the effective thickness (4*90=360min). Nitrogen gas is introduced during the tempering heating and holding stages to prevent oxidation and decarburization of the workpiece surface. After tempering, water cooling is performed for rapid cooling. The second high-temperature tempering disperses carbides, which can slow down the rate at which austenite dissolves cementite in the matrix during subsequent carburizing and quenching holding, slow down grain growth at high temperatures, and reduce carburizing and quenching deformation. It is helpful for grain control in deep carburizing. For the process curve of Example 1, see Example 1. Figure 1 .

[0056] After completion, metallographic physical sample testing was performed on the 18CrNiMo7-6 gear. The data are shown in Table 1. The gear treated with the normalizing process provided by this invention meets the national standards and drawing requirements.

[0057] Table 1 Metallographic test data of precision forged gears

[0058] Inspection Items Require Test results Surface hardness 180-220HB 206HB Oxidative decarbonization <0.2mm 0.05mm Grain size grade ≥6 levels Level 8 Homogeneity of normalized structure ≤3 levels Level 1

[0059] As can be seen from Table 1, the implementation method of Example 1 results in a very high degree of grain refinement after normalizing, and at the same time, the microstructure is extremely uniform, providing the best pre-structure for subsequent carburizing and quenching deformation control.

[0060] Comparative Example 1

[0061] 1. Prepare precision-forged gears, with requirements the same as in Example 1, as follows:

[0062] Material: 18CrNiMo7-6

[0063] Gear module: 6

[0064] Effective thickness: 90mm

[0065] Grinding allowance: 0.2mm

[0066] Single piece weight: 25kg

[0067] 2. 18CrNiMo7-6 is normalized using the following process (with different normalizing holding temperatures):

[0068] (1) The 18CrNiMo7-6 precision forged gear was heated in a nitrogen + methanol atmosphere in the heating chamber of the multi-purpose furnace at a temperature of 960℃. The heating temperature was selected to be 40℃ higher than the subsequent carburizing temperature. The holding time was twice the effective thickness = 2 * 90 = 180 min.

[0069] (2) The 18CrNiMo7-6 precision forged gear treated in step (1) is cooled in a nitrogen + methanol atmosphere in the heating chamber of a multi-purpose furnace to 820°C. The holding temperature is consistent with the quenching temperature after carburizing. The holding time is 1 hour to ensure that the internal and external temperatures of the 18CrNiMo7-6 precision forged gear blank are uniform.

[0070] (3) Cooling treatment is performed on the 18CrNiMo7-6 precision forged gears processed in step (2), including:

[0071] Phase 1: The 18CrNiMo7-6 precision forged gear is transferred to the front chamber of the heating furnace and air-cooled under nitrogen + methanol atmosphere protection until the gear temperature is below 200℃; During the first phase of rapid cooling under nitrogen + methanol atmosphere protection, the 18CrNiMo7-6 precision forged gear is not oxidized during the cooling process from 820℃ to 200℃, and the surface of the workpiece exiting the furnace at 200℃ will no longer show oxidation and decarburization.

[0072] Phase 2: Transfer the 18CrNiMo7-6 precision forged gears at temperatures below 200℃ to a multi-purpose furnace air-cooling table and air-cool them to room temperature.

[0073] (4) Transfer the 18CrNiMo7-6 precision forged gear cooled to room temperature to a tempering furnace for the first high-temperature tempering. The tempering temperature is 680℃ (slightly lower than the Al eutectoid temperature of 690℃), and the tempering time is 3 times the effective thickness, which is 3*90=270min. Nitrogen gas is introduced for protection during the tempering heating and holding stages to prevent oxidation and decarburization of the workpiece surface. After tempering, water cooling is performed for rapid cooling.

[0074] (5) After the first high-temperature tempering and water cooling, the 18CrNiMo7-6 precision forged gear is transferred to a tempering furnace for a second high-temperature tempering. The tempering temperature is 680℃, and the tempering time is 4 times the effective thickness (4*90) = 360 min. Nitrogen gas is introduced during the tempering heating and holding stages to prevent oxidation and decarburization of the workpiece surface. After tempering, water cooling is performed for rapid cooling. For the process curve of Comparative Example 1, see [link to Comparative Example 1]. Figure 2 .

[0075] After completion, metallographic physical sample testing was performed on the 18CrNiMo7-6 precision forged gear treated in this comparative example. The data are shown in Table 2. The gear treated by the normalizing process in this comparative example meets the requirements of national standards and drawings.

[0076] Table 2 Metallographic test data of precision forged gears

[0077] Inspection Items Require Test results Surface hardness 180-220HB 218HB Oxidative decarbonization <0.2mm 0.05mm Grain size grade ≥6 levels Level 6 Homogeneity of normalized structure ≤3 levels Level 2

[0078] As can be seen from Table 2, although Comparative Example 1 can also meet the requirements of the drawing, due to the higher normalizing temperature, some grains grew. Therefore, the grain size grade and the uniformity grade of the normalized structure are worse than those of Example 1, and the deformation of carburizing and quenching is expected to be greater.

[0079] Comparative Example 2

[0080] 1. Prepare precision-forged gears, with requirements the same as in Example 1, as follows:

[0081] Material: 18CrNiMo7-6

[0082] Gear module: 6

[0083] Effective thickness: 90mm

[0084] Grinding allowance: 0.2mm

[0085] Single piece weight: 25kg

[0086] 2. Normalizing 18CrNiMo7-6 is performed using the following process (only one long-term high-temperature tempering):

[0087] (1) The 18CrNiMo7-6 precision forged gear was heated in a nitrogen + methanol atmosphere in the heating chamber of a multi-purpose furnace at a temperature of 930℃ and a holding time of 2 times the effective thickness = 2*90 = 180min. The heating temperature was selected to be 10℃ higher than the subsequent carburizing temperature. The holding time was based on the effective thickness of the workpiece and was 2 times the effective thickness. Sufficient holding time is the key to complete austenitization of the microstructure.

[0088] (2) The 18CrNiMo7-6 precision forged gear treated in step (1) is cooled in a nitrogen + methanol atmosphere in the heating chamber of a multi-purpose furnace to 820°C. The holding temperature is consistent with the quenching temperature after carburizing. The holding time is 1 hour to ensure that the internal and external temperatures of the 18CrNiMo7-6 precision forged gear blank are uniform.

[0089] (3) Cooling treatment is performed on the 18CrNiMo7-6 precision forged gears processed in step (2), including:

[0090] Phase 1: The 18CrNiMo7-6 precision forged gear is transferred to the front chamber of the heating furnace and air-cooled under the protection of nitrogen + methanol atmosphere until the temperature of the 18CrNiMo7-6 precision forged gear is below 200℃; the gear is protected from oxidation during the rapid cooling process from 820℃ to 200℃ in the nitrogen and methanol atmosphere in the first phase, and the surface of the workpiece exiting the furnace at 200℃ will no longer show oxidation and decarburization.

[0091] Phase 2: Transfer the 18CrNiMo7-6 precision forged gears at temperatures below 200℃ to a multi-purpose furnace air-cooling table and air-cool them to room temperature.

[0092] (4) The 18CrNiMo7-6 precision forged gear, cooled to room temperature, is transferred to a tempering furnace for the first high-temperature tempering at 680℃ (slightly lower than the Al eutectoid temperature of 690℃). The tempering time is 4 times the effective thickness, which is 4*90 = 360 min. Nitrogen gas is introduced during the tempering heating and holding stages to prevent oxidation and decarburization of the workpiece surface. After tempering, water cooling is performed for rapid cooling. The process curve for this comparative example is shown in [reference needed]. Figure 3 .

[0093] After completion, metallographic physical sample testing was performed on the 18CrNiMo7-6 gear. The data are shown in Table 3. The gears treated by the normalizing process in this comparative example meet the requirements of national standards and drawings.

[0094] Table 3 Metallographic test data of precision forged gears

[0095]

[0096]

[0097] As can be seen from Table 3, due to the strong tempering resistance of 18CrNiMo7-6 material in this comparative example, even a single long-term tempering cannot guarantee that bainite will be completely and uniformly transformed into pearlite + ferrite; the grain size grade and the uniformity of the normalized structure are slightly worse than those in Example 1, and the deformation of the carburizing and quenching heat treatment is expected to be slightly larger.

[0098] Example 2: Comparison of Deformation During Carburizing and Quenching

[0099] After the normalized gear blanks of Example 1, Comparative Example 1, and Comparative Example 2 were processed using the same machining steps, they were then subjected to carburizing and quenching heat treatment in the same furnace. The process curves of the carburizing and quenching of the three 18CrNiMo7 precision forged gears are shown below. Figure 4 As shown in Table 4, the deformation of the teeth and internal splines of the three schemes are compared.

[0100] Table 4. Gear tooth and spline deformation detection data

[0101]

[0102] As shown in Table 4, the deformation of the gear in Example 1 after carburizing and quenching was small, and the deformation pattern was consistent with that of the gear, with no abnormal abrupt deformation. The gear in Comparative Example 1 did not deform much after carburizing and quenching, but due to the high normalizing temperature, some grains grew, resulting in abnormal deformation in a few locations and inconsistent deformation patterns. The gear in Comparative Example 2 had a consistent deformation pattern after carburizing and quenching, but due to insufficient normalizing, there were structures that did not fully transform to the equilibrium state. There were also unnecessary structural changes during reheating and carburizing, and the structural stress was higher than that of the gears in the other two normalizing methods, resulting in a larger overall deformation.

Claims

1. A normalizing method for 18CrNiMo7-6 precision forged gears, characterized in that, Includes the following steps: (1) The 18CrNiMo7-6 precision forged gear is heated under nitrogen + methanol atmosphere protection. The temperature of the heating treatment exceeds the carburizing and quenching temperature of the subsequent precision gear by 10-20℃. The heating treatment time is at least 2 × the effective thickness of the 18CrNiMo7-6 precision forged gear min. (2) The 18CrNiMo7-6 precision forged gear after heat treatment in step (1) is subjected to furnace cooling treatment at 820-840℃ for 1-2 hours; (3) The 18CrNiMo7-6 precision forged gear after the furnace cooling treatment in step (2) is air-cooled to below 200°C under nitrogen + methanol atmosphere protection, and then air-cooled to room temperature. (4) The 18CrNiMo7-6 precision forged gear after cooling treatment in step (3) is subjected to a first high-temperature tempering under nitrogen protection and water cooling. The temperature of the first high-temperature tempering is slightly lower than the Al eutectoid temperature, and the holding time is at least 3 × the effective thickness of the 18CrNiMo7-6 precision forged gear min. (5) The 18CrNiMo7-6 precision forged gear after step (4) is subjected to a second high-temperature tempering under nitrogen protection and water cooling. The temperature of the second high-temperature tempering is slightly lower than the Al eutectoid temperature, and the holding time is at least 4 × the effective thickness of the 18CrNiMo7-6 precision forged gear min.

2. The normalizing process of the 18CrNiMo7-6 precision forged gear according to claim 1, characterized in that, The heat treatment temperature is 930-950℃, and the heat treatment time is 2-4 hours.

3. The normalizing process of the 18CrNiMo7-6 precision forged gear according to claim 1, characterized in that, The temperature of the first high-temperature tempering is 680-700℃, and the holding time is 4-6 hours.

4. The normalizing process of the 18CrNiMo7-6 precision forged gear according to claim 1, characterized in that, The second high-temperature tempering is performed at a temperature of 680-700℃, and the holding time is 5-8 hours.

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