A carburizing and quenching process for 18CrNiMo7-6 gears

Abstract

The present invention discloses a carburizing and quenching process for 18CrNiMo7-6 gears, which is suitable for gears with a material modulus greater than 15, including: pre-oxidizing the gears: pre-oxidizing the gears in a tempering furnace on a multi-purpose furnace production line, and entering the multi-purpose furnace production line heating furnace with temperature after pre-oxidation; carburizing the gears: the gears enter the heating chamber of the multi-purpose furnace production line heating furnace and are carburized; oil quenching the gears in stages: the gears are transferred from the heating chamber to the quenching chamber for staged oil quenching, and the gears are cooled in the quenching oil tank until the quenching is completed; after the gears are cooled, the oil is drained, cleaned, and tempered twice in the tempering furnace on the multi-purpose furnace production line; and air-cooled to room temperature after being taken out of the furnace. The present invention can effectively control the amount of tooth tip carbides and retained austenite while reducing the heat treatment process by direct carburizing quenching, ensuring that the surface hardness, effective hardened layer, core hardness, tooth tip carbides, and retained austenite levels of the carburized gears meet the requirements of the new process.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment of metal materials, and in particular relates to a carburizing and quenching process for an 18CrNiMo7-6 gear. Background Art

[0002] Currently, the carburizing and quenching process for medium- and large-module gears made of 18CrNiMo7-6 is relatively mature. The process follows: carburizing → slow cooling → high-temperature tempering → reheating and quenching → two low-temperature tempering steps. While reliable, this process is time-consuming and requires multiple steps for carburizing medium- and large-module gears. High-temperature tempering is prone to scale formation on the workpiece, and energy consumption is high. The direct carburizing and quenching process without cryogenic treatment is unsuitable for medium- and large-module 18CrNiMo7-6 gears because 18CrNiMo7-6 has a high alloying element content. Ni, among other elements, is infinitely soluble in austenite, increasing the stability of supercooled austenite. This makes it very likely that a large amount of carbides and retained austenite will form during the quenching process, resulting in unqualified carbides and retained austenite exceeding the specified grade.

[0003] There is an urgent need for a new process that can simplify the process route, reduce production costs, and ensure that the surface hardness, effective hardened layer, core hardness, tooth tip carbide and retained austenite grade of carburized gears meet the requirements. Summary of the Invention

[0004] In order to address the deficiencies of the prior art, the present invention provides a carburizing and quenching process for 18CrNiMo7-6 gears, which realizes direct carburizing and quenching to reduce the heat treatment process while ensuring that the carbide and retained austenite levels of the carburized and quenched gears meet the national standards and drawing requirements.

[0005] In order to solve the deficiencies of the prior art, the present invention provides the following technical solutions:

[0006] This process is suitable for gears with a material modulus greater than 15 and includes the following steps:

[0007] Step 100: Pre-oxidize the gears:

[0008] The gears are pre-oxidized in the tempering furnace on the multi-purpose furnace production line, and then enter the heating furnace of the multi-purpose furnace production line with the temperature after pre-oxidation.

[0009] Step 200: Carburizing the gears:

[0010] The gear enters the heating chamber of the multi-purpose furnace production line heating furnace for carburizing treatment;

[0011] Step 300: Perform oil quenching cooling on the gear in stages:

[0012] The gear is transferred from the heating chamber to the quenching chamber for staged oil cooling quenching. The gear is cooled in the quenching oil tank until the quenching is completed;

[0013] Step 400: After the gears have cooled, they are drained, cleaned, and tempered twice in a tempering furnace on a multi-purpose furnace production line;

[0014] After removing from the oven, cool to room temperature.

[0015] In one embodiment, the oxidation treatment temperature is 400-500° C., and the pre-oxidation time is 1-2 hours.

[0016] In one embodiment, the pre-oxidation time is 1.5 hours.

[0017] In one embodiment, the gear enters the heating chamber of a multi-purpose furnace production line heating furnace and undergoes carburizing treatment, specifically including:

[0018] Step 210: Heating stage: heating the gear in stages to 900-930°C;

[0019] Step 220: Intensive penetration stage: temperature 900-930°C, carbon potential 1.0-1.2%, time 15-20;

[0020] Step 230: Diffusion stage: temperature 900-930°C, carbon potential 0.7-0.8%, time 8-12 hours;

[0021] Step 240: Cooling stage: cooling to 800-850°C, carbon potential of 0.7-0.8%;

[0022] Step 250: Temperature equalization stage: temperature equalization at 800-850° C. for 1-2 hours, with a carbon potential of 0.7-0.8%.

[0023] In one embodiment, heating the gear to 900-930° C. in stages comprises:

[0024] Stage 1: Heat the gear to 750-800℃ without controlling the carbon potential and keep the temperature even for 20-60 minutes;

[0025] Stage 2: Heat the gear to 900-930°C without controlling the carbon potential and keep the temperature even for 20-60 minutes.

[0026] In one embodiment, in the first stage, the gear is heated to 780° C. without controlling the carbon potential and the temperature is maintained for 60 minutes;

[0027] In the second stage, the gear was heated to 920° C. without controlling the carbon potential and the temperature was maintained for 60 minutes.

[0028] In one embodiment, in step S220, the temperature of the strong penetration stage is 920°C, the carbon potential is 1.1%, and the time is 17 hours; in step S230, the temperature of the diffusion stage is 920°C, the carbon potential is 0.8%, and the time is 10 hours.

[0029] In one embodiment, in step S240, the gear is cooled to 810°C, and the carbon potential during the cooling process is 0.7%;

[0030] In step S250, the gear is heated at 810°C for 2 hours, and the carbon potential is 0.7.

[0031] In one embodiment, the gear is transferred from the heating chamber to the quenching chamber for staged oil cooling quenching, and the gear is cooled in the quenching oil tank until the quenching is completed, comprising:

[0032] The gear is transferred to a quenching oil tank; the stirring speed of the quenching oil tank is 1200-1400 rpm, the oil temperature in the quenching oil tank is 40°C-80°C, and the transfer time is 35-45s; the gear is cooled in the quenching oil tank until the quenching is completed.

[0033] In one embodiment, after the gear is cooled, it is drained and cleaned, and tempered twice in a tempering furnace on a multi-purpose furnace production line, and then air-cooled to room temperature after being taken out of the furnace, including:

[0034] After the gears have cooled, drain the oil, clean them, and temper them at low temperature for the first time, then air cool them to room temperature.

[0035] Second low temperature tempering, air cooling to room temperature;

[0036] The temperature of low-temperature tempering is 180-240°C, and the single tempering time is ≈ (intensive penetration time + diffusion time) / 2.5.

[0037] Beneficial effects of the present invention:

[0038] The present invention provides a carburizing and quenching process for 18CrNiMo7-6 medium and large modulus gears, which reduces the internal and external thermal stress difference and tissue stress difference of the gear through pre-oxidation and step-by-step heating; achieves the required carburizing layer depth of the gear through strong infiltration and diffusion to ensure that a sufficient hardened layer depth is obtained after quenching; through the long-term control of the low carbon potential in the cooling and temperature equalization process after diffusion, the carbon concentration of the carburizing layer is reduced to a level close to the carbon concentration balance of the material, and in the subsequent quenching process, it can be ensured that no excessive particulate carbides are precipitated; rapid oil and low oil temperature and high-speed stirring in the quenching cooling process accelerate the cooling and supercooling of the carburizing layer tissue and reduce the content of retained austenite; finally, after two long-term low-temperature tempering and tempering, air cooling stabilizes the carburizing layer tissue, eliminates residual stress and tissue stress, and completes the entire process; after actual cutting inspection, the gear surface hardness is 59-60HRC, the effective hardened layer is 2.5-2.8mm, the core hardness is 36-38HRC, the carbide grade is level 1, and the martensite and retained austenite grade are level 2; all of which meet the national standards and drawing requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The accompanying drawings, as part of this disclosure, are intended to provide a further understanding of the disclosure. The exemplary embodiments of the disclosure and their descriptions are intended to explain the disclosure and do not constitute undue limitations thereon. Obviously, the drawings described below are merely examples, and those skilled in the art can derive other drawings based on these drawings without inventive effort.

[0040] Figure 1 A carburizing and quenching process diagram of an 18CrNiMo7-6 gear provided in one embodiment of the present invention;

[0041] Figure 2 A process diagram of the gear carburizing stage provided by an embodiment of the present invention;

[0042] Figure 3 The process diagram of the gear carburizing stage provided for comparative example.

[0043] It should be noted that these drawings and textual descriptions are not intended to limit the conceptual scope of the present invention in any way, but rather to illustrate the concept of the present invention for those skilled in the art by referring to specific embodiments. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The following embodiments are used to illustrate the present invention but are not used to limit the scope of the present invention.

[0045] The present invention will be further described below with reference to the accompanying drawings.

[0046] Reference Figure 1 As shown, the embodiment of the present invention provides a carburizing and quenching process for an 18CrNiMo7-6 gear, comprising the following steps:

[0047] Step 100: Pre-oxidize the gears:

[0048] The gears are pre-oxidized in the tempering furnace on the multi-purpose furnace production line, and after the pre-oxidation is completed, they are brought into the heating furnace of the multi-purpose furnace production line with temperature.

[0049] In the embodiment of the present application, the gear is pre-oxidized in a tempering furnace at a temperature of 400-500° C. for 1-2 hours, preferably for 1.5 hours.

[0050] It is important to emphasize that carburizing is performed directly after pre-oxidation without cooling. The purpose of pre-oxidation is to activate the gear surface and prepare the surface for carburizing and quenching. It also serves as a preheating stage for the next carburizing stage.

[0051] Step 200: Carburizing the gears:

[0052] The gear enters the heating chamber of the multi-purpose furnace production line heating furnace for carburizing treatment.

[0053] In the embodiment of the present application, the gear enters the heating chamber of the multi-purpose furnace production line heating furnace and undergoes carburizing treatment, which specifically includes:

[0054] Step 210: Heating stage: heating the gear in stages to 900-930°C;

[0055] Step 220: Intensive penetration stage: temperature 900-930°C, carbon potential 1.0-1.2%, time 15-20;

[0056] Step 230: Diffusion stage: temperature 900-930°C, carbon potential 0.7-0.8%, time 8-12 hours;

[0057] Step 240: Cooling stage: cooling to 800-850°C, carbon potential of 0.7-0.8%;

[0058] Step 250: Temperature equalization stage: temperature equalization at 800-850° C. for 1-2 hours, with a carbon potential of 0.7-0.8%.

[0059] Furthermore, in step 210, the gear is heated in stages to a carburizing temperature of 900-930°C, including:

[0060] Stage 1: Raise the temperature to 750-800℃, do not control the carbon potential, and keep the temperature at the same temperature for 20-60 minutes. In stage 1, the temperature is preferably raised to 780℃, do not control the carbon potential, and keep the temperature at the same temperature for 60 minutes.

[0061] Stage 2: Raise the temperature to 900-930°C, do not control the carbon potential, and hold the temperature for 20-60 minutes. In stage 2, preferably, raise the temperature to 920°C, do not control the carbon potential, and hold the temperature for 60 minutes.

[0062] The use of two-stage step heating can reduce the thermal stress generated during the heating process and reduce the gear deformation.

[0063] Furthermore, in the strong infiltration stage, the preferred temperature is 920°C, the carbon potential is 1.1%, and the time is 17 hours. In the strong infiltration stage, the infiltration layer reaches 90% of the infiltration layer required by the drawing.

[0064] Furthermore, the preferred temperature in the diffusion stage is 920°C, the carbon potential is 0.8%, and the time is 10 hours. The purpose of the diffusion stage is to adjust the carbon concentration of the carburized layer to obtain a reasonable carbon concentration gradient.

[0065] Furthermore, during the cooling stage, the temperature is preferably lowered to 810°C, with a carbon potential of 0.7%. During the cooling process, the carbon potential is controlled to evenly and slowly reduce the carbon concentration on the gear surface, slowing down the carbon concentration gradient in the carburized layer.

[0066] Furthermore, the temperature is preferably maintained at 810°C for 2 hours during the temperature equalization stage, with a carbon potential of 0.7%. The quenching temperature is maintained for two hours to effectively reduce the carbon concentration on the workpiece surface while ensuring that the temperature and carbon potential are consistent at all locations throughout the gear.

[0067] It should be noted that during the carburizing treatment, the carburizing equipment can be a MEs150 / 90 / 85 controlled atmosphere sealed box-type multi-purpose furnace, and the carbon potential is controlled by adjusting the flow rate of the carbon source atmosphere. The present invention has no particular limitation on the carbon source atmosphere, and propane, methanol, etc., which are well known to those skilled in the art, can be used as long as the carbon potential requirements of the present invention are met. The use of a carbon source atmosphere is also common knowledge known to those skilled in the art.

[0068] Step 300: Perform oil quenching cooling on the gear in stages:

[0069] The gear is transferred from the heating chamber to the quenching chamber for staged oil quenching, and the gear is cooled in the quenching oil tank until the quenching is completed.

[0070] In an embodiment of the present application, the stirring speed of the quenching oil tank is 1200-1400 rpm, the oil temperature in the quenching oil tank is 40°C-80°C, the transfer time of the gear quenching from the heating chamber to the oil tank is 35-45s, and the gear is cooled in the quenching oil tank until the quenching is completed.

[0071] Step 400: After the gears have cooled, they are drained, cleaned, and tempered twice in a tempering furnace on a multi-purpose furnace production line;

[0072] After removing from the oven, cool to room temperature.

[0073] In the embodiment of the present application, after the gear is cooled, it is drained and cleaned, and tempered twice in a tempering furnace on a multi-purpose furnace production line, and then air-cooled to room temperature after being taken out of the furnace, specifically including:

[0074] After the gears have cooled, drain the oil, clean them, and temper them at low temperature for the first time, then air cool them to room temperature.

[0075] Second low temperature tempering, air cooling to room temperature;

[0076] The temperature of low-temperature tempering is 180-240℃; the single tempering time is ≈ (intensive penetration time + diffusion time) / 2.5.

[0077] It should be emphasized that after both tempering steps, the steel was cooled to room temperature by air. After low-temperature tempering, the retained austenite can be further transformed into tempered martensite through long-term low-temperature tempering, and the carbides can be effectively decomposed and reduced.

[0078] Example 1

[0079] Reference Figure 2 As shown, 1) prepare the gear blank, the requirements are as follows:

[0080] Material: 18CrNiMo7-6

[0081] Surface hardness: 156-217HB

[0082] Grain size grade: ≥6

[0083] Banded tissue level: ≤2

[0084] Gear module: 18

[0085] Unit weight: 100kg

[0086] 2) Use the tempering furnace on the multi-purpose furnace production line to carry out pre-oxidation treatment at 400℃±5℃ for 90min, and after the pre-oxidation is completed, enter the heating furnace of the multi-purpose furnace production line with the temperature;

[0087] 3) After the gear enters the heating chamber of the multi-purpose furnace production line, press Figure 2 Carburizing treatment is carried out by the following process:

[0088] 31) Heat for 1 hour to 780℃±5℃, without controlling carbon potential, and hold the temperature for 60 minutes;

[0089] 32) Heat for 1 hour to 920℃±5℃, without controlling carbon potential, and hold the temperature for 60 minutes;

[0090] 33) Keep at 920℃±5℃, control carbon potential at 1.1%±0.05%, and perform strong penetration for 17 hours;

[0091] 34) Diffusion at 920°C ± 5°C with carbon potential controlled at 0.8% ± 0.05% for 10 hours;

[0092] 35) Cool down for 2 hours, then cool to 810°C ± 5°C, perform quenching and hold, and keep the temperature at the same temperature for 2 hours. The carbon potential is controlled at 0.7% ± 0.05% during cooling and keeping the temperature at the same temperature.

[0093] 4) The gear is transferred from the heating chamber to the quenching chamber for oil quenching (oil temperature: 50°C), and the quenching transfer time is 40s:

[0094] Quenching stage: quenching oil tank stirring is turned on; quenching oil stirring speed is 1400rpm, cooling for 40min;

[0095] 5) After the cleaning is completed, the steel is tempered twice in the tempering furnace on the multi-purpose furnace production line at 220℃±5℃ for 10 hours at low temperature, and then air-cooled to room temperature after being taken out of the furnace.

[0096] After the completion, the gears were subjected to metallographic inspection, and the data are shown in Table 1. The gears treated by the carburizing process provided by the present invention meet the requirements of the national standard.

[0097] Table 1 Metallographic inspection data of gears

[0098]

[0099]

[0100] Comparative Example 1

[0101] Reference Figure 3 As shown, 1) prepare the gear blank, the requirements are as follows:

[0102] Material: 18CrNiMo7-6

[0103] Surface hardness: 156-217HB

[0104] Grain size grade: ≥6

[0105] Banded tissue level: ≤2

[0106] Gear module: 18

[0107] Unit weight: 100kg

[0108] 2) Use the tempering furnace on the multi-purpose furnace production line to carry out pre-oxidation treatment at 400℃±5℃ for 90min, and after the pre-oxidation is completed, enter the heating furnace of the multi-purpose furnace production line with the temperature;

[0109] 3) After the gear enters the heating chamber of the multi-purpose furnace production line, press Figure 1 Carburizing treatment is carried out by the following process:

[0110] 31) Heat for 1 hour to 780℃±5℃, without controlling carbon potential, and hold the temperature for 60 minutes;

[0111] 32) Heat for 1 hour to 920℃±5℃, without controlling carbon potential, and hold the temperature for 60 minutes;

[0112] 33) Keep at 920℃±5℃, control carbon potential at 1.1%±0.05%, and perform strong penetration for 17 hours;

[0113] 34) Diffusion at 920°C ± 5°C with carbon potential controlled at 0.8% ± 0.05% for 10 hours;

[0114] 35) Cool down for 2 hours to 840°C ± 5°C, keep warm, and keep the temperature even for 1 hour. The carbon potential is controlled at 0.8% ± 0.05% during cooling and temperature evenness.

[0115] 4) The gears are put into the multi-purpose furnace antechamber and cooled under air for 2 hours before being taken out of the furnace;

[0116] 5) The gear enters the tempering furnace for high-temperature tempering at 680±10℃ for 8 hours, and then is air-cooled to room temperature after being taken out of the furnace;

[0117] 6) The gear enters the multi-purpose furnace and is heated to 820°C, with the carbon potential controlled at 0.8% ± 0.05%, and kept at this temperature for 2 hours;

[0118] 7) The gear is transferred from the heating chamber to the quenching chamber for oil quenching (oil temperature: 50°C), and the quenching transfer time is 40s:

[0119] Quenching stage: quenching oil tank stirring is turned on; quenching oil stirring speed is 1400rpm, cooling for 40min;

[0120] 8) After the cleaning is completed, the steel is tempered twice in the tempering furnace on the multi-purpose furnace production line at 180℃±5℃ for 10 hours at low temperature, and then air-cooled to room temperature after being taken out of the furnace.

[0121] After completion, the gears were subjected to metallographic testing, and the data are shown in Table 2. The gears treated with the carburizing process provided in this comparative example meet the requirements of national standards and drawings.

[0122] Table 2 Gear metallographic test data

[0123]

[0124] See Table 3 for a comparison of process flow and duration.

[0125] Table 3 Process flow and duration comparison

[0126] Craftsmanship Pre-oxidation carburization High temperature tempering Cleaning Reheating quenching Tempering 1 Tempering 2 total Example 3h 36h none 1h none 13h 13h 66h Comparative Example 3h 36.5h 12h 1h 6h 13h 13h 84.5h

[0127] As can be seen from Table 3, in the embodiment, while ensuring that the microstructure and properties meet the requirements, the process time is shortened by 18.5 hours, one high-temperature tempering and one reheating quenching are reduced, the carburizing and quenching efficiency of the 18CrNiMo7-6 gear is improved, and the heat treatment energy consumption is greatly reduced.

[0128] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

[0129] Furthermore, those skilled in the art will appreciate that, although some embodiments described herein include certain features included in other embodiments but not other features, combinations of features from different embodiments are also intended to fall within the scope of protection of the present invention and form different embodiments. For example, in the above embodiments, those skilled in the art will be able to use them in combination based on the known technical solutions and the technical problems to be solved by this application.

[0130] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as above in terms of a preferred embodiment, it is not intended to limit the present invention. Any technician familiar with this patent can make slight changes or modifications to equivalent embodiments using the above-mentioned technical contents without departing from the scope of the technical solution of the present invention. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the solution of the present invention.

Claims

1. A carburizing and quenching process for 18CrNiMo7-6 gears, characterized in that: This process is suitable for gears with a material modulus greater than 15 and includes the following steps: Step 100: Pre-oxidize the gears: The gears are pre-oxidized in the tempering furnace on the multi-purpose furnace production line, and then enter the heating furnace of the multi-purpose furnace production line with the temperature after pre-oxidation. Step 200: Carburizing the gears: The gear enters the heating chamber of the multi-purpose furnace production line heating furnace for carburizing treatment; Step 300: Perform oil quenching cooling on the gear in stages: The gear is transferred from the heating chamber to the quenching chamber for staged oil cooling quenching. The gear is cooled in the quenching oil tank until the quenching is completed; Step 400: After the gears have cooled, they are drained of oil, cleaned, and tempered twice in a tempering furnace on a multi-purpose furnace production line; after being taken out of the furnace, they are air-cooled to room temperature; The gear enters the heating chamber of the multi-purpose furnace production line heating furnace and undergoes carburizing treatment, specifically including: Step 210: Heating stage: heating the gear to 920° C. in stages; Step 220: Intensive penetration stage: temperature 920°C, carbon potential 1.1%, time 17 h; Step 230: Diffusion stage: temperature 920°C, carbon potential 0.8%, time 10 h; Step 240: Cool the gear to 810°C, and the carbon potential during the cooling process is 0.7%; Step 250: The gear is heated at 810°C for 2 hours, with a carbon potential of 0.7%; After the gears are cooled, they are drained, cleaned, and tempered twice in a tempering furnace on a multi-purpose furnace production line. After being taken out of the furnace, they are cooled to room temperature by air, including: After the gears have cooled, drain the oil, clean them, and temper them at low temperature for the first time, then air cool them to room temperature. Second low temperature tempering, air cooling to room temperature; The low-temperature tempering temperature is 220℃ and the single tempering time is 10 h.

2. The carburizing and quenching process for an 18CrNiMo7-6 gear according to claim 1, characterized in that: The temperature of the pre-oxidation treatment is 400° C., and the pre-oxidation time is 1.5 h.

3. The carburizing and quenching process for an 18CrNiMo7-6 gear according to claim 1, characterized in that: The step of heating the gear to 920°C in stages includes: Stage 1: Heat the gear to 780°C without controlling the carbon potential and hold the temperature for 60 minutes; Stage 2: Heat the gear to 920°C without controlling the carbon potential and keep the temperature at that temperature for 60 minutes.

4. The carburizing and quenching process for an 18CrNiMo7-6 gear according to claim 1, characterized in that: The gear is transferred from the heating chamber to the quenching chamber for staged oil cooling quenching, and the gear is cooled in the quenching oil tank until the quenching is completed, including: The gear was transferred to a quenching oil tank; the stirring speed of the quenching oil tank was 1400 rpm, the oil temperature in the quenching oil tank was 50°C, and the transfer time was 40 s; the gear was cooled in the quenching oil tank until the quenching was completed.

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