Ion nitriding process applied to 25Cr3MoA steel

Abstract

This invention discloses an ion nitriding process for 25Cr3MoA steel. Using a bell-shaped ion nitriding furnace, the process involves placing the workpieces to be processed in equal spaces and loading them into the furnace. Using a vacuum pump, the pressure inside the furnace hood is reduced to below 60 Pa. The voltage and current are then gradually increased to heat the workpieces. After the workpieces reach 200°C and are held at this temperature for 2 hours, the temperature is further increased to 260°C ± 10°C, and heated ammonia gas is introduced into the furnace. After the workpieces reach 400°C and are held at this temperature for 2 hours, the temperature is further increased to 540-580°C, at which point the voltage and current are reduced, the temperature is maintained for 60 hours, and then the temperature is lowered, the ammonia gas flow is stopped, and the voltage and current are turned off. The workpieces are removed from the furnace and air-cooled after cooling to below 160°C. This invention focuses on the nitriding of the outer raceway of a 25Cr3MoA clutch. Based on the material's characteristics, scientific process experiments are conducted to determine the specific nitriding process steps (including temperature, time, etc.) to ensure that the nitriding process for 25Cr3MoA steel meets the operating requirements of the clutch outer raceway.

Description

Technical Field

[0001] The invention relates to a material heat treatment technology, in particular to an ion nitriding process applied to 25Cr3MoA steel. Background Art

[0002] 25Cr3MoA is a high-purity alloy structural steel for aviation use. The high Cr content can improve the hardenability of the material, and it has good strength and toughness after quenching and tempering treatment.

[0003] During use, the outer raceway of a clutch (the shifting device between the engine and transmission) is subjected to cyclical friction under low loads, causing localized temperature rise and frictional wear. This, over time, can lead to a degradation of surface finish. To meet the high wear and corrosion resistance of the 25Cr3MoA clutch surface, and to improve its impact resistance and fatigue life, induction hardening is currently commonly used. However, this treatment method presents several challenges: First, to maintain high hardness, the microstructure is primarily tempered martensite. However, when the operating temperature exceeds the low-temperature tempering temperature, microstructural transformations occur, resulting in a decrease in surface hardness. Second, a heat-affected zone (HAZ) exists between the hardened layer and the matrix. Temperatures in this zone exceed the tempering temperature during quenching and tempering, causing the matrix hardness to decrease, making it a potential source of fatigue cracks. If carburizing and quenching are used, a combination of carburizing and induction hardening is required. This approach also suffers from the problem of decreased matrix hardness at high operating temperatures.

[0004] Ion nitriding is a typical low-temperature precision chemical heat treatment, characterized by minimal deformation, high wear resistance, and corrosion resistance. The nitrided layer, composed of a compound layer and a diffusion layer, offers the advantages of high hardness and low brittleness. Since the ion nitriding process does not alter the workpiece's core structure, which remains primarily composed of quenched and tempered troostite, the workpiece's surface is wear-resistant and the core is impact-resistant, significantly extending its service life. Therefore, ion nitriding is the optimal surface strengthening method for 25Cr3MoA clutches.

[0005] Existing ion nitriding technologies mostly use a one-step heat preservation method, and the nitriding temperature is based on the Fe-N phase diagram. However, the influence of alloying elements on the nitriding temperature needs further research, so the heat treatment process of new materials is mostly carried out in a trial-and-error manner. Secondly, ion nitriding has a fast penetration rate for shallow-layer workpieces, but for deep-layer workpieces and materials with high alloying element content, conventional one-step nitriding is difficult to meet the optimal solution for surface hardness and the highest penetration layer. Therefore, it is necessary to design an ion nitriding process for 25Cr3MoA steel so that the clutch outer raceway of 25Cr3MoA material not only meets the high wear resistance and corrosion resistance surface characteristics, but also significantly improves the impact resistance and fatigue life of the clutch. Summary of the Invention

[0006] The object of the present invention is to provide an ion nitriding process applied to 25Cr3MoA steel to meet the actual use requirements of a clutch made of 25Cr3MoA material.

[0007] To achieve the above purpose, the present invention can adopt the following technical solutions:

[0008] The ion nitriding process for 25Cr3MoA steel described in the present invention adopts a bell-type ion nitriding furnace, and its specific steps are as follows:

[0009] In the first step, the workpiece to be processed is placed in the furnace with equal space, and the pressure inside the furnace cover is pumped down to below 60Pa by a vacuum pump. Then the voltage and current are gradually increased to heat the workpiece.

[0010] The second step is to wait for the workpiece to be heated to 200℃ and then keep the temperature even for 2 hours;

[0011] The third step is to continue heating the workpiece to 260℃±10℃ and start to introduce heated ammonia into the furnace, with the ammonia flow rate maintained at 0.2~0.4L / min;

[0012] Step 4: After the workpiece is heated to 400°C, the temperature is averaged for 2 hours, and the heating rate is ≤30°C / h;

[0013] Step 5: Continue heating the workpiece to 540-580°C, then reduce the voltage to 650-660V, the current to 15-20A, and the ammonia flow rate to 1.0-1.2L / min, and keep warm for 60h.

[0014] Step 6: After the insulation is completed, the temperature begins to drop. When the workpiece temperature drops below 300°C, stop the ammonia flow and turn off the voltage and current. After the furnace cools to below 160°C, the workpiece is taken out of the furnace and air-cooled.

[0015] The present invention takes the nitriding of the outer raceway of the 25Cr3MoA clutch as the starting point, combines the characteristics of the material, conducts scientific process experiments, and determines the specific nitriding process steps (including temperature, time, etc.) to ensure that the nitriding process of the 25Cr3MoA steel material meets the working requirements of the outer raceway of the clutch.

[0016] In order to ensure that the surface hardness of the workpiece does not decrease while increasing the penetration rate, the heating process of the workpiece in the fifth step is divided into three sections: first, the workpiece temperature is raised to 540°C and then kept warm for a period of time, then the temperature is continued to be raised to 580°C and then kept warm for a period of time, and finally the temperature is lowered to 540°C and then kept warm for a period of time. The ratio of the three holding times is 1:2.5:1, and the total holding time is 60h.

[0017] Before entering the furnace, the workpieces undergo a pre-oxidation treatment: After cleaning, they are dried in a trolley furnace at a holding temperature of 280°C to 300°C for (effective thickness / 80-100mm) hours, followed by air cooling to room temperature. Tests have shown that the effective nitriding depth of the pre-oxidized workpieces is significantly increased through three-stage nitriding.

[0018] The heated ammonia gas used in the present invention is ammonia gas heated in an ammonia decomposition furnace at 400°C.

[0019] According to conventional heat treatment methods, the workpiece to be processed in the present invention needs to undergo normalizing treatment, quenching treatment, tempering treatment and stabilization treatment before entering the bell-type ion nitriding furnace; the specific parameters of each treatment step are:

[0020] The temperature of normalizing treatment is 890~910℃, the heating rate is 40~60℃ / h, the holding time is (effective thickness / 45~55mm) hours, and then air-cooled to room temperature.

[0021] The quenching temperature is 860~880℃, the heating rate is 40~60℃ / h, the holding time is (effective thickness / 45~55mm) hours, and then the steel is quickly cooled to room temperature using a water-soluble PAG quenching agent with a true concentration of 10%.

[0022] The tempering temperature is 590~600℃, the heating rate is 40~60℃ / h, the holding time is twice the quenching holding time, and then air-cooled to room temperature.

[0023] The temperature of the stabilization treatment is 580~590℃, the heating rate is 40~60℃ / h, the holding time is the same as the tempering treatment time, and then furnace cooling is adopted, the furnace cooling rate is ≤30℃ / h, and after the temperature drops below 200℃, it is taken out of the furnace and air-cooled to room temperature.

[0024] The advantages of the present invention are:

[0025] The present invention takes the nitriding requirement of the outer raceway of the 25Cr3MoA clutch as the starting point, combines the material properties of 25Cr3MoA, conducts repeated process tests, explores the experimental temperature and scheme that meet the process technology requirements, finds the appropriate nitriding temperature through the isothermal gradient scheme, and then combines the process temperature change time. On the premise that the diffusion layer nitride meets the technical indicators, the technical indicators of nitriding rate and surface hardness are taken into account as much as possible, and finds the most appropriate test scheme for the technical indicators, thereby ultimately ensuring the high hardness and wear resistance of the outer raceway of the clutch.

[0026] The process scheme of the present invention is quick to produce on site, uses hot ammonia (NH3) as a penetrant, which is low in price, and only relies on adjusting the temperature to achieve the improvement of the penetration rate. It has the excellent characteristics of low cost, high efficiency and operability.

[0027] Experiments have shown that after the ion nitriding process of the present invention is used to treat the 25Cr3MoA steel workpiece (clutch), the technical indicators of surface hardness ≥900HV5, effective nitriding layer depth >0.30mm, nitride not exceeding level 2, and brittleness level 1 can be met. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is the tempered microstructure diagram of the workpiece to be processed.

[0029] Figure 2 This is the hardness gradient diagram of the nitriding effective layer for Schemes 1-4.

[0030] Figure 3 -- Figure 6 The following are the nitriding microstructure diagrams of Scheme 1 to Scheme 4 respectively.

[0031] Figure 7 This is the hardness gradient diagram of the nitriding effective layer for Scheme 5 and Scheme 6.

[0032] Figure 8 、 Figure 9 They are the nitriding microstructure diagrams of Scheme 5 and Scheme 6 respectively. DETAILED DESCRIPTION

[0033] The present invention will be described in detail below by taking the workpiece processed, namely, a clutch outer ring (made of 25Cr3MoA, with an outer ring diameter of 120 mm and an outer raceway diameter of 32 mm), as an example to facilitate understanding by those skilled in the art.

[0034] 1. First, determine the composition of the clutch material:

[0035] 25Cr3MoA material must comply with the composition requirements of the "GJB6487-2008 Specification for High-Purity Alloy Structural Steel Bars for Aviation Use." The alloying element content of the clutch outer ring of this invention, measured using an ARL3460 direct-reading spectrometer, is as follows: C 0.20-0.28 (0.278); Mn 0.4-0.7 (0.640); Cr 3.0-3.5 (3.385); Ni ≤ 0.3 (0.106); and Mo 0.5-0.7 (0.588). The measured elements are in parentheses and meet the requirements of the specification.

[0036] 2. Furnace selection: Ion nitriding uses LDMC-150 plasma nitriding furnace.

[0037] 3. The entire process includes: normalizing treatment - quenching treatment - tempering treatment - stabilization treatment - clutch ion nitriding pre-cleaning - pre-oxidation treatment - ion nitriding furnace loading - ion nitriding treatment - ion nitriding result detection.

[0038] The phase transition point of 25Cr3MoA material is calculated based on its composition. The Ac3 phase transition point is 827℃, and parameters such as normalizing temperature and quenching temperature are determined.

[0039] 1. Normalizing: The normalizing temperature is 890~910℃, the heating rate is 40~60℃ / h, the holding time is 2h, and then air-cooled to room temperature.

[0040] 2. Quenching: The quenching temperature is 860~880℃, the heating rate is 40~60℃ / h, the holding time is 2h, and then a water-soluble PAG quenching agent with a true concentration of 10% is used to quickly cool to room temperature.

[0041] 3. Tempering: The tempering temperature is 590~600℃, the heating rate is 40~60℃ / h, the holding time is 4h, and then air-cooled to room temperature. After tempering, the hardness is 31.0HRC using a 200HR-150 Huayin Rockwell hardness tester, which is converted to a Brinell hardness of 295HB.

[0042] 4. Stabilization treatment: Use RT3-380KW trolley resistance furnace, with a holding temperature of 580~590℃, a heating rate of 40~60℃ / h, a holding time of 4h, and then furnace cooling with a cooling rate of ≤30℃ / h. After the furnace cools to below 200℃, take it out of the furnace and air cool it to room temperature.

[0043] 5. Cleaning before ion nitriding: Metal detergent is added to the clutch cleaning fluid, and the liquid temperature is 90°C. After cleaning, ensure that there is no impurities such as cutting fluid, anti-rust oil, iron filings, etc. attached to the clutch surface.

[0044] 6. Pre-oxidation treatment: The cleaned workpiece is dried in a trolley furnace with a holding temperature of 300°C for 1 hour, and then air-cooled to room temperature.

[0045] 7. Ion nitriding furnace: Ion nitriding heating relies on ion bombardment for heating. Each workpiece is a heat source, and heating is subject to the surface area / weight ratio. Combined with the characteristics of the bell-type ion nitriding furnace with low temperature at the top, high temperature at the bottom, high temperature in the middle and low temperature at the outer layer, each clutch is supported by cylindrical pads, and gaps are retained between workpieces. The distance from the cathode disk and furnace cover should also be appropriate to ensure temperature uniformity inside the furnace.

[0046] 8. Ion nitriding treatment:

[0047] The first step is to place the workpieces to be processed in the furnace, turn on the two vacuum pumps, and pump the air pressure inside the furnace cover to below 60Pa. Then gradually increase the voltage (to 700-720V), gradually increase the current (to 50A), and gradually increase the duty cycle value, so that the workpieces inside the furnace begin to arc to remove surface debris and gradually heat up. Note: In this step, the voltage and current are gradually increased. Since there are debris on the surface of the workpieces in the furnace, as the arc cleaning process takes longer, the voltage and current can be gradually increased.

[0048] The second step is to wait for the workpiece to be heated to 200℃ and then keep the temperature even for 2 hours;

[0049] The third step is to continue heating the workpiece to about 260°C (260°C ± 10°C), and introduce ammonia gas heated in a 400°C ammonia decomposition furnace into the furnace. The ammonia flow rate is maintained at 0.2~0.4L / min. The introduced ammonia gas is heated in advance so as not to affect the uniformity of the furnace temperature.

[0050] The fourth step is to turn off one vacuum pump and wait for the workpiece to be heated to 400℃ and then average the temperature for 2 hours. The heating rate in the middle shall not exceed 30℃ / h.

[0051] Step 5: Continue to heat the workpiece to 540-580°C (nitriding temperature), then reduce the voltage to 650-660V, adjust the current to 15-20A, maintain the ammonia flow rate at 1.0-1.2L / min, and keep warm for 60h.

[0052] Step 6: After the insulation is completed, the temperature begins to drop. When the workpiece temperature drops below 300°C, stop the ammonia flow and turn off the voltage and current. After the furnace cools to below 160°C, the workpiece is taken out of the furnace and air-cooled.

[0053] For the convenience of comparison, the applicant designed six schemes for the ion nitriding process:

[0054] Solution 1: The nitriding temperature in the fifth step is set to 520°C and kept at this temperature for 60 hours.

[0055] Solution 2: The nitriding temperature in the fifth step is set to 540°C and kept at this temperature for 60 hours.

[0056] Solution 3: The nitriding temperature in the fifth step is set to 560°C and kept at this temperature for 60 hours.

[0057] Solution 4: The nitriding temperature in the fifth step is set to 580°C and kept at this temperature for 60 hours.

[0058] Plan 5: The fifth step involves heating the workpiece using a three-stage heating method. The first stage involves intensive nitriding, which establishes a high nitrogen concentration on the clutch surface over a longer period of time, forming a large nitrogen concentration gradient from the surface to the core. The temperature of the first stage is set at 540°C and the duration is set at 13.5 hours. The subsequent second stage is diffusion, which increases the nitrogen diffusion coefficient by raising the nitriding temperature based on the intensive nitriding, provided that the hardness of the substrate is not affected. The temperature of the second stage is set at 580°C and the duration is set at 33 hours. The third stage is supplementary nitriding, with a set temperature of 540°C and a set duration of 13.5 hours to compensate for the decreased microhardness of the surface layer after diffusion. The total time for the three nitriding stages remains 60 hours, with the time distribution ratio being 1:2.5:1.

[0059] Solution 6: Before ion nitriding, pre-oxidize the workpiece at 300°C for 1 hour. The remaining operations are the same as Solution 5. Specifically, the pre-oxidation process involves drying the cleaned workpiece in a trolley furnace at 300°C for 1 hour, followed by air cooling to room temperature.

[0060] Among the six schemes designed, Scheme 1 to Scheme 4 are isothermal gradient nitriding, and Scheme 5 to Scheme 6 are three-stage temperature gradient nitriding.

[0061] 9. Ion nitriding test:

[0062] The ion nitriding surface hardness test is carried out in accordance with the "GBT34883-2017 Ion Nitriding" standard, and the clutch surface hardness is tested using the Vickers hardness method.

[0063] The inspection of the ion nitriding layer is carried out in accordance with the inspection standard "GBT11354-2005 Determination of Nitriding Layer Depth and Metallographic Structure of Steel Parts", including the inspection of the compound layer and the effective hardness layer depth of the nitriding layer. The compound layer includes the inspection of the loose level, and the metallographic structure includes the inspection of surface brittleness and vein nitrides.

[0064] The effective depth of the nitriding layer shall be determined by hardness test.

[0065] Test results:

[0066] The workpiece to be nitrided is tempered to a hardness of 295HB and the tempered microstructure is as follows: Figure 1 shown.

[0067] The microstructure after quenching and tempering is tempered bainite and a small amount of free ferrite. According to the relevant atlas in "GB / T13320-2007 Metallographic Structure Rating Chart and Evaluation Method for Steel Die Forgings", it is rated as level 2.

[0068] The hardness and brittleness of the workpiece surface were tested using an HV-10B low-load Vickers hardness tester. The depth of the effective nitriding hardened layer of the workpiece was detected using an HXD-1000TMC / LCD microhardness tester. The microstructure of the nitrided layer was observed using a Leica DMI 3000M optical microscope. The relevant results are shown in Table 1.

[0069] Table 1

[0070]

[0071] Figure 2 This is the hardness gradient diagram of the nitriding effective layer for Scheme 1 to Scheme 4.

[0072] Combine Figure 2 As can be seen from Table 1, when using isothermal gradient nitriding, the effective nitriding layer depth gradually increases with increasing temperature, but the rate of increase slows down in the middle. Furthermore, the surface hardness shows a trend of first increasing and then decreasing. This is because increasing temperature provides a greater driving force for nitrogen atomic diffusion. However, excessively high temperature destroys the coherence relationship between the nitride and the matrix, making it impossible to effectively block dislocation movement. As a result, there is no high-density dislocation field near the nitride material point, which causes the surface hardness to decrease.

[0073] Figure 3 -- Figure 6 These are the microstructure diagrams of the 25Cr3MoA clutch after nitriding at 520℃, 540℃, 560℃, and 580℃ (Scheme 1-Scheme 4).

[0074] According to the inspection method of "GBT11354-2005 Determination of Nitriding Layer Depth and Metallographic Examination of Steel Parts", the effective hardened layers of Schemes 1 to 4 were detected to be 0.275mm, 0.324mm, 0.340mm and 0.389mm respectively.

[0075] Conclusion: Considering the surface hardness and the effective nitriding layer depth, the suitable nitriding temperature is 540℃ and 560℃.

[0076] Although the appropriate nitriding temperature based on the combination of surface hardness and effective nitriding layer depth is 540°C and 560°C, the nitriding layer depth at this time is relatively shallow, so it is necessary to increase the nitriding rate while ensuring that the surface hardness does not decrease. To this end, the present application has designed two solutions, namely Solution 5 and Solution 6.

[0077] Figure 7 It is the hardness gradient diagram of the nitriding effective layer of Scheme 5 and Scheme 6. Table 2 is the nitriding results of Scheme 5 and Scheme 6.

[0078] Table 2

[0079]

[0080] Figure 8 This is the microstructure diagram after nitriding according to Scheme 5. Figure 9 This is the microstructure diagram after nitriding according to Scheme 6.

[0081] From Table 2 and Figure 7-Figure 9 It can be seen that after three-stage nitriding of 25Cr3MoA, the effective nitriding depths of Schemes 5 and 6 were 0.526mm and 0.548mm, respectively. In particular, after pre-oxidation, the effective nitriding depth of the three-stage nitriding reached 0.548mm, exceeding the maximum effective nitriding depth of 0.389mm for isothermal nitriding by 0.159mm, a 40.87% increase. Furthermore, the workpiece surface hardness and nitride grade met technical requirements, significantly extending the clutch's service life.

[0082] The existing ion nitriding process is mainly based on the high chromium content of the material itself. The CrN intermetallic compound formed by Cr and N hinders the dislocation movement inside the grains, thereby achieving strength improvement. Secondly, it relies on adjusting the nitriding temperature to obtain the optimal process solution to achieve technical indicators.

[0083] Based on extrapolation from other conventional materials, further improvements in hardness and nitriding rate can be achieved by introducing new alloying elements such as titanium or rare earth elements. While existing methods can be effective in the nitriding of 25Cr3MoA, both titanium and rare earth elements are relatively expensive, significantly increasing production costs. Furthermore, on-site operation is not convenient, as the placement and content of titanium and rare earth elements affect the final nitriding results. Furthermore, ensuring uniformity of the nitrided layer remains a significant challenge. Therefore, the applicant believes that the present invention is the most suitable ion nitriding process for 25Cr3MoA.

Claims

1. An ion nitriding process for 25Cr3MoA steel, using a bell-type ion nitriding furnace, characterized in that: The specific steps of the ion nitriding process are: In the first step, the workpiece to be processed is placed in the furnace, and the pressure inside the furnace cover is pumped down to below 60 Pa using a vacuum pump. Then, the voltage and current are gradually increased to heat the workpiece. The second step is to wait for the workpiece to be heated to 200℃ and then keep the temperature even for 2 hours; The third step is to continue heating the workpiece to 260℃±10℃ and start to introduce heated ammonia into the furnace, with the ammonia flow rate maintained at 0.2~0.4L / min; Step 4: After the workpiece is heated to 400℃, keep the temperature even for 2 hours; Step 5: Continue heating the workpiece to 540-580°C, then reduce the voltage and current, maintain the ammonia flow rate at 1.0-1.2 L / min, and keep the temperature for 60 hours. Step 6: After the insulation is completed, the temperature begins to drop. When the workpiece temperature drops below 300°C, stop the ammonia flow and turn off the voltage and current. After the furnace cools to below 160°C, the workpiece is taken out of the furnace and air-cooled.

2. The ion nitriding process for 25Cr3MoA steel according to claim 1, characterized in that: The heating process of the workpiece in the fifth step is divided into three sections: first, the workpiece temperature is raised to 540°C and then kept warm for a period of time, then the temperature is continued to be raised to 580°C and then kept warm for a period of time, and finally the temperature is lowered to 540°C and then continued to be kept warm for a period of time. The insulation time ratio of the three sections is 1:2.5:1, and the total insulation time is 60h.

3. The ion nitriding process for 25Cr3MoA steel according to claim 1 or 2, characterized in that: Before the workpiece to be processed enters the furnace, it is first pre-oxidized: the cleaned workpiece is dried in a trolley furnace with a holding temperature of 280℃~300℃ and a holding time of (effective thickness / 80~100mm) hours, and then air-cooled to room temperature.

4. The ion nitriding process for 25Cr3MoA steel according to claim 1 or 2, characterized in that: The heated ammonia gas introduced is ammonia gas heated in an ammonia decomposition furnace at 400°C.

5. The ion nitriding process for 25Cr3MoA steel according to claim 1 or 2, characterized in that: The heating rate of the fourth step is ≤30°C / h.

6. The ion nitriding process for 25Cr3MoA steel according to claim 1 or 2, characterized in that: The workpiece to be processed needs to undergo normalizing treatment, quenching treatment, tempering treatment and stabilization treatment in sequence before entering the bell-type ion nitriding furnace.

7. The ion nitriding process for 25Cr3MoA steel according to claim 6, characterized in that: The normalizing treatment temperature is 890-910° C., the heating rate is 40-60° C. / h, the holding time is (effective thickness / 45-55 mm) hours, and then air-cooling to room temperature is performed.

8. The ion nitriding process for 25Cr3MoA steel according to claim 6, characterized in that: The quenching treatment temperature is 860-880°C, the heating rate is 40-60°C / h, the holding time is (effective thickness / 45-55mm) hours, and then the quenching agent with a true concentration of 10% of the water-soluble PAG is used to quickly cool to room temperature.

9. The ion nitriding process for 25Cr3MoA steel according to claim 6, characterized in that: The tempering treatment temperature is 590-600° C., the heating rate is 40-60° C. / h, the holding time is twice the quenching holding time, and then air-cooling to room temperature is performed.

10. The ion nitriding process for 25Cr3MoA steel according to claim 6, characterized in that: The temperature of the stabilization treatment is 580-590°C, the heating rate is 40-60°C / h, the holding time is consistent with the tempering treatment time, and then furnace cooling is adopted, the furnace cooling rate is ≤30°C / h, and after the temperature drops below 200°C, it is taken out of the furnace and air-cooled to room temperature.

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