Rod type steel part and heat treatment method thereof

Abstract

The invention relates to heat treatment of steel, in particular to a rod type steel part and a heat treatment method thereof, and the heat treatment method comprises the following steps: sequentially carrying out stress relief heat treatment, preheating, vacuum quenching, tempering treatment and aging treatment on the rod type steel part; in the step of vacuum quenching, a gradient temperature rising mode is adopted for heating to the quenching temperature; the tempering treatment comprises first-time tempering treatment, subzero treatment, second-time tempering treatment, subzero treatment and third-time tempering treatment, after the second-time tempering treatment, the hardness of the rod type steel part is measured, and the temperature and the heat preservation time of the third-time tempering treatment are determined according to the measured hardness. The deformation problem of the heat treatment of the rod type steel part can be obviously improved.

Description

Technical Field

[0001] The invention relates to heat treatment of steel, in particular to a rod-type steel part and a heat treatment method thereof. Background Art

[0002] Slender steel rods are much longer than their diameter, with an aspect ratio often exceeding 5:1. Their structure resembles a "slender column." This structure has weak bending resistance under load, and when internal stresses are generated during heat treatment, they are prone to bending and deformation due to insufficient rigidity. Furthermore, if the cross-section of a rod-like steel part is asymmetrical, such as non-circular or with keyways or notches, different parts will heat or cool at different rates during heat treatment, resulting in uneven stress distribution and exacerbated deformation. Summary of the Invention

[0003] The object of the present invention is to provide a rod-type steel part and a heat treatment method thereof, which can significantly improve the deformation problem of the rod-type steel part during heat treatment.

[0004] In order to achieve the above object, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention discloses a heat treatment method for a rod-type steel part, comprising: sequentially performing stress relief heat treatment, preheating, vacuum quenching, tempering treatment, and aging treatment on the rod-type steel part; The vacuum quenching is performed by heating to the quenching temperature in a gradient heating manner; The tempering treatment includes a first tempering treatment, a first cryogenic treatment, a second tempering treatment, a second cryogenic treatment and a third tempering treatment. After the second tempering treatment, the hardness of the rod-type steel parts is measured, and the temperature and holding time of the third tempering treatment are determined based on the measured hardness.

[0005] Furthermore, the vacuum quenching by gradient heating specifically includes: Heat the preheated rod steel parts to 650±50℃ under vacuum state and keep warm for 45±5min; then heat to 850±50℃ and keep warm for 45±5min; then heat to 1000±50℃ and keep warm for 30±5min; finally heat to quenching temperature, that is, heat to 1150±50℃ and keep warm for 10±5min; after the insulation is completed, use nitrogen for gas quenching and cool to room temperature.

[0006] Furthermore, the temperature of the first tempering treatment is 540±50°C, the holding time is 90±10min, and the steel is air-cooled to room temperature; The second tempering treatment is carried out within 30 minutes after the first cryogenic treatment, at a temperature of 540±50°C, a holding time of 90±10 minutes, and air cooling to room temperature; The third tempering treatment is carried out within 30 minutes after the second cryogenic treatment, at a temperature of 580±40°C and a holding time of 80±20 minutes; the temperature and holding time of the third tempering treatment are positively correlated with the hardness of the rod-type steel parts measured after the second tempering treatment; The temperature of the first cryogenic treatment and the second cryogenic treatment is -180±10°C, and the holding time is 180±30min.

[0007] Furthermore, the temperature of the stress relief heat treatment is 580±10° C., and the holding time is 120±30 min.

[0008] Furthermore, the preheating temperature is 350±20° C., and the holding time is 55±5 min.

[0009] Furthermore, the temperature of the aging treatment is 280±10°C, and the holding time is 15±1h.

[0010] In a second aspect, the present invention discloses a rod-type steel part, which is manufactured using the above-mentioned heat treatment method for rod-type steel parts.

[0011] The present invention has the following unexpected beneficial effects: 1. The heat treatment method of the present invention adopts a gradient heating method for vacuum quenching rather than direct heating, thereby reducing the temperature difference between the surface and the core of the part and reducing the thermal stress amplitude. The vacuum environment reduces oxidation and decarburization, while improving heating uniformity and avoiding stress concentration caused by local overheating. The first tempering treatment can eliminate the structural stress generated by quenching, stabilize the martensitic structure, and reduce brittleness; cryogenic treatment promotes the transformation of residual austenite to martensite, refines martensitic grains, and reduces micro-stress concentration; the second tempering further eliminates the stress generated by cryogenic treatment and promotes uniform precipitation of carbides; the third tempering dynamically adjusts the temperature and holding time based on the measured hardness value after the second tempering to ensure that the internal stress of the part is fully released. Through multi-step tempering treatment combined with cryogenic treatment, the internal stress of the part is gradually eliminated, the driving force of deformation is effectively suppressed, and the deformation of rod parts during heat treatment is effectively reduced.

[0012] 2. The heat treatment method of the present invention eliminates the residual stress accumulated during the part processing process through stress relief heat treatment before vacuum quenching, avoiding the aggravation of deformation caused by the superposition of original residual stress and heat treatment stress, and laying a low stress foundation for subsequent heat treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the specific implementation of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the implementation or prior art description. Obviously, the drawings described below are only some embodiments of the present invention.

[0014] Figure 1 A schematic flow chart of a heat treatment method for a rod-type steel part according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0015] The following describes the embodiments of the present invention with reference to the accompanying drawings and preferred embodiments. Those skilled in the art will readily appreciate the other advantages and benefits of the present invention from the disclosure herein. The present invention may also be implemented or applied through various other specific embodiments, and the various details in this specification may be modified or altered based on different viewpoints and applications without departing from the spirit of the present invention. It should be understood that the preferred embodiments are intended only to illustrate the present invention and are not intended to limit the scope of protection of the present invention.

[0016] In one embodiment, the present invention discloses a heat treatment method for a rod-type steel part, see Figure 1 As shown, the method includes: performing stress relief heat treatment, preheating, vacuum quenching, tempering treatment and aging treatment on the rod-type steel parts in sequence.

[0017] The vacuum quenching is performed by heating to the quenching temperature in a gradient heating manner.

[0018] The tempering treatment includes a first tempering treatment, a cryogenic treatment, a second tempering treatment and a third tempering treatment. After the second tempering treatment, the hardness of the rod-type steel parts is measured, and the temperature and holding time of the third tempering treatment are determined based on the measured hardness.

[0019] Stress relief heat treatment is performed before formal heat treatment. Through a low-temperature annealing process, the residual stresses generated during the processing of the parts can be effectively eliminated, creating a low-stress initial state for subsequent processing. If these original residual stresses are not eliminated, they will be superimposed on the newly generated thermal stress and structural stress during subsequent heat treatment, which can easily cause part deformation.

[0020] Vacuum quenching utilizes a gradient heating process, meaning the temperature is gradually increased to the quenching temperature in stages, rather than directly increasing the temperature. This method significantly reduces the temperature difference between the part's surface and core, significantly lowering the magnitude of thermal stress. Furthermore, the vacuum environment ensures uniform heating and avoids stress concentration caused by localized overheating.

[0021] The present invention releases structural stress through multi-step tempering and cryogenic treatment. Specifically, the first tempering treatment can eliminate the structural stress generated by quenching, stabilize the martensitic structure, and reduce brittleness; cryogenic treatment promotes the transformation of retained austenite to martensite, refines the martensitic grains, and reduces micro-stress concentration; the second tempering further eliminates the stress generated by cryogenic treatment and promotes the uniform precipitation of carbides; the third tempering dynamically adjusts the temperature and holding time based on the measured hardness value after the second tempering to ensure that the internal stress of the parts is fully released. Through multi-step tempering treatment combined with cryogenic treatment, the internal stress of the parts is gradually eliminated, the driving force of deformation is effectively suppressed, and the deformation of rod parts during heat treatment is effectively reduced.

[0022] Aging treatment, by maintaining the component at a relatively low temperature for a long period of time, promotes the precipitation of supersaturated solid solutions, eliminating micro-stresses and keeping the dimensional change rate of the component to ≤ 0.01% during long-term use. For slender rod-like components, this treatment effectively prevents deformation caused by stress release during use, maintaining dimensional accuracy. The heat treatment method for rod-type steel parts described in the present invention comprises a series of closely linked steps, from controlling the source of stress generation to gradually releasing it during the process, and ultimately eliminating residual stress. Compared with traditional heat treatment processes, the method significantly reduces the deformation of slender rod-type parts, eliminates the need for additional straightening steps, and greatly improves the dimensional accuracy and processing quality of the parts.

[0023] As a preferred embodiment of the present invention, vacuum quenching is performed by gradient heating, specifically including: heating the preheated rod-type steel parts to 650±50°C in a vacuum state, and keeping warm for 45±5 minutes; then heating to 850±50°C, and keeping warm for 45±5 minutes; then heating to 1000±50°C, and keeping warm for 30±5 minutes; finally heating to the quenching temperature, that is, heating to 1150±50°C, and keeping warm for 10±5 minutes; after the insulation is completed, gas quenching is performed with nitrogen, and then cooled to room temperature.

[0024] This preferred embodiment divides the heating process into four stages: 650±50℃, 850±50℃, 1000±50℃, and 1150±50℃. The heating amplitude and holding time are precisely controlled in each stage, which can significantly reduce the temperature difference between the surface and the core of the part. Specifically, holding at a low temperature of 650±50℃ for 45±5min can slowly preheat the internal structure of the part, avoiding rapid expansion of the surface layer and thermal stress; holding at 850℃ for 45±5min further promotes austenite homogenization and reduces phase transformation stress during subsequent high-temperature heating; holding at 1000±50℃ for 30±5min can fully transform the material structure and prepare for the final quenching temperature. The heating rate in each stage is controllable, usually ≤10℃ / min. Compared with direct heating, the temperature difference between the inside and outside of the part can be reduced to within 20℃, significantly reducing the thermal stress amplitude and the deformation driving force, which is conducive to reducing the heat treatment deformation of rod-type steel parts.

[0025] This preferred embodiment performs gas quenching in a vacuum, avoiding the risk of oxidation and decarburization. It also provides a more uniform temperature distribution within the vacuum furnace, preventing stress concentration caused by localized overheating. At a high quenching temperature of 1150±50°C, conventional air heating can easily lead to the formation of oxide scale on the part surface, resulting in uneven heat dissipation. However, a vacuum environment ensures uniform heating of rod-type steel parts in both the axial and radial directions, further reducing the risk of bending deformation caused by uneven heating.

[0026] This preferred embodiment uses nitrogen for gas quenching. Compared with traditional water quenching and oil quenching, the cooling rate is easier to control, and gentle cooling can be achieved by adjusting the nitrogen pressure according to the material and size of the part. The nitrogen cooling rate is between water quenching (rapid cooling) and air cooling (slow cooling), which can not only meet the quenching phase transformation requirements, but also avoid thermal stress caused by excessive cooling speed. In addition, high-pressure nitrogen can achieve 360° surrounding purge, ensuring a consistent cooling rate on the surface of the part and preventing distortion caused by uneven local cooling. For example, a rod-type steel part that has been insulated is gas quenched for 4 minutes using nitrogen at a pressure of 2 bar, and the total cooling time is 15 minutes.

[0027] As a preferred embodiment of the present invention, the first tempering treatment is performed at a temperature of 540±50°C for a holding time of 90±10 minutes, followed by air cooling to room temperature. The tempering temperature of 540±50°C, which falls within the medium-high temperature tempering range, combined with a holding time of 90±10 minutes, effectively eliminates the significant thermal and structural stresses generated during the quenching process, effectively avoiding deformation caused by stress concentration and laying a stable foundation for subsequent processing.

[0028] The first cryogenic treatment is performed at a temperature of -180±10°C and a holding time of 180±30 minutes. This treatment promotes the transformation of retained austenite into martensite, significantly refining the grain size. Furthermore, during this first cryogenic treatment, microstresses generated by differences in thermal expansion coefficients within the material's various structures are released, further reducing overall stress levels and minimizing the risk of macroscopic deformation caused by microstress accumulation.

[0029] The second tempering treatment is carried out within 30 minutes after the first cryogenic treatment, at a temperature of 540±50°C, a holding time of 90±10 minutes, and air cooling to room temperature. The second tempering is carried out immediately within 30 minutes after the cryogenic treatment to promptly eliminate the new stress generated by the cryogenic treatment and promote the uniform dispersion and precipitation of carbides. The second tempering treatment can further stabilize the retained austenite and optimize the comprehensive mechanical properties of the material. For materials such as high-speed steel, the uniform distribution of carbides can significantly improve hardness and wear resistance, and the stress state in the material is more uniform after the second tempering, avoiding deformation caused by local stress imbalance.

[0030] The temperature of the second cryogenic treatment is -180±10°C, and the holding time is 180±30min. The first cryogenic treatment has caused part of the residual austenite to transform into martensite, but a small amount of residual austenite still remains due to thermodynamic stability. The second cryogenic treatment reduces the temperature to -180°C again, so that the lattice distortion of the retained austenite can accumulate to exceed the critical value, thereby improving the transformation rate of the retained austenite. In addition, the cumulative holding time of 360±60min for the first and second cryogenic treatments provides sufficient time for the diffusion of carbon atoms, prompting the retained austenite to transform into fine lath martensite in a more uniform manner, thereby avoiding phase transformation stress concentration.

[0031] The third tempering treatment is carried out within 30 minutes after the second cryogenic treatment, at a temperature of 580±40°C and a time of 80±20 minutes; the temperature and holding time of the third tempering treatment are positively correlated with the hardness of the rod-type steel parts measured after the second tempering treatment. This adaptive adjustment mechanism can flexibly adjust the process parameters according to the actual performance status of the parts. Specifically, if the hardness is too high after the second tempering, it means that the residual stress has not been fully released. By increasing the third tempering temperature and extending the holding time, the carbide aggregation and growth can be further promoted, the hardness can be reduced and the residual stress can be eliminated; on the contrary, if the hardness is too low, the temperature can be appropriately lowered to avoid excessive performance degradation. By dynamically adjusting the tempering parameters, it is ensured that the internal stress of the parts is completely eliminated, and long-term dimensional changes or deformation during use caused by residual stress can be avoided.

[0032] The tempering and cryogenic treatment steps described in this preferred embodiment are closely linked and synergistic, starting with the initial stress release after quenching, followed by microstructural refinement and microstress elimination through cryogenic treatment. The double tempering process further optimizes stress state and performance, ultimately achieving effective control of part deformation. Compared to the traditional single tempering process, this significantly reduces deformation in slender rod-like parts while significantly improving the stability of comprehensive properties such as hardness and toughness. As a preferred embodiment of the present invention, the stress relief heat treatment is performed at a temperature of 580±10°C and a holding time of 120±30 minutes. At this temperature (580±10°C), atomic diffusion is enhanced, and elastic stresses caused by lattice distortion are released through dislocation slip and climb, reducing residual stresses from machining. During the holding process, grain distortion caused by work hardening gradually recovers, and some fine carbides begin to aggregate, providing a more uniform structural base for subsequent heat treatment and avoiding localized stress concentrations caused by initial structural inhomogeneities.

[0033] This preferred embodiment achieves efficient release of initial stress and structural homogenization through precise temperature-time control without changing the material matrix structure, thereby building a low-stress, high-stability process foundation for subsequent heat treatment processes, suppressing the deformation tendency of rod-type parts from the source, and is particularly suitable for the heat treatment of slender rod-type steel parts.

[0034] As a preferred embodiment of the present invention, the preheating temperature is 350±20°C and the holding time is 55±5 minutes. The preheating treatment avoids the superposition of surface compressive stress and core tensile stress caused by rapid heating.

[0035] As a preferred embodiment of the present invention, the temperature of the aging treatment is 280±10° C., and the holding time is 15±1 h.

[0036] The aging temperature of 280±10°C is within the steel's aging-strengthening range, and combined with a holding time of 15±1h, achieves deep stress relief. During the aging process, carbides uniformly precipitate from the martensite matrix, forming a dispersion-strengthened phase. This not only increases the material's hardness, but also strengthens grain boundaries, hinders dislocation movement, and significantly enhances the material's deformation resistance. Furthermore, aging stabilizes the small amount of untransformed retained austenite, preventing volume expansion caused by stress-induced phase transformation during use and eliminating the risk of deformation.

[0037] In one embodiment, the present invention discloses a rod-type steel part, which is manufactured using the above-mentioned heat treatment method for rod-type steel parts.

[0038] For example, the material of the rod-type steel parts is W6Mo5Cr4V2. After being treated by the above-mentioned heat treatment method for rod-type steel parts, it is measured that the hardness of the W6Mo5Cr4V2 slender rod parts is 57-58HRC, the overheated structure is <level 2, the retained austenite content is ≤1.5%, the part runout is ≤0.20mm, and the elongation is within the design range.

[0039] The above embodiments are only preferred embodiments for fully illustrating the present invention, and the protection scope of the present invention is not limited thereto. Any equivalent substitution or modification made by those skilled in the art based on the present invention is within the protection scope of the present invention.

Claims

1. A heat treatment method for rod-type steel parts, characterized in that: include: The rod-type steel parts are subjected to stress relief heat treatment, preheating, vacuum quenching, tempering and aging treatment in sequence; The vacuum quenching is performed by heating to the quenching temperature in a gradient heating manner; The tempering treatment includes a first tempering treatment, a first cryogenic treatment, a second tempering treatment, a second cryogenic treatment and a third tempering treatment. After the second tempering treatment, the hardness of the rod-type steel parts is measured, and the temperature and holding time of the third tempering treatment are determined based on the measured hardness.

2. The heat treatment method for rod-type steel parts according to claim 1, characterized in that: The vacuum quenching process using a gradient heating method specifically includes: Heat the preheated rod steel parts to 650±50℃ under vacuum state and keep warm for 45±5min; then heat to 850±50℃ and keep warm for 45±5min; then heat to 1000±50℃ and keep warm for 30±5min; finally heat to quenching temperature, that is, heat to 1150±50℃ and keep warm for 10±5min; after the insulation is completed, use nitrogen for gas quenching and cool to room temperature.

3. The heat treatment method for rod-type steel parts according to claim 1, characterized in that: The first tempering treatment temperature is 540±50°C, the holding time is 90±10min, and air cooling is performed to room temperature; The second tempering treatment is carried out within 30 minutes after the first cryogenic treatment, at a temperature of 540±50°C, a holding time of 90±10 minutes, and air cooling to room temperature; The third tempering treatment is carried out within 30 minutes after the second cryogenic treatment, at a temperature of 580±40°C and a holding time of 80±20 minutes; the temperature and holding time of the third tempering treatment are positively correlated with the hardness of the rod-type steel parts measured after the second tempering treatment; The temperature of the first cryogenic treatment and the second cryogenic treatment is -180±10°C, and the holding time is 180±30min.

4. The heat treatment method for rod-type steel parts according to claim 1, characterized in that: The temperature of the stress relief heat treatment is 580±10° C., and the holding time is 120±30 min.

5. The heat treatment method for rod-type steel parts according to claim 1, characterized in that: The preheating temperature is 350±20° C., and the holding time is 55±5 min.

6. The heat treatment method for rod-type steel parts according to claim 1, characterized in that: The temperature of the aging treatment is 280±10° C., and the holding time is 15±1 h.

7. A rod-type steel part, characterized in that: The rod-type steel part is manufactured by the heat treatment method according to any one of claims 1 to 6.

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