A heat treatment method for improving the structure and residual stress of high speed steel punch

Abstract

The application discloses a heat treatment method for improving the structure and residual stress of a high-speed steel punch, and is suitable for M42, M35 and other high-speed steel punches. The method comprises two-stage spheroidizing annealing, strong homogenization austenitizing, sectional quenching, three-stage deep cooling and three-stage gradient tempering. Through the progressive deep cooling of '-90 DEG C -> -150 DEG C -> -196 DEG C', the residual austenite is stably converted into fine martensite, and through 'high-temperature tempering -> medium-temperature tempering -> low-temperature stress stabilization tempering', the direction of the residual stress is controllably adjusted. After the treatment, the carbide size of the punch is significantly refined, the residual austenite is reduced to below 3%, the root stress is changed from the high tensile stress in the traditional process into a controllable low stress state, the hardness reaches 67-70 HRC, and the continuous stamping life is increased by 2-3 times. The application has the advantages of simple process, stability, wide application range and significant industrial application value.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment and microstructure control technology for high-speed steel tool materials. Specifically, it relates to an optimized heat treatment method for ultra-high hardness high-speed steel punches such as M42. In particular, it involves a process route of "multi-stage spheroidization—strong carburizing austenitization—staged cryogenic treatment—gradient tempering composite strengthening" to achieve controllable reversal of residual stress inside the punch, carbide dispersion, and matrix stabilization, thereby significantly improving the punch's anti-chipping performance, fatigue resistance, and service life. This invention is applicable to various high-stress, high-wear metal forming tools such as punches, pins, and precision extrusion dies made of high-speed steels such as M42, M35, W6Mo5Cr4V2, and W18Cr4V, and can serve as an important upgrade technology to existing heat treatment methods. Background Technology

[0002] High-speed steel punches are widely used in cold forming, powder pressing, and high-speed stamping. Their service performance mainly depends on their microstructure and residual stress state. Traditional M42 high-speed steel, due to its high carbon, Co, W, and Mo content, is prone to uneven residual stress formation during extrusion, rough machining, and quenching. The direction and magnitude of this residual stress directly determine the punch's risk of chipping and early fatigue failure. Recent studies have shown that while single measures such as spheroidizing annealing, cryogenic treatment, and multi-stage tempering can improve local microstructure to some extent, they cannot completely solve the stress concentration problem in geometrically abrupt areas such as the punch root. Especially in actual production, common process problems include: uneven phase transformation, carbide segregation, incorrect residual stress direction (e.g., unfavorable compressive or tensile stress superposition at the root), and insufficient cryogenic treatment time. Existing processes generally remain at the level of "spheroidizing + quenching + 2-3 tempering cycles," failing to form a systematic "microstructure-stress synergistic control" system, resulting in limited improvements in punch life. Therefore, there is an urgent need for a systematic heat treatment method that takes into account the homogenization of microstructure, controllability of stress direction, and dispersion of carbides. Summary of the Invention

[0003] This invention proposes a comprehensive process system for high-speed steel punches, consisting of "multi-stage spheroidization pretreatment, high-uniformity austenitization, staged cryogenic transformation, and gradient tempering to stabilize the microstructure". Its core features are: (1) increasing the proportion of fine carbides to more than 60% through two-stage enhanced spheroidization annealing and completely releasing the extrusion stress; (2) using high-temperature short-time high-carburization austenitization to ensure uniform dissolution of alloying elements and avoid carbon-rich areas of residual austenite; (3) immediately performing three-stage cryogenic treatment (-90℃, -150℃, -196℃) after the first quenching to allow the residual austenite to gradually transform without generating severe microstructural stress; (4) constructing a gradient tempering system of "high-temperature first tempering + medium-temperature second tempering + low-temperature stress-stabilizing tempering" to actively transform residual stress into a direction favorable to service; and (5) achieving carbide re-precipitation and dispersion through controlled-temperature rapid cooling. Through the combination of the above technologies, the present invention significantly improves the uniformity of the microstructure of high-speed steel punches, makes the residual stress of the final surface layer and root controllable, and improves the wear resistance, chipping resistance and fatigue life of the punches by 1 to 3 times.

[0004] To achieve the above objectives, the present invention provides the following technical solution:

[0005] This invention provides a heat treatment method for improving the microstructure and residual stress of high-speed steel punches, comprising the following steps:

[0006] (1) Two-stage spheroidizing annealing pretreatment: The combination of "high temperature spheroidizing + low temperature long time spheroidizing" is adopted to make the carbides gradually spheroidize and refine from the original coarse morphology. Specifically, the high-speed steel punch blank is kept at 840-860℃ for 2-4 hours and then cooled to 730-760℃ in the furnace and kept at 6-12 hours to make the coarse carbides spheroidize and refine and eliminate the residual stress of roughing.

[0007] (2) Strong homogenization austenitizing heating: The punch after spheroidizing annealing is heated to 1180-1210℃ and held for 5-12 minutes to introduce a micro carburizing mechanism, so that strong carbide forming elements such as V, Mo, W, and Co are evenly distributed to prevent local solid solution insufficiency.

[0008] (3) Staged quenching and structure-induced phase transformation: First, cool in oil to 250-320°C, then air cool to room temperature to form high-temperature martensite and generate appropriate phase transformation stress;

[0009] (4) Three-stage cryogenic treatment: hold at -90℃ (15-30 minutes), -150℃ (20-40 minutes), and -196℃ (10-25 minutes) in sequence to allow the residual austenite to gradually and smoothly transform into martensite, and avoid stress accumulation caused by one-time cryogenic treatment;

[0010] (5) Gradient tempering (three-stage tempering system): First tempering: 560–580℃, holding for 1.0–1.5 hours; Second tempering: 520–540℃, holding for 1.0–1.5 hours; Third stabilization tempering: 180–220℃, holding for 2–3 hours. Through tempering, the carbides are gradually stabilized and the stress direction is adjusted, ultimately creating a favorable state of slight tensile stress or low stress at the root; and

[0011] (6) Final stress relief cooling and stabilization aging: The punch is kept at 160-200℃ for 3-8 hours to make the structure completely stable.

[0012] Through the above steps, the punch obtains a fine and dispersed carbide distribution, the residual austenite is reduced to below 3%, and a residual stress state that is conducive to resisting chipping is formed.

[0013] In some embodiments, the furnace cooling rate of the spheroidizing annealing step (1) is controlled at 10-25°C / h to avoid abnormal growth of carbides.

[0014] In some embodiments, the strong homogenization austenitization heating in step (2) employs a heating rate of not less than 20°C / min to reduce grain growth and form highly homogenized austenite.

[0015] In some implementations, the time spent at 250-320°C during the quenching and cooling process in step (3) is controlled to be 25-90 seconds to obtain high-temperature martensite and reserve transformable residual austenite.

[0016] In some implementations, the temperature change of the three-stage cryogenic treatment in step (4) adopts a "step cooling" method, and the temperature difference between any two stages is no more than 100°C, so as to avoid sudden stress generated by the cryogenic process.

[0017] In some embodiments, the residual austenite content after the three-stage cryogenic treatment in step (4) is controlled at 1 to 4%, preferably below 2%.

[0018] In some embodiments, the first tempering temperature in step (5) is 20 to 60°C higher than the second tempering temperature, in order to promote the precipitation strengthening of alloying elements and eliminate the internal stress introduced by cryogenic treatment.

[0019] In some implementations, the purpose of the third stabilization tempering in step (5) is to regulate the direction of residual stress so that the root of the punch forms a micro-compressive stress or low stress state, the absolute value of which does not exceed 30 MPa.

[0020] In some embodiments, the size of the punch carbides after the heat treatment method is 60-180 nm, and they are uniformly distributed without forming chain-like or network-like agglomerations.

[0021] In some embodiments, the final hardness of the punch after the heat treatment method is 67-70 HRC, and the hardness fluctuation does not exceed ±0.5 HRC.

[0022] In some embodiments, the residual stress at the root of the punch after the heat treatment method is -20 to +20 MPa, preferably a slight tensile stress state of 0 to +10 MPa.

[0023] In some embodiments, the heat treatment method is applicable to the microstructure and residual stress optimization treatment of high-speed steel punches and their pins, extrusion dies, and precision punching tools such as M42, M35, W6Mo5Cr4V2, and W18Cr4V, which can achieve higher wear resistance, chipping resistance, and fatigue life without changing the chemical composition.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) Significantly reduce the content of retained austenite and stabilize the microstructure. Through three-stage cryogenic treatment, the content of retained austenite is reduced from 8-12% in the traditional process to below 3%, with no abrupt changes in the microstructure. The martensite is refined and uniform, which significantly improves the stability of the punch.

[0026] (2) The direction of residual stress can be controlled, and the root stress can be changed from unfavorable tensile stress to favorable low stress state. Gradient tempering can precisely adjust the stress direction, so that the root residual stress can be reduced from +80 to 150 MPa to -20 to +20 MPa, effectively suppressing chipping, microcrack initiation and fatigue failure;

[0027] (3) Carbide dispersion significantly enhances wear resistance and impact resistance. This invention refines the carbide size to 60-180 nm, ensuring uniform distribution and preventing chain agglomeration, thereby improving the wear resistance of the punch by 30-60% and increasing the continuous stamping life by 1-3 times;

[0028] (4) Stable and consistent performance with significantly reduced batch fluctuations. Due to the adoption of a systematic method of “spheroidization-austenitization-deep cryogenic-gradient tempering”, the hardness fluctuation of batch products is ≤±0.5HRC, the microstructure is uniform, and it is suitable for large-scale industrial applications. Detailed Implementation

[0029] The following is a detailed description of a heat treatment method for improving the microstructure and residual stress of high-speed steel punches according to the present invention.

[0030] The overall process of this invention revolves around "microstructure homogenization, controllable phase transformation, and directional regulation of residual stress." First, a two-stage spheroidizing annealing process is performed on the high-speed steel punch. Initial spheroidization of coarse carbides is achieved in the high-temperature stage, followed by prolonged isothermal treatment in the low-temperature stage, further refining and uniformly dispersing the carbides. Simultaneously, the initial residual stress generated during extrusion and machining is fully released, laying a stable microstructure foundation for subsequent austenitization and quenching treatments.

[0031] Subsequently, a strong homogenization austenitizing heating process is employed, involving rapid solid solution at high temperatures for a short period. This allows strong carbide-forming elements such as V, Mo, W, and Co to fully integrate into the austenite and achieve a uniform distribution. This step significantly reduces alloy segregation and localized insufficient solid solution, preventing the formation of high-carbon residual austenite regions during subsequent quenching and fundamentally improving the final microstructure stability of the punch.

[0032] During the quenching process, this invention employs a "segmented quenching" method. By briefly pausing in the intermediate temperature range of 250–320°C, high-temperature martensite is formed first, and the main phase transformation process is completed without causing drastic changes in structural stress. This measure can effectively reduce the peak internal stress caused by traditional rapid quenching, and improve the geometric stability and impact resistance of the punch after quenching.

[0033] This invention employs a three-stage cryogenic treatment. The cryogenic treatment utilizes a stepped cooling method of -90℃, -150℃, and -196℃, allowing the retained austenite to slowly transform into low-carbon martensite through a "gradual transformation," thereby avoiding abrupt changes in microstructure and stress concentration caused by single-stage cryogenic treatment. After this step, the retained austenite in the punch can be stably reduced to below 3%, resulting in a finer and more uniform matrix structure.

[0034] Finally, by constructing a gradient tempering system of "high-temperature primary tempering – medium-temperature secondary tempering – low-temperature tertiary stabilization tempering," the newly formed martensite during cryogenic treatment is fully tempered, promoting the uniform precipitation of dispersed carbides and enabling controllable adjustment of the direction of residual stress inside the punch. Combined with low-temperature aging treatment, the microstructure is completely stabilized, the hardness difference is controlled within ±0.5 HRC, and the root stress is in a state of slight compressive stress or low stress, significantly improving the punch's resistance to chipping and fatigue life.

[0035] The present invention will be described in detail below through specific embodiments. These embodiments are intended to help understand the present invention and are not intended to limit the scope of the present invention.

[0036] Example 1:

[0037] M42 high-speed steel punch blanks with a diameter of φ12 mm were selected. The chemical composition (mass fraction %) was: C 1.12, W 1.45, Mo 9.1, Co 8.0, V 1.25, Cr 3.8, Si 0.4, balance Fe (details of chemical composition are shown in Table 1 below). First, a two-stage spheroidizing annealing process was performed. The high-temperature stage was held at 850℃ for 3 hours, followed by furnace cooling to 760℃ and holding for 8 hours, transforming the carbides into uniform, fine spherical shapes. Then, rapid heating at 1200℃ for 8 minutes was used for strong solution austenitization, followed by oil quenching to 300℃ and air cooling. A three-stage cryogenic treatment was then performed sequentially at -90℃ for 20 minutes, -150℃ for 30 minutes, and -196℃ for 15 minutes to promote the complete and stable transformation of the retained austenite. The process was then subjected to three tempering processes: holding at 580℃ for 1.2 hours, 540℃ for 1.0 hour, and 200℃ for 2 hours, respectively. Finally, aging was performed at 180℃ for 5 hours, resulting in a hardness of 68.5 HRC, a carbide particle size of approximately 80-150 nm, and a slight compression zone at the root due to residual stress, which is beneficial for preventing chipping. Specific process parameters are shown in Table 2 below. Specific performance test results are shown in Table 3 below.

[0038] Example 2:

[0039] A φ10mm M42 punch blank was selected (chemical composition details are shown in Table 1 below). Spheroidizing annealing was performed at 860℃ for 4 hours followed by 740℃ for 10 hours to refine the carbides. The austenitizing temperature was increased to 1210℃ and held for 6 minutes to prevent grain coarsening. After oil quenching, a deep cryogenic process of -90℃ for 30 minutes, -150℃ for 40 minutes, and -196℃ for 20 minutes was used to reduce the residual austenite content to below 2%. The tempering regime was adjusted to 600℃ for 1.2 hours, 540℃ for 1.0 hour, and 200℃ for 2.5 hours to further stabilize the tempered martensite in the punch matrix. The final result was a hardness of 69.2 HRC, a surface compressive stress of -150 MPa, and a root micro-tensile stress distribution of +20 MPa, significantly improving the impact stability of the punch during actual forming. Specific process parameters are shown in Table 2 below. Specific performance test results are shown in Table 3 below.

[0040] Example 3:

[0041] The punch was made of standard M35 high-speed steel (chemical composition shown in Table 1 below). Spheroidizing annealing was performed at 840℃ for 3 hours followed by 730℃ for 7 hours; austenitization was performed at 1185℃ for 10 minutes; quenching was followed by air cooling at 280℃. Cryogenic treatment was performed at –90℃ for 25 minutes followed by –150℃ for 30 minutes (without liquid nitrogen), followed by tempering at 560℃ for 1 hour, 540℃ for 1.2 hours, and 200℃ for 2 hours. The final hardness reached 67.5 HRC, with a stress reduction of over 40% in the root region of the residual stress distribution. The continuous stamping life of the punch was improved by nearly 80% compared to the factory's current process. Specific process parameters are shown in Table 2 below. Specific performance test results are shown in Table 3 below.

[0042] Comparative Example 1:

[0043] The manufacturer's existing process was adopted: spheroidizing annealing at 850℃ for 3 hours + 740℃ for 6 hours; austenitizing at 1180℃ for 12 minutes; followed by two tempering processes after oil quenching (560℃ for 1 hour + 530℃ for 1 hour). Due to the lack of cryogenic treatment, the retained austenite content was over 10%, resulting in unfavorable tensile stress (60–90 MPa) at the root, which easily led to microcracks during actual stamping. The hardness was 67 HRC, and the fatigue life was significantly insufficient. Specific process parameters are shown in Table 2 below. Specific performance test results are shown in Table 3 below.

[0044] Comparative Example 2:

[0045] The austenitizing process involved austenitization at 1200℃ for 10 minutes, followed by oil quenching and a single-stage deep cryogenic treatment at -196℃ for 20 minutes, followed by tempering at 580℃ for 1.2 hours. Due to the abrupt change in microstructure caused by this single-stage deep cryogenic treatment, severe stress concentration occurred, resulting in unfavorable tensile stress exceeding 150 MPa at the punch root. Although the hardness reached 68 HRC, the toughness decreased, and chipping occurred after 3000 consecutive stamping cycles. Specific process parameters are shown in Table 2 below. Specific performance test results are shown in Table 3 below.

[0046] Comparative Example 3:

[0047] The spheroidizing annealing and austenitizing were the same as in Example 1, but the tempering was only a single tempering at 600℃ for 1.5 hours. Due to the lack of medium-temperature stable tempering and low-temperature stress control steps, the residual stress was difficult to adjust, the stress direction was disordered, and the residual austenite in some areas was insufficiently reversed, ultimately resulting in a hardness of only 66.5 HRC, high root stress, and a punch life reduced by about 50% compared to the example. The specific process parameters are shown in Table 2 below. The specific performance test results are shown in Table 3 below.

[0048] Table 1: Comparison of Chemical Composition of Examples and Comparative Examples (mass percentage, %)

[0049]

[0050] Table 2: Comparison of process parameters between the examples and comparative examples

[0051]

[0052] Table 3: Performance Comparison Table of Examples and Comparative Examples

[0053]

[0054] As can be seen from the examples and comparative examples, the present invention has the following advantages: (1) The residual austenite is significantly reduced and the transformation process is stable and controllable. The examples use three-stage deep cryogenics, and the residual austenite is stably reduced to 2-4%, especially to below 3%. In contrast, the comparative examples, due to the lack of segmented deep cryogenics or the use of only a single deep cryogenics, still have a residual austenite as high as 6-12%, accompanied by abrupt changes in the microstructure and stress concentration. The deep cryogenic path of the present invention can achieve stepwise phase transformation and greatly improve the microstructure stability. (2) The direction of residual stress can be controlled and optimized, and the root stress is greatly improved.

[0055] After gradient tempering, the residual stress at the root of the embodiment was controlled to a favorable low stress range of -20 to +20 MPa, while the comparative example generally had high tensile stress of +80 to 150 MPa, leading to chipping and early fatigue failure. The present invention achieves "stress-oriented control" that cannot be obtained by traditional processes through multi-stage tempering. (3) The carbide has a high degree of dispersion and smaller size, and the wear resistance is significantly improved. In the embodiment, the carbide is refined to 60 to 180 nm and precipitated uniformly, while in the comparative example, the carbide retains a coarse chain or agglomerated morphology, with a size of 180 to 400 nm. The fine carbide obtained by the present invention significantly improves the wear resistance and chipping resistance of the punch surface. (4) The punch life is significantly improved, and the overall performance is significantly better than the existing process.

[0056] The punch life of the embodiment of the present invention can be increased by 1 to 3 times; the comparative example, due to unfavorable residual stress direction, high residual austenite content, or coarse carbides, has a life of only 30% to 50% of that of the embodiment. The present invention achieves a leap in performance without improving the composition or increasing the cost.

[0057] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A heat treatment method for improving the microstructure and residual stress of high-speed steel punches, characterized in that, The heat treatment method includes the following steps: (1) Two-stage spheroidizing annealing: After holding the high-speed steel punch blank at 840-860℃ for 2-4 hours, it is cooled to 730-760℃ in the furnace and then held for 6-12 hours to spheroidize and refine the coarse carbides and eliminate the residual stress of roughing. (2) Heating to homogenize austenitize: Heating the punch after spheroidizing annealing to 1180-1210℃ and holding for 5-12 minutes to allow V, Mo, W and Co alloying elements to be fully dissolved and form uniform austenite; (3) Segmented quenching: Cool in oil to 250-320°C, then air cool to room temperature to obtain high-temperature martensitic structure; (4) Three-stage cryogenic treatment: hold at -90℃ for 15 to 30 minutes, at -150℃ for 20 to 40 minutes, and at -196℃ for 10 to 25 minutes in sequence, so that the residual austenite is gradually and stably transformed into low-carbon martensite; (5) Gradient tempering treatment: including first tempering (560~600℃, holding for 1.0~1.5 hours), second tempering (520~550℃, holding for 1.0~1.5 hours) and third stabilization tempering (180~220℃, holding for 1.5~3 hours). (6) Final stabilization aging: Keep at 160-200℃ for 3-8 hours to further stabilize the residual stress. The heat treatment method described above enables the punch to obtain a fine and dispersed carbide distribution, reduces the residual austenite to below 3%, and forms a residual stress state that is conducive to resisting chipping.

2. The heat treatment method according to claim 1, characterized in that, The furnace cooling rate of the spheroidizing annealing process in step (1) is controlled at 10-25°C / h to avoid abnormal growth of carbides.

3. The heat treatment method according to claim 1, characterized in that, The strong homogenization austenitization heating in step (2) adopts a heating rate of not less than 20℃ / min to reduce grain growth and form highly homogenized austenite.

4. The heat treatment method according to claim 1, characterized in that, In step (3), the time spent at 250-320℃ during the quenching and cooling process is controlled to be 25-90 seconds in order to obtain high-temperature martensite and reserve reusable austenite.

5. The heat treatment method according to claim 1, characterized in that, The temperature changes in the three-stage cryogenic treatment described in step (4) adopt a "step cooling" method, with the temperature difference between any two stages not exceeding 100℃, to avoid sudden stress generated during the cryogenic process; and / or The residual austenite content after the three-stage cryogenic treatment in step (4) is controlled at 1-4%, preferably below 2%.

6. The heat treatment method according to claim 1, characterized in that, In step (5), the first tempering temperature is 20–60°C higher than the second tempering temperature to promote the precipitation strengthening of alloying elements and eliminate the internal stress introduced by cryogenic treatment; and / or The purpose of the third stabilization tempering in step (5) is to control the direction of residual stress so that the root of the punch forms a micro-compressive stress or low stress state, the absolute value of which does not exceed 30 MPa.

7. The heat treatment method according to claim 1, characterized in that, The size of the punch carbides after the heat treatment method is 60-180 nm, and they are uniformly distributed without forming chain or network agglomerations.

8. The heat treatment method according to claim 1, characterized in that, The final hardness of the punch after the heat treatment method is 67-70 HRC, and the hardness fluctuation does not exceed ±0.5 HRC.

9. The heat treatment method according to claim 1, characterized in that, The residual stress at the root of the punch after heat treatment is -20 to +20 MPa, preferably a slight tensile stress state of 0 to +10 MPa.

10. The heat treatment method according to claim 1, characterized in that, The high-speed steel punch is selected from one or more of the following: M42, M35, W6Mo5Cr4V2, W18Cr4V high-speed steel punches and their pins, extrusion dies, and precision punching tools.

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