Gradient hardness hob cutter ring material and heat treatment process

Abstract

The invention provides a hob cutter ring material with gradient hardness and a heat treatment process. The hob cutter ring material with gradient hardness comprises the following components in percentage by mass: 0.65%-0.76% of C, 6.4%-7.4% of Cr, 2.3%-2.8% of V, 1.7%-1.9% of Mo, 0.9%-1.1% of Si, 0.30%-0.34% of Mn, 0.001%-0.006% of Al, less than 0.01% of Ti, less than 0.01% of S, less than 0.03% of P and the balance of Fe. By adding a small amount of V, the as-cast structure of the steel is improved, meanwhile, grains are refined, non-metallic inclusions are deteriorated, the form and the distribution rule of the non-metallic inclusions are improved, and the steel strength and the steel hardness can be obviously improved; in addition, the grain size is controlled by adding a small amount of Mo, the effects of precipitation strengthening and fine grain strengthening are achieved, and the material strength and hardness are improved; in the heat treatment process, the inner diameter of the hob ring is increased once through high-frequency quenching, so that the hob ring reaches the tempering temperature through high-frequency quenching, when the temperature is slowly reduced, the internal hardness of the hob ring is reduced, the purpose of annealing is achieved, and the internal hardness and the external hardness of the hob ring are in a step shape.

Description

Technical Field

[0001] The invention relates to the technical field of heat treatment of steel materials, and in particular to a gradient hardness hob cutter ring material and a heat treatment process. Background Art

[0002] The shield machine is a machine that breaks rocks underground and keeps digging. At present, it has played a vital role in the construction of large-scale tunnel projects. The shield machine cutter is the key to the smooth excavation of the shield machine. The shield machine cutter ring is subject to great wear and impact during operation. Therefore, the research on the cutter ring material is the basis for improving the efficiency of the shield machine. At present, the materials used for the cutter rings of shield machines in my country are mostly H13 steel. H13 steel is the most widely used hot working die steel, with good resistance to cold and hot fatigue and good wear resistance, and good toughness under medium temperature conditions. However, when ensuring that H13 steel reaches a high hardness of more than 56HRC, its toughness and tensile strength both decrease, thus limiting its applicability in the field of shield machines. Therefore, it is particularly important to develop a new shield machine cutter ring material with good strength and toughness and high hardness.

[0003] Due to the complex working conditions of the cutter ring, the hardness of a good cutter ring material should be no less than 58RC, and the impact work should be no less than 45J. LD steel is a high-strength, toughness, and wear-resistant cold-working die steel independently developed by my country. The biggest feature of this steel is that the steel has a high degree of austenite alloying, and the solid solution strengthening and secondary hardening effects are significant. Its hardness can reach more than 60HRC, and its wear resistance is better than H13 steel. If we can improve the mechanical properties of LD steel on the basis of high wear resistance and high hardness, we can develop and develop materials with good strength and toughness, which can meet the strong impact, large load, easy wear and other use requirements of the shield machine hob, as well as a cutter ring with gradient hardness. It will effectively increase the service life of the shield machine hob and reduce the construction cost of tunnel engineering.

[0004] Therefore, the preparation and processing technology of the hob cutter ring material in the prior art still needs to be further improved. Summary of the invention

[0005] In view of this, the present invention proposes a gradient hardness hob cutter ring material and a heat treatment process, which solves the technical problems in the prior art that the hob cutter ring has high hardness but low impact force and is easy to break.

[0006] The technical solution of the present invention is achieved in this way:

[0007] On the one hand, the present invention provides a gradient hardness hob cutter ring material, comprising the following components in mass percentage: C 0.65%-0.76%, Cr 6.4%-7.4%, V 2.3%-2.8%, Mo 1.7%-1.9%, Si0.9%-1.1%, Mn0.30%-0.34%, Al 0.001%-0.006%, Ti<0.01%, S<0.01%, P<0.03%.

[0008] On the basis of the above technical solution, preferably, the gradient hardness hob cutter ring material includes the following components in mass percentage: C 0.75%, Cr 7.37%, V 2.72%, Mo 1.83%, Si 1.02%, Mn 0.33%, Al 0.003%, Ti 0.007%, S 0.002%, and P 0.026%.

[0009] On the basis of the above technical solution, preferably, the gradient hardness hob cutter ring material includes the following components in mass percentage: C 0.72%, Cr 7.21%, V 2.56%, Mo 1.77%, Si 1.00%, Mn 0.31%, Al 0.004%, Ti 0.005%, S 0.003%, and P 0.024%.

[0010] On the other hand, the present invention also provides a heat treatment process for the gradient hardness hob cutter ring material as described in the first aspect, comprising the following steps:

[0011] S1, wear-resistant steel material is prepared and smelted according to the following mass percentage components: C 0.65%-0.76%, Cr 6.4%-7.4%, V2.3%-2.8%, Mo 1.7%-1.9%, Si 0.9%-1.1%, Mn 0.30%-0.34%, Al0.001%-0.006%, Ti<0.01%, S<0.01%, P<0.03%, the tapping temperature is 1580-1620℃, the pouring temperature is 1490-1520℃, and a rough product of a hob cutter ring is prepared;

[0012] S2, placing the rough product of the hob cutter ring in an annealing furnace, heating it to 800-840°C, keeping it warm for 2-3 hours, then cooling it to 700-720°C, keeping it warm for 4-6 hours;

[0013] S3, placing the rough product of the hob cutter ring in a quenching furnace, quenching, tempering, using high-frequency equipment to perform secondary quenching inside the hob cutter ring, taking it out for air cooling, tempering 2-3 times, and obtaining a gradient hardness hob cutter ring.

[0014] On the basis of the above technical solution, preferably, the quenching temperature in step S3 is 1150-1200°C, and the tempering temperature is 500-550°C.

[0015] On the basis of the above technical solution, preferably, the secondary quenching inside the hob cutter ring using high-frequency equipment in step S3 specifically includes the following steps:

[0016] After the first tempering, high-frequency equipment is used to perform secondary quenching inside the hob cutter ring to increase the inner diameter of the cutter ring so that the cutter ring reaches the tempering temperature, and the temperature is slowly lowered to reduce the internal temperature of the cutter ring, and the hardness inside and outside the cutter ring is stepped.

[0017] On the basis of the above technical solution, preferably, the power of the high-frequency equipment is 45kw and the heating time is 5-7min.

[0018] On the basis of the above technical solution, preferably, the temperature of the secondary quenching is 930-950° C., and the quenching depth is 3-5 cm.

[0019] On the basis of the above technical solution, preferably, the hardness of the inner and outer sides of the knife ring is stepped, specifically, the outer side of the knife ring reaches 60-61HRC, and the core reaches 45-47HRC.

[0020] On the basis of the above technical scheme, preferably, the inner diameter of the knife ring and the frequency of the high-frequency equipment are increased, specifically including: the inner diameter of the knife ring is 324-330mm; the power is 45KW, the frequency is 50KHZ, heated to 930-950℃, and the time is 5-7min to air cool to room temperature.

[0021] Compared with the prior art, the gradient hardness hob cutter ring material and heat treatment process of the present invention have the following advantages:

[0022] Beneficial effects:

[0023] (1) The present invention improves the cast structure of steel by adding a small amount of V element, while refining the grains, modifying non-metallic inclusions, and improving their morphology and distribution, which can significantly improve the strength of steel and improve the hardness of steel, and has broad application prospects; in addition, by adding a small amount of Mo element, the grain size is controlled, which plays a role of precipitation strengthening and fine grain strengthening, thereby improving the strength and hardness of the material;

[0024] (2) Add a high-frequency quenching to the inner diameter of the cutter ring by using a high-frequency device to quench the inner ring of the hob cutter to a temperature of 930-950°C. When the temperature slowly decreases, air cool it to room temperature. This reduces the hardness inside the cutter ring, achieving the purpose of annealing, and then performs a normal tempering process. The original external hardness of the cutter ring is maintained at HRC60-62, and the internal hardness reaches a minimum hardness of 40HRC through secondary quenching and air cooling. The hardness inside and outside the cutter ring is stepped;

[0025] (3) After the heat treatment process of the present invention, the hob cutter ring can achieve the following mechanical properties: hardness> 60HRC, overall impact absorption energy ≥ 45J; the cutter ring presents a gradient state. The outer part reaches 61HRC, and the core reaches 45HRC. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0027] Figure 1 This is a picture of spheroidal carbide particles formed after annealing in Example 4 of the present invention;

[0028] Figure 2 This is the SEM image of Example 4 after quenching of the present invention;

[0029] Figure 3 The hardness test result of the hob cutter ring steel material prepared in the embodiment of the present invention. DETAILED DESCRIPTION

[0030] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] The invention provides a gradient hardness hob cutter ring material, comprising the following components in mass percentage: C 0.65%-0.76%, Cr 6.4%-7.4%, V 2.3%-2.8%, Mo 1.7%-1.9%, Si 0.9%-1.1%, Mn 0.30%-0.34%, Al 0.001%-0.006%, Ti <0.01%, S <0.01%, P <0.03%; through the regulation and optimization of rare earth V and Mo elements, combined with spheroidizing annealing, high temperature quenching and high temperature tempering processes, it has excellent mechanical properties, while maintaining a hardness of more than 60HRC, it exhibits an impact energy of more than 45J; at the same time, a new type of high-strength wear-resistant steel material heat treatment process is provided, which can effectively optimize the mechanical properties of the material, adjust the organizational structure, and improve the distribution and uniformity of carbides. Based on LD steel, a gradient hardness cutter ring is developed.

[0032] Example 1

[0033] A gradient hardness hob cutter ring material, comprising the following components in mass percentage: C 0.75%, Cr 7.37%, V 2.72%, Mo 1.83%, Si 1.02%, Mn 0.33%, Al 0.003%, Ti 0.007%, S 0.002%, and P 0.026%.

[0034] Among them, C is one of the most basic elements in steel, and its proportion is usually between 0.008% and 2.11%. The presence of carbon can improve the hardness and strength of steel, but too much carbon will reduce the plasticity and toughness of steel.

[0035] Cr can improve the corrosion resistance and oxidation resistance of steel. The higher the chromium content, the stronger the corrosion resistance of the steel.

[0036] Mo is an important strengthening element. 1. It can improve the strength and hardness of steel. 2. It can improve the hardenability and temper softening resistance of materials. It can occupy the position of impurity elements such as P and S in the austenite grain boundary segregation, and correspondingly improve the bonding force between grain boundaries, thereby improving the unnotched impact performance of high-strength steel. In addition, Mo can also improve the heat resistance and corrosion resistance of steel.

[0037] Si can improve the high temperature resistance and corrosion resistance of steel.

[0038] Mn is an element that can improve the strength and toughness of steel, and can also improve the heat resistance of steel.

[0039] P usually reduces the toughness and ductility of steel, and can also affect the processing performance of steel. Therefore, the P content needs to be controlled within a certain range.

[0040] S also affects the toughness and ductility of steel, and usually needs to be controlled within a lower content range. When the S content is high, it will also reduce the corrosion resistance of steel.

[0041] The role of V in alloy steel

[0042] 1. Strengthening effect: Vanadium can form hard vanadium carbide (VC) particles, strengthen the grain boundaries and intergranular spaces of steel, and improve the hardness and strength of steel.

[0043] 2. Grain refinement: Vanadium can be used as a grain refiner to reduce grain size and improve the plasticity and toughness of the alloy.

[0044] 3. Improve heat treatment performance: Vanadium can improve the heat treatment performance of steel, such as improving quenching hardening ability, so that the alloy has better mechanical properties and wear resistance.

[0045] At the same time, V plays multiple roles in low-carbon alloy steel, such as strengthening, grain refinement, improving heat treatment performance, and improving corrosion resistance, thereby improving the mechanical properties, wear resistance, and corrosion resistance of the alloy.

[0046] The role of Al element in alloy steel

[0047] 1. Anti-oxidation effect: Al can react with oxygen to form a stable Al oxide layer, forming a protective film to prevent ferroalloys from oxidation and corrosion at high temperatures.

[0048] 2. Al can effectively inhibit the grain growth in ferroalloys, refine the grains, and improve the toughness and plasticity of ferroalloys.

[0049] Al in the alloy can not only improve the strength, hardness and thermal stability of the alloy, but also improve the toughness and plasticity of the alloy.

[0050] Role of Ti in Metal Materials

[0051] 1. Deoxidizer and desulfurizer: Ti is often used as a deoxidizer and desulfurizer in the steelmaking process. It reacts with elements such as N, O, and C in molten steel to form stable compounds, thereby improving the mechanical properties and welding properties of steel.

[0052] 2. Grain refinement: Ti can combine with nitrogen in molten steel to form TN, absorb hydrogen, and act as a degassing agent. At the same time, Ti has a strong affinity with N, O, and C. It is a good deoxidizing and degassing agent and an effective element for fixing nitrogen and carbon. It helps to refine the grains of steel and improve the strength and toughness of steel.

[0053] Improve the hardenability of steel: Ti is one of the strong ferrite forming elements, which reduces the austenite phase area. Solid solution Ti improves the hardenability of steel B, while when TiC particles exist, it reduces the hardenability of steel.

[0054] Example 2

[0055] A gradient hardness hob cutter ring material, comprising the following components in mass percentage: C 0.72%, Cr 7.21%, V 2.56%, Mo 1.77%, Si 1.00%, Mn 0.31%, Al 0.004%, Ti 0.005%, S 0.003%, and P 0.024%.

[0056] Example 3

[0057] A heat treatment process for a gradient hardness hob cutter ring material comprises the following steps:

[0058] S1, the hob cutter ring material is prepared and smelted according to the following mass percentage components: C 0.65%, Cr 6.4%, V 2.3%, Mo1.7%, Si 0.9%, Mn 0.30%, Al 0.001%, Ti 0.007%, S 0.004%, P 0.01%, the tapping temperature is 1580°C, the pouring temperature is 1490°C, and the hob cutter ring crude product is prepared;

[0059] S2, placing the rough product of the hob cutter ring in an annealing furnace, heating it to 800°C, keeping it warm for 3 hours, then cooling it to 700°C, keeping it warm for 6 hours;

[0060] S3, placing the rough product of the hob cutter ring in a quenching furnace, quenching, tempering, using high-frequency equipment to perform secondary quenching inside the hob cutter ring, taking it out for air cooling, tempering 2-3 times, and obtaining a gradient hardness hob cutter ring.

[0061] Specifically, the quenching temperature in step S3 is 1150°C, and the tempering temperature is 500°C.

[0062] Specifically, the secondary quenching inside the hob cutter ring using high-frequency equipment in step S3 specifically includes the following steps:

[0063] After the first tempering, high-frequency equipment is used to perform secondary quenching inside the hob cutter ring to increase the inner diameter of the cutter ring so that the cutter ring reaches the tempering temperature, and the temperature is slowly lowered to reduce the internal temperature of the cutter ring, and the hardness inside and outside the cutter ring is stepped.

[0064] Specifically, the power of the high-frequency device is 45kw and the heating time is 5min.

[0065] Specifically, the temperature of the secondary quenching is 930° C., and the quenching depth is 3 cm.

[0066] Specifically, the hardness of the inner and outer parts of the knife ring is stepped, specifically, the outer part of the knife ring reaches 60HRC, and the core reaches 45HRC.

[0067] Specifically, increase the inner diameter of the knife ring and the frequency of the high-frequency equipment, including: the inner diameter of the knife ring is 324-330mm; the power is 45KW, the frequency is 50KHZ, heated to 930-950℃, and the time is 5-7min to air cool to room temperature.

[0068] Example 4

[0069] A heat treatment process for a gradient hardness hob cutter ring material comprises the following steps:

[0070] S1, the hob cutter ring material is prepared and smelted according to the following mass percentage components: C 0.76%, Cr 7.4%, V 2.8%, Mo1.9%, Si 1.1%, Mn 0.34%, Al 0.006%, Ti 0.004%, S 0.006%, P 0.029%, the tapping temperature is 1620°C, the pouring temperature is 1520°C, and a rough hob cutter ring is prepared;

[0071] S2, placing the rough product of the hob cutter ring in an annealing furnace, heating it to 840°C, keeping it warm for 2 hours, then cooling it to 720°C, keeping it warm for 4 hours;

[0072] S3, placing the rough product of the hob cutter ring in a quenching furnace, quenching, tempering, using high-frequency equipment to perform secondary quenching inside the hob cutter ring, taking it out for air cooling, tempering 2-3 times, and obtaining a gradient hardness hob cutter ring.

[0073] Specifically, the quenching temperature in step S3 is 1200°C, and the tempering temperature is 550°C.

[0074] Specifically, the secondary quenching inside the hob cutter ring using high-frequency equipment in step S3 specifically includes the following steps:

[0075] After the first tempering, high-frequency equipment is used to perform secondary quenching inside the hob cutter ring to increase the inner diameter of the cutter ring so that the cutter ring reaches the tempering temperature, and the temperature is slowly lowered to reduce the internal temperature of the cutter ring, and the hardness inside and outside the cutter ring is stepped.

[0076] Specifically, the power of the high-frequency device is 45kw and the heating time is 7min.

[0077] Specifically, the temperature of the secondary quenching is 950° C. and the quenching depth is 5 cm.

[0078] Specifically, the hardness of the inner and outer sides of the knife ring is stepped, specifically, the outer side of the knife ring reaches 61HRC, and the core reaches 47HRC.

[0079] Figure 1 The spherical carbide particles are formed after the material in Example 4 of the present invention is spheroidized and annealed. As shown in the figure, the annealed carbides are fine and evenly distributed; Figure 2 This is the SEM microstructure of the material after quenching in Example 4 of the present invention. It can be seen that fine carbides are precipitated in the grains, which is very helpful in improving the mechanical properties of the material.

[0080] Example 5

[0081] A heat treatment process for a gradient hardness hob cutter ring material comprises the following steps:

[0082] S1, the hob cutter ring material is prepared and smelted according to the following mass percentage components: C 0.72%, Cr 6.8%, V 2.5%, Mo1.8%, Si 1.0%, Mn 0.33%, Al 0.004%, Ti 0.008%, S 0.004%, P 0.025%, the tapping temperature is 1600°C, the pouring temperature is 1500°C, and the rough hob cutter ring is prepared;

[0083] S2, placing the rough product of the hob cutter ring in an annealing furnace, heating it to 820°C, keeping it warm for 2.5 hours, then cooling it to 710°C, keeping it warm for 5 hours;

[0084] S3, placing the rough product of the hob cutter ring in a quenching furnace, quenching, tempering, using high-frequency equipment to perform secondary quenching inside the hob cutter ring, taking it out for air cooling, tempering 2-3 times, and obtaining a gradient hardness hob cutter ring.

[0085] Specifically, the quenching temperature in step S3 is 1160°C, and the tempering temperature is 530°C.

[0086] Specifically, the secondary quenching inside the hob cutter ring using high-frequency equipment in step S3 specifically includes the following steps:

[0087] After the first tempering, high-frequency equipment is used to perform secondary quenching inside the hob cutter ring to increase the inner diameter of the cutter ring so that the cutter ring reaches the tempering temperature, and the temperature is slowly lowered to reduce the internal temperature of the cutter ring, and the hardness inside and outside the cutter ring is stepped.

[0088] Specifically, the power of the high-frequency device is 45kw and the heating time is 6min.

[0089] Specifically, the temperature of the secondary quenching is 940° C., and the quenching depth is 4 cm.

[0090] Specifically, the hardness of the inner and outer parts of the knife ring is stepped, specifically, the outer part of the knife ring reaches 60HRC, and the core reaches 45HRC.

[0091] Example 6

[0092] The difference from Example 5 is that, in step S1, the hob cutter ring material is melted according to the following mass percentages:

[0093] Table 1 Melting composition elements

[0094]

[0095] The annealing temperature is 840°C in the first stage and 720°C in the second stage. In addition, the quenching temperature in step S3 is 1180°C and the tempering temperature is 540°C.

[0096] The results show that the outer part of the cutter ring reaches 62HRC and the impact energy at 25°C is 55AKV. It can be seen that the new high-strength and wear-resistant hob cutter ring material prepared in this embodiment has a hardness of >60HRC and an impact absorption energy of ≥45J. Its performance is far superior to H13 steel and meets the requirements of shield machines for steel materials. Figure 3 The test results of the hob cutter ring steel material prepared in the embodiment of the present invention are shown in FIG. Figure 3 (a) is the hardness value of the hob cutter ring steel material. Figure 3 (b) is the inside and outside cross-section of the cutter ring of the hob cutter ring steel material. It can be seen that the HRC hardness value after heat treatment makes the inside and outside hardness of the cutter ring soft inside and hard outside after the heat treatment process.

[0097] In summary, the present invention provides a gradient hardness hob cutter ring material and a heat treatment process, which can effectively optimize the mechanical properties of the material, adjust the organizational structure, and improve the distribution and uniformity of carbides. A gradient hardness cutter ring is developed based on LD steel. A new type of steel that not only has high strength and a certain toughness, but also has good wear resistance. The new type of steel contains a suitable ratio of alloy elements, and the added alloy elements are all in high domestic resource abundance.

[0098] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A gradient hardness hob cutter ring material, characterized in that: The composition includes the following components in mass percentage: C 0.65%-0.76%, Cr 6.4%-7.4%, V 2.3%-2.8%, Mo 1.7%-1.9%, Si 0.9%-1.1%, Mn 0.30%-0.34%, Al 0.001%-0.006%, Ti<0.01%, S<0.01%, P<0.03%.

2. The gradient hardness hob cutter ring material according to claim 1, characterized in that: The gradient hardness hob cutter ring material includes the following components in mass percentage: C 0.75%, Cr 7.37%, V 2.72%, Mo 1.83%, Si 1.02%, Mn 0.33%, Al 0.003%, Ti 0.007%, S 0.002%, and P 0.026%.

3. The gradient hardness hob cutter ring material according to claim 1, characterized in that: The gradient hardness hob cutter ring material includes the following components in mass percentage: C 0.72%, Cr 7.21%, V 2.56%, Mo 1.77%, Si 1.00%, Mn 0.31%, Al 0.004%, Ti 0.005%, S 0.003%, and P 0.024%.

4. A heat treatment process for a gradient hardness hob cutter ring material according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1, wear-resistant steel material is prepared and smelted according to the following mass percentage components: C 0.65%-0.76%, Cr 6.4%-7.4%, V2.3%-2.8%, Mo 1.7%-1.9%, Si 0.9%-1.1%, Mn 0.30%-0.34%, Al0.001%-0.006%, Ti<0.01%, S<0.01%, P<0.03%, the tapping temperature is 1580-1620℃, the pouring temperature is 1490-1520℃, and a rough product of a hob cutter ring is prepared; S2, placing the rough product of the hob cutter ring in an annealing furnace, heating it to 800-840°C, keeping it warm for 2-3 hours, then cooling it to 700-720°C, keeping it warm for 4-6 hours; S3, placing the rough product of the hob cutter ring in a quenching furnace, quenching, tempering, using high-frequency equipment to perform secondary quenching inside the hob cutter ring, taking it out for air cooling, and tempering 2-3 times to obtain a gradient hardness hob cutter ring.

5. The heat treatment process according to claim 4, characterized in that: The quenching temperature in step S3 is 1150-1200°C, and the tempering temperature is 500-550°C.

6. The heat treatment process according to claim 4, characterized in that: The step S3 of using high-frequency equipment to perform secondary quenching inside the hob cutter ring specifically includes the following steps: After the first tempering, high-frequency equipment is used to perform secondary quenching inside the hob cutter ring to increase the inner diameter of the cutter ring so that the cutter ring reaches the tempering temperature, and the temperature is slowly lowered to reduce the internal temperature of the cutter ring, and the hardness inside and outside the cutter ring is stepped.

7. The heat treatment process according to claim 6, characterized in that: The power of the high frequency device is 45kw, and the heating time is 5-7min.

8. The heat treatment process according to claim 6, characterized in that: The temperature of the secondary quenching is 930-950° C., and the quenching depth is 3-5 cm.

9. The heat treatment process according to claim 6, characterized in that: The hardness of the inner and outer parts of the knife ring is stepped, specifically, the outer part of the knife ring reaches 60-61HRC, and the core reaches 45-47HRC.

10. The heat treatment process according to claim 6, characterized in that: Increase the inner diameter of the knife ring and the frequency of the high-frequency equipment, specifically including: the inner diameter of the knife ring is 324-330mm; the power is 45KW, the frequency is 50KHZ, heating to 930-950℃, the time is 5-7min, and air cooling to room temperature.

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