A method for improving the mixed crystal structure of 17CrNiMo6 material

Abstract

The present invention relates to a method for improving the mixed crystal structure of a 17CrNiMo6 material. The method comprises the following steps: S1, placing a forging in a heating furnace, heating it to 930-950°C, and then holding the temperature for 4-5 hours; S2, normalizing and air-cooling the forging after holding the temperature to 600-620°C, placing the forging again in the heating furnace and isothermally tempering and holding the temperature at 530-560°C for 4-5 hours; S3, placing the forging after tempering and holding the temperature in a heating furnace, raising the temperature of the heating furnace to 850-860°C and holding the temperature for 2-3 hours; S4, normalizing and air-cooling the forging after holding the temperature to 600-620°C, placing the forging again in the heating furnace and isothermally holding the temperature at 530-560°C for 4-5 hours; and S5, air-cooling the forging to room temperature, machining the forging, and then carburizing and quenching the forging. The method for improving the mixed crystal structure of 17CrNiMo6 material of the present invention can effectively improve the mixed crystal situation of 17CrNiMo6 material and ensure the stability of the 17CrNiMo6 material after it is made into products.

Description

Technical Field

[0001] The present invention relates to the technical field of mixed crystal processing of forgings, in particular to a method for improving the mixed crystal structure of a 17CrNiMo6 material. Background Art

[0002] The original structure is a key factor influencing microstructural inheritance. If the original structure of the steel is a non-equilibrium structure, such as martensite, tempered martensite, bainite, or Widmanstätten structure, which retains significant directional characteristics, microstructural inheritance is more likely to occur. During austenitization heating, the original structure of bainite is more heritable than that of martensite for the same steel. The original structure of Widmanstätten structure is also prone to microstructural inheritance. When the original structure is a mixture of ferrite and pearlite, microstructural inheritance generally does not occur.

[0003] 17CrNiMo6, a high-strength alloy steel, boasts excellent mechanical properties and toughness, making it widely used in industries such as mining, shipbuilding, and aerospace, and is often used to manufacture gear components. However, under typical forging processes, which involve heating and holding before air cooling to room temperature, 17CrNiMo6 forgings exhibit severe mixed grains, which can easily lead to the formation of a microstructure that can cause microstructural heredity. Conventional normalizing cannot effectively eliminate this problem. Furthermore, the product's large modulus and significant grinding allowance require deep carburizing, which results in mixed grains after carburizing and quenching, making the components prone to cracking during test runs. Summary of the Invention

[0004] Based on this, it is necessary to provide a method for improving the mixed crystal structure of 17CrNiMo6 material to address the above problems, which can effectively improve the mixed crystal situation of 17CrNiMo6 material and ensure the stability of 17CrNiMo6 material after it is made into products.

[0005] The technical solution is as follows:

[0006] In one aspect, a method for improving the mixed crystal structure of a 17CrNiMo6 material is provided, comprising the following steps:

[0007] S1. Place the forging in a heating furnace, heat to 930-950℃ and keep warm for 4-5 hours;

[0008] S2. After the insulation is completed, the forging is normalized and air-cooled to 600-620℃. The forging is placed in the heating furnace again and is isothermal tempered at 530-560℃ for 4-5 hours.

[0009] S3. Place the tempered forging into a heating furnace, raise the temperature of the heating furnace to 850-860°C and keep it there for 2-3 hours;

[0010] S4. After the insulation is completed, the forging is normalized and air-cooled to 600-620℃, and then placed in the heating furnace again and kept isothermally at 530-560℃ for 4-5 hours;

[0011] S5. Air-cool the forging to room temperature and then perform machining, followed by carburizing and quenching.

[0012] The technical solution is further described below:

[0013] In one embodiment, in step 1, the temperature of the forging after being heated in the heating furnace is kept at 950°C.

[0014] In one embodiment, in step S2, the forging is normalized and air-cooled to 600°C.

[0015] In one embodiment, the temperature at which the forging is heated in the heating furnace in step S3 is 860°C.

[0016] In one embodiment, in step S4, the forging is normalized and air-cooled to 600°C.

[0017] In one embodiment, in step S4, the temperature at which the forging is placed in the heating furnace again for isothermal holding is 550°C.

[0018] In one embodiment, the holding time in step S1 is 4 hours.

[0019] In one embodiment, the holding time in step S2 is 4 hours.

[0020] In one embodiment, the holding time in step S3 is 2 hours.

[0021] In one embodiment, the holding time in step S4 is 4 hours.

[0022] Beneficial effects of the present invention:

[0023] Compared to the prior art, the present invention provides a method for improving the mixed crystal structure of 17CrNiMo6 material. Forgings are heated in a heating furnace. Based on the characteristics of isothermal normalizing during normalizing at 930-950°C, the temperature is lowered to the pearlite transformation starting temperature between 600-620°C. The forgings are then isothermally treated at 530-560°C to eliminate Widmanstätten structure while preventing the formation of upper bainite. Tempering is then performed for 4-5 hours. After the pearlite transformation is complete, the forgings are reheated to 850-860°C. New austenite nucleates and grows at the pearlite boundaries. Due to the rapid reheating temperature and low holding temperature, the forgings achieve complete austenitization but are insufficiently homogenized, limiting grain growth. After a short holding period, the forgings are removed from the furnace and air-cooled. Once the temperature is lowered to the pearlite transformation starting temperature between 600-620°C, the forgings are then isothermally treated at 530-560°C to further eliminate Widmanstätten structure while preventing the formation of upper bainite. After the pearlite transformation is completed, the forgings are removed from the furnace and cooled to room temperature to obtain a mixed structure of ferrite and pearlite, avoiding the occurrence of tissue inheritance in the forgings. After the forgings are air-cooled to room temperature, subsequent machining and carburizing and quenching operations can be carried out, effectively preventing the occurrence of mixed crystals in the forgings during subsequent operations. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The drawings constituting a part of this application are used to provide a further understanding of the present invention. The illustrative embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. 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 creative work.

[0026] Figure 1 This is a schematic diagram of the steps of a method for improving the mixed crystal structure of 17CrNiMo6 material according to the present invention;

[0027] Figure 2 This is the metallographic diagram of the original structure of 17CrNiMo6 forging;

[0028] Figure 3 The metallographic diagram of 17CrNiMo6 forgings produced by the existing technology process;

[0029] Figure 4 This is a metallographic diagram of the 17CrNiMo6 forging produced by the present invention. DETAILED DESCRIPTION

[0030] To make the above-mentioned objects, features, and advantages of the present invention more readily apparent, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. The following description sets forth numerous specific details to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art may make similar modifications without departing from the scope of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0031] like Figure 1 As shown, in one embodiment, a method for improving the mixed crystal structure of 17CrNiMo6 material is provided, comprising the following steps:

[0032] S1. Place the forging in a heating furnace, heat to 930-950℃ and keep warm for 4-5 hours;

[0033] S2. After the insulation is completed, the forging is normalized and air-cooled to 600-620℃. The forging is placed in the heating furnace again and is isothermal tempered at 530-560℃ for 4-5 hours.

[0034] S3. Place the tempered forging into a heating furnace, raise the temperature of the heating furnace to 850-860°C and keep it there for 2-3 hours;

[0035] S4. After the insulation is completed, the forging is normalized and air-cooled to 600-620℃, and then placed in the heating furnace again and kept isothermally at 530-560℃ for 4-5 hours;

[0036] S5. Air-cool the forging to room temperature and then perform machining, followed by carburizing and quenching.

[0037] In this embodiment, the forgings are heated in a heating furnace. Drawing on the characteristics of isothermal normalizing during normalizing at 930-950°C, the temperature is lowered to the pearlite transformation start temperature between 600-620°C. The forgings are then isothermally treated at 530-560°C to eliminate Widmanstätten structure and prevent the formation of upper bainite. Tempering is then performed for 4-5 hours. After the pearlite transformation is complete, the forgings are reheated to 850-860°C. New austenite nucleates and grows at the pearlite boundaries. Due to the rapid reheating temperature and low holding temperature, the forgings achieve complete austenitization but insufficient homogenization, resulting in limited grain growth. After a short holding period, the forgings are removed from the furnace and air-cooled. Once the temperature is lowered to the pearlite transformation start temperature between 600-620°C, the forgings are then reheated to 530-560°C to further eliminate Widmanstätten structure and prevent the formation of upper bainite. After the pearlite transformation is completed, the forgings are removed from the furnace and cooled to obtain a mixed structure of ferrite and pearlite, avoiding the occurrence of tissue inheritance in the forgings. After the forgings are air-cooled to room temperature, subsequent machining and carburizing and quenching operations can be carried out, effectively preventing the occurrence of mixed crystals in the forgings during subsequent operations.

[0038] In one embodiment, the process parameters actually adopted by the present invention are as follows:

[0039] S1. Place the forging in a heating furnace, heat to 950°C and then keep warm for 4 hours;

[0040] S2. After the insulation is completed, the forging is normalized and air-cooled to 600℃, which is conducive to the transformation of pearlite. The forging is placed in the heating furnace again and is isothermal tempered at 550℃ for 4 hours;

[0041] S3. Place the tempered and heat-insulated forging into a heating furnace, raise the temperature of the heating furnace to 860°C and keep it for 2 hours;

[0042] S4. After the insulation is completed, the forging is normalized and air-cooled to 600℃, which is conducive to the transformation of pearlite. The forging is placed in the heating furnace again and is isothermally maintained at 550℃ for 4 hours;

[0043] S5. Air-cool the forging to room temperature and then perform machining, followed by carburizing and quenching.

[0044] Specifically, if Figures 2 to 4 As shown, Figure 2 This is the metallographic diagram of the original material, which has a mixed crystal structure. Figure 3 This is the metallographic diagram of the material obtained by ordinary normalizing process. The mixed crystal structure has not been eliminated. Figure 4 The metallographic diagram of the material obtained by the above process of the present invention shows no mixed crystal structure. In steel, if coarse grains of 1-4 levels and fine grains of 5 or above exist simultaneously in the metallographic field of view, and the grain size difference in the field of view is more than 3 levels, it can be determined to be mixed crystal. Figure 4 As shown, the size of the relevant grains can be seen intuitively, and it can be concluded that the difference between the maximum grain size and the minimum grain size is less than 3 levels, that is, there is no mixed crystal.

[0045] The above embodiments merely illustrate several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A method for improving the mixed crystal structure of 17CrNiMo6 material, characterized in that: The following steps are involved: S1. Place the forging in a heating furnace, heat to 950°C and keep warm for 4 hours; S2. After the insulation is completed, the forging is normalized and air-cooled to 600°C. The forging is placed in the heating furnace again and is isothermal tempered at 550°C for 4 hours. S3. Place the tempered and heat-insulated forging into a heating furnace, raise the temperature of the heating furnace to 860°C and keep it for 2 hours; S4. After the insulation is completed, the forging is normalized and air-cooled to 600°C. The forging is placed in the heating furnace again and is isothermally maintained at 550°C for 4 hours. S5. Air-cool the forging to room temperature and then perform machining, followed by carburizing and quenching.

Images