CN111411203B - Method for obtaining 8Cr4Mo4V steel and optimizing quenching process - Google Patents

Method for obtaining 8Cr4Mo4V steel and optimizing quenching process Download PDF

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CN111411203B
CN111411203B CN202010458322.6A CN202010458322A CN111411203B CN 111411203 B CN111411203 B CN 111411203B CN 202010458322 A CN202010458322 A CN 202010458322A CN 111411203 B CN111411203 B CN 111411203B
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CN111411203A (en
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尚丽娟
于兴福
杨宇晴
柳泽禹
周驰滨
鲍识同
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Shenyang University of Technology
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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    • C21D2211/008—Martensite

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Abstract

The invention relates to a method for obtaining 8Cr4Mo4V steel by an optimized quenching process, which comprises the following steps: (1) determining a heating solid solution process: (2) determining the highest hardness time point of isothermal quenching: (3) carrying out composite quenching treatment on the basis of the step (2): (4) selecting optimized composite quenching process samples: (5) and (4) tempering the optimized composite quenching process sample obtained in the step (4) for 3 to 4 times at 510-550 ℃ for 1.8-2.5h to obtain the final optimized heat-treated 8Cr4Mo4V steel. The method has the advantages of scientificity, feasibility, simple operation and remarkable effect, and improves the space of the performance of the 8Cr4Mo4V steel.

Description

Method for obtaining 8Cr4Mo4V steel and optimizing quenching process
The technical field is as follows:
the invention belongs to the field of special steel heat treatment research, and relates to a quenching technology for effectively improving an 8Cr4Mo4V steel structure and a method for obtaining an optimized quenching process of 8Cr4Mo4V steel.
Background art:
8Cr4Mo4V steel is an important steel for high-temperature bearings, and is widely used for manufacturing high-temperature bearing components such as aircraft engines and gas turbines. In recent years, some researchers have conducted extensive studies on the heat treatment technology of 8Cr4Mo4V steel and have searched for a series of relevant heat treatment protocols. With the continuous development of industrial technology and production level, the performance requirement on 8Cr4Mo4V steel is higher and higher, and the original heat treatment system cannot meet the requirement of the development of times. It is urgent to further refine and quantify the steel structure and to explore the innovation of the heat treatment system corresponding to the refinement.
The 8Cr4Mo4V steel is strengthened without departing from the heat treatment, wherein the quenching treatment is the key of the heat treatment. The quenching form of the 8Cr4Mo4V series steel mainly comprises the following components: oil cooling (medium) quenching, isothermal quenching, vacuum gas quenching, and the like. Among them, oil quenching and isothermal quenching are used more, but the research on the heat treatment of the refined structure is relatively lacked. At present, much effort is put into refining the crystal grains and structure of steel. Patent No. CN201810015019.1 indicates that a large amount of nano-scale carbide structures dispersed in the steel can be obtained by fully austenitizing the steel and then rapidly cooling the steel in a high temperature region, but this method cannot ensure austenite grain refinement and matrix structure control, and the austenite grain size is one of the main factors determining the performance of the steel. The essence of patent No. CN201510150917.4 is that grain refinement of the steel is achieved by subjecting medium and high carbon steel components to a preliminary (quenching) heat treatment, but this method is not suitable for 8Cr4Mo4V steel. At present, reports and patents on more precise control of the structure of 8Cr4Mo4V steel are not found.
The structure obtained by oil-cooling (medium) quenching of 8Cr4Mo4V steel is basically sheet martensite, retained austenite and retained carbide. The tempered troostite (or tempered sorbite), residual carbides and precipitated carbides are obtained after tempering, and the tempered troostite has high hardness, relatively poor toughness and relatively low comprehensive performance. The structure obtained by isothermal quenching is low-temperature bainite, residual austenite and residual carbide. Bainite, tempered troostite (or tempered sorbite), residual carbide and precipitated carbide are obtained after tempering, and the steel has high strength, good toughness, excellent comprehensive performance, lower hardness and unsatisfactory wear resistance. In addition, a single quenching treatment cannot effectively control the steel structure. The most important factors determining the performance of the steel are the grain state, the structure state, the microstructure and the matching among the components of the steel.
The invention content is as follows:
the purpose of the invention is as follows:
the invention discloses a method for obtaining 8Cr4Mo4V steel and optimizing a quenching process, aiming at fundamentally realizing the structure control of the 8Cr4Mo4V steel and effectively improving the performance of the steel.
The technical scheme is as follows:
a method for obtaining 8Cr4Mo4V steel for optimizing a quenching process comprises the following steps:
(1) determining a heating solid solution process: taking W spheroidizing annealed steel samples, respectively preserving heat for different times within the temperature range of 1050-1140 ℃, carrying out heating and solution treatment to obtain W first heating samples, wherein the heat preservation time of the first heating samples ranges from 8min to 90min, taking an arithmetic progression with any tolerance within the range of 8-90 min by taking 8min or 90min as an endpoint as different heat preservation time points of the first heating samples, respectively preserving heat for different times to obtain W first heating samples, and then directly carrying out oil quenching or air cooling on the W first heating samples to 10-30 ℃ to obtain first quenching samples; selecting the heat preservation temperature and the heat preservation time corresponding to the quenching sample with the highest microscopic grain size grade from the 7.6-10 microscopic grain size grades as the heat preservation temperature and the heat preservation time of the heating and solid solution process;
(2) determining the highest hardness time point of isothermal quenching: under the conditions of the heat preservation temperature and the heat preservation time of the heating and solution treatment process obtained in the step (1), heating and solution treatment is carried out on the other M spheroidizing annealed steel samples to obtain a second batch of heating samples; placing a second batch of heating samples into any temperature within the temperature range of 160-280 ℃ for heat preservation for different time for isothermal quenching, wherein the heat preservation time of the second batch of heating samples ranges from 20min to 10h, an arithmetic progression with any tolerance is taken at any point within the range as different heat preservation time points of the second batch of isothermal quenching samples, M second batches of heating samples are obtained after heat preservation for different time respectively, after M second batches of isothermal quenching samples are taken out respectively, air cooling is carried out to 0-30 ℃, the hardness of each second batch of isothermal quenching samples corresponding to different heat preservation time is measured, and the heat preservation time corresponding to the second batch of isothermal quenching samples with the maximum hardness is the heat preservation time point tm；
(3) Carrying out composite quenching treatment on the basis of the step (2): the heat preservation temperature and the heat preservation time of the heating solid solution process obtained in the step (1)Under the condition (1), heating and solid solution treatment are carried out on the other N spheroidizing annealing steel samples to obtain a third batch of heating samples; putting the third batch of heating samples into the heat preservation temperature obtained in the step (2), respectively preserving the heat for different times, and carrying out isothermal quenching, wherein the heat preservation time range of the third batch of heating samples is 0.1 Xtm～0.75×tmAt 0.1 × tmOr 0.75 × tmEnd point is 0.1 × tm～0.75×tmTaking an arithmetic progression with any tolerance as different heat preservation time points of a third batch of isothermal quenching samples within the range, respectively preserving heat of N third batches of heating samples for different time to obtain N third batches of isothermal quenching samples, respectively taking out the N third batches of isothermal quenching samples, and directly quenching the samples by using oil, water, gas or a water-soluble medium to obtain N composite quenching samples;
(4) selecting optimized composite quenching process samples: observing the structure of the composite quenching sample obtained in the step (3) by using a scanning electron microscope, and selecting the sample with the finest structure as an optimized composite quenching process sample from the composite quenching samples of which the matrix structure consists of martensite, low-temperature bainite and residual austenite, more than 80% of the martensite is not more than 5 micrometers, and 80% of the bainite is not more than 6 micrometers;
(5) and (4) tempering the optimized composite quenching process sample obtained in the step (4) for 3 to 4 times at 510 to 550 ℃ for 1.8 to 2.5 hours to obtain the finally optimized heat-treated 8Cr4Mo4V steel.
W in the step (1) is more than or equal to 3.
M is more than or equal to 5 in the step (2).
N is more than or equal to 3 and t is obtained in the step (3)mSelecting 3-5 time points for 0-120 min; t is tmIf the time is more than 120min, 5-10 time points can be selected.
The advantages and effects are as follows:
(1) the invention breaks through the prior single quenching process, scientifically combines the reasonable isothermal quenching process and the medium quenching process, and can obtain the optimized composite quenching process through experimental operation. The method has the advantages of scientificity, feasibility, simple operation and remarkable effect, and improves the space of the performance of the 8Cr4Mo4V steel.
(2) The invention has low quenching temperature and meets the requirement of environmental protection because of oil cooling (medium) quenching after isothermal quenching.
(3) The invention provides reference for further quantifying the steel structure heat treatment in the future.
Description of the drawings:
FIG. 1 is a dynamic frictional wear curve in example 1;
FIG. 2 shows the structure of 8Cr4Mo4V steel in example 1 after heat preservation at 1095 ℃ for 20min to dissolve, heat preservation at 200 ℃ for 50min to austemper and oil quench, and heat preservation at 540 ℃ for 120min to temper for four times;
FIG. 3 is a structural comparison of quenched steels of different processes in example 1;
FIG. 4 is a structural comparison of quenched steels of different processes in example 2;
FIG. 5 is a structural comparison of quenched steels of different processes in example 3;
FIG. 6 is a structural comparison of quenched steels of different processes in example 4.
The specific implementation mode is as follows:
the main idea of the invention is that based on the consideration that the transformation of low-temperature bainite in the isothermal quenching process mainly depends on the diffusion of elements such as carbon and the like, the transformation rate is slow, the transformation of martensite is shear phase transformation, and the transformation rate is fast, under a certain solid solution condition, isothermal quenching is placed before oil cooling (medium) quenching, so that partial low-temperature bainite transformation occurs. Because the isothermal time is limited, the bainite does not grow sufficiently, so that the bainite structure is refined; the proportion of the remaining super-cooled austenite is reduced and is divided by bainite, so that the size of martensite transformed in the subsequent oil-cooled (medium) quenching process is controlled, and the martensite structure is also significantly refined. Meanwhile, the relative contents of low-temperature bainite and martensite can be controlled by controlling the isothermal quenching heat preservation time, the control of the steel structure is fundamentally realized, and the performance of the steel is obviously improved. Through structure observation and comparison, the composite quenching process with fine crystal grains, fine martensite, fine low-temperature bainite and residual carbide structure is determined as the optimized quenching process.
The implementation process of the method for obtaining the 8Cr4Mo4V steel optimized quenching process is as follows:
(1) determining a heating solid solution process: taking W spheroidizing annealed steel samples, respectively preserving heat for different times at any temperature within the temperature range of 1050-1140 ℃, carrying out heating and solution treatment to obtain W first heating samples, wherein the heat preservation time of the first heating samples ranges from 8min to 90min, an arithmetic difference series with any tolerance is taken within the range of 8-90 min by taking 8min or 90min as an endpoint as different heat preservation time points of the first heating samples, respectively preserving heat for different times to obtain W first heating samples, and then directly carrying out oil quenching or air cooling on the W first heating samples to 10-30 ℃ to obtain first quenching samples; selecting the heat preservation temperature and the heat preservation time corresponding to the quenching sample with the highest microscopic grain size grade as the heat preservation temperature and the heat preservation time of the heating solid solution process between the 7.6-10 microscopic grain size grades which accord with the national standard GB/T6394-2017;
(2) determining the highest hardness time point of isothermal quenching: heating and solid solution treatment is carried out on the other M spheroidizing annealed steel samples under the heating and solid solution process condition determined in the step (1) to obtain a second batch of heating samples; placing the second batch of heating samples into an isothermal furnace with any temperature within the temperature range of 160-280 ℃ for isothermal quenching for different time periods respectively, wherein the temperature holding time of the second batch of heating samples ranges from 20min to 10h, arithmetic progression with any tolerance is taken at any point within the range as different temperature holding time points of the second batch of isothermal quenching samples, M second batches of heating samples are respectively subjected to temperature holding for different time periods to obtain M second batches of isothermal quenching samples, after the M second batches of isothermal quenching samples are respectively taken out, air cooling is carried out to 0-30 ℃, the hardness of each second batch of isothermal quenching samples corresponding to different temperature holding time periods is measured, and the temperature holding time corresponding to the second batch of isothermal quenching samples with the highest hardness is a temperature holding time point tm；
(3) Carrying out composite quenching treatment on the basis of the step (2): heating and solid solution treatment is carried out on the other N spheroidizing annealed steel samples under the heating and solid solution process conditions determined in the step (1) to obtain a third batch of heating samples, the third batch of heating samples are placed into the isothermal quenching furnace in the step (2) to be respectively subjected to isothermal quenching at different heat preservation times, and the heat preservation time range of the third batch of heating samples is 0.1 x tm～0.75×tmAt 0.1 × tmOr 0.75 × tmEnd point is 0.1 × tm～0.75×tmTaking an arithmetic progression with any tolerance as different heat preservation time points of a third batch of isothermal quenching samples within the range, respectively preserving heat of N third batches of heating samples for different time to obtain N third batches of isothermal quenching samples, respectively taking out the N third batches of isothermal quenching samples, and directly quenching the samples by using oil, water, gas or a water-soluble medium to obtain N composite quenching samples;
(4) selecting optimized composite quenching process samples: observing the structure of the composite quenching sample obtained in the step (3) by using a scanning electron microscope, and selecting the sample with the finest structure as an optimized composite quenching process sample from the composite quenching samples of which the matrix structure consists of martensite, low-temperature bainite and residual austenite, more than 80% of the martensite is not more than 5 micrometers, and 80% of the bainite is not more than 6 micrometers;
(5) and (4) tempering the optimized composite quenching process sample obtained in the step (4) for 3 to 4 times at the temperature of between 510 and 550 ℃ for 1.8 to 2.5 hours to obtain the finally optimized heat-treated 8Cr4Mo4V steel.
The method comprises the following steps: w in the step (1) is more than or equal to 3; m is more than or equal to 5 in the step (2); n is more than or equal to 3 and t is obtained in the step (3)mSelecting 3-5 time points for 0-120 min; t is tmIf the time is more than 120min, 5-10 time points can be selected.
Example 1:
firstly, the heating and solid solution process of 8Cr4Mo4V steel is determined: taking 3 spheroidizing annealed steel samples, respectively preserving heat at the temperature of 1095 ℃ for 20min, 30min and 40min, carrying out heating and solution treatment, and then carrying out oil quenching to obtain a first batch of quenching samples; the detection shows that the microscopic grain size grade of the sample is highest and 9.0 grade when the temperature is 1095 ℃ and the temperature is kept for 20min, and the temperature keeping time of the heating and solid solution process are determined to be 1095 ℃ and 20min respectively; meanwhile, the austempering temperature is determined to be about 200 ℃ through a series of experiments and structural observation. On the basis, the heat treatment is carried out according to the following steps:
(1) carrying out heating and solution treatment on 7 spheroidized annealed steel samples by keeping the temperature at 1095 ℃ for 20 min;
(2) heating the mixture obtained in the step (1)And putting the sample into a 200 ℃ furnace, preserving heat for 60min, 90min, 120min, 150min, 180min, 210min and 240min respectively, carrying out isothermal quenching to obtain a second batch of isothermal quenching samples, taking out the second batch of isothermal quenching samples, and air cooling to 20 ℃. The relationship between the average hardness of the second austempered test specimens and the soaking time for austempering was determined, and as shown in Table 1, the soaking time t for the maximum hardness was determinedmIt is 150 min.
TABLE 1 relationship between the hardness of austempered 8Cr4Mo4V steel and the holding time for quenching
Figure BDA0002508454080000051
(3) And (3) preserving the heat of the spheroidizing annealing samples at 1095 ℃ for 20min for solution treatment, taking out the heated samples, putting the heated samples into a 200 ℃ furnace, and preserving the heat for 15min, 65min and 115min respectively for isothermal quenching to obtain a third batch of isothermal quenching samples. And taking out the third batch of isothermal quenching samples for direct oil quenching to obtain composite quenching samples.
(4) Observing the composite quenching sample structure obtained in the step (3) through a scanning electron microscope, and observing that: the quenching sample with the heat preservation time of 65min is composed of martensite, low-temperature bainite and residual austenite in a matrix structure, the length of more than 80% of martensite is not more than 5 mu m, the structure of the composite quenching sample with the length of 80% of bainite is not more than 6 mu m is the finest, the heat preservation time of 1095 ℃ is determined as a heating and solution process, the heat preservation time of 200 ℃ is 65min, and the isothermal quenching and water quenching are determined as an optimized quenching process.
(5) And carrying out four times of 540 ℃ isothermal 2h tempering treatment on the sample obtained by the optimized quenching process. The hardness and wear resistance of the tempered steel were measured. As shown in Table 2, the hardness of the steel treated by the optimized composite quenching and tempering process, the optimized isothermal quenching and tempering and the optimized oil quenching and tempering process of the invention is compared, and the hardness of the composite quenching and tempering steel is equivalent to that of the oil-cooled quenching and tempering sample. The dynamic frictional wear curve is shown in figure 1, which shows that the wear resistance of the composite quenching and tempering steel is obviously superior to that of other quenching processes.
TABLE 2 influence of the quenching Process on the hardness of tempered Steel
Figure BDA0002508454080000052
FIG. 2 shows the structure and appearance of 8Cr4Mo4V steel after heat preservation at 1095 ℃ for 20min for solid solution, heat preservation at 200 ℃ for 50min for isothermal quenching and oil quenching, and heat preservation at 540 ℃ for 120min for four tempering. Therefore, austenite disappears, and simultaneously highly dispersed nano-scale carbide is precipitated in the matrix, so that the heat-treated steel integrates fine-grain strengthening, solid solution strengthening and multi-level second phase strengthening.
FIG. 3 is a structural comparison of quenched steels in different processes. Wherein, the drawing (a) is oil-cooled quenching, the drawing (b) is 200 ℃ multiplied by 120min isothermal quenching, and the drawing (c) is 200 ℃ multiplied by 50min isothermal quenching and oil-cooled composite quenching. As can be seen, the austempered steel structure consists of low-temperature bainite, residual micron or submicron-scale primary (including secondary) carbides and residual austenite; the oil-cooled quenched steel structure consists of martensite, residual micron or submicron-scale primary (including secondary) carbide and residual austenite; the optimized composite quenched steel structure consists of low-temperature bainite, martensite, residual micron or submicron-grade primary (including secondary) carbide and trace austenite, and is obviously refined and uniform compared with isothermal quenching and oil quenching sample structures.
Example 2:
firstly, the heating and solid solution process of 8Cr4Mo4V steel is determined: taking 4 spheroidizing annealed steel samples, respectively preserving heat at 1050 ℃ for 60min, 70min, 80min and 90min, heating and carrying out solid solution treatment, and then carrying out oil quenching to obtain a first batch of quenching samples; the detection shows that the microscopic grain size grade of the sample is highest and 9.5 grade when the temperature is 1050 ℃ and the temperature is kept for 60min, and the temperature keeping time of the heating and solid solution process are respectively determined to be 1050 ℃ and 60 min; meanwhile, the austempering temperature is determined to be about 160 ℃ through a series of experiments and structural observation. On the basis, the heat treatment is carried out according to the following steps:
(1) carrying out heating and solid solution treatment on 7 spheroidized annealed steel samples by keeping the temperature at 1050 ℃ for 60 min;
(2) putting the heating sample in the step (1) into a 160 ℃ furnace, and respectively preserving heat for 160min, 200min, 240min, 280min, 320min, 360min and 400min to perform isothermal treatmentQuenching to obtain a second batch of austempered samples, taking out the second batch of austempered samples, and air-cooling to 0 ℃. The relationship between the average hardness of the second austempered test specimens and the soaking time for austempering was determined, and as shown in Table 3, the soaking time t for the maximum hardness was determinedmIt is 240 min.
TABLE 3 relationship between the hardness of austempered 8Cr4Mo4V steel and the holding time for quenching
Figure BDA0002508454080000061
(3) And (3) preserving the heat of another 5 spheroidizing annealing samples at 1050 ℃ for 60min for solution treatment, taking out the heated samples, and putting the heated samples into a 160 ℃ furnace for respectively preserving the heat for 20min, 60min, 100min, 140min and 180min for isothermal quenching to obtain a third batch of isothermal quenching samples. And taking out the third batch of isothermal quenching samples for direct water quenching to obtain composite quenching samples.
(4) Observing the composite quenching sample structure obtained in the step (3) through a scanning electron microscope, and observing that: the quenching sample with the heat preservation time of 140min is formed by martensite, low-temperature bainite and residual austenite in a matrix structure, the length of more than 80% of martensite is not more than 5 mu m, the length of 80% of bainite is not more than 6 mu m, the structure is the finest in the composite quenching sample, the heat preservation time of 1050 ℃ is determined as a heating and solution process, and the heat preservation time of 160 ℃ for 140min is determined as an isothermal quenching and water quenching process as an optimized quenching process.
(5) And carrying out four times of 510 ℃ isothermal tempering for 1.8h on the sample obtained by the optimized quenching process and the sample which is subjected to 160 ℃ heat preservation for 140min and is subjected to isothermal quenching and water quenching. The hardness of the tempered steel was measured. As shown in Table 4, the hardness of the steel treated by the optimized composite quenching and tempering process, the optimized isothermal quenching and tempering and the optimized oil quenching and tempering process of the invention is compared, and the hardness of the composite quenching and tempering steel is obviously superior.
FIG. 4 is a structural comparison of quenched steels in different processes. Wherein, the drawing (a) is oil quenching, the drawing (b) is isothermal quenching with the temperature of 160 ℃ kept for 240min, and the drawing (c) is isothermal quenching with the temperature of 160 ℃ kept for 140min and water quenching. As can be seen from FIG. 4, the structure of the composite quenched steel obtained by isothermal quenching and water quenching at 160 ℃ for 140min consists of low-temperature bainite, martensite, residual micron or submicron-level primary (including secondary) carbide and trace austenite, and is obviously refined and uniform compared with the structure of isothermal quenching and oil quenching samples.
TABLE 4 influence of the quenching Process on the hardness of tempered steels
Figure BDA0002508454080000071
Example 3:
firstly, the heating and solid solution process of 8Cr4Mo4V steel is determined: taking 3 spheroidizing annealed steel samples, respectively preserving heat at 1140 ℃ for 8min, 15min and 22min, carrying out heating and solution treatment, and then carrying out oil quenching to obtain a first batch of quenching samples; the detection shows that the microscopic grain size grade of the sample is the highest grade, 7.6 grade, when the temperature is 1140 ℃ and the heat preservation is carried out for 8min, and the 1140 ℃ and the 8min are respectively determined as the heat preservation temperature and the heat preservation time of the heating and solid solution process; meanwhile, the austempering temperature is about 280 ℃ through a series of experiments and structural observation. On the basis, the heat treatment is carried out according to the following steps:
(1) keeping the temperature at 1140 ℃ for 8min to perform heating solution treatment on 5 spheroidized annealed steel samples;
(2) and (2) putting the heated sample in the step (1) into a 280 ℃ salt bath, preserving the heat for 60min, 120min, 180min, 240min and 300min respectively, carrying out isothermal quenching to obtain a second batch of isothermal quenching samples, taking out the second batch of isothermal quenching samples, and air cooling to 0 ℃. The relationship between the average hardness of the second austempered test specimens and the soaking time for austempering was determined, and as shown in Table 5, the soaking time t for the maximum hardness was determinedmIt is 240 min.
TABLE 5 relationship between the hardness of austempered 8Cr4Mo4V steel and the holding time for quenching
Figure BDA0002508454080000072
(3) And (3) carrying out solution treatment on another 5 spheroidizing annealing samples by keeping the temperature at 1140 ℃ for 10min, taking out the heated samples, putting the heated samples into a salt bath with the temperature of 280 ℃ and respectively keeping the temperature for 20min, 60min, 100min, 140min and 180min for isothermal quenching, and obtaining a third batch of isothermal quenching samples. And taking out the third batch of isothermal quenching samples, and directly air-cooling to 0 ℃ to obtain a composite quenching sample structure.
(4) Observing the composite quenching sample structure obtained in the step (3) through a scanning electron microscope, and observing that: the quenching sample is kept at the temperature for 60min, the matrix structure of the quenching sample consists of martensite, low-temperature bainite and residual austenite, the length of more than 80 percent of the martensite is not more than 5 mu m, the structure of the composite quenching sample is the finest, the heat preservation at 1140 ℃ for 8min is determined as a heating and solution treatment process, and the heat preservation at 280 ℃ for 60min is determined as isothermal quenching and air-cooled quenching as an optimized quenching process.
(5) And carrying out tempering treatment on the sample obtained by the optimized quenching process for three times at 540 ℃ for 2.5 hours. The hardness of the tempered steel was measured. As shown in Table 6, the hardness of the steel treated by the optimized composite quenching and tempering process, the optimized isothermal quenching and tempering and the optimized oil quenching and tempering process of the invention is compared, and the hardness of the composite quenching and tempering steel is higher than that of the samples tempered by the isothermal quenching and tempering and the oil cooling quenching.
FIG. 5 is a structural comparison of quenched steels in different processes. Wherein, the graph (a) is oil-cooled quenching, the graph (b) is isothermal quenching at 280 ℃ for 240min, and the graph (c) is the tissue morphology after the isothermal quenching at 280 ℃ for 60min and air-cooled quenching. It can be seen from fig. 5 that the composite quenched steel structure obtained by isothermal quenching and air-cooled quenching at 280 ℃ for 60min consists of low-temperature bainite, martensite, residual micron or submicron-level primary (including secondary) carbide and trace austenite, and is obviously refined and uniform compared with the sample structure of isothermal quenching and oil quenching.
TABLE 6 influence of the quenching Process on the hardness of tempered steels
Figure BDA0002508454080000081
Example 4:
firstly, the heating and solid solution process of 8Cr4Mo4V steel is determined: taking 3 spheroidizing annealed steel samples, respectively preserving heat at 1095 ℃ for 25min, 35min and 45min, carrying out heating and solution treatment, and then carrying out oil quenching to obtain a first batch of quenching samples; the detection shows that the microscopic grain size grade of the sample is the highest grade and 8 grades when the temperature is 1140 ℃ and the heat preservation is 8min, and the temperature of 1095 ℃ and the temperature of 25min are respectively determined as the heat preservation temperature and the heat preservation time of the heating and solid solution process; meanwhile, the austempering temperature is determined to be about 240 ℃ through a series of experiments and structural observation. On the basis, the heat treatment is carried out according to the following steps:
(1) carrying out heating and solution treatment on 7 spheroidized annealed steel samples by keeping the temperature at 1095 ℃ for 25 min;
(2) and (2) putting the heating sample in the step (1) into a furnace at 240 ℃ and keeping the temperature for 240min, 300min, 360min, 420min, 480min, 540min and 600min respectively to carry out isothermal quenching to obtain a first batch of isothermal quenching samples, taking out a second batch of isothermal quenching samples and air cooling to 30 ℃. The relationship between the average hardness of the second austempered test specimens and the soaking time for austempering was determined, and as shown in Table 7, the soaking time t for the maximum hardness was determinedmThe time is 360 min.
TABLE 7 relationship between the hardness of austempered 8Cr4Mo4V steel and the holding time for quenching
Figure BDA0002508454080000082
(3) And (3) preserving the heat of 7 spheroidizing annealing samples at 1095 ℃ for 25min for solution treatment, taking out the heated samples, and putting the heated samples into a 240 ℃ furnace for respectively preserving the heat for 35min, 75min, 115min, 155min, 195min, 235min and 275min for isothermal quenching to obtain a third batch of isothermal quenching samples. And taking out the third batch of isothermal quenching samples for direct oil quenching to obtain composite quenching samples.
(4) Observing the structure of the composite quenching sample obtained in the step (3) through a scanning electron microscope, and observing that the quenching sample with the heat preservation time of 75min is the thinnest in the composite quenching sample with the matrix structure composed of martensite, low-temperature bainite and residual austenite, more than 80% of the martensite is not more than 5 micrometers, and 80% of the bainite is not more than 6 micrometers, and determining that the heat preservation time of 25min at 1090 ℃ is a heating and solution treatment process, and the heat preservation time of 75min at 240 ℃ is an isothermal quenching and water quenching process as an optimized quenching process.
(5) And carrying out three times of 540 ℃ isothermal tempering treatment for 2h on the sample obtained by the optimized quenching process. The hardness of the tempered steel was measured. As shown in Table 8, the hardness of the steel treated by the optimized composite quenching and tempering process of the invention is compared with that of the steel treated by the optimized isothermal quenching and tempering process and the optimized oil quenching and tempering process, and the hardness of the composite quenching and tempering steel is obviously superior.
FIG. 6 is a structural comparison of quenched steels in different processes. Wherein, the diagram (a) is oil-cooled quenching, the diagram (b) is isothermal quenching with the temperature of 240 ℃ kept for 360min, and the diagram (c) is the tissue morphology of the isothermal quenching and oil quenching sample with the temperature of 240 ℃ kept for 75 min. It can be seen from fig. 6 that the structure of the composite quenched steel obtained by isothermal quenching and oil quenching at 240 ℃ for 75min consists of low-temperature bainite, martensite, residual micron or submicron-level primary (including secondary) carbide and trace austenite, and is obviously refined and uniform compared with the structure of isothermal quenching and oil quenching samples.
TABLE 8 influence of the quenching Process on the hardness of tempered steels
Figure BDA0002508454080000091

Claims (4)

1. A method for obtaining 8Cr4Mo4V steel to optimize a quenching process is characterized by comprising the following steps:
the method comprises the following steps:
(1) determining a heating solid solution process: taking W spheroidizing annealed steel samples, respectively preserving heat for different times within the temperature range of 1050-1140 ℃, carrying out heating and solution treatment to obtain W first heating samples, wherein the heat preservation time of the first heating samples ranges from 8min to 90min, taking an arithmetic progression with any tolerance within the range of 8-90 min by taking 8min or 90min as an endpoint as different heat preservation time points of the first heating samples, respectively preserving heat for different times to obtain W first heating samples, and then directly carrying out oil quenching or air cooling on the W first heating samples to 10-30 ℃ to obtain first quenching samples; selecting the heat preservation temperature and the heat preservation time corresponding to the quenching sample with the highest microscopic grain size grade from the 7.6-10 microscopic grain size grades as the heat preservation temperature and the heat preservation time of the heating and solid solution process;
(2) determining the highest hardness time point of isothermal quenching: heating obtained in step (1)Under the conditions of heat preservation temperature and heat preservation time of the solid solution process, heating and solid solution treatment are carried out on the other M spheroidizing annealing steel samples to obtain a second batch of heating samples; placing a second batch of heating samples into any temperature within the temperature range of 160-280 ℃ for heat preservation for different time for isothermal quenching, wherein the heat preservation time of the second batch of heating samples ranges from 20min to 10h, an arithmetic progression with any tolerance is taken at any point within the range as different heat preservation time points of the second batch of isothermal quenching samples, M second batches of heating samples are obtained after heat preservation for different time respectively, after M second batches of isothermal quenching samples are taken out respectively, air cooling is carried out to 0-30 ℃, the hardness of each second batch of isothermal quenching samples corresponding to different heat preservation time is measured, and the heat preservation time corresponding to the second batch of isothermal quenching samples with the maximum hardness is the heat preservation time point tm；
(3) Carrying out composite quenching treatment on the basis of the step (2): under the conditions of the heat preservation temperature and the heat preservation time of the heating and solution treatment process obtained in the step (1), heating and solution treatment is carried out on the other N spheroidizing annealed steel samples to obtain a third batch of heating samples; putting the third batch of heating samples into the heat preservation temperature obtained in the step (2), respectively preserving the heat for different times, and carrying out isothermal quenching, wherein the heat preservation time range of the third batch of heating samples is 0.1 Xtm～0.75×tmAt 0.1 × tmOr 0.75 × tmEnd point is 0.1 × tm～0.75×tmTaking an arithmetic progression with any tolerance as different heat preservation time points of a third batch of isothermal quenching samples within the range, respectively preserving heat of N third batches of heating samples for different time to obtain N third batches of isothermal quenching samples, respectively taking out the N third batches of isothermal quenching samples, and directly quenching the samples by using oil, water, gas or a water-soluble medium to obtain N composite quenching samples;
(4) selecting optimized composite quenching process samples: observing the structure of the composite quenching sample obtained in the step (3) by using a scanning electron microscope, and selecting the sample with the finest structure as an optimized composite quenching process sample from the composite quenching samples of which the matrix structure consists of martensite, low-temperature bainite and residual austenite, more than 80% of the martensite is not more than 5 micrometers, and 80% of the bainite is not more than 6 micrometers;
(5) and (4) tempering the optimized composite quenching process sample obtained in the step (4) for 3 to 4 times at 510-550 ℃ for 1.8-2.5h to obtain the final optimized heat-treated 8Cr4Mo4V steel.
2. The method for obtaining the 8Cr4Mo4V steel optimized quenching process according to claim 1, wherein the quenching process comprises the following steps: w in the step (1) is more than or equal to 3.
3. The method for obtaining the 8Cr4Mo4V steel optimized quenching process according to claim 1, wherein the quenching process comprises the following steps: m is more than or equal to 5 in the step (2).
4. The method for obtaining the 8Cr4Mo4V steel optimized quenching process according to claim 1, wherein the quenching process comprises the following steps: n is more than or equal to 3 and t is obtained in the step (3)mSelecting 3-5 time points for 0-120 min; t is tmIf the time is more than 120min, 5-10 time points are selected.
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