High-alloy steel and method for determining process parameters for vacuum low-pressure carburization thereof

Abstract

The present application relates to the technical field of steel material heat treatment and process simulation, and particularly relates to a high alloy steel and a method for determining vacuum low-pressure carburizing process parameters of the high alloy steel, which establishes a distribution relationship among total carbon concentration, austenite solid solution carbon concentration and carbide mass fraction; based on the distribution relationship, the change rate of the austenite solid solution carbon concentration to the total carbon concentration is introduced into a diffusion model to construct an apparent diffusion coefficient with the total carbon concentration as a solving variable; the total carbon concentration distribution in the vacuum low-pressure carburizing process of the high alloy steel is solved according to the apparent diffusion coefficient, and the strong carburizing time, diffusion time and pulse cycle number are determined according to the surface total carbon concentration, total carbon concentration at the target depth and carbide mass fraction constraints. The method can avoid the diffusion coefficient inaccuracy caused by directly equating the total carbon concentration to the austenite solid solution carbon concentration, improve the prediction accuracy of the vacuum low-pressure carburizing process parameters of the high alloy steel, and reduce the risk of surface over-carburizing and excessive precipitation of harmful carbides.

Description

Technical Field

[0001] This invention relates to the field of heat treatment and process simulation technology for steel materials, specifically to a method for determining the process parameters of high alloy steel and its vacuum low-pressure carburizing process. Background Technology

[0002] Vacuum low-pressure carburizing is a surface strengthening heat treatment technology widely used in gear steel, bearing steel, mold steel, and high-performance transmission components. This process typically involves introducing carbon-containing gases such as acetylene or propane into a low-pressure vacuum environment. This causes active carbon atoms to adsorb, decompose, and diffuse inwards onto the steel surface, resulting in a strengthened layer with a certain depth and carbon concentration gradient. Compared to traditional gas carburizing, vacuum low-pressure carburizing offers advantages such as less oxidation, higher surface quality, higher carburizing efficiency, and stronger process controllability.

[0003] For ordinary low-alloy carburizing steel, carbon exists mainly in solid solution form in the austenitic matrix at the carburizing temperature, with relatively little carbide precipitation. Therefore, existing carburizing diffusion models typically approximate the total carbon concentration as the austenitic solid solution carbon concentration, and use the austenitic carbon concentration as the only concentration variable in the diffusion model. Correspondingly, the saturated austenitic carbon concentration is often used as the upper limit of the surface carbon concentration in the strong carburizing stage, while the target austenitic carbon concentration is used as the basis for process design in the diffusion stage.

[0004] However, for high-alloy steels containing high levels of alloying elements such as Cr, Mo, V, W, and Co, the above assumptions no longer apply. High-alloy steels are prone to precipitating large amounts of M during vacuum low-pressure carburizing. 23 Carbides such as C6, M7C3, and M6C result in not all carbon entering the material surface being dissolved in austenite; a significant portion is fixed by the carbides. In this case, there is a significant difference between the total carbon concentration and the austenite dissolved carbon concentration. When the total carbon concentration has reached a high level, the austenite dissolved carbon concentration may still be in a low range. If a standard carburizing steel model is continued to be used, and the total carbon concentration is directly substituted into the austenite diffusion coefficient model, it can easily lead to inaccurate diffusion coefficient calculations, inaccurate control of surface carbon concentration, excessively long strong carburizing time, and excessive precipitation of surface carbides.

[0005] Furthermore, traditional diffusion models often use alloying factors calculated based on nominal alloy composition to correct for the diffusion coefficient. However, during the carburizing process of high-alloy steel, strong carbide-forming elements redistribute between austenite and carbides, and the actual alloying element content in the austenite matrix is ​​no longer equivalent to the nominal composition of the steel. Therefore, alloying factors based on nominal composition are difficult to accurately characterize the actual diffusion environment in austenite.

[0006] Therefore, there is an urgent need for a method to determine process parameters suitable for the vacuum low-pressure carburizing process of high alloy steel, which can distinguish between total carbon concentration, austenite solid solution carbon concentration and carbide mass fraction, and on this basis determine the strong carburizing time, diffusion time and pulse cycle number, thereby improving the accuracy and reliability of the design of high alloy steel carburizing process parameters. Summary of the Invention

[0007] The present invention aims to address the technical problem of how to provide a method for determining process parameters applicable to the vacuum low-pressure carburizing process of high alloy steel, which can distinguish between total carbon concentration, austenite solid solution carbon concentration and carbide mass fraction, and determine the strong carburizing time, diffusion time and pulse cycle number based on these parameters, thereby improving the accuracy and reliability of the design of high alloy steel carburizing process parameters.

[0008] To achieve the above objectives, the first aspect of the present invention provides a method for determining the process parameters of vacuum low-pressure carburizing of high alloy steel, comprising the following steps:

[0009] S1. Establish the carbon distribution relationship of high alloy steel at carburizing temperature T based on the basic input parameters. The carbon distribution relationship includes the total carbon concentration C. tot austenite solid solution carbon concentration C γ and carbide mass fraction f carbide The correspondence between them;

[0010] S2. Calculate the rate of change of austenite solid solution carbon concentration with respect to total carbon concentration. ;

[0011] S3. Based on the austenite basic diffusion coefficient, the aforementioned rate of change, and the apparent diffusion correction coefficient, construct a system based on the total carbon concentration C. tot The equation for the apparent diffusion coefficient;

[0012] S4, with total carbon concentration C tot To solve for the variables, a vacuum low-pressure carburizing diffusion equation for high alloy steel was established and solved.

[0013] S5. In the strong infiltration stage of preparing the high alloy steel, carbon flux boundary conditions are used to characterize the carbon input process of carbon-containing atmosphere to the surface of high alloy steel. The total carbon concentration and the mass fraction of surface carbides are judged synchronously at regular intervals. When the total carbon concentration reaches the upper limit of the total carbon concentration or the mass fraction of surface carbides reaches the upper limit of the mass fraction of surface carbides, the strong infiltration stage is stopped and the diffusion stage is carried out.

[0014] That is, when x is 0, the total carbon concentration C tot Reaching the upper limit of total surface carbon concentration, or carbide mass fraction f carbide When the upper limit of the surface carbide mass fraction is reached, the strong infiltration stage is stopped and the diffusion stage begins.

[0015] During the diffusion stage of the high-alloy steel preparation, a boundary condition with no carbon inflow to the surface is used to characterize the state where the surface no longer receives external carbon sources after the input of carbon-containing gas is stopped. The total carbon concentration distribution C along the depth direction of the high-alloy steel is updated at regular intervals. tot (x,t) is used to extract the total carbon concentration on the surface and the total carbon concentration at the target effective infiltration depth. When the total carbon concentration at the target effective infiltration depth reaches the requirement for the total carbon concentration at the target effective infiltration depth, and the total carbon concentration on the surface reaches the range of the target total carbon concentration on the surface, and the mass fraction of surface carbides does not exceed the upper limit of the mass fraction of surface carbides, the process parameters are output. Otherwise, the process is switched to the strong infiltration stage to carry out the next round of strong infiltration-diffusion cycle.

[0016] The process parameters include strong carburizing time, diffusion time, number of pulse cycles, total carburizing time, total carbon concentration distribution, austenite solid solution carbon concentration distribution, carbide mass fraction distribution, and apparent diffusion coefficient distribution.

[0017] The second aspect of the present invention provides high alloy steel obtained by the method for determining the process parameters of the above-mentioned high alloy steel vacuum low-pressure carburizing process.

[0018] The present invention has the following beneficial effects and advantages:

[0019] (1) Compared with existing processes, the determination method provided by the present invention distinguishes between total carbon concentration, austenite solid solution carbon concentration and carbide mass fraction, avoiding the error caused by directly equating total carbon concentration with austenite solid solution carbon concentration in traditional models; with total carbon concentration as a conserved variable, it can be directly compared with the total carbon concentration profile obtained by carbon-sulfur analysis, thereby improving the reliability of model calibration and process prediction.

[0020] (2) The determination method provided by the present invention uses the total surface carbon concentration and the surface carbide mass fraction as the stopping conditions for the strong infiltration stage, which can reduce the risk of over-infiltration and excessive precipitation of harmful carbides on the surface of high alloy steel.

[0021] (3) The determination method provided by the present invention can output the strong carburizing time, diffusion time, number of pulse cycles and total carburizing time, providing a quantitative basis for the design of vacuum low-pressure carburizing process for high alloy steel. Attached Figure Description

[0022] Figure 1 A flowchart illustrating the method for determining the process parameters of vacuum low-pressure carburizing of high alloy steel provided by the present invention.

[0023] Figure 2 The curves showing the total carbon concentration, austenite solid solution carbon concentration, and measured carbon concentration in Example 1 of the present invention are shown.

[0024] Figure 3This is the mass fraction distribution curve of carbides along the depth.

[0025] Figure 4 This is a schematic diagram of a pulse process.

[0026] Figure 5 The carbon concentration curves were calculated to compare the measured carbon concentrations for Comparative Example 1. Detailed Implementation

[0027] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0028] Existing carburizing process models directly equate total carbon concentration to austenite solid solution carbon concentration, leading to inaccurate calculation of diffusion coefficient, high risk of surface over-carburization, and large deviations in process parameter prediction during the carburizing process of high alloy steel.

[0029] The purpose of this invention is to provide a method for determining the process parameters of vacuum low-pressure carburizing of high alloy steel, so as to solve the technical problems of existing carburizing process models that directly equate the total carbon concentration with the austenite solid solution carbon concentration, resulting in inaccurate calculation of the diffusion coefficient, high risk of surface over-carburization, and large deviation in the prediction of process parameters in the carburizing process of high alloy steel.

[0030] Therefore, the first aspect of the present invention provides a method for determining the process parameters of vacuum low-pressure carburizing of high alloy steel, comprising the following steps:

[0031] S1. Establish the carbon distribution relationship of high alloy steel at carburizing temperature T based on the basic input parameters. The carbon distribution relationship includes the total carbon concentration C. tot austenite solid solution carbon concentration C γ and carbide mass fraction f carbide The correspondence between them;

[0032] S2. Calculate the rate of change of austenite solid solution carbon concentration with respect to total carbon concentration. ;

[0033] S3. Based on the austenite basic diffusion coefficient, the aforementioned rate of change, and the apparent diffusion correction coefficient, construct a system based on the total carbon concentration C. tot The equation for the apparent diffusion coefficient;

[0034] S4, with total carbon concentration C tot To solve for the variables, a vacuum low-pressure carburizing diffusion equation for high alloy steel was established and solved.

[0035] S5. In the strong infiltration stage of preparing the high alloy steel, carbon flux boundary conditions are used to characterize the carbon input process of carbon-containing atmosphere to the surface of high alloy steel. The total carbon concentration and the mass fraction of surface carbides are judged synchronously at regular intervals. When the total carbon concentration reaches the upper limit of the total carbon concentration or the mass fraction of surface carbides reaches the upper limit of the mass fraction of surface carbides, the strong infiltration stage is stopped and the diffusion stage is carried out.

[0036] That is, when x is 0, the total carbon concentration C tot Reaching the upper limit of total surface carbon concentration, or carbide mass fraction f carbide When the upper limit of the surface carbide mass fraction is reached, the strong infiltration stage is stopped and the diffusion stage begins.

[0037] During the diffusion stage of the high-alloy steel preparation, a boundary condition with no carbon inflow to the surface is used to characterize the state where the surface no longer receives external carbon sources after the input of carbon-containing gas is stopped. The total carbon concentration distribution C along the depth direction of the high-alloy steel is updated at regular intervals. tot (x,t) is used to extract the total carbon concentration on the surface and the total carbon concentration at the target effective infiltration depth. When the total carbon concentration at the target effective infiltration depth reaches the requirement for the total carbon concentration at the target effective infiltration depth, and the total carbon concentration on the surface reaches the range of the target total carbon concentration on the surface, and the mass fraction of surface carbides does not exceed the upper limit of the mass fraction of surface carbides, the process parameters are output. Otherwise, the process is switched to the strong infiltration stage to carry out the next round of strong infiltration-diffusion cycle.

[0038] The process parameters include strong carburizing time, diffusion time, number of pulse cycles, total carburizing time, total carbon concentration distribution, austenite solid solution carbon concentration distribution, carbide mass fraction distribution, and apparent diffusion coefficient distribution.

[0039] This invention establishes a method for determining the process parameters of vacuum low-pressure carburizing of high-alloy steel based on the coupling relationship between total carbon concentration, austenite solid solution carbon concentration, and carbide mass fraction during the carburizing process. This method uses a carbon concentration distribution model, a carbide mass fraction prediction model, an austenite apparent diffusion model, and a carbon concentration gradient prediction model to quantitatively describe the carbon migration behavior during carburizing. It determines the strong carburizing time based on the carbon input rate, surface total carbon concentration, and carbide mass fraction constraints during the strong carburizing stage; and determines the diffusion time and pulse cycle number based on the total carbon concentration, surface carbon concentration, and carbide mass fraction constraints at the target depth, thereby obtaining the vacuum low-pressure carburizing process parameters that meet the requirements of the target carburized layer.

[0040] According to the present invention, in step S1, the total carbon concentration C tot austenite solid solution carbon concentration C γ and carbide mass fraction f carbide The correspondence between them is as follows:

[0041] For any location at total carbon concentration C tot,i With total carbon concentration C tot,i+1 The total carbon concentration between the two values, and the carbon concentration in the austenite solid solution C. γ and carbide mass fraction f carbide They are represented as follows:

[0042] (1);

[0043] (2);

[0044] In equation (1), C γ,i C represents the austenite solid solution carbon concentration at the i-th time point, in wt.%. γ,i+1 f is the concentration of dissolved carbon in austenite at the (i+1)th time node, in wt.%. In equation (2), f i f represents the carbide mass fraction corresponding to the i-th time node of total carbon concentration. i+1 This represents the carbide mass fraction corresponding to the (i+1)th time node of total carbon concentration.

[0045] According to the present invention, in step S1, the total carbon concentration C tot The total macroscopic carbon concentration is expressed in wt.%.

[0046] According to the present invention, in step S3, the step based on the total carbon concentration C tot The equation for the apparent diffusion coefficient is:

[0047] (3);

[0048] (4);

[0049] In equation (3), Here, is the basic diffusion coefficient of austenite, and is the basic diffusion coefficient of carbon in austenite, in mm. 2 ·s -1 , The frequency factor of the carbon diffusion coefficient in austenite, in mm. 2 ·s -1 R is the molar gas constant, taken as 1.987 cal / (mol·K);

[0050] In equation (4), The apparent diffusion coefficient is described based on the total carbon concentration and is expressed in mm. 2 ·s -1 , This is the apparent diffusion correction coefficient; it is used to correct the deviation between the ideal austenite diffusion model and the actual carbon migration behavior of high alloy steel, and can be calibrated by experimental carbon concentration gradient.

[0051] According to the present invention, in step S4, the vacuum low-pressure carburizing diffusion equation for the high-alloy steel is:

[0052] (5);

[0053] In equation (5), x is the depth of the test site from the surface of the high alloy steel, and t is the carburizing time.

[0054] According to a preferred embodiment of the present invention, the high-alloy steel contains at least one alloying element selected from Cr, Mo, V, W, and Co, wherein the Cr content is 5–18 wt.%, the Mo content is 1–8 wt.%, the V content is 0.1–2 wt.%, the W content is 0–5 wt.%, and the Co content is 0–20 wt.%, and the total content of the above alloying elements is not less than 10 wt.%, and the high-alloy steel is preferably bearing steel, gear steel, or ultra-high-strength steel containing strong carbide-forming elements.

[0055] According to the present invention, in step S1, the carbon distribution relationship is determined by one or more of the following: thermodynamic phase equilibrium calculation, material database, measured total carbon concentration profile, carbon flux by weight gain method, X-ray diffraction analysis, and microstructure analysis.

[0056] According to the present invention, in step S1, the carburizing temperature T is 800~1200℃.

[0057] According to the present invention, the high alloy steel contains at least one element selected from Cr, Mo, V, W and Co, and the total amount of alloying elements is higher than 10 to 40 wt.%.

[0058] According to the present invention, in step S1, the basic input parameters include: steel composition, initial total carbon concentration, density, carburizing temperature, target effective carburizing layer depth, total carbon concentration at the target depth, target total carbon concentration on the surface, upper limit of total carbon concentration on the surface, upper limit of surface carbide mass fraction, carbon flux parameter, and maximum number of pulse cycles.

[0059] According to the present invention, in step S5, during the strong infiltration stage of preparing the high alloy steel, the strong infiltration process is discrete according to time, with each time step being 0.1 to 10 s.

[0060] In this invention, during the strong infiltration stage of preparing the high alloy steel, the time step is the same as the time for synchronously judging the total surface carbon concentration and the surface carbide mass fraction at regular intervals in step S5.

[0061] According to the present invention, in step S5, during the diffusion stage of preparing the high alloy steel, the diffusion process is discretized according to time, with each time step being 1 to 60 s.

[0062] In this invention, during the diffusion stage of preparing the high-alloy steel, the time step is consistent with the periodic updates in step S5 of the total carbon concentration distribution C along the depth direction of the high-alloy steel. tot The time intervals (x, t) are the same.

[0063] According to the present invention, the pressure of the diffusion stage is 0.01 to 100 Pa, and the pressure of the low-pressure carburizing stage is 50 to 1000 Pa.

[0064] The second aspect of the present invention provides high alloy steel prepared by the method for determining the process parameters of the above-mentioned high alloy steel vacuum low-pressure carburizing process.

[0065] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Obviously, the embodiments described herein are only some embodiments of the present invention and are not intended to limit the present invention. All other embodiments implemented by those skilled in the art based on the embodiments of the present invention without creative improvements are within the protection scope of the present invention.

[0066] Example 1

[0067] This embodiment uses a high-alloy steel containing Cr, Mo, V, and Co as the object, with Cr content of 13 wt.%, Mo content of 5 wt.%, V content of 0.6 wt.%, and Co content of 13 wt.%. The diffusion stage pressure is 50 Pa, and the low-pressure carburizing stage pressure is 500 Pa. The true low-pressure carburizing strong carburizing time, diffusion time, and number of pulse cycles are determined as follows:

[0068] Step 1: Obtain the initial information of the high-alloy steel to be processed, including the initial total carbon concentration (C). 0， wt.%) 0.14, Density of steel parts (g / cm³) 3 7.99, Carburizing temperature (T) 950℃, Target effective carburizing layer depth (d) eff , mm)0.5, total carbon concentration at the target depth (C tot, target ,wt.%)0.4, total carbon concentration of surface target (C tot, surface, target 1.1, upper limit of total surface carbon concentration (C wt.%) tot, max ,wt.%)1.7, Upper limit of surface carbide mass fraction (f carbide, max 30, wt.%) Surface carbon flux (J in 6.8×10 -8 g / (mm 2 ×s).

[0069] The carbon distribution relationship of high-alloy steel at a carburizing temperature of 950℃ was established. Specifically, a set of different total carbon concentrations C was set. tot Calculate the austenite solid solution carbon concentration C corresponding to each total carbon concentration.γ and carbide mass fraction f carbid The resulting data is shown in Table 1.

[0070] Table 1. Austenite carbon solution concentration and carbide mass fraction at different total carbon concentrations

[0071]

[0072] Step 2: Based on the carbon distribution relationship, calculate the rate of change of austenite solid solution carbon concentration with respect to total carbon concentration:

[0073] ;

[0074] Step 3: Calculate the basic diffusion coefficient of austenite:

[0075] ;

[0076] Calculate the apparent diffusion coefficient with total carbon concentration as the main variable:

[0077] ;

[0078] Step 4: Establish a one-dimensional computational grid in the spatial direction and initialize the total carbon concentration distribution:

[0079] C tot (x, 0) = C0;

[0080] Where (x=0) is the surface of the steel part, (x=L) is the maximum length of the far end of the computational domain, i.e., the depth direction, and L is 5mm.

[0081] Step 5: During the strong infiltration stage, apply boundary conditions based on the given total surface carbon flux:

[0082] ;

[0083] Solve the following diffusion equation:

[0084] ;

[0085] At each time step, the total surface carbon concentration, the surface austenite solid solution carbon concentration, and the surface carbide mass fraction are calculated. The current strong penetration stage is stopped when any of the following conditions are met:

[0086] ;

[0087] During the diffusion phase, the input of carbon-containing gas is stopped, and a boundary condition with no carbon inflow into the surface is adopted:

[0088] ;

[0089] At each time step, when the total surface carbon concentration reaches the set target range, the total carbon concentration at the target depth meets the requirements, and the surface carbide mass fraction does not exceed the upper limit, the pulse cycle ends; otherwise, the next strong infiltration-diffusion cycle begins.

[0090] Output the vacuum low-pressure carburizing process parameters for high-alloy steel, including the strong carburizing time, diffusion time, number of pulse cycles, total carburizing time for each pulse cycle, and the total carbon concentration distribution, austenite solid solution carbon concentration distribution, carbide mass fraction distribution, and apparent diffusion coefficient distribution at the end of the treatment. Curves showing the total carbon concentration, austenite solid solution carbon concentration, and measured carbon concentration are as follows: Figure 2 As shown.

[0091] from Figure 2 It can be seen that the total carbon concentration is extremely high at the material surface, reaching approximately 1.1 wt.%, and then drops sharply at a very shallow depth, with the concentration falling to near the target level at 0.4 mm. In deeper layers (beyond 3 mm), the total carbon concentration stabilizes at around 0.14 wt.%. Meanwhile, the carbon concentration in austenite remains at a consistently low level. Figure 3 The curve reflects the distribution of carbide mass fraction along depth in the material. Carbide is highly concentrated in the outermost layer, with the highest surface fraction at approximately 0.18. As the depth increases to about 1 mm, the carbide fraction rapidly decreases and approaches zero. This indicates that the process primarily resulted in carbide precipitation in the outermost region. Figure 4 The time composition of the 12 process pulse cycles is visually illustrated using a bar chart. It can be seen that the "boost time" (orange bar) of each cycle is very short, ranging from 9-15 seconds, and remains constant in the later stages; while the "diffusion time" (green bar) shows a significant linear increase as the pulse cycle progresses, increasing from approximately 500 seconds initially to nearly 6000 seconds in the later stages. This indicates that in the later stages of the process, the depth distribution of carbon is mainly controlled through a long diffusion phase.

[0092] Comparative Example 1

[0093] Using a high-alloy steel containing Cr, Mo, V, and Co as the research object, with Cr content of 13 wt.%, Mo content of 5 wt.%, V content of 0.6 wt.%, and Co content of 13 wt.%, the vacuum low-pressure carburizing strong carburizing time, diffusion time, and number of pulse cycles were determined using conventional vacuum carburizing simulation methods.

[0094] Step 1: Obtain the initial information of the high-alloy steel to be processed, including the initial total carbon concentration (C0, wt.%) 0.14, and the density of the steel (g / cm³). 3 7.99, Carburizing temperature (T) 950℃, Target effective carburizing layer depth (d) eff1.6 mm, total carbon concentration at target depth (C) tot, target ,wt.%)0.4, Surface target carbon concentration (C tot, surface, target ,wt.%)0.9, Surface carbon flux (J in 6.8×10 -8 g / (mm 2 ×s);

[0095] Step 2: Calculate the basic diffusion coefficient of austenite:

[0096] ;

[0097] Step 3: Establish a one-dimensional computational grid in the spatial direction and initialize the total carbon concentration distribution:

[0098] C(x, 0) = C0;

[0099] Where (x=0) represents the surface of the steel part, (x=L) represents the far end of the computational domain, and L is 3mm;

[0100] Step 4: During the strong infiltration stage, a boundary condition with a given total surface carbon flux is applied:

[0101] ;

[0102] Solve the following diffusion equation:

[0103] ;

[0104] The surface austenite solid solution carbon concentration is calculated at each time step. When the surface carbon concentration reaches the austenite saturation carbon concentration, the current strong infiltration stage is stopped.

[0105] During the diffusion phase, the input of carbon-containing gas is stopped, and a boundary condition with no carbon inflow into the surface is adopted:

[0106] ;

[0107] At each time step, when the surface carbon concentration reaches the set target range and the total carbon concentration at the target depth meets the requirements, the pulse cycle ends; otherwise, the next strong infiltration-diffusion cycle begins.

[0108] Output the vacuum low-pressure carburizing process parameters for high-alloy steel, including the strong carburizing time, diffusion time, number of pulse cycles, total carburizing time for each pulse cycle, and the carbon concentration distribution at the end of the process. Compare the predicted values ​​with the measured values. Figure 5 The graph shows a comparison between the calculated and measured carbon concentrations. The calculated carbon concentration distribution differs significantly from the actual carbon concentration distribution.

[0109] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for determining the process parameters of vacuum low-pressure carburizing of high alloy steel, characterized in that, Includes the following steps: S1. Establish the carbon distribution relationship of high alloy steel at carburizing temperature T based on the basic input parameters. The carbon distribution relationship includes the total carbon concentration. Ctot austenite solid solution carbon concentration C γ and carbide mass fraction fcarbide The correspondence between them; S2. Calculate the rate of change of austenite solid solution carbon concentration with respect to total carbon concentration. ; S3. Based on the austenite basic diffusion coefficient, the aforementioned rate of change, and the apparent diffusion correction coefficient, construct a system based on total carbon concentration. Ctot The equation for the apparent diffusion coefficient; S4, based on total carbon concentration Ctot To solve for the variables, a vacuum low-pressure carburizing diffusion equation for high alloy steel was established and solved. S5. In the strong infiltration stage of preparing the high alloy steel, carbon flux boundary conditions are used to characterize the carbon input process of carbon-containing atmosphere to the surface of high alloy steel. The total carbon concentration and the mass fraction of surface carbides are judged synchronously at regular intervals. When the total carbon concentration reaches the upper limit of the total carbon concentration or the mass fraction of surface carbides reaches the upper limit of the mass fraction of surface carbides, the strong infiltration stage is stopped and the diffusion stage is carried out. That is, when x is 0, the total carbon concentration Ctot Reaching the upper limit of total surface carbon concentration, or carbide mass fraction. fcarbide When the upper limit of the surface carbide mass fraction is reached, the strong infiltration stage is stopped and the diffusion stage begins. During the diffusion stage of the high-alloy steel preparation, a boundary condition with no carbon inflow to the surface is used to characterize the state where the surface no longer receives external carbon sources after the input of carbon-containing gas is stopped. The total carbon concentration distribution C along the depth direction of the high-alloy steel is updated at regular intervals. tot (x,t) is used to extract the total carbon concentration on the surface and the total carbon concentration at the target effective infiltration depth. When the total carbon concentration at the target effective infiltration depth reaches the requirement for the total carbon concentration at the target effective infiltration depth, and the total carbon concentration on the surface reaches the range of the target total carbon concentration on the surface, and the mass fraction of surface carbides does not exceed the upper limit of the mass fraction of surface carbides, the process parameters are output. Otherwise, the process is switched to the strong infiltration stage to carry out the next round of strong infiltration-diffusion cycle. The process parameters include strong carburizing time, diffusion time, number of pulse cycles, total carburizing time, total carbon concentration distribution, austenite solid solution carbon concentration distribution, carbide mass fraction distribution, and apparent diffusion coefficient distribution; In step S1, the total carbon concentration Ctot austenite solid solution carbon concentration C γ and carbide mass fraction fcarbide The correspondence between them is as follows: For any location at the total carbon concentration Ctot,i With total carbon concentration Ctot,i+1 Total carbon concentration, austenite solid solution carbon concentration C γ and carbide mass fraction fcarbide They are represented as follows: （1）； （2）； In equation (1), C γ,i The concentration of dissolved carbon in austenite at the i-th time point is expressed in wt.%. C γ,i+1 The concentration of austenite solid solution carbon at the (i+1)th time node is expressed in wt.%, where Equation (2) is... fi Let be the mass fraction of carbides corresponding to the i-th time node of total carbon concentration. fi+1 This represents the carbide mass fraction corresponding to the (i+1)th time node of total carbon concentration. In step S1, the total carbon concentration Ctot The total carbon concentration is expressed in wt.%; In step S3, the step based on total carbon concentration Ctot The equation for the apparent diffusion coefficient is: （3）； （4）； In equation (3), Here, is the basic diffusion coefficient of austenite, and is the basic diffusion coefficient of carbon in austenite, in mm. 2 ·s -1 , The frequency factor of the carbon diffusion coefficient in austenite, in mm. 2 ·s -1 R is the molar gas constant, taken as 1.987 cal / (mol⋅K); In equation (4), The apparent diffusion coefficient is described based on the total carbon concentration and is expressed in mm. 2 ·s -1 , This is the apparent diffusion correction factor; To correct the discrepancy between the ideal austenite diffusion model and the actual carbon migration behavior of high alloy steel, it can be calibrated by experimental carbon concentration gradient; In step S4, the vacuum low-pressure carburizing diffusion equation for the high-alloy steel is: （5）； In equation (5), x is the depth of the test site from the surface of the high alloy steel, and t is the carburizing time.

2. The method for determining the process parameters of vacuum low-pressure carburizing of high-alloy steel according to claim 1, characterized in that, In step S1, the carbon distribution relationship is determined by one or more of the following: thermodynamic phase equilibrium calculation, material database, measured total carbon concentration profile, carbon flux by weight gain method, X-ray diffraction analysis, and microstructure analysis.

3. The method for determining the process parameters of vacuum low-pressure carburizing of high alloy steel according to claim 1, characterized in that, In step S1, the carburizing temperature T is 800~1200℃.

4. The method for determining the process parameters of vacuum low-pressure carburizing of high alloy steel according to claim 1, characterized in that, The high alloy steel contains at least one element selected from Cr, Mo, V, W, and Co, and the total amount of alloying elements is 10 to 40 wt.%.

5. The method for determining the process parameters of vacuum low-pressure carburizing of high alloy steel according to claim 1, characterized in that, In step S1, the basic input parameters include: steel composition, initial total carbon concentration, density, carburizing temperature, target effective carburizing layer depth, total carbon concentration at the target depth, target total carbon concentration on the surface, upper limit of total carbon concentration on the surface, upper limit of surface carbide mass fraction, carbon flux parameters, and maximum number of pulse cycles.

6. The method for determining the process parameters of vacuum low-pressure carburizing of high alloy steel according to claim 1, characterized in that, In step S5, during the strong infiltration stage of preparing the high alloy steel, the strong infiltration process is discretized according to time, with each time step being 0.1~10s.

7. The method for determining the process parameters of vacuum low-pressure carburizing of high alloy steel according to claim 1, characterized in that, In step S5, during the diffusion stage of preparing the high alloy steel, the diffusion process is discretized over time, with each time step lasting 1 to 60 seconds.

8. The method for determining the process parameters of vacuum low-pressure carburizing of high alloy steel according to claim 1, characterized in that, The pressure during the diffusion stage is 0.01–100 Pa, and the pressure during the low-pressure carburizing stage is 50–1000 Pa.

9. High alloy steel prepared by the method for determining the process parameters of vacuum low-pressure carburizing of high alloy steel according to any one of claims 1-8.

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