CN110263406B - Heat treatment method and optimization method for ultra-large module gear under low-speed heavy load - Google Patents
Heat treatment method and optimization method for ultra-large module gear under low-speed heavy load Download PDFInfo
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Abstract
The invention discloses a heat treatment method and an optimization method for an ultra-large module gear under low-speed heavy load. The method solves the phase balance, the thermophysical and physical properties, the mechanical properties and the phase transformation of the steady state and the metastable state, more accurately predicts the physical and thermophysical properties of the alloy material in the heating and cooling stages, then predicts the material property change of the 18CrNiMo7-6 low-carbon alloy steel in the heating and cooling processes, and designs a reasonable carburizing and quenching heat treatment process flow so as to effectively improve the hardness of the gear.
Description
Technical Field
The method relates to the field of casting, in particular to a heat treatment method and an optimization method for an ultra-large module gear under low-speed heavy load.
Background
The control of the heat treatment process of the high-speed heavy-duty gear and the effect generated by the heat treatment are directly related to the quality of gear parts, and the service life of the gear parts is determined. The carburization and quenching surface hardening technology is generally adopted by the industry nowadays to improve the mechanical properties, and the numerical calculation process of the carburization and quenching heat treatment is relatively complicated, so the complexity is generated mainly because the heat treatment process is controlled by various variables, and the whole technology is slowly developed. The main problems faced by the heat treatment numerical calculation are the establishment of a related material database, the development of numerical simulation software, simulation result verification and the like, the cooling rate of quenching has a great influence on the hardness of the gear, but the component proportion of each gear is different, the optimal cooling rate is also different, and a method for controlling the cooling rate according to different component proportions does not exist at present, and the invention aims to solve the problems.
DEFORM-3D software: DEFORM is a finite element based process simulation system for analyzing metal forming and its associated various forming and heat treatment processes. The two decades of industrial practices prove that DEFORM based on a finite element method has excellent accuracy and stability, a simulation engine conforms to actual production in the aspects of large flow, stroke load, product defect prediction and the like, the exclamatory precision DEFORM-3D is kept to be comprehensively modeled, formed, thermally conducted, formed equipment characteristics and the like in the same integrated environment, the method is mainly used for analyzing the flow condition of three-dimensional materials in the forming process of various complex metals, and the method is suitable for hot, cold and warm forming and provides valuable process analysis data.
Jmatpro software: a powerful material performance simulation Software developed by Sente Software company can solve the phase balance, solidification performance, thermophysical and physical properties, mechanical properties and phase transformation of a steady state and a metastable state, and can accurately predict the physical and thermophysical properties of an alloy material in the heating and cooling stages.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a heat treatment method of an ultra-large module gear under low-speed heavy load and an optimization method thereof. The method solves the phase balance, the thermophysical and physical properties, the mechanical properties and the phase transformation of the steady state and the metastable state, more accurately predicts the physical and thermophysical properties of the alloy material in the heating and cooling stages, then predicts the material property change of the 18CrNiMo7-6 low-carbon alloy steel in the heating and cooling processes, and designs a reasonable carburizing and quenching heat treatment process flow so as to effectively improve the hardness of the gear.
The technical scheme adopted by the invention for solving the technical problems is as follows:
a heat treatment optimization method for an ultra-large module gear under low-speed heavy load comprises the following steps:
step one, calculating the proportion of each phase in the non-carburized gear at each temperature when each phase is balanced and the alloy component of each phase;
calculating to obtain the correlation performance of each phase;
step three, calculating the overall performance of each related performance of the gear;
inputting the chemical components and proportion of the non-carburized gear, the relevant performance of each phase and the overall performance of the relevant performance of the gear into DEFORM-3D software to generate a material performance model; heating the gear to austenitize, setting ferrite and pearlite as initial phases before heat treatment of the gear, starting transformation of the ferrite and the pearlite into austenite in an austenitizing stage, and homogenizing the carbon content on the surface of the gear in an austenitizing stage;
step five, inputting the set carburizing process into DEFORM-3D software to calculate the carbon flux C after the gear is carburized A ；
Step six, DEFORM-3D software converts carbon flux C A Inputting a material performance model, and then inputting a phase change model into DEFORM-3D software for simulation to obtain corresponding volume ratios of ferrite, martensite, bainite and pearlite at different temperatures;
step seven, calculating to obtain the composite structure hardness H of ferrite, martensite, bainite and pearlite under different volume ratios T ；
Step eight, obtaining H T The proportion of each phase when the value is the maximum value or the set value;
step nine, selecting H T JmatPro software is input into each phase when the value is the maximum value or the set value, and each phase is obtained so that H T The maximum value or the set value is the cooling curve.
In a further improvement, the first step includes the following steps: the chemical composition and the ratio of the manufactured gear were input into the JmatPro software, and the ratio of each phase at equilibrium in the gear and the alloy composition of each phase at each temperature were determined.
In a further improvement, in the second step, the relevant properties include expansion coefficient, thermal conductivity, young's modulus, poisson's ratio, specific heat capacity, and transformation plasticity coefficient.
In a further improvement, in the third step, the method for calculating the overall performance of each related performance of the gear is as follows:
P 1 ＝x α P α1 +x β P β1 +......+F S P I1 (1.14)
P 2 ＝x α P α2 +x β P β2 +......+F S P I2 (1.15)
P 3 ＝x α P α3 +x β P β3 +......+F S P I3 (1.16)
P 4 ＝x α P α4 +x β P β4 +......+F S P I4 (1.17)
P 5 ＝x α P α5 +x β P β5 +......+F S P I5 (1.18)
P 6 ＝x α P α6 +x β P β6 +......+F S P I6 (1.19)
wherein, the alpha, beta … is the phase structure of the 18CrNiMo7-6 material under the set temperature; x is the number of α ,x β … is alpha, beta … phase structure accounts for the mass fraction of 18CrNiMo7-6 material, and the sum of the two is 1; p is 1 、P 2 、P 3 、P 4 、P 5 、P 6 The overall properties of the material are respectively expansion coefficient, thermal conductivity, young modulus, poisson ratio, specific heat capacity and density; p α1 、P β1 Expansion coefficients for the alpha, beta phases; p α2 、P β2 Thermal conductivity divided into alpha, beta phases; p α3 、P β3 Young's modulus divided into alpha, beta phases; p α4 、P β4 Poisson's ratio divided into alpha, beta phases; p is α5 、P β5 Specific heat capacity divided into alpha, beta phases; p α6 、P β6 Density divided into alpha, beta phases; f S Is the degree of tissue dispersion; p I1 、P I2 、P I3 、P I4 、P I5 、P I6 The properties of the expansion coefficient, the thermal conductivity, the Young modulus, the Poisson ratio, the specific heat capacity and the density related to the tissue topological structure; p I1 、P I2 、P I3 、P I4 、P I5 、P I6 All calculated by software JmatPro.
In a further improvement, the fourth step includes the following steps:
introducing the overall performance of each related performance into DEFORM-3D software as a material performance model, heating the gear to austenitize, setting the initial phase as ferrite and pearlite before heat treatment of the gear, and setting the volume fractions as 0.75 and 0.25 respectively; in the austenitizing stage, ferrite and pearlite begin to be transformed into austenite, the carbon content of the gear surface in the austenitizing stage is homogenized to be 0.2, and the phase transformation is calculated by using a formula (1.7);
in the formula: t is a unit of s Lower critical point temperature, T, for phase transition e Is the upper limit temperature of the phase transition, beta A And T is the gear temperature.
In a further improvement, in step five, the carbon flux is C A Solving by Fick law:
wherein s is carbon solubility; d e The diffusion quantity of carbon element on the surface of the workpiece in the carburizing process; φ is the carbon diffusion rate; p is the surface pressure; x is the carburization direction;
is the unit carbon diffusivity in the x-direction; t is the temperature;
is the pressure difference in the x direction; k s Is the influence factor of temperature on the diffusion rate; k is p Is the influence factor of pressure on the diffusion rate;
K p and K s Is determined by:
in the formula, K s The factor of the temperature on the diffusion rate, K p Is the influence factor of pressure on the diffusion rate; c. s is the external carbon concentration, carbon solubility, respectively; t is the surface temperature of the gear in the carburizing process;
is the rate of change of carbon solubility with temperature;
is the rate of change of carbon solubility with surface pressure.
In a further improvement, in the sixth step, the phase change model is as follows:
the volume fractions of ferrite, pearlite and bainite were solved using JMAK equation (1.9).
In the formula: beta is a beta F Represents a ferrite volume fraction; beta is a beta P Represents the volume fraction of pearlite; beta is a B Represents the volume fraction of bainite; t is t F A transformation time that is the ferrite transformation temperature; t is t B A transformation time that is the bainite transformation temperature; t is t P The transformation time for pearlite at the transformation temperature; k is a radical of F 、n F Is a ferrite material parameter; k is a radical of formula B 、n B Taking bainite material parameters; k is a radical of P 、n P Is a pearlite material parameter, and k and n are material parameters related to phase transformation temperature, parent phase composition and grain size respectively; values of cooling stages k and nAs shown in table 1.1.
Table 1.1: values of k and n
Wherein: in Table 1.1 and formulae (1.9), (1.10), (1.11), B s The bainite onset transition temperature; c A Carbon content in untransformed austenite;
respectively austenite isothermal transformation temperatures; AGS is austenite grain size; t is the gear temperature;
the martensitic transformation is a non-diffusion phase transformation and depends only on the temperature change, and is solved using equation (1.12).
In the formula: m is a group of s A martensite start transition temperature; beta is a M Is the volume fraction of martensite; beta is a F Is ferrite volume mass fraction; beta is a P Is the volume fraction of pearlite; beta is a beta B Is bainite volume fraction; t is the gear surface temperature.
In a further improvement, the seventh step includes the following steps:
calculating the tissue hardness of the material according to the mixing law of the composite material, assuming that the material is isotropic, firstly calculating tissue components and volume fractions at different temperature moments, and then solving and calculating a hardness value by using a weighted average value to obtain the composite tissue hardness of the material;
H T ＝H P ·ω P +H B ·ω B +H A ·ω A +H F ·ω F +H M ·ω M (1.26)
table 1.2: specific value of hardness of each phase
Formula (1.14) and table 1.2: h T 、H P 、H B 、H A 、H F And H M Respectively, the composite structure hardness, pearlite hardness, bainite hardness, austenite hardness, ferrite hardness and martensite hardness, omega P 、ω B 、ω A 、ω F 、ω M Pearlite volume fraction, bainite volume fraction, austenite volume fraction, ferrite volume fraction, and martensite volume fraction, respectively;
a heat treatment method for an ultra-large module gear under low-speed heavy load comprises the following steps:
step one, a gear made of 8CrNiMo7-6 low-carbon alloy steel is placed into a continuous gas carburizing furnace to be heated, the furnace temperature is set to be 880 ℃, and the gear is heated for 1 hour; the size parameters of the gear are as follows: the modulus is 5mm, the number of teeth is 25, the pressure angle is 20 °, and the helix angle is 16 °.
Step two, introducing carburizing gas into a carburizing furnace, controlling the external carbon potential to be 1%, and keeping for 3 hours; keeping the temperature at 880 ℃ for 3 hours;
step three, strong infiltration: heating the carburizing furnace to 920 ℃, and then keeping the temperature of the gear in the environment with the carbon potential of 1.15% for 2.5 hours;
step four, diffusion: the temperature and the carbon potential are respectively reduced to 900 ℃ and 0.9%, and the temperature is kept for 2 hours.
Step five, adopting a precooling quenching process route: reducing the furnace temperature to 830-870 ℃, reducing the internal stress and distortion generated by quenching, then stopping introducing carburizing gas, reducing the furnace temperature to 830-870 ℃, and preserving the temperature for 600s; step six, primary quenching: the gear is immersed in water and the water is stirred, the water and the tooth surface carry out heat convection, and the gear is cooled to the room temperature of 30 degrees.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2a, FIG. 2b, FIG. 2c, FIG. 2d, FIG. 2e and FIG. 2f are graphs of the expansion coefficient, thermal conductivity, young's modulus, poisson's ratio, specific heat capacity and phase change plasticity coefficient, respectively, of each phase;
FIG. 3 is a TTT graph of tissue transition;
fig. 4 is a graph showing the temperature decrease curve obtained in the present embodiment.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The material property of the gear is as follows:
finite element analysis sets for the whole gear to satisfy the heat conduction equation, discretizes the whole space domain, divides into finite unit, and every unit satisfies the heat conduction equation simultaneously. The unit is composed of a plurality of nodes, the temperature of each point in the unit is obtained by the product of the node temperature and the shape function, and the whole temperature field can be represented by the node temperature. The model DEFORM-3D software has a special Heat Treatment module (Heat Treatment), and is widely applied to simulating the Heat Treatment of workpieces.
A standard helical gear with the modulus of 5mm, the tooth number of 25, the pressure angle of 20 degrees and the helix angle of 16 degrees is used as a research object, 18CrNiMo7-6 is selected as a gear material, and the chemical components are shown in Table 9.1.
TABLE 9.1 chemical composition of 18CrNiMo7-6 low carbon alloy steel
The helical gear is made of low-carbon alloy steel 18CrNiMo7-6, JMatPro is powerful material performance simulation Software developed by Sente Software company, and can solve phase balance, solidification performance, thermophysical and physical performance, mechanical performance and phase transition of a stable state and a metastable state, and accurately predict physical and thermophysical performances of the alloy material in the heating and cooling stages, so that JMatPro is used for predicting material performance change of the low-carbon alloy steel 18CrNiMo7-6 in the heating and cooling process.
Dynamics calculation
The heat treatment multi-field coupling numerical calculation relates to material phase change, different phases have different Ji Pusi free energy, and according to the thermodynamic principle, the system can reach the balance at constant temperature and constant pressure under the general conditions that: (1) The total Gibbs free energy G of the system reaches a minimum value G min (ii) a (2) The chemical potentials of component i in each phase are equal, so the Ji Pusi free energy for each phase is calculated as:
in the formula, X i Is the mass fraction of the component i,
the sum of Gibbs free energy of pure components represents the Gibbs free energy obtained by pure mechanical mixing;
free energy increase due to ideal mixing entropy; omega v Is the coefficient of interaction (v is 0-1);
excess free energy from ideal solutions.
The primary phase before the heat treatment of the low-carbon alloy steel is mainly ferrite and pearlite, and after the low-carbon alloy steel is heated to the austenite transformation temperature, the primary phase is gradually transformed into austenite. FIG. 1 shows the phase composition of 18CrNiMo7-6 low-carbon alloy steel at different temperatures.
Physical/thermophysical performance calculations
The heat treatment multi-field coupling numerical calculation relates to material phase change, the thermophysical property of the material is not only changed along with the temperature, but also the phase composition determines the property change of the material, and the physical/thermophysical property of the material is influenced by the difference of the structure composition.
The thermal physical properties of the material are determined by a composite material mixing law, namely the comprehensive thermal physical properties of the material are calculated by a weighted average method according to the thermal physical properties and components of a composition phase, and the thermal physical properties of 18CrNiMo7-6 are obtained by JMatPro.
The alloy composition of each phase is first obtained, and based on the alloy composition of each phase, the formula for calculating the relevant properties of the phase is as follows:
in the formula, P α1 、P α2 、P α3 、P α4 、P α5 、P α6 The coefficient of expansion, thermal conductivity, young modulus, poisson ratio, specific heat capacity and transformation plasticity coefficient of the material are respectively; i is any one component of different phases, X i Is the mass fraction of i component, P i 0 Is the property of the i component(s),
is the sum of the properties of pure components, and represents the properties obtained by pure mechanical mixing; omega v Is the coefficient of interaction (v is 0-1);
excess performance from the ideal solution.
The thermophysical properties of the phases obtained by JMatPro are shown in FIGS. 2 a), 2 b), 2c, 2d, 2e, 2f,
Calculating the overall performance of the material by using a mixing law according to the phase composition and the performance of each phase of the material, wherein the following formula is a calculation formula:
P 1 ＝x α P α1 +x β P β1 +......+F S P I1
P 3 ＝x α P α3 +x β P β3 +......+F S P I3
P 2 ＝x α P α2 +x β P β2 +......+F S P I2
P 4 ＝x α P α4 +x β P β4 +......+F S P I4
P 5 ＝x α P α5 +x β P β5 +......+F S P I5
P 6 ＝x α P α6 +x β P β6 +......+F S P I6
wherein, the alpha, beta … is the phase structure of the 18CrNiMo7-6 material at the temperature; x is the number of α ,x β … are respectively alpha, beta … phase structure accounts for the mass fraction of 18CrNiMo7-6 material, and the sum of the alpha phase structure and the beta … phase structure is 1; p 1 、P 2 、P 3 、P 4 、P 5 、P 6 The overall properties of the material are respectively expansion coefficient, thermal conductivity, young modulus, poisson ratio, specific heat capacity and density; p α1 、P β1 Expansion coefficients for the alpha, beta phases;P α2 、P β2 thermal conductivity divided into alpha, beta phases; p is α3 、P β3 Young's modulus divided into alpha, beta phases; p α4 、P β4 Poisson's ratio divided into alpha, beta phases; p α5 、P β5 Specific heat capacity divided into alpha, beta phases; p α6 、P β6 Density divided into alpha, beta phases; f S Is the degree of tissue dispersion; p is I1 、P I2 、P I3 、P I4 、P I5 、P I6 The properties of the expansion coefficient, the thermal conductivity, the Young modulus, the Poisson ratio, the specific heat capacity and the density related to the tissue topological structure; f S P I1......6 Reflecting the influence of the tissue morphology on the material performance.
Diffusion model:
the carbon flux on the surface of the workpiece depends on two parameters of temperature and pressure, and if the external conditions are kept unchanged, the carbon flux value on the surface of the workpiece is kept unchanged in the carburizing process.
Carbon flux C according to external temperature and pressure A Solving by using Fick law:
where φ, c, p, and s are the carbon diffusion rate, external carbon concentration, surface pressure, and carbon solubility, respectively.
The effect of carburization temperature and pressure on carbon diffusion is shown in equation (9.12).
In the formula, K s The factor of the temperature on the diffusion rate, K p Is the influence factor of pressure on the diffusion rate.
Through the above steps, the process of carburizing can be determined. Therefore, the carburization process can be obtained by back-stepping according to the required carbon flux, and specifically comprises the following steps: the gear carburizing process for the patent comprises the following steps: introducing carburizing gas into a carburizing furnace, controlling the external carbon potential to be 1%, and keeping the external carbon potential for 3 hours; the temperature is 880 ℃ for 3 hours.
The volume of each phase at different temperatures is determined by the following formula:
tissue transformation model (phase transformation model)
Before the heat treatment is started, the helical gear primary phase is set as ferrite and pearlite and the volume fractions are set as 0.75 and 0.25, respectively, with reference to the element distribution and the microstructure distribution of the low-carbon alloy steel with similar material properties. The processes of transforming ferrite and pearlite into austenite and transforming austenite into bainite, pearlite and ferrite are all diffusion phase transformation, and in the heating stage, when the temperature exceeds the lower critical point A c1 At this point, the phases will begin to transform to austenite. In the heating stage, the carbon content is equal in different parts, so that the kinetic parameters of ferrite and pearlite transformation into austenite are functions only related to the temperature, and the transformation of ferrite and pearlite into austenite is calculated by using the following formula.
In the formula: beta is a A Is the volume fraction of austenite, T s Is the lower critical point temperature, T e The upper limit temperature, T, is the gear temperature. During austenite transformation, the phase volume of each component changes along with the temperature change, and the latent heat of transformation released at different transformation temperatures also differs.
The tissue transformation in the cooling stage was calculated using the tissue transformation point obtained by the Jmatpro method and the following formula, and the tissue transformation points obtained according to Jmatpro are shown in Table 9.2.
Table 9.2: tissue transformation point of cooling phase
For non-diffusive phase transformations that depend only on temperature changes, the martensitic transformation is solved using equation (9.6).
In the formula: m s Starting transformation temperature for martensite
In the cooling stage, due to the heterogeneous distribution of carbon concentration, the transformation power of austenite is different from that of the formula, and in the isothermal transformation process of austenite, the volume fractions of ferrite, pearlite and bainite are solved by using a JMAK equation.
β＝1-exp(-kt n )
Table 9.3: values of cooling stages k and n
In Table 9.3, C A 、
Respectively the carbon content in the non-transformed austenite and the austenite isothermal transformation temperature, B s AGS and T are the bainite transformation start temperature, austenite grain size and temperature, respectively.
The tissue hardness is calculated according to the composite material mixing law, the hardness change is set to be isotropic, and the weighted average value is calculated according to the volume fraction of each phase and the hardness value of each phase. On the other hand, according to the previous research, the martensite hardness is related to the carbon content of the martensite hardness, and the mathematical formula of the martensite hardness HB under the conditions of different carbon contents is determined; the martensite hardnesses were 44.87 and 65.25, respectively, when the carbon contents were 0.25 and 0.85, and the other structures were set to constant values, and specific values of the respective phase hardnesses are shown in table 9.5.
H T ＝H P ·ω P +H B ·ω B +H A ·ω A +H F ·ω F +H M ·ω M (1.1)
Formula (9.10) and table 9.4: h T 、H P 、H B 、H A 、H F And H M Respectively, the composite structure hardness, the pearlite hardness, the bainite hardness, the austenite hardness, the ferrite hardness and the martensite hardness, omega P 、ω B 、ω A 、ω F 、ω M Pearlite volume fraction, bainite volume fraction, austenite volume fraction, ferrite volume fraction, and martensite volume fraction, respectively.
Table 9.4: specific value of hardness of each phase
To obtain H T The proportion of each phase when the value is the maximum value or the set value;
selection of H T JmatPro software is input into each phase when the value is the maximum value or the set value, and each phase is obtained so that H T The cooling curve at the maximum or set point is shown in fig. 4.
The above embodiment is only one specific embodiment of the present invention, and is not meant to be a limitation of the present invention, and simple substitutions made by the present invention are within the scope of the present invention.
Claims (9)
1. A heat treatment optimization method for an ultra-large module gear under low-speed heavy load is characterized by comprising the following steps:
step one, calculating the proportion of each phase in the non-carburized gear at each temperature when each phase is balanced and the alloy component of each phase;
step two, calculating to obtain the correlation performance of each phase;
step three, calculating the overall performance of each related performance of the gear;
inputting the chemical components and proportion of the non-carburized gear, the relevant performance of each phase and the overall performance of the relevant performance of the gear into DEFORM-3D software to generate a material performance model; heating the gear to austenitize, setting ferrite and pearlite as initial phases before heat treatment of the gear, starting transformation of the ferrite and the pearlite into austenite in an austenitizing stage, and homogenizing the carbon content on the surface of the gear in an austenitizing stage;
step five, inputting the set carburizing process into DEFORM-3D software to calculate the carbon flux C after the gear is carburized A ；
Step six, DEFORM-3D software converts carbon flux C A Inputting a material performance model, and then inputting a phase change model into DEFORM-3D software for simulation to obtain corresponding volume ratios of ferrite, martensite, bainite and pearlite at different temperatures;
step seven, calculating to obtain the composite structure hardness H of ferrite, martensite, bainite and pearlite under different volume ratios T ；
Step eight, obtaining H T The proportion of each phase when the value is the maximum value or the set value;
step nine, selecting H T JmatPro software is input into each phase when the value is the maximum value or the set value, and each phase is obtained so that H T The maximum value or the set value is the cooling curve.
2. The method for optimizing the heat treatment of the extra-large module gear under the low speed and heavy load as claimed in claim 1, wherein the step one comprises the following steps: the chemical composition and the ratio of the manufactured gear were input into the JmatPro software, and the ratio of each phase at equilibrium in the gear and the alloy composition of each phase at each temperature were determined.
3. The method for optimizing heat treatment of a super large module gear under low speed and heavy load as claimed in claim 1, wherein in the second step, the related properties include expansion coefficient, thermal conductivity, young's modulus, poisson's ratio, specific heat capacity, and transformation plasticity coefficient.
4. The method for optimizing heat treatment of a super-large module gear under low speed and heavy load as claimed in claim 1, wherein in the third step, the method for calculating the overall performance of each related performance of the gear is as follows:
P 1 ＝x α P α1 +x β P β1 +......+F S P I1 (1.1)
P 2 ＝x α P α2 +x β P β2 +......+F S P I2 (1.2)
P 3 ＝x α P α3 +x β P β3 +......+F S P I3 (1.3)
P 4 ＝x α P α4 +x β P β4 +......+F S P I4 (1.4)
P 5 ＝x α P α5 +x β P β5 +......+F S P I5 (1.5)
P 6 ＝x α P α6 +x β P β6 +......+F S P I6 (1.6)
wherein, the alpha, beta … is the phase structure of the 18CrNiMo7-6 material under the set temperature; x is the number of α ,x β … is alpha, beta … phase structure accounts for the mass fraction of 18CrNiMo7-6 material, and the sum of the two is 1; p 1 、P 2 、P 3 、P 4 、P 5 、P 6 The overall properties of the material are respectively expansion coefficient, thermal conductivity, young modulus, poisson ratio, specific heat capacity and density; p is α1 、P β1 Expansion coefficients for the alpha, beta phases; p is α2 、P β2 Thermal conductivity divided into alpha, beta phases; p α3 、P β3 Young's modulus divided into alpha, beta phases; p α4 、P β4 Poisson's ratio divided into alpha, beta phases; p α5 、P β5 Specific heat capacity divided into alpha, beta phases; p α6 、P β6 Density divided into alpha, beta phases; f S Is the degree of tissue dispersion; p I1 、P I2 、P I3 、P I4 、P I5 、P I6 Respectively expansion coefficient, thermal conductivity, young modulus, poisson ratio, specific heat capacity, density and tissue topological junctionA structure-related property; p I1 、P I2 、P I3 、P I4 、P I5 、P I6 Are calculated by software JmatPro.
5. The method for optimizing heat treatment of a very large module gear under low speed and heavy load as claimed in claim 1, wherein said step four comprises the steps of:
introducing the overall performance of each related performance into DEFORM-3D software as a material performance model, heating the gear to austenitize, setting the initial phase as ferrite and pearlite before heat treatment of the gear, and setting the volume fractions as 0.75 and 0.25 respectively; in the austenitizing stage, ferrite and pearlite begin to be transformed into austenite, the carbon content of the gear surface in the austenitizing stage is homogenized to be 0.2, and the phase transformation is calculated by using a formula (1.7);
in the formula: t is a unit of s Lower critical point temperature, T, for phase transition e Is the upper limit temperature of the phase transition, beta A Austenite volume fraction, T is gear temperature.
6. The method for optimizing heat treatment of a very large modulus gear under low speed and heavy load as claimed in claim 1, wherein in said step five, the carbon flux C A Solving by Fick law:
wherein s is carbon solubility; d e The diffusion quantity of carbon element on the surface of the workpiece in the carburizing process; phi is the carbon diffusion rate; p is the surface pressure; x is the carburization direction;
in the x directionSpecific carbon diffusivity; t is the temperature;
is the pressure difference in the x-direction; k s Is the influence factor of temperature on diffusion rate; k p Is the influence factor of pressure on the diffusion rate;
K p and K s Is determined by:
in the formula, K s The factor of the temperature on the diffusion rate, K p Is the influence factor of pressure on the diffusion rate; c. s is the external carbon concentration, carbon solubility, respectively; t is the surface temperature of the gear in the carburizing process;
is the rate of change of carbon solubility with temperature;
is the rate of change of carbon solubility with surface pressure.
7. The method for optimizing the heat treatment of the extra-large module gear under the low speed and heavy load as claimed in claim 1, wherein in the sixth step, the phase change model is as follows:
the volume fractions of ferrite, pearlite and bainite were solved using JMAK equation (1.9).
In the formula: beta is a F Represents the volume fraction of ferrite; beta is a beta P Represents the volume fraction of pearlite; beta is a B Represents the volume fraction of bainite; t is t F A transformation time that is the ferrite transformation temperature; t is t B A transformation time that is the bainite transformation temperature; t is t P The transformation time for pearlite at the transformation temperature; k is a radical of F 、n F Is a ferrite material parameter; k is a radical of B 、n B Taking bainite material parameters; k is a radical of P 、n P Is a pearlite material parameter, and k and n are material parameters related to phase transformation temperature, parent phase composition and grain size respectively; the values of the cooling stages k and n are shown in table 1.1,
table 1.1: values of k and n
Wherein: in Table 1.1 and formulae (1.9), (1.10), (1.11), B s The bainite onset transition temperature; c A Carbon content in unconverted austenite;
respectively austenite isothermal transformation temperatures; AGS is austenite grain size; t is the gear temperature;
the martensitic transformation is a non-diffusion phase transformation and depends only on the temperature change, and is solved using equation (1.12).
In the formula: m s A martensite start transition temperature; beta is a M Is the volume fraction of martensite; beta is a F Is ferrite volume mass fraction; beta is a P Is the volume fraction of pearlite; beta is a B Is bainite volume fraction; t is the gear surface temperature.
8. The method for optimizing the heat treatment of a super large module gear under low speed and heavy load as claimed in claim 1, wherein said seventh step comprises the steps of:
calculating the tissue hardness of the material according to the mixing law of the composite material, assuming that the material is isotropic, firstly calculating tissue components and volume fractions at different temperature moments, and then solving and calculating a hardness value by using a weighted average value to obtain the composite tissue hardness of the material;
H T ＝H P ·ω P +H B ·ω B +H A ·ω A +H F ·ω F +H M ·ω M (1.13)
wherein H T 、H P 、H B 、H A 、H F And H M Respectively, the composite structure hardness, the pearlite hardness, the bainite hardness, the austenite hardness, the ferrite hardness and the martensite hardness, omega P 、ω B 、ω A 、ω F 、ω M Pearlite volume fraction, bainite volume fraction, austenite volume fraction, ferrite volume fraction, and martensite volume fraction, respectively.
9. A heat treatment method for an ultra-large module gear under low-speed heavy load is characterized by comprising the following steps:
step one, a gear made of 8CrNiMo7-6 low-carbon alloy steel is placed into a continuous gas carburizing furnace to be heated, the furnace temperature is set to be 880 ℃, and the gear is heated for 1 hour; the size parameters of the gear are as follows: a modulus of 5mm, a number of teeth of 25, a pressure angle of 20 DEG, a helix angle of 16 DEG,
step two, introducing carburizing gas into the carburizing furnace, controlling the external carbon potential to be 1%, and keeping for 3 hours; keeping the temperature at 880 ℃ for 3 hours;
step three, strong infiltration: heating the carburizing furnace to 920 ℃, and then keeping the temperature of the gear in the environment with the carbon potential of 1.15% for 2.5 hours;
step four, diffusion: the temperature and the carbon potential are respectively reduced to 900 ℃ and 0.9 percent, and the temperature is kept for 2 hours.
Step five, adopting a precooling quenching process route: reducing the furnace temperature to 830-870 ℃, reducing the internal stress and distortion generated by quenching, then stopping introducing carburizing gas, reducing the furnace temperature to 830-870 ℃, and preserving the temperature for 600s;
step six, primary quenching: the gear is immersed in water and the water is stirred, the water and the tooth surface carry out heat convection, and the gear is cooled to the room temperature of 30 degrees.
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| JP2005315703A (en) * | 2004-04-28 | 2005-11-10 | Nippon Steel Corp | Method for predicting material in steel material |
| CN104060081A (en) * | 2014-07-05 | 2014-09-24 | 扬州大学 | Method for preventing heat treatment deformation of carburized gear from being out of tolerance |
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| JP2005315703A (en) * | 2004-04-28 | 2005-11-10 | Nippon Steel Corp | Method for predicting material in steel material |
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