RU2413777C1 - Procedure for thermal treatment of items out of steel and alloys - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: procedure for production of metal items with specified structure condition after thermal treatment consists in laboratory determination of temperature curve for heat treatment of material sample of metal item. This curve should facilitate obtaining specified structure state of item material. Further, the procedure consists in defining intervals of thermal treatment under conditions of following modes of thermal treatment corresponding to obtained temperature curve, in considering cooling effect on surface of item with control of heat flow along surface of item in each interval of heat treatment, and in facilitating compliance of obtained and specified temperature curves by method of iteration in volume of item with consideration of energy release in volume of item during physical and chemical conversion. Heat flow is determined out of expression: Q = [(H'+H₀'-qΔm)-(H+H₀)]/Δt, J/f, where H and H₀ are initial enthalpies of phases, J, H' and H₀' are enthalpies of phases, J, in a time interval, Δt, s; Δm is change of weight of phases, kg, during specified interval of time; q is specific energy release, J/kg.

EFFECT: stabilisation of process effects, minimisation or complete avoiding rejects during production of items with required mechanical properties.

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Description

The invention relates to the field of heat treatment of parts made of metal materials, including those having a complex shape.

A known method of heat treatment of parts of metal materials having a complex shape, in particular rails, including continuous cooling of the head, followed by controlled cooling of the rail profile elements. The rail from rolling heating is cooled to a temperature of 820-870 ° C and cooled in two environments: initially compressed air from the surface of the head for 20-30 s at an air flow rate of 3000-4000 m ³ / h, at an air temperature of 10-25 ° C and pressure of 0.55 MPa, then the head is cooled with a water-air mixture at a water flow rate of 25-30 l / min, a water temperature of 10-30 ° C and a pressure of 0.3-0.4 MPa, while the rail head is cooled, the sole is cooled by a water-air mixture at a water temperature of 10-30 ° C, a flow rate of 6-7 l / min and a pressure of 0.08-0.09 MPa (RU 2280700 [1]). The disadvantage of this method is the limited application, due to the fact that the proposed heat treatment modes are selected experimentally based on the requirements for the microstructure, straightness of the products, mechanical properties and hardness of carbon steel and are suitable only for a specific type of product (rails) and a specific steel grade. In addition, this method is focused on the specific configuration of the cooling system, when changing the design of which (for example, the number, location and throughput of the refrigerant nozzles), most of these digital data will change and turn out to be inapplicable to cooling equipment of another design.

A known method of heat treatment of parts made of metal materials having a complex shape, in particular railway wheels, including heating to an austenitizing temperature, intermittent cooling of the surface layer of the rim, subsequent exposure of the wheel to air and tempering. Cooler consumption in the first 20-40 s increases linearly from 0.001-0.003 l / (cm ² · s) to the optimum value determined by the content (C + 1 / 4Mn), where C and Mn are the percentage of carbon and manganese in steel, respectively ( RU 2140997 [2]).

If the rim of the wheel is subjected to accelerated cooling in this method, the wheel hub will remain unprocessed, as a result of which the normalization process is applied here. All this leads in the zone of the wheel rim to the corresponding changes in structure and strength in the radial direction. Known installations have, as a rule, time control, i.e. the volume and / or pressure and thereby, basically, also the temperature of the coolant are constant, i.e. firmly installed, so that the structure and strength of the wheel rim are exerted during a predetermined cooling time. Temperature measurement or temperature-controlled on-line cooling is not known in this regard. The effect on the desired structural or mechanical properties is unsatisfactory, while the internal stresses in the rim zone of the wheel, if at all possible, are very inaccurate and therefore also unsatisfactory, since it is practically impossible to control the plasticization processes, which decisively determine the internal stresses, in the zone transition between the cooling wheel rim and the hot disk due to the fact that only the wheel rim is controlled, and the structure or internal voltage of the disk is not are read. In addition, the disadvantage of this method is its limited use, due to the fact that the proposed heat treatment modes are selected experimentally based on the requirements for microstructure, mechanical properties and hardness of carbon steel and are suitable only for a specific type of product (wheel), a specific nomenclature of steel grades and construction equipment.

A known method of heat treatment of parts having the form of bodies of revolution, of metallic materials, in particular wheels, rims and similar disks and rings, etc. (RU 2277132 [3]). To obtain products with desired properties, the method involves heating the product to the desired temperature and its subsequent cooling, which is carried out differently for different parts of the volumes or surface of the workpiece so that the specified areas of the part are cooled with control or regulation on-line through the use of a controlled volume flow cooling medium.

For each functional area of a part, such as a wheel, it is possible to purposefully control cooling in order to achieve a certain and reproducible establishment of mechanical properties in the corresponding zone, due to the cooling time, type of cooling medium, amount of cooling medium and / or cooling sequence, for example, strength, viscosity and , in particular, internal stresses in the entire wheel or in the volume of the wheel brace with the inclusion of all functional areas of the part, i.e. hub, disc and rim.

The technical result of the invention is to simplify laboratory research and reduce material costs in the development of heat treatment technology, stabilize the process of implementing technological influences, minimize or completely eliminate rejection in the process of obtaining products with the required mechanical properties.

To achieve a technical result, the claimed method for producing metal products with a given structural state after heat treatment, including laboratory determination of the temperature curve of the heat treatment of a sample of metal material, ensuring the achievement of a given structural state of the material of the product, setting heat treatment intervals from the conditions of compliance with the heat treatment modes corresponding to the obtained temperature curve and cooling impact on the surface of the product with adjustable vaniem heat flux over the surface of each product in the range of heat treatment and with ensuring receipt by iteration in a volume of temperature matching products obtained and the target temperature curves with the energy in the bulk product in chemical and phase transformation, wherein said heat flux is determined from the expression

Q = [(H ′ + H ₀ ′ -qΔm) - (H + H ₀ )] / Δt, J / s,

where Н and Н ₀ are the initial enthalpies of phases in J, Н ′ and Н ₀ ′ are the enthalpies of phases in J over a time interval Δt in seconds, Δm is the change in the mass of phases in kg during a given time interval, q is the specific energy release, J / kg

When implementing the method, the calculation of the cooling effect is carried out by solving a system of equations that includes the heat equation in general

dQ = L · ∇T · dS · dt,

where dQ is the heat flux through the surface dS, m ² during the time dt in s, measured in J, L is the thermal conductivity coefficient, J / K · m · s, ∇T is the temperature gradient in K / m; or the heat balance equation in general

,

,

Where Qs is the heat flux through the element dS, m ^{2 of the} surface S, m ² bounding the volume V, m ³ measured in J / m ² · s, dh is the change in the specific enthalpy of the volume dV, m ³ during the time dt, s., measured in J / m ³ ;

energy conservation equation that takes into account energy release due to changes in the chemical and phase composition of the material of the processed metal

H + H ₀ = H ′ + H ₀ ′ -qΔm,

where H and H ₀ are the initial enthalpies of the phases, J, H ′ and H ₀ ′ are the enthalpies of the phases, J after time Δt, c, q is the specific energy release during the phase transformation, J / kg, Δm is the change in the mass of the phases, kg during a specified period of time;

для случая, когда не произошло столкновения границ зерен растущей фазы или фронтов концентрационных возмущений и

для случая, когда столкновение уже произошло, где η - безразмерное локальное относительное содержание растущей фазы, k_i=k_i(T, ΔT) - кинетические коэффициенты, с^-1, Т - локальная температура, а ΔТ - отклонение локальной температуры от температуры фазового равновесия, К, q - удельное энергвыделение, Дж/кг.and kinetic equations describing phase and chemical transformations in the volume of a product of the form

for the case when there was no collision of grain boundaries of the growing phase or fronts of concentration perturbations and

for the case when the collision has already occurred, where η is the dimensionless local relative content of the growing phase, k _i = k _i (T, ΔT) are kinetic coefficients, s ^-1 , T is the local temperature, and ΔT is the deviation of the local temperature from the phase temperature equilibrium, K, q - specific energy release, J / kg.

,

, где ΔT - отклонение температуры от температуры фазового равновесия, ΔТ₀ - отклонение температуры от температуры фазового равновесия в момент начала фазового превращения; c₁, c₂ - кинетические константы.The indicated result is also achieved by the fact that when calculating the thermal effect in the system of equations, kinetic coefficients are used, having the form k ₁ = c ₁ × ΔT ₀ ,

,

where ΔT is the temperature deviation from the temperature of phase equilibrium, ΔT ₀ is the temperature deviation from the temperature of phase equilibrium at the moment of the beginning of the phase transformation; c ₁ , c ₂ are kinetic constants.

In addition, in some cases, between the stages of heating and cooling, other technological operations can be carried out, for example, machining (rolling, drawing, forging, pressing, etc.), and accelerated cooling of the car from rolling heating can be used.

The laboratory determination of the temperature curves of heat treatment leading to the achievement of the desired properties of the product material is necessary in order to further calculate the thermal effect on the surface of the product and ensure their implementation in each selected section of the product volume, taking into account that the requirements for the properties of different sections of the product can be various, up to the fact that the product may have a complex shape or consist of parts made of different materials. In addition, this makes it possible to significantly simplify the method and reduce material costs for its implementation, since the determination of the temperature curves of heat treatment can be carried out on samples of a simple form - rods, plates, etc.

Calculation of the necessary thermal effect on the surface of a real product, ensuring the implementation of the obtained temperature curves in the volume of the product and the thermal effect on the product in accordance with the calculation, allows us to abandon preliminary expensive experiments on real products necessary to determine their mechanical properties in different parts of the volume products obtained depending on the cooling conditions.

Calculation of thermal effects by solving the heat equation in general

dQ = L · ∇T · dS · dt,

or heat balance equations in general

,

,

allows the simplest way to determine the necessary thermal effect, providing cooling of the product according to temperature curves, leading to the formation of structures with specified mechanical properties in them.

для случая, когда не произошло столкновения границ зерен растущей фазы и

для случая, когда столкновение уже произошло, и совместно с уравнением сохранения энергии, учитывающим энерговыделение за счет изменения химического и фазового состава материала обрабатываемого металлоизделияThe solution of the above equations together with kinetic equations describing phase transformations in the volume of a product of the form

for the case when there was no collision of the grain boundaries of the growing phase and

for the case when the collision has already occurred, and together with the energy conservation equation that takes into account energy release due to changes in the chemical and phase composition of the material of the processed metal

H + H ₀ = H '+ H ₀ ' + qΔm,

It is necessary in order to take into account the heat release in the product volume during phase transformations in the product material, which is a necessary condition for the correct thermophysical calculation and control of the real heat treatment technology.

The determination of the necessary law for regulating the distribution of the heat flux over the product’s surface by a predictive calculation with the forecasting depth for the entire heat treatment period is necessary in order to check the feasibility of realizing the temperature curves obtained in laboratory conditions when obtaining real products; to develop a technology for the calculated thermal and technological impact on the surface of the product during its heat treatment and to adjust the technology in real time, fending off the deviations and disturbances.

Dividing the heat-treatment period into intervals makes it possible to identify the process areas where the technology is unchanged and thereby organize the iterative process of calculations, set the technological effects on the surface of the product in the form of their dependence on time.

Determine the thermal effect on the surface of the product by iterations, achieving the coincidence of the results of the calculated and set temperature values on the selected interval - the specified action (iterative process) provides the calculation of the required heat fluxes on the surface of the product, since there is no possibility of their direct calculation due to the fact that from a mathematical point of view, this is an inverse incorrect problem.

The duration of the intervals is chosen on the basis of the need to comply with the tolerances for the heat treatment regimes determined by the temperature curve, since at too long intervals the constant value of the heat fluxes on the surface of the product will inevitably lead to the temperature of any part of its volume going beyond the tolerances. However, too short intervals increase the requirements for the used computing power and the control system of the heat treatment installation.

The essence of the proposed method is illustrated by an example of its implementation.

The task was to determine the cooling regimes necessary to obtain a pearlite structure in a wire with a diameter of 2 mm from steel 80 having the following chemical composition: 0.81% C; 0.52% Mn; 0.017% S; 0.28% Si; 0.009% P; 0.08% Cr; 0.05% Ni; 0.006% Al; 0.04% Cu; the rest is Fe.

For a wire of the indicated diameter, it is not possible to carry out a model experiment with fixing the temperature according to the readings of a thermocouple stamped into the product. Therefore, for the purpose of laboratory determination of the temperature curves of heat treatment, leading to the achievement of the desired properties of the material of the products, we took a bar of the specified steel with a diameter of 8 mm and a length of at least 30-40 diameters to exclude longitudinal heat transfer. A thermocouple was minted in the middle of the bar to a depth of 0.5 radius (2 mm). The thermocouple readings during the experiment were recorded using a computer recording system. They placed the bar in the furnace and heated it to a temperature of 1070 ° C, exceeding the maximum possible temperature in real technology, thereby ensuring guaranteed obtaining of an austenitic structure. After heating, the bar was cooled in air, subjecting the bar material to the austenite-perlite phase transformation, since the steel has a chemical composition close to eutectoid. The results of recording thermocouple readings in such an experiment are shown in the graph (see drawing).

On the cooling curve in the region of the 100th and 620th seconds of the experiment, S-shaped regions corresponding to the heat released during the phase transformation are clearly visible. Thus, to calculate the thermal effect on the bar corresponding to the experimentally obtained temperature curve, it is necessary to apply a system of equations describing the heat transfer along the thickness of the bar sample and the phase transformation in the material.

For quantitative processing, we selected a portion of the curve corresponding to its cooling in air (570th - 700th seconds of the experiment).

In order to determine the necessary thermal effect, providing cooling of the product according to temperature curves, leading to the formation of structures with specified mechanical properties in them, the heat equation in general form was written

dQ = L · ∇T · dS · dt,

where dQ is the heat flux through the surface dS in m ² during the time dt in seconds, measured in J, L is the thermal conductivity in J / K · m · s, ∇T is the temperature gradient in K / m.

To do this, the entire cross-section of the bar was divided by concentric circles with a step of no more than 1/50 of the radius. On each of the obtained layers, the thermodynamic conditions and the phase state of the material were considered the same.

The heat equation in this case will take the form:

Q =

, где λ - коэффициент теплопроводности (справочное значение) [Дж/м·с·град], dT/dr - градиент температуры по радиусу образца

where λ is the thermal conductivity coefficient (reference value) [J / m · s · deg], dT / dr is the temperature gradient along the radius of the sample for internal heat flow (between layers) ε · σ · T ⁴ , where ε equal to 0.8 is the dimensionless blackness coefficient of steel, σ is the Boltzmann constant, having a dimension of J / m ² · s · deg ⁴ , T is the temperature of the sample surface for external heat flux (for the outer side of the surface layer)

The energy conservation equation, taking into account energy release due to changes in the chemical and phase composition of the material of the processed metal:

H + H ₀ = H '+ H ₀ ' -qΔm-QΔt,

where Н and Н ₀ are the initial enthalpies of phases in J, Н ′ and Н ₀ ′ are the initial enthalpies of phases in J after a time Δt in seconds, q is the specific energy release during phase transformation in J / kg, Δm is the change in the mass of phases in kg in for a given period of time, Q is the influx of heat from the environment in J / s,

for each layer of the difference grid will take the form:

where c _m (in J / kg · deg) is the heat capacity of the initial phase, s _n (in J / kg · deg) is the heat capacity of the final phase, m ₀ (in kg) is the mass of the finite difference mesh layer, q (J / kg ) is the thermal effect of the phase transformation, Q (J) is the heat flux outside, η is the dimensionless mass fraction of the final phase in the material (dimensionless local relative content of the growing phase). Data without a dash and a dash refer to the initial and final values of the selected time interval, respectively.

The concentration of the final phase state (perlite) during the phase transformation was described by the kinetic equation:

for the case when less than half of the initial phase state (austenite) is processed.

for the case when more than half of the initial phase state (austenite) is processed.

Here:

k ₁ = c ₁ × ΔT ₀

ΔT, K, is the deviation of the temperature from the temperature of phase equilibrium, ΔT ₀ , K is the deviation of the temperature from the temperature of phase equilibrium at the beginning of the phase transformation, c ₁ (having a dimension of deg ^-1 ) and c ₂ (measured in deg ^-4/3 ) are kinetic constants.

Solving numerically the system of the indicated equations, we achieved the selection of the corresponding parameters (heat capacity, critical temperature, energy defect, induction time, kinetic constants) for the coincidence of the experimental and calculated curves. For this experiment, the specific heat at a temperature of 737 ° C (before the start of the phase transformation) is 630 J / kg · deg, at a temperature of 617 ° C (after the completion of the phase transformation) - 660 J / kg · deg, the energy defect is 7.7 · 10 ⁴ J / kg, critical temperature 752 ° С, induction time 26 s, kinetic constants c ₁ = 10 ^-7 , c ₂ = 3.5 · 10 ^-4 .

Then, material science studies of the model sample used in the experiment were carried out in order to determine its structural state. The entire sample had a pearlite structure. Calculation of the heat flux from the surface using the above equations and the thermokinetic parameters found above showed that the heat flux at the time of the phase transformation was ~ 34.4 kW / m ² · s (at a temperature of 660 ° C).

Thus, to obtain a pearlite structure of a wire with a diameter of 2 mm, it is necessary to heat it to a temperature above the austenization temperature (in this case, above 737 ° C), and then cool it. The cooling rate to a temperature of 737 ° C is unprincipled. Further cooling is carried out in any way that ensures the outflow of heat from the metal surface in the range of 26.3-47.2 kW / m ² · s until the complete phase transformation is completed (for example, until it reaches a temperature of 600 ° C). Further, the cooling mode may also be arbitrary.

Claims (1)

A method of obtaining metal products with a given structural state after heat treatment, including laboratory determination of the temperature curve of the heat treatment of a sample of metal material, ensuring the achievement of a given structural state of the material of the product, setting heat treatment intervals from the conditions for observing the heat treatment modes corresponding to the obtained temperature curve, and performing a cooling effect on the surface of the product with regulation of heat flow over the surface of the product in each heat treatment interval and ensuring that iterations in the product volume match the temperatures of the obtained and given temperature curves, taking into account the energy release in the product’s volume during chemical and phase transformation, the heat flux being determined from the expression:
Q = [(H ′ + H ₀ ′ -qΔm) - (H + H ₀ )] / Δt, J / s,
where H and H ₀ are the initial enthalpies of phases, J;
H ′ and H ₀ ′ are the enthalpies of phases, J, over a time interval Δt, s;
Δm is the change in phase mass, kg, over a given time interval;
q is the specific energy release, J / kg.

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