EA036583B1 - Method for alloyed structural steel case-hardening - Google Patents

Abstract

The invention relates to the field of metalworking and can be used for production of gears of alloyed structural steels of grades 18CrGT, 25CrGM, 20CrNiM, 20CrNi2M, 15CrGNi2TA, 20CrNi3A, etc., for transmissions of motor and tractor vehicles operated under increased specific loads. The invention can be used for production of bearing elements and sliding supports, spherical and cylindrical joints, and gears used in agricultural machine building, machine tool engineering, metallurgical industry. According to the proposed method for case-hardening of the alloy structural steel, the steel is heated up to hardening temperature according to the mode including heating in the range of 20-680°C at a speed of 6 deg/min, heating in the range of austenitic transformation (680-800°C) at a speed of 1 to 3.5 deg/min, and final heating up to a hardening temperature of 1,000-1,020°C at a speed of 6 deg/min.

Description

The invention relates to the field of metalworking and can be used in the manufacture of gears from structural alloy steels of the type 18KhGT, 25KhGM, 20KhNM, 20KhN2M, 15KhGN2TA, 20KhN3A, etc. for transmissions of automotive vehicles operating at increased specific loads. The invention can be used in the manufacture of elements of bearings and sliding bearings, spherical and cylindrical joints, as well as gears used in agricultural machinery, machine tool building, metallurgical industry. To ensure the required wear resistance and cyclic durability of gears using the methods of chemical thermal treatment (CHT), namely carburizing, carbon saturation of the surface layers of the parts to be strengthened is carried out, which ensures the formation of the required complex of physical and mechanical characteristics of gears.

Currently, the overwhelming majority of parts are carburized in automated pass-through and chamber units with high productivity and ensuring the entire cycle of chemical treatment from heating to quenching in controlled gas atmospheres without air access [1].

The known method [2], including heating cemented parts to temperatures of 930-950 ° C, saturation of the surface layers of parts with carbon using a controlled endothermic gas atmosphere (which is prepared by converting natural gas methane or propane with air in special generators) for a given time, and also quenching in oil after cooling to a temperature of 800-860 ° C, depending on the steel grade.

The disadvantage of this method is the long duration of isothermal holding in the process of saturation of the surface layers of steel parts with carbon, which leads to low productivity of the CTO process and its high cost. In particular, the holding time of structural case-hardened steels at a saturation temperature of 930 ° C, required to achieve a thickness of the case-hardened layer of 1.2-1.7 mm, is 12-13 hours [3]. In this regard, work is underway to improve the cementation technology and, in particular, to create high-performance and energy-saving cementation methods. At the same time, an increase in the temperature of the saturation process is the most effective method of increasing productivity during CTH, increasing the efficiency of thermal production and, above all, reducing the consumption of energy resources. In particular, an increase in the cementation temperature from 950 ° C to 1000 and 1050 ° C leads to a decrease in the saturation time to a layer depth of 1.5 mm, respectively, by 1.5 and 3 times.

The high-temperature carburizing process is carried out in shaft furnaces and through-feed units at 960-1050 ° C. At the same time, high-temperature carburizing at a temperature of 1000-1040 ° C with direct hardening after cooling to 860 ° C leads to coarsening of grains in structural steels and, in this connection, obtaining a coarser microstructure both in the layer and in the core, as well as to an increased the content of retained austenite in the microstructure of the layer [4], which hinders the spread of high-temperature carburizing technology. In particular, the results of the studies carried out show [5] that the steels 25KhGT, 20KhN3A, 20KhNR, 15KhGN2TA (GOST 4543-71), widely used in the automotive industry, are not applicable for high-temperature carburizing due to their tendency to the growth of austenite grains at temperatures above 950 ° C. ... In this regard, after high-temperature vacuum carburizing, phase recrystallization or thermal cycling is used [4].

The known method [3] of carrying out vacuum carburizing using a furnace type VSQ-182436 firm Heiss (carburizer - natural gas, process temperature - 1050 ° C). The vacuum carburizing process includes two periods: saturation at maximum gas supply and pressure of 21 MPa and diffusion without gas supply (residual pressure). In this mode, in 30 minutes of saturation, the effective thickness of the cemented layer reaches 1.2 mm, and in 50 minutes - 1.4 mm. After the end of the processing process, the parts are cooled to 500-600 ° C, and then subjected to reheating for quenching and cooling. This mode contributes to the refinement of the austenite grain and obtaining high mechanical properties of the products.

The disadvantage of this method is the need for re-heating and re-hardening, which significantly reduces the energy-saving efficiency of the high-temperature carburizing process and increases the warpage of parts.

The closest in technical essence to the claimed one is the method of high-temperature carburizing in ModulTherm 7/1 vacuum furnaces from ALD Vacuum Technologies GmbH at 1000-1050 ° C with cyclic supply of acetylene during saturation, followed by cooling the cemented parts to 850 ° C and their quenching in an inert gas - helium at a pressure of 2.0 MPa [6].

The disadvantage of this method is the growth of austenite grains in structural steels [5], which leads to the need to lower the carburizing temperature to 950-960 ° C and increases the processing time, or it is required to reheat and re-quench after high-temperature carburizing at 1000-1050 ° C, which causes a significant increase in the cost of chemical treatment, as well as an increase in warpage of the processed parts, which affects the quality of their manufacture.

The objective of the invention is:

- 1 036583 improving the quality of parts manufacturing, increasing the productivity of the CHT method, increasing the efficiency of thermal production and, in particular, reducing the consumption of energy resources.

To solve this problem, in the carburizing method, including heating steel to the carburizing temperature, saturation with carbon at the carburizing temperature, as well as cooling up to 850 ° C and quenching the carburized parts, according to the invention, heating the steel to the carburizing temperature is carried out according to a mode including heating in the interval 20 -680 ° C at a rate of> 6 deg / min, heating in the range of austenitic transformation (680-800 ° C) at a rate of 1 to 3.5 deg / min and final heating to a cementation temperature of 1000-1020 ° C at a rate of> 6 deg / min.

In this case, heating steel in the range of austenitic transformation at a rate of <1 deg / min is not economically feasible, since this greatly increases the processing time, and heating at a rate of> 3.5 deg / min leads to rejects due to the occurrence of island grain size in case hardened steel (Fig. . 1, fig. 2).

In the inventive method, the heating of steel according to the specified modes ensures the preservation of a high-quality fine-grained austenitic structure in the cemented layer and in the core of the parts during high-temperature carburizing, which eliminates the need for repeated phase recrystallization (hardening) of cemented parts and provides an increase in the quality of parts processing and higher economic indicators of the proposed HTO technology.

The invention is illustrated by the figures.

FIG. 1 shows the microstructure of 20KhN3A steel after heat treatment according to the regime including heating to 1000 ° C at a rate of 5-6 deg / min, holding for 1 h and quenching in oil.

FIG. 2 shows the microstructure of steel 20KhGNR after heat treatment according to a regime including heating to 1000 ° C at a rate of 5-6 deg / min, holding for 1 h and quenching in oil.

FIG. 3 shows the microstructure of the cemented layer of 20KhN3A steel after high-temperature carburizing at 1000 ° C according to the experimental regime, including heating in the range of austenitic transformation (680-800 ° C) at a rate of 1.3 deg / min.

FIG. 4 shows the microstructure of the core of a 20KhN3A steel specimen after high-temperature carburizing at 1000 ° C according to the experimental regime, including heating in the range of austenitic transformation (680-800 ° C) at a rate of 1.3 deg / min.

FIG. 5 shows the distribution of microhardness along the depth of the cemented layer of steel 20KhN3A. Cementation was carried out according to the experimental regime.

An example of implementation of the invention.

Experimental high-temperature chemical-thermal treatment of parts made of steel 20XN3A was carried out on a ModulTherm 7/1 vacuum line using step heating to saturation temperature, including accelerated heating to a temperature of 680 ° C, slow heating at a rate of 1.3 deg / min in the range 680-800 ° C, accelerated heating from 800 ° C to a carburizing temperature of 1000 ° C, followed by isothermal holding for 2.5 h at the stage of saturation with carbon and further quenching in an inert gas (helium) medium in a quenching chamber after cooling to a temperature of 850 ° C. At the final stage of the cycle of chemical-thermal treatment of steel 20KhN3A, low tempering was carried out at 170 ° C for 2.5 h.

The saturation with carbon is carried out by any method known from the prior art, which is applicable in this case, for example, saturation in the medium of a hydrocarbon-containing gas with acetylene in a cyclic mode of its supply.

FIG. 3 shows the microstructure of the cemented layer of 20KhN3A steel. It can be seen that the steel has a fine-grained structure with an average grain size of 50 µm in the cemented layer and 43 µm in the core (Fig. 4). The distribution of microhardness over the depth of the cemented layer is shown in Fig. 5. Cemented layer depth is 1.5-1.6 mm, and a surface microhardness of 780 HV _{0, 2} (61-62 HRC). The specified characteristics of the structural state and microhardness of the surface layer fully meet the requirements of STP 257 - 2188 - 2004 and are achieved in a significantly shorter cementation time, which ensures energy savings with high quality cemented parts.

For comparison, FIG. Tables 1 and 2 show the microstructures of 20KhN3A and 20KhGNR steel samples after heating to 1000 ° C (1 h) in the usual mode (without a slow heating section in the range of austenitic α ^ γ transformation). It can be seen that, despite a significantly shorter high-temperature holding interval, the average austenite grain size of steels after conventional heating is> 2 times larger (»100-120 μm) than the grain size after the proposed processing mode with delayed heating in the α ^ γ range transformation.

Information sources

1. Lakhtin Yu.M., Arzamasov B.N. Chemical heat treatment of metals. Moscow: 1985, 256 p.

2. Sagaradze B.C. Improving the reliability of cemented parts. M .: Mashinostroenie, 1975 .-- 216 p.

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3. Rosenberg S.E., Susin A.A. Defects in the structure of diffusion-hardened layers. Minsk, Belarusian Science, 1997 .-- 224 p.

4. Zinchenko V.M. Engineering of the surface of gears by chemical thermal treatment. M .: MVTU im. N.E. Bauman, 2001.-304 p.

5. Valko A.A., Rudenko S.P., Chichin A.N. Influence of high-temperature vacuum carburizing on the grain size of structural steels // Actual problems of mechanical engineering: collection of articles. scientific. tr. / Combine. Institute of Mechanical Engineering of the National Academy of Sciences of Belarus; editorial board: S.N. Poddubko [and others]. 2014.-Issue 3. - S. 343-346.

6. High-temperature vacuum carburizing - a reserve for reducing the energy intensity of production and improving the quality of gear wheels of transmissions of power-saturated machines / А.А. Shipko, SP. Rudenko, A.L. Valko, A.N. Chichin // Casting and metallurgy. - 2016. - No. 2. - S. 104-109.

Claims (1)

CLAIM The method of carburizing structural alloy steel, including heating the steel to the carburizing temperature, saturation with carbon at the carburizing temperature, as well as hardening to 850 ° C and quenching, characterized in that the steel is heated to the carburizing temperature according to a mode including heating in the range of 20-680 ° C at a rate> 6 deg / min, heating in the range of austenitic transformation 680-800 ° C at a rate from 1 deg / min to 3.5 deg / min and final heating to a cementation temperature of 1000-1020 ° C at a rate of> 6 deg / min.