SU1477750A1 - Method of thermal treatment of articles of grey iron - Google Patents

Abstract

The invention relates to heat treatment of parts made of pearl ferrite gray cast iron (ferrite up to 35%) and can be used in the tractor and automotive industries when processing mainly cylinder liners. The purpose of the invention is to obtain high hardness, wear resistance and a large transition zone of the hardened layer. The heat treatment method includes heating up to 1050–1075 ° C at a speed of 250–275 ° C / s, cooling for 2–3 s, quenching with a water shower at 18–30 ° C, and an electrical outlet at 250–275 ° C for 20 s for every 3 mm of the total depth of the hardened layer, air cooling. Heat treatment of parts by this method allows to increase their operational properties. 1 tab.

Description

The invention relates to heat treatment of parts made of perlite-ferritic gray iron (ferrite up to 35%) and can be used in the tractor and automotive industries when processing mainly cylinder liners.

The aim of the invention is to obtain a high hardness, wear resistance and a large transition zone of the hardened layer.

The method includes heating up to 1050–1075 ° C at a speed of 250–275 ° C / s, cooling for 2–3 s, quenching with a water shower at 18–30 ° C, holding at 250 ° C – 250 ° C for a temperature 20 s on

every 3 mm total depth of the hardened layer, air cooling.

The choice of the quenching temperature of 1050-1075 ° C at heating rates of 250-2755 ° C / s is due to the need to complete the conversion of high-silicon areas of ferrite located in places of silicon segregation into austenite. At lower temperatures, these areas do not turn into austenite, and at higher temperatures than 1075 ° C, austenite grain grows rapidly.

The choice of heating rates in the range of 250-275 ° C / s is due to the need to obtain fine-grained austenite with a displacement of completion

Consequently, it is possible to create conditions for cooling with air cooling to temperatures lying above Ac. At lower heating rates, the temperature of the end of the transformation decreases, which eliminates the possibility of intensification, and at large it is impossible to obtain a fine-grained structure of austenite.

With the duration of precooling, it is possible to saturate austenite with carbon by dissolving graphite and obtaining a fine-grained structure of austenite, and during quenching, fine-needle martensite having high hardness. With a decrease in the cooldown time (less than 1.5 s), the carbon content in austenite decreases and, as a result of quenching, low-carbon | martensite with low hardness is formed. With an increase in the time of undercooling (more than 2 s), coarse-grained austenite is formed in connection with a large growth rate of austenite grain, and during quenching, coarse-grained martensite, which has a low hardness.

The holding time at the electrospray for 20 s for every 3 mm of the total depth of the hardened layer is due to the need to sufficiently reduce the residual directions of stretching without significantly reducing the hardness of the hardened layer. At lower shutter speeds, residual stresses are slightly reduced, which can lead to the formation of cracks under operating conditions, & with an increase, the hardness of the hardened layer sharply decreases due to the decomposition of low-carbon martensite into troostite.

The choice of the tempering temperature is justified by the need to obtain a high uniform hardness. At lower temperatures, carbon diffusion, and especially silicon, proceeds at a very low speed, which requires long exposures to reduce stress, and at large, high carbon martensite is transformed into low carbon and low carbon austenite is depleted in carbon, which significantly reduces hardness.

The procedure for carrying out the procedure is as follows.

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Pre-machined castings are installed in an inductor-sprayer and fixed at the centers of the quenching machine. Pressing the Start button turns on the heat, the duration of which is controlled automatically by a time relay. When the hardening temperature reaches 1050-1075 C, the time switches off the heating and the cooling occurs in air for 2-3 seconds, and the castings are not removed from the inductor. At the end of the time for delaying, the time relay switches on the cooling with a water shower at 18-30 ° C, and after the cooling time has elapsed, the castings are removed from the inductor and installed s мо a transformer, which is installed on the semi-automatic machine. The start button is pressed and the power frequency is heated to a temperature of 250–275 ° C electric outflow. When the temperature of the self-tempering is reached, the heating is turned off with the help of an electric potentiometer and held at that temperature. The condition of the temperature of the electric outlet is maintained automatically by an electric potentiometer, and the duration of exposure is controlled by a time relay. At the end of the exposure (20 s for every 3 mm of the total depth of the hardened layer) the time switch turns off the semi-automatic machine, the castings are removed and cooled in air, then subjected to final machining. I

Under production conditions, the depth of the hardened layer is controlled by the distance from the surface of the part to the zone in whose structure 50% martensite is contained.

Example. Conduct heat treatment of parts made of gray iron composition, wt.%: C 3.3-3.5; Si 1.4-2.2; Mp 0.7-1.0; P 0.2; S 0.15; iron (ferrite 30%) the rest.

Heating for quenching is carried out with a current of 8000 Hz at a speed of 250-275 ° C / s, which corresponds on average to a power of 1.38-1.41 kW / cm2. The results of hardness measurement and wear tests are shown in the table.

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The proposed method allows to obtain a higher hardness and increases the wear resistance of parts in comparison with the known, as well as the magnitude of the transition zone, with an increase in which the maximum value of tensile stresses is removed from the surface of the hardened layer and, thereby, eliminates the possibility of cracked or chipped hardened layer under operating conditions. This effect increases the reliability of the parts.

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formula of invention

The method of heat treatment of gray iron products, mainly with ferrite content up to 35%, including induction heating to quenching temperatures, cooling with a water shower, and tempering, which, in order to increase hardness, wear resistance, after heating to quenching temperatures podstuzhivanie carried out in air

2-3

s, and the vacation is spent

during

within 20 seconds for every 3 mm of the total depth of the hardened layer.

275 250 275

250 250 275

2

3.3

3

2.8 3.2

2.7

3.2 3.5 3.3

58 58 59

56 56 57

0,006

Claims (1)

SUMMARY OF THE INVENTION A method for heat treating gray cast iron products predominantly with a ferrite content of up to 35%, including induction heating to quenching temperatures, cooling with a water shower and tempering, characterized in that, in order to increase hardness, wear resistance, they are cooled after quenching in air for 2-3 s, and tempering is carried out for 20 s for every 3 mm of the total depth of the hardened layer. Speed Temperature Time Hold Total depth Hardness, 'HRC heating electret * .substitute FROM hardened layer, mm - —Η ———— ° C / s vacation, C living. ------- g ------— to self after from hardened transitional vacation self layer. zones vacation 1 ----: Iekos, mm Quenching temperature, C 1075 275 250 2 40 2,8 3.2 58 56 1050 250 250 3.3 45 3.2 3,5 58 56 0.006 1075 275 275 3 40 2.7 3.3 59 57 J