SU1650729A1 - Method of heat treatment of medium-carbon steel castings - Google Patents

Abstract

The invention relates to ferrous metallurgy and can be used in the heat treatment of castings mainly from medium carbon silicon-manganese steels. The purpose of the invention is to increase the crack resistance of castings while maintaining the strength properties. The method of heat treatment of castings includes austenization, aging, cooling and tempering, with cooling of castings from 20 to 100 mm thick from the austenitic state at a rate of 2-14 ° C / s, back proportional to the thickness of the castings so that with an increase in thickness for every 20 mm, the cooling rate in the specified interval is reduced by 2-3 ° C / s. These cooling parameters ensure the formation of a ferrite-quasi-eutectoid structure, Table 1.

Description

The invention relates to ferrous metallurgy and can be used in the heat treatment of castings, mainly from medium-carbon silicon-manganese steels.

The purpose of the invention is to increase the crack resistance while maintaining the strength properties of the casting.

Medium carbon silicon-manganese steel tend to axial and zonal segregation. Depending on their thickness, the carbon content and some alloying elements in the axial zone of the castings may increase by two or more times, which causes a considerable unevenness of the properties of the peripheral and axial zones of the castings, and also increases their tendency to crack formation.

It was established experimentally that if with increasing thickness of the cast billet (casting) from 20 to 100 mm, the cooling rate from the austenitic state decreases from 14 to 2 ° C / s, and with increasing thickness for every 20 mm, the cooling rate in the specified interval decreases by 2-3 ° C / s, for castings of various thickness, the decomposition of supercooled austenite occurs according to one mechanism with the formation of a dispersed ferrite-quasi-eutectoid structure, which ensures a high uniformity of properties in the direction of the thickness of the casting and the complete absence of in.

If cooling is performed at a speed of more than 14 ° C / s, then in castings even 20 mm thick, part of the supercooled austenite decays by an intermediate mechanism with the formation of bainite, which can

ate about

v |

Yu

It can cause cracking and reduces the ductility and toughness of steel. If the cooling is carried out at a rate of less than 2 ° C / s, then the required strength characteristics of the product are not provided.

If, with an increase in casting thickness for every 20 mm, the cooling rate is reduced by less than 2 ° C / s, then bainite is formed in the axial zone, resulting in the occurrence of cracks, and the heterogeneity of properties in the thickness direction of the casting increases. If the cooling rate is reduced by less than 2 ° C / s with increasing casting thickness for every 20 cm, then bainite is formed in the axial zone, resulting in cracks, and the heterogeneity of properties increases in the direction of the casting thickness. If, with an increase in the casting thickness for every 20 mm, the cooling rate reduces by more than 3 ° C / s, then the required strength properties of the casting are not ensured.

An example. Castings of various thicknesses of steel 27C GTL are heat treated in accordance with known and proposed methods.

When implementing a known casting method with a thickness of 20, 60 and 100 mm, it is heated to 920 ° C, kept at the rate of 2.5 minutes per 1 mm of thickness, cooled at the same speed to 680 ° C, isothermally aged at this temperature, followed by cooling to air and subjected to stepwise tempering with a consistent decrease in temperature in the range from AP - (tO-30) ° C (630-850 ° C) to AG1- (80-100) ° C and (560-580 ° C).

A part of castings with a thickness of 20, 60 and 100 mm is also subjected to austenitization according to the described modes, and cooling is carried out

differentially for castings of each thickness. Thus, castings with a thickness of 20 mm are cooled at a rate of 14 ° C / s, and with increasing thickness for every 20 mm,

cooling is reduced by 2-3 ° C / s, which is when cooling castings with a thickness of 60 mm 8-10 ° C / s, and 100 mm 2 ° C / s.

The adjustment of the cooling rates in the indicated ranges is carried out by means of water-air cooling by changing the water-air percentage ratio. Next, the casting is subjected to tempering at 600 ° C (the proposed method).

After complete heat treatment of casting

investigated for cracks, studied the mechanical properties.

Processing modes and properties of castings from steel 27C GTL are given in the table. Analysis of the research results shows that the optimal modes of the proposed method (modes 2, 4, 5, 7) as compared with the known one allow, when the required complex of mechanical properties is achieved, significantly increase

crack resistance of products.

Claims (1)

Invention Formula The method of heat treatment of castings from medium carbon steels, predominantly alloyed with manganese and 20-100 mm thick, including austenitization, aging, cooling and tempering, characterized in that, in order to increase the crack resistance while maintaining strength properties, cooling is conducted at a speed of 2-14 ° C per second is inversely proportional to the thickness of the casting, and with an increase in the thickness of the casting for every 20 mm, the cooling rate in the specified interval is reduced by 2-3 ° C / s.