Influence of components and 20GL steel microstructure hardness on impact toughness of car castings

Abstract

Microstructural analysis of low-alloy 20GL steel showed the need to improve heat treatment (normalization) in terms of impact on grain quantity, size and hardness, average grain area and properties, in particular impact toughness of wagon castings «Side frame» and «Truck bolster». Relationships between the 20GL low-alloy steel microstructure parameters and toughness at negative temperature have been established. The method of optical microscopy was used to carry out metallographic and statistical analyzes of the ferrite (F) and pearlite (P) microstructure of 20GL steel samples of car castings for «Side frame» and «Truck bolster» according to GOST 5639-82 using the Vestra Image System program. Graphical dependences are constructed between the number and average area of grains on the section, the number of F and average area of grains on the section, and the impact toughness of 20GL samples of open-hearth and electric arc melting steel. For open-hearth melts with the same values of the amount and average area of grain on the section, the amount of F and the average area of grain on the section, the impact toughness can be both KCV – 60 ≤ 2.0 and KCV – 60 > 2.0∙102 kJ/m2. Analysis of electric arc melts showed that low values of the average area of ferrite grains do not increase the impact toughness of KCV – 60> 2.0∙102 kJ/m2. The problem of obtaining satisfactory results of impact toughness KCV – 60 > 2.0∙102 kJ/m2, associated with heat treatment in 20GL steel samples, was established.