Non-metallic inclusions as an indicator of the impact toughness of carriage castings from steel 20GL

Abstract

The correlation dependence between impact toughness and non-metallic inclusions (NI) in fractures of specimens made of low-alloy cast steel 20GL produced in open-hearth and electric arc furnaces for carriage castings "bogie side frame" and "bolster beam" is considered. Metallographic and statistical analyzes of NI according to GOST 1778–70 (method L) based on the Vestra image system program were carried out using the method of optical microscopy on samples of low-alloy cast steel 20GL with different impact toughness, with the help of which one of the reasons for the low values of impact toughness (KCV–60 < 2∙102 kJ/m2) — impurity by NI of 20GL steel samples. Using a scanning electron microscope, a combined approach to the study of NI and a metal matrix of fractures in samples of low-alloy cast steel 20GL with different impact toughness was implemented. An X-ray microspectral analysis of NI at the fracture point of specimens melted in open-hearth and electric arc furnaces was carried out, which showed the presence of large accumulations of NI in fractures of specimens made of 20GL steel with low values of impact toughness (KCV–60 < 2∙102 kJ/m2) both in open-hearth and electric arc melts. The use of a scanning electron microscope made it possible to obtain fractograms of fractures in samples with a clearly expressed diverse morphology of NI and various reliefs of the metal matrix. A significant role in the results of impact toughness plays the metal matrix of samples of openhearth and electric arc melts, which is clearly confirmed by the presence of cleavage facets with a stream pattern and tongues at brittle and small depressions at ductile-brittle fractures, as well as the presence of various shapes and sizes of NI. On the basis of X-ray microanalysis, graphs of the weight content of chemical elements contained in NI, metal matrix, fractures of samples of open-hearth and electric arc melts with different impact toughness are given.