Structure, defect substructure and fracture surface of low-carbon alloy steel welds

Abstract

By the methods of scanning and transmission electron microscopy a structural phase state, defect substructure and fracture surface of low carbon alloy steel welds produced with use of carbon-containing addition and without it are investigated. The study of etched metallographic section structure of welds revealed a presence of a large amount of second phase particles (carbides, sulfides, oxides, etc.). It is shown that in weld with a carbon-containing addition the particles locate chaotically, and in weld without addition they decorate boundaries of ferrite grains. Dimensions of second phase particles vary in the limits from 0.8 μm to 5 μm and are identical for both types of welds. Analysis of fracture surface structure of welds is carried out. A presence of nonmetallic inclusions is detected. They are typical of mainly weld produced without a carbon-containing addition which may indicate its increased brittleness. Amount of micropores in fracture of weld with carbon-containing addition is many times greater than that in weld without addition, which is indicative of gas removal in use of carbon-containing addition. The quantitative analysis of structure and dislocation substructure parameters of weld metal is carried out, the estimates of contributions of scalar and excess dislocation density to strength of welds are performed. It is shown that higher values of scalar and excess dislocation density in the weld formed without carbon-containing addition in flux may be responsible for material embrittlement. A large quantity of stress concentrators in the weld without carbon-containing addition may result in material embrittlement.