The effect of inclusion parameters on the fracture toughness of two X-70 Pipeline steels

Abstract

This is the second of a two-paper contribution on the effect of inclusion parameters on the elastic and elastic-plastic fracture toughness of two types of X-70 pipeline steel. In this paper, the fracture toughness is measured under static strain rate conditions (10−4/s, K = 10 ksi √in. (11 MPa √m) for X-70 steels having different inclusion morphologies as obtained by examining a fully killed, calcium treated steel and various stages of reduction viz 38 pct, 70 pct, 88 pct and 97 pct of a semi-killed conventional pipeline steel. The inclusion parameters, the area fraction (AF), the average inclusion length (Co), the inter-inclusion spacing (D), the aspect ratio (FF), and the density (N) were quantitatively analyzed using a Quantimet 720 image analyzing computer and direct microstructural measurements. The elastic and the elastic-plastic fracture toughness of the specimens were measured by the unloading compliance J-integral technique. It was observed that an increase in the area fraction (AF) and to a less extent the maximum average inclusion length (Co), the aspect ratio (FF) and the density of inclusions (N) decreased the elastic-plastic fracture toughness of X-70 steel and an increase in the inter-particle distance increased the elastic-plastic fracture toughness of the X-70 steels. The inclusion parameters markedly affected the elastic-plastic fracture toughness properties of the X-70 pipeline steels, particularly in the upper shelf and transition temperature region where failure occurred by ductile fracture processes. The effect of the inclusion parameters on the elastic fracture toughness in the brittle temperature region, where fracture occurs by brittle fracture processes, was insignificant. The most significant inclusion parameters affecting the elastic-plastic fracture toughness were the area fraction (AF) and the inter-inclusion spacing (D).