Determination of the Root Causes for Cracking in a Large-Size Cast Ingot of AISI 4317 Steel Using Microstructural Analysis

Abstract

The objective of the present work is to study the root causes for the cracking and material detachment from the external surface of a 10 metric tons high-strength low-alloy steel ingot after solidification and annealing processes. Failure analysis investigations were conducted on different samples from the cracked areas as well as from non-cracked ones. A combination of optical and electron microscopies, x-ray diffraction, and microhardness techniques were used to study and analyze the nature, morphology, and composition of the different microstructural components in the cracked regions. Thermodynamic software, Thermo-Calc Ò was used to corroborate and interpret the experimental findings with the existing theories. The results indicated a strong intergranular character of the cracks accompanied by the presence of second-phase particles at prior austenite grain boundaries. Segregation of chromium and manganese was demonstrated through local chemical analysis of the grain boundaries. It was concluded that a rapid cooling rate combined with the weakening of grain boundaries was at the origin of crack initiation.

Subsurface Crack Initiation and Propagation in an Open Dieforged High Strength Steel: A Microstructural Analysis

“…Due the fact that the cracks were discovered during the machining step, the exact determination of their origin becomes quite complex. Indeed, several possible sources have been identified for initiation and propagation of cracks during casting [9], open die forging [13] and/or heat treatment [1] operations. Moreover, it is also possible that cracks initiate in one step (e.g.…”

“…As a result, often only post-mortem analysis of industrial cracking cases can be investigated. Recently, Loucif et al [9] showed that high cooling rates combined with weakened grain boundaries resulted in the crack development in an as-cast large size ingot. In another work, Bitterlin et al [1] investigated the root cause for cracking of a large size nickel rich medium carbon low alloy steel ingot and found that abnormal grain size may be at the origin of the observed cracks.…”