Cracks-Running Hot and Cold a Tale of Weld Metal Integrity

Abstract

The welding of steel is a casting process in which molten steel solidifies within the confines of a steel mould formed by the plates being joined. Without counteractive measures it is therefore subject to common casting defects such as segregation, cold shuts, contamination, oxidation, gas and shrinkage porosity and hot tearing. However, welding differs in several major respects from batch or continuous casting processes. One obvious difference is that weld ng is a joining process in which the cast metal unifies with the metal mould and, as a consequence, stresses develop from the restraint to shrinkage imposed by the mould. In addition, the mould material melts and alloys with the weld filler metal to form a “diluted†slug of weld metal. Welding can also be regarded as a mini-electric-arc steelmaking process in which molten metal interacts with elements in the arc plasma and with fluxing materials to produce a weld bead with a high volume fraction of non-metallic inclusions, compositional gradients and segregation, as well as relatively high concentrations of the gaseous elements oxygen, nitrogen and hydrogen.

This gassiness, the effect of restraint and compositional segregation increase the susceptibility to “hot’ cracking at temperatures near the primary solidus and to “cold†cracking at temperatures well below the primary A_r3 temperature of the weld metal. This paper reviews hot cracking of carbon steel weld metals and discusses recent research results on hot and cold cracking of pipeline ischolar_main pass girth welds, the interactivity between the two types of cracking and welding conditions which reduce the likelihood of cracking.