Ductility-dip cracking susceptibility in dissimilar weld metals of alloy 690 filler metal and low alloy steel

Abstract

The effect of dilution ratio and chemical composition on the ductility-dip cracking (DDC) susceptibility of dissimilar overlay welds of Alloy 690 filler wire and low alloy steel (LAS) was investigated. We measured the hot ductility curve using a hot tensile test coupled with an in-situ measurement system. The minimum critical strain of Alloy 690 was 4.0% at 943 °C. The minimum strain and the temperature decreased with increasing LAS content from 3.2% (901 °C) to 2.4% (842 °C) and 2.2% (802 °C) at LAS contents of 25%, 50%, and 75%, respectively. The formation of tortuous grains resulting from microconstituents containing niobium improved the DDC susceptibility. With increasing dilution of LAS, the niobium content decreased, resulting in a reduction in the tortuosity ratio at the cracked grain boundaries. In addition, the grain boundaries embrittled with increased dilution ratio owing to an increased phosphorous and sulfur content. Therefore, at high LAS content, the combination of low tortuosity ratio of the grain boundaries and high phosphorous and sulfur content increases DDC susceptibility.