Brittle Fracture Initiation from a Notch of Poor Penetration in a Weldment of HT80 Steel

Abstract

To assess the degree of embrittlement in a transformed HAZ and the possibility of additional embrittlement caused by hot straining, brittle fracture initiation behavior from a notch of poor penetration in a weldment of HT 80 steel of 31 mm thickness has been investigated by measuring critical crack opening displacement (COD) at fracture. Three points bend specimens of 15 mm thickness were extracted from butt joint with weld defect of poor penetration, and the tip of the notch was located on a fusion' line in modified single=bevel joints, or, it was located on a weld metal in modified single-Vee joints. In these butt joints the degree of hot straining was altered by changing a intensity of tensile restraint in modified Lehigh restraint test specimens.
Heat input had large effect on the fracture toughness of fusion line, and reference temperature (Tr: critical COD of 0.25 mm) was markedly riled with increasing the heat input up to 42 KJ/cm. Fusion line and coarse-grained region of the HAZ was susceptible to hot straining embrittlement whereas the weld metal was not. The quantitative relation between the intensity of tensile restraint and the increase in reference temperature (ΔTr) was obtained. Small region ahead of the poor penetration in the joint of high tensile restrain received severe hot straining and showed high Tr temperature, and it was concluded that this severe hot straining was caused by restricting the free contraction of the weldments.
In the as welded and hot strained conditions, all the specimens with notch of poor penetration were more brittle than the specimens with fatigue pre-crack extracted from parent material, and thermal stress relief had disadvantage effect on fracture toughness of fusion line. In the most specimens extracted from modified double-bevel joints, crack propagated along fusion line.