On the Cracking in HT 100 Weldments (1st Report)

Abstract

Causes and mechanisms of the cracking in HT 100 weldments are investigated by use of three kinds of test method. Both the hot and cold cracking are not observed in the heat-affected-zone, but mainly observed in the weld metal.
From metallographic examinations conducted on these fractured specimens, it can be found that the hot cracking is located in cellular dendrite boundaries, where solutes, such as Ni, Mn, and Mo, are segregated.
Even when none of hot cracking is observed, the special etchant with a surface active agent reveals darken-broading cellular dendrite boundaries in the weld metal that are sustained plastic deformation during solidifing.
The plastic deformation generated during the cooling process has also a deleterious effect to the delayed cracking. It is found that under the presence of hydrogen, the weld metal cracking initiated at abovementioned cellular dendrite boundaries transfers to the austenite grainboundaries, and then into the grains. In paticular, these cracks tend to occur at the place where the cellular dendrite boundary coincides with the austenite grainboundary. It is estimated to be due to the solute segrigation, dislocation pile-up and the precipitation of carbide.