Effect of preheat on the microstructure, hardness and toughness of HT-9 weldments

Prior studies have shown that HT-9 plates up to 1'' thick can be successfully welded with preheats as low as 1000C. The use of a 1000C preheat of the base metal prior to welding HT-9 results in an increased toughness over that for a 4000C preheat by about 13.5 Joules and a ductile-brittle transition temperature of -120C which is 140C lower than for 4000C preheated condition. SEM examinations showed that there were significant increases in the dendrite spacing and increased segregation due to the slower cooling rates in the 4000C preheat weld. This segregation is commensurate with depletion of the ferritizers Cr, Mo, and W in the surrounding matrix accompanied by increases in the Ni concentrations. These results indicate the lowest preheat compatible with good welding practice should be used