Observation of twinned martensite in weld heat affected zone of Grade 100 low carbon microalloyed steel

Abstract

The microstructural changes caused by a low nominal heat input of 0.5 kJ mm^-1 in the coarse grained heat affected zone (CGHAZ) of Grade 100 microalloyed steel were investigated. Microhardness measurements suggested that the CGHAZ was martensite of maximum theoretical hardness for the carbon content of the steel. The bulk of the CGHAZ was lath martensite containing none of the small and few of the intermediate sized Nb precipitates responsible for strength and grain size control in the steel plate. Twinned martensite was unexpectedly observed in local areas of the CGHAZ. The formation of twins, which are normally seen in steels with a higher level of carbon, is explained by a combination of the rapid heating rates, high peak temperatures, precipitate dissolution and dispersion, and rapid cooling rates.