Localized ageing in the heat affected zone of welded X5CrNiCuNbl6-4 and X4CrNiSiTi14-7 sheets

Localized ageing and corresponding microstructural developments as a result of welding heat in the heat-affected zone (HAZ) of two different precipitation-hardened Cr-Steels (X5CrNiCuNb16-4 and X4CrNiSiTi14-7) have been studied. The X5CrNiCuNb16-4 sheet was in solution annealed condition and X4CrNiSiTi14-7 sheet was in peak aged condition. The results showed that despite of initial heat treated condition, the fusion zone formed in both welded sheets has typical cast structure. The HAZ has different microstructure compared to fusion zone and base metal. The HAZ is found to be sensitive to the welding heat and was aged locally due to thermal effects of welding. This localized ageing forms regions in HAZ varying from over-aged to under aged, depending upon the initial ageing condition of the base sheet