Microstructural change of a 5% Cr steel weld metal during tempering

The microstructure of an ultra-low carbon 5% Cr steel weld metal was studied in welded and tempered conditions using optical microscopy and analytical electron microscopy. The tempering was carried out at 400, 500, 600, 700 and 750°C for 4 h after welding. Transmission electron microscopy (TEM) showed that martensite in the weld metal started to recrystallize at 500°C and had changed to a coarse polygonal ferrite after tempering at 750°C. Except for some cementite in bainite, the main precipitates in the weld metal were M7C3, M2X and M23C6. M2X was not stable and transformed to M23C6 during tempering at temperatures above 600°C. The process of the microstructural development in the 5% Cr weld metal is discussed.