Metallurgical characterisation of cobalt-base weld hardfacings for hot tooling: Influence of welding parameters

Improving service life of tooling by welding high performance alloys on their surfaces is a current practice in hot forming. Cobalt-base or Nickel-base superalloys are often used as weld-deposited hardfacings for their outstanding strength, resistance to wear and oxidation at high temperatures (> 550 degrees C). This paper describes the procedure of welding Cobalt-base superalloy layers on steel substrate as well as their metallurgical properties according to the welding parameters. During the welding process, the first phase to form from the liquid state consists of cobalt-rich dendrites. The remaining liquid, rich in alloy elements, is solidified by an eutectic reaction into the interdendritic area. Indeed, the hardening elements such as Mo and Cr precipitate by forming carbides in the interdendritic region. The welding parameters are closely linked to the cooling speed which is responsible for variations in chemistry composition between the different welded layers or microstructural and mechanical changes able to impairing machinability of these weld-hardfacing deposits.