Microstructural changes in weld joint of COST F and FB2 steels after long term creep tests

Abstract

A trial weld joint of COST F and FB2 steels was produced using the GTAW HOT-WIRE method in conditions used in industry for welding steam turbine rotors. Long-term creep tests to the rupture of smooth cross-weld samples were carried out. Creep strength and hardness profiles across the weld joint were evaluated and correlated with the microstructure. Microstructures of ruptured samples were investigated using light, scanning and transmission electron microscopy and compared with virgin samples. Fracture occurred in COST F steel and its heat affected zone. Precipitation of Laves phase and structure recovery were the main reasons for the failure. Quantitative evaluation of M23C6 carbides and Laves phase particles in different parts of the weld joint was carried out. In the initial conditions, small Laves phase particles occurred only exceptionally in the weld metal. During creep at temperatures above 575 °C Laves phase precipitated in all parts of the weld joint and especially in the heat affected zones. Coarse Laves phase particles and their clusters with chromium carbides served as nucleation centres for cavities. As the fine grained heat affected zone of COST F steel was the softest part of the weld joint, many cavities originated and cracks were propagated.