Precipitation behaviour in weld metal of W-containing 9Cr ferritic heat-resisting steel

Abstract

Summary This paper describes a study of the precipitation behaviour in the weld metal of W-containing 9Cr ferritic heat-resisting steel during post weld heat treatment (PWHT) and creep rupture testing through analysis of the extracted residues sampled from the weld metals after PWHT and creep rupture testing. The analytical results are compared with thermodynamic calculation software (THERMOCALC) predictions. The THERMOCALC predicts that M23C6 and MX type carbonitrides are precipitated at PWHT temperatures in equilibrium states. In addition to M23C6 and MX type carbonitrides, the Laves phase is predicted by the THERMOCALC to be precipitated at creep rupture test temperatures in equilibrium states. X-ray diffraction analysis results show that M23C6 and MX type carbonitrides are precipitated in the weld metal during PWHT as predicted by the THERMOCALC. X-ray diffraction analysis results further show that M23C6 and MX type carbonitrides are detected in weld metals after less than 100 hr in creep rupture tests at 873 K. The Laves phase is precipitatedin the range from around 100 hr to more than 1000 hr in creep rupture tests at 873 K. X-ray diffraction analysis results show the presence of the Laves phase in the weld metal after creep rupture testing at 873 K × 1126.7 hr. The W content of the extracted residues from the weld metals after creep rupture tests at 873 K sharply increases after a time of around 100 hr to more than 1000 hr. This increase in the W content is probably due to precipitation of the Laves phase. The W and Mo contents of extracted residues from weld metals after creep rupture tests at 873 K attain constant values after more than around 10000 hr, these results showing good agreement with those predicted by the THERMOCALC. The time dependence of the W content of the extracted residues from weld metals in creep rupture tests at 873 and 923 K is in good agreement with those based on the Johnson-Mehl-Avrami's equation.