Failure analysis of a 2.25Cr-1Mo-0.25V steel heavy wall-thickness multi-pass welded component

Crack-like defects were found in a low-alloy 2.25Cr-1Mo-0.25V steel multipass welded heavy wallthickness component prior to its usage. After welding, the component had been subjected to local Intermediate Stress Relief (ISR) heat treatment at 600-650°C, with the aim at removing diffusible hydrogen and enhancing partial tempering of the weldment microstructure before the final PWHT. The objective of the present paper was to investigate the actual fracture micromechanism of the discovered damage associated with the manufacturing stage of the component, in order to explain the inherent causes of failure. The metallographic and fractographic studies demonstrated (i) cracks propagating through the weld solidification structure as quasi-cleavage fractures, (ii) the presence of micro-cracks at thereby 'opened' solidification boundaries, as well as (iii) occasional appearance of ductile 'ridges' at the fracture surface; all that were characteristic of hydrogen-induced cold cracking. In line with this, Vickers hardness measurements revealed maximum hardness as great as 381-382 HV and 371-378 HV in cases of the CGHAZ and weld metal microstructures, respectively. Furthermore, hardness traverses in the weld thickness direction revealed higher hardness values in the weld intermediate thickness than closer to the surface or the root, which was ascribed to inadequacy of the thermal effects of the ISR heat treatment. The occurrence of hydrogen cracking was attributed to simultaneous co-existence of several adverse factors: (i) excessively high weldment hardness, (ii) accidentally high initial hydrogen content of the applied SMAW electrode and (iii) inherently high structural rigidity and restraint of the component.