Failure Analysis of HAZ Cracking in Low C–CrMoV Steel Weldment
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This case study describes the failure analysis of steel nozzle in which cracking was observed after a circumferential welding process. The nozzle assembly was made from low C–CrMoV alloy steel that was subsequently single pass butt welded using gas tungsten arc welding. No cracks were found in visual inspection of the welds; however, X-ray radiography showed small discontinuous cracks on the surface in the area adjacent to weld bead, i.e. heat affected zone. The welding of nozzle parts made of same material was a routine process and this type of cracking did not occur in the past. Therefore, it became essential to determine the root cause of the failure. A detailed investigation including visual examination, non-destructive testing, optical microscopy, microhardness measurements and residual stress measurements were carried out to find out the primary cause of failure and to identify actions required to avoid its reoccurrence in future. Results of the investigation revealed that the principal cause of failure was the presence of coarse untempered martensite in the heat affected zone due to localized heating. The localized heating was caused by high welding heat input or low welding speed and resulted in the high transformation stresses. These transformation stresses combined with the thermal stresses and the constraint conditions to cause intergranular brittle fracture.
KeywordsNozzle Heat affected zone Weld cracking Residual stresses GTAW Untempered martensite
This failure analysis was carried out in the Materials Research and Testing Laboratory of Pakistan Space and Upper Atmosphere Research Commission (SUPARCO). The authors wish to thank Mr. Muhammad Saraf (Deputy Chief Manager) and Dr. Sajid Mirza (Senior Chief Manager) for their valuable suggestions and Mr. Raza Hussain (Chairman SUPARCO) for his guidance and provision of facilities throughout this investigation. The authors also like to thank Mr. Badar-ul-Hassan (Technical Officer) for his technical assistance throughout the experimental work.
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