Failure Analysis of HAZ Cracking in Low C–CrMoV Steel Weldment

Abstract

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, nondestructive 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.

Effect of Preheating Temperature on Microstructure and Properties of 42CrMo4/38MnVS6 Heterogeneous Laser Welded Joint

“…First, the quenched and hardened microstructure of weld should be avoided during welding. Second, welded joints need to have high strength and toughness [8][9][10][11]. Yoo et al studied the influence of welding parameters on the weld penetration, the aspect ratio of the weld and the initiation of defects in the weld for the case of the laser welding of S45C medium carbon steel, the relationship between effective heat input and weld penetration was obtained through a large number of tests, and it was determined that…”