Abstract Scope |
Susceptibility to hot cracking during welding of austenitic stainless steels (ASS) is well-known. The metallurgical phenomenon of ASS weld hot cracking has been investigated for several decades. A familiar preventive measure to the hot cracking is to have typically 4 - 10 vol.% delta ferrite, directly solidified from the molten metal. The prediction of hot-crack-susceptibility of a base material or a welding consumable of ASS is well established using a constitutional diagram (e.g., WRC-92 diagram) and the ratio, Creq/Nieq.
The 316L material used in this investigation fulfills the requirements of both delta ferrite content (WRC-92) and Creq/Nieq in the modified Suutala diagram (Pacary et al. [1]) for pulsed-laser welding to prevent hot cracking. Nevertheless, hot cracking occurred in the material during the autogenous pulsed-laser welding, unpredictable and at random locations. The investigation established that the base metal consists of micro-segregation bands, where the microstructure is mostly austenitic and thus highly susceptible to hot cracks. It is believed that these micro-segregation bands in the base plate occurred during the initial production process, slab casting. When a tiny weld, is largely confined to the austenite-predominant segregation-region, then cracking occurs in the weld metal and in the heat affected zones of the base metal. The fracture surface of these cracks shows a dendritic morphology, particles and regions enriched with phosphorous and alloying elements Si, Cr and Mo. The analysis shows that both solidification cracking and liquation cracking occurs during pulsed-laser welding. It is certain that the requirements of defect free micro-welds in engineering applications demand material’s composition and microstructure homogeneity to a high-level.
Reference
1. Pacary, G. Moline, M. and Lippold, J. C. 1990. A diagram for predicting the weld solidification cracking susceptibility of pulsed-laser welds in austenitic stainless steel. EWI Research Brief No. B9008, Edison Welding Institute. |