Experimental test set-up for studying hot cracking in multi pass laser hybrid welding of thick section austenitic stainless steel

Although the austenitic stainless steel grades are commonly considered to be quite easily weldable, there are certain applications which make exception to the above statement. As an example, which has also attributed to this study, it could be mentioned a welded assembly which forms a very rigid structure. In above mentioned structure, a risk of solidification cracking (i.e. hot cracking) in produced welds could be a significant problem, if necessary precautions are not taken into account well in advance. It is generally known that hot cracking of austenitic stainless steel during welding is very much coupled to chemical composition and the strains formed during solidification stage of the weld. The level of strains is dependent on e.g. a groove design, used welding parameters and the rigidity of the structure to be welded. In this work the main objective was to find a method for studying hot cracking susceptibility when a thick section austenitic stainless steel is welded using laser hybrid welding process (3 kW Nd:YAG-laser + GMAW) and multi pass technique. The tested parent material was a specially customized heat: AISI 316L(N)-IG ITER-grade austenitic stainless steel. During this study the test system was first developed and tested. It consists of a very rigid clamping system and specially designed 60 mm thick test piece which was planned to be rigid as itself. The test welds were evaluated with macroscopic and microscopic examination. In addition NDT was used as well. The results of welding tests showed that the developed test set-up can produce strains high enough to promote hot cracking in produced test welds. According to welding test results, hot cracking occurred in these rigid weld arrangements. The test set-up is described and the results of hot cracking tests are reported. The effect of chemical composition of the used parent/filler material and prevailing micro structure of weld metal on the risk of hot cracking susceptibility is discussed as well.