Steel microstructures in autogenous laser welds

Abstract

Autogenous Nd:YAG laser welding was carried out on austenitic stainless steel (316LN) and duplex stainless steel (2205). The structures were examined using optical and color etching metallography. This showed optically that the austenitic steel solidified as a totally austenitic structure, but the transmission electron microscopy (TEM) work confirmed the presence of ferrite. In the case of the duplex stainless steel, the solidification structure was almost 100% ferrite. However, the weld metal properties required were met. Dissimilar autogenous Nd:YAG laser welding was performed on a duplex stainless steel to carbon steel joint and on an austenitic stainless steel to carbon steel joint. This revealed the presence of preferential fluid flow to the stainless steel side in the weld metal, and areas of excessive hardness. The results of this study were that filler wire would be required to counteract the high hardness effects induced by the autogenous laser welding process, without having to resort to postweld heat treatment. An evaluation has also taken place of autogenous CO2 laser welds of D and DH 36 steel. Mechanical properties of the DH 36 steel have been found to be acceptable, and the structure of the D grade material has been examined using TEM, where pearlite and bainite have been identified as being present.