Microstructure and mechanical properties of laser-welded high-strength AISI 301LN steel in reversion-treated and temper-rolled conditions

Reversion treatment is an efficient method to refine the grain size of metastable austenitic stainless steels and enhance their mechanical properties. A piece of an AISI 301LN 56% cold-rolled 3.4 mm thick sheet was annealed at 790 degrees C for 80 s on a 600 kW pilot induction heating line resulting in the grain size of 0.9 nm. A commercial 3.0 mm thick temper-rolled 301LN (+CP700 / C1000) sheet together with data for a hot-rolled steel from a previous study were used for comparisons. The sheets were bead-on plate autogenously welded using a diode pumped Yb:YAG laser at a constant power of 4 kW. Microstructure examinations were performed employing optical microscopy and electron backscatter diffraction. Hardness and tensile tests as well as strain-controlled fatigue tests were carried out to determine the mechanical properties. Microstructure evolution revealed that only slight grain growth occurred in a very narrow zone in the heat-affected zone (HAZ) in the reversion-treated sheet during welding. In the HAZ of the temper-rolled sheet both recrystallization and grain growth took place. The HAZ was also narrower in the reversion-treated sheet. The minimum hardness level was about 240 HV in the both welds. The yield strengths of the weld seams of the reversiontreated and temper-rolled sheets were equal but twice as high as the yield strength of hot-rolled steel without welding. The fatigue strength was identical in the both structures, impaired by welding, but about the same as that of the hot-rolled steel without welding. The welded specimens failed in the weld metal either along the fusion line between the HAZ and the weld metal or inside the weld metal. It can be assumed that the microsegregation and/or the grain structure provide some weaker sites for crack nucleation, but also the grain size and structure are different from those of the base metals. (C) 2019 The Authors. Published by Elsevier B.V.