High Manganese Austenitic Steel (HMnAS) is a novel steel exhibiting excellent mechanical properties at arctic temperatures. Defense Material Organisation (DMO) is interested in the feasibility of using HMnAS in their navy's ships alongside EH36 high-strength shipbuilding steel, using 307PF stainless steel weld filler metal.
The feasibility of using HMnAS is investigated by performing mechanical tests on both the base materials and the weld. Microstructure and fractographs were analysed using optical and electron microscopy, and the chemical composition was measured using EDS, EPMA and XRD. The results of mechanical tests were compared to current standards for EH36.
This study yielded three main observations. The first important result is the finding of severe anisotropy in the HMnAS. Impact toughness in the L-T direction was threefold the amount in the T-L direction. This is thought to be because of stringer MnS inclusions, elongated in the rolling direction. The second observation was the inadequate mechanical behaviour at the EH36 fusion line and the weld, these did not meet the requirements of IACS standards. Lastly, after observing a crack path along the fusion line of the CTOD fatigue pre-cracking, microhardness tests over the fusion lines were conducted and pointed out a hard band along the EH36 fusion line. This band is found to be a martensitic band (15-108 microns in width) that hampers mechanical performance.
This investigation concludes that the current weld joint is not as desired. More research is required to improve the weld joint before HMnAS can be implemented on navy ships. Future research for DMO should mainly focus on the welding characteristics and the filler metal composition, to improve weld performance.