Superior mechanical properties in high-Ni transformation induced plasticity steels after friction stir welding

The high-Ni TRIP steel plates were butt joined by friction stir welded. For this aim, the rotational and traverse speeds were set at 600 rpm-100 mm-min−1 and 950 rpm-50 mm-min−1. The optical microscopy, X-Ray diffraction (XRD, orientation image microscopy, and tensile test were employed for the characterization of the joints. The results showed that friction stir welding replaced the large grains of the base metal with fine and equiaxed grains by continuous dynamic recrystallization (CDRX). In addition, friction stir welding lead to the stabilization of the austenite. The lower heat input condition caused finer grain sizes. Using lower heat inputs resulted in more retained austenite by means of both work hardening and finer grain size mechanisms. The friction stir welded joints showed superior UTS and elongation of 1130 MPa-115 % and 1050 MPa-76 % respectively for the low and high heat input conditions compared to that of the base metal i.e., 880 MPa-60 % due to stabilization of austenite and formation of more transformation induced plasticity (TRIP) effect. The low heat input joint showed superior tensile properties thanks to more austenite stabilization.