Achieving balanced strength and toughness in 2.5Ni-Cr-Mo steel via submerged arc additive manufacturing by regulating intrinsic heat treatment

Abstract

Submerged arc additive manufacturing (SAAM) is attractive for the production of large-size, medium-complex parts. The approach enables the manipulation of ‘full-layer-penetrated’ intrinsic heat treatment (IHT) and thus the resulting metal's microstructure, by controlling the processing parameters. We recently found that this IHT can trigger repeated recrystallizations involved with allotropic transformation in the high-strength low-alloy steel. Here, we report a 2.5Ni-Cr-Mo steel, inherently sensitive to grain boundary-segregated impurities and blocky martensite/austenite (M/A) phase, of which microstructure and properties are tailor-designed via SAAM. The α-Fe phase matrix of this steel is refined in situ by multiple recrystallization whilst impurities are desegregated as the prior-γ grain boundaries disappeared. M/A phase is then finer and is deliberately dispersed via in situ element partitioning, followed by an long-duration tempering. Control of multiple recrystallization, impurity desegregation and dispersion of M/A phase leads to a near homogeneous microstructure, enabling the substantial improvement on impact toughness (∼9 J–∼42 J at −60 °C, for half-sized specimen). Our material has a tensile strength of 820 MPa, yield ratio of 0.65–0.72, and ductile-brittle transition temperature of −80 °C, exhibiting superior mechanical properties to those of common engineering steels. SAAM approach and the principles of in situ microstructure control provides new idea and avenues for new additive manufacturing and the development of heavyweight steel parts for various industrial applications.