Ultrafine-grained Stainless Steel Research Articles

Ultrafine-grained stainless steel with wear-resistant nanoborides decorating grain boundaries produced by laser powder bed fusion

Stainless steel composites combine corrosion resistance and toughness of the metallic matrix with wear resistance granted by hard reinforcing nanoparticles. Given their near-net-shape manufacturing characteristics, additive manufacturing techniques are attractive to produce these hard and wear-resistant classes of materials with low machinability. In this work, from a SAF 2205 duplex stainless steel with boron addition, laser powder bed fusion (LBPF) was employed to produce an ultrafine-grained (∼1 µm), dense (porosity ∼0.1 %), and crack-free ferritic-induced matrix composite with Cr2B-nanoborides reinforcing the grain boundaries (GBs) without formation of Cr-depleted regions. The composite showed significant higher hardness (up to 456 HV0.5) and wear resistance (4.4 x 10−5 mm3 N−1 m−1) in sliding condition compared to a hot-rolled (225 HV0.5 and 2.9 x 10−3 mm3 N−1 m−1) and a LPBF-produced SAF 2205 (314 HV0.5 and 3.3 x 10−4 mm3 N−1 m−1).

Microstructural evolution in adiabatic shear band in the ultrafine-grained austenitic stainless steel processed by multi-axial compression

Adiabatic shear bands play the most important role in the deformation and failure of high strength AISI 201 austenitic stainless steel. The microstructure and microtexture of the adiabatic shear band in ultrafine-grained stainless steel are investigated by means of optical microscopy, TEM and EBSD. The shear bands can be generated at about 54μs after the true flow stress reaches the peak value of about 1135MPa. The width of the shear bands is about 9μm, and the grains sizes 50–200nm in the boundary of the shear band are elongated along the shear direction; the grain subdivision on approaching the shear band can be observed, and the core of the shear band consists of recrystallized equiaxed grains (sizes 30–80nm) with new microtextures and the ultrafine grains with deformed microtextures and with high dislocation density. The grain boundaries in the adiabatic shear band are geometrical necessary boundaries with high-angles. The calculated temperature in the shear band is estimated to reach 0.58 Tm (968K). It takes 7μs for the shear band to cool down from 968K to the room temperature when the shear localization ceases, and the cooling strain rate is calculated as high as 9.6×107K/s. Kinetic calculations indicate that during the deformation process, the recrystallized nanosized grains can be formed in the shear band by way of the subgrain boundaries rotation when the subgrains׳ sizes are lower than 80nm or the temperature in the shear band is higher than 0.5 Tm (834K), and they do not undergo significant growth by grain boundary migration at the cooling stage. These results indicate that the microstructure development within shear band is controlled by both dynamic recovery and rotational dynamic recrystallization.