The evolution of the microstructure changes during hot deformation of high-chromium content of stainless steels (martensitic stainless steels) is reviewed. The microstructural changes taking place under high-temperature conditions and the associated mechanical behaviors are presented. During the continuous dynamic recrystallization (cDRX), the new grains nucleate and growth in materials with high stacking fault energies (SFE). On the other hand, new ultrafine grains could be produced in stainless steel material irrespective of the SFE employing high deformation and temperatures. The gradual transformation results from the dislocation of sub-boundaries created at low strains into ultrafine grains with high angle boundaries at large strains. There is limited information about flow stress and monitoring microstructure changes during the hot forming of martensitic stainless steels. For this reason, continuous dynamic recrystallization (cDRX) is still not entirely understood for these types of metals. Recent studies of the deformation behavior of martensitic stainless steels under thermomechanical conditions investigated the relationship between the microstructural changes and mechanical properties. In this review, grain formation under thermomechanical conditions and dynamic recrystallization behavior of this type of steel during the deformation phase is discussed.

In this study, dissimilar sheets including AA3003 aluminum and A441 AISI steel were welded via cooling-assisted friction stir welding (FSW). Three different cooling mediums including forced CO₂, forced water, and forced air were employed, and a non-cooled sample was processed to compare the cooling-assisted condition with the traditional FSW condition. The highest cooling rate belongs to CO₂ and the lowest cooling rate belongs to the non-cooled sample as FSW. The best macrograph without any segregation at interface belongs to the water-cooled sample and the poorest joint with notable segregation belongs to the CO₂ cooling FSW sample. The CO₂ cooling FSW sample exhibits the smallest grain size due to the suppression of grain growth during dynamic recrystallization (DRX). The intermetallic compound (IMC) thickening was suppressed by a higher cooling rate in CO₂ cooling sample and just Al-rich phase was formed in this joint. The lowest cooling rate in the FSW sample exhibits formation of the Fe rich phase. The IMC layers were thicker at the top of the weld due to closeness with the heat generation source. The water cooling sample exhibits the highest tensile strength due to proper mechanical bonding simultaneously with optimum IMC thickness to provide appropriate metallurgical bonding. Fractography observation indicates that there is a semi-ductile fracture in the water cooling sample and CO₂ cooling sample exhibits more brittle fracture. Hardness evaluation reveals that the higher the cooling rate formed, the higher the hardness in stir zone, and hardness changes in the aluminum side were higher than the steel side.