Architecting an ultrahard surface layer on maraging steel by a surface thermomechanical approach

Maraging steels with ultra-high strength and superior fracture toughness are extensively applied as structural materials in modern industries. Various mechanical surface processes have been developed to enhance their surface hardness and therefore such properties as fatigue and wear resistances. However, the enhancements are limited mostly due to the difficulties in plastic deformation and low work-hardening capabilities of such kinds of materials. In this work, a peak-aged 18Ni maraging steel was submitted to surface mechanical grinding treatments (SMGT) at room and elevated (∼673 K) temperatures, both resulting in the formation of gradient microstructure and hardness enhancement in the surface layer. In comparison, the surface hardness on samples treated at elevated temperature (∼8.9 GPa) is much higher than that on samples treated at room temperature (∼6.2 GPa). Microstructure observations revealed that the extraordinary hardening effect is related to the enrichment of alloying elements at grain boundaries and microstructure relaxation in the near surface layer of samples treated at elevated temperature, in addition to the formation of nanostructured martensite and intensive dislocations which also occurs in samples treated at room temperature. Accordingly, a thermomechanical strategy combining SMGT and heating processes was proposed to architect an ultrahard surface layer on maraging steels.