1Harden the surface of (a material)

1. 1.1 Give a hard surface to (iron or steel) by carburizing it.
   用渗碳法使（铁，钢）表面硬化
   the grade of steel was unsuitable for the type of heat treatment process used to case-harden the gears
2. 1.2 Make (someone) callous or tough.
   〈喻〉使（人）无情（或冷酷）
   years of surgical experience did not case-harden him to suffering