20Cr2Ni4A History of engineering & technology PVD coating Vanadis 6 die steel carbon diffusion carburizing catalysis cryo-processing cryo-treatment cryogenic treatment fractography fracture toughness grade 92 steel weldment hardness hydrogen embrittlement ion implantation ledeburitic tool steels mechanical properties metallography and fractography microstructure nitriding post-welding heat treatment rare earths rare-earth element pre-implantation sub-zero treatment at −75 °C sub-zero treatments surface finish tensile straining toughness vacuum carburizing