Martensitic Stainless Steels
The grades in this group are the only significant alloy group with 11-17% chromium content. These ferritic groups exhibit similarities. However, the 0.10 - 0.65 % carbon content radically changes the reaction of the group. High-carbon enables it to be hardened through heat treatment.Grade 420, like most higher-carbon non-stainless steels it can be hardened by heat treatment. It contains a minimum of 12 per cent chromium, just sufficient to give corrosion resistance properties. It has good ductility in the annealed condition but is capable of being hardened up to Rockwell Hardness 50 through heat treatment. Its best corrosion resistance is achieved when the metal is hardened and surface ground or polished. Martensitic stainless steels are best suited for high hardness. It has poor weldability and usually also requires a final harden and temper heat treatment. Corrosion resistance is lower than the common austenitic grades. Their useful operating temperature range is limited by their loss of ductility at sub-zero temperatures and loss of strength at elevated temperatures In the annealed condition this grade is relatively easily machined. But if hardened (to especially above 30HRC) machining becomes more difficult.
The most common American norms ; AISI 410 - 420