AK Steel Type 420 is a martensitic stainless steel that provides corrosion resistance similar to Type 410 plus increased strength and hardness. It is magnetic in both the annealed and hardened conditions. Maximum corrosion resistance is attained only in the fully hardened or fully hardened and stress relieved condition. It is never used in the annealed condition.

Applications requiring good corrosion resistance and high hardness are ideal for this alloy. Typical uses include cutlery, surgical and dental instruments, scissors, tapes and straight edges. The alloy is not normally used at temperatures exceeding 800°F (427°C) due to rapid softening and loss of corrosion resistance.