AISI 403(UNS S40300, X7Cr13) stainless steel is a martensitic stainless steel formulated for primary forming into wrought products. 403 is the AISI designation for this material. S40300 is the UNS number. Additionally, the British Standard (BS) designation is 403S17.

This straight chromium stainless steel offers reasonable corrosion resistance and high mechanical characteristics. The 403 Series offers heat resistance and a scaling temperature of about 1300°F. It is a corrosion and heat resistant 12% Cr martensitic stainless steel. This alloy offers a wide range of mechanical properties that are obtainable by appropriate heat treatments. It is easily hardened by oil, fan or air cooling and may be tempered at temperatures of up to 1350°F (732°C) to improve fracture toughness. Oil or fan cooling is required in thicker sections
This material is well established: the Further Reading section below cites a number of published standards, and that list is not necessarily exhaustive.

It has the lowest base cost and a moderately low embodied energy among wrought martensitic stainless steels.

The properties of AISI 403 stainless steel include two common variations. This page shows summary ranges across both of them. For more specific values, follow the links immediately below. The graph bars on the material properties cards further below compare AISI 403 stainless steel to: wrought martensitic stainless steels (top), all iron alloys (middle), and the entire database (bottom). A full bar means this is the highest value in the relevant set. A half-full bar means it's 50% of the highest, and so on.