600 series stainless steels are also called as precipitation hardening stainless steels. They have advantages of both austenitic stainless steels and martensitic stainless steels – that is, corrosion resistance, hardness, and strength. Originally, 600 series stainless steels were designed out for aerospace and aircraft industries. Nowadays, they are applied into making precision turned components. Before machining, they are usually supplied in annealed conditions and are hereafter doing heat treatment. 17-4PH is now the most extensively applied with industries.

Stainless Steels Chemical Composition, %
TYPE C  Si  Mn  P  S Ni  Cr Mo Cu other
630 ≤0.07  ≤1.0  ≤1.0 ≤0.030 ≤0.030 3.00~5.00  15.00~17.50    3.00~5.00  Nb 0.15~0.45
631 ≤0.09  ≤1.00  ≤1.00  ≤0.04  ≤0.03  6.50~7.75  16.00~18.00      Al 0.75~1.50