Precipitation Hardening Stainless Steel Basics
Precipitation Hardening Stainless Steel Basics - What are precipitation hardening stainless steels?
PH stainless steels are heat treated by precipitation hardening to provide high strength, excellent corrosion resistance and simplified heat treatment compared to conventional martensitic stainless steels.
Introduction to Precipitation Hardening (PH)
Precipitation hardening is a heat treatment process that is different from conventional heat treatment.
The initial "solution treatment" is performed at high temperature, usually 1900°F (about 1038°C), to ensure that all alloying elements are evenly distributed.
The cooling rate after the solution treatment maintains the distribution of the hardening elements.
The structure may be martensitic, semi-austenitic or austenitic, depending on the specific alloy chemistry.
Types of Precipitation Hardening Alloys
Martensitic Alloys: Solution treated to form a low carbon, more brittle martensitic structure, which should not be used in the solution treated state.
By heating again to the aging temperature, the particles formed further strengthen the structure, improving toughness and corrosion resistance.
Heat Treatment Conditions
The heat treatment condition is defined by the letter "H" followed by the aging temperature, for example: H900 means aged at 900°F (about 480°C) after solution treatment.
The hardness increases, and the yield strength can reach a minimum of 170ksi (about 1170MPa) with this two-step simple heat treatment.
Aging Temperature Range
The heat treatment conditions range from H900 to H1150, and even double HH1150, meaning two aging treatments at 1150°F (about 620°C).
Material Conditions
H1150M is the over-aged condition, which produces the lowest hardness.
Solution Treated, Solution Annealed, Annealed, and Cond A are synonymous in these stainless steels and mean the same thing.
Applications
Typically, these types of materials are solution treated by the production steel mill and then aged after additional processing.
If the material is already in the desired aged condition, no further heat treatment is required.
Common Stainless Steels
Common stainless steels include 17-4 (also known as 630), 15-5, 13-8, 450, and 455.
15-5 and 13-8 are examples of advanced vacuum melted grades, where additional melting under vacuum minimizes harmful impurities and is suitable for critical applications such as highly stressed aerospace components.
