A stainless martensitic alloy should have the following characteristics:
- It must have at least 10.5% chromium to qualify as stainless, and even more for better corrosion resistance.
- It should be fully austenitic at some temperature.
- The temperature at which austenite forms on heating should be sufficiently high to permit tempering above the temper embrittlement range.
These criteria are somewhat challenging to meet. The two graphs show iron-chromium phase diagrams at two low-carbon levels.
The figure on the left shows that at low-carbon (0.05%) levels, austenite is stable up to about 12% chromium, above which some δ-ferrite tends to be stable at all temperatures below the melting point.
Increasing carbon slightly, to 0.1%, shown on the right, expands the chromium level at which the steel becomes fully austenite.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Heat Treating of Martensitic Stainless Steels ⇒ Alloying
