Chromium in Martensitic Stainless Steels

September 26, 2016
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The interplay between chromium and carbon is further explained in the two graphs of the iron-chromium phase diagram. The iron-chromium phase diagram at (a) shows 12% chromium and (b) shows 17% chromium. Higher chromium alloys, the range over which full austenitization can occur is further restricted.

The variety of martensitic stainless steels would be very limited if only chromium and carbon were available as alloying elements, but fortunately nickel can make an important contribution. Nickel greatly expands the chromium levels and temperatures at which austenite is stable,

Volume 4D, Heat Treating of Irons and Steels â‡’ Heat Treated High-Alloy Steels ⇒ Heat Treating of Martensitic Stainless Steels ⇒ Alloying

One Minute Mentor: Martensitic Stainless Steels

September 16, 2016
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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 â‡’ Heat Treated High-Alloy Steels â‡’ Heat Treating of Martensitic Stainless Steels â‡’ Alloying