4. Austenitic-ferritic stainless steels

4.1 Chemical composition, structure and general properties

Austenitic-ferritic stainless steels, known as "duplex" steels, contain 20-30% chromium, 2-10% nickel and between 0.1 and 0.3% nitrogen (table 7 ). They may also contain other elements such as molybdenum (up to approx. 4.5%), copper (up to 2%) and tungsten.

Modern duplex steels have two main characteristics:

• a two-phase structure (45% ferrite, 55% austenite) obtained by balancing alphagenic (Cr, Mo, Si...) and gammagenic (Ni, N, Cu, C) elements and a judicious choice of heat treatment temperature. This structure is in the hyper-quenched state, free from precipitation of brittle phases (σ, χ...)...