TABLE OF CONTENTS Some elements extend the ?-loop in the iron-carbon equilibrium diagram (see Chapter 4), e.g. nickel and manganese. When sufficient alloying element is added, it is possible to preserve the face-centred cubic (fcc) austenite at room temperature, either in a stable or metastable condition. Chromium added alone to a plain carbon steel tends to close the ?-loop and favour the formation of ferrite. However, when chromium is added to a steel-containing nickel it retards the kinetics of the ? ? ? transformation, thus making it easier to retain austenite at room temperature. The presence of chromium greatly improves the corrosion resistance of the steel by forming a very thin stable oxide film on the surface, so that chromium-nickel stainless steels are now the most widely- used materials in a wide range of corrosive environments both at room and elevated temperatures. Added to this, austenitic steels are readily fabricated and do not undergo a ductile/brittle transition which causes so many problems in ferritic steels. This has ensured that they have become a most important group of construction steels, often in very demanding environments. Nevertheless, there are also some important ferritic stainless steels which will be discussed in this chapter.
The binary iron-chromium equilibrium diagram (Fig. 12.1) shows that chromium restricts the occurrence of the ?-loop to the extent that above 13 wt% Cr the binary alloys are ferrite over the whole temperature range, while there is a narrow ( ? +
