Austenitic Steels

Some elements extend the γ-loop in the iron-carbon equilibrium diagram, e.g. nickel and manganese. When sufficient alloying element is added, it is possible to preserve the face-centered cubic austenite at room temperature, either in a stable or metastable condition.

Chromium added alone to plain carbon steel tends to close the γ-loop and favor the formation of ferrite. However, when chromium is added to a steel containing nickel it retards the kinetics of the γ → α 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.

Simple austenitic steels usually contain between 18 and 30% Cr, 8 to 20% Ni, and between 0.03 and 0.1% carbon. The binary iron-chromium equilibrium diagram (Fig. 1) shows that chromium restricts the occurrence of the γ-loop to the extent that above 13% Cr the binary alloys are ferritic over the whole temperature range, while there is a narrow (α + γ) range between 12% and 13% Cr. The ferrite is normally referred to as delta ferrite, because in these steels the phase can have a continuous existence from the melting point to room temperature.

The addition of carbon to the binary alloy extends the γ-loop to higher chromium contents (Fig. 2), and also widens the (α + γ) phase field up to 0.3% C. When carbon is progressively added to an 18% Cr steel, in the range up to about 0.04% C, the steel is fully ferritic (Fig. 2) and cannot be transformed. Between 0.08 and 0.22% C, partial transformation is possible leading to (α + γ) structures, while above 0.40% C the steel can be made fully austenitic if cooled rapidly from the γ - loop region.

In austenitic steels, M₂₃C₆ is the most significant carbide formed and it can have a substantial influence on corrosion resistance.

If nickel is added to a low carbon iron-18% Cr alloy, the γ -phase field is expanded until at about 8% Ni the γ-phase persists to room temperature leading to the familiar group of austenitic steels based on 18% Cr 8% Ni. This particular composition arises because a minimum nickel content is required to retain γ at room temperature. With both lower and higher Cr contents more nickel is needed. For example, with more corrosion resistant, higher Cr steels, e.g. 25% Cr, about 15% Ni is needed to retain the austenite at room temperature.

Lack of complete retention is indicated by the formation of martensite. A stable austenite can be defined as one in which the Ms is lower than room temperature. The 18Cr8Ni steel, in fact, has an Ms just below room temperature and, on cooling, e.g. in liquid air, it will transform very substantially to martensite.

Manganese expands the γ -loop and can, therefore, be used instead of nickel. However, it is not as strong a γ -former but about half as effective, so higher concentrations are required. In the absence of chromium, around 12% Mn is required to stabilize even higher carbon (1-1.2%) austenite, achieved in Hadfields steel which approximates to this composition. Typically Cr-Mn steels require 12-15% Cr and 12-15% Mn to remain austenitic at room temperature if the carbon content is low.

Like carbon, nitrogen is a very strong austenite former. Both elements, being interstitial solutes in austenite, are the most effective solid solution strengtheners available. Nitrogen is more useful in this respect as it has less tendency to cause intergranular corrosion. Concentrations of nitrogen up to 0.25 %are used, which can nearly double the proof stress of Cr-Ni austenitic steel.

One of the most convenient ways of representing the effect of various elements on the basic structure of chromium-nickel stainless steels is the Schaeffler diagram, often used in welding. It plots the compositional limits at room temperature of austenite, ferrite and martensite, in terms of nickel and chromium equivalents (Fig. 3).

At its simplest level, the diagram shows the regions of existence of the three phases for iron-chromium-nickel alloys. However, the diagram becomes of much wider application when the equivalents of chromium and of nickel are used for the other alloying elements. The chromium equivalent has been empirically determined using the most common ferrite-forming elements:

Cr equivalent = (Cr) + 2(Si) + 1.5(Mo) + 5(V) + 5.5(Al) + 1.75(Nb) + 1.5(Ti) + 0.75(W)

while the nickel equivalent has likewise been determined with the familiar austenite-forming elements:

Ni equivalent = (Ni) + (Co) + 0.5(Mn) + 0.3(Cu) + 25(N) + 30(C)

All concentrations being expressed in weight percentages.

The large influence of C and N relative to that of the metallic elements should be particularly noted. The diagram is very useful in determining whether particular steel is likely to be fully austenitic at room temperature. This is relevant to bulk steels, particularly to weld metal where it is frequently important to predict the structure in order to avoid weld defects and excessive localized corrosive attack.