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
Fig.1: The Fe-Cr equilibrium diagram
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.
Fig.2: Effect of carbon on the Fe-Cr diagram
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
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
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
Ni equivalent = (Ni) + (Co) + 0.5(Mn) + 0.3(Cu) + 25(N) + 30(C)
All concentrations being expressed in weight percentages.
Fig.3: Schaeffler diagram. Effect of alloying elements on the basic structure
of Cr- Ni stainless steels.
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.