During the last fifty years engineers have
demanded steels with higher and higher tensile strength, together with adequate ductility. This has been particularly so where
lightness is desirable, as in the automobile and aircraft
industries. An increase in carbon content met this demand in a
limited way, but even in the heat-treated condition the maximum
strength is about 700 MPa above which value a rapid fall in
ductility and impact strength occurs and mass effects limit the
Heattreated alloy steels provide high strength, high yield point,
combined with appreciable ductility even in large sections. The use
of plain carbon steels frequently necessitates water quenching
accompanied by the danger of distortion and cracking, and even so
only thin sections can be hardened throughout. For resisting
corrosion and oxidation at elevated temperatures, alloy steels are
The Alloy Steels Research Committee adopted the following
definition: “Carbon steels are regarded as steels containing not
more than 0,5% manganese and 0,5% silicon, all other steels being
regarded as alloy steels”.
The principal alloying elements added to steel in widely varying
amounts either singly or in complex mixtures are nickel, chromium,
manganese, molybdenum, vanadium, silicon and cobalt.
The effect of the alloying element in the steel may be
one or more of the following:
(1) It may go into solid solution in the iron, enhancing the
strength. The general effectiveness is shown in Fig. 1.
carbides associated with Fe,C may be formed.
(3) It may form
intermediate compounds with iron, e.g. FeCr (sigma phase), Fe,W,.
(4) It may influence the critical range in one or more of the
(a) Alter the temperature. For example, 3% nickel lowers
the Ac points some 30°C, while 12% chromium raises the Ac1,
temperature to about 800°C and also forms a range of 150/200°C above
this in which the pearlite changes to austenite. Fig. 2 shows the
effect of alloys on the eutectoid temperature.
(b) Alter the carbon content of the eutectoid (Fig. 2).
The carbon content of the pearlite in a 12% chromium steel is 0,33%,
as compared with 0,87 in an ordinary steel. Nickel also reduces the
amount of carbon in the pearlite and consequently increases the
volume of this constituent at the expense of the weaker ferrite.
(c) Alter the “critical cooling velocity”, which is the
minimum cooling speed which will produce bainite or martensite from
austenite. Typical critical speeds obtained by quenching from 950°C
are given in Table 1.
||Figure 1. Hardening effects of
alloying elements in solid solution in fully annealed ferrite
||Figure 2. Effects of alloying elements on the carbon
content and temperature of the eutectoid point
Table 1. Effect of alloying on the critical cooling
speed of steel
Alloying Element, %
Cooling Speed to form Martensite, °C per sec
The efficiency of the additions of the various alloy elements in
reducing the effect of mass during quenching may be judged by the
relative reduction of the critical velocity of the steel. Chromium
and manganese respectively are far more effective than nickel.
(5) Combinations of elements can be chosen so that the volume
change is reduced and also the risk of quench cracking. It may
produce effeets characteristic of the alloying element.
(a) It may render the alloy sluggish to thermal changes,
increasing the stability of the hardened condition and so producing
tool steels which are capable of being used up to 550°C without
softening and in certain cases may exhibit an increase in
(b) It may have a chemical effect on the impurities. Under
suitable slag conditions vanadium, in quite small quantities,
"cleans" the steel and renders it free from slag inclusions.
Manganese and zirconium form sulphides.
(c) Certain elements such as chromium, Aluminium, silicon and
copper tend to produce adherent oxide films on the surface of the
steel which increase its resistance to corrosion and oxidation at
(d) Creep strength may be increased by the presence of a
dispersion of fine carbides, e.g. molybdenum.
Classification of alloying additions
Classification of alloying metals according to their effect
in the steel is difficult, because the influence varies so widely
with each addition depending on the quantity used and other elements
present. A useful grouping, however, is based upon the effect of the
element on (a) the stability of the carbides and (b) the stability
of the austenite.
(1) Elements which tend to form carbides.
Chromium,tungsten,titanium, columbium, vanadium, molybdenum and
manganese. The mixture of complex carbides is often referred to as
(2) Elements which tend to graphitise the carbide.
Silicon, cobalt, aluminium and nickel. Only a small proportion of
these elements can be added to the steel before graphite forms
during processing, with attendant ruin of the properties of the
steel, unless elements from group 1 are added to counteract the
(3) Elements which tend to stabilise austenite.
Manganese, nickel, cobalt and copper.
These elements alter the critical points of iron in a similar way
to carbon by raising the A4 point and lowering the
A3 point, thus increasing the range in which austenite is
stable, and they also tend to retard the separation of carbides.
They have a crystal lattice (f.c.c.) similar to that of g-iron in which they are more soluble than in
(4) Elements which tend to stabilise ferrite. Chromium,
tungsten, molybdenum, vanadium and silicon .
These elements are more soluble in a-iron than in g-iron.
They diminish the amount of carbon soluble in the austenite and thus
tend to increase the volume of free carbide in the steel for a given
carbon content. On the binary equilibrium diagram of these elements
with pure iron the A4 point is lowered and A3
raised (although it may be lowered initially), until the two points
merge to form a “closed gamma loop”.
Thus, with above, a certain amount of each of these elements the
austenite phase disappears and ferrite exists from the melting-point
down to room temperature. No critical points exist and such steels
(e.g. 18% chromium irons) are not amenable to normal heat treatment,
except recrystallisation after cold work. This effect, however, can
be counteracted by adding elements from group 3. For example, 2% of
nickel is added to the 18% chromium stainless steel to enable it to
be refined by normal heat-treatment; carbon has the same effect.
Aluminium has the reverse effect in 12 % chromium steel.