The equilibrium diagram does not
tell us what form is taken by the ferrite or cementite ejected from
the austenite on cooling. Without going too deeply into the matter,
it may be considered that the ferrite has a choice of three
different positions, which, in order of degree of supercooling or
ease of forming nuclei,
(1) boundaries of the austenite crystals; (Fig. 1)
certain crystal planes (octahedral); (Fig. 2)
inclusions (Fig. 3).
Thus, ferrite starts to form at the grain boundaries, and if
sufficient time is allowed for the diffusion phenomena a ferrite
network structure is formed, while the pearlite occupies the centre,
as in Fig. 1. The size of the austenite grains existing above
A3 is thereby betrayed.
If the rate of cooling is faster, the complete separation of the
ferrite at the boundaries of large austenite grains is not possible,
and ejection takes place within the crystal along certain planes,
forming a mesh-like arrangement known as a Widmanstätten structure,
shown in Fig. 2. In steels containing more than 0,9% carbon,
cementite can separate in a similar way and Widmanstätten structures
are also found in other alloy systems.
Steel with Widmanstätten structures are characterised by
(1) low impact value, (2) low percentage elongation since the strong
pearlite is isolated in ineffective patches by either weak ferrite
or brittle cementite, along which cracks can be readily propagated.
This structure is found in overheated steels and cast steel, but the
high silicon used in steel castings modifies.
It is highly desirable that Widmanstätten and coarse network
structures generally be avoided, and as these partly depend upon the
size of the original austenite grain, the methods of securing small
grains are of importance. Large austenite grains may be refined by
(a) hot working, (b) normalising.
Such refined austenite grains are liable to coarsen when the
steel is heated well above the Ac3 temperature, in such
operations as welding, forging and carburising unless the grain
growth is restrained. This restraint can be brought about by a
suitable mode of manufacture of the steel.
Controlled grain size
It is now possible to produce two steels of practically identical
analysis with inherently different grain growth characteristics so
that at a given temperature each steel has an "inherent austenite
grain size", one being fine relative to the other. The so-called
"fine-graine" steel increases its size on heating above
Ac3 but the temperature at which the grain size becomes
relatively coarse is definitely higher than that at which a
"coarse-grained" steel develops a similar size.
The fine-grained steels are "killed" with silicon together with a
slight excess of aluminium which forms aluminium nitride as
submicroscopic particles that obstruct austenite grain growth and is
an example of a general phenomenon.
At the coarsening temperature the AIN goes into solution rapidly
above 1200°C and virtually completely at 1350°C. The austenite grain
size is frequently estimated by the following tests:
(1) McQuaid-Ehn Test. Micro-sections of structural
steels carburised for not less than 8 hours at 925°C and slowly
cooled to show cementite networks are photographed at a
magnification of 100. Comparison is made with a grain-size chart
issued by the American Society for Testing Materials. This test is
also valuable in detecting "abnormality" of pearlite.
(2) The Quench and Fracture test consists in heating
normalised sections of the steel, above Ac3 quenching
them at intervals of 30°C. An examination of the fractured surface
enables the depth of hardness and grain size to be estimated by
comparison with standard frac tures.