|
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, are:
(1) boundaries of the austenite crystals; (Fig. 1)
(2) certain crystal planes (octahedral); (Fig. 2)
(3) about 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.
|
