temperatures where the cooling rate is sufficient to suppress the higher transformation temperature processes of ferrite, pearlite and bainite. The transformation reaction occurs by a diffusionless shear mechanism forming a body-centred tetragonal structure. The ' formation of martensite is largely an athermal
transformation in that the extent of the transformation is independent of the time at temperature. This is the result of the very rapid rate of reaction; the rate of movement of the martensite/austenite interface can reach the speed of sound. Lowering the temperature of
transformation does not result in the existing martensite growing but causes new martensite laths to nucleate. The morphology of the martensite nucleated is dependent upon
the carbon content of the alloy, being lath like in low carbon alloys and lenticular in higher carbon alloys.
(i) Lath Martensite (31)
Lath martensite occurs in plain carbon steels with up to•=£!=.0.4% carbon. The laths tend to form in sheaves due to sympathetic nucleation and have low angle boundaries between each lath. However, they form high angle boundaries on contact with other martensite sheaves. The long straight interfaces of the laths are semi-coherent and conform to the habit plane of a Kurdjumov-Sachs type
relationship,.adopting the {111}^ habit plane. The 19
internal structure of the laths consists of a high density of dislocations which form tangled arrays but can, if the transformation temperature is high enough, form dislocation substructures. The
structure of the lath martensite is only slightly tetragonal and is often regarded as body-centred cubic.
(ii) Lenticular Martensite (32)
This type of martensitic structure appears as
individual lens-shaped plates rather than lath type sheaves. The plates often touch at their tips in a zig-zag array, resulting from a burst phenomenon. In the carbon range 0.5 to 1.4%C the martensite obeys the Kurdjumov-Sachs relationship, adopting the {225}^ habit plane. It is the adoption of the several variants of this habit plane, within a
small volume, automatically nucleated, that results in the burst phenomenon. Internally the plates normally contain micro-twins which form due to the lower martensite start (Ms ) temperature caused by the increased carbon content. Lenticular
martensite is truly a body-centred tetragonal
structure due to the martensite inheriting all the carbon from the austenite during the
transformation. The exact nature of the
transformation is complex and has been studied by various authors (31-33).
In steels with greater than 1.4% carbon the orientation relationship between the austenite and martensite changes from the Kurdjumov-Sachs to the Nishiyama-Wassermann
relationship and the {22.5}^ habit plane changes to the [25*1]y plane. The Nishiyama (34)-Wassermann relationship (35)
The martensite remains lenticular, forming by the burst mechanism and is generally accompanied by an audible click. However, the plates tend to be far more heavily twinned in nature than at lower carbon concentrations. 2.2.1.4 The Austenite to Bainite Transformation
Bainite is formed by the transformation of austenite at a temperature between that for the ferrite-pearlite
transformation and the martensite transformation. The transformation process is essentially a shear
transformation (36) but is also dependent upon the diffusion of carbon. The bainite transformation is generally regarded as a duplex transformation process, similar to pearlite, in that the resulting microstructure contains ferrite and cementite. However, two different
is
bainite structures have been observed:-
(i) Upper bainite
(ii) Lower bainite (i) Upper Bainite
Upper bainite tends to occur at the higher
temperatures in the bainitic transformation range and forms as elongated ferrite laths with
cementite forming between the laths. The
formation of the ferrite laths is the result of a shear transformation similar to that observed in martensitic structures (36). The laths form in packets due to sympathetic nucleation, which are only slightly misoriented and therefore form low angle boundaries. However, a number of different packets may be nucleated within the same austenite grain resulting in high angle boundaries between the different packets.
It is considered that the bainitic carbide results from the partitioning of carbon from the bainitic ferrite to the austenite, during the
transformation causing a build up of carbon at the austenite/ferrite interface. Consequently a stage is reached where the cementite precipitates
directly from the austenite.
along their broad faces, similar to Widmanstatten ferrite. It would therefore be expected that the interfaces would be semi-coherent and thus have an orientation relationship with the austenite from which the bainitic ferrite formed. It is found that within the accuracy of electron diffraction, upper bainite obeys either the Kurdjumov-Sachs or the Nishiyama-Wassermann orientation relationship. The orientation relationship between austenite and cementite has been shown to conform to the Pitsch relationship. Therefore, it would be expected that an orientation relationship between the bainitic ferrite and cementite would exist.
However, no unique relationship has been observed. (ii) Lower Bainite
Lower bainite tends to form at lower
transformation temperatures than experienced in the formation of upper bainite. Once again the bainitic ferrite forms as laths by a shear
process. However, because of the lower
transformation temperature the laths are more frequently nucleated within the austenite grains than for upper bainite. The sympathetic
nucleation observed in upper bainite is not so apparent in the lower bainite structure.
The bainitic carbide in lower bainite does not precipitate at the ferrite lath boundaries but rather within the laths. The carbides form as parallel arrays of plates which adopt a specific
o
angle of approximately 55 to the axis of the ferrite laths. There has been two alternative theories proposed for the occurrence of this carbide morphology. The first theory involves precipitation of cementite from supersaturated
(37) ferrite while the second involves
precipitation at the austenite/bainitic ferrite interface, during the transformation, by an interphase precipitation mechanism (38).
The former mechanism is due to the lower diffusion rate of carbon causing supersaturation of the
ferrite laths. Consequently, a high driving force is required for the ferritic growth to continue and so the transformation process relies upon the precipitation of carbides within the laths to
lower the supersaturation. However, this type of mechanism does not explain the unique orientation at which the carbides form within the ferrite laths.
The interphase precipitation mechanism involves the precipitation of the cementite at small ledges
which have been observed on the broad faces of the ferrite laths. It has been suggested that the carbides obey a Bagaryatskii orientation
relationship with the bainitic ferrite, which is the relationship generally found in the tempering of martensite. However, in contrast to tempered martensite, the carbides in lower bainite exhibit only one variant of the relationship such that the precipitates form parallel arrays at one angle of 55° to the axis of the banite laths. The
occurrence of the single variant strongly suggests a ledge type mechanism at the austenite/ferrite interface (38). However, Ohmori (39)
suggested that the precipitates obeyed the Isaichev relationship, namely:-
which is the relationship produced when cementite is formed on the ferrite surface, once again
inferring a ledge type mechanism. However, Bhadeshia (40) examined the
austenite/ferrite interface and found that contrary to the expectations of interphase precipitation theory, the cementite was essentially confined to the ferrite, suggesting cementite nucleation within supersaturated ferrite. Therefore, it is very
clear that much further work is required before the 25
exact nature of the bainite transformations can be conclusively determined.
2.2.2 The Effect of Alloying Additions