Martensite Definition
Martensite is formed when specimen is quenched in water above the critical cooling rate from the authenticate region (i.e. above A3 temperature) to room temperature.
Martensite Temperature
Martensite start temperatures are usually relatively easy to calculate as long because the steels have low alloy content (Kung and Rayment, 1978; Wang et al., 2000).
Martensite Structure
The martensitic structure formed after austenitisation and tempering isn't stable; on exposure at envisaged service temperatures around 600 °C, recovery processes occur, the dislocation density within the martensite laths decreases, and a ferrite subgrain structure is made . it's been found that only after 3000 h exposure at 600 °C a more or less stable microstructure is achieved, as illustrated in, which shows the dislocation density and subgrain width as functions of exposure time at 600 and 650 °C. The creep strengthening effect of the high dislocation density therefore diminishes. This effect has important implications for the extrapolation of creep rupture data to envisaged service lifetimes. In short-term testing, say below 3000 h duration, the transient strengthening effect dominates. If such data are used for extrapolation, the long-term strength are going to be over-estimated. this is often illustrated in, which shows the results of a Larson-Miller parameter extrapolation for data sets of P92 steel, differentiated by the test duration ranges:
Predicted 100 000 h stress rupture strengths at 550, 600 and 650 °C for P92, with different ranges of measured rupture times.
●
less than 1 000 h;
●
less than 3000 h;
●
all data (up to 60 000 h);
●
1000–60 000 h;
●
3000–6 0000 h.
The extrapolations administered using results for specimens during which the transient transformation strengthening is dominant (< 3000 h) yield the very best stress rupture strengths; the strain rupture strengths derived from the tests with durations greater than 3000 h provide the foremost reasonable estimates of the long-term strength. As only stress rupture data from tests with durations above 3000 h enable a satisfactory prediction of long-term stress rupture behaviour, the event of latest steels becomes rather time-consuming. Short duration creep rupture tests are of little value in selecting compositions for further development.
Martensite Transformation
The martensite transformation is diffusionless, and thus martensite forms with none interchange within the position of neighbouring atoms. Accordingly, the observed orientation relationships are an immediate consequence of the atom movements that occur during the transformation. the primary suggestion of a possible transformation mechanism was made by Bain in 1934. He suggested that since austenite could also be considered a body-centred tetragonal structure of axial ratio, the transformation merely involves a compression of the c-axis of the austenite unit and expansion of the a-axis. The interstitially dissolved carbon atoms prevent the axial ratio from going completely to unity, and, counting on composition, the c/a ratio are going to be between 1.08 and 1.0. Such a mechanism only gives rise to 3 martensite orientations whereas, in practice, 24 resulted. To account for this, Kurdjumov and Sachs proposed that the transformation takes place not by one shear process but by a sequence of two shears, first along the weather , then a minor shear along the weather ; these elements are the twinning elements of the fcc and bcc lattice, respectively. This mechanism predicts the right orientation relations but not the right habit characteristics or relief effects. Accordingly, Greninger and Troiano in 1941 proposed a special two-stage transformation, consisting of an initial shear on the irrational habit plane which produces the relief effects, along side a second shear along the twinning elements of the martensite lattice. If slight adjustments in spacing are then allowed, the mechanism can account for the relief effects, habit plane, the orientation relationship and therefore the change of structure.
Shear mechanisms of Kurdjumov and Sachs. (a) Face-centred austenite with {1 1 1}γ in horizontal plane, (b) body-centred tetragonal martensite (α′) and (c) cubic ferrite (α).
Further additions to those theories are made in an attempt to supply the perfect general theory of the crystallography of martensite transformation. Bowles, for instance , replaces the primary shear of the Greninger–Troiano mechanism by the overall sort of homogeneous deformation during which the habit plane remains invariant, i.e. all directions during this plane are unrotated and unchanged long . However, altogether such cases the matter resolves itself into one among determining whether a homogeneous strain can transform the γ-lattice into the α-lattice, while preserving coherency at the boundary between them. The homogeneous strain doesn't do that , in order that some reasonable additional sort of strain has got to be added.
This shear can occur either by twinning or by slip, the mode prevailing counting on the composition and cooling rate. Between carbon contents of 0.2% and 0.5% the martensite changes from dislocated martensite arranged in thin lathes or needles to twinned acicular martensite arranged in plates. within the martensite formed at low C contents (e.g. Fe–Ni alloys) the skinny lathes lie parallel to every other, with a {1 1 1}γ habit, to make pockets of massive martensite with jagged boundaries thanks to the impingement of other nearby pockets of lathes. The inhomogeneous shear produced by deformation twinning occurs on {1 1 2} planes within the martensite, in order that each martensite plate is formed from parallel twin plates of thickness 2–50 nm. By operation of such a posh transformation mode with a high index habit plan the system maintains an invariant interfacial plane.
Martensite Microstructure
Microstructure is that the arrangement of the phases on the microscopic scale. A microscope are often wont to observe a material's microstructure. The microstructure of martensite contains many needle-shaped features, which cause martensite to be very brittle.
0 Comments