Martensite in Steels : its Significance, Recent Developments and Trends

Abstract

Abstract:Martensite is generally known as a hard but brittle microstructure. This is only true for high carbon plate martensite. Recently developed steels with a lath martensite microstructure offer an excellent toughness at yield strength of 1000 MPa yield strength. A transformation into lath martensite by glide as invariant shear mechanism is only possible at a carbon content below 0,03 %. The source of both high strength and good toughness is the high dislocation density and the narrow lath width off less than 1 pm. By a thermomechanical treatment, that leads to a finer lath structure both strength and ductility can be improved to a yield strength of 1150 MPa and an elongation of 18 % . As, unlike high carbon plate martensite, the hardness of lath martensite is not achieved by the distortion of the tetragonal cell by carbon atoms, the hardness of lath martensite remains stable up during an annealing treatment up to 600°C. This thermal stability of the lath martensit microstructure makes an additional increase of hardness by the precipitation of different types of intermetallic phases possible. The increase of the hardness from 300 HV to 600 HV by precipitation without volume changes and good cold deformability reveals many new application in manufacturing. In plate martensite too, comparatively high toughness values can be achieved, if carbon is replaced by nitrogen. The refining influence of nitrides on the austenite grain sizes and the precipitation of fine nitrides during the annealing process leads to impact values three times higher than those of comparable high carbon plate martensite.

Austenite reconstruction via EBSD measurements: a tool to understand low Carbon martensite steel properties

“…The RAQ880 material shows the lowest fraction of low angle misorientations. This indicates a higher dislocation density in martensite [11] with reducing FRT, which is in line with the expectation that martensite inherites the dislocations formed in austenite [12,13]. This higher dislocation density in austenite for the DQ 860 is visualized in Figure 8 as higher misorientation within a PAG compared to the RAQ 880 condition.…”

EBSD study of early fractured phenomena in a 350 grade Maraging steel elbows exposed to hydrofluoric acid

“…The hardness and prior austenite grain size were measured as 650 ± 25 HV and 34 ± 6 μm, respectively. It is worth mentioning that the dispersion of fresh martensite through the very low tetragonal tempered martensite improved mechanical behaviour (increasing both strength and ductility, simultaneous) [23,24]. Some microcracks that led to early failure were initiated and propagated through the investigated sample, and they are shown in Fig.…”

Effect of cooling rate on the high strain rate properties of boron steel

“…4a and b. The magnified sections of the micrographs highlight the previous austenite grain boundaries and the packets of parallel lath crystals that are characteristics of the martensite [14][15][16][17]. The microstructure of the specimen cooled at 2200 C/s appears to contain a finer and more randomly oriented structure.…”