Martensite needles

Fig. 6.53. Micrograph of a hardened C60 steel. The martensite needles can be observed...

Figure 10.17 Transmission electron micrograph showing the structure of bainite. A grain of bainite passes from lower left to upper right corners it consists of elongated and needle-shape particles of FejC within a ferrite matrix. The phase surrounding the bainite is martensite. (From Metals Handbook, Vol. 8,8th edition, Metallography, Structures and Phase Diagrams, 1973. Reproduced by permission of ASM International, Materials Park, OH.)...

In addition, the phase that surrounds the needle is martensite, the topic addressed by a subsequent section. Furthermore, no proeutectoid phase forms with bainite. 

Figore 10.21 Photomicrograph showing the martensitic microstructure. The needle-shape grains are the martensite phase, and the white regions are austenite that failed to transform during the rapid quench. 

Martensite Body-centered, tetragonal, single phase Needle-shaped grains Very hard and very brittle... 

Martensite—this has platelike or needle-like grains of an iron-carbon solid solution that has a body-centered tetragonal crystal structure. Martensite is produced by rapidly quenching austenite to a sufficiently low temperature so as to prevent carbon diffusion and the formation of pearlite and/or bainite. 

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