Composition: Carbon steel (0.35%C hypoeutectoid).

Processing: Hot-rolled. Annealing (870ºC - 1h), Water quenching.

Etching: 10-15 s immersion in 2% nital.

This is the same steel as the one in specimens X19 and X24, but in this case water quenching was carried out after annealing. This implies a much faster cooling rate and, as a result, the precipitation of proeutectoid ferrite and the eutectoid reaction are both avoided due to the lack of time for diffusion to occur. Austenite, which is not stable at room temperature, transforms into martensite, a metastable phase that is hard and brittle. The martensite that is formed in this steel shows a needle-like morphology. Some areas of this specimen may show upper bainite (at the grain boundaries) with a feather-like morphology. This a sign of cooling rates below the critical quench rate. The last micrograph shows an example of this.

Fe-Fe₃C diagram

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