Microstructure Analysis of Quenched Steel

The question asks for the microstructure of a 0.2 wt.% plain carbon steel after heating in the inter-critical region and subsequent rapid quenching.

Phase Identification in Inter-critical Region

The inter-critical region for steels lies between the lower critical temperature (A1) and the upper critical temperature (A3). For a 0.2 wt.% carbon steel:

• Heating into this region causes partial transformation of ferrite ($\alpha$) to austenite ($\gamma$).
• The equilibrated microstructure at this stage consists of a mixture of proeutectoid ferrite (the phase present below A1) and austenite ($\gamma$).

Effect of Rapid Quenching

Instant water quenching provides a very high cooling rate. This rate is crucial for phase transformations:

• The austenite ($\gamma$) phase, upon rapid cooling below its martensite start (Ms) temperature, transforms primarily into martensite. This transformation occurs with minimal diffusion.
• The proeutectoid ferrite ($\alpha$) present before quenching is relatively stable and does not significantly transform into martensite under these conditions. It remains as ferrite in the final microstructure.

Final Microstructure Determination

Combining the results of the quenching process:

• The austenite transforms to martensite.
• The proeutectoid ferrite remains as ferrite.

Therefore, the resulting microstructure is a combination of proeutectoid ferrite and martensite.