Decarburisation during high-temperature exposure of steel: Which statements correctly describe the mechanism and conditions that favour decarburisation in furnaces?

Introduction / Context:
Decarburisation is the loss of carbon from the surface layers of steel when heated in oxidising or certain neutral atmospheres. It alters surface hardness and strength, and is critical in heat treatment, forging, and hot rolling operations.

Given Data / Assumptions:

• Steel heated above critical temperatures for significant time.
• Furnace atmosphere contains oxidising species (e.g., CO₂, H₂O, O₂).
• No protective coatings or inert atmospheres are used.

Concept / Approach:
At temperature, carbon in austenite and iron carbide (Fe₃C) reacts with oxidisers to form CO or CO₂, reducing surface carbon content. This changes phase transformation behaviour and microstructure upon cooling, affecting properties like hardness and wear resistance. The reaction rate rises with CO₂ and H₂O partial pressure and time-at-temperature.

Step-by-Step Solution:

Verification / Alternative check:
Metallography shows a decarburised layer with reduced pearlite/martensite content near the surface; hardness profiles confirm lower surface hardness.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing decarburisation with oxidation scale. They often coexist, but decarburisation specifically refers to carbon loss in the metal, not only oxide formation.

Final Answer:
all (a), (b) and (c).