What carburising does: Carburising is a case-hardening process in which a low-carbon steel is held at a high temperature (typically around 900–950 °C, in the austenite range) in a carbon-rich environment (gas, pack, or liquid). Carbon atoms diffuse inward from the surface, raising the surface carbon content so that after quenching the case transforms into hard, wear-resistant martensite while the low-carbon core stays tough. The depth and carbon concentration of the case both increase with holding time, since diffusion depth grows roughly with the square root of time.

Why excessive time causes retained austenite: If the part is carburised for too long, the surface carbon content becomes excessively high (well above ~1.0–1.2%). High carbon strongly depresses the martensite-start (M_s) and martensite-finish (M_f) temperatures. When M_f falls below room temperature, quenching to room temperature cannot complete the austenite-to-martensite transformation. The result is excessive retained austenite at the surface — which is the correct answer.

Why this matters: Retained austenite is a soft, unstable phase. It lowers surface hardness and wear resistance, and it can later transform to martensite during service, causing volume change, dimensional instability, and residual stresses. (It is often reduced by sub-zero/cryogenic treatment or tempering.)

Why the other options are wrong:

• Increased porosity is a casting or powder-metallurgy defect; carburising is a solid-state diffusion process and does not create through-thickness porosity.
• Coarse pearlite across the section would result from slow cooling, not from over-carburising followed by quenching — and it would affect the core structure through cooling rate, not carbon time.
• Intergranular corrosion in the core is associated with sensitised stainless steels (chromium-carbide precipitation), not with carburising a plain/low-alloy carbon steel.