Furnace atmosphere control – Primary purpose: Controlling furnace atmosphere is mainly intended to prevent which types of surface attack during heating of steel components?

Introduction / Context:
Heat treatment quality depends not only on temperature and time but also on the furnace atmosphere. Uncontrolled atmospheres cause surface degradation, dimensional change, and property loss, undermining product performance and post-process finishing.

Given Data / Assumptions:

• Steel components are heated in furnaces for annealing, normalising, hardening, or tempering.
• Atmosphere composition (O2, H2O, CO/CO2, H2, CH4) influences surface reactions.
• Main concerns in general heat treatment are scaling and decarburisation.

Concept / Approach:
Oxidation/scaling occurs when oxygen potential is high at temperature, forming iron oxides. Decarburisation occurs when the carbon activity of the atmosphere is below that of the steel, pulling carbon out and reducing surface hardness after quench. Proper control (neutral/reducing, dew-point control, endogas/nitrogen-methanol systems) suppresses both.

Step-by-Step Solution:
Identify principal defects addressed by atmosphere control: scaling and decarburising.Relate to atmosphere chemistry: adjust CO/CO2 and H2/H2O ratios to a carbon potential matching the steel, and minimise free oxygen.Select the option that covers both major mechanisms.

Verification / Alternative check:
Shop practice uses neutral atmospheres to avoid surface loss; verification includes metallographic inspection for decarburised layer and visual inspection for scale.

Why Other Options Are Wrong:

• Oxidation/scaling only: incomplete; decarburising is equally critical.
• Decarburising only: incomplete; preventing scale is also essential.
• Sulphur penetration: generally controlled via low-sulphur fuels and materials; not the primary universal purpose across heat treatments.

Common Pitfalls:
Assuming a single atmosphere works for all steels; neglecting dew-point control; ignoring carbon potential matching for carburising steels.

Final Answer:
Both oxidation/scaling and decarburising