Furnace atmosphere and surface reactions in steel heat treatment Pick the wrong statement regarding decarburising, carburising, and nitriding tendencies of common furnace atmospheres.

Introduction / Context:
Heat-treatment quality is tightly linked to furnace atmosphere chemistry. Depending on the ratios of CO, CO2, H2, H2O, CH4, and N2 (sometimes with NH3), the surface of steel can decarburise, carburise, or nitride. The question asks which statement is wrong among several known effects.

Given Data / Assumptions:

• Mild to high-carbon steel at annealing or carburising temperatures.
• Standard practice atmospheres containing H2, N2, hydrocarbons, and occasionally NH3.
• Surface reactions governed by carbon potential and nitrogen potential at temperature.

Concept / Approach:
Hydrocarbon additions such as methane raise the carbon potential (through cracking to CO/H2 and carbon), suppressing net carbon loss (decarburisation). Nitrogen plus steam can be decarburising because H2O raises the oxygen potential, driving C + H2O → CO + H2 at the surface. Hydrocarbon-rich atmospheres at annealing conditions carburise the surface. Ammonia dissociation produces atomic nitrogen that diffuses to form nitrides, boosting hardness (nitriding). Therefore, options (a), (b), and (c) reflect accepted metallurgical behavior; none is wrong.

Step-by-Step Solution:

Verification / Alternative check:
Carbon/nitrogen potential charts and Ellingham-type equilibria confirm the qualitative directions for these reactions at typical annealing temperatures.

Why Other Options Are Wrong:

• Methane additions always oxidise the surface: Incorrect; methane increases carbon potential rather than oxidising the surface.

Common Pitfalls:
Confusing nitrogen’s inert transport role with active nitrogen from NH3 cracking; ignoring steam’s strong influence on decarburisation via H2O/CO2 equilibria.

Final Answer:
none of these.