Surface engineering – Purpose of nitriding steel: In thermochemical surface treatments, what is the principal objective of nitriding a suitable steel component?

Introduction / Context:
Nitriding is a low-temperature thermochemical treatment where nitrogen diffuses into steel to form hard nitride phases. It is widely used for gears, shafts, and wear-critical components requiring a hard, fatigue-resistant surface while maintaining a tough core.

Given Data / Assumptions:

• Nitriding performed typically at 500–550°C in ammonia or plasma atmospheres.
• Alloy steels containing nitride-forming elements (Cr, Mo, Al, V) respond best.
• Objective centers on surface property enhancement.

Concept / Approach:
During nitriding, nitrogen reacts with alloying elements to form fine nitrides in a diffusion zone beneath the surface, producing high surface hardness, improved wear resistance, and better fatigue performance. Because the temperature is relatively low, distortion is minimized compared to higher-temperature case-hardening processes.

Step-by-Step Solution:
Identify the mechanism: nitrogen uptake and nitride formation.Connect nitride formation to high surface hardness and wear resistance.Conclude that the primary purpose is surface hardening (case formation) without significantly affecting the core toughness.

Verification / Alternative check:
Hardness profiles after nitriding show a steep rise near the surface (compound layer + diffusion zone), confirming the treatment’s intent.

Why Other Options Are Wrong:

• Softening: annealing or tempering at suitable regimes softens, not nitriding.
• “Improve reliability” is vague and not the direct mechanism; the improvement stems from increased hardness and fatigue resistance.
• None of these: incorrect because surface hardening is explicit.

Common Pitfalls:
Attempting to nitride plain carbon steels without nitride-forming elements; response will be limited compared to alloy steels designed for nitriding.

Final Answer:
To harden its surface (case hardening via nitrides)