Question

The heat treatment process used to enrich the steel surface by addition of Nitrogen is known as

• A. Annealing
• B. Surface hardening
• C. Tempering
• D. Nitriding

Hint:

The clue is right in the name! - Nitrogen addition $\rightarrow$ Nitriding. - Carbon addition $\rightarrow$ Carburizing. Nitriding produces an exceptionally hard case because the formed nitrides are inherently hard and stable, avoiding the need for an aggressive oil or water quench.

Answer 1

The Correct Option is D

Concept: Surface hardening (or case hardening) techniques alter the chemical composition and microstructure of a component's outer surface layer while maintaining a tough, ductile core. This is achieved by diffusing specific interstitial elements into the steel matrix at elevated temperatures. When the surface layer is enriched with Nitrogen, the process is called Nitriding. During nitriding, the steel part is exposed to a nitrogen-rich environment (such as ammonia gas, $\text{NH}_3$, or a molten cyanide salt bath) at temperatures typically ranging from 500 $^\circ$C to 575 $^\circ$C. The nitrogen atoms diffuse into the ferrite phase of the steel and react with alloying elements (such as Aluminum, Chromium, or Molybdenum) to form extremely hard, finely dispersed interstitial alloy nitrides. This process significantly increases surface hardness, wear resistance, and fatigue life without requiring a subsequent liquid quench.

Step 1: Review the purpose of each listed heat treatment process.
Let's analyze the standard definitions of the choices to differentiate their underlying chemical and structural mechanisms:
• Annealing: A thermal process where a material is heated to a high temperature, held there, and then cooled very slowly inside the furnace. Its primary goals are to soften the metal, relieve internal residual stresses, and improve ductility. It does not alter the surface chemistry.
• Surface Hardening: A broad category that includes any process used to harden the outer shell of a component while leaving the core soft. Examples include flame hardening, induction hardening, carburizing, and nitriding. While nitriding falls under this category, it is a generic term rather than the specific process defined by adding nitrogen.
• Tempering: A process performed after quenching to reduce excess hardness, restore toughness, and relieve internal stresses in martensitic steel by heating it below the lower critical temperature.
• Nitriding: The specific case-hardening process that relies explicitly on the thermal diffusion and enrichment of Nitrogen into the surface layer.

Step 2: Choose the most precise answer matching the chemical element.
Since the question asks for the specific process that introduces Nitrogen to enrich the surface, Nitriding is the most accurate and precise selection. This corresponds to Option (D).