A precision injection mold component requires an extremely hard, wear-resistant surface, but cannot tolerate any dimensional change during heat treatment. Which case hardening method is most appropriate?
ACarburizing, because it produces the deepest and hardest case of all methods
BInduction hardening, because it heats only the surface layer without affecting dimensions
CNitriding, because hard nitride phases form during the diffusion anneal without requiring a subsequent quench
DThrough-hardening, because uniform hardness prevents stress concentrations
Nitriding is the correct choice because it achieves surface hardness through formation of nitride phases during the diffusion process itself — no quench is required. Quenching is the primary source of distortion in carburizing, as the rapid thermal gradient causes uneven contraction. Induction hardening also requires quenching. Nitriding's low treatment temperature (500–575°C) and quench-free process makes it the standard method for precision tooling and gears where dimensional stability is critical.
Question 2 Multiple Choice
Why does carburizing start with a low-carbon steel (0.1–0.25% C) rather than a medium- or high-carbon steel?
ALow-carbon steel has a higher diffusion coefficient for carbon, so treatment time is shorter
BThe goal is to produce a surface with high carbon content while keeping the core low in carbon, so the core remains tough and ductile — high-carbon steel would make the entire part brittle after quenching
CLow-carbon steel is cheaper and the carbon added during carburizing is the expensive part of the process
DHigh-carbon steel cannot be austenitized at the temperatures used in carburizing
The entire point of case hardening is to create a gradient: hard surface, tough core. If you started with high-carbon steel, quenching would produce martensite throughout — a through-hardened part that is hard but brittle everywhere. Starting with low-carbon steel means only the carbon-enriched surface layer (0.7–0.9% C) forms martensite on quenching, while the low-carbon core (0.1–0.25% C) remains ferritic and tough. The carbon gradient created by diffusion is what makes the functional architecture possible.
Question 3 True / False
Nitriding achieves surface hardness without a subsequent quench because hard nitride phases form in place during the diffusion anneal itself, unlike carburizing where the hard phase must be created by rapid quenching.
TTrue
FFalse
Answer: True
This is the fundamental mechanistic distinction between nitriding and carburizing. In carburizing, diffused carbon is not inherently hard — it must be 'frozen' as martensite by rapid quenching. Nitriding introduces nitrogen at 500–575°C, and the nitrogen reacts with iron and alloying elements (chromium, aluminum, vanadium) to form hard nitride precipitates directly during the anneal. The hardness is already there when the part cools slowly; no quench is needed. This is why nitriding causes minimal distortion and is preferred for precision components.
Question 4 True / False
A deeper case depth is generally preferable in case hardening because it provides more wear-resistant material and a larger safety margin against surface damage.
TTrue
FFalse
Answer: False
This is a common misconception. An excessively deep case makes the component behave like a through-hardened part — hard and brittle throughout — losing the tough core advantage that is the whole purpose of case hardening. Gears and bearings need the hard surface to resist wear and contact fatigue, and the tough core to absorb shock loads without fracturing. If the case extends too deep, there is no tough material left to absorb impact, and the component becomes brittle overall. Case depth is engineered to match the service loads: just deep enough to handle the surface stresses, no deeper.
Question 5 Short Answer
Explain why case hardening can achieve both a hard surface and a tough core simultaneously, while through-hardening cannot provide the same combination of properties.
Think about your answer, then reveal below.
Model answer: Case hardening selectively modifies only the outer layer of the part, either by enriching its chemistry (carburizing adds carbon, nitriding adds nitrogen) or by locally heat-treating only the surface zone (induction hardening). This creates a gradient: the surface has the microstructure (martensite or nitride compounds) needed for hardness and wear resistance, while the interior retains the original low-carbon or medium-carbon microstructure (ferrite, pearlite) that is tough and ductile. Through-hardening treats the entire part uniformly — quenching a high-carbon steel transforms everything to martensite, which is hard but brittle everywhere. There is no way to make the surface hard and the core tough if both have the same composition and cooling history.
This also explains the selection logic for starting materials: carburizing uses cheap low-carbon steel for the tough core, then enriches only the surface. Induction hardening uses medium-carbon steel (0.4–0.6% C) because the core needs moderate toughness but the surface carbon must be sufficient to form martensite on rapid quenching.