Question 7 of 7: Question VII: Three Surface-Hardening Approaches in Modern Manufacturing

Question VII: Three Surface-Hardening Approaches in Modern Manufacturing (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Surface hardening puts a hard, wear- and fatigue-resistant case on a part while leaving a tougher, more ductile core — the three families below achieve that by three genuinely different mechanisms: adding carbon, adding nitrogen, and reheating only the surface.

7.1 — Carburizing (thermochemical, composition change)

A low-carbon steel part is held at an elevated temperature (typically 900–950 °C, in the austenite range) in a carbon-rich environment — a solid pack, a carburizing gas atmosphere, or a molten cyanide/carbonate salt bath — so that carbon diffuses inward from the surface, building up a carbon-concentration gradient (highest at the surface, falling to the core's original low level over the case depth). The part is then quenched (directly or after reheating), transforming the now high-carbon SURFACE layer into high-carbon, high-hardness martensite, while the low-carbon CORE — which never picked up enough carbon to harden the same way — stays comparatively soft and tough. The hardening mechanism is therefore the standard martensitic-hardness-vs-carbon-content relationship (Question IV.3 of this exam), applied selectively only where the carbon diffused in.

7.2 — Nitriding (thermochemical, composition change)

The part (typically an alloy steel containing strong nitride-forming elements — Al, Cr, Mo, V) is held at a comparatively LOW temperature (roughly 500–550 °C, well below the austenitizing range, in a nitrogen-rich gas or salt/plasma environment) so that nitrogen diffuses into the surface and reacts directly with those alloying elements to precipitate an extremely fine dispersion of hard alloy nitrides (AlN, CrN, and related compounds) within the case, plus a thin, very hard iron-nitride "white layer" right at the surface. Because the whole process runs below the austenitizing temperature, NO PHASE TRANSFORMATION (and no subsequent quench) is needed at all — the hardening mechanism is pure precipitation/dispersion strengthening from the fine nitride particles, which also means nitriding produces very little distortion compared with a process that requires a quench.

7.3 — Induction (or flame) hardening (selective heat treatment, no composition change)

A medium-carbon (or low-alloy medium-carbon) steel part — one that already has enough carbon to form hard martensite on its own — is heated ONLY in a shallow surface layer, either by eddy currents induced by a high-frequency induction coil held close to the surface or by a direct oxy-fuel flame played rapidly over the surface, and is then immediately quenched (often by an integrated water spray) before the heat can conduct far into the cool core. Only the thin surface layer that was actually raised above $A_3$ transforms to austenite and then to martensite on the quench; the core, which never got hot enough to austenitize, is unaffected and remains in its original tougher condition (and, because the cool core acts as an internal heat sink, the process is inherently self-quenching/self-tempering to some degree). Unlike carburizing and nitriding, NO new element is added — the hardening mechanism is exactly the ordinary carbon-martensite hardening of Question IV, applied selectively by controlling WHERE the steel is heated rather than by changing its composition.