Carburizing and Case Hardening: The Basics

The idea

Some parts need two contradictory properties: a surface that resists wear and contact fatigue, and a core that absorbs shock without cracking. Carburizing delivers both from one piece of low-carbon steel. The part is held at roughly 880–950 °C in a carbon-rich atmosphere; carbon diffuses into the surface, and a subsequent quench hardens only that enriched layer — the case — while the low-carbon core stays comparatively soft and tough.

Case depth

Case depth is specified as the depth at which hardness falls to a defined level — commonly 550 HV in European practice (CHD, case hardening depth). Two depths matter:

• Effective case depth (to 550 HV): typically about 0.3–2.5 mm for gears and shafts, matched to the contact stress and module of the gear.
• Total case depth: the full diffusion zone, always deeper than the effective depth.

Deeper cases need disproportionately longer furnace time, because diffusion depth grows with the square root of time — doubling the depth takes roughly four times as long.

Carbon potential

The furnace atmosphere is controlled to a carbon potential — the surface carbon level the atmosphere will produce in pure iron at temperature. It is typically run somewhat below or near the eutectoid-to-saturation region, often around 0.8–1.0% C at the surface in the boost stage. Too high a potential forms soot, carbide networks and excess retained austenite; too low gives a shallow, soft case. Production cycles often use a boost–diffuse pattern: high potential to drive carbon in fast, then a lower-potential stage to set the final surface carbon and gradient.

Core versus case after quenching

After carburizing, the part is quenched (directly, or after a reheat for grain refinement) and tempered at low temperature, roughly 150–200 °C. The high-carbon case transforms to hard martensite, typically around 58–62 HRC at the surface. The core — still only about 0.15–0.25% carbon — hardens only as far as its hardenability allows, staying much softer and providing the toughness the part is designed around.

Steels designed for carburizing

Case-hardening steels are low-carbon by design, alloyed for core strength and hardenability:

• 8620 (AISI): the general-purpose North American gear steel, Ni-Cr-Mo core.
• 20CrMnTi (GB): the standard Chinese gear steel; titanium pins the grain during long carburizing cycles.
• 16MnCr5 (EN 1.7131): the European workhorse for small and medium gears.
• Higher-duty variants add nickel (e.g. 18CrNiMo7-6) where core strength in large sections is critical.

Specifying a carburized part means specifying case depth, surface hardness and core hardness together — any one alone is an incomplete requirement.

Gas, pack and low-pressure variants

Traditional pack carburizing in solid carbon compound is now largely historical. Modern production uses gas carburizing in atmosphere furnaces, where carbon potential is measured and controlled continuously, or low-pressure (vacuum) carburizing, where hydrocarbon gas pulses in a vacuum furnace give very uniform cases on complex gear geometries with no surface oxidation — typically followed by high-pressure gas quenching. The metallurgy the designer specifies is the same in all three; what changes is uniformity, cleanliness, cycle economics and the distortion behaviour that gear finishing must absorb.