Hard Surface, Tough Core — When and How Surface Hardening Is Applied
Through-hardening by quench and temper treats the entire section of a component to a uniform hardness. For many mining and industrial components — crusher wear parts, structural castings, conveyor structural components — this is appropriate because wear or load is distributed across the full section.
For a different class of components, the requirement is different: high surface hardness to resist wear or contact fatigue at a specific interface, combined with a tough, lower-hardness core that resists fracture under impact or bending loads. A gear tooth must resist surface pitting at the contact line while the tooth root must resist bending fatigue. A track pin must resist abrasive wear at the pin-bush interface while the core must absorb impact loads without cracking. Through-hardening to a hardness that satisfies the surface requirement would make these components too brittle at the section scale. Surface hardening is the engineering solution.
Induction Hardening
Induction hardening uses electromagnetic induction to heat a defined surface zone rapidly to the austenitising temperature, followed by immediate quenching. Only the heated zone transforms to martensite — the core, which was not heated, remains in its pre-treatment condition. The depth of the hardened layer (case depth) is controlled by the induction frequency, power, and dwell time. Higher frequencies produce shallower case depths; lower frequencies penetrate deeper.
Typical applications for induction hardening include shaft journals and bearing seats (where surface hardness resists fretting and wear at the bearing contact), track pin and bushing contact surfaces, gear tooth flanks on medium-to-large module gears where the geometry is suitable for induction coil design, and localised wear surfaces on structural components where only a specific zone requires hardening.
Case depth after induction hardening is verified by cross-section hardness traverse — hardness measured at defined intervals from the surface through the hardened layer into the core, establishing the effective case depth (typically defined as the depth at which hardness drops to 550 HV or the specified threshold). Surface hardness is verified by Vickers or Rockwell measurement on the treated surface.
The transition zone between hardened case and unhardened core is a stress concentration under cyclic loading — the hardness gradient introduces a zone of lower fatigue resistance. Induction hardening parameters are set to produce a gradual transition rather than an abrupt one, reducing the stress concentration effect. For high-fatigue applications, the case depth specification must account for this.
Carburising and Case Hardening
Carburising diffuses carbon into the surface of a low-carbon steel component at elevated temperature (typically 850–950°C), raising the surface carbon content to 0.7–1.0%. The component is then quenched, transforming the high-carbon surface layer to hard martensite while the low-carbon core remains tough and ductile. Case depths of 0.5–3 mm are achievable depending on carburising time, temperature, and the specific carbon potential of the atmosphere.
Carburising is appropriate for gears, sprockets, and drive components where the combination of high surface hardness (58–63 HRC), deep case depth, and tough core is required — requirements that induction hardening cannot always satisfy due to geometry constraints or the need for uniform case depth on complex tooth profiles.
After carburising and quenching, a tempering cycle at 150–200°C relieves quench stresses and reduces the risk of grinding cracking in subsequent finish grinding operations, while having minimal effect on achieved surface hardness.
Nitriding
Nitriding diffuses nitrogen into the surface of alloy steel at relatively low temperature (500–550°C), producing a very hard but thin surface layer (typically 0.1–0.5 mm) without a quench step. Because no quench is involved, dimensional distortion is minimal — nitriding is used for precision components where post-treatment grinding is not desired or feasible.
Nitriding is used for precision shaft journals, precision gear components, and tooling where dimensional stability after treatment is critical. It is not appropriate for high-impact applications — the nitrided layer is harder but thinner and more brittle than a carburised case, and is subject to spalling under heavy impact loads.
Process Selection
The selection between induction hardening, carburising, and nitriding depends on the component geometry, the required case depth and surface hardness, the base material, the allowable distortion after treatment, and the load character in service. For components where surface hardening is part of the specification, the process route is confirmed at the engineering review stage before production begins. Specifying surface hardness and effective case depth as performance requirements — rather than prescribing a specific process — allows the appropriate method to be selected for the component geometry and material.
For surface hardening specifications or to discuss the appropriate process for a specific component, contact our engineering team. See also: Heat Treatment Capabilities.