case harden

Case hardening creates a hard, wear-resistant outer layer on a softer, tougher inner core. The work piece itself remains relatively ductile, while the surface skin becomes brittle enough to resist abrasion, impact, and fatigue. This is done by introducing carbon or nitrogen into the outer material, then quenching to lock in the hardness. The result is a composite structure: a case of high-carbon steel around a body of low-carbon steel.

The standard methods differ in how they deliver carbon or nitrogen to the surface. Carburizing, the most common, heats steel in a carbonaceous atmosphere (often methane in a sealed furnace) at 900 to 950 degrees Celsius for several hours, allowing carbon to diffuse inward. The depth of the hardened layer, called the case depth, is controlled by time and temperature and typically ranges from 0.3 to 1.5 millimeters for parts like gears and bearings. Nitriding, slower but more precise, uses ammonia gas and lower temperatures around 500 to 650 degrees Celsius, producing very hard, shallow cases. Pack carburizing, an older method, buries parts in a carbon-rich powder inside a closed container.

After the carbon or nitrogen soak, the work must cool fast. Oil quenching is standard; water is avoided because the thermal shock between the hard case and soft core can cause cracking. Tempering, a mild reheating to 150 to 300 degrees Celsius, relieves some internal stress without sacrificing hardness. The goal is a case with hardness around 58 to 62 HRC (Rockwell hardness) bonded to a core around 30 HRC.

Why this matters in production

Case hardening solves a real metallurgical problem: you cannot make a part that is both hard and tough using a single steel grade. A fully hardened part becomes brittle and fails catastrophically under shock. By hardening only the surface, case hardening creates components that resist wear where it matters most while remaining forgiving where stress concentration occurs. Gears, camshafts, spindle shafts, and bearing races are classic case-hardened parts.

The process is slow and costs more than simple hardening, but it extends service life significantly. A case-hardened gear lasts much longer than a fully hardened one of the same material. The main risk is uneven case depth, which creates weak spots, and quench cracking if cooling rates are not carefully controlled. Quality control typically includes metallographic cross-sections to verify case depth and microstructure.