Heat Treatment Reference

Hardening produces martensite by heating the steel to its austenitising temperature and quenching rapidly. The resulting structure is hard but brittle and almost always requires tempering before use.

• Austenitising temperature: typically 800–900°C for carbon steels; 950–1050°C for tool steels and many alloy grades.
• Quench medium: water, oil, polymer, or air, chosen to balance hardness against distortion and quench-cracking risk.
• Section size: drives both heating time and the through-hardness achievable.

Tempering is the controlled reheat that follows hardening. It transforms the brittle as-quenched martensite into a tougher tempered microstructure, trading a small amount of hardness for substantial gains in ductility and impact resistance.

• Low tempering (150–250°C): retains high hardness; used for cutting tools and bearings.
• Medium tempering (300–500°C): balanced strength and toughness; used for springs and dies.
• High tempering (550–700°C): emphasises toughness and creep resistance; used for shafts and structural components.

Case hardening produces a hard surface on a tough, ductile core. It is widely used for gears, shafts, and pins that need wear resistance at the contact surface but cannot tolerate brittleness through the section.

• Carburising: diffusion of carbon into low-carbon steel at 880–950°C.
• Carbonitriding: simultaneous diffusion of carbon and nitrogen at 760–870°C; thinner case than carburising.
• Nitriding: nitrogen diffusion at 500–550°C; very hard, very thin case with minimal distortion.

Martempering interrupts the quench at a temperature just above the martensite-start point, holds long enough to equalise temperature through the section, and then air-cools to form martensite. The result is a fully hardened part with substantially less distortion and quench-cracking risk than direct quenching.

It is particularly suited to thin or geometrically complex tool steel parts where dimensional control matters as much as final hardness.