Annealing, Normalizing, Quenching, Tempering: How to Distinguish Heat Treatments

Heat treatment improves the mechanical properties of metal, eliminates residual stresses and improves machinability. It is divided into two categories by purpose: preparatory heat treatment and final heat treatment.

Annealing and normalizing: applied to hot-worked blanks. For carbon and alloy steels with carbon above about 0.5%, annealing is used to soften the steel for cutting; below about 0.5% C, normalizing avoids over-softening (sticky cutting). Both refine the grain and homogenise the structure in preparation for final treatment; typically placed after blank making, before rough machining;

Ageing (stress relief): eliminates stresses from casting and machining. General precision parts get one ageing cycle before finishing; high-precision parts (e.g. coordinate-boring machine housings) get two or more; slender precision parts (e.g. precision screws) are aged repeatedly between roughing and semi-finishing; straightening operations are followed by ageing;

Quenching-and-tempering (tempering): the quench plus high-temperature temper produces a uniform tempered sorbite structure - good preparation for subsequent surface hardening or nitriding, reducing distortion. Because of its balanced properties it can also serve as a final treatment for parts needing moderate hardness and wear resistance.

Quenching: overall or surface. Surface (induction/flame) quenching is widely used - small deformation, oxidation and decarburization, with a hard wear-resistant case and a tough impact-resistant core; often preceded by tempering or normalizing;

Carburizing quenching: for low-carbon and low-alloy steels - enriches the surface carbon, then quenches for a hard case (0.5–2 mm) with a tough core. Process: material → forging → normalizing → rough/semi-finishing → carburizing → quenching → finishing; locally carburized parts use copper-plating or anti-carburizing coatings, with excess case removed before quenching;

Nitriding: nitrogen atoms penetrate the surface to form a nitride layer (≤ 0.6–0.7 mm), improving hardness, wear resistance, fatigue strength and corrosion resistance with minimal distortion - placed after stress-relief high-temperature tempering and as late as possible in the process.

material → forging → normalizing (annealing) → rough machining → tempering → semi-finishing → surface hardening → finishing.

Preparatory treatments manage machinability, stress and microstructure; final treatments deliver the specified hardness, wear and strength. Choose annealing vs normalizing by carbon content, and surface hardening vs carburizing vs nitriding by the required case depth, distortion limit and core properties.