- heating on at required temperature - dwell at temperature - cooling Thermal processing of metal alloys Control changes of mechanical, physical, electrical and other properties – always connected with structural or substructural changes in treated material Each heat treatment consist of three main periods: - heating on at required temperature - dwell at temperature - cooling Aim of heat treatment is to achieve more or less thermodynamically equilibrium state (stable or metastable)

Thermal processing of metal alloys Heat treatment (temperature effect) annealing (to get more stable state) reduce internal stress, reach softer and ductile structure, … hardening (to get metastable state) increases strength, hardness, wear resistance, … Thermomechanical treatment (effect of temperature and deformation) control of final structure and mechanical properties Chemical heat treatment (effect of temperature and changes of the chemical composition) to get different properties of surface layer as in core of the piece – higher hardness, better wear or corrosion resistance, …

Heat treatment of steels Annealing Lower critical temperature (without transformation) Process (recrystallization) annealing Stress relief annealing Spheroidizing Upper critical temperature (partial or whole transformation) Normalizing Homogenization

Process (recrystallization) annealing 550 – 700 °C, 1- 5 hours, cooling in air To change the structure and properties of cold worked (strain hardened) steel. Recovery and recrystallization processes occur. Softening, increase ductility and uniform fine grain structure is achieved. Stress relief annealing 400 – 650 °C, 2 - 10 hours, very slow cooling in furnace Reduce internal residual stresses (after machining, heat treatment, …)

Spheroidizing (soft annealing) 700 °C, 5 – 25 hours, slow cooling to 600 °C in furnace, then cooling may continue in air. Spheroidite structure is developed Used to improve machineability and toughness.

Normalizing Full annealing Homogenization 50 – 80 °C above upper critical temperature (phase diagram), cooling in air. Austenitization of the steel is required and cooling in accordance with CCT diagram to get uniform and fine grain structure. Full annealing over A3 and A1 with furnace cooling. Homogenization 1100 - 1200 °C (200 °C under solidus!) To reduce structure and chemical composition inhomogeneities after casting.

Hardening of steel Purpose: improve strength, hardness, wear resistance, … Way: Reach the martensite or bainite structure or their combination How: By quenching

CCT diagram

CCT diagram

Hardenability of the steel The martensitic transformation have to bee possible! Carbon steel with at least 0,25 wt %. Hardened structure contain at least 50 % of martensite. Hardenability is defined by depth, in which the hardened structure is achieved.

Jominy end quench test