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:

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

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