Heat treatment of steel - general definitions and terms

Heat treatment of steel - general definitions and terms

Without heat treatment

Rolled, forged or cast steel without heat treatment

Controlled cooling

Cooling from an elevated temperature (after hot plastic deformation) in a  predetermined way. Tnis process produces a definite microstructure and mechanical properties and it prevents strain hardening, cracking and internal damages

Soft annealing

Annealing at the temperature range A1 or oscillating around A1 foolowed by slow cooling. The purpose is to obtain the struture of spheroidized  perlite and to reduce hardness for further machining or cold forming. This ensures the most suitable structure for further heat treatment.

Soft annealing with recrystallization

Heating just above A3 and rapid cooling to lower perlite or upper bainite range with spheroidizing just below Ac1. The process diminishes austenite grains and leads to a 100% spheroidizing of perlite which is most suitable for further heat treatment.

Grain coarsening

This is used especially for low-carbon steels which are machined after heat treatment. The temperature range of this annealing is 100-200°C above Ac3. Cooled to ferritic-perlitic range. The purpose is to produce large ferritic-perlitic grains with lamellar perlite improving machinability of heat-treatable steel.

Annealing to a definite strenght

Heating to 800-950°C, adequate cooling, then tempering at 500-650°C. The purpose is to produce heat-treatable steel and case-hardening steel of a definite strength.

Annealing to ferritic-perlitic structure

After austenitizing, steel is cooled down to temperature of perlite transformation and is then held isothermally. Cooling rate depends on hardness requirement. The purpose of this annealing is to produce a ferrite-perlite structure of case-hardening steel.

Stress-relief annealing

Annealing at a suitable temperature followed by a very slow cooling (to reduce and prevent internal stresses). Steel should be stress-relived at least 25°C below the tempering temperature. This annealing reduces internal stresses induced by rolling, forging, casting, cold forming, irregular cooling, welding, machining.

Homogenizing

Annealing at high temperatures just below solidus (1000-1300°C), depending on steel grade. This process diminishes inhomogeneity in steel and equlizes local diffrences in concentrations of chemical elements in crystal segregation.

Dehydrogenation

A long anneal cycle of steel containing hydrogen which is therefore subjected to form "flakes" or temperature oscillating above A1, followed by slow cooling down to room temperature. The purpose is to reduce hydrogen content and to avoid the danger of "flakes".

Recrystallization

Annealing after cold work to renew the ability of steel for cold work. Recrystallization temperature is above 600°C. The purpose is to obtain polygonal grains suitable for further cold plastic deformation.

Normalizing

Heating the hypoeutectoid steel just above Ac3 and hypereutectoid steel above Ac1 followed by air cooling. The purpose is to obtain a small anduniform perlitic structure throughout the whole piece (after rolling, forging and casting) improving mechanical properties. In casting, it eliminates a rough cast structure. Structure is conditioned for further heat treatment.

Quench hardening

Heat treatment comprised of austenitzing followed by cooling in air, oil, water or some other medium that allows the transformation of austenite to martensite or bainite with the highest possible hardness.

Austempering/Martempering

Austempering-Steel cooling from austenitizing temperature to the bainte transformation temperature where it is held until a suitable isothermal transformation into bainite is implemented, followed by cooling down to the ordinary temperature.

Martempering-Heat treatment comprised of austenitizing followed by step quenching at a rate fast enough to avoid the formation of ferrite, perlite or bainite, to a temperature slightly above Ms and soaking for long enough to ensure that the temperature is uniform but short enough to avoid the formation of bainite.

Case hardening, surface hardening

Quench hardening treatment after surface heating. According to the type of heating, we destnguish between flame hardening and induction hardening. The purpose is to achieve and abrasive resistant surface and to retain the tough core part.

Step quenching

Quenching during which the process of cooling is temporarly interrupted by soaking in a medium at a suitable temerature. The purpose is to reduce the danger of deformation and cracking in quenching process.

Hardening after case hardening

Blank hardening

Herdening of uncarburized case-hardening steels to determine mechanical properties of ucarburized regions of parts.

Direct quenching

Hardening of a carburized part directly after carburization (or after cooling) from the temperature suitable for a carburized layer.

Single quenching

Single hardening of case or core after carburizing

Double quenching

Double hardening of carburized steels. The first hardening from the austenitizing temperature of core, the second one from the temperature which is suitable for case hardening. The purpose is to obtain an abrasive resistant case.

Tempering

It consists of heating to specific temperatures below Ac1 for one or more times and holding at these temperatures. Followed by appropriate cooling rate. It should be temperd immediately after quenching. The purpose is to transform a hardend structure, to improve mechanical properties and to reduce internal stresses.

Quenching and tempering

Quench hardening followed by tempering at high temperature with aim of obtaining the desired mechanical properties and, in particular, good ductility.

Quenching

Heating to a definite temperature depending on steel composition without changing its ferritic or austentic structure, holding at that temperature and rapid cooling. The result is a homogeneous structure (dissolved carbides), maximum toughness and an improved corrosion resistance.

Precipitation hardening

Heating to a definite temperature (determined by the cemical composition of steel), holding at thet temperature for several hours and cooling. The aim is to reach the  percipitation of special phases to obtain higher hardnes.

Thermomechanical treatment

Combination of hot plastic deformation of heat treatment. The purpose is to obtain special mechanical properties. Change in resistance to deformation and cracks.

Aging

Age hardening-Hardening at room temperature after rapid cooling or cold working.

Artificial hardening-Aging above room temperature. The purpose is to stabilaze structure stresses and dimension.