Heat treatments

Heat treatments may lead to considerable changes to the properties of steel which, sometimes, can exceed those induced by changes to the chemical composition of the steel. From a practical point of view, the transformations which heat treatment can cause to the strength, yield point, elongation, contraction, toughness and module of elasticity values are interesting.
Alloy steel which is subjected to incorrect heat treatment may give poorer results than those which may be obtained with correctly treated carbon steel.

A list of the main types of heat treatment is given below, but it should be noted that the technical data sheets for the individual steels give the parameters which experience recommends.

ANNEALING
The main requirement of this heat treatment is to achieve a lowering of the hardness of hot deformed, rolled and cold drawn materials. Annealing is introduced in some cases to eliminate stresses or non-homogeneous structures. The temperature is held for 1 hour 30 minutes for every inch of thickness (e.g. 300×100 flat; dwell time 5 hour).

CASE-HARDENING
Heat and chemical treatment to obtain a higher carbon content on the surface. Carbon increases hardness and contrasts wear.

CHROMIUM PLATING
Electroplating treatment to form a film of extremely hard chromium on ground bars. This treatment provides abrasion and corrosion resistance, and a lower friction coefficient between moving parts; especially with rubber gaskets.

HEAT TREATMENT
Series of heat operations in order to change the properties and/or the structure of a ferrous material.

NITRIDING
Heat and chemical treatment to obtain higher nitrogen content on the surface. Nitrogen increases hardness and contrasts wear.

NORMALIZATION
This is carried out at a temperature just higher than Ac3 +50/70 °C (Ac1 for the hypereutectoid steels C% > 0.80) followed by cooling in calm air. The main aim is to homogenise the structure and to reduce the size of the grain enlarged by previous hot transformation operations. This treatment is not recommended for tool steels and self hardening steels. Normalization is also used to regenerate the structure damaged by the hardening and tempering heat treatment, when the desired mechanical values have not been obtained. It is recommended that hardening and tempering not be carried out more than twice on the same material. If necessary, carry out the normalization before repeating the hardening and tempering. Repeated treatments, in oxidising environments, inevitably create an addition of decarburization and this fact must be taken into due account.

PRE-HEATING
Heating of the material with intermediate breaks (400 – 600 °C) before reaching the pre-set temperature for austenitization. It is mainly used to reduce stresses and differential expansion induced by hot deformation cycles and machining. The break at the pre-heating temperature must ensure a uniform temperature throughout the entire section.

QUENCHING
This is the cooling of a ferrous product faster than in calm air. It is good practice not to use a quenching medium which is more drastic than necessary as the faster the cooling the greater will be the stresses induced in the part. Quenching baths must be stirred to prevent vapour bubbles adhering to the material. The most commonly used baths are: gas mixtures (for treatment below freezing), water, salt baths, polymers (water with additives), oil, forced or calm air. The weight of the baths must be at least 10-15 times greater than that of the material to be quenched. The temperature of the bath at the end of quenching must not exceed 49 °C. The temperature is normally maintained for 30 minutes for every inch of thickness (e.g. 300×100 flat; time 2 hours).

QUENCHING AND TEMPERING
Hardening treatment composed of quenching and tempering to obtain the desired combination of mechanical properties and good ductility and toughness. It should be noted that, if it is necessary to repeat the hardening and tempering on the same material, the temperature of the first treatment must be higher than the second. See also quenching, tempering.

SECONDARY HARDENING
Hardening achieved after one or more tempering operations (550-600 °C) which precipitate a compound (oversaturated carbides) that destabilises the austenite due to the thermal effect and it transforms into in martensite or bainite during the cooling. In this way there is an increase in hardening and the phenomenon is called secondary hardening.

SOFT ANNEALING
This is carried out at 30-50 °C below the Ac1 point. This treatment does not modify the structure but adequately softens and eliminates stresses due to previous processes. The cooling (approx. 10 °C/h, normally 5-10 °C per minute for carbon steels and 20-40 °C per hour for alloy steels) may be carried out either in a furnace or in air.

STRESS RELIEF
Treatment aimed at reducing stresses (due to cold straightening, sudden cooling, machining, etc.) without reducing the hardness. It is generally carried out at 50 °C below the temperature of the last tempering carried out on the hardened parts or products which are used with very high strengths. The cooling must be carried out very slowly, generally in a furnace.

TEMPERING
This is the heat treatment which a ferrous product undergoes after hardening by quenching, to achieve the desired mechanical properties. After quenching, the material is highly stressed and these stresses must be eliminated as their force, if it exceeds the failure load, could break the material.
This is one of the purposes of tempering. The second is to lower the strength until a compromise is reached between a good failure load and good toughness (impact strength). The temperature is normally maintained for 1 hour for every inch of thickness (e.g. 300×100 flat; time 4 hours).

THERMOCHEMICAL TREATMENT
Process performed in a suitably selected medium-environment to change the chemical composition of the base material.