Quenching after surface heating
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Principle
Hardening after surface heating aims at heating quickly the surface of a steel part, without letting the heat diffuse towards the heart of the part, then cooling very quickly (by sprinkling or immersion). This treatment allows a hardening of the surface while keeping a malleable core.
The treatment is done in a unitary way but can be realized in series (enfilade, carousel...).
Since hardening is only done on the surface, the large size of a part is usually no longer a limiting factor.
The heating temperature is generally higher than for quenching in the mass, between 900 and 1100°C, in order to obtain a homogeneous austenite after a heating time of between a fraction of a second and a few tens of seconds. The stresses generated by the high thermal gradients during heating and quenching are added to the stresses formed during the martensitic transformation during quenching.
The most commonly used means of heating are:
Induction
The flashlight
The laser
The principle of induction consists in the use of an electromagnetic coil surrounding the part to be treated. When the electric current is fed to the coil, eddy currents are formed on the surface of the part which, through the Joule effect, cause the heated layer to heat up very quickly.
The steels used to obtain treated depths of about 2mm are, for example, carbon steels such as C35E, C45E or C55E. Steels with a higher content of additive elements (Cr Mo W V) such as 42CrMo4 are used when greater depths are required.
Typical treatment features
The surface hardness depends on the carbon content of the selected steel and varies between 500 and 700HV.
The hardness of the core does not change, which allows it to retain an interesting impact resistance;
The residual surface stresses are in compression and range from -250MPa to -700MPa which increases the resistance to mechanical fatigue. Around the treated parts, there are areas of high tensile stresses at the end of the profile. If these areas are stressed in fatigue, the endurance limit of the part may be affected;
The treatment depths vary between a few tenths of a mm and a few cm depending on the material, the frequency of the electric current used, the heating time, the cooling mode, and the geometry of the part..;
The surface fatigue strength is greatly increased because the stressed surface area has a high yield point;
The deformations after treatment are low because the core is not affected by the treatment;
The treatment is suitable for large parts;
The price of the treatment depends on the size of the series.
Applications
Mechanical parts of great series.
Gears, pinions and drive shafts ;
Wind turbine bearing ring ;
Rolls of rolling mills.