HEAT TREATMENT OF FERROUS COMPONENTS
HARDENING AND TEMPERING
Metal hardening processes are used to impart specific mechanical properties to a component in order to render it fit for use.
Tempering is a low temperature (150ºC to 650ºC) heat treatment intended to remove the stresses and brittleness caused by quenching and to develop the required mechanical properties.
Application & Material
The maximum hardness of a steel grade, which is obtained by hardening, gives the material a low toughness. Tempering reduces the hardness in the material and increases the toughness. Through tempering you can adapt materials properties (hardness/toughness ratio) to a specified application.
Tempering temperature may vary, depending on the requirements and the steel grade, from 150°C to 650°C or higher. Tempering is normally performed in furnaces which can be equipped with a protective atomosphere will prevent the surface from oxidation during the process
CASE HARDENING WITH SUBSEQUENT HARDENING OPERATION
Carburising and quench produce hard surfaces which are resistant to wear. Moreover, failure from impact loading is avoided due to a softer core.
Application and Materials
A wide variety of steels can be carburised. The unique combination of a hard wear resistant surface and a tough core can be controlled by the choice of alloy elements and process parameters.
Typical applications include transmission gears and shafts for automotive, wind turbine and pump components and all applications where components are required to operate for long periods and under high impact loads.
Carburising is a thermochemical diffusion process which adds carbon to the surface of a low carbon steel (typically 0.25% carbon) with other alloying elements..
This process in a furnace and quench to enrich the surface in a carbon environment in a temperature range of 850 to 980°C.Hardening is achieved after quenching either in oil, polymer/water..
A tempering operation follows. Tempering temperature and time at heat depend on product requirement and application. The tempering process minimises peak stresses and reduces crack initiation.
A process to reduce internal residual stresses in a metal object by heating the object to a suitable temperature and holding for a proper time at that temperature. This treatment may be applied to relieve stresses induced by casting, quenching, normalizing, machining, cold working or welding. Stress relieving does not change the material’s structure and does not significantly affect its hardness.
Application & material
All Steel parts and some of cases Copper and brass components can also be stress relieved. For stainless steels a high temperature solution heat treatment is normally necessary.
The stress relieving temperature is normally between 550 and 650°C for steel parts. Soaking time is about one to two hours. After the soaking time the components should be cooled down slowly in the furnace or in air. A slow cooling speed is important to avoid tensions caused by temperature differences in the material, this is especially important when stress relieving larger components.
- Process details
The stress relieving temperature is normally between 550 and 650°C for steel parts. Soaking time is about one to two hours. After the soaking time the components should be cooled down slowly in the furnace or in air. A slow cooling speed is important to avoid tensions caused by temperature differences in the material, this is especially important when stress relieving larger components. The temperature for stress relieving copper parts is, depending on the alloy, 150-275°C and for brass components 250-500°C.
Annealing is a heat treatment process which alters the microstructure of a material to change its mechanical or electrical properties. Typically, in steels, annealing is used to reduce hardness, increase ductility and help eliminate internal stresses.
- Application & materials
One of the main applications of annealing is reversing the effects of work hardening. During cold forming, drawing, bending etc. the material can become hardened to the point where further working can be impossible or result in cracking. An annealing operation at this stage will make the material more ductile, permitting further forming. In a similar manner, annealing is utilized to remove the internal stresses which occur when welds solidify.
Besides steels, other metals may also benefit from annealing such as copper, aluminium, and brass.
- Process details
Annealing is a generic term and can be further classified according to temperature and atmosphere.
For steels, subcritical annealing takes place at 550C – 650C, so there is no crystal structure change. Intermediate annealing is carried out at 650°C - 750°C, so there is some transformation to austenite and full annealing involves completely austenitizing the work at 800°C - 900°C.
After, Annealing in air is employed where surface finish is not an important factor otherwise slow cooling in the furnace needed.
Normalising aims to give the steel a uniform and fine-grained structure. The process is used to obtain a predictable microstructure and an assurance of the steel’s mechanical properties
After forging, hot rolling or casting a steel’s microstructure is often unhomogeneous consisting of large grains, and unwanted structural components such as bainite and carbides. Such a microstructure has a negative impact on the steel’s mechanical properties as well as on the machinability. Through normalising, the steel can obtain a more fine-grained homogeneous structure with predictable properties and machinability.
Application & materials
Normalisation is mainly used on carbon and low alloyed steels to normalise the structure after forging, hot rolling or casting. The hardness obtained after normalising depends on the steel dimension analysis and the cooling speed used .
- Process details
During normalising, the material is heated to a temperature approximately equivalent to the hardening temperature (800-920°C).
After heating and a short soaking time the components are cooled freely in air.. During cooling, new ferritic grains are formed with a further refined grain size.