Surface Hardening is a process whereby the metallic structure in the outer layer of a component is by heating and quenching to improve properties in only a selected area of the changed component.
For instance, the bearing track only in a bearing element; the gear tooth only on a gear; the slideway surface only on a guideway; or the cam track area only of a cam. It is neither a coating process nor a finishing process, i.e. it does not add any further elements to the material or lay on different materials.
“If correct procedures are followed, induction and flame hardening probably offers the lowest level of distortion of all the available range of heat treatment processes,” says Roger Haw, managing director of heat treatment subcontract specialist Flame Hardeners.
The process is highly energy efficient, since heat is applied only to those areas requiring improved properties and not to the whole bulk of the component. Many engineering components can fulfil their desired functions satisfactorily without a particularly high core strength in the material providing that the surface under load is adequate.
Examples of such components include steel wheels on railway rolling stock or on pleasure park and fairground rides. The process is also highly regarded in the mining and quarrying industries, where abrasive materials in the working environment cause excessive wear.
The necessary heating of components is obtained by either electro-magnetic induction techniques or directly applied high intensity flames, the technique being to heat the required area as quickly as possible. Irrespective of the method of heat application selected, the process is a machine operation requiring careful control of the temperature range and duration of application, and also of the rate of quenching.
The hardness level that can be achieved by direct hardening is entirely controlled by the amount of carbon in the steel. Generally, the higher the carbon content, the higher the resultant hardness. Increased carbon content reduces ductility, while increasing the tensile strength and the ability of the steel to harden.
Among the many advantages of the process are that it is highly energy efficient and that it can produce considerably less distortion than conventional heat treatment processes using readily available steels.
Steels suitable for surface hardening are either medium carbon or low alloy. The material is selected to suit the application and design characteristics required, such as tensile strength, hardenability, toughness, wear and abrasion resistance. The successful hardening of a component is controlled by the chemical analysis of the actual material supplied.
Components which could distort should be stress relieved prior to hardening, e.g. slender shafts or starter rings. The hardening operation should be included as early as possible in the production sequence on all components.
“When choosing a surface hardening supplier, you should look for a company that can offer both technical support and production capability,” Mr Haw adds. “The company should employ engineers and technicians who are well versed in the production of engineering components, with a sound knowledge of materials, metallurgy and material testing techniques.
“Such a combination of skills will provide you with the necessary advice on your choice of treatments and materials, together with an assessment of the likely advantages and disadvantages of your intentions. When designs have been finalised the same skills will provide you with assurance that your products are satisfactory, as technicians involved with treatment will control repeatability and testing functions, such as hardness testing and NDT examinations.”