Harden and Tempering

Hardening and tempering of engineering steels is performed to provide components with mechanical properties suitable for their intended service. Steels are heated to their appropriate hardening temperature {usually between 800-900°C), held at temperature, then “quenched” (rapidly cooled), often in oil or water. This is followed by tempering (a soak at a lower temperature) which develops the final mechanical properties and relieves stresses. The actual conditions used for all three steps are determined by steel composition, component size and the properties required.
Hardening and tempering can be carried out in “open” furnaces (in air or combustion products), or in a protective environment (gaseous atmosphere, molten salt or vacuum) if a surface free from scale and decarburisation (carbon loss) is required (“neutral hardening”, also referred to as “clean hardening”).
Two specialised quenching options can be applied in special circumstances:
Martempering (also known as “marquenching”) uses an elevated-temperature quench (in molten salt or hot oil) which can substantially reduce component distortion. This process is limited to selected alloy-containing steels and suitable section sizes.
Austempering can be applied to thin sections of certain medium- or high-carbon steels or to alloy-containing steels of thicker section. It requires a high temperature quench and hold, usually in molten salt, and results in low distortion combined with a tough structure that requires no tempering. It is widely used for small springs and presslngs.
What Are The Benefits?
Hardening and tempering develops the optimum combination of hardness, strength and toughness in an engineering steel and offers the component designer a route to savings in weight and material. Components can be machined or formed in a soft state and then hardened and tempered to a high level of mechanical properties.
Hardening from open furnaces is often employed for products such as bars and forgings that are to be fully machined into components afterwards. Neutrally clean hardening is applied to components that require surface integrity to be maintained; examples include nuts, bolts, springs, bearings and many automotive parts. Neutral clean hardening is carried out under tightly-controlled conditions to produce a precision component needing the minimum of final finishing.
What Are The Limitations?
Every steel has a “limiting” section size (“ruling section”) above which full hardening cannot be achieved. A higher grade of steel will be required to ensure optimum properties in a larger section.
It may be possible to harden larger components in lower-grade steels by using non-standard treatments such as faster quench rates or lower-temperature tempers. Faster quench rates always increase the risk of distortion or cracking, and low-temperature tempers can seriously impair mechanical properties such as toughness. Serious consideration should be given to these facts before asking for non-standard treatments to be carried out.