While it is not uncommon to hear people interchange the terms hardening and tempering steel, they are actually different in both techniques as well as the desired results.
All hardening processes and there are several, are used to create a required change in the mechanical or physical properties of the steel or metal. The degree of heat, the time of exposure at the specific temperature, and the cooling or quenching process will vary to create the various changes required by each hardening process.
Hardening can be completed in vacuum furnaces or controlled atmosphere furnaces, both which eliminate the need for additional finishing of parts after the process is completed.
Rather than using heat from an external source, induction hardening can be carefully controlled to harden only specific areas of the part or component. It is used on the surface of the part to develop a highly wear resistant surface on the specific areas where this is required.
This is a process for hardening and tempering steel that is completed as one series of treatments. This includes a high heat exposure followed by quenching and then tempering. Through this process, a bainite structure, which is strong and resistant to stress, is developed in the metal.
Neutral hardening can include both oil quenching or water quenching. Both options allow for a fast, continuous process to help to reduce costs of the processing as well as increase the life of the parts.
Tempering, on the other hand, is done at lower temperatures. It is common for hardening and tempering steel to be done in sequence with the same part. The hardening process changes the physical properties, then low temperature heating allows the stress to be released and the brittleness eliminated for increased strength and durability of the part.