Spring steels are supplied either hardened or tempered or annealed to allow component manufacture prior to the hardening process.
The optimum combination of hardness, strength and toughness is developed throughout the cross section of an engineering product made from steel, by means of hardening and tempering. This treatment consists of heating the work-piece to an appropriate hardening temperature, which is dependant upon the particular steel analysis involved, holding for sufficient time to ensure the whole work-piece is at temperature and then rapidly cooling it in a suitable medium (quenching). This medium can be air, oil, water, molten salt, fluidised bed or a pressurised inert gas, such as nitrogen. Selection of the quench medium is dependant upon steel analysis, component geometry, the heat treatment furnace used and the manufacturing stage at which hardening and tempering is carried out. The resultant temperature changes induce physical transformation of the steel, resulting in mechanical property changes.
Following quenching, the work-piece is in its hardest but brittle condition and therefore requires a further thermal treatment (tempering, or drawing), to produce the optimum balance of properties. This consists of re-heating the work-piece to a lower temperature and holding for a specific time. The choice of time and temperature depends upon the amount of tempering or ‘softening back’ the work-piece requires. Hardening of engineering steels (in the carbon range 0.3 to 0.55%, ranges between 800 and 900 deg.C.. Tempering is generally carried out between 400 and 700 deg. C. Tools made from higher alloy steels are also hardened and tempered but require significantly higher temperatures, up to 1300 deg. C. for hardening and multiple tempering treatments are often required.
For any steel analysis and quenching medium there is a section size, above which the work-piece will not satisfactorily through harden. This is known as the limiting ruling section and is the main design parameter that needs to be considered, in combination with the geometry and property requirements of the work-piece, when specifying a hardening and tempering treatment. As quench severity increases, as it does if air is replaced by oil, and oil is replaced by water, the limiting ruling section increases for a particular steel composition. However, the use of more severe quenching is limited in turn by the increased risk of distortion or cracking during quenching, due to the higher thermal stresses induced in the work-piece.
Although it can be applied to bars, forgings and castings, hardening and tempering is often left to a late stage of manufacture, in order to minimise manufacturing costs and maximise the properties produced. Often, the only post treatment operation is grinding. As a result, the environment within the furnace surrounding the work-piece has to be controlled, in order to prevent unwanted chemical changes, such as oxidation, decarburisation and scaling, adversely affecting the work-piece surfaces. For this reason Bodycote operate vacuum furnaces and controlled atmosphere pit and continuous furnaces, sealed quench furnaces and fluidised beds, all of which are suitable for hardening and tempering with complete control of surface chemistry.
The need for precise control of temperature and time, as well as environmental chemistry, has led to the application of computerised control systems for most types of furnace. This is a development that was pioneered in the UK by Bodycote and continues to be an important tool for both process improvement and cost control.