Spring steels are supplied in one of two conditions: they are either supplied in annealed condition – to allow component manufacture prior to the hardening process – or hardened and tempered.
The hardening and tempering 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.
The hardening and tempering 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 quenching it in a suitable medium, cooling the steel.
The quenching medium can be air, oil, water, molten salt, fluidised bed or a pressurised inert gas, such as nitrogen.
Selection of the quenching medium is dependent 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.
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.
Following quenching, the work-piece is in its hardest – but most 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.
Uses for hardened and tempered steel
Spring steel in the hardened and tempered condition is used mainly for the production of flat springs, blades and saws, and is it very difficult to form. If you require spring steel for forming before heat treatment, we stock a range of steels in annealed condition as well.
BSS stock a range of hardened and tempered spring steel including, CS70, C75, CS80, CS95 and CS100.
If you require technical assistance to help you select the correct condition or grade, use our friendly telephone advice service. Call 0114 244 0527.