While certain materials have come to be regarded as helical spring materials, they are not specially designed alloys. Spring materials are high strength alloys which often exhibit the greatest strength in the alloy system. For example: In steels, medium and high-carbon steels are regarded as spring materials. Beryllium copper is frequently specified when a copper base alloy is required. For titanium, cold-worked and aged Ti-13V-11 Cr-3A1 is used. The energy storage capacity of a helical spring is proportional to the square of the maximum operating stress level divided by the modulus. An ideal spring material has high strength, a high elastic limit and a low modulus. Because springs are resilient structures designed to undergo large deflections, spring materials must have an extensive elastic range. Other factors such as fatigue strength, cost, availability, formability, corrosion resistance, magnetic permeability and electrical conductivity can also be important and must be considered in light of cost/benefit. Consequently, careful selections must be made to obtain the best compromise.
Surface quality has a major influence on fatigue strength and is often not clearly delineated on national specifications. It is important to use only those materials with the best surface integrity for fatigue applications, particularly those in the high cycle region.
In steel alloys, for which processing costs are a large fraction of product cost, surface quality can vary over an appreciable range. Depth of surface imperfections, such as seams, pits and die marks, can be up to 3.5% of diameter for commercial spring wire grades. Various intermediate qualities can be obtained. Highest levels are represented by music and valve spring quality grades which are virtually free of surface imperfections. Decarburization, which can also adversely affect fatigue performance, follows a similar pattern. Surface quality of spring materials is a function of the care exercised in their production and processes employed. Materials produced with a high level of surface integrity are more costly than commercial grades.
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