These steels have a carbon-content ranging from about 0.5% to 1.1%. In general it may be taken that, the higher the carbon-content, the better the spring properties that may be obtained.
In the manufacture of flat springs and the heavier coil springs, it is usual to form the spring from annealed material and subsequently to heat-treat it. However, it is sometimes possible to manufacture flat springs from material which is already in the hardened and tempered condition, and this latter technique may give a lower production cost than the former.
For light coil springs, the material loosely known as piano wire is used; this is a spring wire which obtains its physical properties from cold-working, and not from heat-treatment. Springs made from this wire require only a low-temperature stress-relieving treatment after manufacture. Occasionally wire known as 'oil-tempered' is used—this is a wire which is hardened and tempered in the coil form, and again requires only a low-temperature stress relief after forming.
Plain-carbon steel springs are satisfactory in operation up to a temperature of about 180°C. Above this temperature they are very liable to take a permanent set, and alternative materials should be used.
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