MATERIAL FOR HELICAL SPRINGS
The selection of the spring material depends upon the service for which helical springs are used may be broadly classified as severe, average, and light Although the classification in particular application depends largely on the designer's judgment, in general , SERVER service includes rapid continuous loading where the ratio of minimum to maximum stress is one half or less, as in valve springs, Average Service includes the same stress range as in severe service but with only intermittent operation, as in engine governor springs Light Service includes springs subjected to loads that are static very infrequently varied, as in safety valve springs Music wire is suitable for small springs made of wire up to 0.7 mm in diameter, or if necessary for strength and finish up to 3.175mm Oil tempered wire is suitable for low stresses and non-critical uses.
Valve spring wire is used when long life is important, or alloy steel wire may be used. For high temperature application, tungsten high speed steel or molybdenum type is suitable. Non ferrous material include phosphor bronze, beryllium copper, silicon bronze, nickel silver alloys or nickel alloys, such as Monel, K Monel. Iconel, Iconel X, and Z nickel may be used in special cases to increase fatigue resistance, and corrosion resistance. As to forming, springs made of wire less than 13mm in diameter are formed cold; wire over 13mm is hot formed.