Elastic limit and yield limit
The maximum stress that the steel wire can withstand without permanent residual deformation is called the elastic limit.
The steel wire with high elastic limit has large elastic force. According to the use state of the spring, the elastic limit that affects the elastic force can be divided into torsional elastic limit (τe) and tensile elastic limit (Re). The torsional elastic limit is used for compression and tension coil springs, and the tensile elastic limit is used for spring pads and leaf springs.
An important function of a spring is to absorb and store energy. The energy absorbed and stored is called deformation energy. The deformation energy of the spring is proportional to the square of the elastic limit (U=2τe2/2G or U=2Re2/2E), so the elastic limit has a great influence on the spring characteristics.
It is difficult to accurately measure the elastic limit of a steel wire in a tensile test, and the yield limit is generally used to measure the elastic limit.
Yield limit (ReL) refers to the minimum stress at which the steel wire begins to produce unrecoverable plastic deformation during the stretching process. The yield point of carbon spring steel wire is very inconspicuous, and the stress when the steel wire produces 0.2% residual deformation is usually taken as the yield limit (RP0.2).
Under annealing or solid solution conditions, the elastic limit and the yield limit of the steel wire are very close. The steel wire drawn by a large reduction in surface rate or after quenching often has a high yield limit due to internal stress, but the spring limit is very low. . Only the elastic limit of the steel wire after stress relief annealing or tempering treatment is close to the yield limit.
The elastic limit is generally proportional to the tensile strength. The tensile elastic limit and torsional elastic limit of common spring steel are shown in Table 2.
Table 2 The elastic limit is the percentage of tensile strength (%)
|Material||Tensile elastic limit||Torsional elastic limit|
|Cold drawn carbon spring steel wire||60～75||45～55|
|Oil quenched and tempered carbon spring steel wire||80～90||45～55|
|Oil quenching and tempering65Mn||85～90||50～60|
|50CrV（Oil quenching and tempering）||88～93||65～75|
|55CrSi（Oil quenching and tempering）||88～93||65～75|