Oil Tempered Spring wire is produced by taking usually a silicon-chrome steel and hardening inline to tensiles of around 2000MPa. Oil tempered wire is used widely in the automotive industry for a number of different applications but predominantly suspension springs and valve springs. There are different classes of oil tempered wires, and they are classified according to the fatigue stress/kinetic requirements of the spring. For example, valve springs will be exposed to high levels of kinetic movement and so requires VD quality material. The specification for VD requires all wire to be final Eddy Current Tested to ensure that any inclusions in the wire are found and highlighted to the spring manufacturer. The number of inclusions found in the wire can be greatly reduced by shaving the wire rod prior to drawing. This is where around 20-40 microns of material is removed from the outer surface of the wire, therefore physically removing inclusions. Wire can then be further processed to super clean.
Wire for suspension applications where the fatigue stresses are not as high use a FD or TD class of material. For FD class of material, eddy current test is not essential and is often not specified by spring manufacturers, as the risk for failure in static applications is not as high as for springs with high kinetic stresses. It is vital that high quality wire rod is used for oil tempered wires as the number of inclusions from poorer quality rod manufacturers can be high, therefore increasing the risk of a spring failure.
Another application for oil tempered wire is for clutch springs. This is classed as medium to high fatigue stress and TD is usually specified for this application. TD is final ECT reducing the chance of inclusions being used in the spring, reducing the risk of a failure greatly.
Springs are usually produced by cold coiling and then further stress relieving. The wire has to be handled with the correct equipment due to the high tensile strength of the material. Avocet Precision Metals are a UK leading supplier of oil tempered wires for the spring industry.