Cold coiling of pre-hardened wire for suspension springs

The demand for weight reduction in automobiles to reduce fuel consumption pushed the spring manufacturer to choose higher-strength martensitic steels to reduce the resisting section of the wire, and for this reason the weight of the spring. The traditional approach in forming such materials is based on the hot coiling of the wire at high temperature to increase material formability and to reduce the required forming forces. Another approach is the cold forming process wherein the wire is heat treated before the spring is formed to improve its formability. In both cases the coiled spring has to be hardened and tempered to reach the target strength. Energy and time consumption due to the final heat treatment slow production and increase the manufacturing cost of the spring. The two-fold target of shortening the process cycle and reducing the weight of the spring was met successfully in cold forming the material in its final condition (hardened and tempered). In this case the coiling process becomes more critical because the formability of these high-strength martensitic steels is not very high. The trend of using high-strength steels (between 1800 and 2100 MPa) increases the difficulties in the cold forming of such wires and can lead to the failure of the coiled wire caused by the breakage of the spring.1-3 The forming forces required in the coiling of the wire increase with the high strength of the material causing the feeding force to also increase. The feeding rolls should provide enough force to avoid the slip of the wire between the rolls. For this reason the friction between the wire and the feeding rolls should be at a high level to provide enough tangential force to avoid any slip of the wire. On the other hand, high friction increases the required forming force due to the higher contribution of friction forces developing in the stationary dies where the wire slides as in the guiding plate, in the bending die and in the bar controlling the axial pitch of the spring. The contribution made by the bending die can be reduced using two bending rolls where the sliding friction is substituted by a prevalent rolling friction. This solution has been successfully used by spring manufacturers, who still face the problem of friction in the remaining components of the coiling machine: in the feeding system the friction should be high and elsewhere should be low. The insertion of a lubricating station between feeding and forming can solve the problem, but can be difficult in the industrial application, generating a dirty area of the press and requiring a cleaning step after coiling to remove residual lubricant from the spring’s external surface. For these reasons, the spring manufacturer asks for a wire with an external oxide, which should act as a thin solid lubricant layer. If this layer is too thin, it can be broken when the wire is passing between the feeding rolls, causing a possible slip of the wire and an increase of friction in the forming section of the press. Slipping in the feeding rolls can be eliminated by increasing the radial pressure of the rolls on the wire, but this action can introduce damage to the wire and eventually can lead to failure in the coiling of the spring. The investigation has been focused on the effect of friction as well as on the set-up of the coiling machine, and it is based on a preliminary experimental analysis of different wires. Different samples of the same steel with different Fe-oxide layers have been tested in the coiling machine. Most of them allow the correct manufacture of the spring, but some result in the breakage of the spring during coiling. The samples have been characterized in the laboratory relative to mechanical properties, microstructure, thickness of the oxide layer and its morphology and type of oxide and its adherence to the steel underneath. Afterwards, a numerical model of the spring coiling process has been developed introducing the description of the Lemaitre damage model in the material properties. Relative damage has been used as the index to estimate the risk of failure when different friction and process parameters are used.