Investigation of the impact of AISI D2 tool steel wear and lubrication type on the evolution of the friction coefficient during cold winding process

In the automotive industry, lightning vehicles is both an economic and an ecological necessity. The manufacturing of springs that meet these requirements requires the use of high strength martensitic steels SAE 9254. This has the advantage of enabling more complex spring designs and increasing productivity. However, this increase of productivity is accompanied by significant wear on the forming dies, which results in undetectable surface defects on the finished products as the main effect. The objective of this study is twofold, starting with a characterization of the wear of the AISI D2 tool steel and its damage modes, and then the evaluation of the impact of wear and changes in lubrication conditions on the evolution of the friction coefficient during cold winding process. Results shows that, to avoid the creation of those defects, it is necessary, on one hand, to change the tooling when approaching 20,000 parts produced and, on the other hand, to leave the oxide accumulated in the tooling, which acts as a thermal barrier and protect the wire from damage.