Alloy Selection

In the original design, BMW needed a hardness of 50-56 Rockwell C and specified an 8640 steel alloy (iron-carbon with manganese, chromium, nickel, and molybdenum) for the castings.

But when test components went into engine testing, six months before full production, the original specified alloy had wear problems on the eccentric shaft.
Engine tests 3 months prior to production on the modified shaft indicated wear on the intermediate lever arm. The alloy on the intermediate lever arm had to be changed to be compatible with the new shaft.

BMW and Hitchiner engineers worked together to respond to this unexpected challenge, doing a joint study to choose and test an alternate alloy with higher hardness and improved required wear resistance.

The development team chose and validated a European alloy -- a 30Cr-MoV9 tool steel -- to meet the performance requirements of the lever arm. A plasma nitriding treatment produces hard, wear resistance surface layers on the casting.

There was one negative factor with the new alloy -- lower yield. The 30CR-MoV9 steel has lower fluidity which required higher casting temperatures for good fill. Higher temperatures required longer solidification time, which increased the casting time and produced fewer daily casting cycles.