THIS STUDY WAS DEVELOPED in response to a component fatigue strength test which determined that if the current #3 engine bearing cap were used in the 2.5L Turbo application, it would be over stressed. Proposed solutions for solving this problem included: a redesigned grey iron cap with additional material in the highly stressed areas, or a cap made from either nodular iron or a free machining steel using current specifications. One of the manufacturing concerns about switching materials is the perceived difference in the machinability of nodular iron and steel.
A single point turning evaluation was carried out by Chrysler Motors' Machinability Development Laboratory to compare the machinability of various materials proposed for use in engine bearing caps. Materials tested included: SAE G2500 grey cast iron, the current production material; SAE D45-12 nodular cast iron; and SAE steel grades 1117, 1137, 1215, 12L14, 1215, and 1215 modified (Incut 200). To better replicate production experiences, the steel materials were supplied from two steel suppliers (Inland and LTV) and were tested as-received and normalized.
Results indicated the steel grades, in both the as-received and normalized conditions, performed better than the grey cast iron, which in turn, machined better than the nodular.
The turning evaluation does not prove free machining steels will have'the same improvement in broaching and drilling operations, but does indicate that certain free machining steel grades can exceed the machinability of some commonly used cast irons. As a result, additional machining evaluations should be considered.