Effects of Chemical Composition, Heat Treatment, and Microstructure in Splittable Forged Steel Connecting Rods

Fracture split forged steel connecting rods are utilized in many new high performance automotive engines to increase durability. Higher strength levels are needed as the power density increases. Fracture splitting without plastic deformation is necessary for manufacturability. Metallurgical design is a key for achieving the required performance levels. Several medium carbon steels containing 0.07 wt pct P, 0.06 wt pct S and various amounts of Mn, Si, V, and N were produced by vacuum induction melting laboratory heats and hot working the cast ingots into plates. The plates were cooled at varying rates to simulate typical cooling methods after forging. Microstructures were generally ferrite and pearlite as evaluated by light optical and scanning electron microscopy. Mechanical properties were determined by standard tensile tests, high strain rate notched tensile tests, and Charpy V-notch impact tests to assess “splittability”. The test results indicate that large prior austenite grain sizes achieved during heating for rolling and increased cooling rates after hot working decreased the fraction of ferrite, leading to higher strength levels. Increased levels of V and N also increased the strength. At higher strength levels the amount of fracture deformation was reduced as measured in the notched tensile test results, and impact transition temperatures were higher. An additional steel containing Cr, Ti, and B resulted in a bainitic microstructure with a higher tensile strength than the ferrite-pearlite microstructures and maintained low levels of plastic deformation in the notched tensile test.