Fatigue Anisotropy in Forged Components

Doctoral thesis, 2007

Ever-growing requirements on combustion engine efficiency of motor vehicles demand increasing service loads in powertrain components. Optimization of component material is therefore inevitable. A major detriment with many forged transmission components is their anisotropic mechanical behavior, not least during cyclic loading. Any forging operation will introduce material flow and therewith orientation into the material. With the bulk material also non-metallic inclusions will be deformed. This orientation of the microstructure is held responsible for mechanical anisotropy. Manganese sulfides (MnS) in engineering steel are indigenous non-metallic inclusions which appear according to chemical specifications. These sulfides may generate benefits during component machining and are therefore widely accepted. Microstructural investigations on several materials showed that manganese sulfides deform excessively during any hot deformation operation of the steel. However, by solid solution hardening of the sulfides with calcium (Ca), original inclusion shapes can be maintained throughout a deformation operation. The present investigation is concerned with the examination of the influence of deformed MnS inclusions on fatigue anisotropy. Experimental as well as commercial medium carbon steels with standard and low sulfur (S) contents and therewith different sulfide populations have been hot deformed. Hardened test specimens of those steels, taken in short-transverse and longitudinal directions with respect to the deformation direction, were then fatigue tested. For material with standard S content, fatigue limits in longitudinal test direction are generally twice as high as in short transverse direction. Fractographic investigations showed that fatigue crack initiation is dominated by sulfide inclusions which ultimately can be held responsible for material anisotropy. The very poor fatigue resistance in short transverse direction can be attributed to the appearance of clustered sulfides, where the geometry of such clusters is not accessible by any established metallographic procedure. Diminishing of sulfide inclusions did improve isotropy. Yet, the most powerful measure to increase isotropy of the material proved to be an additional Ca treatment of the sulfides. Thus, quasi isotropic material could be created. In-situ tensile testing in a scanning electron microscope chamber showed that manganese sulfides bond very poorly to the steel matrix. Manganese sulfides in a steel matrix should therefore be seen as inherent micro-cracks. The nature of discoid sulfides being micro-cracks matters also during material hardening where current production material shows a considerably higher quench crack propensity as compared with low S material. Quench cracks initiate at flattened sulfides. Machinability of low S material did not show to be problematic at high cutting speeds. It is believed that an optimization of cutting parameters can balance the disadvantages of low S material during machining operations with low cutting speeds.