Anisotropy of forged steel components is especially adverse when it concerns rotationally symmetric components. Manganese sulfides (MnS) in steels may be desired for their improvement of machining properties; however, they also deteriorate fatigue behavior. A quantification of the effect of MnS on anisotropy is necessary to find an optimum for component dimensioning. To isolate the influence of MnS on anisotropy only, high cleanness of the test material is required. The test material in the current investigation was molten in a vacuum furnace to high-cleanness composition. Materials with two different S levels were produced to detect variations in anisotropy according to amount, shape, and distribution of the MnS inclusions. The two batches were cross-rolled to plates with a deformation ratio of 4.5. The MnS phase constitutes, upon forging or rolling, pancake-shaped inclusions. In the case of cross-rolling, an in-plane rotational symmetry of the inclusions could be created. The shape and size of these inclusions are essential for the mechanical behavior of the material. Push-pull fatigue testing was performed in longitudinal (in plane) and short transversal directions relative to the rolling plane. The results showed strong anisotropy of the fatigue behavior with inferior performance in short transverse directions where the principal stress is perpendicular to the flattened inclusions. The anisotropy was somewhat more pronounced for the high-S material, resulting from a different fatigue crack growth mechanism.

The bonding between manganese sulfide (MnS) inclusions and the surrounding steel matrix was investigated by in-situ tensile testing in a scanning electron microscope (SEM) at room temperature. Tests were carried out for two different orientations of the inclusions with respect to the loading axis. The orientation was created during a hot cross rolling operation of the test material. Straining was performed along both longitudinal (L) and short transverse (S) directions. The investigation showed that the bond between the MnS inclusions and the matrix is weak. This was particularly seen in the S test direction where the sulfides, lying perpendicular to the load axis, delaminated from the matrix at very low applied stresses. The MnS inclusions in longitudinal tests instead fractured at high stress levels close to the yield stress.

Deformation and forging operations often introduce microstructural orientation and, therewith, mechanical anisotropy to steel. Flattened manganese sulfide inclusions are held responsible for a great part of fatigue anisotropy. Isotropic-quality (IQ) steel maintains the mechanical isotropy of the material, even after a deformation operation. Isotropic material generally contains little S and, therewith, few manganese sulfides. Further, the IQ steels used in this investigation were Ca treated. The Ca treatment improves the shape stability of the sulfides, even during a hotworking deformation. Two commercial materials were compared for their fatigue response, a standard medium-carbon steel with 0.04 wt pct S and a low-sulfur variant that underwent IQ treatment. The two batches were cross-rolled to plates with a deformation ratio of 4.5, leading to in-plane isotropy. Tension-compression fatigue testing was performed in longitudinal and short transversal directions relative to the rolling plane. The results showed strong anisotropy of the fatigue behavior for the standard material. The performance in the short transverse direction, with the principal stress perpendicular to the flattened inclusions, was inferior. The IQ material with nearly spherical inclusions was almost perfectly isotropic, with only slightly worse fatigue response in the short transverse direction.

A B S T R A C T Forged steel generally suffers from strong fatigue anisotropy. This anisotropy is more pronounced in standard sulphur (SS) steels (typically 0.04 wt percent) with a dense manganese sulphide (MnS) population as compared with low sulphur (LS) steels (down to 0.004 wt percent or less). Anisotropy due to MnS arises because of the flattened shape which sulphides obtain upon a forging operation. The higher grade of anisotropy in SS materials is attributed to the occurrence of sulphide clusters that are exclusive to these materials. It is shown that the prediction of fatigue limit with respect to conventionally recorded inclusion size is highly non-conservative when it concerns cluster-affected material, but only when the clusters are oriented perpendicular to the loading axis. Fatigue failures occurred at stresses much lower than expected. An interaction of the single sulphide within one cluster and an interaction of the clusters within the test volume are believed to cause the inferior fatigue performance. √ area = square root of projected effective inclusion area on the fracture surface according to Murakami (μm) A 5 = elongation (%) C = constant describing the position of the inclusion (−) HV = hardness in Vickers (HV) K I = stress intensity factor for mode I loading (MPa √ m) k t = stress concentration factor (−) N f = number of stress cycles to failure (−) r = fatigue strength ratio (−) R = load ratio (−) α = dimensionless exponent in Murakami's model; dependent on hardness (−) σ f = recorded fatigue strength for the specimen at a defined lifetime N f (MPa) σ m = ultimate tensile strength (MPa) σ p0.2 = yield strength (MPa) σ W = Murakami fatigue limit for the material (calculated) (MPa)

The bending fatigue strength of case-hardened pulsator test gear wheels has been investigated for wheels in fatigue isotropic 20NiMo10 (Ovako 158Q) steel. The pulsator gears where either in un-peened (but shot-cleaned), single shotpeened or double shot-peened condition. Single shot peening increased the bending fatigue strength of the gears by some 11 per cent as compared with the un-peened. Double shotpeening was not successful and did not result in any further increase of bending fatigue strength. That was explained with wrongly chosen parameters of the second shot peening moment and therewith over-peening of the component. That second moment was a trial and had never been tested before. 20NiMo10 can be used on current production lines and can be case-hardened in current production furnaces. If availability of the steel is guaranteed, 20NiMo10 has the potential to replace current production material.