The biaxial fatigue of decarburized steel

A completely reversed loading fatigue test program was used to determine the effect of the type of loading (bending or torsion) on the fatigue properties of SAE 1042 steel. The program utilized two series of specimens, decarburized (with a 0.042 inch decarburized layer) and non-decarburized. The results of the fatigue tests were analyzed using modern statistical curve fitting techniques. The results of the fatigue tests in bending and torsion were compared using multiaxial failure criteria, either the maximum shear stress criterion or the maximum octahedral shear stress criterion.

Examination of the fractured fatigue specimens by scanning electron microscopy (SEM) revealed important information on the macroscopic fracture modes of specimens tested in bending and torsion.

Some higher magnification SEM pictures are included to show the microscopic fracture appearance. Because of the microstructure of the material used, identification of features unique to fatigue failure was difficult.

In addition, surface roughness tests, tensile tests, microhardness tests, and metallography were used to characterize the properties and microstructure of the specimens.

In summary, the study showed that:

1. Decarburization lowers the hardness and decreases the amount of pearlite present in the surface layer of SAE 1042 steel.

2. Decarburization has only a slight effect on monotonic tensile properties.

3. The presence of a decarburized surface layer reduces fatigue strength and the fatigue limit in both bending and torsion.

4. When comparing fatigue strengths and the fatigue limit in bending and torsion, it was found that failure criteria based on an equivalent stress concept can be applied at 10⁷ cycles, but not for fatigue strengths at less than 10⁶ cycles.

5. Macroscopic fracture modes of specimens tested in bending and torsion were found to be quite different, although for both types of loading, crack propagation was observed on planes of maximum normal stress.

6. Using previous data as a comparison, it was found that the present data for fatigue life are dominated by cycles spent in fatigue crack propagation.

The fatigue properties were discussed in relation to fatigue processes and the fatigue testing machine.