A Study of Decarburization in SAE 1042 Steel: Its Effect on Fatigue Life, Tensile Properties, and Fatigue Fractography

The R. R. Moore rotating-beam fatigue test was used to determine the effects of-increased surface decarburization on the fatigue life of SAE 1042 steel. This test was conducted on three series of test specimens: (1) specimens having no surface decarburization (series D-1), (2) specimens having a 0.038 inch (0. 97 mm) decarburized surface layer (series D-8), and (3) specimens having a 0.080 inch (2.03 mm) decarburized surface layer (series D-24). In addition to the fatigue tests, the change in various tensile properties resulting from surface decarburization was investigated for each of the three test series utilizing the standard ASTM tensile test.

Careful study of the fractured fatigue specimens by scanning electron microscopy (SEM) revealed important information as to the nature and morphology of fatigue striations present in medium carbon steels, and established the "quasi-striation" pattern as the primary microscopic identity present in normalized SAE 1042 steel.

Results of the fatigue tests indicated that the initial decarburization depth had the greatest effect on the fraction of life degradation, while the tensile properties tests proved that the tensile properties, ultimate tensile strength CUTS) and yield strength (YS), originally reduced by surface decarburization, could be restored with proper machining practices.

In essence the study showed:

1. A decarburized surface layer severely reduces fatigue life.

2. Fatigue life decreases with an increase in the decarburization depth.

3. Removal of the decarburized surface layer completely restores the initial tensile properties.

4. Macroscopic fracture surface appearance is not affected by the presence of decarburization.

5. Three distinct fracture surface appearances exist in the applied stress range of a fatigue curve: (1) a "jagged" fracture surface at high stresses (above the YS), (2) a "ratcheted" fracture surface at medium stresses (approximately 1/2 of the UTS), and (3) a "smooth" fracture surface at low stresses (approximately 1/2 of the YS).

6. Striations are invariably more prone to form in primary ferrite.

7. Striation density is enhanced by (1) a reduction in the applied stress, and (2) an increase in the depth of surface decarburization.

8. No direct correlation between applied stress and striation spacing can be determined for materials which exhibit primarily "quasistriation" fatigue patterns.

Particular emphasis was given the implementation of computerized statistical data analysis. The unique combination of the statistical analysis system (SAS) and curve fitting programs, specifically developed for this study is discussed.