介在物軟質化した弁ばね鋼に対する疲労試験による介在物検査の妥当性

抄録

Inclusion inspection conducting fatigue tests was applied to a JIS-SWOSC-V valve spring steel in which oxide type inclusions were composite and softened. In this inspection, inclusion sizes were measured on the fracture surfaces of fish-eye fracture after fatigue tests. The fracture surfaces after fatigue tests revealed a TiN inclusion at the origin in most cases, and in some cases, no inclusion was found at the origin. The TiN inclusion sizes at the fracture origin were precisely measured after etching of fracture surfaces. Even in cases of the origin without an inclusion, a TiN inclusion mostly appeared after the etching of fracture surfaces. The composite oxide inclusions have never caused fish-eye fracture, while the composite oxide inclusions were detected in a conventional inclusion inspection scanning polished surfaces with an optical microscope. These results meant that inclusion inspection conducting fatigue tests were effective for the inclusion-controlled steel. In addition, three tempering conditions, that were 200 and 430°C furnace tempering and 460°C induction tempering, were employed for the fatigue tests. These fatigue test results, then, showed little difference in spite of the difference in hardness and microstructure. Convenient tempering conditions could, therefore, be selected in the inclusion inspection conducting fatigue tests.