Specimen Size Effect on Gigacycle Fatigue Properties of SUP7 Spring Steels

Ultrasonic fatigue tests up to gigacycle regimes were conducted for two heats of SUP7 spring Steels using phi 7x20 mm and phi 3 mm specimens whose risk volumes were 912 and 33 mm(3), respectively. As the result, the phi 7x20 mm specimens revealed much lower fatigue strength than the phi 3 mm specimens, showing fish-eye fracture regardless of the specimen types. The fish-eye fracture origins were all Al(2)O(3) inclusion in most cases, while in case of the phi 3 mm specimens of heat C, a TiN inclusion and matrix itself also caused fish-eye fracture. The Al(2)O(3) inclusion sizes of the phi 7x20 mm specimens were larger than those of the phi 3 mm specimens, which was the reason why the phi 7x20 mm specimens showed lower fatigue strength. The minimum size of the Al(2)O(3) inclusion observed at the fish-eye fracture origin was about 10 mu m. This implied that when the specimen contained no Al(2)O(3) inclusion exceeding 10 mu m in the risk volume, a TiN inclusion or matrix itself could Cause fish-eye fracture as in case of the phi 3 mm specimens of heat C. These results meant that the effects of the specimen size were large in case of fish-eye fracture, recommending us to use large specimens, such as the phi 7x20 mm specimens, in conducting gigacycle fatigue tests on high-strength steel.