Coaxing Effect of High Strength Steels with Different Strength Levels
Masaki NAKAJIMA, Yuki NAKAMURA, Toshihiro SHIMIZU and Yu MOCHIZUKI
Abstract:Effect of strength level on the coaxing behavior of high strength steels was studied. The materials used were high strength steels, SCM435, SNCM439 and SUJ2. Stress incremental tests were performed using cantilever-type rotary bending fatigue testing machines. Fatigue limits obtained by the conventional fatigue tests were 525MPa and 575MPa for SCM435 and SNCM439, respectively. On the other hand, since a step-wise S-N curve was seen in SUJ2, stress-incremental tests were started from a stress level just below the transition stress, 1275MPa, where failure mode was changed from surface to subsurface. It was found that SCM435 and SNCM439 indicated a marked coaxing effect and non-propagating cracks were not seen. In SUJ2, a noticeable coaxing effect was hardly appeared but subsurface fracture was found in failed specimens. Based on hardness and static strain aging ability measurements, it was suggested that the coaxing effect in SCM435 and SNCM439 was attributed to strain-aging, but not work hardening. The stress-incremental tests results were analyzed by Minerfs rule. In SCM435 and SNCM439, the cumulative damage values in all stress-incremental test results are more than unity and increase linearly with increasing the number of cycles in a step. In SUJ2, the cumulative damage values are independent of the number of cycles in a step and are small compared to those of SCM435 and SNCM439. Key Words:Coaxing effect, High strength steel, Strain aging, Work-hardening, Minerfs rule, SUJ2, SNCM439, SCM435