Influence of decarburisation on the fatigue behaviour of shot peened spring steels

The influence of decarburization on the high cycle fatigue behaviour of shot peened spring steel was investigated. The material studied was a quenched and tempered carbon steel (0.55C, 1.9Si and 0.9 Mn). The specimens were decarburized to depths of 0.40 and 0.50 mm. Shot peening with full coverage then took place, leading to a thickness of the compressed surface layer of about 0.25 mm. The specimens were tested in bending fatigue to fatigue lives in the range Nf = 1·10 to 3·10 cycles. Following fatigue limits were then achieved: 655 – 675 MPa for the two decarburized levels and 880 MPa for the non-decarburized specimens. Crack nucleation generally took place in the very surface zone. Stress relaxation during fatigue loading and surface roughness of the specimens were measured and correlated to the fatigue behaviour.