Effect of Tensile Strength and Microstructure on Notch-fatigue Properties of Ultrafine-grained Steels

Abstract

Fatigue strength is one of the key properties in the practical use of ultrafine grained steels. Fatigue tests were conducted on notched specimens by conventional electromagnetic resonance fatigue testing machines. The electromagnetic resonance fatigue testing was carried out at 150 Hz up to 107 cycles. The investigated steels had different levels of carbon, 0.15 wt%, 0.30 wt% and 0.60 wt% with tensile strengths of 850 MPa, 950 MPa and 1105 MPa, respectively. With increasing in carbon content, the tensile strength increased and the total elongation decreased. The notched specimens never showed internal fracture and showed a clear fatigue limit. The notch fatigue limit increased with an increase in tensile strength from 850 MPa to 970 MPa, when the carbon content was 0.15 and 0.30 wt% with microstructures consisting of ultrafine ferrite grains and cementite particles. On the other hand, when the carbon content was 0.60 wt%, the notch fatigue limit decreased, though tensile strength increased to 1105 MPa. Retained pearlite grains were observed in 0.60 wt%C steel in addition to ultrafine ferrite grains and cementite particles. These retained pearlite grains which were coarse and high angle grain boundary poor regions were attributed to the lower notch fatigue limit.