The influence of tempering temperature on small fatigue crack behavior monitored with surface acoustic waves in quenched and tempered 4140 steel

Small surface fatigue crack growth in specially designed cantilevered bending samples of high-purity 4140 steel quenched and tempered to various strength levels was investigated. Tempering temperatures of 200 °C, 400 °C, 550 °C, and 700 °C were used to produce a range of yield strengths and microstructures. Crack propagation and crack closure were monitored with a surface acoustic wave ultrasonic technique. The small crack results were compared to those of long cracks in compact tension samples. Small cracks in the 200 °C and 400 °C tempers grew at ΔK levels above their long crack thresholds. Small cracks in the 550 °C tempers grew at ΔK levels slightly below the long crack threshold. The surface cracks in the 700 °C temper grew well below the long crack threshold showing the “small crack effect.” Small crack growth occurred in a very narrowda/dN- ΔK scatter band showing much less variation with microstructure than the long crack data. The differences between the long and small crack data were due to the high, relatively similar closure values for the small cracks and the variation of long crack growth with microstructure.