Statistical Comparison of Fatigue Lives of Forged Ck45 Steel in Normalized, Quenched and Tempered Condition with Those in Quenched and Tempered Condition

A common process in auto-parts manufacturing consists of forging, normalizing, quenching and tempering. Normalizing treatment is usually employed to improve machinability of steel, and to homogenize and refine the microstructure. For simple parts, production cost would be reduced if normalizing stage were eliminated. In this paper, the effect of normalizing treatment on fatigue life of a quenched and tempered carbon steel has been studied. Samples were taken from forged, quenched and tempered parts at industrial conditions. Sixty four fatigue specimens were tested using a rotary bending fatigue machine (R=-1) at four different stress amplitudes between 500 and 600MPa. Results showed minor effect of normalizing on microstructure, hardness and tensile properties. However, specimens normalized after forging indicated improved fatigue life at the same stress amplitude. Average fatigue lives at 508, 516, 534 and 583MPa were 261000, 194000, 119000 and 54000 cycles respectively for normalized quenched and tempered samples and 209000, 138000, 101000 and 46000 cycles for quenched and tempered samples. Due to variation of fatigue life results, Student's t-test was employed to prove any meaningful difference of fatigue life for specimens with or without normalizing treatment. Student's t-test is often used to determine if the averages of two samples are significantly different. The test approved the difference with 95% confidence. Fatigue life of normalized samples was estimated at least 18% higher than the life of samples without normalizing.