EFFECTS OF PROCESSING ON THE TRANSVERSE FATIGUE PROPERTIES OF LOW-SULFUR AISI-4140 STEEL

The effects of inclusions due to steelmaking processes on the fatigue lite of AISI 4140 have been investigated. The test matrix consisted of three commercially produced heats of AISI 4140 of comparable cleanlin ess: one was conventionally cast (CC), and two were inert gas-shielded /bottom-poured (IGS). One of the IGS heats was calcium-treated to expl ore the effects of inclusion shape control (IGS/SC). All heats were ho t-rolled and reduced over 95 pct to produce bar stock of 127 to 152 mm (5 to 6 in.) in diameter. Transverse axial specimens conforming to AS TM E466 were machined, quenched, and tempered to approximately 40 HRC, and they were fatigue tested in tension-tension cycling (R = 0.1). Te st results and statistical analyses of the stress-life data show that the IGS grade has several times the fatigue strength of the CC grade a t 10(7) cycles. Lower-limit fatigue strengths calculated at a 99.9 pct probability were 518.5 MPa (75.2 ksi) for IGS vs 55.6 MPa (8.1 ksi) f or the CC grade. The IGS/SC grade had the best performance at all stre ss and life levels. The results obtained indicate that fatigue perform ance can be improved by choosing a processing method that reduces the incidence of exogenous oxides and by controlling the shape of the sulf ides.