Abstract
The potential to reduce the susceptibility to quench embrittlement, a fracture mechanism that promotes intergranular fracture in high carbon steels, and to improve the bending fatigue performance of vacuum carburized modified SAE 4320 steels was evaluated. Data were obtained on an industrially produced SAE 4320 steel and four laboratory produced steels based on the 4320 composition but with additions of Si (1.0 or 2.0 wt pct) and B (0 or 17 ppm). All five alloys were vacuum carburized together and gas quenched with three different cooling rates as controlled by the gas quench conditions: 10 bar nitrogen, and 15 and 20 bar helium. Modified Brugger fatigue samples of each alloy and quench condition were tested in cantilever bending and failed samples were analyzed with scanning electron and Auger spectroscopy. Standard S-N curves and endurance limits were obtained and the fracture surfaces were evaluated using both light and electron microscopy techniques to determine fracture initiation sites and fracture growth mechanisms, both in the stable fatigue crack growth zone and in the overload zone. The percentage of transgranular fracture in the carburized case was quantified and used as a measure of the susceptibility to quench embrittlement. The susceptibility to quench embrittlement was observed to be independent of quench rate and boron additions, but depended on Si content. With an increase in Si content, the extent of intergranular fracture decreased, indicating a decrease in the susceptibility to quench embrittlement. However, with an increase in Si content to 2 wt pct, significant grain growth occurred producing prior austenite grain sizes 2 to 3 times those observed in the base or 1 pct Si alloys. The grain growth experienced by the high Si alloys was interpreted to result from the effects the retardation of cementite nucleation and growth at austenite grain boundaries. The fatigue properties were shown to be essentially independent of cooling rate and differences in fatigue performance were assessed primarily based on a consideration of alloy additions. Fatigue crack nucleation in all samples exhibited similar characteristics, i.e. intergranular crack nucleation at a small cluster of surface grains. The larger grain sizes in the 2 wt pct Si alloys were shown to be the primary factor that affected endurance limits. The 2 wt pct Si alloys exhibited endurance limits of approximately 915 MPa while the baseline 4320 alloy and the modified 1 wt pct Si alloys exhibited higher endurance limits of approximately 1070 MPa. In comparison to fatigue data on gas carburized samples reported in the literature the samples in this study exhibited more variability in measured lifetimes with many samples exhibiting runout at stress levels significantly greater than the measured endurance limits. Implications of this study with respect to the development of potential new carburizing alloys are discussed.
