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Technical Paper

Effect of Thermal Treatments and Carbon Potential on Bending Fatigue Performance of SAE 4320 Gear Steel

This project investigated the effect of carburizing carbon-potential and thermal history on the bending fatigue performance of carburized SAE 4320 gear steel. Modified-Brugger cantilever bending fatigue specimens were carburized at carbon potentials of 0.60, 0.85, 1.05, and 1.25 wt. pct. carbon, and were either quenched and tempered or quenched, tempered, reheated, quenched, and tempered. The reheat treatment was designed to lower the solute carbon content in the case through the formation of transition carbides and refine the prior austenite grain size. Specimens were fatigue tested in a tension/tension cycle with a minimum to maximum stress ratio of 0.1. The bending fatigue results were correlated with case and core microstructures, hardness profiles, residual stress profiles, retained austenite profiles, and component distortion.
Technical Paper

Effects of Strain Path on Formability and Microstructural Evolution in Low-Carbon Sheet Steels

The effects of strain path on formability and microstructural evolution with strain in two low-carbon steels were examined. The steels include a 0.008 wt. pct. C batch annealed 0.81 mm thick sheet and a 0.031 wt. pct. C continuously annealed0.74 mm sheet with essentially equivalent mechanical properties (YS: 230 MPa; UTS: 350 MPa; n: 0.18). The steels were subjected to various increments of prestrain in either uniaxial or biaxial tension, and forming limits were assessed in the samples after a strain path change to biaxial or uniaxial tension, respectively. Biaxial stretching prestrain lowers the uniaxial tension forming limit, while uniaxial tensile prestrain raises the biaxial stretching forming limit. The differences in forming response were also correlated with distinct dislocation cell structures. The effects of strain path on formability were shown to correlate with predictions based on a redundant strain model and a critical thickness strain model.
Technical Paper

Optimized Carburized Steel Fatigue Performance as Assessed with Gear and Modified Brugger Fatigue Tests

The effectiveness of three different techniques, designed to improve the bending fatigue life in comparison to conventionally processed gas-carburized 8620 steel, were evaluated with modified Brugger bending fatigue specimens and actual ring and pinion gears. The bending fatigue samples were machined from forged gear blanks from the same lot of material used for the pinion gear tests, and all processing of laboratory samples and gears was done together. Fatigue data were obtained on standard as-carburized parts and after three special processing histories: shot-peening to increase surface residual stresses; double heat treating to refined austenite grain size; and vacuum carburizing to minimize intergranular oxidation. Standard room-temperature S-N curves and endurance limits were obtained with the laboratory samples. The pinions were run as part of a complete gear set on a laboratory dynamometer and data were obtained at two imposed torque levels.