THE HIGH CYCLE FATIGUE and FINAL FRACTURE BEHAVIOR OF ALLOY STEEL 4140 USED IN HYDROGEN PRESSURE VESSELS: Influence of Copper Plating

摘要

In this paper the results of a study aimed at understanding the extrinsic influence of exposure to hydrogen gas on high cycle fatigue properties and final fracture behavior of alloy steel 4140, a viable candidate for use in hydrogen-containing pressure vessels, that has been plated with copper is presented and discussed. The specific heat treatment given to the chosen alloy steel was to get a quenched and tempered condition. Test specimens of the alloy were precision machined and conformed to the standards specified in ASTM E466. Test specimens both without a notch and with a notch were prepared from the alloy steel bar stock. The notch can be visualized as a state of potentially high local stress concentration. Test specimens of the alloy in both the smooth condition and the notched condition were then plated with a thin layer of copper (0.001 inch thick). The copper plated test specimens were subsequently exposed to gaseous hydrogen in a specially designed and constructed environmental chamber for 120 minutes at a temperature of 375 F. The unexposed and exposed samples were deformed both in tension to get the tensile properties and in cyclic fatigue over a range of maximum stress, at a load ratio of 0.1, and the number of cycles to failure recorded. The significance of exposure to gaseous hydrogen prior to mechanical testing on cyclic fatigue life of both the smooth and notched specimens of alloy steel 4140 is presented. The fatigue fracture surfaces were examined in a scanning electron microscope to establish the macroscopic fracture mode and to concurrently characterize the intrinsic features on the fracture surfaces. The conjoint influence of microstructure, exposure to gaseous hydrogen, and the presence and role of notch on cyclic fatigue life is discussed.