Influence of dissolved hydrogen on the fatigue crack growth behaviour of AISI 4140 steel

In this investigation, the influence of dissolved hydrogen on the fatigue crack growth behaviour of a low alloy steel (AISI 4140) was examined. The investigation also examined the influence of microstructure on the fatigue threshold and the fatigue crack growth behaviour of this material. Compact tension (CT) specimens prepared from AISI 4140 steel as per ASTM standard E-647; were tested in two different heat treated conditions, i.e., cold rolled and annealed as well as austempered condition. The specimens were precharged with hydrogen using ex situ cathodic charging technique at a constant current density for three different time periods ranging from 150 to 250 h. The samples were then subjected to fatigue crack growth test in the threshold region as well as in the linear region. The near threshold fatigue crack growth rate, fatigue threshold and the crack growth rate in the linear region of these specimens were determined. The fatigue crack growth behaviour of the hydrogen charged samples were compared with specimens without any dissolved hydrogen. The current test results show that the near threshold crack growth rate increases with the increase in dissolved hydrogen content. It was also observed that the fatigue threshold decreases as the dissolved hydrogen content increases. The crack growth rate was found to be higher in the austempered samples compared to the annealed samples. An interesting phenomenon was observed in the case of annealed specimens charged with hydrogen for 250 h. These specimens had unusually high fatigue threshold (ΔKth).