Effect of Environment on the Fatigue Behavior of a Medium Carbon Steel

The effects of environment on the fatigue behavior of a quenched and tempered medium carbon steel have been studied. Alternating tension, low cycle fatigue tests were performed at room temperature in humid and dry hydrogen, hydrogen sulfide, and argon, as well as in ambient air. With respect to behavior in air, humid hydrogen, and to a lesser extent, argon caused an increase in fatigue life, while hydrogen sulfide caused a decrease. These results are explained by consideration of the processes occurring at the tip of a growing fatigue crack where the freshly exposed clean metal surface interacts with the environment. In some cases, such as humid hydrogen, it is believed that the presence of a condensed water film can act as a shield to prevent reactions with hydrogen or oxygen which would otherwise increase crack growth rates. In the presence of hydrogen sulfide or dry hydrogen, however, chemical reactions are believed to occur leading to hydrogen embrittlement. The result is a change in the fracture mode, as observed by fractography, and a consequent decrease in fatigue life.