Mechanisms of environment induced subcritical flaw growth in AISI 4340 steel

Abstract

The results of an experiment designed to study the effect of several variables on the subcritical crack growth rate of a high-strength steel in a water environment are presented in this paper. Double cantilever beam specimens of an AISI 4340 steel, heat treated to a yield strength level of 217,000 psi with a fracture toughness of K Ic = 51.5 ksi (in.) 1 2 , were employed to study crack extension rates. The environmental variables included temperature, pH of the liquid environment, electric charging, and the combined effect of pH and electric charging. Tapered double cantilever beam specimens were designed so that the ℷ level was maintained constant at a constant load, and independent of crack length. Thus, steady-state crack growth measurements were possible in constant load and constant environment experiments. With this specimen design, it was possible to make a series of measurements that covered the entire range of effects of a particular variable using only one specimen. It was concluded from the results that both hydrogen embrittlement and chemical corrosion assisted crack growth. The relative contributions of the two mechanisms were dependent on the pH and the applied potential.