Stress corrosion crack growth behavior of 4130 (UNS(1) G41300) and 4340 (UNS GG43400) steels was examined as a function of temperature in deaerated distilled water and deaerated 0.6 NaCl solution to determine whether electrochemical reactions controlled crack growth in the aqueous environments. Particular attention was given to the K-independent (Stage II) crack growth rate. Independent measurements of the rates of electrochemical reactions of bare steel surfaces with the chloride solution were made as a function of temperature to assist in this determination.
The results showed that Stage II crack growth was thermally activated, with an apparent activation energy of 34 ±3 kJ/mol (at the 95% confidence level). Crack growth rates were found to be more rapid in distilled water than in the sodium chloride solution and were faster in the higher strength 4340 steel. The apparent activation energy for crack growth compared well with a pooled value of 37 ± 6 kJ/mol (95% confidence level) for reactions with water vapor and electrochemical reactions in the chloride solution. The agreement in apparent activation energies provides support for the hypothesis that crack growth in high-strength steels in the aqueous environments is controlled by the rate of electrochemical/chemical reactions, with the overall reaction rate controlled by either the anodic or the hydrolysis reactions. The results also indicated that hydrogen embrittlement is the responsible mechanism for crack growth enhancement. The difference in Stage II crack growth rates between 4130 and 4340 steels is tentatively attributed to a difference in the propensity for hydrogen segregation to the prior-austenite grain boundaries, which depends on the density of grain boundary traps for hydrogen.
