DELAYED FAILURE OF PH13-8MO STEEL PLATED WITH AL-IVD

We have experienced failure of the bolts for aircraft made of PH13-8Mo steel. The failure occurred a short time after the bolts had been fastened for assembly of aircraft parts. These bolts had been aluminum plated with ion vapor deposition (IVD) process. Based on this trouble, delayed failure of PH13-8Mo steel plated with aluminum by IVD was studied to clarify the cause of the trouble. Sustained load tests were performed in distilled water with notched specimens made of PH13-8Mo steel on which aluminum was plated by the same IVD process as defective bolts. From the tests carried out on many heats of PH13-8Mo steel, aged at 783 K (510-degrees-C), the following results were obtained: (1) heats containing more than 1.16 pct aluminum exhibited premature delayed failure with a low stress level threshold; (2) the heats containing less than 0.025 pct carbon and less than 1.16 pct aluminum showed extensive scatter in the values of threshold stress; and (3) the heats containing more than 0.025 pct carbon and less than 1.16 pct aluminum did not exhibit the premature delayed failure even at high stress levels. From the measurement of electrical resistivity changes associated with isothermal aging of PH13-8Mo steel, it is evident that the low-temperature phase formed by aging plays an important role in delayed failure and formation of the phase is significantly influenced by carbon and aluminum contents in the steel. Microcracks were found to form on the aluminum film plated by IVD when stress was applied to the specimen. Therefore, a galvanic couple would be constructed between coated aluminum and PH13-8Mo steel with the presence of distilled water which penetrated through the cracks initiated when the load was applied to the specimen. Hydrogen necessary to give rise to the delayed failure is produced by the cathodically polarized condition. Notch toughness was also controlled by carbon and aluminum contents in PH13-8Mo steel; however, it is not thought that the decrease in notch toughness plays a significant role in the cause of delayed failure. It is particularly noteworthy that carbon in the steel is useful to prevent the delayed failure and to keep excellent notch toughness in PH13-8Mo steel aged to high strength levels.