Microstructural effects on the hydrogen embrittlement of high strength steels

Andrew W. DeVillier

doi:10.17918/etd-3405

High-strength, high-toughness steels are used in critical components of Naval aviation systems. These critical applications include landing gear components in tactical aircraft and rotary components in helicopters where large stresses must be sustained and the probability of failure due to flaws in the part are kept minimal. To prevent corrosion and wear, high strength steels are plated. When parts undergo repair/rework, the plating is stripped in an acidic bath, and the part is re-plated. During stripping and plating, as well as in-service duty, the steel is exposed to hydrogen-rich environments, which have been identified as a potential cause of premature failures in aviation system components. A high strength, precipitation hardened steel, PH 13-8Mo, one of six candidate steels, was examined in three different conditions - solution annealed (SA), H1000 (538oC aged), and H1150 (621oC aged) in order to compare the susceptibility of the different microstructures to hydrogen embrittlement and determine the stress-life curve for PH13-8 Mo in the H1000 condition. The PH 13-8Mo was selected as a result of a recent in-service failure of a reworked component. Fatigue pre-cracked compaction-tension (CT) specimen were loaded in displacement control and seated in a 10% strength hydrochloric acid bath. The load and crack mouth opening displacement (CMOD) were monitored as a function of time. From the load and CMOD versus time behavior, failure criterion were developed to determine points of crack initiation. Fractured samples were examined via scanning electron microscopy (SEM). Polished samples of the H1000, SA and H1150 condition were also analyzed by electron backscatter diffraction (EBSD). EBSD analysis showed that the packets of reverted austenite that grew in the H1150 condition material possessed an orientation that tended towards [111], which may indicate a Kurdjumov-Sachs or Nishiyama-Wasserman orientation relationship. In addition, packets of austenite within a local region also shared a common orientation with one another. An explanation for this behavior is proposed. The hydrogen embrittlement results show that the H1000 condition is highly susceptible to crack initiation over a wide range of applied stress values. A comparison of the different conditions showed that the H1150 and SA conditions possessed similar equal or better resistance to crack initiation in the hydrogen-rich environment at equivalent values of applied stress intensity (K) than the H1000 condition. Comparing equal percentages of applied (apparent) fracture toughness (KQ or KIC), only the SA condition had better resistance to initiation than the H100 condition. Resistance to crack propagation for the H1150 and SA conditions was superior to that of the H1000 condition. Finally, the threshold stress intensity, Kth, below which no cracking occurs for the H1000 condition, is on the order of 10% KIC or less, which is in good agreement with what has been established in literature.

Microstructural effects on the hydrogen embrittlement of high strength steels

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Microstructural effects on the hydrogen embrittlement of high strength steels

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