Failure Beneath Cannon Thermal Barrier Coatings by Hydrogen Cracking; Mechanisms and Modeling

摘要

Army experience with hydrogen cracking failures of cannons is described, including extensive testing of high strength steel and nickel-iron base alloys to address the failures. Cracking of cannon pressure vessel steels just under bore thermal barrier coatings is now common, and can be explained by the combined action of hydrogen-bearing combustion gases and thermally induced tensile residual stresses. Above-yield transient thermal compression and resultant residual tension stresses beneath the coating are shown to give good predictions of crack arrays observed under the coatings. A similar array of hydrogen cracks in a prototype cannon has recently been explained by contact of combustion gases with uncoated high strength steel that has been yielded by mechanical compressive stresses, leading to residual tension and cracking. The use of nickel-plated hydrogen barrier coatings was shown to eliminate this type of cracking. Recent cannon experience provides a basis for a summary of mechanisms of hydrogen cracking beneath cannon barrier coatings. The near-bore transient temperature distributions due to cannon firing are calculated by finite difference calculations using temperature-dependent thermal and physical properties and validation by comparison with the known temperatures and the observed depths of microstructural damage. Solid mechanics calculations of transient thermal compressive stresses and resultant residual tensile stresses are made, taking account of temperature dependent coating properties and yielding of the steel substrate near the bore surface. Effects of coating material, coating thickness, and the temperature and duration of firing gases on the depth of thermal damage below the coating are investigated. Direct comparisons between observed and predicted thermal damage and hydrogen cracking are made for coating and firing conditions that correspond to modern cannon firing. This comparison suggests changes in cannon bore coatings to handle the more extreme thermal conditions in modern cannon firing, including thicker or more durable thermal barrier coatings to minimize thermal stresses in the steel substrate and different coating materials that can serve as hydrogen barrier coatings.