Analysis of a Piston Experiencing Environmentally-Assisted Cracking as a Result of Compressive Overloading

摘要

A piston used in service in a closed-ended pressure vessel, as a component of the seal for the vessel, cracked after as few as two high-pressure loading cycles. As a further complication, the finite element analysis was predicting compression on the cracked piston face during the loading cycle. The cracks in the piston emanated in a radial fashion and were measured at 1.5-mm deep. Upon subsequent loading, the cracks grew in length (in large incremental advances), but did not propagate any deeper than the initial 1.5-mm. In all instances the robust design did not leak. A closer look at the finite element analysis revealed that the piston, made from Maraging 200 steel, was loaded in compression to above the compressive yield point of the material. The initial loading set up a residual tensile stress as a result of the compressive yielding. On the next successive loading, this tensile residual stress field, coupled with the hydrogen-rich products in the pressure vessel and a highly susceptible material, resulted in cracking. The fact that the cracks were only 1.5-mm deep suggests that this was the extent of the compressive yielding, and that the cracks arrested themselves once the neutral axis or zero stress was encountered. The fracture morphology of the cracks revealed that they were intergranular in nature, and likely to be the result of hydrogen-induced cracking. At this point in the program the design of the piston was firmly established, so the only alternatives were to prevent the hydrogen from getting to the susceptible material, or to change to a less susceptible material. Consideration of the second alternative revealed no material that possessed the unique combinations of strength, toughness, hydrogen resistance, or producibility that were required. Therefore, plating of the piston with various nickel and chromium combinations was investigated. Laboratory tests were devised using a modified bolt-loaded compact specimen for conducting plating studies.