Compressive thermal yielding leading to hydrogen cracking in a fired cannon

investigation of environmental cracking of a 1100-MPa yield strength A723 s teel cannon tube subjected to prototype firings is described. Metallographi c results show cracking of the steel beneath a 0.12-mm protective layer of chromium. Cracks undermine and remove sections of chromium and lead to loca lized erasion that ruins the cannon. Key features of the firing thermal dam age and cracking are: (i) recrystalization of the chromium to a depth of up to 0.08 mm: (ii) steel transformation to 0.19 mm below the chrome surface; (iii) two different periodic arrays of cracks normal to the hoop and axial directions with mean depths of 0.23 and 0.46 mm, respectively. Time-temper ature-depth profiles for the firing cycle were derived via bi-material fini te difference analysis of a semi-infinite solid which incorporated cannon c ombustion gas temperatures and material properties that vary as a function of temperature. The temperature and depth associated with the steel transfo rmation were used to solve iteratively for the convective heat transfer coe fficient. This value was further confirmed by the depths of chromium recrys talization and of the crack arrays in the two orientations. A profile of ma ximum temperature versus depth is used to determine the near-bore applied a nd residual stress distributions within the tube. The measured volume chang e of steel transformation is used to determine an upper limit on applied an d residual stresses. These stresses are used to determine crack-rip stress intensity factors for the observed crack arrays, and hence provide some exp lanation for the differential depths of cracking. The near-bore temperature and residual stress distributions are used to help determine the cause hyd rogen cracking and measures to prevent cracking. Compressive yielding due t o thermal loading produces near-bore tensile residual stresses, and thereby causes hydrogen cracking. Prevention of cracking is discussed in relations hip to hydrogen crack growth rate tests of alternative alloys and coatings.