An alloy commonly used for large pressure vessels, known as SA508 Gr. 3, has a microstructure after heat treatment consisting of a mixture of tempered bainite and martensite at fast cooled regions near surfaces subject to water quenching. These two phases are conventionally recognised to consist of fine platelets, each of which is approximately 0.2 μm in thickness; enhancing strength and leading to good toughness properties.
We have discovered in our experimental work that there are circumstances where the adjacent platelets of a similar orientation can coalesce as the austenite transforms, to produce much coarser structures which are believed to be detrimental to toughness. An examination of published micrographs reveals that such coalesced regions existed but were not noticed in previous studies. The mechanism of coalescence is described and methods to ameliorate the coarsening are discussed.