The Titanic disaster and the continuing effort to improve the impact toughness of ferritic steels

Professor Debalay Chakrabarti of the Indian Institute of Technology Kharagpur, India, provides a historical context to the brittle failure of ferritic steels, and then goes on to explain the fundamental science of the ductile-brittle transition in steels. He contrasts, using dislocation theory, the difficulty in moving dislocations in ferrite against the relative ease in austenite. This explains why the ductile-brittle transition is generally absent in austenitic steels. Detailed fracture mechanics arguments are presented to estimate the fracture stress as a function of variables such as the grain size, hard-particle cleavage, stress triaxiality etc. Specific comments added after the lecture, on the role of manganese sulphides: Being softer than the matrix, MnS inclusions elongate along the direction of metal-flow during hot-deformation and that creates anisotropy in ductility in toughness, particularly affecting the properties along transverse direction of rolled / forged plates. It was mentioned that MnS decreases toughness at room temperature, but the accurate statement is that it significantly reduces the upper shelf energy level and thus brings down the entire transition curve towards lower energy side.