Phase transformations in heat-affected zones of ferritic heat-resistant steels @bhadeshia123

bhadeshia123 | Phase transformations in heat-affected zones of ferritic heat-resistant steels @bhadeshia123 | Uploaded March 2022 | Updated October 2024, 1 hour ago.
A talk by Professor Cong Wang of the oxide metallurgy group at Northeastern University in China. After introducing power plant issues he describes the need for new creep-resistant materials that would be needed to increase steam temperatures in the context of thermodynamic efficiency. This in turn leads to welding, and problems associated with the microstructures within the heat-affected zones of the welds, leading to cracking. The focus then shifts to acicular ferrite that nucleates intragranularly in austenite grains containing non-metallic inclusions. Confocal laser microscopy is used to monitor the growth of plates. The properties in the heat-affected zones are also explained.

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