Hot Workability and Microstructure Control in Monel K 500 in as Cast Condition: An Approach Using Processing Maps

Monel K500 is a high strength, precipitation hardenable, nickel-copper alloy with additions of Al and Ti, having excellent corrosion resistance and ignition resistance to high pressure gaseous oxygen. However, this alloy is highly sensitive to hot workability and is crack prone during hot deformation. This study investigates the effect of hot workability parameters such as temperature, strain rate, and overall strain on the microstructure evolution. The hot deformability of this alloy was studied using isothermal hot compression tests in the temperature range of 850 °C to 1150 °C and at strain rates ranging from 10^− 3 to 10 s^− 1, using a Gleeble 3800 thermo-mechanical simulator. The flow behaviour was analysed using stress-strain and strain hardening plots. Initial microstructure of the material has as-cast dendritic structure, while microstructural analysis of hot deformed samples revealed gradual reconstitution with increasing temperature and decreasing strain rate. Hot deformed samples showed traces of recrystallized grains and carbides across the matrix at high temperatures and low strain rates. EBSD GROD mapping further elucidates the variation of microstructural features with variation of strain rate. In accordance with the Ziegler instability criterion, processing maps were constructed for a true strain of 0.65, encompassing deformation temperatures between 850 °C and 1150 °C, and strain rates ranging from 0.001 to 10 s^− 1. Through an examination of strain rate sensitivity map, processing map and analysis of deformation activation energy, both undesirable (unstable) and potentially favourable (stable) hot deformation parameters were identified. Instability regions in the processing maps were validated with the microstructural features of deformed samples of cast Monel K500 alloy.

Graphical Abstract