Abstract
When approving large castings for use, mechanical values are measured by room temperature tensile test on test bodies made from the centres of the castings. These test bodies often exhibit lower values than required, in particular ultimate tensile strength and elongation. The presence of fragile niobium carbides in the structure of the alloy was identified as the main cause of this situation, which originated as a result of the slow speed cooling. Because of large grain size compared to gauge length diameter, crystal orientation of individual grains also plays an important role. Interdendritic carbides are oriented parallel with dendrites thus parallel with directions in Ni superalloy crystal lattice. Grain oriented with axis parallel to tensile direction has platelet carbides oriented perpendicular to the tensile axis. In that position carbides crack easily at low grain deformation. Longitudinal metallographic cuts of selected tested specimen gauge lengths were prepared and grain orientation of individual grains was investigated by EBSD, Specimens, where grain oriented with axis parallel to tensile direction were found, performed poor elongation values 3%, compared to 7% where no such oriented grains were found.
Highlights
IN713LC nickel superalloy is currently used widely, for example, for manufacturing of turbine wheels in turbochargers
When approving large castings for use, mechanical values are measured by room temperature tensile test on test bodies made from the centres of the castings
Grain oriented with axis parallel to tensile direction has platelet carbides oriented perpendicular to the tensile axis
Summary
IN713LC nickel superalloy is currently used widely, for example, for manufacturing of turbine wheels in turbochargers. Customers demand production of increasingly larger impeller parts than before. This requirement has resulted in a greater castings weight implying different solidification and cooling conditions during casting process. Modified castings microstructure arises, e.g. large grains, larger gamma prime phase particles and long interdendritic carbides. When approving large castings for use, mechanical values are measured by room temperature tensile test on test bodies made from the centres of the castings. These test pieces often exhibit lower values than required, in particular ultimate tensile strength and elongation. An investigation was performed to find the cause of such poor mechanical properties
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