Weld Cracking of Precipitation Hardening Ni-based Superalloys - Investigation of repair welding characteristics and susceptibility towards strain age cracking
Sammanfattning: High temperature resistance and strength requirements make nickel-based superalloys the material of choice for the hot section of aero engines. Fabrication in terms of combining wrought and cast parts in the manufacturing of hot structural components enables component optimisation via the use of wrought high-strength parts, where geometrical constraints allow, and cast parts to produce complex geometries. Such an approach requires that the materials involved are weldable. Due to the complex microstructure of precipitation hardening nickel-based superalloys, welding comes with the risk of weld cracking, more specifically solidification cracking, heat affected zone (HAZ) liquation cracking and strain age cracking (SAC). While the first two types require a liquid phase to be present, SAC occurs during heating to post-weld heat treatment, in which age-hardening reactions coincide with the relaxation of weld residual stresses. Increasing engine operating temperatures as well as the intermittent cycling of land-based gas and steam turbines motivates research on the weldability of highly temperature-stable alloys. Hence, the main objective of this work has been the investigation and analysis of microstructural changes and their effect on weldability in terms of susceptibility towards weld cracking of the nickel-based superalloys Haynes® 282® and ATI 718Plus®. This has been addressed by the means of repair-welding studies and a simulative test approach using a Gleeble system. Microstructural changes were found to significantly affect HAZ cracking in cast ATI 718Plus®, where high amounts of Laves phase showed an increased resistance towards cracking. Haynes® 282® shows good weld-cracking resistance, as no HAZ cracks were present after multi-pass weld operations and subsequent post weld heat treatments. A simulative Gleeble test was developed to provide more data on ductility in the SAC temperature range and its dependence on ongoing microstructural changes during thermal exposure. Comparison with Waspaloy showed that the high resistance of Haynes® 282® towards SAC is correlated with the moderate age-hardening kinetics of the alloy and the rapid formation of a grain boundary strengthening carbide network. Furthermore, grain size was found to be a major factor affecting ductility and hence SAC susceptibility.
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