Influence of Hot Isostatic Pressing on the Hot Ductility of Cast Alloy 718: The Effect of Niobium and Minor Elements on
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, FABIAN HANNING
, and JOEL ANDERSSON
The influence of two hot isostatic pressing (HIP) treatments on liquation behavior was investigated and compared with regard to the extent of heat-affected zone liquation cracking in cast Alloy 718. The extent of liquation was seen to increase after HIP treatment at 1190 C due to solute changes caused by the homogenization of Nb, which contributed to extensive grain boundary melting. The HIP treatment at 1120 C exhibited lower liquation with contributions from particle liquation of the Laves phase and constitutional liquation of NbC carbides. This was also reflected in a lower ductility recovery temperature, with slower recovery for the former due to the extensive liquation. Interestingly, the nil ductility temperatures were both below the predicted equilibrium solidus of the alloy, which suggests that the ductility drop is related to liquation caused by solute segregation at the grain boundaries. https://doi.org/10.1007/s11661-020-06004-8 The Author(s) 2020
I.
INTRODUCTION
ALLOY 718 is one of the most commonly used superalloys. It is available in four material forms— wrought, cast, powder, and additive manufactured— owing to rapid technological advances in recent years. The development of the Fe-Ni-based superalloy Alloy 718 during the 1960s[1] was motivated by strain age cracking issues encountered in gamma prime (c¢) hardening Ni-based superalloys during post-weld heat treatments, due to the rapid precipitation of the strengthening phase. A cast version was developed in a later stage following progress made with vacuum technology and hot isostatic pressing (HIP) technology. In cast Alloy 718, weld cracking in the heat-affected zone (HAZ) caused by the liquation of secondary phases was found to be a major concern. The presence of liquid along grain boundaries in concomitance with thermal stresses promotes cracking. The wider the temperature range at which the liquid exists, the more detrimental is the cracking behavior. The nonequilibrium Laves phase was found to exacerbate the HAZ liquation cracking susceptibility when present in a high volume fraction. Consequently, homogenization and HIP treatment SUKHDEEP SINGH is with the Department of Industrial and Materials Science, Chalmers University of Technology, 412 96, Gothenburg, Sweden. Contact e-mail: [email protected] FABIAN HANNING and JOEL ANDERSSON are with the Department of Engineering Science, University West, 461 81, Trollha¨ttan, Sweden. Manuscript submitted June 7, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
studies tailored for cast structural components were conducted to find the optimal material and mechanical properties. The results led to the general understanding that the Nb-rich Laves phase needs to be reduced as much as possible to decrease issues regarding HAZ liquation cracking.[2–4] Another concern was about the role of minor elements in weldability. Trace elements such as S, P, and B were found to increase the liquation susceptibility of the grain boundaries by lowering the initial tem
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