Abstract
Solidification cracking is a major obstacle when joining dissimilar alloys using additive manufacturing. In this work, location-specific solidification cracking susceptibility has been investigated using an integrated computational materials engineering (ICME) approach for a graded alloy formed by mixing P91 steel and Inconel 740H superalloy. An alloy mixture of 26 wt.% P91 and 74 wt.% Inconel 740H, with high configurational and total entropy, was fabricated using wire arc additive manufacturing. Microstructure characterization revealed intergranular solidification cracks in the FCC matrix, which increased in length along with the enrichment of Nb (~27 to 56 wt.%) and Cu (~87 wt.%) in the middle and top regions. DICTRA simulations to model location-specific solidification cracking susceptibility showed that the top region with the highest cooling rate (270 K/s) has the highest solidification cracking susceptibility in comparison with the middle and bottom regions. This is in good agreement with the experimentally observed varying crack length. From Scheil simulations, it was deduced that enrichment of Nb and Cu affected the solidification range as high as ~77%, in comparison with the matrix composition. The overall solidification cracking susceptibility and freezing range was highest for the 26 wt.% P91 alloy amongst the mixed compositions between P91 steel and 740H superalloy, proving that solidification characteristics play a major role in alloy design for additive manufacturing.
Highlights
The high energy efficiency of steam power cycles in advanced ultra-supercritical power plants requires a steam temperature above 700 ◦C and a steam pressure up to 35 MPa
It can be observed that the longest cracks were observed in the top portion of the sample and the crack length decreased in the middle region
It is evident from the inverse pole figure (IPF) map that the grain structure was columnar, and the crack was intergranular since it propagated along the grain boundary
Summary
The high energy efficiency of steam power cycles in advanced ultra-supercritical power plants requires a steam temperature above 700 ◦C and a steam pressure up to 35 MPa. Ni-based superalloys, such as Inconel 740H (referred as 740H hereafter), were designed with high Co (~20 wt.%) and Cr (~24 wt.%) content to achieve superior creep properties up to 850 ◦C [2]. P91 steels can be employed in sections with operating temperatures below 650 ◦C, while Inconel 740H can be utilized for parts operating at a higher temperature, especially, for repairing and multifunctional purposes. This strategy involves joining P91 steel with 740H, which is challenging because these materials possess strikingly different crystal structures and properties. This necessitates using an interlayer for joining these dissimilar materials which possesses intermediate properties to ensure a smooth transition between the parent materials
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