Creep-Resistant Ni-Based Superalloy Casting via FGH Method
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Solution Overview
Problem
Existing Ni-based superalloys in cast form fail to meet long-term high-temperature creep strength requirements for advanced supercritical carbon dioxide power cycles, particularly in coal-fired boilers and steam/CO2 turbines, due to poor creep performance compared to their wrought counterparts.
Innovation Solution
A Fine Grain Homogenized (FGH) casting method is developed, involving vacuum induction melting, electroslag remelting, and a computationally optimized homogenization heat treatment to produce a NiCrCoAlTi superalloy with a fine-grained equiaxed microstructure, reducing chemical segregation and enhancing creep resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional casting methods are used to produce Ni-based superalloy, then manufacturing complexity is reduced and production cost decreases, but creep strength and mechanical properties deteriorate significantly compared to wrought alloys
Solution Approach 1:
The manufacturing process is segmented into multiple distinct stages: vacuum induction melting (VIM) for initial alloy production, electroslag remelting (ESR) for purification and homogenization, and controlled cooling for microstructure development. Each stage addresses specific requirements separately, allowing the final product to achieve wrought-like properties through cumulative refinement rather than attempting to achieve all goals in a single process step.
Solution Approach 2:
The alloy undergoes preliminary processing steps before final casting, including VIM to establish base composition, ESR to refine purity and distribute elements uniformly, and controlled cooling rates to pre-establish the desired microstructure. These preliminary actions ensure that when the final casting occurs, the material is already optimized for creep resistance, eliminating the need for extensive post-processing.
2Duration of action of moving object
If conventional casting methods are used, then production cost and manufacturing simplicity are improved, but creep life and high-temperature performance deteriorate
Solution Approach 1:
The process systematically changes critical parameters at each stage: VIM controls composition within tight tolerances, ESR adjusts purity levels and element distribution, and controlled cooling rates (specified in degrees per minute) dictate microstructure formation. By precisely controlling these parameters throughout the manufacturing sequence, the material achieves extended creep life comparable to wrought alloys while maintaining cast form manufacturability.
3Stability of the object's composition
If standard casting processes are applied, then manufacturing simplicity is maintained, but chemical homogeneity and microstructure quality deteriorate
Solution Approach 1:
The manufacturing process maintains continuous refinement of chemical homogeneity through sequential operations. VIM establishes uniform distribution of alloying elements, ESR continues this refinement by removing impurities and further homogenizing composition, and controlled cooling preserves this homogeneity while forming the desired microstructure. This continuous action ensures chemical uniformity throughout the final product despite the multi-step process complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The FGH casting method significantly improves creep life and mechanical properties of NiCrCoAlTi superalloy, matching or exceeding those of wrought IN740H, with a three-fold increase in creep life and enhanced tensile strength, while maintaining chemical homogeneity and carbide distribution.
Implementation Method 1
melting the feed stock in a furnace under vacuum to form liquid metal
Implementation Method 2
vacuum induction melting (VIM)
Implementation Method 3
pouring the liquid metal into a preheated mold
Implementation Method 4
solidifying the molten metal in the preheated mold, forming the creep-resistant Ni-based superalloy
Implementation Method 5
c computationally optimized homogenization heat treatment to produce a NiCrCoAlTi superalloy with a fine-grained equiaxed microstructure, reducing chemical segregation
Data Source
AI summary
One or more embodiments relates to a method of casting a creep-resistant Ni-based superalloy and a homogenization heat treatment for the alloy. The method includes forming a feed stock having Nickel (Ni) and at least one of Chromium (Cr), Cobalt (Co), Aluminum (Al), Titanium (Ti), Niobium (Nb), Iron (Fe), Carbon (C), Manganese (Mn), Molybdenum (Mo), Silicon (Si), Copper (Cu), Phosphorus (P), Sulfur (S) and Boron (B). The method further includes fabricating the creep-resistant Ni-based superalloy in a predetermined shape using the feed stock and at least one process such as vacuum induction melting (VIM), electroslag remelting (ESR) and/or vacuum arc remelting (VAR).