Key Takeaways & Executive Findings
- •• Optimal melt superheating at 1,650°C for 5 min yields the finest grain size (949 μm) and SDAS (25.38 μm) in K424 superalloy, enhancing mechanical properties. • Melt treatment temperature and duration significantly influence undercooling, elemental segregation, and solidification microstructure, with higher temperatures reducing segregation. • Room-temperature tensile strength and stress rupture life are markedly improved by specific melt treatment conditions, with 1,650°C/5 min providing superior strength and 1,600°C/10 min balancing strength and stability. • The study provides practical guidance for optimizing melt treatment parameters to refine microstructure and improve performance of K424 superalloy castings.
Abstract
The effect of melt superheating treatment on the solidification microstructure and mechanical properties of the γ' phase precipitation-strengthened K424 superalloy was investigated. Differential scanning calorimetry (DSC) experiments were conducted to explore the influence of melt treatment temperature on the undercooling of the superalloy. Additionally, pouring experiments were carried out to assess how alterations in both the temperature and duration of melt treatment impacted the grain size, secondary dendrite arm spacing (SDAS), elemental segregation, and mechanical properties of the alloy. Metallographic analysis, scanning electron microscopy, energy dispersive spectroscopy (EDS) and Thermo-Calc software were employed for microstructure characterization. The test specimens were subjected to tensile testing at room temperature and stress rupture testing at 975 °C under 196 MPa. The findings reveal that appropriate melt treatment conditions result in decreased grain size, refined SDAS, minimized elemental segregation, and significant improvements in mechanical properties. Specifically, the study demonstrates that a melt treatment at 1,650 °C for 5 min results in the smallest average grain size of 949 μm and the smallest SDAS of 25.38 μm. Furthermore, the room temperature tensile properties and creep resistance are notably affected by the melt treatment parameters. It is shown that specific melt treatment conditions, such as holding at 1,650 °C for 5 min, result in superior room temperature strength and extended stress rupture life of the K424 superalloy, while a balance between strength and stability is achieved at 1,600 °C with a holding time of 10 min. These findings offer guidance for optimizing the melt treatment parameters for the K424 superalloy, laying a foundation for further investigations.
1. Introduction
Nickel-based polycrystalline superalloys are utilized in the production of aero-engine turbine blades due to their substantial strength, excellent structural stability, superior heat resistance, and lower production costs [1-4]. However, technological advancements in the aerospace industry have led to continuous enhancements in the thrust-to-weight ratio and operating temperatures of combustion chambers, thereby imposing more stringent demands on the performance of high-temperature structural materials [5].
Heat treatment processes can be utilized to enhance the comprehensive mechanical properties of superalloys by refining the as-cast solidification structure, reducing segregation, promoting the precipitation of strengthening phases, and thus achieving a uniform distribution of solid solution strengthening elements and phases within the matrix [6]. Nevertheless, given that solidification fundamentally involves a liquid-solid phase transition, the impact of melt characteristics on the solidification structure of castings should not be ignored. Research results showed that the liquid metal consists of atomic clusters with a specific number of nearest neighbors, and these clusters exhibit a crystal-like structure, interspersed with free atoms and electrons [7]. The melt structures could be influenced by variations in melt temperature [8-10], and these structural changes are proved through alterations in the physical parameters of melt [11, 12]. The melt structure, which is modified by melt superheating treatment, positively influences the subsequently formed solidification microstructure, thereby enhancing the comprehensive mechanical properties of alloy. Kolotukhin et al. [13] and Tyagunov et al. [14] have demonstrated that the melt structure undergoes alterations with an increase in temperature, which can effectively refine the microstructure and augment the mechanical properties of the superalloy. Jie et al. [15] investigated the solidification structure of IN718C superalloy across melt treatment temperatures ranging from 1,380 °C to 1,680 °C. The study revealed that higher melt treatment temperatures corresponded to a marked reduction in grain size and elemental segregation.
In summary, extensive research has been conducted to examine the impact of melt temperature on the melt structure, as well as the resulting alterations in microstructure and mechanical properties of alloys after solidification. However, the influence of melt treatment duration, a factor of equal significance, has not been sufficiently explored. This study focuses on the widely used material for high-pressure turbine blades, i.e., the γ' phase precipitation-strengthened K424 superalloy [16]. The influence of melt treatment temperature on the undercooling of this superalloy was studied through DSC experiments.
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Gao-yang Jing, Ao-qi Li, Xun Sun, Lei Jin, Cheng Zhou, Dong-ping Zhan, Ji-hang Li (2025). Effect of melt superheating on solidification microstructure and mechanical properties of K424 superalloy. China Foundry. https://doi.org/10.1007/s41230-025-4041-1
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Frequently Asked Questions
What is the optimal melt superheating temperature and time for K424 superalloy?
The study found that melt treatment at 1,650°C for 5 minutes yields the finest grain size (949 μm) and SDAS (25.38 μm), leading to superior room temperature strength and extended stress rupture life. A balance between strength and stability is achieved at 1,600°C with a holding time of 10 minutes.
How does melt superheating affect the solidification microstructure of K424 superalloy?
Melt superheating refines the solidification structure by reducing grain size and secondary dendrite arm spacing, and minimizing elemental segregation. This is attributed to changes in the melt structure, which influence nucleation and growth during solidification.
What are the mechanical property improvements observed after melt superheating treatment?
Appropriate melt treatment conditions significantly improve room-temperature tensile properties and creep resistance. Specifically, treatment at 1,650°C for 5 min results in superior strength and extended stress rupture life, while 1,600°C for 10 min provides a balance between strength and stability.
Why is melt treatment duration important in superalloy processing?
The duration of melt treatment influences the extent of structural changes in the melt, affecting the final solidification microstructure and mechanical properties. The study highlights that both temperature and duration must be optimized to achieve desired outcomes.
What methods were used to characterize the microstructure in this study?
The researchers employed metallographic analysis, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Thermo-Calc software to characterize the microstructure and elemental segregation of the K424 superalloy.
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