# Effect of Heat Treatment on Microstructure and Creep Behavior of Fe-40 Ni-24 Cr Alloy
## 1 Introduction  
Superalloys are known to be heat resistant alloys; they are named nickel-based, iron nickel-based and cobalt-based alloys. They are generally used at a temperature above 540 \({}^{\circ}\)C [1, 2, 3, 4, 5] and some exceed 85% of their initial melting temperatures, such as gas turbines and steam generators [6, 7, 8]. These superalloys need to exhibit combinations of good high strength and high creep resistance, and the potential to work at high temperatures for long periods [9, 10, 11, 12, 13, 14]. Generally, for application at very high temperatures, nickel-based alloys are used, followed by cobalt-based alloys and then lastly iron-nickel alloys. The strengthening of superalloys can be achieved by solid solution hardening mechanism and precipitation hardening [15, 16, 17, 18, 19, 20, 21]. A combination of precipitate hardening, the existence of carbides at the grain boundaries and solid solution hardening can strengthen both Ni and Fe-Ni-based superalloys [22, 23]. In this perspective, the majority of Fe-Ni-based superalloys consist of 25-45% Ni and 15-60% Fe [24]. For oxidation resistance at high temperature, 15 to 28% Cr is added, whereas, for solid solution strengthening, 1-6% Mo is added. Meanwhile, for precipitation hardening effects, Al, Ti and Nb elements are added [25]. At high temperatures, Ni alloys provide longer service life and higher strength compared to most of the alloys available today [26, 27]. Fe-Ni-Cr alloys are included inthe family of Ni-based superalloys that offer excellent strength at high temperature with oxidizing and carburizing environments. This renders them suitable for such parts as blades and disks in gas turbine engines [26; 27; 28]. In this context, Fe-40 Ni-24 Cr alloy (HAYNES(r) HR-120(r)) is a new solid-solution strengthened Fe-Ni-Cr-based superalloy which has excellent strength at high temperatures [15; 29; 30]. The alloy has a high resistance to sulfidizing and carburizing environments and its strength, at 1095 \({}^{\circ}\)C, is significantly higher compared to Ni-Cr alloys. Hence, Fe-40 Ni-24 Cr alloy was developed as a more affordable alternative to other Ni-based superalloys and also acted as an upgrade to stainless steels [31; 32; 33]. This alloy has been widely applied in industrial heating, gas-turbine, heat-treating equipment, and fixtures, thermal processing equipment such as bar frame heat-treating baskets, muffles retorts, radiant tubes, cast link belt pins, and mineral processing, land-based gas-turbine industries, and waste incinterior internals [6; 29; 34]. Since Fe-40 Ni-24 Cr alloy was exposed to a critical environment, it is important to fully understand the creep damage behaviour for the design and safety evaluation process of high-temperature structural material [6; 35]. Creep can occur at temperatures that exceed absolute zero [35]. Creep often occurs in the material under load over an extended period with continuous deformation with slow behavior. For that reason, the creep behavior and properties of Fe-Ni-Cr alloy have been extensively studied. In this respect, creep can occur at temperatures approximately higher than 0.5 Tm in which Tm is the absolute melting temperature of the alloy [31; 36]. Upon reaching up to the temperature above 0.4 Tm, the dislocations are no longer restricted to move in their slip planes. The dislocation climb process will occur in this free dislocation movement [36]. In this context, a total failure or rupture will occur even with the slightest millimeters of crack induced by creep if the crack is not being controlled [15; 37; 38; 39; 40; 41]. A great deal of research was performed on developing mechanisms for creep deformation. The creep deformation mechanisms were classified as dislocation creep, dislocation glide, grain boundary sliding and diffusion creep [3; 4]. Creep crack initiation and subsequent growth is a principal failure mechanism of components operating at elevated temperatures [5].