Abstract
The Ni–Fe–Cr system is the basis of a series of commercial alloys featuring chemical–physical characteristics that allow them to be used in a variety of fields where excellent resistance to aggressive environments is required. In this scenario, the CU5MCuC alloy, the foundry counterpart of Alloy 825, is proving successful in the petrochemical field thanks to its good corrosion resistance in acidic and highly oxidizing environments. Intergranular corrosion resistance, critical for this material, is ensured by the stabilization treatment that allows precipitation of Nb carbides. Strengthening of this alloy takes place only via a solid solution. Therefore, its mechanical properties depend on the solution annealing treatment: often this treatment alone does not make it possible to reach the UTS imposed by the ASTM-A494 standard. In this work, the possibility of using stabilization treatment to increase mechanical strength as well was considered. Treatments, with different combinations of time and temperature, were carried out in order to modify the material’s microstructure. After the thermal treatments, microstructural analyses, mechanical tests and (pitting and intergranular) corrosion and resistance tests were carried out to identify optimal treatment parameters in order to promote the evolution of microstructural constituents capable of improving mechanical strength without decreasing corrosion resistance. The treatment that achieves the best compromise between mechanical properties and corrosion resistance is stabilization at 970 °C for 4 h.
Highlights
Nickel-based alloys are widely used in the modern industry because of their ability to work in the presence of elevated temperatures, high stresses and corrosive environments or combinations of the same.[1,2] They have applications in critically important components in chemical, petrochemical and nuclear processes as well as in marine and aeronautical applications.[3,4] High corrosion resistance makes nickel an excellent base for the development of specialized alloys, such as CU5MCuC, which gains superior corrosion resistance thanks to the additions of chromium and molybdenum.[5]
Mo and Nb avoid the precipitation of Cr23C6 at the grain boundary after solution heat treatment: the chromium concentration remained almost constant in the entire matrix avoiding deleterious depletion at grain boundaries and so the tendency to intergranular corrosion.[18,19]
Some suggestions can be given in order to improve alloy performance and to obtain final products that satisfy ASTM-A494 requirements
Summary
Nickel-based alloys are widely used in the modern industry because of their ability to work in the presence of elevated temperatures, high stresses and corrosive environments or combinations of the same.[1,2] They have applications in critically important components in chemical, petrochemical and nuclear processes as well as in marine and aeronautical applications.[3,4] High corrosion resistance makes nickel an excellent base for the development of specialized alloys, such as CU5MCuC, which gains superior corrosion resistance thanks to the additions of chromium and molybdenum.[5]. CU5MCuC is a foundry alloy, the casting structure has intrinsic problems due to solidification structures, i.e. segregation, dendritic structure, and porosity and cannot be processed using thermomechanical processing (forging, rolling, etc.) to obtain a uniform and fine microstructure that guarantees good mechanical properties.[5] On the other hand, nickel is characterized by an austenitic FCC-matrix, able to dissolve extensive amounts of elements in various combinations For this reason, solid-solution strengthening is the way to improve the mechanical properties of CU5CuMC. Tensile tests were performed in accordance with ASTM E8/E8M-16a (Specimen 3).[17] Tests were carried out at room temperature applying a cross-head speed of 2 mm/min, both on solution annealed samples (SA) and on stabilized ones. Samples are weighted at the end of the test in order to evaluate their weight loss
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