Superalloys, or high-performance alloys, are non-ferrous alloys that exhibit outstanding strength and surface stability at high temperatures. Their ability to operate safely at a high fraction of their melting point (up to 85% of their melting points (Tm) expressed in degrees Kelvin, 0.85) is their key characteristics. Superalloys are generally used at temperatures above 540 °C (1000 °F), as at these temperatures ordinary steel and titanium alloys are losing their strengths, also corrosion is common in steels at this temperature. At high temperatures, superalloys retain mechanical strength, resistance to thermal creep deformation, surface stability, and resistance to corrosion or oxidation. Some nickel-based superalloys can withstand temperatures beyond 1200°C, depending on the composition of the alloy. Superalloys are often cast as a single crystal, while grain boundaries may provide strength, they decrease creep resistance.
They were initially developed for use in aircraft piston engine turbosuperchargers. Today, the most common application is in aircraft turbine components, which must withstand exposure to severely oxidizing environments and high temperatures for reasonable time periods. Current applications include:
- Aircraft gas turbines
- Steam turbine power plants
- Medical applications
- Space vehicles and rocket engines
- Heat-treating equipment
- Nuclear power plants
Inconel 718 – Nickel-based Superalloy
In general, Inconel is a registered trademark of Special Metals for a family of austenitic nickel-chromium-based superalloys. Inconel 718 is a nickel-based superalloy that possesses high strength properties and resistance to elevated temperatures. It also demonstrates remarkable protection against corrosion and oxidation. Inconel’s high temperature strength is developed by solid solution strengthening or precipitation hardening, depending on the alloy. Inconel 718 is composed of 55% nickel, 21% chromium, 6% iron, and small amounts of manganese, carbon, and copper.
Common uses of superalloys are in the aerospace and some other high-technology industries. With the combination of corrosion resistance and material strength in the face of extreme heat, this kind of superalloy works well in the nuclear industry. Some nuclear plants use nickel-based superalloys for the reactor core, control rod, and similar parts. In nuclear industry, especially low-cobalt superalloys (due to possible activation of cobalt-59) are used. Some of structural parts of nuclear fuel assemblies, such as top and bottom nozzle, may be produced from superalloys such as Inconel. Spacing grids are usually made of a corrosion-resistant material with low absorption cross section for thermal neutrons, usually zirconium alloy (~ 0.18 × 10–24 cm2). First and last spacing grid may be also made of low-cobalt Inconel, which is a superalloy well suited for service in extreme environments subjected to pressure and heat.
Hardness of Superalloy – Inconel 718
Brinell hardness of superalloy – Inconel 718 depends on heat treatment process, but it is approximately 330 MPa.
In materials science, hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, resistance to abrasion, resistance to indentation or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.
Brinell hardness test is one of indentation hardness tests, that has been developed for hardness testing. In Brinell tests, a hard, spherical indenter is forced under a specific load into the surface of the metal to be tested. The typical test uses a 10 mm (0.39 in) diameter hardened steel ball as an indenter with a 3,000 kgf (29.42 kN; 6,614 lbf) force. The load is maintained constant for a specified time (between 10 and 30 s). For softer materials, a smaller force is used; for harder materials, a tungsten carbide ball is substituted for the steel ball.
The test provides numerical results to quantify the hardness of a material, which is expressed by the Brinell hardness number – HB. The Brinell hardness number is designated by the most commonly used test standards (ASTM E10-14 and ISO 6506–1:2005) as HBW (H from hardness, B from brinell and W from the material of the indenter, tungsten (wolfram) carbide). In former standards HB or HBS were used to refer to measurements made with steel indenters.
The Brinell hardness number (HB) is the load divided by the surface area of the indentation. The diameter of the impression is measured with a microscope with a superimposed scale. The Brinell hardness number is computed from the equation:
There are a variety of test methods in common use (e.g. Brinell, Knoop, Vickers and Rockwell). There are tables that are available correlating the hardness numbers from the different test methods where correlation is applicable. In all scales, a high hardness number represents a hard metal.