Production of Exotic Alloys

Hot Rolling

Hot rolling consists of three main types of rolls: strip rolls; sheet rolls; and rod rolls. This method is used to reduce the size of wrought-iron ingots or bars. The Bessemer process was made mandatory in the rolling technique so manufacturers could keep pace with the high demand of these materials needed for the market. Hot rolling is used for larger or thicker amounts of metal. The machines used for this application are very similar to cold rolling, but the requirements are not as important - such as keeping a precise thickness or exact edge dimensions. Hot rolling is mainly used with non-ferrous metals during the beginning stages of the breakdown of alloys. 

Cold Rolling (fig 1.1)

Cold rolling steel is generally used at the end of a metal forming process. The following functions are applied to obtain and enable accurate dimensions in the finished product. Cold rolling also ensures a smooth, clean appearance or finish and the straightness of the edges of the product. Cold rolling is to help obtain a degree of hardness known otherwise as temper in alloys. Once the desired temper is obtained it cannot be re-heated or it will lose properties intended for specific jobs.

Cold rolling is applied during the end process of production for strip and section rolled metals, such as foil. The cold roll mills are to obtain the desired thickness and also to limit and prevent any excess thinning of the already thin material. When rolling these alloys they are made long enough to make sure there are no creases or tears in the metal. The end result after the process of cold rolling will acquire the metal thickness and temper which is needed for each type of work.

 

Common Types of Exotic Alloys

Hastelloy®

Hastelloy® is the trademark name of Haynes International.  It is a series of nickle based, high-strength, and highly corrosion resistant metal alloys. The principal alloy is nickel, however other prominent components include molybdenum and chromium. The main function of Hastelloy® is surviving under high temperature and high stress service. It has a melting point range of 2417-2498°F and a density of 554.98 lb/ft.

Hastelloy’s main benefit is its excellent resistance to pitting, stress-corrosion cracking, and oxidizing atmospheres up to 1900°F.  Because of this it is well suited for chemical applications. Hastelloy® is available in many different forms, including sheet, foil, rod, or wire.  The B and C alloys are used the most often out of the group. Hastelloy® B does not contain chromium and as a result should only be used in reducing environments.  On the other hand, Hastelloy® C has excellent resistance to a very wide range of chemicals, including oxidizing and reducing chemicals.

Applications

The Hastelloy® alloy is used in nuclear reactors, chemical reactors, pipes, and valves. One of the most important applications of Hastelloy® is its use in manufacturing chemical process equipment. It is especially useful for process use at high temperatures where stainless steels do not have adequate strength and/or corrosion resistance.

 

Inconel®

Inconel® is another nickel-based superalloy. It most commonly exists as an alloy containing 72% nickel, 16% chromium, and 8% iron, but other combinations and grades also exist.  It was originally developed by Huntington Alloys, which is now called Special Metals Corporation. Inconel® has a melting point of 2000° F and a density of 525.31lb/ft3.

Inconel® is known for being highly oxidation and corrosion resistant, even at very high temperatures, and it maintains a high mechanical strength in high heat conditions as well.  Because of this heat resistance, Inconel® is often put to use in extreme conditions, such as aircraft engine parts, turbocharger turbine wheels, chemical processing and pressure vessels. There are a total of about a dozen different Inconel® alloys.  It is available in a variety of forms including sheet, plate, bar, and a seamless tube.

 

Waspalloy

Waspalloy, like Hastelloy®, is primarily nickel-based. It is an age-hardenable superalloy with exceptional high-temperature strength and good corrosion resistance, particularly to oxidation. It derives its strength from the presence of molybdenum, cobalt, and chromium, and its corrosion resistance from its age-hardening elements, aluminum and titanium. It has a melting point range of 2425-2475°F and a density of 511.91 lb/ft3.

Because of its excellent resistance to corrosion, Waspalloy is used frequently in gas turbines and aircraft jet engines (and other critical rotating applications), where temperatures reach up to 1600°F. Its weldability is limited because of its vulnerability to strain age cracking, however it can be fusion welded by argon-arc methods using a filler metal. It is also among the more difficult of the superalloys to machine. Other applications include compressor and rotor discs, shafts, rings and casings, fasteners and other miscellaneous engine hardware, airframe assemblies, and missile systems. Waspalloy is available in round bar, forging stock, extruded section, and wire forms.

 

Titanium

Titanium is a strong, corrosion-resistant metal with a white-silvery-metallic color. It is twice as strong as steel but 45% lighter and twice as strong as aluminum but 60% heavier. Titanium is available in many different forms including sheet, wire, rod, foil, and sponge.

Because of its high tensile strength (even at high temperatures), light weight, extraordinary corrosion resistance, and ability to withstand extreme temperatures, titanium alloys are used in aircraft, armor plating, naval ships, spacecraft and missiles.

Corrosion Resistance

Titanium is immune to corrosion from salt-water, most industrial and organic chemicals. This is due to titanium's very thin, tenacious and highly protective surface oxide Ti02. If worn or scratched, the oxide layer will immediately restore itself in the presence of air or water. Titanium dioxide is one of the most basic materials in of daily life. Commercially pure titanium resists corrosion from acid rain and is growing in popularity for architectural application. As a building material, titanium is free from pitting corrosion, crevice corrosion or stress corrosion cracking. This is very important when comparing titanium to stainless steel, which is susceptible to these forms of corrosion.