Stellite

Last updated

Stellite alloys are a range of cobalt-chromium alloys designed for wear resistance. "Stellite" is also a registered trademark of Kennametal Inc. and is used in association with cobalt-chromium alloys.

Contents

History

As noted above, Stellite is a trademark of Kennametal Inc. [1] Prior to that, it was owned by Union Carbide, Stellite Division. [2] Invented by Elwood Haynes [3] in the early 1900s as a material for making cutlery that wouldn't stain or require constant cleaning.

Composition

Stellite alloys include a range of cobalt-based alloys, with significant proportions of chromium (up to 33%) and tungsten (up to 18%). Some of the alloys also contain nickel or molybdenum. Most of them are fairly high carbon content when compared to carbon steels.

Properties

Stellite alloys are a family of completely non-magnetic and corrosion-resistant cobalt alloys of various compositions that have been optimised for different uses. Stellite alloys are suited for cutting tools, for example, is Stellite 100, because this alloy is quite hard, maintains a good cutting edge at high temperature, and resists hardening and annealing. Other Stellite alloys are formulated to maximize combinations of wear resistance, corrosion resistance, or ability to withstand extreme temperatures.

Stellite alloys display outstanding hardness and toughness, and are also usually very resistant to corrosion. Typically, a part produced with a Stellite alloy is precisely cast so that only minimal machining is necessary. Due to the very high hardness many Stellite alloys are primarily machined by grinding, as cutting operations in some alloys cause significant tool wear even with carbide inserts. Stellite alloys also tend to have extremely high melting points due to the cobalt and chromium content.

Applications

Typical applications for Stellite alloys include saw teeth, hardfacing, and acid-resistant machine parts. Stellite alloys were a major improvement in the production of poppet valves and valve seats for the valves, particularly exhaust valves, of internal combustion engines. By reducing their erosion from hot gases, the interval between maintenance and re-grinding of their seats was dramatically lengthened. Stellite alloys have also been used in some engines for the cam followers, particularly by the Norton Motorcycle Company.

The first third of the M2HB machine gun and M60 machine gun barrels (starting from the chamber) are lined with a Stellite alloy. [4] [5] The locking lugs and shoulders of Voere Titan II rifles also include a Stellite alloy. In the early 1980s, experiments were done in the United Kingdom to make artificial hip joints and other bone replacements out of precision-cast Stellite alloys. Stellite alloys are also used for making the cast structure of dental prostheses.

Stellite alloys have also been used in the manufacture of turning tools for lathes. With the introduction and improvements in tipped tools it is not used as often, but it was found to have superior cutting properties compared to the early carbon steel tools and even some high-speed steel tools, especially against difficult materials such as stainless steel. Care was needed in grinding the blanks and these were marked at one end to show the correct orientation, without which the cutting edge could chip prematurely.

While Stellite alloys remain the material of choice for certain internal parts in industrial process valves (valve seat hardfacing), cobalt alloys have been discouraged in nuclear power plants. In piping that can communicate with the reactor, tiny amounts could be released into the process fluid and eventually enter the reactor. There the cobalt would be activated by the neutron flux in the reactor and become cobalt-60, a radioisotope with a five-year half life that releases very energetic gamma rays. This phenomenon is more problematic in boiling water reactor (BWR) plants, since the steam is in direct contact with both the reactor and the steam turbine. Pressurized water reactor (PWR) designs are less susceptible. While not a hazard to the general public, about a third to a half of nuclear worker exposures could be traced to the use of a cobalt alloy and to trace amounts of cobalt in stainless steels.

Stellite alloys have also used as the cage material for the first commercially available artificial heart valve, the Starr-Edwards caged-ball valve, first implanted in 1960.

Varieties

Notes

Related Research Articles

<span class="mw-page-title-main">Stainless steel</span> Steel alloy resistant to corrosion

Stainless steel, also known as inox, corrosion-resistant steel (CRES) and rustless steel, is an alloy of iron that is resistant to rusting and corrosion. It contains at least 10.5% chromium and usually nickel, as well as 0.2 to 2.11% carbon. Stainless steel's resistance to corrosion results from the chromium, which forms a passive film that can protect the material and self-heal in the presence of oxygen.

<span class="mw-page-title-main">Martensitic stainless steel</span> One of the 5 crystalline structures of stainless steel

Martensitic stainless steel is a type of stainless steel alloy that has a martensite crystal structure. It can be hardened and tempered through aging and heat treatment. The other main types of stainless steel are austenitic, ferritic, duplex, and precipitation hardened.

<span class="mw-page-title-main">Carbon steel</span> Steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

<span class="mw-page-title-main">Tool steel</span> Any of various steels that are particularly well-suited to be made into tools and tooling

Tool steel is any of various carbon steels and alloy steels that are particularly well-suited to be made into tools and tooling, including cutting tools, dies, hand tools, knives, and others. Their suitability comes from their distinctive hardness, resistance to abrasion and deformation, and their ability to hold a cutting edge at elevated temperatures. As a result, tool steels are suited for use in the shaping of other materials, as for example in cutting, machining, stamping, or forging.

<span class="mw-page-title-main">High-speed steel</span> Subset of tool steels

High-speed steel is a subset of tool steels, commonly used as cutting tool material.

<span class="mw-page-title-main">Tungsten carbide</span> Hard, dense and stiff chemical compound

Tungsten carbide is a chemical compound containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into shapes through sintering for use in industrial machinery, cutting tools, chisels, abrasives, armor-piercing shells and jewelry.

<span class="mw-page-title-main">Titanium carbide</span> Chemical compound

Titanium carbide, TiC, is an extremely hard refractory ceramic material, similar to tungsten carbide. It has the appearance of black powder with the sodium chloride crystal structure.

Cryogenic hardening is a cryogenic treatment process where the material is cooled to approximately −185 °C (−301 °F), usually using liquid nitrogen. It can have a profound effect on the mechanical properties of certain steels, provided their composition and prior heat treatment are such that they retain some austenite at room temperature. It is designed to increase the amount of martensite in the steel's crystal structure, increasing its strength and hardness, sometimes at the cost of toughness. Presently this treatment is being used on tool steels, high-carbon, high-chromium steels and in some cases to cemented carbide to obtain excellent wear resistance. Recent research shows that there is precipitation of fine carbides in the matrix during this treatment which imparts very high wear resistance to the steels.

Alacrite is a family of cobalt-based alloys. The alloy exhibits useful mechanical properties and is oxidation- and sulfidation-resistant.

<span class="mw-page-title-main">Chromium(II) carbide</span> Chemical compound

Chromium(II) carbide is a ceramic compound that exists in several chemical compositions: Cr3C2, Cr7C3, and Cr23C6. At standard conditions it exists as a gray solid. It is extremely hard and corrosion resistant. It is also a refractory compound, which means that it retains its strength at high temperatures as well. These properties make it useful as an additive to metal alloys. When chromium carbide crystals are integrated into the surface of a metal it improves the wear resistance and corrosion resistance of the metal, and maintains these properties at elevated temperatures. The hardest and most commonly used composition for this purpose is Cr3C2.

<span class="mw-page-title-main">Intergranular corrosion</span> When crystallite boundaries are more corrosive than their interiors

In materials science, intergranular corrosion (IGC), also known as intergranular attack (IGA), is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides.

<span class="mw-page-title-main">SAE steel grades</span> Standard alloy numbering system for steel grades

The SAE steel grades system is a standard alloy numbering system for steel grades maintained by SAE International.

Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties.

Boriding, also called boronizing, is the process by which boron is added to a metal or alloy. It is a type of surface hardening. In this process boron atoms are diffused into the surface of a metal component. The resulting surface contains metal borides, such as iron borides, nickel borides, and cobalt borides, As pure materials, these borides have extremely high hardness and wear resistance. Their favorable properties are manifested even when they are a small fraction of the bulk solid. Boronized metal parts are extremely wear resistant and will often last two to five times longer than components treated with conventional heat treatments such as hardening, carburizing, nitriding, nitrocarburizing or induction hardening. Most borided steel surfaces will have iron boride layer hardnesses ranging from 1200-1600 HV. Nickel-based superalloys such as Inconel and Hastalloys will typically have nickel boride layer hardnesses of 1700-2300 HV.

<span class="mw-page-title-main">Haynes International</span> Producer of corrosion-resistant and high-temperature alloys

Haynes International, Inc., headquartered in Kokomo, Indiana, is one of the largest producers of corrosion-resistant and high-temperature alloys. In addition to Kokomo, Haynes has manufacturing facilities in Arcadia, Louisiana, Laporte, Indiana, and Mountain Home, North Carolina. The Kokomo facility specializes in flat products, the Arcadia facility in tubular products, and the Mountain Home facility in wire products. In fiscal year 2018, the company's revenues were derived from the aerospace (52.1%), chemical processing (18.2%), industrial gas turbine (12.0%) and other (12.3%) industries. The company's alloys are primarily marketed under the Hastelloy and the Haynes brands. They are based on nickel, but also include cobalt, chromium, molybdenum, tungsten, iron, silicon, manganese, carbon, aluminum, and/or titanium.

<span class="mw-page-title-main">Cemented carbide</span> Type of composite material

Cemented carbides are a class of hard materials used extensively for cutting tools, as well as in other industrial applications. It consists of fine particles of carbide cemented into a composite by a binder metal. Cemented carbides commonly use tungsten carbide (WC), titanium carbide (TiC), or tantalum carbide (TaC) as the aggregate. Mentions of "carbide" or "tungsten carbide" in industrial contexts usually refer to these cemented composites.

NOREM is a hardfacing material developed by the Electric Power Research Institute to deal with radiation safety issues associated with the use of cobalt alloys in nuclear power station coolant systems. It is a solid-solution strengthened austenitic matrix with a continuous network of eutectic and non-eutectic carbides at the grain boundaries. It is intended to be deposited by various welding processes.

<span class="mw-page-title-main">Cobalt-chrome</span> Alloy of cobalt and chromium used in medical implants

Cobalt-chrome or cobalt-chromium (CoCr) is a metal alloy of cobalt and chromium. Cobalt-chrome has a very high specific strength and is commonly used in gas turbines, dental implants, and orthopedic implants.

Havar, or UNS R30004, is an alloy of cobalt, possessing a very high mechanical strength. It can be heat-treated. It is highly resistant to corrosion and is non-magnetic. It is biocompatible. It has high fatigue resistance. It is a precipitation hardening superalloy.