Cobalt based alloy
Inside the tomb of the pharaoh Tutankhamun (Tharakhatu), who ruled from 1361-1352 BC, is a small glass object with dark blue and cobalt. Cobalt blue was known even earlier in China and was used in pottery glazes.
In 1730, the Stockholm chemist Georg Brandt became interested in the dark blue ore of some local copper mines, which he eventually proved to contain a hitherto unrecognized metal, which he gave to him by the Germans. The name of the ore cursed by the miners. Sometimes mistaken for a silver mine. He published his findings in 1739. Over the years, his claim that he had discovered a new metal was questioned by other chemists who said his new element was indeed a compound of iron and arsenic, but was eventually recognized as an element itself.
Cobalt based alloy
As a whole, cobalt-based alloys can often be described as wear-resistant, corrosion-resistant and heat-resistant (strong even at high temperatures).
As a whole, cobalt-based alloys can often be described as wear-resistant, corrosion-resistant and heat-resistant (strong even at high temperatures). Many of the alloy's properties arise from the crystallographic properties of cobalt (especially its response to stress), the solid solution strengthening effects of chromium, tungsten and molybdenum, the formation of metal carbides, and the corrosion resistance imparted by cobalt to chromium. Typically, softer and harder compositions are used for high temperature applications, such as gas turbine blades and blades. Harder grades are used to resist wear.
Historically, many commercialized cobalt-based alloys originated from cobalt-chromium-tungsten and cobalt-chromium-molybdenum ternary alloys first studied by Elwood Haynes in the early 20th century. He discovered the high strength and stainless steel properties of binary cobalt-chromium alloys, and later he discovered that tungsten and molybdenum were powerful enhancers in cobalt-chromium systems. When he discovered these alloys, Haynes named them Stellite alloys (after the Latin stella (star)) because of their star-like luster. Haynes also promoted the use of Stellite as a material for cutting tools after discovering its high strength at high temperatures.
Cobalt-based wear-resistant alloy
Today's cobalt-based wear resistant alloys are little changed from Elwood Haynes' earlier alloys. The most important difference has to do with the control of carbon and silicon (which were defects in earlier alloys). Indeed, the main difference in current Stellite alloy grades is the carbon and tungsten content (and therefore, the amount and type of carbide formation in the microstructure during solidification). Carbon content affects hardness, ductility and wear resistance. Tungsten also plays an important role in these properties.
Wear type. There are several different types of wear, generally divided into three broad categories:
The type of wear encountered in a particular application is an important factor in the selection of wear resistant materials.
Abrasive wear occurs when hard particles or hard protrusions (on the opposite side) are forced against and move relative to a surface. The terms high stress wear and low stress wear refer to the state of the abrasive media (whether hard particles or protrusions) after interacting with the surface. If an abrasive medium is crushed, it is said to be in a state of high stress. If the grinding media remains intact, the process is called low stress wear. Typically, high-stress wear is caused by hard particles being sandwiched between metal surfaces (relative motion), while low-stress wear is encountered when moving surfaces come into contact with packed abrasives such as soil and sand.
In alloys containing hard phases, such as cobalt-based wear-resistant alloys, wear resistance generally increases with the volume fraction of hard phases. However, wear resistance is strongly influenced by the size and shape of the hard phase precipitates within the microstructure in the hard structure and the size and shape of the abrasive species.
Sliding wear. Of the three main wear types, sliding is probably the most complex and is not a conceptual issue, but rather how different materials respond to sliding conditions. Sliding wear can occur whenever two surfaces are pressed together and moved relative to each other. If both surfaces are metallic in nature and have little or no lubrication, the chance of damage is greatly increased.
Cobalt-Based Metallic Glass
Recently, ultra-strong cobalt-based metallic glasses have been observed. The topology of metallic glasses without metalloids is strongly determined by dense atomic packing, whereas the topology of metal-metalloid metallic glasses is also influenced by strong covalent bonds. The design of metal-nonmetallic cobalt-based superglasses is challenging due to the complex chemical-topology-property relationship. The metal-metalloid cobalt-based metallic glasses discussed in this paper have unique local atomic configurations, resulting in ultra-high fracture strengths of over 5000 MPa and high Young's modulus of 268 GPa. It has been inferred in the literature that properties such as glass formability, magnetic properties and mechanical properties can be enhanced through chemically induced topological changes. It has also been reported that Young's modulus as well as elastic limit exhibit strong topological dependence. In the case of Co-based metallic glasses, it has been observed that the short-range order of metal-metalloids strongly affects the mechanical properties.
For many years, the main user of superalloys has been the gas turbine industry. For aircraft gas turbines, the main material requirements are high temperature strength, thermal fatigue resistance and oxidation resistance. Sulfur resistance is a major concern for land-based gas turbines that typically burn low-grade fuels and operate at lower temperatures.
Today, as greater efficiencies are sought from the combustion of fossil fuels and wastes, and with the development of new chemical processing technologies, the use of superalloys has become more diverse.
Although cobalt-based alloys are not as widely used in high-temperature applications as nickel and nickel-iron alloys, cobalt-based superalloys still play an important role because of their excellent sulfidation resistance and strength at high temperatures in excess of 200°C. Those precipitates with γ' and γ' double bottoms dissolved in nickel and nickel-iron alloys. Cobalt is also used as an alloying element in many nickel-based superalloys.
Cobalt-Based Corrosion Resistant Alloys
Although cobalt-based wear-resistant alloys have some resistance to water corrosion, they are limited by grain boundary carbide precipitation, lack of important alloying elements in the matrix (after carbide or Laves precipitation), and in the following situations: casting and Weld surfacing material by chemical segregation in the microstructure.
Due to their uniform microstructure and lower carbon content, deformed cobalt-based superalloys (which typically contain tungsten instead of molybdenum) are more resistant to water corrosion, but are still much less corrosive than nickel-chromium-molybdenum alloys.
To meet the industrial demand for alloys with excellent resistance to water corrosion while at the same time having the properties of cobalt as the basis of the alloy (resistance to various forms of wear and high strength over a wide temperature range) , forging several low-carbon alloys to produce cobalt-nickel-chromium-molybdenum alloys.