Alloy Steel

 

Low Alloy Steel

Alloy steel is steel alloyed with other elements in amounts of between 1 and 50% by weight to improve its mechanical properties. Alloy steels can be splitted in two sections of Alloys: low alloy steels and high alloy steels. The differentiation between low alloy steels and high alloy steels is somewhat arbitrary; Smith and Hashemi define the difference at 4%, while Degarmo, et al., define it at 8%.[1][2] However, most commonly alloy steel refers to low alloy steel.

These steels have greater strength, hot hardness, toughness, hardness, hardenability, or wear resistance compared to carbon steel. However, they may require heat treatment in order to achieve such properties. Common alloying elements are molybdenum, manganese, nickel, chromium, vanadium, silicon and boron.

Low alloy steels are usually used to achieve better hardenability, which in turn improves its other mechanical properties. They are also used to increase corrosion resistance in certain environmental conditions.

With medium to high carbon levels, low alloy steel is difficult to weld. Lowering the carbon content to the range of 0.10% to 0.30%, along with some reduction in alloying elements, increases the weldability and formability of the steel while maintaining its strength. Such a metal is classed as a high-strength low-alloy steel.

Alloying elements are being added to achieve certain properties in the material. As a guideline, alloying elements are added in lower percentages (less than 5%) to increase strength or hardenability, or in larger percentages (over 5%) to achieve special properties, such as corrosion resistance or even extreme temperature stability.

Manganese, silicon, or aluminium are added during the steelmaking process to remove dissolved oxygen from the melt. Manganese, silicon, nickel, and copper are added to increase strength by forming solid solutions in ferrite. Chromium, vanadium, molybdenum, and tungsten increase strength by forming second-phase carbides. Nickel and copper improve corrosion resistance in small quantities. Molybdenum helps to resist embrittlement. Zirconium, cerium, and calcium increase toughness by controlling the shape of inclusions. Manganese sulfide, lead, bismuth, selenium, and tellurium increase machinability.

The alloying elements tend to either form compounds or carbides. Nickel is very soluble in ferrite, therefore it forms compounds, usually Ni3Al. Aluminum dissolves in the ferrite and forms the compounds Al2O3 and AlN. Silicon is also very soluble and usually forms the compound SiO2MxOy. Manganese mostly dissolves in ferrite forming the compounds MnS, MnOSiO2, but will also form carbides in the form of (Fe,Mn)3C. Chromium forms partitions between the ferrite and carbide phases in steel, forming (Fe,Cr3)C, Cr7C3, and Cr23C6. The type of carbide that chromium forms depends on the amount of carbon and other types of alloying elements present. Tungsten and molybdenum form carbides if there is enough carbon and an absence of stronger carbide forming elements (i.e. titanium & niobium), they form the carbides Mo2C and W2C, respectively. Vanadium, titanium, and niobium are strong carbide forming elements, forming the carbides V3C3, TiC, and NiC, respectively.

Alloying elements also have an affect on the eutectoid temperature of the steel. Manganese and nickel lower the eutectoid temperature and are known as austenite stabilizing elements. With enough of these elements the austenitic structure may be obtained at room temperature. Carbide forming elements raise the eutectoid temperature; these elements are known as ferrite stabilizing elements

High-strength low-alloy steel is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. HSLA (High-strength low-alloy ) steels vary from other steels in that they aren't made to meet a specific chemical composition, but rather to specific mechanical properties. They have a carbon content between 0.050.25% to retain formability and weldability. Other alloying elements include up to 2.0% manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare earth elements, or zirconium. Copper, titanium, vanadium, and niobium are added for strengthening purposes. These elements are intended to alter the microstructure of carbon steels, which is usually a ferrite-pearlite aggregate, to produce a very fine dispersion of alloy carbides in an almost pure ferrite matrix. This eliminates the toughness-reducing effect of a pearlitic volume fraction, yet maintains and increases the material's strength by refining the grain size, which in the case of ferrite increases yield strength by 50% for every halving of the mean grain diameter. Precipitation strengthening plays a minor role.. Their yield strengths can be in between 250590 megapascals (36,00086,000 psi). Due to their higher strength and toughness HSLA steels usually require 25 to 30% more power to form, as compared to carbon steels.

Copper, silicon, nickel, chromium, and phosphorus are added to increase corrosion resistance. Zirconium, calcium, and rare earth elements are added for sulfide-inclusion shape control which increases formability. These are needed because most HSLA steels have directionally sensitive properties. Formability and impact strength can vary significantly when tested longitudinally and transversely to the grain. Bends that are parallel to the longitudinal grain are more likely to crack around the outer edge because it experiences tensile loads. This directional characteristic is substantially reduced in HSLA steels that have been treated for sulfide shape control.[2]

They are used in cars, trucks, cranes, bridges, roller coasters and other structures that are designed to handle large amounts of stress or need a good strength-to-weight ratio. HSLA steels are usually 20 to 30% lighter than a carbon steel with the same strength.

HSLA steels are also more resistant to rust than most carbon steels, due to their lack of pearlite the fine layers of ferrite (almost pure iron) and cementite in pearlite.[citation needed] The Angel of the North at Gateshead, England is a well known example of an unpainted HSLA structure (the actual alloy used is called COR-TEN and includes a small amount of copper). HSLA steels usually have densities of around 7800 kg/m


High Performance Alloys

A superalloy, or high-performance alloy, or high Temperature alloy is an alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Super alloys typically have a matrix with an austenitic face-centered cubic crystal structure. A super alloy's base alloying element is usually nickel, cobalt, or nickel-iron. Super alloy development has relied heavily on both chemical and process innovations and has been driven primarily by the aerospace and power industries. Typical applications are in the aerospace, industrial gas turbine and marine turbine industry, e.g. for turbine blades for hot sections of jet engines. Mainly these alloys are being used to make blades and other components for such industries. These alloys plays a very crucial role for their own uses.

Examples of super alloys are Hastelloy, Inconel, Waspaloy, Rene alloys (e.g. Rene 41, Rene 80, Rene 95, Rene 104), Haynes alloys, Incoloy, MP98T, TMS alloys, and CMSX single crystal alloys. These all are common Grades in Super Alloys, High Performance Alloys or High Temperature Alloys there are gain many grades there into.

Super alloys are metallic materials for service at high temperatures, particularly in the hot zones of gas turbines. Such materials allow the turbine to operate more efficiently by withstanding higher temperatures. Turbine Inlet Temperature (TIT), which is a direct indicator of the efficiency of a gas turbine engine, depends on the temperature capability of 1st stage high pressure turbine blade made of Ni base super alloys exclusively.

One of the most important super alloy or High Temperature Alloys properties is high temperature creep resistance. Other crucial material properties are fatigue life, phase stability, as well as oxidation and corrosion resistance.

Super alloys develop high temperature strength through solid solution strengthening. Oxidation and corrosion resistance is provided by the formation of a protective oxide layer which is formed when the metal is exposed to oxygen and encapsulates the material, and thus protecting the rest of the component. Oxidation or corrosion resistance is provided by elements such as aluminum and chromium. By far the most important strengthening mechanism is through the formation of secondary phase precipitates such as gamma prime and carbides through precipitation strengthening.

Creep resistance is dependent on slowing the speed of dislocations within the crystal structure. In Ni-base super alloys the gamma prime phase [Ni3(Al,Ti)] present acts as a coherent barrier to dislocation motion and is a precipitate strengthener. Chemical additions such as aluminum and titanium promote the creation of the gamma prime phase. The gamma prime phase size can be precisely controlled by careful precipitation hardening heat treatments. Many super alloys have a two phase heat treatment which creates a dispersion of square gamma prime particles known as the primary phase with a fine dispersion between these known as secondary gamma prime. Many other elements, both common and exotic, (including metals also metalloids and nonmetals) can be present; chromium, cobalt, molybdenum, tungsten, tantalum, aluminum, titanium, zirconium, niobium, rhenium, carbon, boron or hafnium are just a few examples. Cobalt base super alloys or High Temrature, or High Performance Alloys do not have a strengthening secondary phase like gamma prime.

The historical developments in super alloy processing have brought about considerable increases in High Performance Alloys operating temperatures. Super alloys were originally iron based and cold wrought prior to the 1940s. In the 1940s investment casting of cobalt base alloys significantly raised operating temperatures. The development of vacuum melting in the 1950s allowed for very fine control of the chemical composition of super alloys and reduction in contamination and in turn led to a revolution in processing techniques such as directional solidification of alloys and single crystal super alloys.

Within gas turbine engines many forms of super alloys are present. Polycrystalline Ni-base super alloys are used for the disks of the high pressure turbine which can be created using powder metallurgy or casting technology. Turbine blades can be polycrystalline, have a columnar grain structure, or being single crystal. Polycrystalline blades are formed using casting technology into a ceramic mold. Columnar grain structured blades are created using directional solidification techniques and have grains parallel to the major stress axes.

Single-crystal super alloys (SC super alloys) are formed as a single crystal using a modified version of the directional solidification technique, so there are no grain boundaries in the material. The mechanical properties of most other alloys depend on the presence of grain boundaries, but at high temperatures, they would participate in creep and must be replaced by other mechanisms. In many such alloys, islands of an ordered intermetallic phase sit in a matrix of disordered phase, all with the same crystalline lattice. This approximates the dislocation-pinning behavior of grain boundaries, without introducing any amorphous solid into the structure.

 

Research and development of new super alloys


The availability of super alloys during past decades has led to a steady increase in the turbine entry temperatures and the trend is expected to continue. Sandia National Laboratories is now a days studying a new method for making super alloys, known as radiolysis. It introduces an entirely new area of research into creating alloys and super alloys through nanoparticle synthesis. This process holds promise as a universal method of nanoparticle formation. By developing an understanding of the basic material science behind these nanoparticle formations, there is speculation that it might be possible to expand research into other aspects of super alloys.

There may be considerable disadvantages in making alloys by this method. About half of the use of super alloys is in applications where the service temperature is close to the melting temperature of the alloy. It is common therefore to use single crystals. The above method produces polycrystalline alloys which will suffer from unacceptable level of creep.

 

Applications of High Performance Nickel Alloys


Super alloys are commonly used to manufacture gas turbine engines in regions that are subject to high temperatures which require high strength, excellent creep resistance, as well as corrosion and oxidation resistance. In most turbine engines this is in the high pressure turbine, blades here can face temperatures approaching if not beyond their melting temperature. Thermal barrier coatings (TBCs) play an important role in blades allowing them to operate under such conditions, protecting the base material from the thermal affects as well as corrosion and oxidation. Additional applications of super alloys include the following: gas turbines (commercial and military aircraft, power generation, and marine propulsion); space vehicles; submarines; nuclear reactors; military electric motors; chemical processing vessels, and heat exchanger tubing.

 

Metallurgy of High Performance Alloys

The super alloys of the first generation were intended for operation up to 700 C (973 K). The up-to-date super alloys of the fourth generation are used as single or Monocrystals and are extra alloyed, especially with ruthenium. They can operate up to 1100 C (1373 K).

The structure of most precipitation strengthened nickel-base super alloys consists of the gamma matrix, and of intermetallic ?' precipitates. The ?-phase is a solid solution with a face-centered crystal (fcc) lattice and randomly distributed different species of atoms.

By contrast, the ?'-phase has an ordered crystalline lattice of type LI2. In pure Ni3Al phase atoms of aluminum are placed at the vertices of the cubic cell and form the sublattice A. Atoms of nickel are located at centers of the faces and form the sublattice B. The phase is not strictly stoichiometric. There may exist an excess of vacancies in one of the sublattices, which leads to deviations from stoichiometry. Sublattices A and B of the ?'-phase can solute a considerable proportion of other elements. The alloying elements are dissolved in the ?-phase as well. The ?'-phase hardens the alloy through an unusual mechanism called the yield stress anomaly. Dislocations dissociate in the ?'-phase, leading to the formation of an anti-phase boundary. It turns out that at elevated temperature, the free energy associated with the anti-phase boundary (APB) is considerably reduced if it lies on a particular plane, which by coincidence is not a permitted slip plane. One set of partial dislocations bounding the APB cross-slips so that the APB lies on the low-energy plane, and, since this low-energy plane is not a permitted slip plane, the dissociated dislocation is now effectively locked. By this mechanism, the yield strength of ?'-phase Ni3Al actually increases with temperature up to about 1000 C, giving super alloys their currently unrivalled high-temperature strength.

In addition, it is often beneficial for a grain boundary containing nickel-base alloy to contain carbides for improvements in creep strength. Where the carbides (e.g. MC where M is a metal and C is a carbon atom) are precipitated at the grain boundaries, they act to pin the grain boundaries and improve the resistance to sliding and migration that would occur during creep diffusion. However if they precipitate as a continuous grain boundary film, the fracture toughness of the alloy may be reduced, together with the ductility and rupture strength.

 

Coating of Super Alloys

Super alloy products that are subjected to high working temperatures and corrosive atmosphere (such as high pressure turbine region of jet engines) are coated with various kinds of coating. Mainly two kinds of coating process are applied: pack cementation process and gas phase coating. Both are a type of CVD. In most cases, after the coating process near-surface regions of parts are enriched with aluminum, the matrix of the coating being nickel aluminize.

Pack cementation process Coating on Super Alloys

The pack cementation process is carried out at lower temperatures, about 750C). The parts are loaded into boxes that contain a mixture of powders: active coating material, containing aluminum, activator (chloride or fluoride), and thermal ballast, like aluminum oxide). At high temperatures the gaseous aluminum chloride (or fluoride) is transferred to the surface of the part and diffuses inside (mostly inward diffusion). After the end of the process the so-called "green coating" is produced, which is too thin and brittle for direct use. A subsequent diffusion heat treatment (several hours at temperatures about 1080C) leads to further inward diffusion and formation of the desired coating.

Gas phase coating on High Performance Alloys

This process is carried out at higher temperatures, about 1080C. The coating material is usually loaded onto special trays without physical contact with the parts to be coated. The coating mixture contains active coating material and activator, but usually does not contain thermal ballast. As in the pack cementation process, the gaseous aluminum chloride (or fluoride) is transferred to the surface of the part. However, in this case the diffusion is outwards. This kind of coating also requires diffusion heat treatment.

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