One Minute Mentor Archive

One Minute Mentor Archive 

December 18, 2008 - Metallographic Sectioning
Sectioning a specimen for metallographic examination can alter the material condition at the test location. Ideally, changes in microstructure from the sectioning process should be avoided. However, because some hot and cold working inevitably occurs from most sectioning methods, compromises must be made to get the job done. The damage to the specimen during sectioning depends on the material being sectioned, the nature of the cutting devise used, the cutting speed and feed rate, and the amount and type of coolant used. On some specimens, surface damage near the cut is inconsequential and can be removed during subsequent grinding and polishing. The depth of damage varies with material and sectioning method.

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December 11, 2008 - Microstructure of Steel
The solidification and as-cast microstructure of steel is a function of chemical composition and cooling rate. For plain carbon and low alloy steels the solidification structure consists of austenite grains. However, during cooling to room temperature after solidification, a peritectic and then solid-state transformation occur that almost entirely conceal the original as-cast structure. For carbon steel, the austenite transforms into ferrite and pearlite if the composition is hypoeutectoid and into cementite and pearlite for hypereutectoid steel. Depending on cooling rate and composition, low-alloy steels can have various microstructures consisting of different forms and combinations of pearlite, bainite, martensite, cementite, and ferrite. High-alloy steels may have an austenite structure even after cooling to room temperature.

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December 4, 2008 - Temperature Measurement a Vacuum Furnace
Hot zone temperature measurement in a vacuum furnace typically is done using thermocouples located in the vicinity of the heating elements. Normally a minimum of two are used, one for furnace control and the other connected to an independent hot-zone power supply shutdown and alarm, for over-temperature control. In practice, the size of the hot zone may dictate multiple zone control, with multiple thermocouples for each function. An optical pyrometer may be used where process temperatures exceed normal thermocouple ranges, or in special situations such as noncontacting applications in a semicontinuous vacuum furnace. Hot zone control thermocouple signals in the millivolt range are generally transmitted to a process temperature controller-programmer, often connected in parallel with temperature recorders, data logging instruments, or computers.

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November 20, 2008 - Gas Quenching in a Vacuum Furnace
Gas quenching follows the final thermal soak in vacuum. The furnace is quickly backfilled with inert gas to atmospheric or positive pressure and the gas, driven by a powerful fan or blower and continuously recirculated, flow at high velocity over the workload and through a gas-to-water heat exchanger. The inert gases used for gas quenching are nitrogen, argon, and helium. Quench rates are enhanced through the use of cooling gas at greater than atmospheric pressures. The advantage of higher pressure cooling is a denser gas, with increased mass flow and therefore greater thermal conductivity, all of which add up to improved cooling rates. In addition, gas blowers and heat exchangers operate at better efficiency at increased pressure.

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November 13, 2008 - Vacuum Furnace Insulation
Two types of insulation are used in vacuum furnaces: the radiation shield and a hot pack of solid or fibrous insulation. In the radiation shield, each time a vacuum space is provided between two adjacent parallel sheets of matter, the heat loss is reduced. Heat can transfer between the two only by radiation. The amount of heat transferred depends primarily on the temperature difference between them, and also on their surface quality. Usually, the hot face and one or more inner shields are a refractory metal, or graphite for the highest temperatures, and the remaining shields are of a less expensive material, such as stainless steel. The hot pack insulation uses various combinations of metal and graphite or conventional insulating fiberboard of blanket. The insulating properties of blankets improve in vacuum because the spaces between fibers are evacuated.

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November 6, 2008 - Volatilization in a Vacuum Furnace
In a vacuum furnace, materials can be pressed at temperatures and pressures at which the vapor pressure of the materials becomes an important consideration. Vapor pressure, which is the gas pressure exerted when a substance is in equilibrium with its own vapor, increases rapidly with temperature because the amplitude of molecular vibration increases with temperature. Some molecules in the outer surface of the solid material have higher energies than others, and they escape as free molecules or vapor. If a solid substance is contained in an enclosure devoid of any other material, molecules will continue to escape from the solid surface until their rate of escape is exactly balance by the rate of condensation or recapture of the gaseous molecules. The equilibrium pressure developed is the vapor pressure of the substance at that temperature. The vapor pressure of a metal is dependent on the temperature and pressure only but the effect is time dependent. It is normally desirable to use a vacuum-temperature combination that accelerates the desorption of a gases without producing the vaporization of more volatile alloy constituents.

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October 30, 2008 - Effect of Carburizing Temperature on Carbon Penetration
The penetration of carbon into the steel in gas carburizing depends on the carburizing temperature, the time at temperature, and the carburizing agent. Because the solubility of carbon is greatest above the Ac3 temperature, carburization takes place most readily above this temperature. Furthermore, the higher the temperature is, the greater the rate of carbon penetration will be, because the rate of diffusion is greater. Thus, it is customary to select a temperature approximately 40°C (70°F) above the Ac3 point. The time at the carburizing temperature is the most influencing factor in the control of the depth of carbon penetration. Temperatures as low as 790°C and as high as 985°C (1450 and 1800°F) have been used for carburizing.

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October 23, 2008 - Nature of a Carburized Case
At a temperature of approximately 925°C (1700°F), a steel surface is extremely active, and if the carbon content of its environment is higher than that of the steel, the steel absorbs carbon to achieve equilibrium with the environment. However, if the carbon potential of the environment is lower than that of the steel, the steel loses carbon to its environment (decarburization). However, equilibrium conditions prevail only at the steel surface, and as distance from the surface increases, carbon concentration decreases gradually to the original carbon content of the steel. The drop in carbon content below the surface is important, because finishing operations (grinding, for example) must be planned carefully because too much stock removal in finishing removes the most valuable part of the case. A general rule is to plan operations so no more than 10% (per side) of the case is removed during finishing.

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October 16, 2008- Internal Oxidation Process

At a given temperature during gas carburizing, the oxygen content and the depth of oxygen penetration are strongly influenced by the oxygen potential of the atmosphere (the limiting oxygen potential being that at which iron begins to oxidize). However, as the carbon potential rises, the oxygen potential falls. Consequently, with high carbon-potential carburizing, the oxidizing effect is reduced depending on the duration of carburizing. In commercial case-hardening steels, the depths at which oxides are detected using conventional optical microscopy typically are less than 25 µm (i.e., for carburized total case depths of 1 to 2 mm). Deeper cases will produce deeper penetrating oxides; for example, an 8 mm total case depth in a Cr-Ni-Mo steel would likely have an oxide penetration depth of 75 to 100 µm.

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October 9, 2008- Classification of Case Hardening
Case hardening-the production of parts that have hard, wear-resistant surfaces, but with softer and/or tougher cores-can be accomplished using two distinct methods.One approach is to use a grade of steel that already contains sufficient carbon to provide the required surface after heat treatment. The surface areas requiring the higher hardness are then selectively heated and quenched. The second method is to use a steel that normally is not capable of being hardened to the desired degree, then alter the composition of the surface layers by diffusion so it either can be hardened or, in some instances, becomes hard during processing. Precise classification of case hardening is difficult, but for most practical purposes, case hardening treatments can be broadly classified into four groups including carburizing, carbonitriding, nitriding, and nitrocarburizing.

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October 2, 2008- Internal Oxidation from Carburizing
Gas carburizing is normally carried out at a temperature within the range of 900 to 950°C using an endothermic carrier gas generated by the controlled combustion of another gas (such as natural gas and liquid propane gas) with air in the presence of a catalyst at a high temperature.

Prepared from natural gas (methane), the endothermic atmosphere has a typical composition of 40% H, 20% CO, 0.46% CH4, 0.27% CO2, and 0.77% H2O (vapor; dew point, 4°C), with a balance of nitrogen. The balance of the component gases ensures that the endothermic atmosphere is reducing to iron. For those alloying elements in solid solution in the steel that have a greater affinity for oxygen than iron does, the atmosphere is potentially oxidizing. Water vapor and carbon dioxide are the offending component gases that provide the oxygen for the internal oxidation processes. The oxidation potentials of the main elements used for alloying can be derived from the ratios of partial pressures of the oxidizing and reducing constituents in the atmosphere.

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September 11, 2008 - Heat Extraction and Cooling Rates
Two important factors influence cooling rates or the rates at which heat can be removed from a steel part. One is the ability of the heat to diffuse from the interior to the surface of the steel specimen, and the other is the ability of the quenching medium to remove heat from the surface of the part. The ability of a steel to transfer heat is characterized by its thermal diffusivity (units of area per unit time) or the ratio of its thermal conductivity to the volume specific heat. The thermal diffusivity of austenite transformation products increases with decreasing temperature. The slower cooling rates at positions removed from the surface of a bar permit more time for diffusion-controlled transformations, and it is this type of cooling behavior that results in the lower center hardness of bars, especially in the larger sizes.

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September 4, 2008 - Quenching Process

Quenching is defined as the controlled extraction of heat. A quenchant is any medium that extracts heat from the part, and can be a liquid, solid, or gas. Three stages of quenching when a hot part comes into contact with a liquid quenchant are (1) vapor stage (Stage A or vapor blanket), (2) boiling stage (Stage B or nucleate boiling), and (3) convection stage (Stage C or convection cooling). The vapor stage is encountered when the hot part surface initially comes into contact with the liquid quenchant, and the part becomes surrounded with a blanket of vapor. The second stage encountered in quenching is nucleate boiling, where the vapor stage starts to collapse and all liquid in contact with the component surface erupts into boiling bubbles. This is the fastest stage of quenching. The boiling stage stops when the temperature of the component's surface reaches a temperature below the boiling point of the liquid. The final stage of quenching is the convection cooling, which occurs when the component reaches a temperature lower than the quenchant boiling temperature. The convection stage is usually the slowest stage, and is typically where most distortion occurs.

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August 28, 2008 - Plasma Nitriding

A plasma is an ionized gas, which is formed when sufficient energy is applied to free electrons from atoms or molecules. Plasma, or ion, nitriding uses glow discharge technology to introduce nascent (elemental) nitrogen into the surface with subsequent diffusion into the material. The process is carried out in a vacuum under high voltage, and the ions in the plasma that is formed are accelerated for impingement on the workpiece. The ion bombardment process heats up the workpiece and cleans the surface providing active nitrogen under the influence of the glow discharge to form the nitrided case. The treatment can be performed at temperatures as low as 350°C (660°F) due to plasma activation (which does not exist in gas nitriding).

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August 21, 2008 - Nitrided Case Characteristics

Nitrided case formation consists of a series of nucleated growth areas on the steel surface, which eventually become what is known as the compound layer, or more commonly the white layer. This layer is very hard and brittle and comprises two intermixed phases. The layer does not diffuse into the steel, but remains on the surface and grows thicker with time. Carbon in the steel changes the morphology of the nucleation process causing a mixed phase formation at the steel surface. The region beneath the white layer is called the diffusion zone, which consists of stable nitrides formed by the reaction of nitrogen with nitride- forming elements. Below this region is the core of the steel consisting of tempered martensite.

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August 14, 2008 - Nitriding Steels

Certain steels respond better to nitriding from a metallurgical standpoint in terms of surface hardness, core hardness, distortion, cycle time at temperature, and the formation of stable nitrides. Certain alloying elements respond more readily than others to form stable nitrides during the nitriding process. Of the alloying elements commonly used in commercial steels, aluminum, chromium, vanadium, tungsten, and molybdenum are beneficial in nitriding because they form nitrides that are stable at nitriding temperatures. Unalloyed steels, such as mild steels and low-carbon steels, also can be nitrided, but will have lower surface hardness because the case formation is limited to pure iron nitride. In general, most stainless can be nitrided, but with some adverse effects on corrosion resistance.

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August 7, 2008 - Nitriding Metallurgical Considerations

Nitriding is a ferritic thermochemical method of diffusing nascent nitrogen into the surface of steels and cast irons. The diffusion process is based on the solubility of nitrogen in iron, the limit of which is temperature dependent. An iron-base alloy at a temperature of 450°C (840°F) will absorb up to 5.7 to 6.1% N. Beyond this amount, the surface phase formation on alloy steels tends to be predominantly epsilon (e) phase. The potential for epsilon phase formation is strongly influenced by the carbon content of the steel; the greater the carbon content, the greater the potential. The limit of solubility of nitrogen begins to decrease at a temperature of approximately 680°C (1256°F).

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July 31, 2008 - Induction Heating Power Sources

There are many different types of induction heating power sources. Most induction power supplies sold for heat treating are either some type of solid state or oscillator (vacuum) tube. Regardless of the electronics technology, the power supplies perform a common function; they basically are frequency changers that change 60 Hz (U.S.), three-phase current furnished by the electric utility into a higher frequency, single-phase current for induction heating. These power supplies often are referred to as converters, inverters, or oscillators, depending on the circuits and electronic devices used, with many possible combinations of conversion techniques. The frequency furnished by the power supply is critical to the intended induction heat treating process because of the relationship between the size of the part being heated and the depth of heating of the frequency being used.

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July 24, 2008 - Eddy Current Characteristics

At any moment, the direction of the induced current in the workpiece is approximately opposite to that in the inductor coil, and in general its flow pattern will describe a kind of "shadow image" of the coil conductors. The induced currents also generate their own magnetic fields, which are in opposition to the field generated by the coil, and, thereby, prevent the field from penetrating to the center of the object. Therefore, the eddy currents are more concentrated at the surface and decrease in strength toward the center of the object. This phenomenon of the eddy currents traveling closer to the surface of a conductor is called the "skin effect.".

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July 17, 2008 - Induction Heating Applications

Induction heating is a method of heating electrically conductive materials by the application of a varying magnetic field whose lines of force enter the workpiece. The magnetic field induces an electric potential (voltage), which can then create an electric current depending on the shape and the electrical characteristics of the workpiece. These so-called eddy currents dissipate energy and produce heat by flowing against the resistance of an imperfect conductor. Because all metals are fair electrical conductors, induction heating is applicable to several types of metal processing operations such as melting, welding, brazing, heat treating, stress inducement, zone refining, and heating prior to hot working. The technique also lends itself to a variety of nonmetal applications including adhesive bonding, graphitizing carbon, drying, curing, and superheating glass. Heat treating predominates in terms of number of units used, with surface hardening of steel and cast iron being the most prevalent use.

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July 10, 2008 - Transverse-Flux Induction Coil

Transverse-flux (also called proximity) coils are not as widely used and are used to heat workpieces where the cross sectional thickness is less than four times the reference depth. The workpiece essentially is placed in between turns of the inductor coil in which the current is flowing in the same direction. By doing this, there is no current cancellation effect and thin pieces can be heated effectively. Coil design is application specific, so the type of coil to be used should be selected before designing fixturing. The type of process used, such as whether the workpiece is heated single shot or scanned, influences coil selection.

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July 3, 2008 - Longitudinal-Flux Induction Coil

The coil, also called an inductor or induction work coil, basically is a transformer primary that induces high-frequency output of an induction power supply into a workpiece, which effectively is the transformer secondary. Longitudinal flux (reverse current flow) induction coils are the most widely used type of coil, with solenoid types of coils most commonly used. The workpiece is surrounded or enveloped, with the turns on opposite sides so that induced current flows around the workpiece. When the air gap between the coil and workpiece is reasonable for the frequency and load conditions involved, heating is efficient because the flux lines tend to be confined. There are many different forms, shapes, and adaptations of solenoid coils, with the circular or enveloping coil the simplest form. Solenoid coils require the workpiece thickness to be at least four times the reference depth for efficient operation.

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June 26, 2008 - Heat Treating of Maraging Steels

Maraging steels are highly alloyed low-carbon iron-nickel martensites that possess an excellent combination of strength and toughness superior to that of most hardened carbon steels. Hardened carbon steels derive their strength from transformation-hardening mechanisms (such as martensite and bainite formation) and the subsequent precipitation of carbides during tempering. In contrast, maraging steels derive their strength from the formation of a very low-carbon, tough, and ductile iron-nickel martensite, which can be further strengthened by subsequent precipitation of intermetallic compounds during age hardening. The term marage was coined based on the age hardening of the martensitic structure.

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June 19, 2008 - Tempering Medium-Carbon Low-Alloy Steels

Type 4340 steel, the most popular steel in the medium-carbon low-alloy steels class, is deep-hardening steel. In thin sections, the steel is air hardening although in practice it usually is oil quenched. Hardening consists of heating to 800-845°C (1475-1550°F) and holding 15 min for each 25 mm (1 in.) of thickness; quench in warm oil at 25-60°C (75-140°F). Temper at least 2 h at 455-650°C (850-1200°F); air cool. Double tempering is recommended to optimize yield strength and impact properties. Temperature and time at temperature depend mainly on desired strength or hardness. Tempering below 455°C (850°F) is not recommended because of susceptibility to temper embrittlement.

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June 12, 2008 - Stress Relieving Cast Magnesium Alloys

Precision machining of Mg-alloy castings to close dimensional limits, the necessity of avoiding warpage and distortion, and the desirability of preventing stress-corrosion cracking in welded Mg-al casting alloys make it mandatory that cast components be substantially free from residual stresses. Although Mg castings do not normally contain high residual stresses, the low modulus of elasticity of Mg alloys means that comparatively low stresses can produce appreciable elastic strains. Residual stresses can arise from contraction due to mold restraint during solidification, from nonuniform cooling after heat treatment, and from quenching. Machining operations also can result in residual stress and require intermediate stress relieving prior to final machining. Weld repairs can introduce severe stresses and should be followed by some type of heat treatment.

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June 5, 2008 - Stress Relieving Wrought Magnesium Alloys

Stress relieving is used to remove or reduce residual stresses induced in wrought magnesium products by cold and hot working, shaping and forming, straightening, and welding. Stress-relieving temperatures and times recommended for wrought magnesium alloys are based on obtaining assemblies having maximum freedom from stress. When extrusions are welded to hard-rolled sheet, lower stress-relieving temperature and longer time should be used to minimize distortion; for example, use 150°C (300°F) for 60 minutes rather than 260°C (500°F) for 15 minutes.

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May 29, 2008 - Medium-Alloy Air-Hardening Steels

Ultrahigh-strength steels H11 Mod and H13, which also are known as 5% Cr hot-work die steels, are similar in composition, heat treatment, and many properties. They have deep hardenability and can be hardened through in large section by air cooling. Air hardening results in minimal residual stresses after hardening. Both H11 and H13 are secondary hardening steels, and, therefore, develop optimum properties when tempered at temperatures above the secondary hardening peaks at about 510°C (950°F). These high tempering temperatures provide substantial stress relief and stabilization of properties so steels can be used to advantage at elevated temperatures. This also enables heat treated parts to be warm worked or preheated for welding at temperatures as high as 55°C (100°F) below the prior tempering temperature.

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May 22, 2008 - Heat Treating Ultrahigh-Strength Steels

Ultrahigh-strength steels are those commercial structural steels capable of a minimum yield strength of 1,380 MPa (200 ksi). Three types of these steels are medium-carbon low-alloy, medium-alloy air-hardening, and high-alloy hardenable steels. The medium-carbon low-alloy steels, such as 4130, 4140, 4340, and 8640, generally are supplied by the mill in either the normalized and tempered or annealed condition and are readily forged. Alloy 4130 is a water-hardening alloy steel of low to intermediate hardenability. Because 4130 steel has low hardenability, section thickness must be considered when heat treating to high hardness or strength.

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May 15, 2008 - Annealing Wrought Magnesium Alloys

Annealing of products lowers tensile properties considerably and increases ductility, thereby facilitating some types of fabrication. Wrought magnesium alloys in various conditions of strain hardening of temper can be annealed by heating at 290 to 455°C (550 to 850°F), depending on the alloy, for one or more hours. This procedure usually will provide a product with the maximum anneal that is practical. Because most forming operations on magnesium alloys are carried out at elevated temperature, the need for fully annealed wrought material is less than with many other metals.

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May 8, 2008 - Heat Treatments Applied to Wrought Magnesium Alloys

In most wrought alloys, maximum mechanical properties are developed through strain hardening, and these alloys generally are used either without subsequent heat treatment or merely aged to a T5 temper. Occasionally, solution treatment, or a combination of solution treatment with strain hardening and artificial aging, will substantially improve mechanical properties. Wrought alloys that can be strengthened by heat treatment are grouped into five general classes according to composition including: Magnesium-aluminum-zinc (e.g., AZ80A) Magnesium-thorium-zirconium (e.g., HK31A) Magnesium-thorium-manganese (e.g., HM21A and HM31A) Magnesium-zinc-zirconium (e.g., ZK60A) Magnesium-zinc-copper (e.g., ZC71A) Source: ASM Handbook, Vol. 4, Heat Treating, ASM International, 1991, p 899.

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May 1, 2008 - Heat Treatments Applied to Cast Magnesium Alloys

The mechanical properties of most casting alloys can be improved by heat treatment. Casting alloys are grouped into seven general classes of commercial importance on the basis of composition including: Magnesium-aluminum-manganese (e.g., AM100A) Magnesium-aluminum-zinc (e.g., AZ63A and AZ91C) Magnesium-zinc-zirconium (e.g., ZK51A and ZK61A) Magnesium-rare earth metal-zinc-zirconium (e.g., EZ33A and ZE41A) Magnesium-rare earth metal-silver-zirconium, with or without thorium (e.g., QE22A and QH21A) Magnesium-thorium-zirconium, with or without zinc (HK31A and ZH62A) Magnesium-zinc-copper (e.g., ZC63A)

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April 24, 2008 - Heat Treating Magnesium Alloys

Magnesium alloys usually are heat treated to improve mechanical properties or as a means of conditioning for specific fabricating operations. The type of heat treatment selected depends on alloy composition and product form (cast or wrought) and on anticipated service conditions. The types of heat treatment commonly used for various magnesium alloys, both cast and wrought, include annealing, stress relieving, solution treating and aging, and reheatreating. Basic temper designations are used to indicate the various types of heat treatments, which are the same as those applied to aluminum alloys. For certain magnesium alloys, development of properties depends almost entirely on heat treatment.

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April 17, 2008 - Alpha-Beta Aluminum Bronzes

Complex alpha-beta aluminum bronzes are those whose normal microstructures contain more than one phase to the extent that beneficial quench and temper treatments are possible. These alloys, with and without iron, are heat treated using procedures somewhat similar to those used to heat treat steel, and have isothermal transformation diagrams that resemble those of carbon steels. For these alloys, the quench-hardening treatment essentially is a high-temperature soak intended to dissolve all of the alphaphase into the beta phase. Quenching results in a hard room-temperature beat-martensite structure, and subsequent tempering reprecipitates fine alpha needles in the structure, forming a tempered beta martensite.

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April 10, 2008 - Aging of Spinodal Hardening Alloys

The microstructures and consequent heat treatabilitites of aluminum bronzes vary with aluminum content much the same as these characteristics vary with carbon content in steels. Alpha-aluminum bronzes contain less than 9% Al, or less than 8.5% Al with up to 3% Fe, and are essentially single-phase alloys. Effective strengthening of these alloys can be achieved only by cold work, and annealing and/or stress relieving are the only heat treatments of practical use. The most prevalent alloys of this group are C60600, C61000, C61300, and C61400. Annealing of alpha-aluminum bronzes is carried out at temperatures between 540 to 870°C (1000 to 1600°F), with the iron-containing alloys requiring temperatures nearer the high end of the range. Alloys of intermediate composition (containing small amounts of beta phase), such as C61900, are typically annealed at 5495to 650°C (1100 to 1200°F).

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April 3, 2008 - Aging of Spinodal Hardening Alloys

The copper-nickel-tin (Cu-Ni-Sn) alloys are hardened by treating in the rather narrow temperature range of 350 to 360°C (660 to 680°F). The development of the optimum properties requires the careful control of temperature and time at temperature. The use of hardness alone to evaluate results may not be adequate because high hardness may be maintained where excessive aging causes a decrease in elastic properties. Variations in tensile properties of 70 to 100 MPa (10 to 15 ksi) are possible without a significant hardness change. A combination of cold working and heat treatment (microduplexing) can be used to impart moderately high tensile properties and significantly greater ductility. The process consists of cold working to significant reductions (typically 40 to 60%) and partially solution treating below the single-phase boundary, typically at 725°C (1340°F). The alloy is then aged at the higher spinodal-hardening temperature level of 425°C (800°F) for an extended time.

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March 27, 2008 - Heat Treatment that Causes Spinodal Decomposition

The heat treatment that causes the spinodal decomposition of a spinodal alloy is to homogenize at a temperature above the miscibility gap so that only statistical variation in composition exist within the specimen followed by rapid cooling to a temperature within the spinodal region, and holding at that temperature, or continuously cooling from the solution temperature to room temperature. It is important to maintain control within the specified solution treatment temperature range for a particular alloy to obtain the proper heat treating response in the subsequent spinodal aging treatment. Exceeding the upper limit can result in brittle material that does not respond to spinodal hardening. Solution treating below the minimum temperature results in incomplete solution and failure of the material to harden fully during the spinodal aging treatment.

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March 20, 2008 - Spinodal-Hardening Copper Alloys

Spinodal structures are composed of a fine, homogeneous mixture of two phases that form by the growth of composition waves in a solid solution during a suitable heat treatment. The phases of the spinodal product differ in composition from each other and from the parent phase, but have the same crystal structure as the parent phase. The fineness of the spinodal structure is characterized by the distance between regions of identical composition, which is of the order of 50 to 1,000 angstroms. To form a spinodal structure, the particular alloy system must have a miscibility gap (either stable or metastable), and the atoms of the two component metals must possess sufficient mobility at the heat treating temperature. If an alloy decomposes within the spinodal region by a diffusional process that allows composition variations to increase in magnitude, it is said to decompose spinodally.

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March 13, 2008 - Quality Control of Aged Wrought Copper-Beryllium Alloys

The close control of temperature is critical in the conventional aging of copper-beryllium alloys. A change in temperature affects the time required to develop maximum properties. Also, the higher temperatures can result in lower property values. In most instances, the completeness of aging can be verified by harness testing. Exceptions are tensile testing of specimens taken from large parts and simulated service testing to determine elastic performance. Hardness measurements always should be made using the method and load most suitable for the thickness of the material and the normal level of hardness expected. Hardness test methods typically suggested for testing various thicknesses of hardened beryllium-copper alloy include diamond pyramid, Rockwell superficial 15N, and Rockwell B or C.

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March 7, 2008 - Precipitation Hardening Wrought Copper-Beryllium Alloys

The cold working of solution-treated copper-beryllium alloys influences the strength attainable through subsequent aging. The greatest response to aging occurs in material in the cold-rolled hard temper condition. In general, work hardening offers no advantage beyond the hard temper condition because formability is poor and control of the precipitation-hardening treatment for maximum strength is critical. However, in some applications wire is drawn to higher levels of cold work prior to precipitation hardening. Special combinations of properties can be obtained by varying either the aging time or the aging temperatures. As tensile strength increases, elongation decreases and does not recover substantially with overaging.

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February 29, 2008 - Solution Treating Wrought Copper-Beryllium Alloys

Solution-treating temperature limits for wrought copper-beryllium alloy mill products must be adhered to if optimum properties are to be obtained from the precipitation-hardening treatment. Solution treating below the specified minimum temperature results in insufficient solution of the beryllium-rich phase. This results in lower hardness after precipitation hardening. Also, solution treating must be carefully controlled to produce the desired grain size, dimensional tolerances, and mechanical properties and to prevent oxidation. Exceeding the upper temperature limit causes grain coarsening in wrought and cast materials. A coarse grain size impairs formability; overheating results in a brittle material that does not fully respond to precipitation hardening. To minimize grain growth, it is recommended that wrought alloys be held at temperature 1 hour for each inch or fraction of an inch of section thickness. The optimum time for a specific application must be determined by mechanical testing and microscopic examination of the alloy.

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February 22, 2008 - Wrought Copper-Beryllium Alloy Mill Products

Wrought copper-beryllium alloy mill products generally are supplied solution treated or solution treated and cold worked. Material in these conditions can be fabricated without further heat treatment. Thus, solution treating typically is not a part of the fabricating process unless it is necessitated by a special requirement such as softening of the material for additional forming, or is used as a salvage operation for parts that have been incorrectly heated for precipitation hardening. Cu-Be alloys in the quenched condition are easily fabricated using standard production methods. Even though fully solution-annealed material is the softest form available, better age hardening properties can be obtained if the material is cold worked after the final solution anneal. The selection of a proper cold-worked temper for a particular application is based on the severity of cold forming and the mechanical property requirements.

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February 15, 2008 - Copper-beryllium Alloys

Copper-beryllium alloys are precipitation hardenable because the solid solubility of beryllium in an alpha-copper matrix decreases with decreasing temperature. Heat treatment typically consists of solution annealing followed by precipitation hardening. Optimum mechanical and physical properties for specific applications can be achieved by varying the typical recommended heat treating schedules. Better age hardening characteristics can be obtained if the material is cold worked after the solution anneal. In addition to the wrought Cu-Be alloys, there is a wide variety of copper-base casting alloys (C81300 through C82800) that contain beryllium. Appropriate solution treating and aging schedules for these alloys are dictated by the levels of beryllium and other additives.

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February 8, 2008 - Aging and Stress-Relieving Treatment of Cu Alloys

Because of the necessity for close temperature control, forced convection (recirculating air) and salt-bath furnaces are commonly used for aging and stress relieving. Salt baths can reduce total furnace time by up to 30% compared with that required using atmosphere furnaces. Salt baths are particularly valuable when the age hardening time is of short duration and when precise control of time at the aging temperature is required. Commercially available nitrate-nitrite salt mixtures (40 to 50% sodium nitrate, remainder sodium or potassium nitrite) that melt at 143°C (290°F) are used for aging and stress relieving. All material to be heated in salt should be properly cleaned and dried before being immersed in the molten salt; any organic substance (such as oil or grease) will react violently with the nitrate-nitrite salt.

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February 1, 2008 - Heat Treating Equipment for Cu Alloys

Batch-type atmosphere furnaces can be heated electrically and by using oil or gas. Furnaces heated using oil or gas that have a protective atmosphere sometimes use a muffle to contain the atmosphere and prevent air infiltration by maintaining positive pressure when explosive atmospheres, such as hydrogen, are used. Direct natural gas-fired furnaces can be used if some surface oxidation and discoloration can be tolerated. Parts annealed in reducing atmospheres require cleaning to restore luster. Continuous atmosphere furnaces have a vestibule that provides a seal for the atmosphere, a heating chamber long enough to ensure complete solution treating, and a cooling or quenching chamber that also serves as an atmosphere seal. Salt baths consisting of molten neutral salts also are used for annealing, stress relieving, solution heat treating, and aging of copper alloys.

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January 25, 2008 - Precipitation-Hardening Cu Alloys

Alloys that harden during low to intermediate-temperature treatments following solution quenching include precipitation-hardening, spinodal-hardening, and order-hardening types. Most Cu alloys of the precipitation hardening type are used in electrical and heat-conduction applications. Thus, the heat treatment must impart the necessary mechanical strength and electrical conductivity. Copper alloys harden via elevated-temperature treatment rather than ambient-temperature (natural) aging. Hardness increases, reaches a peak, and then decreases with time. Electrical conductivity increases continuously with time until some maximum is reached, typically in the fully precipitated condition. The optimum condition generally preferred results from treating at a temperature and time just beyond those corresponding to the hardness aging peak. Cold working prior to precipitation aging tends to improve heat treated hardness.

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January 18, 2008 - Hardening Cu Alloys

Two general types of copper alloys that are hardened by heat treatment are: those that are softened by high-temperature quenching and hardened by lower temperature precipitation heat treatments, and those that are hardened by quenching from high temperatures through martensitic-type reactions. Alloys that harden during low to intermediate-temperature treatments following solution quenching include precipitation-hardening, spinodal-hardening, and order-hardening types. Quench-hardening alloys comprise aluminum bronzes, nickel-aluminum bronzes, and a few special copper-zinc alloys. Usually quench-hardened alloys are tempered to improve toughness and ductility and reduce hardness in a manner similar to that used for alloy steels.

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January 11, 2008 - Stress Relieving Cu Alloys

Stress relieving is a process intended to relieve internal stress in materials or parts without appreciably affecting their properties. Stress-relief treatments are carried out at temperatures below those normally used for annealing. From a practical standpoint, higher-temperature/shorter time treatments are preferable. However, to guarantee the preservation of mechanical properties, lower temperatures and longer times are sometimes necessary. The optimum cycle produces adequate stress relief without adversely affecting properties. Thermal stress relief reduces residual stress by eliminating part of the residual elastic strain. Some alloys may undergo slight increases in property values during stress relief heat treatment.

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January 4, 2008 - Annealing Cu Alloys to Specific Properties

Although specific properties are most frequently produced by the controlled cold working of annealed material, there are occasions in which annealing to temper is necessary or advantageous. An anneal is used to alter hardness and tensile properties to levels between those of hard and fully annealed tempers, with reasonably predictable results. For most copper alloys, the rapid drops in tensile properties and hardness occur with an increase in temperature in the annealing range, with special precautions taken to avoid any overheating. Tensile strengths and hardness levels similar to those of 1/8, 1/4, and 1/2 hard cold-worked tempers can be produced by annealing cold-worked brasses, nickel silvers, and phosphor bronzes.

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December 28, 2007 - Annealing Copper Alloys

Annealing is a heat treatment intended to soften and to increase the ductility and/or toughness of metals and alloys. Annealing is applied to wrought products, during and after mill processing, and to castings. Annealing of cold-worked metal is accomplished by heating to a temperature that produces recrystallization and, if desirable, by heating beyond the recrystalliztion temperature to initiate grain growth. Annealing primarily is a function of metal temperature and time at temperature. The source and application of heat, furnace design, furnace atmosphere, and shape of the workpiece have a significant influence on part finish, cost of annealing, and uniformity of results obtained.

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December 21, 2007 - Heat Treating of Copper Alloys

Heat treating processes applied to copper and copper alloys include homogenizing, annealing, stress relieving, solution treating, precipitation (age) hardening, and quench hardening and tempering. Homogenizing involves prolonged high-temperature soaking to reduce chemical or metallurgical segregation commonly known as coring, which occurs as a natural result of solidification in some alloys. The process is applied to copper alloys to improve hot and cold ductility of cast billets for mill processing, and occasionally is applied to castings to meet specified hardness, ductility, or toughness requirements. Typical soak times vary from 3 to over 10 h. Temperatures typically are above the upper annealing range, to within 50°C (90°F) of the solidus temperature. Homogenization slowly decreases tensile and yield strengths and hardness, and significantly increases elongation.

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December 14, 2007 - Effects of Cryogenically Treated Steel

The absence of a clear-cut understanding of the mechanism(s) by which cryogenic treatment improves performance has hindered its widespread acceptance in the industry. Nevertheless, studies have been conducted to determine the effects of cryogenic treatment. Theories about the reasons for the effects of cryogenic treatment include a more nearly complete transformation of retained austenite into martensite; precipitation of submicroscopic carbides; and a reduction in internal stresses in martensite that occurs when the submicroscopic carbide precipitation occurs. A reduction in microcracking tendencies resulting from reduced internal stresses is suggested as a reason for improved properties.

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December 7, 2007 - Cold and Cryogenic Treating of Steel

Cold treatment of steel consists of exposing the ferrous material to subzero temperatures to either impart or enhance specific conditions or properties of the material. Unlike heat treating, which requires that temperature be precisely controlled to avoid reversal, successful transformation through cold treating depends only on the attainment of the minimum low temperature (-84°C, or -120°F) and is not affected by lower temperatures. Typical cryogenic treatment consists of a slow cool-down (~2.5°C/min, or 4.5°F/min) from ambient temperature to liquid nitrogen temperature. When the material reaches approximately 80K (-315°F), it is soaked for an appropriate time (generally 24 h). At the end of the soak period, the material is removed from the liquid nitrogen and allowed to warm to room temperature in ambient air. Temperature can be controlled accurately and thermal shock to the material is avoided by conducting the cool-down cycle in gaseous nitrogen.

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November 30, 2007 - Annealing Ductile Cast Iron

Ductile iron castings generally are given a full ferritizing anneal when maximum ductility and good machinability are required and high strength is not required. The microstructure is converted to ferrite, and the excess carbon is deposited on the existing nodules. This treatment produces ASTM grade 60-40-18. Amounts of manganese, phosphorus, and alloying elements such as chromium and molybdenum should be as low as possible if superior machinability is desired, because these elements retard the annealing process. Three types of annealing treatment are full anneal for unalloyed 2-3% Si iron having no eutectic carbide, full anneal with carbides present, and subcritical anneal to convert pearlite to ferrite.

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November 16, 2007 - Austenitizing Ductile Cast Iron

The objective of austenitizing is to produce an austenitic matrix having as uniform a carbon content as possible prior to thermal processing. For a typical hypereutectic ductile cast iron, an upper critical temperature must be exceeded so the austenitizing temperature is in the two-phase (austenite-graphite) field; this temperature varies with alloy content. The "equilibrium" austenite carbon content in equilibrium with graphite increases with increasing austenitizing temperature. The ability to select (within limits) the matrix austenite carbon content makes austenitizing temperature control important in processes that depend on carbon in the matrix to drive a reaction. This is particularly true in structures to be austempered, in which the hardenability (or austemperability) depends to a significant degree on matrix carbon content. Austenitizing temperatures in the range of 900 to 940°C (1650 to 1750°F) typically are used with times ranging from 1 to 3 h.

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November 9, 2007 - Properties of Austempered Ductile Iron

 ADI is a unique cast iron material, having tensile properties attributable to ?H, but with the fine dispersion of ferrite. Austempering is accomplished by heating the casting to a temperature in the austenite-phase range (usually 815 to 925°C, or 1500 to 1700°F), holding for the time required to saturate the austenite with carbon, cooling to a temperature above the Ms temperature at a rate sufficient to avoid the formation of pearlite or other mixed structures, and them holding at that austenitizing temperature for the time required to produce the optimum structure of acicular ferrite and carbon-enriched austenite. The properties of ADI can be varied by changing the austempering temperature.

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November 2, 2007 - Austempering of Ductile Irons

 To produce austempered ductile iron (ADI), austenitizing is followed by rapid quenching (usually in molten salt) to an intermediate temperature range for a time that allows the unique metastable carbon-rich (~2%C) austenitic matrix (gamma subscript H) to evolve simultaneously with nucleation and growth of a plate-like ferrite (alpha) or ferrite plus carbide, depending on the austempering temperature and time at temperature. The austempering reaction progresses to a point at which the entire matrix has been transformed to the metastable product (stage I), and that product is "frozen in" by cooling to room temperature before the true banitic ferrite plus carbide phases can appear (stage II). The presence of 2-3%C prevents the rapid formation of iron carbide (Fe3C), and, thus, the carbon rejected during ferrite formation in stage I enters the matrix austentite, enriching it and thermally stabilizing it to prevent martensite formation upon subsequent cooling.

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October 26, 2007 - Heat Treating of Ductile Irons

 Ductile cast iron (also known as nodular or spheroidal graphite iron) is heat treated primarily to create matrix microstructures and associated mechanical properties not readily obtained in the as-cast condition. The microstructures achievable can be depicted using a continuous cooling transformation (CCT) diagram and cooling curves for furnace cooling, air cooling, and quenching. Slow furnace cooling results in a ferritic matrix (the desired product of annealing); whereas air cooling, or normalizing, results in a pearlitic matrix; and quenching produces a matrix microstructure consisting mostly of martensite with some retained austenite. Tempering softens the normalized and quenched conditions, resulting in microstructures consisting of the matrix ferrite with small particles of iron carbide (or secondary graphite). Actual annealing cycles usually involve more than just furnace cooling, depending on alloy content and prior structure.

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October 19, 2007 - Quench Severity for Aluminum Alloys

Quench severity is commonly expressed in terms of an H-value (or Grossmann number), where the H-value is related to the thermal conductivity (k) of the parts(s) and the coefficient of heat transfer (C) between the quenchant and the part, related by the equation H = C/2k. Water can achieve cooling rates up to about 200°C/s (360°F/s) at the midplane of 25-mm (1 in.) thick plate. Lower cooling rates are achieved by immersion in heated water, reducing the velocity of quenchant around the part, lowering surface tension, and increasing the stability of the vapor film around the part. Polymer quenchants retard cooling rates by the formation of films around the part.

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October 12, 2007 - Spray Quenching of Aluminum Alloys

 The quench rate for spray quenching is controlled by both the velocity and volume of water per unit area per unit time of impingement of the water on the workpiece; rate of travel of the workpiece through the sprays is an important variable. Local increases in temperature that occur within the first few seconds of quenching, caused by a phenomenon such as plugged spray nozzles, are particularly deleterious. The remaining "internal heat" could be sufficient to reheat the surface region, which causes a large loss in strength at the previously quenched surface. The loss of strength in the affected area of a heavy part is more severe than that caused by an inadequate quenching rate alone.

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October 5, 2007 - Water-Immersion Quenching of Aluminum Alloys

 Water-immersion quenching typically is controlled in practice by stipulating maximum quench-delay time and maximum water temperature. The first requirement controls the cooling rate during transfer and, for high-strength alloys, often is based on the criterion of complete immersion before the metal cools below 415°C (775°F). This specified temperature is based on a critical temperature for alloy 7075, which has one of the more severe C-curves. Therefore, the criterion for complete immersion of other alloys might be based on a temperature lower than the 415°C specification, depending on the characteristics of the particular C-curve.

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September 28, 2007 - Effects of Quench Delay of Solution Treated Aluminum Alloys

Whether the transfer of parts from the furnace to the quench is performed manually or mechanically, it must be completed in less than the specified maximum time. The maximum allowable transfer time or "quench delay" varies with the temperature and velocity of the ambient air and the mass and emissivity of the parts. Maximum quench delays can be determined from cooling curves that will ensure complete immersion before the parts cool below 400°C (750°F).

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September 21, 2007 - Quench Rate vs. Properties of Solution Treated Aluminum Alloys.

Average quench rates are useful to compare experimental results from various quench methods. However, average quench rates only compare results in a "critical" temperature range, where precipitation is most likely to occur. This method is not entirely accurate because significant precipitation can also occur outside the specified critical temperature range of average quench rates. In addition, for high-strength alloys, toughness and corrosion resistance could be impaired without significant loss of tensile strength. Therefore, the more sophisticated comparison called quench-factor analysis is required for quantitative property prediction or property optimization.

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September 14, 2007 - Quenching Solution Treated Aluminum Alloys

The objective of quenching solution treated components is to preserve the solid solution formed at the solution heat treating temperature by rapidly cooling to some lower temperature, usually near room temperature. This applies not only to retaining solute atoms in solution, but also to maintaining a certain minimum number of vacant lattice sites to assist in promoting the low-temperature diffusion required for zone formation. As a broad generalization, the highest strengths achievable and the best combination of strength and toughness are those associated with the most rapid quench rates. The effect of quench rate on mechanical properties also can depend on the desired temper. For example, in the underaged condition, a slow quench rate is more detrimental on ductility and fracture toughness. Strength would be more affected after near-to-peak aging. The relative effects of quench methods can be compared in terms of average quench rates.

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September 7, 2007 - Solution Treating Time for Aluminum Alloys

Heat treatment to increase the strength of aluminum alloys involves solution heat treatment (dissolution of soluble phases), quenching (development of supersaturation), and age hardening (precipitation of solute atoms). The time at the nominal solution heat treating temperature (soak time) required to effect a satisfactory degree of solution of undissolved or precipitated soluble phase constituents and to achieve good homogeneity of the solid solution is a function of microstructure before heat treatment. The time can vary from less than a minute for thin sheet product to as much as 20 h for large sand or plaster-mold castings. The time required to heat the load to the treatment temperature in furnace heat treatment also increases with section thickness and furnace loading, and, thus, total cycle time increases with these factors.

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August 31, 2007 - GP Zones in Precipitation Hardening Aluminum Alloys

The exact size, shape, and distribution of Guinier-Preston (GP) zones depend on the alloy in which they form and on the thermal and mechanical history of the specimen. GP zones essentially are distorted regions on the matrix lattice, rather than discrete particles of a new phase having a different lattice. Thus, they are completely coherent with the matrix, imposing local but often large strains on it. These mechanical strains, as well as the presence of a locally solute-rich, sometimes ordered lattice, can account for large changes in mechanical properties of the alloy before any long-range microstructural changes occur. GP zones are metastable, and, thus, dissolve in the presence of a more stable precipitate. This dissolution causes a precipitate-free, visibly denuded region to form around the stable precipitate particles.

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August 24, 2007 - Precipitation from Solid Solution in Aluminum Alloys

An essential feature of a precipitation-hardening alloy system is a temperature-dependent equilibrium solid solubility characterized by increasing solubility with increasing temperature. The major aluminum alloy systems with precipitation hardening include:

Aluminum-copper systems with strengthening from CuAl2
Aluminum-copper-magnesium systems (magnesium intensifies precipitation)
Aluminum-magnesium-silicon systems with strengthening from Mg2Si
Aluminum-zinc-magnesium systems with strengthening from MgZn2
Aluminum-zinc-magnesium-copper systems
The general requirement for precipitation strengthening of supersaturated solid solution involves the formation of finely dispersed precipitates during aging treatments. Aging must be accomplished not only below the equilibrium solvus temperature, but also below the metastable miscibility gap called the Guinier-Preston (GP) zone solvus line.

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August 17, 2007 - Effect of Precipitation on Aluminum Alloy Properties

Formability is the extent to which a material can be deformed in a particular process before the onset of failure. Aluminum sheet and aluminum shapes usually fail by localized necking or by ductile fracture. Precipitation-strengthened aluminum alloys usually are formed in the naturally aged (T4) condition, or in the annealed (O) condition, but only very rarely in the peak-strength (T6) condition where both the necking and fracture limits are low. Curves can be plotted for most of the precipitation-strengthened alloys in the 2xxx and 6xxx series showing the effect of a wide range of precipitation structures on some of the forming properties.

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August 10, 2007 - Artificial (Over)Aging Aluminum Alloys

Artificial aging includes exposure to temperatures above room temperature to produce the transitional (metastable) forms of the equilibrium precipitate of a particular alloy system, which remain coherent with the solid-solution matrix, thus contributing to precipitation strengthening. Further heating at the temperatures that cause strengthening, or at higher temperatures causes the precipitates to grow, but even more importantly, to convert to equilibrium phases, which generally are not coherent. These changes soften the material, and if carried further, produce the softest or annealed condition.

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August 3, 2007 - Artificial Aging in Aluminum Alloys

The rates and amounts of the changes in the strength and hardness of Al-Cu alloys can be increased by holding the alloys at moderately elevated temperatures (for alloys of all types, the useful range is about 120 to 230°C, or 250 to 450°F). This treatment is called artificial aging or precipitation heat treating. In the Al-Cu system, alloys having as little as 1% Cu, slowly quenched, start to harden after about 20 days at a temperature of 150°C (300°F). The alloys of this system having less than about 3% Cu show little or no natural aging after low cooling-rate quenching, which introduces little stress.

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July 27, 2007 - Natural Aging in Aluminum Alloys

Natural aging refers to spontaneous formation of a Guinier-Preston (G-P) zone structure during exposure at room temperature. Solute atoms either cluster or segregate to selected atomic lattice planes, depending on the alloy system, to form the G-P zones, which are more resistant to movement of dislocations through the lattice, and, therefore, are stronger. Of the binary alloys, aluminum-copper alloys undergo natural aging after being solution heat treated and quenched. The amounts by which strength and hardness increase become larger with time of natural aging and with the copper content of the alloy, from about 3% to the limit of solid solubility (i.e., 5.67%).

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July 20, 2007 - Heat-Treatable Aluminum Alloys

Heat-treatable (precipitation-hardening) aluminum alloys for wrought and cast products contain elements that decrease in solubility with decreasing temperature, and in concentrations that exceed their equilibrium solid solubility at room temperature and moderately higher temperatures. Heat treatment for precipitation strengthening includes a solution heat treatment at a high temperature to maximize solubility, followed by rapid cooling or quenching to a low temperature to obtain a solid solution supersaturated with both solute elements and vacancies. The heat treatment is designed to maximize the solubility of elements that precipitate in subsequent aging treatments, which may include either natural aging or artificial aging.

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July 13, 2007 - Strain Hardening of Aluminum Alloys

Strain hardening by cold rolling, drawing, or stretching is a highly effective way to increase the strength of non-heat treatable aluminum alloys. The increases in strength that accompany increasing reduction by cold rolling are obtained at the expense of ductility, as measured by percent elongation in a tensile test and by reducing formability in operations such as bending and drawing. All mill products can be supplied in the strain-hardened condition, although there are limitations on the amount of strain hardening that can be applied to products.

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July 6, 2007 - Second-Phase Strengthening of Aluminum Alloys

Elements and combinations that form predominantly second-phase constituents with relatively low solid solubility include Fe, Ni, Ti, Mn, and Cr, and combinations thereof. The presence of increasing volume fractions of intermetallic-compound phases formed by these elements and the elemental Si constituent formed by Si during solidification or by precipitation in the solid state during post-solidification heating also increase strength and hardness. These irregularly shaped particles form during solidification and occur mostly along grain boundaries and between dendrite arms. For alloys that consist of both solid solution and second-phase constituents and/or dispersoid precipitates, all of these microstructural components contribute to strength in a roughly additive manner.

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June 29, 2007 - Solid-Solution Strengthening of Aluminum Alloys

The predominant objective in the design of aluminum alloys is to increase strength, hardness, and resistance to wear, creep, stress relaxation, and fatigue. Strengthening in non-heat treatable alloys is achieved through solid-solution formation, second-phase microstructural constituents, dispersoid precipitates, and/or strain hardening. The principal alloys strengthened by alloying elements in solid solution are those in the Al-Mg (5xxx) series, ranging from 0.5 to 6 wt% Mg. These alloys often contain small additions of transition elements, such as Cr and Mn, and less frequently Zr, to control the grain or subgrain structure, and Fe and Si impurities, which usually are present in the form of intermetallic particles.

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June 22, 2007 - Strengthening Aluminum Alloys

Two most common methods to increase the strength of aluminum alloys are to:

Disperse second-phase constituents or elements in solid solution and cold work the alloy (non heat treatable alloys)
Dissolve alloying elements into solid solution and precipitate them as coherent submicroscopic particles (heat treatable or precipitation-hardening alloys)
Elements most commonly present in commercial aluminum alloys to provide increased strength, particularly when coupled with strain hardening by cold working or with heat treatment (or both) are copper, magnesium, manganese, silicon, and zinc. These elements all have significant solid solubility in aluminum, and in all cases, the solubility increases with increasing temperature. Of all the elements, zinc has the highest solid solubility in aluminum (a maximum of 66.4 at.%). The maximum solid solubility in aluminum alloys occurs at the eutectic, peritectic, or monotectic temperature.

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June 15, 2007 - Principal Commercial Aluminum Alloys

Aluminum alloys encompass more than 300 commonly recognized alloy compositions and many additional variations developed in supplier/customer relationships. The principal types of alloys are age-hardening alloys, casting alloys, and work-hardening alloys. All commercial aluminum alloys contain some iron and silicon, as well as two or more elements intentionally added to enhance properties. The principal types of aluminum alloys achieve strengthening through the alloying effects of cooper, magnesium, manganese, silicon, and zinc, in conjunction with strain hardening, heat treatment, or both.

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June 8, 2007 - Laser Hardening Gray Cast Iron

Laser surface heat treatment (or laser transformation hardening) is used to harden localized areas of gray iron machine components. The heat generated by the absorption of the laser beam is controlled to prevent melting, and, therefore, is used in the selective austenitization of local surface regions, which transform to martensite as a result of rapid cooling (self-quenching) by the conduction of heat into the bulk material of the workpiece. No change in chemical composition is produced by laser transformation hardening, and the process, as with induction and flame hardening, provides an effective technique to harden ferrous materials selectively.

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June 1, 2007 - Induction Hardening Gray Cast Iron

Gray cast iron can be surface hardened using the induction heating method, but there may be considerable variation in the hardness due to a variation in the combined carbon content. A minimum combined carbon content of 0.40 to 0.50% C (as pearlite) is recommended. The recommended minimum induction hardening temperature for gray iron is 870 and 925°C (1600 and 1700°F). The surface hardness achieved is influenced by the carbon equivalent (%C + 1/3% Si) when hardness is measured using conventional Rockwell tests. The more graphite present in the microstructure, the lower the surface hardness will appear to be after hardening.

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May 25, 2007 - Flame Hardening Gray Cast Iron

Flame hardening is a method of surface hardening commonly applied to gray iron, resulting in a hard, wear-resistant outer layer of martensite and a core of softer gray iron. Both unalloyed and alloyed gray irons can be flame hardened. One of the most important aspects of chemical composition is the combined carbon content, which should be in the range of 0.50 to 0.70%. Gray iron castings to be flame hardened should be as free as possible from porosity, sand, and slag, which can produce a rough surface or result in cracking after hardening. The surface of flame-hardened gray iron typically has a slightly lower hardness than the metal immediately below the surface, possibly caused by the retention of relatively soft austenite at the surface. The choice of quenching medium is influenced by the flame hardening method used.

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May 18, 2007 - Effect of Austempering Temperature on Properties

Gray iron usually is quenched in salt or oil at a temperature between 230 and 425°C (840 and 900°F) for austempering. The quench bath usually is held at a temperature between 230 and 290°C (450 and 550°F) when high hardness and wear resistance are the ultimate aim of the treatment. The required holding time to achieve maximum transformation is determined by the temperature of the quenching bath and the chemical composition of the iron. The effect of chemical composition on the holding time can be considerable, and alloy additions such as Ni, Cr, and Mo increase the time required for transformation. Casting shape and section thickness determine the amounts of the added hardenability agents (Ni, Cu, Mo), because cooling must be fast enough to prevent any transformation of austenite until the casting reaches the temperature of the bath.

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