Carburizing is a process of adding Carbon to the surface. This is done by exposing the part to a Carbon rich atmosphere at an elevated temperature and allows diffusion to transfer the Carbon atoms into steel. This diffusion will work only if the steel has low carbon content, because diffusion works on the differential of concentration principle. If, for example the steel had high carbon content to begin with, and is heated in a carbon free furnace, such as air, the carbon will tend to diffuse out of the steel resulting in Decarburization.
Pack Carburizing: Parts are packed in a high carbon medium such as carbon powder or cast iron shavings and heated in a furnace for 12 to 72 hours at 900 ºC (1652 ºF). At this temperature CO gas is produced which is a strong reducing agent. The reduction reaction occurs on the surface of the steel releasing Carbon, which is then diffused into the surface due to the high temperature. When enough Carbon is absorbed inside the part (based on experience and theoretical calculations based on diffusion theory), the parts are removed and can be subject to the normal hardening methods.
The Carbon on the surface is 0.7% to 1.2% depending on process conditions. The hardness achieved is 60 - 65 RC. The depth of the case ranges from about 0.1 mm (0.004 in) upto 1.5 mm (0.060 in). Some of the problems with pack carburizing is that the process is difficult to control as far as temperature uniformity is concerned, and the heating is inefficient.
Gas Carburizing: Gas Carburizing is conceptually the same as pack carburizing, except that Carbon Monoxide (CO) gas is supplied to a heated furnace and the reduction reaction of deposition of carbon takes place on the surface of the part. This processes overcomes most of the problems of pack carburizing. The temperature diffusion is as good as it can be with a furnace. The only concern is to safely contain the CO gas. A variation of gas carburizing is when alcohol is dripped into the furnace and it volatilizes readily to provide the reducing reaction for the deposition of the carbon.
Liquid Carburizing: The steel parts are immersed in a molten carbon rich bath. In the past, such baths have cyanide (CN) as the main component. However, safety concerns have led to non-toxic baths that achieve the same result.
source : http://www.efunda.com/processes/heat_treat/hardening/diffusion.cfm#PageTop
Sabtu, 03 Januari 2009
Carburizing 2
Carburizing
Abstract:
Carburizing is the addition of carbon to the surface of low-carbon steels at temperatures generally between 850 and 950°C (1560 and 1740°F), at which austenite, with its high solubility for carbon, is the stable crystal structure. Hardening is accomplished when the high-carbon surface layer is quenched to form martensite so that a high-carbon martensitic case with good wear and fatigue resistance is superimposed on a tough, low-carbon steel core.
Carburizing steels for case hardening usually have base-carbon contents of about 0.2%, with the carbon content of the carburized layer generally being controlled at between 0.8 and 1% C. However, surface carbon is often limited to 0.9% because too high a carbon content can result in retained austenite and brittle martensite.
Carburizing is the addition of carbon to the surface of low-carbon steels at temperatures generally between 850 and 950°C (1560 and 1740°F), at which austenite, with its high solubility for carbon, is the stable crystal structure. Hardening is accomplished when the high-carbon surface layer is quenched to form martensite so that a high-carbon martensitic case with good wear and fatigue resistance is superimposed on a tough, low-carbon steel core.
Case hardness of carburized steels is primarily a function of carbon content. When the carbon content of the steel exceeds about 0.50% additional carbon has no effect on hardness but does enhance hardenability. Carbon in excess of 0.50% may not be dissolved, which would thus require temperatures high enough to ensure carbon-austenite solid solution.
Case depth of carburized steel is a function of carburizing time and the available carbon potential at the surface. When prolonged carburizing times are used for deep case depths, a high carbon potential produces a high surface-carbon content, which may thus result in excessive retained austenite or free carbides. These two microstructural elements both have adverse effects on the distribution of residual stress in the case-hardened part. Consequently, a high carbon potential may be suitable for short carburizing times but not for prolonged carburizing.
Carburizing steels for case hardening usually have base-carbon contents of about 0.2%, with the carbon content of the carburized layer generally being controlled at between 0.8 and 1% C. However, surface carbon is often limited to 0.9% because too high a carbon content can result in retained austenite and brittle martensite.
Most steels that are carburized are killed steels (deoxidized by the addition of aluminum), which maintain fine grain sizes to temperatures of about 1040°C. Steels made to coarse grain practices can be carburized if a double quench provides grain refinement. Double quenching usually consists of a direct quench and then a requench from a lower temperature.
Many alloy steels for case hardening are now specified on the basis of core hardenability. Although the same considerations generally apply to the selection of uncarburized grades, there are some peculiarities in carburizing applications.
First, in a case-hardened steel, the hardenability of both case and core must be considered. Because of the difference in carbon content, case and core have quite different hardenabilities, and this difference is much greater for some steels than for others.
Moreover, the two regions have different in-service functions to perform. Until the introduction of lean alloy steels such as the 86xx series, with and without boron, there was little need to be concerned about case hardenability because the alloy content combined with the high carbon content always provided adequate hardenability. This is still generally true when the steels are direct quenched from carburizing, so that the carbon and alloying elements are in solution in the case austenite. In parts that are reheated for hardening and in heavy-sectioned parts, however, both case and core hardenability requirements should be carefully evaluated.
The relationship between the thermal gradient and the carbon gradient during quenching of a carburized part can make a measurable difference in the case depth as measured by hardness. That is, an increase in base hardenability can produce a higher proportion of martensite for a given carbon level, yielding an increased measured case depth. Therefore, a shallower carbon profile and shorter carburizing time could be used to attain the desired result in a properly chosen steel.
Core Hardness. A common mistake is to specify too narrow a range of core hardness. When the final quench is from a temperature high enough to allow the development of full core hardness, the hardness variation at any location will be that of the hardenability band of the steel at the corresponding position on the end-quenched hardenability specimen.
In standard steels purchased to chemical composition requirements rather than to hardenability, the range can be 20 or more HRC points; for example, 8620 may vary from 20 to 45 HRC at the 4/16 in.(6.35mm) position. The 25-point range emphasizes the advantage of purchasing to hardenability specifications to avoid the intolerable variation possible within the ranges for standard chemistry steels. Another way to control core hardness within narrow limits without resorting to the use of high-alloy steels is to use a final quench from a lower temperature so that full hardness in the case will be developed without the disadvantage of excessive core hardness.
Gears are almost always oil quenched because distortion must be held to the lowest possible level. Therefore, alloy steels are usually selected, with much debate about which particular alloy. The lower-alloy steels such as 4023, 5120, 4118, 8620, and 4620, with a carbon range between 0.15 and 0.25%, are widely used and generally satisfactory. Usually, the first choice is one of the last two steels mentioned, either of which should be safe for all ordinary applications. The final choice, based on service experience or dynamometer testing, should be the least expensive steel that will do the job. For heavy-duty applications, higher-alloy grades such as 4320, 4817, and 9310 are justifiable if based on actual performance tests. The life testing of gears in the same mountings used in service to prove both the design and the steel selection is particularly important.
In other applications, when distortion is not a major factor, the carbon steels described above, water quenched, can be used up to a 50 mm (2 in.) diameter. In larger sizes, low-alloy steels, water quenched, such as 5120, 4023, and 6120 can be used, but possible distortion and quench cracking must be avoided.
Carburizing Methods. While the basic principle of carburizing has remained unchanged since carburizing was first employed, the method used to introduce the carbon into the steel has been a matter of continuous evolution.
In its earliest application, parts were simply placed in a suitable container and covered with a thick layer of carbon powder (pack carburizing). Although effective in introducing carbon, this method was exceedingly slow, and as the demand for greater production grew, a new process using a gaseous atmosphere was developed.
In gas carburizing, the parts are surrounded by a carbon-bearing atmosphere that can be continuously replenished so that a high carbon potential can be maintained. While the rate of carburizing is substantially increased in the gaseous atmosphere, the method requires the use of a multicomponent atmosphere whose composition must be very closely controlled to avoid deleterious side effects, for example, surface and grain-boundary oxides. In addition, a separate piece of equipment is required to generate the atmosphere and control its composition. Despite this increased complexity, gas carburizing has become the most effective and widely used method for carburizing steel parts in large quantities.
In efforts required to simplify the atmosphere, carburizing in an oxygen-free environment at very low pressure (vacuum carburizing) has been explored and developed into a viable and important alternative. Although the furnace enclosure in some respects becomes more complex, the atmosphere is greatly simplified. A single-component atmosphere consisting solely of a simple gaseous hydrocarbon, for example methane, may be used. Furthermore, because the parts are heated in an oxygen-free environment, the carburizing temperature may be increased substantially without the risk of surface or grain-boundary oxidation. The higher temperature permitted increases not only the solid solubility of carbon in the austenite but also its rate of diffusion, so that the time required to achieve the case depth desired is reduced.
Although vacuum carburizing overcomes some of the complexities of gas carbunzing, it introduces a serious new problem that must be addressed. Because vacuum carburizing is conducted at very low pressures, and the rate of flow of the carburizing gas into the furnace is very low, the carbon potential of the gas in deep recesses and blind holes is quickly depleted. Unless this gas is replenished, a great nonuniformity in case depth over the surface of the part is likely to occur. If, in an effort to overcome this problem, the gas pressure is increased significantly, another problem arises, that of free-carbon formation, or sooting.
Thus, in order to obtain cases of reasonably uniform depth over a part of complex shape, the gas pressure must be increased periodically to replenish the depleted atmosphere in recesses and then reduced again to the operating pressure. Clearly, a delicate balance exists in vacuum carburizing: The process conditions must be adjusted to obtain the best compromise between case uniformity, risk of sooting, and carburizing rate.
A method that overcomes both of these major problems, yet retains the desirable features of a simple atmosphere and permissible operating temperature is plasma or ion carburizing.
To summarize, carburizing methods include:
* Gas carburizing
* Vacuum carburizing
* Plasma carburizing
* Salt bath carburizing
* Pack carburizing
These methods introduce carbon by the use of gas (atmospheric-gas, plasma, and vacuum carburizing), liquids (salt bath carburizing), or solid compounds (pack carburizing). All of these methods have limitations and advantages, but gas carburizing is used most often for large-scale production because it can be accurately controlled and involves a minimum of special handling.
Vacuum carbunzing and plasma carburizing have found applications because of the absence of oxygen in the furnace atmosphere. Salt bath and pack carburizing arc still done occasionally, but have little commercial importance today.
Process characteristics of the above-mentioned carburizing methods fall into two general groups:
* Conventional methods, which introduce carbon by gas atmospheres, salt baths or charcoal packs
* Plasma methods, which impinge positive carbon ions on the surface of a steel part (the cathode)
The main difference between conventional and plasma methods is the reduced carburizing times achieved in plasma-assisted methods. The quickly attained surface saturation also results in faster diffusion kinetics. Furthermore, plasma carburizing produces very uniform case depths, even in parts with irregular surfaces.
With the conventional methods, carburization always takes place by means of a gaseous phase of carbon monoxide; however, each method also involves different reaction and surface kinetics, producing different case-hardening results.
In general, with conventional methods, carbon monoxide breaks down at the steel surface:
2CO ↔ CO2 + C
The liberated carbon is readily dissolved by the austenite phase and diffuses into the body of the steel. For some process methods (gas and pack carburizing), the carbon dioxide produced may react with the carbon atmosphere or pack charcoal to produce new carbon monoxide by the reverse reaction.
Carburizing is most frequently performed between 850 and 950°C (1550 and 1750°F), but sometimes higher temperatures are used to reduce cycle times and/or produce deeper depths of the high-carbon surface layer.
A comprehensive model of gas carburization must include algorithms that describe:
* Carbon diffusion
* Kinetics of the surface reaction
* Kinetics of the reaction between endogas and enriching gas
* Purging (for batch processes)
* The atmosphere control system.
source :http://steel.keytometals.com/Articles/Art114.htm
Carbonitriding
Carbonitriding process is most suitable for low carbon and low carbon alloy steels. In this process, both Carbon and Nitrogen are diffused into the surface. The parts are heated in an atmosphere of hydrocarbon (such as methane or propane) mixed with Ammonia (NH3). The process is a mix of Carburizing and Nitriding.
Carburizing involves high temperatures (around 900 ºC, 1652 ºF) and Nitriding involves much lower temperatures (around 600 ºC, 1112 ºF). Carbonitriding is done at temperatures of 760 - 870 ºC (1400 - 1598 ºF), which is higher than the transformation temperatures of steel that is the region of the face-centered Austenite.
It is then quenched in a natural gas (Oxygen free) atmosphere. This quench is less drastic than water or oil-thus less distortion. However this process is not suitable for high precision parts due to the distortions that are inherent. The hardness achieved is similar to carburizing (60 - 65 RC) but not as high as Nitriding (70 RC). The case depth is from 0.1 to 0.75 mm (0.004 to 0.030 in). The case is rich in Nitrides as well as Martensite. Tempering is necessary to reduce the brittleness.
source : http://www.efunda.com/processes/heat_treat/hardening/diffusion.cfm#PageTop
Selecting a Gauge Material to Match Application Requirements
Strain gauges are sensors that rely on the change in resistance of the gauge to determine the strain within the part that the gauge is attached to.
A strain gauge consists of a wire matrix to maximize the effects of the strain on the gauge. Selecting the right gauge wire material for an application depends on the degree of strain to be measured, environmental variables such as temperature, and non-material considerations such as cost.
Strain Gauge Alloys
There are four primary alloys used in strain gauge fabrication: Constantan, annealed Constantan, isoelastic, and Karma.
* Constantan - Constantan is a copper-nickel alloy and the most commonly used alloy used for strain gauge matrices. Constantan has a very stable resistance over a large temperature range, making it the perfect choice for general-purpose gauges. Constantan can be processed to provide temperature compensation for a variety of coefficients of thermal expansion. Constantan-based gauges are generally the least expensive of the strain gauge alloys, due to their wide-scale usage.
* Annealed Constantan - Annealed Constantan alloy is used when large strains, usually over 5%, are expected. This alloy will experience resistance drift under high cyclic strain, so gauges that use annealed Constantan should not be used in cyclic loading applications.
* Isoelastic - Isoelastic alloy is used for dynamic strain measurements and cyclic loading applications. Gauges that use isoelastic alloys have a larger gauge factor than general purpose gauges, and provide a larger signal to noise ratio. Isoelastic alloy is sensitive to thermal effects, so it is best used in temperature-controlled environments
* Karma - Karma alloy gauges are extremely stable over time and are appropriate for long term monitoring applications. Karma alloy provides better accuracy at extreme temperatures than general purpose gauges, but is difficult to solder. Karma alloy can be processed to provide temperature compensation for a variety of coefficients of thermal expansion.
In addition to these alloys, some other materials are used for very specific applications. For example, titanium wire is used in very high temperature applications due to its high melting point.
Choosing a Strain Gauge Material
Because each of the alloys provide different benefits, the variables within the measurement application need to be considered when choosing a specific gauge material. Some considerations and the appropriate alloy choice include:
* General Purpose Applications - Constantan
* High-Strain Applications - Annealed Constantan
* Dynamic Strain Measurements - Isoelastic
* Long-Term Monitoring - Karma
* Extreme Temperatures - Karma
* Temperature Compensation Needed - Constantan, Karma
The accuracy of a strain gauge measurement depends on the ability of the gauge to maintain stability under environmental and operational effects. Choosing the right strain gauge alloy for a specific measurement application will result in more stable and accurate data.
Sources
eFunda website - Strain Gauge: Materials
Vishay-Micromeasurements website
Vishay Tech Note TN505-4: Strain Gauge Selection
Jumat, 26 Desember 2008
Strain Gauge Materials and Performance
Selecting a Gauge Material to Match Application Requirements
Strain gauges are sensors that rely on the change in resistance of the gauge to determine the strain within the part that the gauge is attached to.
A strain gauge consists of a wire matrix to maximize the effects of the strain on the gauge. Selecting the right gauge wire material for an application depends on the degree of strain to be measured, environmental variables such as temperature, and non-material considerations such as cost.
Strain Gauge Alloys
There are four primary alloys used in strain gauge fabrication: Constantan, annealed Constantan, isoelastic, and Karma.
- Constantan - Constantan is a copper-nickel alloy and the most commonly used alloy used for strain gauge matrices. Constantan has a very stable resistance over a large temperature range, making it the perfect choice for general-purpose gauges. Constantan can be processed to provide temperature compensation for a variety of coefficients of thermal expansion. Constantan-based gauges are generally the least expensive of the strain gauge alloys, due to their wide-scale usage.
- Annealed Constantan - Annealed Constantan alloy is used when large strains, usually over 5%, are expected. This alloy will experience resistance drift under high cyclic strain, so gauges that use annealed Constantan should not be used in cyclic loading applications.
- Isoelastic - Isoelastic alloy is used for dynamic strain measurements and cyclic loading applications. Gauges that use isoelastic alloys have a larger gauge factor than general purpose gauges, and provide a larger signal to noise ratio. Isoelastic alloy is sensitive to thermal effects, so it is best used in temperature-controlled environments
- Karma - Karma alloy gauges are extremely stable over time and are appropriate for long term monitoring applications. Karma alloy provides better accuracy at extreme temperatures than general purpose gauges, but is difficult to solder. Karma alloy can be processed to provide temperature compensation for a variety of coefficients of thermal expansion.
In addition to these alloys, some other materials are used for very specific applications. For example, titanium wire is used in very high temperature applications due to its high melting point.
Choosing a Strain Gauge Material
Because each of the alloys provide different benefits, the variables within the measurement application need to be considered when choosing a specific gauge material. Some considerations and the appropriate alloy choice include:
- General Purpose Applications - Constantan
- High-Strain Applications - Annealed Constantan
- Dynamic Strain Measurements - Isoelastic
- Long-Term Monitoring - Karma
- Extreme Temperatures - Karma
- Temperature Compensation Needed - Constantan, Karma
The accuracy of a strain gauge measurement depends on the ability of the gauge to maintain stability under environmental and operational effects. Choosing the right strain gauge alloy for a specific measurement application will result in more stable and accurate data.
Sources
eFunda website - Strain Gauge: Materials
Vishay-Micromeasurements website
Vishay Tech Note TN505-4: Strain Gauge Selection
Kamis, 11 Desember 2008
surface treatment
Surface Treatment Of Aluminum And Aluminum Alloys
| Abstract: Aluminum alloys are divided into two major categories: wrought and casting alloys. A further differentiation for each category is based primary on mechanism of property development. Many alloys respond to thermal treatment based on phase solubility. These treatments include solution heat treatment, quenching and precipitation, or age hardening. In order to improve surface properties of final products, such as wear resistance, corrosion resistance, reflectivity etc., different types of surface treatment were designed. All of them are divided into several groups, such as electrochemical treatments, chemical treatments and coatings. In this article their terms and definitions will be explained. |
Aluminum alloys are divided into two major categories: wrought and casting alloys. A further differentiation for each category is based primary on mechanism of property development. Many alloys respond to thermal treatment based on phase solubility. These treatments include solution heat treatment, quenching and precipitation, or age hardening.
In order to improve surface properties of final products, such as wear resistance, corrosion resistance, reflectivity etc., different types of surface treatment were designed. All of them are divided into several groups, such as electrochemical treatments, chemical treatments and coatings. In this article their terms and definitions will be explained.
Electrochemical treatment
Electrochemical brightening: Electrochemical treatment to improve the optical reflectivity of a surface.Electropolishing: Polishing of a metal surface by making it anodic in an appropriate electrolyte.
Anodized metal Metal with an anodic coating, produced by an electrolytic oxidation process in which the metal is converted to a mainly oxide coating having protective, decorative or functional properties.
Clear anodized metal: Metal with a substantially colorless, translucent anodic oxidation coating.
Color anodized metal: Anodized metal colored either during anodizing or by subsequent coloring processes.
Integral color anodized metal: Metal that has been anodized using an appropriate (usually organic acid based) electrolyte which produces a colored coating during the anodizing process itself.
Electrolytically colored anodized metal: Metal with an anodic oxidation coating that has been colored by the electrolytic deposition of a metal or metal oxide into the pore structure.
Dyed anodized metal: Metal with an anodic oxidation coating colored by absorption of dye-stuff or pigments into the pore structure.
Combination color anodized metal: Metal with an anodic oxidation coating that is colored by electrolytic coloring or produced by integral color anodizing followed by absorption dyeing.
Interference color anodized metal: Metal with an anodic oxidation coating colored by means of optical interference effects.
Bright anodized metal: Anodized metal with a high specular reflectance as the primary characteristic.
Protective anodizing: Anodizing where protection against corrosion or wear is the primary characteristic and appearance is secondary or of no importance.
Decorative anodizing: Anodizing where a decorative finish with a uniform or a esthetically pleasing appearance is the primary characteristic.
Architectural anodizing: Anodizing to produce an architectural finish to be used in permanent, exterior and static situations where both appearance and long life are important.
Hard anodized metal: Anodized metal on which the anodic oxidation coating has been produced with wear and/or abrasion resistance as the primary characteristic.
Sealing: Treatment of anodic oxidation coatings on metal to reduce porosity and the absorption capacity of the coating by hydrothermal processes carried out after anodizing.
Cold impregnation: Treatment of anodic oxidation coatings on metal to plug the pores and reduce the absorption capacity of the coating by chemical processes carried out at low temperatures after anodizing.
Significant surface: The part of the product covered or to be covered by the coating and for which the coating is essential for serviceability and/or appearance.
Chemical treatment
Chemical brightening: Chemical treatment to improve the optical reflectivity of a surface.Chemical polishing: Polishing of a metal surface by immersion in a solution of chemical reagents.
Degreasing: Removal of oil or grease, usually by a suitable organic solvent or an aqueous detergent.
Etching: Roughening of the surface of a metal by overall or selective dissolution in acid or caustic media.
Pickling: Removal of a thin surface layer of a metal by chemical action, mainly by treatment in a caustic solution.
Coating
Coating (organic): Method in which a coating material is applied on a metallic substrate. This process includes cleaning and chemical pre-treatment and either:- one-side or two-side, single or multiple application of liquid or powder coating materials which are subsequently cured or
- laminating with plastic films.
Backing coat: Single coating of any type with no particular requirements for appearance, malleability, corrosion protection, etc. usually on the reverse side of the coated product.
Chemical conversion coating: Treatment of a metal with chemical solutions by dipping or spraying to build up an oxide film containing chromates or phosphates.
Priming: Application of a priming paint often pigmented with a corrosion inhibitor such as zinc chromate, after suitable pretreatment.
Pretreatment priming: Application of a solution containing a resin, a chromate and an acid, which is allowed to dry on and provide the key for subsequent painting.
Single coat system: Single coating either with requirements on appearance, malleability, corrosion protection, subsequent painting, etc., or as a primer with special properties regarding adhesion and corrosion protection for post-painting applications.
Multiple coat system: System comprising a primer or a base coat, possibly intermediate coat(s), and a top coat with particular requirements on appearance, malleability, corrosion protection, etc.
Organic coating: Dry paint film of the coated product or the organic film metal laminate.
Film coating: Organic film applied to a substrate to which an adhesive and, if appropriate, a primer has been applied beforehand.
Lacquering: Coating with a formulation based on a dissolved material which forms a transparent layer primarily after drying by evaporation of the solvent.
