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In the foundry industry there are many techniques that have been developed to aid in assuring that quality is built into the castings. Below is a short discussion outlining some of the basic techniques. This is meant to provide some familiarity with the terminology and is not a complete survey. METALLURGICAL CONTROLS Certified Ingot- ingot that comes directly from a smelter or metal refiner and that is certified to meet specific specifications. Certified ingot provides a high degree of reliability. Chemical Analysis- a spectrometer is the most common and readily accepted method of testing metal chemistries. A spectrometer works by analyzing the "spectrum" of light emitted from a sample of material when it is burned. A polished sample is burned by a high energy spark in an inert atmosphere of argon. The light emitted from the burn passes through a series of lenses and a diffraction grating. The diffraction grating breaks the light down into its constituent wavelengths. Each metallic element emits unique wavelengths of light when burned. Photo receptors are placed where they can receive one band of light that is indicative of each element. The relative intensity of the light is measured by the photoreceptor and compared to a known standard. From this the amount of each element present in the sample is measured. Spectrometry is normally sufficient for foundry purposes. It is claimed to be accurate within plus or minus one percent of the measured amount. If greater accuracy is required other testing methods must be used. Independent Laboratories- occasionally chemical analyses must be done by an independent laboratory. This may be done because very high accuracy is required, to use specific problem solving methods or to get a second, independent opinion. Outside labs may utilize exotic testing equipment including atomic absorption and Scanning Electron Microscopy (SEM) with special scanning devices that can detect the presence and composition of individual elements and map the locations of the concentrations on a photograph. Leco Chemistries- some elements, especially carbon in cast iron and sulphur in many metals, are not always accurately measured by spectrometers. Testing equipment manufactured by Leco Corporation was created to solve these problems economically for the foundryman. This equipment is considered very reliable. Metallography- analysis of the microstructure of metals. An investigative methodthat can reveal many things about chemistry, strength and process. In metallography a polished sample of the metal is observed through a microscope. The sample is often etched to reveal more of the detail of the microstructure. Test Bars- test bars are used to determine mechanical properties of cast metal. There are many types of test bar that can be used depending on the particular property being investigated. Test bars can be poured in separately made molds or they can be cut directly from a casting. The latter, of course, results in the destruction of the casting. Test bars can serve two purposes. One is to assure that the process is performing as it should. The second is to assure that a specific casting or group of castings meets the required properties. SAND TESTING METHODS Clay Content- must be controlled within a given range. Excess clay requires additional water to temper it. The extra water can contribute to rough finish and hydrogen blowholes in the casting. Normally clay is measured using a methylene blue test. Methylene blue uses a simple titration method to indicate the amount of clay. Compactability- measures the amount that a sand can be compacted under a given kinetic load. Sand is funneled into a metal tube about 1.5 inches inside diameter by 4 inches long. Excess sand is struck away from the top of the tube. A heavy weight is raised a known distance by a cam and dropped on the sand 3 times. The amount of compaction is measured as a percent of the original height of the column of sand. This is the compactability. It is a very good test of the effective balance between clay and water in the sand. It is one of the easiest and most useful tests that can be performed in sand control. It is also an excellent candidate for SPC. Grain Distribution- grain size is a measure of the size of individual sand grains in fractions of an inch. An 80 size grain means the grains pass through a screen with 80 openings per inch. The size of the grains is measured by shaking them through a stack of successively finer screens. The amount remaining on each screen is measured and recorded. The relative amount of sand on each screen is the distribution. Normally sands have about the same amount left on four adjacent screens and then taper off for larger and smaller grains. If graphed the distribution of a good sand would make a bell shaped curve. Grain Fineness Number- a method developed by the American Foundrymen's Society for evaluating the typical grain size of a sample of sand. Green Compression Strength- a measure of the maximum load required to break a sample of tempered sand. It is an important indicator of how well a sand will hold shapes in molds. This is especially important when a large amount of sand is being suspended in the cope side of a mold. LOI- an acronym for Loss On Ignition. It is a measure of the combustible constituents of a sand. A sample of sand is heated to very high temperatures so that anything that can burn off will do so. The sample is weighed before and after heating and the amount lost is expressed as a percentage of the original weight. Moisture Content- moisture must be controlled as closely as possible. Excess moisture will result in rough castings and hydrogen blows. Too little moisture will result in weak sand. Moisture is normally measured by taking a 100 gram sample of sand, evaporating the moisture, then weighing it. If the dried sample weighs 97 grams then the moisture content was 3%. Mold Hardness- a test to evaluate the thoroughness of the compression of sand in a mold by measuring the mold's hardness. Permeability- tests the degree to which gasses can pass through a sand. A sample is placed on a blower that measures the resistance to air passing through the sample. The permeability reading is read from a scale on the blower. Shear Strength- measures the maximum shear stress that a sand is capable of developing. Tensile Strength- measures the maximum tensile stress that a sample of sand is capable of sustaining. PROCESS CONTROL METHODS Incoming Shipment Audits- foundries should be auditing their receipts of raw materials to assure they meet the standards set between the foundry and the supplier. SPC- statistical process control refers to statistical methods used by many progressive foundries today to assure that quality is being built into the castings as they are made rather than inspected in after they are made. One method involves the measurement of a given parameter on a periodic basis. Statistical studies are done on the results of the measurements . The capabilities of the process are found from the statistics. If a measurement occurs outside of the normal capabilities of the process it is said to be out of control and action is taken to determine and correct the cause of the deviation. Supplier Evaluation- many supplier evaluation forms are intended for evaluation of machine shops and do not apply to foundries. This is even true of Mil I45208-A. Many foundries can comply with the applicable portions of those surveys. A few have also developed their own conformance form. CASTING INSPECTION METHODS (NON-DESTRUCTIVE) Brinell Hardness- a method of checking the hardness of a casting by indenting the metal with a 10 mm ball under either 500 or 3000 kg of force. The diameter of the indentation is measured and then translated into a relative value of hardness. Brinell is the recommended method of hardness evaluation for castings. Dye Penetrant Inspection- castings are coated with a fluorescent dye. The dye is then washed from the surface. If there are any fissures or voids in the casting thedye will remain in them after washing. The dye glows under special lighting revealing the defects. Eddy Current- uses an electric coil to induce a current in the casting. Changes in reactance and resistance in the coil indicate discontinuities in the casting. Magnaglo and Magnaflux- both proprietary names for magnetic crack detection methods. Magnetic force and fine iron powder are applied to the casting. The powder collects in cracks where it can be readily detected. In Magnaglo the powder is treated so it glows under ultraviolet light. Radiography- x-rays or gamma rays are used to measure the soundness of a casting. The military has developed a rating system for the size and frequency of voids found. Rockwell Hardness- method of determining hardness by measuring the indentation in the metal left by a steel ball or diamond cone under pressure. Not recommended for heterogenous metals. Ultrasonic Testing- high frequncy sound is used to locate discontinuities in castings. CERTIFICATIONS OF CASTINGS Often certifications are required to assure the customer they are getting what they expect. Certifications can range from merely sending a certificate of conformance to doing destruction and testing of one or more castings from every heat of metal. The costs of certifying can be as wide ranging. Bear in mind that any certification costs money and even if the foundry doesn't charge you outright for it, you can be sure you are paying for it. Many certifications are very expensive. If you don't absolutely need it, don't ask for it. It will quickly inflate the net cost of your castings. A certificate of conformance is a document which states that the foundry has met the requirements of the blueprint and the purchase order. No testing has been required. It is a formal way of giving the foundry's word that they have performed as required. A certificate of conformance is often all that is needed. It usually is done without charge. It should, however be requested at the time of order to be sure it will be done with the order. A foundry with good process control will verify the heat of castings against their process testing records when doing certificates of conformance. When requesting certifications it is a good idea to request date coding of castings so different shipments can be identified in your plant even after a long period of time. |
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