Testing and inspection: the final step in assuring casting quality.
In this final installment of our 12-part series on the Metalcasting Process, a variety of destructive and nondestructive tests that are commonly used by foundries are discussed.
Every step in the metalcasting process--from part design to cleaning and finishing--plays a role in the ultimate quality level of a casting. Testing and inspection affords the foundryman with a last opportunity to verify that his product is defect free and what the customer expects.
Despite the growing influence of process control procedures, foundries must still be able to verify that the final casting meets the requirements of the customer. This is why testing and inspection remain a critical step in the metalcasting process.
Foundries use a variety of destructive and nondestructive testing techniques for identifying defects and determining physical and mechanical properties of castings and some of these are described below.
While destructive testing offers several advantages compared to nondestructive methods, like the reliable measure of the serviceability of metalcasting, it also possesses a number of limitations. Chief among these is the cost of such tests in terms of man-hours for properly preparing the part for testing and then carrying it out.
Also, the destruction of the part itself is a consideration, as is the cost of the equipment necessary to carry out the testing. Whether or not a foundry performs such tests should depend on customer requirements. Foundries without the capabilities of performing such tests should locate a reliable metallurgical laboratory with expertise in casting testing.
Tensile Testing--Tensile testing falls into the gray area between destructive and nondestructive testing. If separately cast test bars or coupons are used in the test, it may be considered nondestructive, but some argue that test bars and coupons do not accurately reflect the tensile strength of the actual castings in question.
On the other hand, if an actual casting section is used to provide a better indication ol casting strength, then obviously the casting becomes unusable. Under these conditions, tensile testing would have to be considered a destructive technique. However the test is utilized, tensile testing has long been recognized as an important method for determining properties of a casting (ultimate tensile strength, yield, elongation) and remains as the standard measurement of casting strength.
Tensile strength is the maximum load in tension that a material will withstand prior to fracture. In the case of ductile materials, fracture is preceded by elongation and consequent reduction in the cross-sectional area of the piece being tested. The maximum stress is reached just prior to the necking down of the test piece.
Tensile strength is then calculated from the maximum load applied during the test, divided by the original cross-sectional area, and is usually expressed in pounds per square inch (psi). Adaptation of computer techniques and other automated devices have added to the speed, accuracy and use of the tensile tester.
Impact Testing--Impact testing is a measurement of the ability of a metal to withstand sudden shock or impact. This test is considered an excellent criteria of performance for metals subjected to varying temperatures, including subzero temperatures.
The specially machined Charpy impact test specimen (V-notch or keyhole) is placed in position and the heavy head or pendulum of the impact testing machine is released so that it swings downward and fractures the test piece in a guillotine-like stroke. The tougher and stronger the material, the more energy will be consumed and the pendulum will have to travel a shorter distance past the point of impact. The reported result is in foot-pounds of energy absorbed by the test piece.
A variety of other destructive tests can be performed on a casting to determine physical and mechanical properties, but like all destructive testing techniques, they should be used judiciously. Sectioning (particularly for prototype and first run castings), fatigue testing and fracture toughness all are destructive testing methods that can be used in the properly equipped foundry or metallurgical laboratory when required for customer assurance.
Nondestructive testing (NDT), in which many of the technologies used are the result of advances made in just the last few decades, is one of the newer testing sciences. Long thought of as the province of the skilled craftsman, NDT used to be regarded as a shop floor activity, and only recently has it come within the sphere of the scientist/engineer.
Most NDT techniques for castings are designed for detecting external or internal imperfections or discontinuities, and are used most often with castings destined for critical load applications. In a technological era demanding metal components and structures of unprecedented reliability and efficiency and often requiring that constituent metals be exploited close to their ultimate capability, NDT takes on a critical role for the foundryman.
For many years, the most common nondestructive tests included liquid penetrant, magnetic particle, x-ray radiography, ultrasonic, eddy current testing and the old standby, hardness testing. The advent of the nuclear and space exploration ages expanded these to include leak detection and strain sensing, and refinements such as thermal and holographic testing.
Liquid Penetrant--Liquid penetrant inspection is a very sensitive process for locating surface defects in metals by the observance of highly visible liquid that penetrates exposed openings such as cracks, tears, pits, laps and into porous surfaces. The liquid is retained by these occlusions and remains during a surface rinsing operation.
Trapped liquid migrates to the surface after a thin coating of absorbent material, or developer, is applied to the casting under test. The visibility of the trace amount of liquid withdrawn from the defect into the developer is enhanced by an additive that may be a very bright dye or a compound that radiates ultraviolet light.
Used most often with steel, copper-based and aluminum castings, as well as other high alloys, it is one of the most widely used NDT inspection systems. Some drawbacks, such as it being dependent on the skill of the inspection operator, its sensitivity to high and low temperatures and being prone to error due to penetrant solution contamination, are, nonetheless, well within the average cost and experience parameters of many foundries.
Magnetic Particle--Magnetic particle testing uses the low-frequency surface magnetic field of a casting to detect cracks, seams, laps, voids, porosity and inclusions. Relatively simple and inexpensive, it has the advantage of sensing shallow subsurface flaws, as well, but is limited to use on ferromagnetic castings only.
The basic steps for using the magnetic particle testing procedure consist of magnetizing the test casting, applying magnetic particles in powder form to the test specimen, examining the surface area for tell-tale clusters of powder and, finally, demagnetizing the casting.
The proper choice of magnetizing current level is critical to the system's effectiveness. Too much current and the field gradient may be strong enough, even in flawless areas, to attract and hold particles over the entire casting surface, masking flaw indications. Too little current will fail to produce a field gradient sufficient to hold the particles in place around flaws.
The magnetic powders are available either colored with bright dye or treated with a fluorescent pigment. The particles vary in diameter (0.0025-0.0015 in.) and can be either round or elongated. The round ones move freely over the test surface; the elongated ones tend to be held more strongly to flaws. They are applied as a loose, dry powder or as a suspension in a liquid, hence, known as "wet" and "dry" methods. The powder is applied while the current is flowing through the casting (continuous) and when the current has stopped (residual).
The magnetic particle system, though highly efficient, has limitations:
* the empirical current level is critical to successful
* it is most effective when flaws and electrical current flow
are perpendicular to each other;
* it cannot be used on austenitic iron castings;
* it requires careful test surface cleaning;
* it is operator-dependent.
Eddy Current--Eddy current inspection is used to detect surface or subsurface flaws in electricity-conducting castings. It also can be used in evaluating such characteristics as hardness, heat treat conditions, alloy composition and variations in mass, shape, conductivity and permeability. It is called eddy current because of the action of electrical currents formed when a coil conducting an electric current is placed on or near the surface of a sample casting.
The electrical currents thus set up behave like circling water currents. Flaws within the test sample cause distortions in the electrical currents much like water swirls around a rock protruding above its surface. These flaw-induced electrical disturbances are reflected back to and can be read by an appropriate test coil and compared to an acceptable reference sample.
Advantages of eddy current inspection include:
* detects surface and subsurface flaws quickly;
* requires no special operator skill or training;
* adapts to product sorting by easily gaging products by
size, shape, plating or insulation thicknesses;
* indication of flaws is immediate;
* lends itself to automation and high-speed inspection;
* no probe contact is necessary.
Limitations of the eddy current system are its shallow depth penetration (in the range of 1/4 in.), the materials inspected must be electrically conductive and readouts can be influenced by more than one variable.
Ultrasonics--Ultrasonic inspection beams high frequency acoustic energy into a casting to detect surface and subsurface flaws and to measure the thickness of the part or the distance to the flaw. An ultrasonic beam will travel through a casting until it strikes a discontinuity, or flaw, such as a void, inclusion or crack.
Through either pulse-echo or other transmission techniques, the presence, location and relative size of casting discontinuities can be determined. These flaws interrupt the beam and reflect a portion of it back to a CRT imager or other read out scale for interpretation by an inspector who "maps" the returning sound waves to precisely locate the flaw.
The casting and the beam of high-frequency acoustic energy must be linked by a thin layer of liquid or semiliquid between the active surface of the electrical transducer, or beam generator, and the test casting. This layer is called a coupling medium or a coulplant. The amount of energy reflected is a function of the type and orientation of the flaw.
In application, a series of electrical pulses is applied to a transducer which converts the pulses into mechanical energy. The transducer is located in a holder so it can transmit the waves into the casting through the couplant. The assembly of the transducer, a wear surface and holder with an electrical connector, is called the search unit.
Pulsed energy is transmitted into the casting and is reflected back into the search unit by the boundaries of the casting or by interrupting discontinuities. The echoes return to the search unit, where they are converted back from mechanical to electrical energy, and amplified by a receiver. By adjusting the pulsed sweep of the beam, the operator can pinpoint the location of reflected flaws.
A wide range of ultrasonic frequency testing equipment is available. High frequencies can detect smaller defects, but low frequencies have greater penetrating ability. This is particularly true in the case of iron castings, because graphite scatters the ultrasonic pulse. Low frequencies are better for coarser flake graphite irons or for penetrating greater thickesses of iron. Ultrasonic testing is most often used to test cast iron, aluminum and steel castings.
The advantages of ultrasonic testing include:
* high sensitivity, ideal for detection of minute cracks;
* great penetrating power, good for testing thick
* accuracy in measuring flaw position and size.
Radiography--Radiographic testing is based on exposure of a casting to differential absorption rates of penetrating radiation in the form of x rays or gamma rays. Because of differences in density and variations in thickness of the test part or because of differences in absorption characteristics caused by variations in composition, different portions of a test piece absorb different amounts of penetrating radiation.
Unabsorbed radiation passing through the casting can be recorded on film or photosensitive (xerographic) paper, viewed continuously (real time) on a fluorescent screen or monitored by various types of electronic radiation detectors. On radiographic film, the image of a discontinuity or void appears in most instances as a dark shadow, representing the local increase in the transmission of radiation because of the effective reduction in metal thickness in the path of the beam.
Some inclusions in light alloys, notably aluminum oxide, reduce transmission and their images appear lighter than the matrix. Radiographic testing can be used with virtually any type of casting, usually for detecting conditions such as porosity and shrinkage. While very accurate in metalcasting inspection, it requires a skilled operator to successfully interpret results.
Sonic Testing--The sonic method of NDT testing is used to assess the graphite form in iron castings and test bars. It is most profitably applied when it is necessary to check the form of nodular graphite in ductile iron castings. It can be used to predict tensile strength when, under routine testing conditions, the matrix structure remains constant but when the form of the graphite may vary.
Sonic testing relies on a measurement of a resonant frequency of the casting or test bar. This resonant frequency is governed by the elastic constants of the material and by its shape and size. The elastic constants vary with the graphite form; nodular graphite irons have the lowest elastic moduli.
Castings or test bars are placed between two transducers with small air gaps between the transducer and test piece. Electronic methods of determining the resonant frequency are then used with either a manual tuning system or automatic frequency selection. This NDT technique does have certain limitations. Calibration curves are needed for each casting design to be tested.
To produce the curves, destructive tests to assess the tensile strength are required. The iron matrix also must remain constant. Castings that have been heat treated are usually not suitable for sonic testing unless other tests (hardness or eddy current) are used to check the matrix structure. Also, sonic testing is not easy to use on heavy section castings and does not detect defects such as pinholes, porosity and cracks.
Hardness Testing--Technically a mechanical test procedure, hardness testing is the simplest and often the most versatile testing tool available to the failure analyst. In the case of metals, hardness is commonly considered to be the resistance to penetration and is generally used as an index of strength and wear resistance.
Among its many applications, hardness testing can be used to evaluate heat treating, to provide an approximation of tensile strength and to detect work hardening or softening or hardening caused by overheating, decarburization or carbon or nitrogen pickup.
The most common method of measuring hardness in the foundry is the Brinell test, which is particularly effective on soft materials. A steel ball (normally 10mm in diameter, 0.304 in.) is pressed onto the test casting using a constant pressure (usually 3000 kg or approximately 3-1/4 tons) for a specific time period (about 30 sec minimum).
The diameter of the resultant impression is then measured with a microscope and a Brinell hardness number (Bhn) calculated according to a predetermined formula. On very hard materials, the steel ball tends to deform slightly in which case hardened or tungsten balls are used.
The Rockwell hardness test measures the depth of residual penetration by a steel or diamond cone under fixed load conditions. A minor preload of ten kg is applied in order to seat the penetrator. A major load of 60,100 lb or 50 kg is then applied and released and the difference between the major and minor load is automatically registered by an indicator which reads directly the Rockwell hardness number.
The Rockwell C scale is used for hard materials (Brale penetrator and 150 kg of major load). The B scale is used for soft steels (1/16 in. diameter steel ball and 100 kg major load). For softer materials, the E scale is used (1/8 in. steel ball and 100 kg major load). For extremely hard nitrided materials, the A scale is used (Brale penetrator and 60 kg of major load).
The Vickers diamond pyramid test (dph), originated in the aircraft industry, is an effective method for precision work. It is suited to test extremely hard materials, but generally is not used for on-site production use.
The principle is similar to the Brinell test with a square base pyramid-shaped, chipped diamond being used to produce the impression which is read with a microscope. The hardness number is calculated from the load and area so that the result is not affected by variations in the load. The load, therefore, may be varied to suit the thickness of the test specimen.
The Knoop hardess test determines the hardness from the resistance of metal to indentation by a pyramidal diamond indentor with edged angles of 170 [degrees] 30 min and 130 [degrees] which makes a rhomboid impression with one long and one short diagonal. The impression also is read with the aid of a microscope.
Conversion tables, which show the relationship between diamond pyramid hardness, Rockwell hardness, superficial Rockwell and Brinell hardness numbers, are available. The use of such tables is recommended in converting results obtained from Rockwell, Brinell and diamond pyramid only on flat surfaces. Conversion of hardness values should be used only when it is impossible to test the material under the conditions specified.
When conversion is made, it should be done with discretion and under controlled conditions. Each type of hardness test is subject to certain errors, but, if precautions are carefully observed, the reliability of hardness readings made on instruments of the indentation type will be found comparable. Important guidelines for hardness testing can be found in ASTM specifications.
File Test--Another chill test than can be done once the casting has solidified and been removed from the mold is the file test. This is a quick and inexpensive test used to determine if iron castings have chilled edges.
Chilled edges can occur when liquid metal solidifies too quickly for the carbon to come out of solution as graphite. Instead, the carbon forms carbides, resulting in hard spots in the casting. This is most apt to happen on casting edges because of the faster cooling rates in those areas. Carbides are very hard and cause difficulty in machining.
The file test consists merely of trying to make a notch in a casting with the corner of a common metal file. If the metal solidified properly, without massive carbides, the file will notch the casting easily. If the test edge is chilled, the file will skip over the metal without making an impression.
Leak Testing--When castings will be required to withstand pressures, they may be leak tested in the foundry in a variety of ways, including listening, submerged bubble testing, soap bubble testing, flow detection and specific gas detection.
In one method, air is pumped at a specified pressure into the casting, which is then submerged in water at a given temperature. Any escaping air can be observed as a string of bubbles escaping through a faulty section.
Another method is to fill the casting cavity with pressurized parafin, which can penetrate into the smallest crevices or porous metal occlusions. Escaping parafin will show up as an oily or moist patch on the casting surface.
One of the earliest ways of detecting escaping air through a cast metal pressure vessel involved the use of soap bubbles applied to a test surface and observing the formation of the bubbles. Depending on the foundry customer's requirements, leak detection can be as simple as detecting bubbles or smelling the particular odor of an escaping gas.
Others might involve using materials that change color in contact with certain gases, or more sophisticated instruments that measure changes in air/gas mixtures, trace-sensitive gas chromatography, infrared radiation detection, mass spectrometers or even nuclear radiation detectors used with radioactive trace gases.
PHOTO : An inspector, checking the hardness of a metal casting, uses a specially-calibrated
PHOTO : microscope to gage the size of the indentation caused by a steel ball pressed into the
PHOTO : metal casting.
PHOTO : A real-time x-ray image processor station uses a preprogrammed inspection sequencer that
PHOTO : responds to touch-screen commands by the inspector. It allows for recall of set
PHOTO : inspection routines or lets the operator program special procedures. The color monitor
PHOTO : shows the casting as it moves through a computer-controlled inspection cycle.
PHOTO : An operator prepares a metal test bar sample in a tensile testing machine to test its
PHOTO : mechanical properties by measuring the bar's resistance to the stresses induced by being
PHOTO : pulled apart.
PHOTO : Using a computer-guided layout inspection machine, this operator can validate the accuracy
PHOTO : of a die prior to committing it to production.
David P. Kanicki, Publisher/Editor Tom Bex, Senior Editor
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|Title Annotation:||The Metalcasting Process, Part 12 of 12.|
|Date:||Dec 1, 1989|
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