Nondestructive Evaluation and Quality Control (1998) Part 12 pps

Fig. 14 Schematic of the effect of casting shapes on reflection and oscilloscope screen display of sound beams. See text for discussion. Many castings contain cored holes and changes in section, and echoes from holes and changes in section can interfere with echoes from discontinuities. As shown in Fig. 14(c), the echo from the cored hole overlaps the echo from the discontinuity on the oscilloscope screen. The same effect is shown in Fig. 14(d), in which echoes from the discontinuity and the casting fillets at a change in section are shown overlapping on the oscilloscope. Curved surfaces do not permit adequate or easy coupling of the flat search units to the casting surface, especially with contact double search units. This can be overcome to some extent by using a suitable viscous couplant, but misleading results may be produced because multiple reflections in the wedge of fluid between the search unit and the surface can result in echoes on the screen in those positions where discontinuity echoes may be expected to appear. Because the reflections inside the couplant use energy that would otherwise pass into the casting, the back echo decreases, and this decrease might be interpreted as confirmation of the presence of a discontinuity. On cylindrical surfaces, the indication will change as a double search unit is rotated. The wedge effect is least when the division between the transmitting and receiving transducers is parallel to the axis of the cylinder. Wedge effects in the couplant are a particular problem on castings curved in two directions. One solution in this case is to use a small search unit so that the wedge is short, although the resolution and sensitivity may be reduced. If the surface of the casting to be inspected is of regular shape, such as the bore of a cylinder in an engine block, the front of the search unit can be shaped to fit the curvature of the surface. These curved shapes form an acoustic lens that will alter the shape of the sound beam, but unless the curvature is severe, this will not prevent adequate accuracy in the inspection. Cast-on flat metal pads for application of the ultrasonic search unit are very effective and allow particular areas of the casting to be inspected. Subsurface Defects. Defects such as small blowholes, pinholes, or inclusions that occur within depths of 3 or 4 mm (0.1 or 0.15 in.) of a cast surface are among the most difficult to detect (Ref 1). They are beyond the limits of sensitivity of conventional magnetic particle methods and are not easily identified by eddy current techniques. They usually fall within the dead zone (the surface layer that cannot be inspected) of conventional single-crystal ultrasonic probes applied directly to a cast surface, although some improvement can be obtained by using twin crystal probes focused to depths not too far below the surface. The other alternative using contact methods of ultrasonic testing is to employ angle probes, but this complicates the procedures and interpretation methods to the point at which they can only be applied satisfactorily under the close control of skilled operators. Freedom from such surface defects is, however, a very important aspect of the quality of castings. Apart from their effect in reducing bending fatigue properties, such defects are frequently revealed at late stages in the machining of a component, leading to its rejection. Ultrasonic methods for detecting subsurface defects are much more successful when the dead zone beneath the as-cast surface is virtually eliminated by using immersion methods in which the probe is held away from the cast surface at a known controlled distance, with coupling being obtained through a liquid bath. To make such methods consistent and reliable, the test itself must be automated. Semiautomatic equipment has been developed for examining castings such as cylinder heads by this method (Ref 1). With this equipment, the casting is loaded into a cradle from a roller track and is then transferred using a hoist into the immersion tank until the surface of the casting to be inspected is just submerged in the liquid. Depth of immersion is closely controlled because the customer will not permit liquid to be left in the internal passageways of the cylinder head. The immersed surface of the casting is then scanned manually using an ultrasonic probe held at a fixed distance from the casting surface. This equipment is suitable for testing any casting requiring examination over a flat surface. Internal Defects. Ultrasonic inspection is a well-established method for the detection of internal defects in castings. Test equipment developments, automated testing procedures, and improvements in determining the size and position of defects, which is essential to assessing whether or not their presence will likely affect the service performance of the casting, have contributed to the increasing use of ultrasonic test equipment. For determining the position and size of defects, the usual method of presentation of ultrasonic data is an A-scan, in which the amplitude of the echoes from defects is shown on a time base and has well-known limitations (Ref 1). Sizing relies on measuring the drop in amplitude of the echo as the probe is passed over the boundary of a defect or measuring the reduction in the amplitude of the back wall echo due to the scattering of sound by the defect. In most cases, sizing is approximate and is restricted to one or two dimensions. Improvements in data presentation in the form of B-scans and C- scans that present a plan view through the section of the component provide a marked improvement in defining defect positions and size in two or three dimensions. Such displays have been used for automated defect characterization systems in which porosity, cracks, and dross have been distinguished. Because of the requirement to scan the probe over the surface, the application of B-scan and C-scan methods has generally been limited to simple geometric shapes having good surface finish, such as welded plate structures. Application to castings is currently restricted, but greater use of B- scan and C-scan methods is likely with either improved scanning systems or arrays of ultrasonic probes. Structure evaluation is an area of growing importance for foundry engineers. Ultrasonic velocity measurements are widely used as a means of guaranteeing the nodularity of the graphite structure and, if the matrix structure is known to be consistent, guaranteeing the principal material properties of ductile irons (Ref 1). Velocity measurements have been used to evaluate compacted graphite iron structures to ensure that the desired properties have been consistently obtained. The use of ultrasonic velocity measurements for structure evaluation is discussed in the section "Ductile Iron Castings" in this article. Reference cited in this section 1. P.J. Rickards, Progress in Guaranteeing Quality Through Nondestructive Methods of Evaluation, Foundryman Int., April 1988, p 196-209 Nondestructive Inspection of Castings By the ASM Committee on Nondestructive Inspection of Castings * Leak Testing Castings that are intended to withstand pressures can be leak tested in the foundry. Various methods are used, according to the type of metal being tested (see the article "Leak Testing" in this Volume). One method consists of pumping air at a specific pressure into the inside of the casting in water at a given temperature. Any leaks through the casting become apparent by the release of bubbles of air through the faulty portions. An alternative method is to fill the cavities of a casting with paraffin at a specified pressure. Paraffin, which penetrates the smallest of crevices, will rapidly find any defect, such as porosity, and will show quickly as an oily or moist patch at the position of the fault. Liquid penetrants can be poured into areas of apparent porosity and time allowed for the liquid to seep through the casting wall. The pressure testing of rough (unmachined) castings at the foundry may not reveal any leaks, but it must be recognized that subsequent machining operations on the casting may cut into porous areas and cause the casting to leak after machining. Minor seepage leaks can be sealed by impregnation of the casting with liquid or by filling with sodium silicate, a synthetic resin, or other suitable substance. As-cast parts can be impregnated at the foundry to seal leaks if there is to be little machining or if experience has shown that machining does not affect the pressure tightness. However, it is usually preferable to impregnate after final machining of the casting. Nondestructive Inspection of Castings By the ASM Committee on Nondestructive Inspection of Castings * Inspection of Ferrous Castings Ferrous castings can be inspected by most of the nondestructive inspection methods. Magnetic particle inspection can be applied to ferrous metals with excellent sensitivity, although a crack in a ferrous casting can often be seen by visual inspection. Magnetic particle inspection provides good crack delineation, but the method should not be used to detect other defects. Nonrelevant magnetic particle indications occasionally occur on ferrous castings, especially with a strong magnetic field. For example, a properly fused-in steel chaplet can be indicated as a defect because of the difference in magnetic response between low-carbon steel and cast iron. Even the graphite in cast iron, which is nonmagnetic, can cause a nonrelevant indication. Standard x-ray and radioactive-source techniques can be used to make radiographs of ferrous castings, but the typical complexity of shape and varying section thicknesses of the castings may require digital radiography or computed tomography. Ultrasonic inspection for both thickness and defects is practical with most ferrous castings except for the high-carbon gray iron castings, which have a high damping capacity and absorb much of the input energy. The measurement of resonant frequency is a good method of inspecting some ductile iron castings for soundness and graphite shape. Electromagnetic testing can be used to distinguish metallurgical differences between castings. The criteria for separating acceptable from unacceptable castings must be established empirically for each casting lot. Gray Iron Castings Gray iron castings are susceptible to most of the imperfections generally associated with castings, with additional problems resulting from the relatively high pouring temperatures. These additional problems result in a higher incidence of gas entrapment, inclusions, poor metal structure, interrupted metal walls, and mold wall deficiencies. Gas entrapment is a direct result of gas being trapped in the casting wall during solidification. This gas may be in the metal prior to pouring, may be generated from aspiration during pouring, or may be generated from core and mold materials. Internal defects of this type are best detected by radiography, but ultrasonic and eddy current inspection methods are useful when the defect is large enough to be detected by these methods. Inclusions are casting defects in which solid foreign materials are trapped in the casting wall. The inclusion material can be slag generated in the melting process, or it can be fragments of refractory, mold sand, core aggregate, or other materials used in the casting process. Inclusions appear most often on the casting surface and are usually detected by visual inspection, but in many cases the internal walls of castings contain inclusions that cannot be visually detected. Internal inclusions can be detected by eddy current, radiographic, or ultrasonic inspection; radiography is usually the most reliable method. Poor Metal Structure. Many casting defects resulting from metal structure are related to shrinkage, which is either a cavity or a spongy area linked with dendrites or a depression in the casting surface. This type of defect arises from varying rates of contraction while the metal is changing from a liquid to a solid. Other casting defects resulting from varying rates of contraction during solidification include carbide formation, hardness variations, and microporosity. Internal shrinkage defects are best detected by radiography, although eddy current or ultrasonic inspection can be used. Soft or hard gray iron castings are usually detected by Brinell hardness testing; electromagnetic methods have proved useful on some castings. Interrupted Metal Walls. Included in this category are such flaws as hot tears, cold shuts, and casting cracks. Cracking of castings is often a major problem in gray iron foundries because of the combination of casting designs and high production rates. Visual inspection or an aided visual method such as liquid penetrant or magnetic particle inspection is used to detect cracks and cracklike flaws in castings. Mold wall deficiencies are common problems in gray iron castings. They result in surface flaws such as scabs, rattails, cuts, washes, buckles, drops, and excessive metal penetration into the spaces between sand grains. These flaws are generally detected by visual inspection. Size and Quantity of Graphite. In cast iron, the length of the lamellae (flakes) that is, the coarseness of the graphite is expressed by code numbers from 1 to 8, as described in ASTM A 247 and in Casting, Volume 15 of ASM Handbook, formerly 9th Edition Metals Handbook. These numbers correspond to lamellar lengths ranging from 1.25 to 0.01 mm (0.05 to 0.0004 in.), as viewed in micrographs of the cast iron structure. The dependence of the longitudinal wave sonic velocity on the size and quantity of graphite is shown in Fig. 15; with decreasing coarseness and quantity of graphite, the velocity approaches that in steel. In microlamellar cast iron, the amount of graphite is usually smaller. Therefore, the finer the graphite, the higher the sonic velocity. In both lamellar and spheroidal cast irons, the transverse wave sonic velocity is related to the longitudinal wave velocity, as shown in Fig. 16. Fig. 15 Variation of longitudinal wave sonic velocity with graphite size for lamellar cast iron containing different percentages of free graphite. Scale at right indicates ratio of sonic velocity in cast iron to corresponding sonic velocity in steel. Fig. 16 Relation of longitudinal wave and transverse wave sonic velocities for spheroidal and lamellar cast irons and for steel Malleable Iron Castings Blowholes and spikes are defects often found in malleable iron castings. Spikes are a form of surface shrinkage not normally visible to the naked eye, but they appear as a multitude of short, discontinuous surface cracks when subjected to fluorescent magnetic particle inspection. Unlike true fractures, spikes do not propagate, but they are not acceptable where cyclic loading could result in fatigue failure. Spikes are usually seen as short indications about 1.6 mm ( in.) long or less and never more than 75 μm (0.003 in.) deep. These defects do not have a preferred orientation, but a random pattern that may or may not follow the direction of solidification. Shrinkage or open structure in the gated area is a defect often found in malleable iron castings that may be overlooked by visual inspection, although it is readily detected by either liquid penetrant or magnetic particle inspection. Ductile Iron Castings Ductile iron is cast iron in which the graphite is present in tiny balls or spherulites instead of flakes (as in gray iron) or compacted aggregates (as in malleable iron). The spheroidal graphite structure is produced by the addition of one or more elements to the molten metal. Soundness and Integrity. Cracks and fine tears that break the surface of the casting but are difficult to detect visually can be revealed with dye penetrants or magnetic particle inspection. Modern techniques of magnetizing the casting, followed by the application of fluorescent magnetic inks, are very effective and widely used. Methods of sonic testing that involve vibrating the casting and noting electronically the rate of decay of resonant frequency or damping behavior are also used to detect cracked or flawed castings. Internal unsoundness, when not immediately subsurface, can be detected with ultrasonic inspection by the failure to observe a back wall echo when using reflected radiation or by a weakening of the signal in the transmission through the casting. Coupling of the probes and interpretation of the results involve operator skill, but methods are available that consist of partial or total immersion of the casting in a liquid, automatic or semiautomatic handling of the probes, and computer signal processing to ensure more reliable and consistent interpretation of results. Problems arise in detecting very-near-surface defects and when examining thin castings, but the use of angled probes and shear wave techniques has yielded good results. The soundness of the ductile iron can also be assessed by x-ray or γ-ray examination. The presence of graphite, especially in heavy sections, makes the method more difficult to evaluate than for steels, but the use of image intensification by electronic means offers considerable promise, especially for sections up to 50 mm (2 in.) thick. Confirmation of Graphite Structure (Ref 4). Both the velocity of ultrasonic transmission and the resonant frequency of a casting can be related to the modulus of elasticity. In cast iron, the change from flake graphite to nodular graphite is related to an increase in both modulus of elasticity and strength; therefore, ultrasonic velocity or resonant frequency measurement can be employed as a guide to modularity, strength, and other related properties. Because the microscopic estimation of nodularity is a subjective measurement, these other nondestructive examination methods may provide a better guide to some properties provided the matrix remains constant (Ref 5). Figure 17 illustrates how ultrasonic velocity may vary with graphite nodularity. Fig. 17 Ultrasonic velocity versus visually assessed modularity in ductile iron castings Ultrasonic transmission measurement is conducted with two probes on either side of the casting. This method provides a guide to the local properties. It must be coupled with a thickness measurement, and automatic equipment is commonly used, often involving immersion of the casting in a tank of fluid (Fig. 18). The calculation does not require calibration of the castings. Simple caliper devices have also been used for examining castings and simultaneously measuring their thickness to provide a calculated value of ultrasonic velocity and a guarantee of modularity (Ref 6). Fig. 18 Ultrasonic test equipment used for determining the thickness, nodularity, and integrity of ductile iron castings. (a) Schematic of setup for the ultrasonic velocity testing for structure eva luation. (b) Photograph of test instrument used for integrity/nodularity studies showing controls and instrumentations. Courtesy of J. Johnston, Krautkramer Branson Sonic testing involves measurement of the resonant frequency of the casting or rate of decay of resonance of a casting that has first been excited by mechanical or electrical means. This method evaluates the graphite structure of the entire casting and requires calibration of the castings against standard castings of known structure. It is also necessary to maintain casting dimensions within a well-controlled narrow range. Some foundries use sonic testing as a routine method of final inspection and structure guarantee (Ref 7). The relationship between modularity, resonant frequency, or ultrasonic transmission velocity and properties has been documented for tensile strength, proof strength, fatigue, and impact strength. Examples are shown in Fig. 19 and 20. Fig. 19 Ultrasonic velocity versus strength in ductile iron castings Fig. 20 Strength versus resonant frequency in the nondestructive evaluation of ductile iron test bars The presence of carbides can also be detected with sonic or ultrasonic measurements provided enough carbides are present to reduce the graphite sufficiently to affect the modulus of elasticity. Discrimination between the effects of graphite variation and carbide amount would require an additional test, such as a hardness test. Properties Partially Dependent on Graphite Structure (Ref 7). When the matrix structure of ductile iron varies, this variation cannot be detected as easily as variations in graphite structure, and sonic and ultrasonic readings may not be able to reflect variations in mechanical properties. A second measurement, such as a hardness measurement, is then needed to detect matrix variations in the same way as would be necessary to confirm the presence of carbides. Eddy current or coercive force measurements can be used to detect many changes in casting structure and properties; but the indications from such measurements are difficult to interpret, and the test is difficult to apply to many castings unless they are quite small and can be passed through a coil 100 to 200 mm (4 to 8 in.) in diameter. Eddy current indications are, however, useful for evaluating pearlite and carbide in the iron matrix. Multifrequency eddy current testing uses probes that do not require the casting to pass through a coil, it is less sensitive to casting size, and it allows automatic measurements and calculations to be made; but the results remain difficult to interpret with reliability in all cases. It may, however, be a very good way to detect chill and hard edges on castings of reproducible dimensions. References cited in this section 4. A.G. Fuller, P.J. Emerson, and G.F.Sergeant, A Report on the Effect Upon Mechanical Properties of Variation in Graphite Form in Irons Having Varying Amounts of Ferrite and Pearlite in the Matrix Structure and the Use of Nondestructive Tests in the Assessments of Mechanical Properties of Such Irons, Trans. AFS, Vol 88, 1980, p 21-50 5. A.G. Fuller, Evaluation of the Graphite Form in Pearlitic Ductile Iron by Ultrasonic and Sonic Testing and the Effect of Graphite Form on Mechanical Properties, Trans. AFS, Vol 85, 1977, p 509-526 6. P.J. Rickards, "Progress in Guaranteeing Quality Through Non- Destructive Methods of Evaluation," Paper 21, presented at the 54th International Foundry Congress, New Delhi, The International Committee of Foundry Technical Associations (CIATF), Nov 1987 7. A.G. Fuller, Nondestructive Assessment of the Properties of Ductile Iron Castings, Trans. AFS, Vol 88, 1980, p 751-768 Nondestructive Inspection of Castings By the ASM Committee on Nondestructive Inspection of Castings * Inspection of Aluminum Alloy Castings Effective quality control is needed at every step in the production of an aluminum alloy casting, from selection of the casting method, casting design, and alloy to mold production, foundry technique, machining, finishing, and inspection. Visual methods, such as visual inspection, pressure testing, liquid penetrant inspection, ultrasonic inspection, radiographic inspection, and metallographic examination, can be used to inspect for casting quality. The inspection procedure used should be geared toward the specified level of quality. Information on casting processes, solidification, hydrogen content, silicon modification, grain refinement, and other topics related to aluminum alloy castings is provided in the articles "Solidification of Eutectic Alloys: Aluminum-Silicon Alloys," "Nonferrous Molten Metal Processes," and "Aluminum and Aluminum Alloys" in Casting, Volume 15 of ASM Handboook, formerly 9th Edition Metals Handbook. Stages of Inspection. Inspection can be divided into three stages: preliminary, intermediate, and final. After tests are conducted on the melt for hydrogen content, for adequacy of silicon modification, and for degree of grain refinement, preliminary inspection may consist of the inspection and testing of test bars cast with the molten alloy at the same time the production castings are poured. These test bars are used to check the quality of the alloy and the effectiveness of the heat treatment. Preliminary inspection also includes chemical or spectrographic analysis of the casting, thus ensuring that the melting and pouring operations have resulted in an alloy of the desired composition. Intermediate inspection, or hot inspection, is performed on the casting as it is taken from the mold. This step is essential because castings that are obviously defective can be discarded at this stage of production. Castings that are judged unacceptable at this stage can then be considered for salvage by impregnation, welding, or other methods, depending on [...]... High Pressure Coining, and High Pressure Repressing of P/M Parts," Paper presented at the Prevention and Detection of Cracks in Ferrous P/M Parts Seminar, Metal Powder Industries Federation, 1988 Nondestructive Inspection of Powder Metallurgy Parts R.C O'Brien and W.B James, Hoeganaes Corporation Summary of Defect Types in P/M Parts The four most common types of defects in P/M parts are ejection cracks,... in this section 5 F.V Lenel, Powder Metallurgy Principles and Application, Metal Powder Industries Federation, 1980, p 112 Nondestructive Inspection of Powder Metallurgy Parts R.C O'Brien and W.B James, Hoeganaes Corporation Nondestructive Tests and Their Applicability to P/M Processing As described below, applicable inspection methods for P/M parts can be broadly classified into the following categories:... techniques for the nondestructive evaluation of both green and sintered P/M parts In addition to detecting cracks in green parts, as well as part- to -part density variation, studies have shown that changes in resistivity due to poor carbon pickup during sintering were also detectable (Ref 31) Resistivity testing has also been used later in the processing sequence to screen heat-treated parts for incomplete... promising nondestructive testing methods for P/M applications include electrical resistivity testing, eddy current and magnetic bridge testing, magnetic particle inspection, ultrasonic testing, x-ray radiography, gas permeability testing, and -ray density determination The capabilities and limitations of each of the techniques are evaluated in this article Nondestructive Inspection of Powder Metallurgy Parts... Visual inspection and pressure testing The techniques covered in this article are summarized in Table 1 Additional information on these procedures can be found in the Section "Methods of Nondestructive Evaluation" in this Volume Table 1 Comparison of the applicability of various nondestructive evaluation methods to flaw detection in P/M parts Method Measured/detected Applicability to P/M parts(a) Green... information on the principles and applications of this technique Gamma-Ray Density Determination Local variations in the density of P/M parts have been detected by measuring the attenuation of γ-rays passing through the part (Ref 9) Depending on the material and the dimensions of the part, density can be measured to an accuracy of ±0.2 to ±0.7%, and the technique has been used by P/M parts fabricators in place... in P/M parts and their resonant frequencies have been related to density, yield strength, and tensile strength Plain carbon steel P/M specimens were used in one series of tests and the correlation was found to be close enough for the test to be used as a quick check for the degree of sintering in production P/M parts (Ref 12) Other work has demonstrated the relationship between sound velocity and tensile... 751-768 Nondestructive Inspection of Powder Metallurgy Parts R.C O'Brien and W.B James, Hoeganaes Corporation Introduction THE PROBLEM of forming defects in green parts during compaction and ejection has become more prevalent as parts producers have begun to use higher compaction pressures in an effort to achieve high-density, high-performance powder metallurgy (P/M) steels In this article, several nondestructive. .. indicated by magnetic particle testing The prior test method consisted of sintering, sectioning, and magnetic particle inspection, a 2-h process This part was also the subject of a series of experiments demonstrating that the resistivity test method had high reproducibility and was not operator sensitive Fig 24 Resistivity-measurement device for examining P/M parts Courtesy of R.A Ketterer and N.F McQuiddy,... variations, microlaminations, and poor sintering Ejection Cracks When a part has been pressed, there is a large residual stress in the part due to the constraint of the die and punches, which is relieved as the part is ejected from the die The strain associated with this stress relief depends on the compacting pressure, the green expansion of the material being compacted, and the rigidity of the die . Principles and Application, Metal Powder Industries Federation, 1980, p 112 Nondestructive Inspection of Powder Metallurgy Parts R.C. O'Brien and W.B. James, Hoeganaes Corporation Nondestructive. poor gating design and pouring practice can produce turbulence and can generate nonmetallic inclusions. Sand inclusions may also be evident as the result of improper sand and core practice refining, and casting practices is available in Casting, Volume 15 of ASM Handbook, formerly 9th Edition Metals Handbook. Nondestructive Inspection of Castings By the ASM Committee on Nondestructive