Nondestructive Evaluation and Quality Control (1998) Part 12 pps

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.. Shrinkage or open structur

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 evaluation (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

the type of flaw present and the end use of the casting More complex castings usually undergo visual and dimensional inspection after the removal of gates and risers

Final inspection establishes the quality of the finished casting through the use of any of the methods previously mentioned Visual inspection also includes the final measurement and comparison of specified and actual dimensions Dimensions of castings from a large production run can be checked with gages, jigs, fixtures, or coordinate measuring systems

Liquid penetrant inspection is extensively used as a visual aid for detecting surface flaws in aluminum alloy castings Liquid penetrant inspection is applicable to castings made from all the aluminum casting alloys as well as castings produced by all methods One of the most useful applications, however, is the inspection of small castings produced in permanent molds from alloys such as 296.0, which are characteristically susceptible to hot cracking For example, in cast connecting rods, hot shortness may result in fine cracks in the shank sections Such cracks are virtually undetectable by unaided visual inspection, but are readily detectable by liquid penetrant inspection

All the well-known liquid penetrant systems (that is, water-washable, postemulsifiable, and solvent-removable) are applicable to the inspection of aluminum alloy castings In some cases, especially for certain high-integrity castings, more than one system can be used Selection of the system is primarily based on the size and shape of the castings, surface roughness, production quantities, sensitivity level desired, and available inspection facilities

Pressure testing is used for castings that must be leaktight Cored-out passages and internal cavities are first sealed off with special fixtures having air inlets These inlets are used to build up the air pressure on the inside of the casting The entire casting is then immersed in a tank of water, or it is covered by a soap solution Bubbles will mark any point of air leakage

Radiographic inspection is a very effective means of detecting such conditions as cold shuts, internal shrinkage, porosity, core shifts, and inclusions in aluminum alloy castings Radiography can also be used to measure the thickness of specific sections Aluminum alloy castings are ideally suited to examination by radiography because of their relatively low density; a given thickness of aluminum alloy can be penetrated with about one-third the power required for penetrating the same thickness of steel

Aluminum alloy castings are most often radiographed with an x-ray machine, using film to record the results Real-time (digital) radiography and computed tomography are also widely used and are best suited to detecting shrinkage, porosity, and core shift (Fig 12 and 13) Gamma-ray radiography is also satisfactory for detecting specific conditions in aluminum castings Although the γ-ray method is used to a lesser extent than the x-ray method, it is about equally as effective for detecting flaws or measuring specific conditions Aluminum alloy castings are most often radiographed to detect approximately the same types of flaws that may exist in other types of castings, that is, conditions such as porosity or shrinkage, which register as low-density spots or areas and appear blacker on the film or real-time image screen than the areas of sound metal

Aluminum ingots may contain hidden internal cracks of varying dimensions Depending on size and location, these cracks may cause an ingot to split during mechanical working and thermal treatment, or they may appear as a discontinuity in the final wrought product Once the size and location of such cracks are determined, an ingot can be scrapped, or sections free from cracks can be sawed out and processed further Because the major dimensions of the cracks are along the casting direction, they present good reflecting surfaces for sound waves traveling perpendicular to the casting direction Therefore, ultrasonic methods using a wave frequency that gives adequate penetration into the ingot provide excellent sensitivity for 100% inspection of that part of the ingot containing critical cracks Because of ingot thickness (up to 400

mm, or 16 in.) and the small metal separation across the crack, radiographic methods are impractical for inspection

Ultrasonic Inspection. Aluminum alloy castings are sometimes inspected by ultrasonic methods to evaluate internal soundness or wall thickness The principal uses of ultrasonic inspection for aluminum alloy castings include the detection

of porosity in castings and internal cracks in ingots

Nondestructive Inspection of Castings

By the ASM Committee on Nondestructive Inspection of Castings*

Inspection of Copper and Copper Alloy Castings

The inspection of copper and copper alloy castings is generally limited to visual and liquid penetrant inspection of the surface, along with radiographic inspection for internal discontinuities In specific cases, electrical conductivity tests and ultrasonic inspection can be applied, although the usual relatively large cast grain size could prevent a successful ultrasonic inspection

Visual inspection is simple yet informative A visual inspection would include significant dimensional measurements as well as general appearance Surface discontinuities often indicate the presence of internal discontinuities

For small castings produced in reasonable volume, a destructive metallographic inspection on randomly selected samples

is practical and economical This is especially true on a new casting for which foundry practice has not been optimized and a satisfactory repeatability level has not been achieved

For castings of some of the harder and stronger alloys, a hardness test is a good means of estimating the level of mechanical properties Hardness tests are of less value for the softer tin bronze alloys because hardness tests do not reflect casting soundness and integrity

Because copper alloys are nonmagnetic, magnetic particle inspection cannot be used to detect surface cracks Instead, liquid penetrant inspection is recommended Ordinarily, liquid penetrant inspection requires some prior cleaning of the casting to highlight the full detail

For the detection of internal defects, radiographic inspection is recommended Radiographic methods and standards are well established for some copper alloy castings (for example, ASTM E 272 and E 310)

As a general rule, the method of inspection applied to some of the first castings made from a new pattern should include all those methods that provide a basis for judgment of the acceptability of the casting for the intended application Any deficiencies or defects should be reviewed and the degree of perfection defined This procedure can be repeated on successive production runs until repeatability has been ensured

Gas Porosity. Copper and many copper alloys have a high affinity for hydrogen, with an increasing solubility as the temperature of the molten bath is increased Conversely, as the metal cools in the mold, most of this hydrogen is rejected from the metal Because all the gas does not necessarily escape to the atmosphere and may become entrapped by the solidifying process, gas porosity may be found in the casting

In most alloys, gas porosity is identified by the presence of voids that are relatively spherical and are bright and shiny inside Visible upon sectioning or by radiography, they may either be small, numerous, and rather widely dispersed or fewer in number and relatively large Regardless of size, they are seldom interconnected except in some of the tin bronze alloys, which solidify in a very dendritic mode In these alloys, the gas porosity tends to be distributed in the interstices between the dendrites

Shrinkage voids caused by the change in volume from liquid to solid in copper alloys are different only in degree and possibly shape from those found in other metals and alloys All nonferrous metals exhibit this volume shrinkage when solidifying from the molten condition

Shrinkage voids may be open to the air when near or exposed to the surface, or they may be deep inside the thicker sections of the casting They are usually irregular in shape, compared to gas-generated defects, in that their shape frequently reflects the internal temperature gradients induced by the external shape of the casting

Hot Tearing. The tin bronzes as a class, as well as a few of the leaded yellow brasses, are susceptible to hot shortness; that is, they lack ductility and strength at elevated temperature This is significant in that tearing and cracking can take place during cooling in the mold because of mold or core restraint In aggravated instances, the resulting hot tears in the

part appear as readily visible cracks Sometimes, however, the cracks are not visible externally and are not detectable until after machining In extreme cases, the cracks become evident only through field failure because the tearing was deep inside the casting

Nonmetallic inclusions in copper alloys, as with all molten alloys, are normally the result of improper melting and/or pouring conditions In the melting operation, the use of dirty remelt or dirty crucibles, poor furnace linings, or dirty stirring rods can introduce nonmetallic inclusions into the melt Similarly, 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 All commercial metals, by the nature of available commercial melting and molding processes, usually contain very minor amounts of small nonmetallic inclusions These have little or no effect on the casting Inclusions of significant size or number are considered detrimental A thorough review of copper alloy melting,

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 Inspection of Castings*

References

1 P.J Rickards, Progress in Guaranteeing Quality Through Nondestructive Methods of Evaluation,

Foundryman Int., April 1988, p 196-209

2 P.M Bralower, Nondestructive Testing Part I The New Generation in Radiography, Mod Cast., Vol 76

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 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 inspection methods are evaluated, with the aim of identifying those that are practical for detecting defects as early as possible in the production sequence

The most 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

R.C O'Brien and W.B James, Hoeganaes Corporation

Current Status of P/M Testing

In the ceramics industry, the fraction of the finished-part cost that arises from scrap due to flaws introduced during processing is estimated to average 50%, and it can be as high as 75% (Ref 1) Although the ceramics industry has been mobilized for the past 15 years toward the use of nondestructive evaluation in processing, the P/M industry has built up only a scattered background of experience (Ref 2)

To remain competitive, P/M parts producers have increasingly turned to simplified processing It has been shown that the physical properties of P/M parts, especially the fatigue strength, are always improved by increasing the density (Ref 3) The need for densification by double pressing can often be avoided by pressing to high density in a single step However, the use of higher compaction pressures requires the utmost attention to materials selection, tool design, and press setup (Ref 4) A quick, preferably nondestructive method of crack detection would be of great benefit during press setup and for testing the integrity of parts as early as possible in their production sequence

The growth of nondestructive testing in the 1980s has been explosive, and the field has benefited greatly from computerized image reconstruction techniques applied to radiography, ultrasonic, and even magnetic particle inspection Commercial test systems are being marketed as fast as the technology is developed, and the metal powder industry should find solutions to its on-line testing needs by reviewing methods being used by other parts fabrication technologies

In preparing this article, a number of test methods presented themselves as having potential for crack detection in green (unsintered) P/M compacts, and these are recommended for further investigation For a detailed overview of P/M

technology, the reader is referred to Powder Metal Technologies and Applications, Volume 7 of the ASM Handbook

References cited in this section

1 J.W McCauley, Materials Testing in the 21st Century, in Nondestructive Testing of High Performance Ceramics, Conference Proceedings, American Ceramics Society/American Society for Nondestructive

Testing, 1987, p 1

2 R.W McClung and D.R Johnson, Needs Assessment for NDT and Characterization of Ceramics:

Assessment of Inspection Technology for Green State and Sintered Ceramics, in Nondestructive Testing of High Performance Ceramics, Conference Proceedings, American Ceramics Society/American Society for

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, density 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 Green expansion, also known as spring out, is the difference between the ejected-part size and the die size A typical value of green expansion for a powder mix based on atomized iron powder pressed at relatively high pressure (600 to 700 MPa, or

45 to 50 tsi) is 0.20% In a partially ejected compact, for example, the portion that is out of the die expands to relieve the residual stress, while the constrained portion remains die size and a shear stress is imposed on the compact When the ability of the powder compact to accommodate the shear stress is exceeded, ejection cracks such as the one shown in Fig

1 are formed

The radial strain can be alleviated to a degree by increasing the die rigidity and designing some release into the die cavity However, assuming that the ejection punch motions are properly coordinated, the successful ejection of multilevel parts depends to a large degree on the use of a high-quality powder that combines high green strength with low green expansion and low stripping pressure

Density Variations. Even in the simplest tool geometry possible a solid circular cylinder conventional pressing of a part to an overall relative density of 80% will result in a distribution of density within the part ranging from 72 to 82% (Ref 5) The addition of simple features such as a central hole and gear teeth presents minor problems compared with the introduction of a step or second level in the part Depending on the severity of the step, a separate, independently actuated punch can be required for each level of the part During the very early stage of compaction, the powder redistributes itself

by flowing between sections of the die cavity However, when the pressure increases and the powder movement is restricted, shearing of the compact in planes parallel to the punch axis can only be avoided by proper coordination of punch motions When such shear exists, a density gradient results

The density gradient is not always severe enough for an associated crack to form upon ejection However, a low-density area around an internal corner, as shown in Fig 2, can be a fatal flaw, because this corner is usually a point of stress concentration when the part is loaded in service

Fig 2 Density gradient around an internal corner in a part made with a single-piece stepped punch Unetched Fig 1 Ejection crack in sintered P/M steel Unetched

Microlaminations. In photomicrographs of unetched part cross sections, microlaminations such as those shown in Fig

3 appear as layers of unsintered interparticle boundaries that are oriented in planes normal to the punch axis They can be the result of fine microcracks associated with shear stresses upon ejection; such microcracks fail to heal during sintering Because of their orientation parallel to the tensile axis of standard test bars, they have little influence on the measured tensile properties of the bars, but are presumed to be a cause of severe anisotropy of tensile properties

Fig 3 Microlaminations in sintered P/M steel Unetched

Poor Sintering. When unsintered particle boundaries result from a cause other than shear stresses, they are usually present because of insufficient sintering time or sintering temperature, a nonreducing atmosphere, poor lubricant burn-off, inhibition of graphite dissolution, or a combination of these A severe example is shown in Fig 4 Unlike microlaminations, defects associated with a poor degree of sintering are not oriented in planes

Fig 4 Poor degree of sintering in P/M compact Unetched

Reference cited 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:

• Radiographic techniques

• Acoustic methods

• Thermal inspection

• Electrical resistivity inspection

• 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

Applicability to P/M parts (a)

pinpoint defect location

Extremely high initial cost; highly trained operator required; radiation hazard

Gamma-ray density

determination

Density variations A A High resolution and

accuracy; relatively fast

High initial cost; radiation hazard

Flat or convex surfaces only

Electrical resistivity Subsurface cracks, density

variations, degree of sinter

A A Low cost, portable, high

potential for use on green compacts

Sensitive to edge effects

Under development;

Slow; operator sensitive

Liquid dye penetrant

A A Low cost, simple, fast Gas-tight fixture required;

cracks in green parts must intersect surface

(a) A, has been used in the production of commercial P/M parts; B, under development for use in P/M; C, could be developed for use in P/M, but

no published trials yet; D, low probability of successful application to P/M

Radiographic Techniques

X-Ray Radiography. Any feature of a part that either reduces or increases x-ray attenuation will be resolvable by x-ray radiography Some types of flaws and their x-ray images are shown in Fig 5

Fig 5 Schematic of flaws and their x-ray images Defect types that can be detected by x-ray radiography are

those that change the attenuation of the transmitted x-rays Source: Ref 6

The ability to detect defects depends on their orientation to the x-ray source A crack parallel to the x-rays will result in reduced attenuation of the rays, and the x-ray film will be darker in this region A thin crack perpendicular to the x-ray will hardly influence attenuation and will not be detected

Historically, flaw detection by x-ray radiography has been an expensive and cumbersome process suited only to critical and high added value parts The process has been considerably improved by the development of real-time imaging techniques that replace photographic film Real-time imaging means that parts can be tested rapidly and accepted or rejected on the spot

safety-Real-time x-ray systems include image intensifiers or screens that convert x-rays into visible light and discrete detector arrays that convert x-rays into electronic signals (which are reconstructed by computer for video display) The image in all these systems can be recorded and digitized for image enhancement The ability of the system to detect flaws is, however, still sensitive to defect orientation Additional information is available in the article "Radiographic Inspection"

in this Volume

Computed tomography is a recently developed version of x-ray radiography that includes highly sophisticated analysis of the detected radiation A tomographic setup consists of a high-energy photon source, a rotation table for the specimen, a detector array, and the associated data analysis and display equipment, as shown in Fig 6 The ability to rotate the specimen increases the chance of orienting a defect relative to the x-rays such that it will be detected The x-ray source and detector array can be raised or lowered to examine different planes through the sample

Fig 6 Schematic of computed tomography, which is the reconstruction by computer of a series of tomographic

planes (slices) of an object The transmitted intensity of the fan-shaped beam is processed by computer and the resulting image is displayed on a terminal Source: Ref 7

In a typical system, the photon source can be a radioisotope such as 60Co, depending on the energy requirements of the individual specimens The lead aperture around the source acts as a collimator to produce a fan-shaped beam about 5 mm (0.2 in.) thick The sample is rotated in incremental steps, and the transmitted radiation is detected at each step by computer-controlled detectors situated one every 14 mm (0.55 in.) in a two-dimensional array

The computer then reconstructs the object using intensity data from a number of scans at different orientations The output is in the form of a two-dimensional plan in which colors are mapped onto the image according to the intensity of the transmitted radiation The resolution available depends on the difference in density between the various features of the object For example, steel pins embedded in polyvinyl chloride plastic are more easily resolvable than aluminum pins of the same diameter (Ref 7) Experiments with P/M samples have shown that density can be measured to better than 1% accuracy, with a spatial resolution of 1 mm (0.04 in.) (Ref 8) The article "Industrial Computed Tomography" in this Volume contains more 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 of immersion density tests as an aid in tool setting

The apparatus consists of a vertically collimated -ray beam originating from a radioisotope The beam passes through the sample as shown in Fig 7 and reaches a detector via a 1 mm (0.04 in.) diam aperture, where the transmitted intensity

is measured The detector consists of a sodium iodide scintillation crystal, which in turn excites a photomultiplier Exposure time is 1 to 2 min; a 4 mm (0.15 in.) aperture can reduce this time to 30 s at the expense of some resolution The radiation source of the Gamma Densomat is Americium 241 (60 keV) For high-energy beams, Cesium 137 (660 keV) can be substituted

Fig 7 Schematic of the Gamma Densomat Source: Ref 9

This method has been shown to be particularly useful in cases where the section of the part to be checked is too small for immersion density measurements (Ref 10) Tool life was extended when the method was used for part density checks in order to avoid overloading

Ultrasound Transmission in Green Compacts. The characterization of green compacts by ultrasonic techniques appears to be hindered by problems of extreme attenuation of the incident signal In one case, signals of 1 to 20 MHz were transmitted through an 8 mm (0.3 in.) thick compact of atomized iron with 0.2% graphite added (Ref 11) Although the attenuation did not allow back-wall echo measurement, through-transmission measurements indicated that the transmitted intensity had a maximum at 4 to 5 MHz (Fig 8) Density was found to influence the transmitted intensity, with specimens at 95% relative density allowing some degree of transmission over the entire range of frequencies tested, while specimens at 87% relative density damped the incident signals entirely

Fig 8 Ultrasonic spectrum analyzer output showing change in transmitted intensity with density of green

compact Source: Ref 11

Figure 9 shows that the velocity of ultrasonic waves in green compacts is about half the velocity in sintered compacts and that it is essentially invariant with density (Ref 13) It has also been shown that the velocity of ultrasound in green parts is highly anisotropic and that the experimental reproducibility is very poor (Fig 10) It has been proposed that the anisotropy in velocity is due to the orientation of porosity (Ref 15)

Fig 9 Effect of density on ultrasonic velocity in green and sintered cylindrical Ancorsteel 1000-B specimens

Source: Ref 12

Fig 10 Anisotropy of ultrasound in green transverse rupture strength bars Source: Ref 14

The variation in the velocity of ultrasound with applied pressure during the compaction of ceramic powders has been

measured in situ by fixing transducers to the ends of the punches (Ref 16) Unlike the case of finished green P/M

compacts, a clear relationship was found between longitudinal wave velocity and compacting pressure (Fig 11), probably because the constraint of the punches and die forced the individual particles together, providing an efficient acoustic coupling between particles

Fig 11 Ultrasonic wave velocity in ceramic powders, measured during compaction Source: Ref 16

Ultrasound Transmission in Sintered Parts. Early work relating the physical properties of cast iron to the velocity

of sound waves suggested the potential for evaluating P/M steels in the same way (Ref 17) As expected, both the velocity

of sound 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 strength in porous parts (Fig 12) The same types of relationships have also been documented in powder forgings Ref 19

Fig 12 Correlation of ultrasonic velocity with tensile strength of sintered steel Source: Ref 18

Sintered parts have been found to transmit ultrasound according to the relationships shown in Fig 13 The highest wave velocities occurred in the pressing direction An additional distinction was found between the velocities in the longitudinal and lateral axes of an oblong specimen, and these results were shown to be reproducible between different powder lots and specimen groups The anisotropy of velocity diminished at higher densities and disappeared above 6.85 g/cm3

Fig 13 Anisotropy of ultrasound velocity in sintered transverse rupture strength bars Source: Ref 14

Ultrasonic Imaging: C-Scan. The C-Scan is a form of ultrasonic testing in which the testpiece is traversed by the ultrasound transducer in a computer-controlled scan protocol (Fig 14) The transmitted intensity is recorded and analyzed

by computer, and a gray-mapped image is output

Fig 14 Schematic of a C-Scan scanning protocol for an adhesive-bonded structure Source: Ref 20

In one trial, seeded oxide inclusions were detected in porous sintered steels using a C-Scan (Ref 21) The inclusions consisted of admixed particles of chromium oxide and alumina at concentrations of 65 to 120 particles per square centimeter Inclusions as small as 50 μm in diameter were detected Additional information on the C-Scan can be found in the articles "Ultrasonic Inspection" and "Adhesive-Bonded Joints" in this Volume

Ultrasonic Imaging: Scanning Acoustic Microscopy (SAM). Ultrasonic waves can be focused on a point using a transducer and lens assembly, as shown in Fig 15 and described in the article "Acoustic Microscopy" in this Volume In this way, the volume of the specimen being examined is highly limited, so that reflections from defects can be closely located at a given depth and position in the specimen In SAM, the specimen is moved by stepper motors in a raster pattern, and an image of the entire structure can be built up Scanning acoustic microscopy has been shown to be capable

of resolving small surface and subsurface cracks, inclusions, and porosity in sintered, fully dense ceramics (Ref 22)

Fig 15 Schematic of the image-forming process in scanning acoustic reflecting microscopy Source: Ref 22

Ultrasonic Imaging: Scanning Laser Acoustic Microscopy (SLAM). When a continuous plane wave impinges

on a sample that is roughly flat in shape, it propagates through and is emitted from the sample with relatively little scattering, retaining its planar nature When the plane wave is emitted from the sample, it contains information on variations in properties that were encountered in the interior of the sample, which takes the form of variations in intensity with position in the plane A scanning laser acoustic microscope detects these variations as distortions in a plastic sheet that is placed in the path of the plane wave The information is gathered by a laser that scans a reflective coating on one side of the sheet, as shown in Fig 16 and explained in the article "Acoustic Microscopy" in this Volume

Fig 16 General configuration used in scanning laser acoustic microscopy Source: Ref 23

Therefore, although ultrasonic testing is not appropriate for evaluating green P/M parts, it is applicable to the assessment

of sintered components Optimum results dictate careful selection and placement of the transducers because the orientation of the defects influences the ability to detect them Small defects close to the specimen surface can be masked

by surface echoes Although enhanced image analysis techniques appear beneficial, it is unlikely that the more sophisticated techniques, such as C-Scan and SLAM, will be cost effective for most ferrous P/M parts in the near future

Resonance Testing. When a structural part is tapped lightly, it responds by vibrating at its natural frequency until the sound is damped Both the damping characteristics and the natural frequency change with damage to the structure Changes in the natural frequency can be detected with a spectrum analyzer, as shown in Fig 17

Fig 17 Schematic of resonance test configuration Source: Ref 24

Sintered P/M parts behave in a similar manner, and the minimum defect size that can be detected has been determined experimentally by testing the resonant frequency after milling narrow grooves of various depths in the parts (Ref 24) It was found that defects covering 2% of the cross section could always be detected and that smaller defects (down to 0.5%) could be detected under favorable conditions of part geometry This was later shown to apply to real defects as well as machined grooves (Ref 25) There is no record of the technique having been tried on green parts However, the extremely high sound-damping capacity of green parts would appear to preclude its use As with ultrasonic techniques, resonance testing has been used to determine physical properties such as the elastic modulus of materials as well as their defect structure (Ref 26)

Acoustic emissions are sounds generated in a material as stored elastic energy is released in a noncontinuous mode by mechanisms such as transformation and twinning, slip, and fracture (see the article "Acoustic Emission Inspection" in this Volume) The acoustic emission spectra have been characterized for the compressive deformation of powder-forged 4600 steels with carbon contents ranging from 0.3 to 0.9% (Ref 27)

If P/M tooling were monitored for acoustic emissions of the powder during compaction and ejection, it might be possible

to distinguish emission peaks due to the release of stored energy as cracks are formed However, a developmental program would be required to evaluate this concept and practical application is not anticipated

Thermal Inspection

Thermal Wave Imaging. When a pulsed laser impinges on a surface, the rapidly alternating heating and cooling of the surface is conducted into the body of the specimen, as shown in Fig 18 These thermal waves have been shown to possess many of the same characteristics as electromagnetic or mechanical waves They can be reflected and refracted, they can form interference patterns, and they interact with irregularities contained in the transmitting medium In coincidence with the thermal wave formation, acoustic waves are formed by the alternating expansion and contraction of the area of impingement of the laser on the surface These photoacoustic waves have the same frequency as the thermal waves (typically 1 MHz) but have a much longer wavelength They are also affected by scattering and reflection of the thermal waves in the volume immediately surrounding the laser impingement point, and it is this effect that allows detection of flaws Thermal wave imaging has been used to detect delamination and microcracking in silicon integrated circuits (Ref 28)

Fig 18 Transmission of thermal and acoustic waves in thermal wave imaging Source: Ref 28

Another method of detecting the interactions of thermal waves with defects is optical beam deflection, or the mirage effect (Ref 29) The impingement point of the laser on the surface heats rapidly, and the air around this point is also heated If there are no irregularities present beneath the surface, this volume of lower-density heated air is roughly hemispherical in shape A second laser beam that transits this low-density air volume by skimming closely parallel to the specimen surface, as shown in Fig 19, will be refracted by the density gradient in the same way as it would be by a conventional lens A four-quadrant detector array gathers the beam deflection data as the specimen surface is scanned by the laser Subsurface defects are detected as changes in the shape of the density gradient "lens."

Fig 19 Detection of interactions between thermal waves and flaws by optical beam deflection (mirage effect)

Source: Ref 29

Although there is no record of thermal wave imaging having been applied to P/M parts, the damping capacity of green compacts would appear to restrict the potential application of the technique to sintered components only Full details on the principles and applications of thermal wave imaging can be found in the article "Thermal Inspection" in this Volume

Electrical Resistivity Testing

Direct Current Resistivity Testing. A voltage field within a conductive solid will create currents that are influenced

by structural irregularities, including cracks and porosity This characteristic has been used to measure carburized case depth in wrought steels (Ref 30) The arrangement shown in Fig 20 is used to measure the voltage drop in a current field localized between two electrode probes This method has been used to detect seeded defects in laboratory specimens It has also been successfully applied to the production of sintered steel parts (Ref 31), as described in Examples 1, 2, and 3

Fig 20 Four-point probe used in the resistivity test The outer probe pins are the current leads; the inner pins

are the potential leads Source: Ref 30

Although the resistivity of green compacts is an order of magnitude higher than that after sintering, the same technique has been shown to apply (Ref 30) Green-state specimens with laboratory-simulated cracks of the type shown in Fig 21 have been subjected to resistivity inspection with encouraging results If the probe electrodes span the plane containing the defects and if a series of measurements is made along the edge of the plane, the resistivity varies when defects are present, as shown in Fig 22 Other tests on green parts are described in Ref 30 and 31

Fig 21 Defects in green P/M compacts (a) Artificial defect caused by the inclusion of a fine wax sliver in the

die fill Unetched (b) Artificial defect produced by compacting a partially filled die at 345 MPa (25 tsi), completing the fill, and carrying out final compaction of the entire part at 620 MPa (45 tsi) Unetched See also Fig 22

Fig 22 Variation in resistivity in a green compact was used to locate artificial defects of the type shown in Fig

21(a) and 21(b) Source: Ref 30

There are two potential contributors to variability in the resistivity test First, in addition to cracks, the edges and corners

of the parts distort the current fields The internal corners of parts are often the sites of green cracks Testing the volume

of material immediately underlying the corners necessitates the use of specially made electrode probe sets Another variable influencing the resistivity inspection of green compacts is the nature of the oxide layers on the particles When the oxide layer is altered with a thermal treatment, the resistivity of the green part decreases (Ref 32)

Another study has yielded the relative density/conductivity relationship shown in Fig 23, suggesting that resistivity tests could be used as a rapid check for localized density variations As with ultrasound, the elastic modulus and the toughness

of porous steels can also be distinguished by resistivity checks (Ref 34)

Fig 23 Variation in resistivity with relative density in sintered iron Source: Ref 33

The direct current resistivity test can be used on any conductive material; it is not limited to ferromagnetic materials Although further development is needed, resistivity measurements appear to be one of the most promising 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 transformation to martensite Several uses for resistivity testing are given in the following examples (Ref 31)

Example 1: Automotive Air Conditioner Compressor Part

The resistivity-measuring equipment and hand-held probe are shown in Fig 24 The part, shown in Fig 25, was tested for green cracking in the locations marked in Fig 25(b), which were suspect because of prior experience The parts could then be sorted for cracks by comparing the measured resistivity with limiting resistivity values that had previously been determined using parts with cracks 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,

Ferraloy

Fig 25 Automotive air conditioner compressor part examined by resistivity measurement (a) Actual part (b)

Cross section showing flawed areas (c) Six location test fixture Courtesy of R.A Ketterer and N.F McQuiddy, Ferraloy

Example 2: Automotive Transmission Spacer

The resistivity test was used to screen the parts shown in Fig 26 for incomplete transformation to martensite upon heat treating The test is based on the lower resistivity of pearlitic microstructures compared with martensitic microstructures

of the same chemistry To determine a resistivity criterion for the screening of these parts, resistivity was correlated with hardness measurements A resistivity of 60 μΩ· cm was associated with a hardness of 30 HRC, and a go/no-go test strategy was used The prior test methods for this part were hardness measurements and metallography

Fig 26 Automatic transmission spacer examined by resistivity measurement Courtesy of R.A Ketterer and

N.F McQuiddy, Ferraloy

Example 3: Automatic Transmission Clutch Plate

The part, shown in Fig 27, was pressed, sintered, and sized The resistivity test was then used to screen for part-to-part density variations to levels below 6.8 g/cm3, which was shown to be a minimum density level for achieving the radial crush strength specification for the part Again, a limiting resistivity value was determined for the part; resistivity values below 27.5 μΩ· cm were considered acceptable

Fig 27 Automatic transmission clutch plate examined by resistivity measurement Courtesy of R.A Ketterer

and N.F McQuiddy, Ferraloy

Eddy Current Testing. Another form of resistivity testing is the eddy current test In this test, instead of producing currents in the part by direct contact with electrodes, eddy currents are induced in the part by an alternating electromagnetic field from an induction coil, as described in the article "Eddy Current Inspection" in this Volume

Single-Coil Tests. Disruptions in the eddy current path due to any defect that changes the resistivity of the material are detected as extraneous induced voltages in the induction coil Alternatively, a separate detector coil can be placed in the magnetic field around the testpiece

The alternating current in the induction coil can vary from 1 to 1000 kHz The depth of penetration varies with frequency, with the highest frequencies yielding the smallest depths (skin effect) The way in which the eddy current varies as a function of depth is also described in the article "Eddy Current Inspection" in this Volume

The output of eddy current testing is in the form of an oscilloscope display An eddy current inspection system can detect changes from point to point in single testpieces (for example, welded tubes) as they move through the coil For cases where the testpieces consist of a series of discrete parts, a second coil containing a reference can be added to the system; this configuration is called a magnetic bridge comparator

Magnetic Bridge Comparator Testing. When a ferromagnetic part is placed in the core of a coil with an alternating current, a unique set of harmonics characteristic to the part can be detected in the coil Some of the variables influencing the harmonics are alloy type, core or surface hardness, case depth, and porosity (Ref 35)

In the magnetic bridge comparator arrangement, the harmonic signals from two like coils are compared The coils are similar and carry the same excitation waveform One coil contains the part to be inspected and the other a reference part chosen at random from the group to be tested Differences between the harmonic characteristics of the two parts are displayed as the displacement of a dot from the center of an oscilloscope screen; no displacement means the two parts are alike

Although the magnetic susceptibility of porous sintered steels is reduced by the pinning of domain-boundary walls by pores, P/M parts are also capable of being analyzed by the magnetic bridge comparator In one study, 120 P/M production parts were tested in a magnetic bridge comparator Seventeen of the parts were singled out on the basis of a displacement

of the oscilloscope indication, as shown in Fig 28 These parts were tested for chemistry, hardness, crush strength, and pressed height For comparison, 25 parts selected at random from the remaining specimens were also tested Statistically significant differences were found between the groups with regard to carbon content and hardness (Ref 36) The technique has also been successfully applied to powder-forged parts (Ref 37) Although there are no published trials, there

is a possibility that the comparator could also be used for testing green compacts

Fig 28 Magnetic bridge comparator display for a set of 120 sintered parts, in which 17 parts were indicated as

differing from the reference part Source: Ref 36

Visual Inspection and Pressure Testing

Magnetic Particle Inspection. Cracks that exist on or close to the surface of a ferromagnetic material in the magnetic field act as magnetic poles, creating localized increases in the field intensity Iron particles suspended in a fluid at the surface will be preferentially attracted to these high-intensity areas, and these particles can be used to mark the locations

of the flaws The detectability of the particles themselves can in turn be improved by coating with a pigment that contrasts with the part surface or fluoresces under ultraviolet light (see the article "Magnetic Particle Inspection" in this Volume)

This method has been used to inspect finished P/M parts for cracks originating in processing, and it may also be applicable to green compacts It is also possible to automate the inspection process by using digital image processing (Ref 38)

Liquid Dye Penetrant Inspection. A liquid that wets the surface of the material being inspected will lower its surface energy by residing preferentially in surface cracks and cavities In the liquid penetrant inspection technique, cracks are detected by removing the dye from the flat surface of the specimen The dye that is left behind in the cracks is then wicked out onto the surface by a fine particulate layer in which the pore radius is even lower than that of the crack The penetrant in this particulate developer layer can be detected visually because of its high contrast with the white developer, or it can be mixed with a dye that fluoresces under ultraviolet light This process is described in the article

"Liquid Penetrant Inspection" in this Volume

The dye penetrant equipment found in P/M shops is generally used only for checking parts of the tooling and machinery for cracks The dye does not preferentially reside at cracks in P/M parts, because the pore radius and the crack radius are equivalent However, there might be an application for green parts because the surfaces of green parts are sealed against penetration by liquids through smearing of the metal powder against the die wall and through the formation of a thin coating of dry powder lubricant on the surface Cracks intersecting the surface may form an opening in this layer that could be detected by the dye penetrant

Pore Pressure Rupture Testing of Green Compacts. A novel test is available for detecting ejection cracks in green compacts (Ref 39) A pressure seal is formed around a corner or area of a part where experience has shown that cracks are likely to occur The area is then pressurized to about 3.5 MPa (500 psi) using a fixture such as that shown in Fig 29 If a crack is present, the gas pressure in the crack will be sufficient to propagate the crack the rest of the way

through the part This would be classed as a proof test rather than a nondestructive test because the part is destroyed if defects are present

Fig 29 Pore pressure rupture test for crack detection in green parts Source: Ref 39

The test can be used in a nondestructive manner on sintered parts The gas permeability of the pressurized area is measured at reduced pressures, and the presence of cracks or low-density areas is indicated by high permeability, as shown in Fig 30

Fig 30 Detection of flawed compact using the gas permeability technique Source: Ref 39

References cited in this section

6 C Rain, Uncovering Hidden Flaws, High Technol., Feb 1984

7 B Chang et al., Spatial Resolution in Industrial Tomography, IEEE Trans Nuclear Sci., NS30 (No 2),

April 1983

8 H Heidt et al., Nondestructive Density Evaluation of P/M Objects by Computer Tomography, in Horizons

of Powder Metallurgy, 1986 International Powder Metallurgy Conference Proceedings, Part II, p 723

9 G Schlieper, W.J Huppmann, and A Kozuch, Nondestructive Determination of Sectional Densities by the

Gamma Densomat, Prog Powder Metall., Vol 43, 1987, p 351

10 C.T Waldo, Practical Aspects of the Gamma Densomat, in Horizons in Powder Metallurgy, 1986

International Powder Metallurgy Conference Proceedings, Part II, p 739

11 J.L Rose, M.J Koczak, and J.W Raisch, Ultrasonic Determination of Density Variations in Green and

Sintered Powder Metallurgy Components, Prog Powder Metall., Vol 30, 1974, p 131

12 B Patterson, C Bates, and W Knopp, Nondestructive Evaluation of P/M Materials, Prog Powder Metall.,

Vol 37, 1981, p 67

13 M.F Termine, "Ultrasonic Velocity Measurements on Green and Sintered P/M Compacts," Unpublished Report, Hoeganaes Corporation, 1985