ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control

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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

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

(No. 7), July 1986, p 21-23

3. R.A. Armistead, CT: Quantitative 3-D Inspection, Adv. Mater. Process.,

Vol 133 (No. 3), March 1988, p

42-48

4. A.G. Fuller, P.J. Emerson, and G.F.Sergeant, A Report on the Effect Upon M

echanical 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

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

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

R.W. McClung and D.R. Johnson, Needs Assessment for NDT and Characterization of C

eramics:

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 Testing, 1987, p 33

R.C. O

'Brien, "Fatigue Properties of P/M Materials," SAE Technical Paper 880165, Society of Automotive

Engineers, March 1988

G.F. Bocchini, "High Pressure Compaction, High Pressure Coining, and High Pressure Repressing of P/M

Parts," Paper presented at the P

revention 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, 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

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)

Method Measured/detected

Green

Sintered

Advantages

Disadvantages

X-ray radiography Density variations, cracks,

inclusions

C C Can be automated

Relatively high initial cost;

radiation hazard

Computed

tomography

Density variations, cracks,

inclusions

C C Can be automated;

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

Ultrasonic imaging:

C-scan

Density variations, cracks D B Sensitive to cracks; fast

Coupling agent required

Ultrasonic imaging:

SLAM

Density variations, cracks D C Fast; high resolution

High initial cost; coupling

agent required

Resonance testing Overall density, cracks D B Low cost; fast

Does not give information on

defect location

Acoustic emission Cracking during pressing and

ejection

C D Low cost

Exploratory

Thermal wave

imaging

Subsurface cracks, density

variations

D C No coupling agent

required

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

Eddy current/magnetic

bridge

Cracks, overall density,

hardness, chemistry

C A Low cost, fast, can be

automated; used on P/M

valve seat inserts

Under development;

Magnetic particle

inspection

Surface and near-surface

cracks

C A Simple to operate, low

cost

Slow; operator sensitive

Liquid dye penetrant

inspection

Surface cracks C D Low cost

Very slow; cracks must

intersect surface

Pore pressure

rupture/gas

permeability

Laminations, ejections,

cracks, sintered density

variations

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 safety-

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.

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

Co, 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.

Acoustic Methods

Ultrasonic Testing. Many characteristics of solids can be determined from the behavior of sound waves propagating in

them. Ultrasonic signals impinged on a sample at one surface are transmitted at speeds and attenuated at rates determined

by the density, modulus of elasticity, and continuity of the material. The sound waves are reflected from other surfaces of

the sample, including cracks as well as free surfaces. They can be picked up and amplified for display on a CRT screen,

as described in Ref 6 and the article "Ultrasonic Inspection" in this Volume. The height and position of the flaw/defect

peak indicate its size and location. Although much effort has been directed toward relating sound propagation to the

physical properties of P/M materials, little has been written on the detection of cracks by the ultrasonic inspection of

porous materials.