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

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Surface only but independent of size

Visual

Replica

Digital enhancement

Liquid penetrant

Shallow depth or thin object (thickness 1 mm, or 0.04 in.)

Magnetic particle

Magnetic field

Magabsorption

Eddy current

Increased thickness (thickness 3 mm, or 0.12 in.)

Microwave

Optical holography

Speckle metrology

Acoustic holography

Acoustic microscopy

Increased thickness (thickness 100 mm, or 4 in.)

X-ray computed tomography

Increased thickness (thickness 250 mm, or 10 in.)

Neutron radiography

(a)

X-ray radiography

Thickest (dimension 10 m, or 33 ft)

Ultrasonic

(a)

All NDE methods suitable for thick objects can be used on thin objects, except neutron radiography, which is not useful for most thin objects.

Table 5 Comparison of NDE techniques based on shape of object to be evaluated

Simplest shape

Optical holography

Acoustic holography

Acoustic microscopy

Thermography

Microwave

Eddy current

Magnetic particle

Magnetic field

Magabsorption

Neutron radiography

X-ray radiography

Ultrasonic

Liquid penetrant

Digital enhancement

Replica

Visual

X-ray computed tomography

Most complex shape

The characteristics of the material that may affect NDE method selection are highly dependent on the specific NDE

method under consideration. Table 6 lists a number of NDE methods and the characteristic of critical importance for each.

Table 6 NDE methods and their important material characteristics

Method Characteristic

Liquid penetrant Flaw must intercept surface.

Magnetic particle Material must be magnetic.

Eddy current Material must be electrically conductive or magnetic.

Microwave Microwave transmission

Radiography and x-ray computed tomography

Changes in thickness, density, and/or elemental composition

Neutron radiography Changes in thickness, density, and/or elemental composition

Optical holography Surface optical properties

The specific NDE method can be selected by applying all the previously discussed factors. Because each NDE method

has a specific behavior, it is often desirable to select several NDE methods having complementary detection capabilities.

For example, ultrasonic and radiographic methods can be used together to ensure the detection of both planar flaws (such

as cracks) and volumetric flaws (such as porosity).

Guide to Nondestructive Evaluation Techniques

John D. Wood, Lehigh University

Leak Detection and Evaluation

Because many objects must withstand pressure, the nondestructive determination of leakage is extremely important. The

NDE area known as leak detection utilizes many techniques, as described in the article "Leak Testing" in this Volume.

Each technique has a specific range of applications, and a particular leak detection technique should be selected only after

careful consideration of the factors discussed in the article "Leak Testing".

Guide to Nondestructive Evaluation Techniques

John D. Wood, Lehigh University

Metrology and Evaluation

The measurement of dimensions, referred to as metrology, is one of the most widely used NDE activities, although it is

often not considered with other conventional NDE activities, such as flaw detection. Although conventional metrology is

not specifically discussed in this Volume, modern high-technology metrology is covered in the articles "Laser

Inspection," "Coordinate Measuring Machines," and "Machine Vision and Robotic Evaluation."

The selection of a metrology system is highly dependent on the specific requirements of a given application. Standard

reference works on the topic should be consulted for conventional metrology, and the articles "Laser Inspection,"

"Coordinate Measuring Machines," and "Machine Vision and Robotic Evaluation." should be studied for selecting new

technology. In addition, other NDE methods, such as eddy current, ultrasonic, optical holography, and speckle metrology,

often find application in the field of metrology. Selection of these methods for metrology application can be assisted by

the information in the articles "Eddy Current Inspection," "Ultrasonic Inspection," "Optical Holography," and "Speckle

Metrology" in this Volume.

Guide to Nondestructive Evaluation Techniques

John D. Wood, Lehigh University

Location Determination and Evaluation

An occasional problem is whether an assembled unit (one that contains several parts) actually contains the necessary

components. This type of inspection has resulted in an NDE activity that can be termed location determination. The most

commonly employed NDE techniques for location determination are x-ray radiography, x-ray computed tomography, and

neutron radiography. These techniques and their selection are discussed in separate articles in this Volume.

Guide to Nondestructive Evaluation Techniques

John D. Wood, Lehigh University

Structure or Microstructure Characterization

Another interesting area of NDE is microstructural characterization, which can be done in situ without damaging the

object by using replication microscopy (discussed in the article "Replication Microscopy Techniques for NDE" in this

Volume) or by using conventional optical microscopy techniques with portable equipment, including polishing, etching,

and microscopic equipment. In addition, it is possible to characterize the microstructure through the correlation with some

type of NDE information. For example, the transmission of ultrasonic energy has been correlated with the microstructure

of gray cast iron.

Microstructure can often be characterized by determining physical or mechanical properties with NDE techniques

because there is usually a correlation among microstructure, properties, and NDE response. Characterizing microstructure

from NDE responses is a relatively recent area of NDE application, and new developments are occurring frequently.

Guide to Nondestructive Evaluation Techniques

John D. Wood, Lehigh University

Estimation of Mechanical and Physical Properties

As discussed previously, the prediction of mechanical and physical properties with NDE techniques is a relatively new

application of NDE. Eddy current, ultrasonic, x-ray and neutron radiography, computed tomography, thermography, and

acoustic microscopy phenomena are affected by microstructure, which can be related to some mechanical or physical

properties. In addition, microwave NDE can be related to the properties of plastic materials. Several technical meetings

are held each year to discuss advances in NDE, and these meetings often feature session on characterizing microstructure

and mechanical and physical properties with NDE techniques. Some of the meetings are listed below:

• Annual Review (every spring), Center for Nondestructive Evaluation, The Johns Hopkins University

•

Annual Review of Progress in Quantitative NDE (every summer), The Center for NDE, Iowa State

University

• Symposium on Nondestructive Evaluation (every other spring), Nondestructive Testing Inform

ation

Center, Southwest Research Institute

• Spring and Fall Meetings, American Society for Nondestructive Testing

Guide to Nondestructive Evaluation Techniques

John D. Wood, Lehigh University

Stress/Strain and Dynamic Response Determination

The local strain at a specific location in an object under a specific set of loading conditions can be determined by using

strain sensing methods such as photoelastic coatings, brittle coatings, or strain gages. These methods are discussed in the

article "Strain Measurement for Stress Analysis" in this Volume. If the stress-strain behavior of the material is known,

these strain values can be converted into stress values.

A number of methods have also been developed for measuring residual stresses in materials. These include x-ray

diffraction, ultrasonics, and electromagnetics. Surface residual stresses can be measured by x-rays as described in the

article "X-Ray Diffraction Residual Stress Techniques" in Materials Characterization, Volume 10 of ASM Handbook,

formerly 9th Edition Metals Handbook. Practical application of ultrasonic techniques for characterizing residual stresses

have not yet materialized. A number of electromagnetic techniques have, however, been successfully used as described in

the articles "Electromagnetic Techniques for Residual Stress Measurements" and "Magabsorption NDE" in this Volume.

Dynamic behavior of an object can be evaluated during real or simulated service by employing strain sensing technology

while the object is being dynamically loaded. In addition, accelerometers and acoustic transducers can be used to

determine the dynamic response of a structure while it is being loaded. This dynamic response is called a signature and

evaluation of this signature is called signature analysis. The nature of this signature can be correlated with many causes,

such as machine noise, vibrations, and structural instability (buckling or cracking).

Replication Microscopy Techniques for NDE

A.R. Marder, Energy Research Center, Lehigh University

Introduction

SURFACE REPLICATION is a well-developed electron microscopy sample preparation technique that can be used to

conduct in situ measurements of the microstructure of components. The in situ determination of microstructural

deterioration and damage of materials subjected to various environments is an objective of any nondestructive evaluation

(NDE) of structural components. The need to assess the condition of power plant and petrochemical metallic components

on a large scale recently led to the application of surface replication to the problem of determining remaining life. The

usual method of metallographic investigation, which may involve cutting large pieces from the component so that

laboratory preparation and examination can be performed, usually renders the component unfit for service or necessitates

a costly repair. As a result, metallographic investigations are avoided, and important microstructural information is not

available for evaluating the component for satisfactory performance. Therefore, an in situ or field microscopy

examination is needed to aid in the proper determination of component life.

The replica technique for the examination of surfaces has been extensively used for studying the structure of polished-

and-etched specimens and for electron fractographic examination (see the article "Transmission Electron Microscopy" in

Fractography, Volume 12 of ASM Handbook, formerly 9th Edition Metals Handbook for a discussion of replication

techniques in fractography). Surface replication was the predominant technique in electron microscopy prior to being

supplemented by thin-foil transmission and scanning electron microscopy. Recently, the replication microscopy technique

has become an important NDE method for microstructural analysis, and an American Society for Testing and Materials

specification has been written for its implementation (Ref 1).

Acknowledgements

The author would like to acknowledge the contributions of his colleagues A.O. Benscoter, S.D. Holt, and T.S. Hahn in the

preparation of this article.

Reference

"Standard Practice for Production and Evaluation of Field Metallographic Replicas," E 512-87,

Annual Book

of ASTM Standards, American Society for Testing and Materials

Replication Microscopy Techniques for NDE

A.R. Marder, Energy Research Center, Lehigh University

Specimen Preparation

Mechanical Polishing Methods. Components in service usually have a well-developed corrosion or oxidation

product or a decarburized layer on the surface that must be removed before replication. Coarse-grinding equipment can be

used as long as the proper precautions are taken to prevent the introduction of artifacts into the structure due to

overheating or plastic deformation. Sandblasting, wire wheels, flap wheels, and abrasive disks have all been used. After

the initial preparation steps are completed, standard mechanical polishing techniques can be used. Field equipment is

commercially available to help the metallographer reproduce the preparation steps normally followed in the laboratory.

Depending on the material, various silicon carbide abrasive disks of different grit size, together with polishing cloth disks

with diamond paste or alumina of varying grit size, can be used to prepare for the etching step. Finally, any appropriate

etchant for the material being examined can be applied to develop the microstructure. For the proper identification of such

microstructural features as creep cavities, a maximum double or triple etch-polish-etch procedure should be used (Ref 2).

The etchants used for the various materials investigated by the replication technique are described in Metallography and

Microstructures, Volume 9 of ASM Handbook, formerly 9th Edition Metals Handbook and in Ref 3.

Electrolytic Preparation Technique. Although electrolytic polishing and etching techniques have often been

employed as the final mechanical polish step in sample preparation, inherent problems still exist in this process. The

electropolishing technique uses an electrolytic reaction to remove material to produce a scratch-free surface. This is done

by making the specimen the anode in an electrolytic cell. The cathode is connected to the anode through the electrolyte in

the cell. Specimens can be either polished or etched, depending on the applied voltage and current density, as seen in the

fundamental electropolishing curve in Fig. 1. However, the pitting region must be avoided so that artifacts are not

introduced into the microstructure. It is virtually impossible to prevent pitting without precise control of the polishing

variables, and pits can often be mistakenly identified as creep voids.

Fig. 1 Current density-voltage curve for electropolishing

Several portable electropolishing units are commercially available. The most important variables (time, bath temperature,

electrolyte composition, and the current density-voltage relationship) have been investigated for a selected group of

electrolytes (Ref 4). A direct comparison of electropolishing units and the precautions necessary for handling certain

electrolytes are given in Ref 5.

It should be noted that there are areas in both fossil and nuclear plants in which neither acid etches nor electropolishing

methods and materials are allowed because of the potential for intergranular stress-corrosion cracking. Stainless steel

piping in nuclear plants can be replicated to determine defects by manual polishing without etchants. Generator retaining

rings have been replicated by manual polishing to resolve NDE indications, because they are extremely sensitive to stress-

corrosion cracking and no acids or caustics are allowed to be used (Ref 6).

References cited in this section

A.M. Bissel, B.J. Cane, and J.F. DeLong, "Remanent Life Assessment of Seam

Welded Pipework," Paper

presented at the ASME Pressure Vessel and Piping Conference, American Society of Mechanical Engineers,

June 1988

G.F. Vander Voort, Metallography: Principles and Practice, McGraw-Hill, 1984

T.S. Hahn and A.R. Marder, Effect of Electropolishing Variables on the Current Density--

Voltage

Relationship, Metallography, Vol 21, 1988, p 365

M. Clark and A. Cervoni, "In Situ Metallographic Examination of Ferrous and Non-

Ferrous Components,"

Canadian Electrical Association, Nov 1985

J.F. DeLong, private communication

Replication Microscopy Techniques for NDE

A.R. Marder, Energy Research Center, Lehigh University

Replication Techniques

Replication techniques can be classified as either surface replication or extraction replication. Surface replicas provided

an image of the surface topography of a specimen, while extraction replicas lift particles from the specimen. The

advantages and disadvantages of some typical replication techniques are given in Table 1.

Table 1 Comparison of replica techniques

Type Advantages

Disadvantages

Surface replicas

Acetate Excellent resolution

Coating required

Acrylic Direct viewing

Adhesion

Rubber Easy removal

Resolution

Extraction replicas

Direct stripped plastic

Easy preparation

Particle retention

Positive carbon Excellent particle retention with two-stage etching

Coating required

Direct carbon Excellent resolution Not applicable to in situ studies

Surface Replicas. Replication of a surface can involve either direct or indirect methods. In the direct, or single-stage,

method, a replica is made of the specimen surface and subsequently examined in the microscope, while in the indirect

method, the final replica is taken from an earlier primary replica of the specimen surface. Only the direct method will be

considered in this article because it lends itself more favorably to on-site preparation. The most extensively used direct

methods involve plastic, carbon, or oxide replica material. All direct methods except plastic methods are destructive and

therefore require further preparation of the specimen before making additional replicas.

Plastic replicas lend themselves to in-plant nondestructive examination because of their relative simplicity and short

preparation time. Plastic replicas can be examined with the light optical microscope, the scanning electron microscope,

and the transmission electron microscope, depending on the resolution required. As illustrated in Fig. 2, the plastic replica

technique involves softening a plastic film in a solvent, applying it to the surface, and then allowing it to harden as the

solvent evaporates. After careful removal from the surface, the plastic film contains a negative image, or replica, of the

microstructure that can be directly examined in the light microscope or, after some preparation, in the electron

microscope. Double-faced tape is used to bond the replica to the glass slide in order to obtain large, flat, undistorted

replica surfaces.

Fig. 2 Schematic of the plastic replica technique

There are some significant advantages of the replica technique over the use of portable microscopes in the field (Ref 5):

• A permanent record of the specimen is obtained

• Better resolution and higher magnification can be used

• Contamination of the polished surface is minimized

• Time spent in an unpleasant or hazardous environment is minimized

• Scanning electron microscopy can be utilized

Several materials, including acetate, acrylic resin, and rubber, can be used in the surface replica technique (Ref 5). The

choice of material depends on the geometry of the component and the microstructural features to be examined.

In the acetate method, an acetate tape is wetted with acetone and applied to the surface; other less volatile solvents, such

as methyl acetate, can be used when large areas are replicated. For improved resolution, the back side of the replica can

be painted with any fast-drying black paint or ink prior to removal, or for the same effect, evaporated coatings of carbon,

aluminum, or gold can be applied at a shadow angle of 45° to the front side of the replica after removal.

In the acrylic casting resin method, dams are required because a powder is mixed with a liquid on the surface to be

replicated. After hardening, the replica can be examined directly in an optical microscope without further processing. If

adhesion is a problem, a composite replica can be made of an initial layer of Parlodian lacquer before the acrylic layer is

applied.

In the dental impression rubber method, uncured liquid rubber material (for example, GE RTV60 silicon rubber

compound) is poured onto the surface to be replicated and is contained by a dam. After removal, the replica can be

examined directly or can be coated for better resolution.

Extraction Replicas. Several different extraction replica techniques can be used to characterize small particles that are

embedded in a matrix, such as small second-phase particles in a steel (see the article "Analytical Transmission Electron

Microscopy" in Materials Characterization, Volume 10 of ASM Handbook, formerly 9th Edition Metals Handbook).

More detailed descriptions of the various extraction replica techniques can be found in Ref 7 and 8.

After careful preparation of the surface using normal polishing methods, the first step in producing an extraction replica is

to etch the alloy heavily to leave the particles of interest in relief. In the positive carbon extraction replica, as shown in

Fig. 3, a piece of solvent-softened polymeric film (cellulose acetate tape) is pressed onto the surface exposed by this first

etch (Ref 5). Once the solvent has evaporated, one of two steps can be taken. The tape can be carefully pulled from the

specimen to produce a negative of the surface, or the specimen can undergo a second etch to free the particles exposed by

the first etch (Fig. 3). In the second etch, the specimen can be etched through the plastic; most plastics are quite

permeable to etching solutions, and the specimen etches almost as rapidly as without the plastic film (Ref 9). Carbon is

then evaporated in a vacuum onto the plastic replica. The carbon and plastic containing the particles now make up the

positive replica. The cellulose acetate is then dissolved, and the positive carbon replica is allowed to dry. It should be

noted that for the negative carbon extraction replica technique, vacuum deposition of carbon onto the surface of the

specimen is required, and therefore this replica method is not applicable to NDE.

Fig. 3 Positive carbon extraction replication steps. (a) Pl

acement of plastic after the first etch. (b) After the

second etch. (c) After the deposition of carbon. (d) The positive replica after the plastic is dissolved

References cited in this section

M. Clark and A. Cervoni, "In Situ Metallographic Examination of Ferrous and Non-

Ferrous Components,"

Canadian Electrical Association, Nov 1985

D. Kay, Ed., Techniques for Electron Microscopy, Blackwell Scientific Publications, 1965

J.W. Edington, Practical Electron Microscopy in Materials Science, Van Nostrand Rheinhold, 1976

G.N. Maniar and A. Szirmae, in Manual of Electron Metallography Techniques,

STP 547, American Society

for Testing and Materials, 1973

Replication Microscopy Techniques for NDE

A.R. Marder, Energy Research Center, Lehigh University

Microstructural Analysis

Crack determination is important to help establish the root cause of a potential failure in a component. After a

preliminary evaluation of the crack to assess crack shape and length by using magnetic flux or dye penetrant, the replica

method is then used on unetched specimens to assist in the crack evaluation. Figure 4 schematically shows the

propagation of different types of cracks in a steel structure (Ref 10). Each crack has its own characteristics, and it is often

possible to make a correct determination of crack type. It is important to determine whether the crack is the original defect

or has been caused by service conditions or damage. Once the crack type is identified, the proper corrective action, such

as eliminating a corrosive environment or reducing stress levels, can be attempted. Figure 5 shows the replication of

surface cracks in a boiler tube.

Fig. 4 Propagation of different crack types. (a) Creep. (b) Fatigue. (c) Stress corrosion. (d

) Intergranular

corrosion