Replication Microscopy Techniques for NDE

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

SURFACE REPLICATION is a well- developed electron microscopy sample preparation technique that can be used to conduct in situ measurements of the micro- structure of components. The in situ deter- mination of microstructural deterioration and damage of materials subjected to vari- ous environments is an objective of any nondestructive evaluation (NDE) of struc- tural components. The need to assess the condition of power plant and petrochemical metallic components on a large scale recent- ly led to the application of surface replica- tion to the problem of determining remain- ing 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 compo- nent unfit for service or necessitates a cost- ly repair. As a result, metallographic inves- tigations are avoided, and important microstructural information is not available for evaluating the component for satisfacto- ry performance. Therefore, an in situ or field microscopy examination is needed to aid in the proper determination of compo- nent 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 ex- amination (see the article "Transmission Electron Microscopy" in Volume 12 of the 9th Edition of Metals Handbook for a discus- sion of replication techniques in fractogra- phy). Surface replication was the predomi- nant technique in electron microscopy prior to being supplemented by thin-foil transmis- sion and scanning electron microscopy. Re- cently, the replication microscopy technique has become an important NDE method for microstructural analysis, and an American Society for Testing and Materials specifica- tion has been written for its implementation (Ref 1).

Specimen Preparation Mechanical Polishing Methods. Compo-

nents in service usually have a well-devel-

oped 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 struc- ture due to overheating or plastic deforma- tion. 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 metallog- rapher reproduce the preparation steps nor- mally 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. Final- ly, any appropriate etchant for the material being examined can be applied to develop the microstructure. For the proper identifi- cation 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 Volume 9 of the 9th Edition of Metals Handbook and in Ref 3.

Electrolytic Preparation Technique. Al- though electrolytic polishing and etching techniques have often been employed as the final mechanical polish step in sample prep- aration, inherent problems still exist in this process. The electropolishing technique uses an electrolytic reaction to remove ma- terial 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 electro- lyte in the cell. Specimens can be either polished or etched, depending on the ap- plied 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 im- possible to prevent pitting without precise control of the polishing variables, and pits

#

~Etchingl Polishing / i ~ ~,~ >1 ~

f Pitting

Voltage -,u©". 1 Current density-voltage curve for electropol-

ishing

can often be mistakenly identified as creep voids.

Several portable electropolishing units are commercially available. The most im- portant variables (time, bath temperature, electrolyte composition, and the current density-voltage relationship) have been in- vestigated for a selected group of electro- lytes (Ref 4). A direct comparison of elec- tropolishing 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 deter- mine defects by manual polishing without etchants. Generator retaining rings have been replicated by manual polishing to re- solve NDE indications, because they are extremely sensitive to stress-corrosion cracking and no acids or caustics are al- lowed to be used (Ref 6).

Replication Techniques Replication techniques can be classified

as either surface replication or extraction replication. Surface replicas provide an im- age of the surface topography of a speci-

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T a b l e 1 C o m p a r i s o n of rep l i ca t e c h n i q u e s 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 Coating required retention with two-stage etching

Direct carbon.. Excellent resolution Not applicable to in situ studies

men, while extraction replicas lift particles from the specimen. The advantages and disadvantages of some typical replication techniques are given in Table 1.

5 u d a t e Replicas. Replication of a sur- face 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 fa- vorably 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

R e p l i c a t i o n M i c r o s c o p y T e c h n i q u e s f o r N D E / 5 3

electron microscope, and the transmission electron microscope, depending on the res- olution 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 microstruc- ture 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, fiat, undistorted replica surfaces.

There are some significant advantages of the replica technique over the use of porta- ble microscopes in the field (Ref 5): * A permanent record of the specimen is

obtained • Better resolution and higher magnifica-

tion 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 uti-

lized 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, resolu- tion, 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,

Softened acetate tape / ~ Tape applied t o surface and dried

\ Polished-and- / etched part ~ [ ~

Tape removed with negative replica of surface

Fig. 2 Schematic of the plastic replica technique

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 exam- ined directly in an optical microscope with- out further processing. If adhesion is a problem, a composite replica can be made of an initial layer of Parlodian lacquer be- fore the acrylic layer is applied.

In the dental impression rubber method, uncured liquid rubber material (for exam- ple, 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.

Plastic /

'---~u~ i,i - - First etch/" • " "~" 'JJ'. ~./nc;usion ~ °~ irst etch • \ \ . ® / / . e

• (~ ~) ~ , / / / ~) " Metal

(a) (b)

Carbon ? ~ / J "nL • , , ~ ~ J

Carbon

(c) (d)

''ul¢|~" 3 Positive carbon extraction replication steps, (a) Placement of plastic after the first etch. (b) After the second etch. (c) After the deposition of carbon. (d) The positive replica offer the plastic is dissolved

54 / M e t h o d s of N o n d e s t r u c t i v e E v a l u a t i o n

(a)

(c) =.===-.. 4 Propagation of different crock types. (a)

corrosion

Extraction Replicas. Several different ex- traction replica techniques can be used to characterize small particles that are embed- ded in a matrix, such as small second-phase particles in a steel (see the article "Analyt- ical Transmission Electron Microscopy" in Volume 10 of the 9th Edition of 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 pro- duce a negative of the surface, or the spec- imen 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 plas- tic replica. The carbon and plastic contain- ing the particles now make up the positive replica. The cellulose acetate is then dis- solved, and the positive carbon replica is allowed to dry. It should be noted that for the negative carbon extraction replica tech-

(b)

(d) Creep. (b) Fatigue• (c) Stress corrosion. (d) Intergranular

nique, vacuum deposition of carbon onto the surface of the specimen is required, and therefore this replica method is not applica- ble to NDE.

Microstructural A n a l y s i s Crack determination is important to help

establish the root cause of a potential failure in a component. After a preliminary evalu- ation of the crack to assess crack shape and length by using magnetic flux or dye pene- trant, 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 pos- sible 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 elim- inating a corrosive environment or reducing stress levels, can be attempted. Figure 5 shows the replication of surface cracks in a boiler tube.

Creep Damage. Creep defects cause the majority of failures in power plant compo- nents operating under stress and thermal load, and the replica method is especially suitable for the detection of these defects. Therefore, the replica method has become an especially important tool in the deter- mination of remaining life in such compo- nents as boiler tubes, steam piping, and

@

j -

(a)

% ~ ,

• 20__.~m

(b) Surface crack in a boiler tube. Comparison Fig. 5 of the (a) actual microstructure and (b) the

replica of the crack

turbine components. The replica method reveals defects due to creep at a much earlier stage than other NDE techniques. Creep defects begin as small holes or cav- ities at grain boundaries or second phases. With time and stress, these holes or cavi- ties can link up and form cracks that eventually lead to failure of the component (Fig. 6). Creep cracks are usually very localized, and they form in welds, bends, or other highly stressed regions. Determin- ing the remaining life of components nor- mally depends on assessments of regular inspections, as indicated in Table 2. Figure 7 shows a comparison of creep voids in a surface replica and the corresponding bulk microstructure.

Precipitate Ana lys is . The detection of various deleterious precipitates in compo- nents subjected to high temperature and stress can lead to improved life assessment

R e p l i c a t i o n M i c r o s c o p y T e c h n i q u e s f o r N D E / 5 5

(a) (b) (c) (d)

F i g , 6 Schematic of creep crock formation. Small cavities (a) link up over time (b) and form intergranulor crocks (c) and eventually macrocracks (d).

Table 2 Creep d a m a g e classif icat ion Class Nature Action

1 . . . . . . . . . . . . . . . . . . . . . . . . . No creep defects 2 . . . . . . . . . . . . . . . . . . . . . . . . . A few cavities 3 . . . . . . . . . . . . . . . . . . . . . . . . . Coalescent cavities 4 . . . . . . . . . . . . . . . . . . . . . . . . . Microscopic creep cracks 5 . . . . . . . . . . . . . . . . . . . . . . . . . Macroscopic creep cracks

Source: Ref 11

None Reinspection after 20 000 h of service Reinspection after 15 000 h of service Reinspection after 10 000 h of service Management must be informed immediately

(a)

(a) (b)

F i g . 8 Comparison of cT-phase formation as seen in (a) a replica and (b) the actual microstructure

(b)

Fig. 7 Comparison of creep voids in (a) a replica and (b) the actual microstructure

analysis of these components. The extrac- tion replication technique is an excellent nondestructive method of detecting these precipitates.

Sigma phase is a deleterious FeCr com- pound that can form in some stainless steels, and its presence can severely limit remaining life. Extraction replicas have been used to determine the amount of (r phase in the microstructure (Ref 12), and the amount of ~ phase has been directly related to the creep rate (Ref 13). Figure 8 shows an example of cr phase in an extrac- tion replica.

The composition of carbides, and their stability with time and temperature of expo- sure, can indicate the remaining life of a component. Extraction replicas have been used to evaluate carbides, and it has been suggested that changes in morphology and

chemistry can be used to assist the estima- tion of effective exposure temperature for use in determining the remaining life of components (Ref 14). Figure 9 shows an example of precipitates extracted from a 200 000-h exposed sample, together with the accompanying chemical analysis.

A C K N O W L E D G M E N T

The author would like to acknowledge the contributions of his colleagues A.O. Ben- scoter, S.D. Holt, and T.S. Hahn in the preparation of this article.

5 6 / M e t h o d s o f N o n d e s t r u c t i v e E v a l u a t i o n

(a)

F i g . 9 Extraction replica of the microstructure (a) and the

(b)

precipitate microchemical analysis (b) from an extraction replica

REFERENCES

1. "Standard Practice for Production and Evaluation of Field Metallographic Replicas," E 512-87, Annual Book of ASTM Standards, American Society for Testing and Materials

2. A.M. Bissel, B.J. Cane, and J.F. De- Long, "Remanent Life Assessment of Seam Welded Pipework," Paper pre- sented at the ASME Pressure Vessel and Piping Conference, American Soci- ety of Mechanical Engineers, June 1988

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

4. T.S. Hahn and A.R. Marder, Effect of Electropolishing Variables on the Cur- rent Density--Voltage Relationship, Metallography, Vol 21, 1988, p 365

5. M. Clark and A. Cervoni, " In Situ

Metallographic Examination of Fer- rous and Non-Ferrous Components," Canadian Electrical Association, Nov 1985

6. J.F. DeLong, private communication 7. D. Kay, Ed., Techniques for Electron

Microscopy, Blackwell Scientific Publi- cations, 1965

8. J.W. Edington, Practical Electron Mi- croscopy in Materials Science, Van Nostrand Rheinhold, 1976

9. G.N. Maniar and A. Szirmae, in Man- ual on Electron Metallography Tech- niques, STP 547, American Society for Testing and Materials, 1973

10. P.B. Ludwigsen, Non-Destructive Ex- amination, Structure, Sept 1987, p 3

11. B. Neubauer and U. Wedel, NDT: Rep- lication Avoids Unnecessary Replace- ment of Power Plant Components, Pow- er Eng., May 1984, p 44

12. F. Masuyama, K. Setoguchi, H. Haneda, and F. Nanjo, Findings on Creep-Fatigue Damage in Pressure Parts of Long-Term Service-Exposed Thermal Power Plants, in Residual Life Assessment Nondestructive Examina- tion and Nuclear Heat Exchanger Ma- terials, PVP-Vol 98-1, Proceedings of the Pressure Vessels and Piping Confer- ence, American Society of Mechanical Engineers, 1985, p 79

13. T. Fushimi, "Life Evaluation of Long Term Used Boiler Tubes," Paper pre- sented at the Conference on Boiler Tube Failures in Fossil Plants (Atlanta), Electric Power Research Institute, Nov 1987

14. A. Afrouz, M.J. Collins, and R. Pilking- ton, Microstructural Examination of ICr-0.5Mo Steel During Creep, Met. Technol., Vol 10, 1983, p 461