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REFERENCE NO. PNRA-CNS-NDT-50-13 MAY 2013

TECHNICAL REPORT ON

REPLICA METALLOGRAPHY AND PENETRANT TESTING

PAKISTAN NUCLEAR REGULATORY AUTHORITY P.O. BOX 1912, ISLAMABAD

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Contents

1. INTRODUCTION ........................................................................................................................................ 1

2. REPLICA METALLOGRAPHY ................................................................................................................ 1

2.1 GENERAL STEPS FOLLOWED IN REPLICA METALLOGRAPHY ........................................................................ 2A. SURFACE PREPARATION .................................................................................................................................... 2B. PREPARATION OF REPLICA ................................................................................................................................ 2C. MICROSTRUCTURAL ANALYSIS AND INTERPRETATION OF RESULTS ................................................................. 3

3. LIQUID PENETRANT TESTING (PT) ...................................................................................................... 4

3.1 BASIC STEPS PT ............................................................................................................................................ 43.2 ADVANTAGES AND DISADVANTAGES OF PENETRANT TESTING .................................................................... 7

4. CONCLUSION ............................................................................................................................................ 8

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List of Figures

Figure 1: Portable Grinder/Polisher 2Figure 2: Transcopy Replica Kit 3Figure 3: Micrographs; Specimen (Left) and Prepared Replica (Right) 4Figure 4: Application of penetrant after surface preparation 5Figure 5: Indication development after application of developer 6Figure 6: Inspection of indication and position measurement 7

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1. INTRODUCTION

Nondestructive testing (NDT) consists of test methods used to examine an object, material or system without impairing its future usefulness. A limited scale NDT laboratory has been established in PNRA for training purpose of nuclear inspectors and to enhance the quality of regulatory oversight of nuclear power plants in Pakistan. The following equipments are available in the laboratory:

1. Ultrasonic flaw detector 2. Ultrasonic thickness gauge 3. Portable hardness tester 4. Eddy current test equipment 5. Coating thickness gauge 6. Penetrant testing 7. Universal hardness tester 8. Replica Metallographic Equipment

A PNRA task force comprising officers of CNS and SNRS was established to make the equipment functional. This report is submitted after completion of self training on the Replica Metallographic Equipment and Penetrant Technique. The following team was engaged in this activity:

1. Muhammad Rafiq (SE, CNS) 2. Aliullah Jan (SE, CNS) 3. Mansoor Arshad (AE, CNS) 4. Zafar Abbas (AE, SNRS)

The training program comprised of the following activities:

1. Study of Equipment Manual of Replica Metallography 2. Study of PT Techniques 3. Practical Work

2. REPLICA METALLOGRAPHY 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.

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This technique gives the flexibility of observing microstructures in the comfort of a lab instead of harsh plant conditions (heat, radiation etc.).

2.1 General Steps Followed In Replica Metallography In Replica Metallography steps followed are similar to that of common methods used in laboratory metallography with the exception that the part to be viewed is not required to be brought to the laboratory

2.2 Surface Preparation Surface preparation holds key in revealing the microstructure to be studied. It involves steps like grinding, polishing and etching. 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. In Situ, Replication Metallography utilizes portable grinding and polishing unit to serve this purpose. Grinder/Polisher unit is shown in figure 1.

Figure 1: Portable Grinder/Polisher

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2.3 Preparation of Replica

During work at NDT Lab SNRS these steps were omitted because the equipment was not in working condition. Instead already prepared specimens were used for preparation of 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 effort because it lends itself more favorably to on-site preparation. Replication of surface was performed by using Transcopy Replica-Kit. Replica of the sample was prepared as:

i. Two drops of replication fluid were applied to the replica foil.

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ii. Replica strip was pressed firmly against the prepared sample surface for at least 30 seconds so that clear impression of the specimen can be obtained for observation under microscope.

iii. Prepared replica was fixed on a glass slide by removing its back cover tape.

Figure 2: Transcopy Replica Kit

2.4 Microstructural Analysis and Interpretation of Results Microstructural observation is the key step for which all the specimen preparation and replication is done. It is the step which reveals type of microstructure and the extent of damage sustained by the component since it went into service. Crack determination is important to help establish the root cause of a potential failure in a component. Creep defects cause the majority of failures in power plant components operating under stress and thermal load, and the replica method is especially suitable for the detection of these defects. The detection of various deleterious precipitates in components subjected to high temperature and stress can lead to improved life assessment analysis of these components.

Prepared replicas were viewed by using portable microscope and were compared with actual microstructure of the specimens as shown in figure 3. Basic technique of utilizing the equipment for preparation and observation of replicas has been practiced whereas interpretation of results will require the team to be trained at advanced level to interpret the results.

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Figure 3: Micrographs; Specimen (Left) and Prepared Replica (Right)

3. LIQUID PENETRANT TESTING (PT)

Liquid penetrant inspection is a method that is used to reveal surface breaking flaws by bleed out of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer applied. This acts as a blotter. It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrant require good white light while fluorescent penetrant need to be used in darkened conditions with an ultraviolet "black light"

3.1 Basic Steps PT Surface Preparation: One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear metal over the flaw opening and prevent the penetrant from entering.

Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the part in a penetrant bath.

Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected for. Minimum dwell times typically range from five(5) to sixty (60) minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry.

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Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treating the part with an emulsifier and then rinsing with water. These steps are shown in figure 4.

Figure 4: Application of penetrant after surface preparation

Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers).

Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes. Significantly longer times may be necessary for tight cracks.

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Figure 5: Indication development after application of developer

Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present.

Liquid penetrant inspection can only be used to inspect for flaws that break the surface of the sample. Some of these flaws are listed below:

• Fatigue cracks • Quench cracks • Grinding cracks • Overload and impact fractures • Porosity • Seams • Pin holes in welds • Lack of fusion or braising along the edge of the bond line

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Figure 6: Inspection of indication and position measurement

Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

3.2 Advantages and Disadvantages of Penetrant Testing

Like all nondestructive inspection methods, liquid penetrant inspection has both advantages and disadvantages. The primary advantages and disadvantages when compared to other NDE methods are summarized below:

Advantages:

• The method has high sensitivity to small surface discontinuities. • The method has few material limitations, i.e. metallic and nonmetallic, magnetic and

nonmagnetic, and conductive and nonconductive materials may be inspected. • Large areas and large volumes of parts/materials can be inspected rapidly and at low cost. • Parts with complex geometric shapes are routinely inspected. • Indications are produced directly on the surface of the part and constitute a visual

representation of the flaw. • Aerosol spray cans make penetrant materials easily portable. • Penetrant materials and associated equipment are relatively inexpensive.

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Disadvantages:

• Only surface breaking defects can be detected. • Only materials with a relatively nonporous surface can be inspected. • Pre-cleaning is critical since contaminants can mask defects. • Metal smearing from machining, grinding and grit or vapor blasting must be removed

prior to LPI. • The inspector must have direct access to the surface being inspected. • Surface finish and roughness can affect inspection sensitivity. • Multiple process operations must be performed and controlled. • Post cleaning of acceptable parts or materials is required. • Chemical handling and proper disposal is required.

4. CONCLUSION

Two experiments were performed each by Replica Metallography and Penetrant Testing technique. Penetrant testing experiment was performed successfully as a whole. It is to be noted that grinding/polishing of specimens was not possible due to inoperable condition of portable polishing unit. Steps should be taken to ensure that this equipment is in operational condition. On the plus side the basic process of replication and microscopy was performed on already polished samples available in the lab. Advance steps of Metallography i.e. interpretation of results require advanced level of training and skill in the field of physical metallurgy and require more sophisticated tools.

FUTURE TASKS

The future tasks include specialized training on:

i. Use of Coating thickness gauge ii. Use of Universal hardness tester