ASNT Level III- Visual & Optical Testing

My Level III Self Study Notes Reading 4 Section 4B

My Level III Self Study Notes Reading 4 Section 4B


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<strong>ASNT</strong> <strong>Level</strong> <strong>III</strong>- <strong>Visual</strong> & <strong>Optical</strong> <strong>Testing</strong><br />

My Pre-exam Preparatory<br />

Self Study Notes Reading 4 Section 4B<br />

2014-August<br />

Charlie Chong/ Fion Zhang

For my coming <strong>ASNT</strong> <strong>Level</strong> <strong>III</strong> VT Examination<br />

2014-August<br />

Charlie Chong/ Fion Zhang<br />


At works<br />

Charlie Chong/ Fion Zhang

Reading 4<br />

<strong>ASNT</strong> Nondestructive Handbook Volume 8<br />

<strong>Visual</strong> & <strong>Optical</strong> testing- Section 4B<br />

For my coming <strong>ASNT</strong> <strong>Level</strong> <strong>III</strong> VT Examination<br />

2014-August<br />

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang<br />

Fion Zhang<br />


SECTION 4<br />



Charlie Chong/ Fion Zhang



1.1 Effects of the Test Object<br />


2.1 Range of Characteristics<br />

2.2 Low Power Microscopes<br />

2.3 Medium Power Systems<br />

2.4 High Power Systems<br />

Charlie Chong/ Fion Zhang


3.1 Fiber Optic Borescopes<br />

3.2 Rigid Borescopes<br />

3.3 Special Purpose Borescopes<br />

3.4 Typical Industrial Borescope Applications<br />

3.5 Borescope <strong>Optical</strong> Systems<br />

3.6 Borescope Construction<br />

Charlie Chong/ Fion Zhang


4.1 Lighting Techniques<br />

4.2 <strong>Optical</strong> Filtering<br />

4.3 Image Sensors<br />

4.4 Image Processing<br />

4.5 Mathematical Morphology<br />

4.6 Image Segmentation<br />

4.7 <strong>Optical</strong> Feature Extraction for High Speed <strong>Optical</strong> Tests<br />

4.8 Conclusion<br />

Charlie Chong/ Fion Zhang


5.1 Cellulose Acetate Replication<br />

5.2 Silicon Rubber Replicas<br />

5.3 Conclusion<br />

Charlie Chong/ Fion Zhang


6.1 Other Temperature Indicators<br />

6.2 Certification of Temperature Indicators<br />

6.3 Applications for Temperature Indicators<br />

Charlie Chong/ Fion Zhang


7.1 Test Object Selection<br />

7.2 Surface Preparation<br />

7.3 Etching<br />

7.4 Using Etchants<br />

7.5 Conclusion<br />

Charlie Chong/ Fion Zhang


1.0 General<br />

SKIP the whole part.<br />

Charlie Chong/ Fion Zhang


5.0 General<br />

Replication is a valuable tool for the analysis of fracture surfaces and<br />

microstructures and for documentation of corrosion damage and wear. There<br />

is also potential for uses of replication in other forms of surface testing.<br />

Replication is a method used for copying the topography of a surface that<br />

cannot be moved or one that would be damaged in transferal. A police officer<br />

making a plaster cast of a tire print at an accident scene or a scientist malting<br />

a cast of a fossilized footprint are common examples of replication. These<br />

replicas produce a negative topographic image of the subject known as a<br />

single stage replica. A positive replica made from the first cast to produce a<br />

duplicate of the original surface is called a second stage replica. Many<br />

replicating mediums are commercially available. The types commonly used in<br />

nondestructive testing typically fall into one of two categories: cellulose<br />

acetate replicas and silicone rubber replicas. Both have advantages and<br />

limitations but both can also provide valuable information without altering the<br />

test object.<br />

Charlie Chong/ Fion Zhang

5.1 Cellulose Acetate Replication<br />

Acetate replicating material is used for surface cleaning, removal and<br />

evaluation of surface debris, fracture surface microanalysis and for<br />

microstructural evaluation. Single stage replicas are typically made, creating<br />

a negative image of the test surface. A schematic diagram of microstructural<br />

replication is shown in Fig. 56.<br />

5.1.1 Cleaning and Debris Analysis<br />

Fracture surfaces should be cleaned only when necessary. Cleaning is<br />

required when the test surface holds loose debris that could hinder analysis<br />

and that cannot be removed with a thy air blast. Cleaning debris from fracture<br />

surfaces is useful when the test object is the debris itself or the fracture<br />

surface. Debris removed from a fracture can be coated with carbon and<br />

analyzed using energy dispersive spectroscopy. This provides a<br />

semiquantitative analysis when a particular element is suspected of<br />

contributing to the fracture.<br />

Charlie Chong/ Fion Zhang

Removal of loose surface particles is usually done by wetting a piece of<br />

acetate tape on one side with acetone, allowing a short period for softening<br />

and applying the wet side of the tape to the area of interest. Thicker tapes of<br />

0.013 mm (0.005 in.) work best for such cleaning applications (thin tapes tend<br />

to tear). Following a short period, the tape hardens and is removed. This<br />

procedure is normally repeated several times until a final tape removes no<br />

debris from the surface.<br />

Charlie Chong/ Fion Zhang

5.1.2 Fracture Surface Analysis<br />

The topography of fracture surfaces can be replicated and analyzed using an<br />

optical microscope, scanning electron microscope or transmission electron<br />

microscope. The maximum useful magnification obtained using optical<br />

microscopes depends on the roughness of the fracture but seldom exceeds<br />

100 x . The scanning electron microscope has good depth of field at high<br />

magnifications and is typically used for magnification of 10,000 x or less. The<br />

transmission electron microscope has been used to document microstructural<br />

details up to 50,000 x . In general, scanning electron microscope analysis of a<br />

replica provides information regarding mode of failure and, in most instances,<br />

is sufficient for completion of this kind of analysis.<br />

Charlie Chong/ Fion Zhang

An example of a replicated fracture surface is shown in Fig. 57. The<br />

transmission electron microscope is used in instances where information<br />

regarding dislocations and crystallographic planes is needed. Both single<br />

stage (negative) and second stage (positive) replicas can be used for failure<br />

analysis. Some scanning electron microscope manufacturers offer a reverse<br />

imaging module that provides positive images from a negative replica. This<br />

eliminates the need to think and interpret in reverse. This feature has also<br />

proven valuable for evaluating microstructures through replication.<br />

As with the removal of surface debris, it has been found that the thicker<br />

replicas provide better results, for the same reasons. The procedure for<br />

replication of fracture surfaces is identical to that for debris removal. On rough<br />

surfaces, however, difficulty may be encountered when trying to remove the<br />

replica. This can cause replication material to remain on the fracture surface<br />

but this can easily be removed with acetone.<br />

Charlie Chong/ Fion Zhang

Replicas, in the as-stripped condition, typically do not exhibit the contrast<br />

needed for resolution of fine microscopic features such as fatigue striations.<br />

To improve contrast, shadowing or vapor deposition of a metal is performed.<br />

The metal is deposited at an acute angle to the replica surface and collects at<br />

different thicknesses at different areas depending on the surface topography.<br />

This produces a shadowing that allows greater resolution at higher<br />

magnifications. Shadowing with gold or other high atomic number metals<br />

enhances the electron beam interaction with the sample and greatly improves<br />

the image in the scanning electron microscope by reducing the signal-tonoise<br />

ratio.<br />

Charlie Chong/ Fion Zhang

5.1.3 Microstructural Interpretation<br />

To date, the greatest advances in the use of acetate replicas<br />

for nondestructive testing have come from their use in microstructural<br />

testing and interpretation. Replication is an integral part of visual tests in the<br />

power generation industries as well as in refining, chemical processing and<br />

pulp and paper plants. Replication, in conjunction with microstructural<br />

analysis, is used to quantify microstrain over time and to predict the remaining<br />

useful life of a component. Future applications are not limited by material type.<br />

In industry, tests are carried out at preselected intervals to assess the<br />

structural integrity of components in their systems. These components can be<br />

pressure vessels, piping systems or rotating equipment. Typically these<br />

components are exposed to stresses or an environment that limits their<br />

service life. Replication is used to evaluate such systems and to provide data<br />

regarding their metallurgical condition.<br />

Charlie Chong/ Fion Zhang

Microstructural replication is done in two steps: surface preparation followed<br />

by the replication procedure. Surface preparation involves progressive<br />

grinding and polishing until the test surface is relatively free of scratches<br />

(metallurgical quality). Depending on the material type and hardness, this<br />

can be obtained by using a I to 0.05 p.m (0.04 to 0.002 mil) polishing<br />

compound as the final step. Electrolytic polishing can increase efficiency if<br />

many areas are being tested. Surfaces can be electropolished with a 320-400<br />

grit finish. The disadvantages of electropolishing are that (1) the equipment is<br />

costly, (2) with most systems only a small area can he polished at one time<br />

and (3) pitting has been known to occur with some alloy systems containing<br />

large amounts of carbides.<br />

Next, the polished surface is etched to provide microstructural topographic<br />

contrast which may be necessary for evaluation. Etchants vary with material<br />

type and can be applied electrolytically, by swabbing or spraying the etchant<br />

onto the surface. With some materials, a combination of etch-polish etch<br />

intervals yields the most favorable results.<br />

Charlie Chong/ Fion Zhang

To replicate the surface microstructure, an area is wetted with acetone and a<br />

piece of acetate tape is laid on the surface. The tape is drawn by capillary<br />

action to the metal surface, producing an accurate negative image of the<br />

surface microstructure. Thin acetate tape at 0.025 mm (0.001 in.) provides<br />

excellent results and gives the best resolution at high magnifications. Thicker<br />

tapes must be pressed onto the test surface and, depending on the expertise<br />

of the inspector, smearing can result. Thicker tapes are more costly and the<br />

resolution of microscopic detail does not match thinner tapes. Studies of<br />

carbide morphology and creep damage mechanisms have been performed at<br />

magnifications as high as 10,000 x with thin tape replicas. Before removal of<br />

the tape from the test object, the back is coated with paint to provide a<br />

reflective surface that enhances microscopic viewing. The replica is removed<br />

and can be stored for future analysis.<br />

Charlie Chong/ Fion Zhang

If analysis with the scanning electron microscope is needed, replicas should<br />

be coated to prevent electron charging. This is accomplished by evaporating<br />

or sputter coating a thin conductive film onto the replica surface. Carbon, gold,<br />

gold-palladium and other metals are used for coating. There are differences in<br />

the sputtering yield from different elements and this should he remembered<br />

when choosing an element or when attempting to calculate the thickness of<br />

the coating. The main advantage of sputter coating over evaporation<br />

techniques is that it provides a continuous coating layer. Complete coating is<br />

accomplished without rotating or tilting the replica. With evaporation, only line<br />

of sight areas are coated and certain areas typically are coated more than<br />

others.<br />

Charlie Chong/ Fion Zhang

Some examples of replicated microstructures, documented with both a<br />

scanning electron microscope and with conventional optical microscopy, are<br />

shown in Figs. 58 to 61. Replication is used for detection of high temperature<br />

creep damage, stress corrosion cracking, hydrogen cracking mechanisms,<br />

as well as the precipitation of carbides, nitrides and second phase<br />

precipitates such as sigma or gamma prime. Replication is also used for<br />

distinguishing fabrication discontinuities from operational discontinuities.<br />

Charlie Chong/ Fion Zhang

Sputter deposition<br />

is a physical vapor deposition (PVD) method of thin film deposition by<br />

sputtering. This involves ejecting material from a "target" that is a source onto<br />

a "substrate" such as a silicon wafer. Resputtering is re-emission of the<br />

deposited material during the deposition process by ion or atom<br />

bombardment. Sputtered atoms ejected from the target have a wide energy<br />

distribution, typically up to tens of eV (100,000 K). The sputtered ions<br />

(typically only a small fraction — order 1% — of the ejected particles are<br />

ionized) can ballistically fly from the target in straight lines and impact<br />

energetically on the substrates or vacuum chamber (causing resputtering).<br />

Alternatively, at higher gas pressures, the ions collide with the gas atoms that<br />

act as a moderator and move diffusively, reaching the substrates or vacuum<br />

chamber wall and condensing after undergoing a random walk. The entire<br />

range from high-energy ballistic impact to low-energy thermalized motion is<br />

accessible by changing the background gas pressure. The sputtering gas is<br />

often an inert gas such as argon.<br />

Charlie Chong/ Fion Zhang

For efficient momentum transfer, the atomic weight of the sputtering gas<br />

should be close to the atomic weight of the target, so for sputtering light<br />

elements neon is preferable, while for heavy elements krypton or xenon are<br />

used. Reactive gases can also be used to sputter compounds. The<br />

compound can be formed on the target surface, in-flight or on the substrate<br />

depending on the process parameters. The availability of many parameters<br />

that control sputter deposition make it a complex process, but also allow<br />

experts a large degree of control over the growth and microstructure of the<br />

film.<br />

Charlie Chong/ Fion Zhang

Sputtering<br />

http://en.wikipedia.org/wiki/Sputter_deposition<br />

Charlie Chong/ Fion Zhang

Sputtering<br />

http://en.wikipedia.org/wiki/Sputter_deposition<br />

Charlie Chong/ Fion Zhang

Sputtering<br />

http://clearmetalsinc.com/technology/<br />

Charlie Chong/ Fion Zhang

Brief Introduction to Coating Technology for Electron Microscopy<br />

Coating of samples is required in the field of electron microscopy to enable or<br />

improve the imaging of samples. Creating a conductive layer of metal on the<br />

sample inhibits charging, reduces thermal damage and improves the<br />

secondary electron signal required for topographic examination in the SEM.<br />

Fine carbon layers, being transparent to the electron beam but conductive,<br />

are needed for x-ray microanalysis, to support films on grids and back up<br />

replicas to be imaged in the TEM. The coating technique used depends on<br />

the resolution and application<br />

Charlie Chong/ Fion Zhang<br />


Sputter Deposition<br />

Charlie Chong/ Fion Zhang

Sputter deposition<br />

Charlie Chong/ Fion Zhang

5.1.4 Strain Replication<br />

The replication technique can be used to evaluate and quantify the<br />

occurrence of localized strain in materials exposed to elevated temperatures<br />

and stresses over time (materials susceptible to high temperature creep),<br />

Replication allows monitoring for accumulated strain before detectable<br />

microstructural changes occur. Strain replication involves inscribing a grid<br />

pattern onto a previously polished surface. A reference grid pattern is<br />

replicated using material with a shrinkage factor that has been quantified<br />

through analysis. This known shrinkage factor is included in future<br />

numerical analysis of strain. The grid is then coated to prevent surface<br />

oxidation during use. After a predetermined period of operation, the coating is<br />

removed and the area is again replicated. The grid intersection points on the<br />

two replicas are compared for dimensional changes and the changes are<br />

then correlated to units of strain. This technique does not yield absolute<br />

values of strain but does provide the change in strain calculated over time.<br />

This gives the operator information that can help approximate where the<br />

component is in its service life.<br />

Charlie Chong/ Fion Zhang

Strain replication is especially useful for materials that do not exhibit creep<br />

void formation until late in their service life. As long as the strain calculations<br />

indicate a linear relationship between strain and time, the material is still said<br />

to be in the second stage region on the creep curve (see Fig. 62 and Table<br />

14). When the relationship deviates from linearity, the material has begun<br />

third stage or tertiary creep, where the strain rate can become unstable.<br />

Charlie Chong/ Fion Zhang

FIGURE 56. Principles of acetate tape replication producing a negative image of the surface: (a)<br />

microstructure cross section, In softened acetate tape applied, (cJ replica curing and (d) replica<br />

removal<br />

Charlie Chong/ Fion Zhang

FIGURE 57. Fracture surface documentation using replication shows fatigue striations on the<br />

surface at magnifications originally of (a) 2,000 x and (b) 10,000 x<br />

Charlie Chong/ Fion Zhang

Fatigue striations<br />

Charlie Chong/ Fion Zhang

FIGURE 58. Comparison of optical microscopy to scanning electron microscopy in the<br />

documentation of a replicated microstructure; evidence of creep damage is visible in the grain<br />

boundaries; etchant is aqua regia; 100 x original magnification: (a) optical microscope image and<br />

(b) scanning electron microscope image<br />

Charlie Chong/ Fion Zhang

FIGURE 59. Documentation of creep damage: (al a weld viewed originally at 500 x in an optical<br />

microscope; the microstructure consists of an austenitic matrix, precipitated nitrides and carbides;<br />

linked creep voids can be observed; and lb) the alloy in Fig. 58 viewed originally at 1,000 x in a<br />

scanning electron microscope; grain boundary carbides, creep voids and particles believed to be<br />

nitrides can be observed in the matrix<br />

Charlie Chong/ Fion Zhang

Creep Void<br />

Charlie Chong/ Fion Zhang

FIGURE 60. Documentation of stress corrosion cracking found in the welds of an anhydrous<br />

ammonia sphere; 3 percent nital etch at 200 x original magnification<br />

Charlie Chong/ Fion Zhang

FIGURE 61. Documentation of heat affected zone cracking in A516 grade 70 steel; cracking<br />

associated with a nonstress relieved repair weld; the presence of this repair weld was not known<br />

until in-field metallography and replication were performed; 3 percent nital etch at 100 x original<br />

magnification<br />

Charlie Chong/ Fion Zhang

TABLE 14. Action required for creep damage in typical stressed material (see Figure 62)<br />

Damage Parameter<br />

isolated cavities<br />

Oriented cavities<br />

Microcracks<br />

Macrocracks<br />

Action Required<br />

no action until next major<br />

scheduled maintenance outage<br />

replica test at specified intervals<br />

limited service until repair<br />

immediate repair<br />

Charlie Chong/ Fion Zhang

FIGURE 62. Creep damage curve showing the typical relationship of strain to time for a material<br />

under stress in a high temperature atmosphere; note that development of creep related voids in<br />

this alloy occurs early in service life; their eventual linkage is shown schematically on the curve<br />

[see reference 27): fa) isolated cavities, (b) oriented cavities, (c) microcracks and (d)<br />

macrocracks (see Table 14)<br />

http://www.scielo.br/scielo.php?pid=S1516-14392004000100021&script=sci_arttext<br />

Charlie Chong/ Fion Zhang

Cellulose Acetate Replication<br />

http://corrosionhelp.com/failures.htm<br />

Charlie Chong/ Fion Zhang

Replication Microscopy Techniques for NDE<br />

Fig. 2 Schematic of the plastic replica technique<br />

Charlie Chong/ Fion Zhang<br />


Replication Microscopy Techniques for NDE<br />

Figure 3 Positive carbon extraction replication steps, (a) Placement of plastic after the first etch.<br />

(b) After the second etch. (c) After the deposition of carbon. (d) The positive replica offer the<br />

plastic is dissolved<br />

Charlie Chong/ Fion Zhang<br />


Replication Microscopy Techniques for NDE<br />

Figure 3 Positive carbon extraction replication steps, (a) Placement of plastic after the first etch.<br />

(b) After the second etch. (c) After the deposition of carbon. (d) The positive replica offer the<br />

plastic is dissolved<br />

Charlie Chong/ Fion Zhang

Replication Microscopy Techniques for NDE<br />

Figure 3 Positive carbon extraction replication steps, (a) Placement of plastic after the first etch.<br />

(b) After the second etch. (c) After the deposition of carbon. (d) The positive replica offer the<br />

plastic is dissolved<br />

Charlie Chong/ Fion Zhang

5.2 Silicone Rubber Replicas<br />

Silicone impression materials have been used extensively in medicine,<br />

dentistry and in the science of anthropology. In nondestructive testing,<br />

silicone materials are used as tools for documenting macroscopic and<br />

microscopic material detail. Quantitative measurements can be obtained for<br />

depth of pitting, wear, surface finish and fracture surface evaluation. Silicone<br />

material is made with varying viscosities, setting times and resolution<br />

capabilities. Compared to an acetate replica, the resolution characteristics of<br />

a silicone replica is limited. With a medium viscosity compound, fine features<br />

visible at 50 x can be resolved but difficulties are encountered at higher<br />

magnifications. With a low viscosity compound, slightly better resolution is<br />

obtained but curing times are long and not suited to field applications. The<br />

lower viscosity medium is also known to creep with time and is not<br />

recommended for applications where very accurate dimensional studies are<br />

needed.<br />

Charlie Chong/ Fion Zhang

5.2.1 Use of Silicone Replicating Materials<br />

Silicone replicating materials are supplied in two parts: a base material and<br />

an accelerator. Although it is best to follow the recommended mixing ratios,<br />

these can be altered slightly to change the working time of the material. The<br />

two parts are mixed thoroughly and spread over the subject area. Additional<br />

material can he added to thicken the replica. Molding clay can also be used to<br />

build a dam around a replicated area. The dam supports the replica as its<br />

sets and allows thicker replicas to be made. Measurements of pit depth and<br />

surface finish can be obtained easily because of the silicone's ability to flow<br />

into crevices on the test object. To evaluate pit depth and surface finish, the<br />

replica is cut and the cross section is examined with a microscope or a<br />

macroscopic measuring device (a micrometer or an optical comparator).<br />

Charlie Chong/ Fion Zhang

Wear can be determined in a similar manner by replicating and comparing a<br />

worn surface to an unworn surface (see Fig. 63). Fracture surfaces with rough<br />

contours can be easily replicated with silicone (taking an acetate replica of<br />

such surfaces is difficult). However, the resolution characteristics of a silicone<br />

replica are not as good as acetate replicas and this limits the amount of<br />

interpretation that can be performed. Macroscopic details such as chevron<br />

markings can be easily located with the silicone technique to determine crack<br />

propagation direction or to trace a fracture path visually to its origin.<br />

Charlie Chong/ Fion Zhang

FIGURE 63. Silicon replicas used to determine wear variance on a failed pinion gear<br />

Charlie Chong/ Fion Zhang

Brittle Failure Chevron Marking<br />

Charlie Chong/ Fion Zhang

Brittle Failure Chevron Marking<br />

Charlie Chong/ Fion Zhang

5.3 Conclusion<br />

Cellulose acetate tape and silicone impression materials are commonly used<br />

for nondestructive visual tests of surface phenomena such as corrosion, wear,<br />

cracking and microstructures. Both types of replicating material have<br />

advantages and limitations but when used in the correct application, can<br />

provide valuable information. In terms of resolution, the silicone replica<br />

typically does not have the capability to copy fine detail above 50 x . The<br />

acetate replica can reveal detail up to 50,000 x on a transmission electron<br />

microscope. The acetate replica is limited, however, by the roughness of the<br />

topography it can copy. On rough fracture surfaces, difficulty is encountered<br />

in both applying and removing an acetate replica. The silicone material is not<br />

as restrictive in terms of the surface features it can copy. The need for fine,<br />

resolvable detail versus macroscopic features normally indicates whether<br />

acetate or silicone replicas are best for the application.<br />

Charlie Chong/ Fion Zhang


6.0 General<br />

A temperature indicating stick (chalk or crayon) is typically made of materials<br />

with calibrated melting points and temperature measuring accuracies to ± 1<br />

percent. Indicators are available in closely spaced increments over a range<br />

from 38 °C (100 °F) to 1,370 °C (2,500 °F). The workpiece to be tested is<br />

marked with the stick. When the workpiece attains the predetermined melting<br />

point of the indicator mark, the mark instantly liquefies, notifying the observer<br />

that the workpiece has reached that temperature.<br />

Charlie Chong/ Fion Zhang

Pre-marking with a stick is not practical under certain circumstances- when a<br />

heating period is prolonged a highly polished surface does not readily accept<br />

a mark or the marked material gradually absorbs the liquid phase of the<br />

indicator. In such instances, the operator frequently marks the workpiece with<br />

the stick. The desired temperature is noted when one ceases to make dry<br />

marks and begins to leave a liquid smear. A similar procedure can be<br />

employed to indicate temperature during a cooling cycle. But a melted mark,<br />

on cooling, will not solidify at the exact same temperature at which it melted,<br />

so solidification of a melted indicator mark cannot be relied on for temperature<br />

indication. Temperature ratings are in increments as small as 3.4 °C (6 °F)<br />

but increments ranging from 14 to 28 °C (25 to 50 °F) are typically used for<br />

welding applications. For most applications, a jump of 28 to 56 °C (50 to 100<br />

°F) and a range of sticks up to 650 °C (1,200 °F) are usually adequate.<br />

Charlie Chong/ Fion Zhang

Temperature indicating sticks were developed in America by a metallurgist<br />

working on submarine hulls in the 1930s. At the time, preheat was measured<br />

with so-called melting point standards, granules of substances with known<br />

melting points used to calibrate heat sensing instruments. The engineer used<br />

the granules directly, spreading them on the preheated metal and using their<br />

melt as a signal to proceed with welding.<br />

The melting point granules were next formed into sticks held together with<br />

organic hinders. Different temperature ratings were added and some<br />

refinements have been made but the principle of indicators has remained<br />

unchanged. The sticks make physical contact with the heated test object,<br />

reach thermal equilibrium rapidly and do not conduct heat away from the test<br />

surface. For temperature ratings less than 340 °C (650 °F), indicator marks<br />

can usually be removed with water or alcohol. For ratings above 340 °C<br />

(650 °F), water is preferred. If the mark has been heated well above the<br />

rated temperature and has become charred, abrasion may be needed for<br />

complete removal.<br />

Charlie Chong/ Fion Zhang

6.1 Other Temperature Indicators<br />

In addition to the stick, temperature indicating pellets and liquids are available.<br />

The liquid indicator is brushed on before welding starts and is useful on highly<br />

polished surfaces or for making large marks viewed at a distance. Heat<br />

indicating pellets, about the size and shape of an aspirin, have greater mass<br />

than stick or lacquer marks (see Fig. 64). Pellets are sometimes selected for<br />

use with large, heavy pieces requiring prolonged heating- applications where<br />

stick or lacquer marks could fade with time.<br />

Charlie Chong/ Fion Zhang

<strong>Level</strong>s of<br />

Charlie Chong/ Fion Zhang

FIGURE 64, Temperature indicating pellets<br />

Charlie Chong/ Fion Zhang

6.2 Certification of Temperature Indicators<br />

Temperature indicating sticks are mixtures of organic and inorganic<br />

compounds. The purity of the source materials directly affects the accuracy of<br />

the predicted melting point. There is the possibility of contamination with trace<br />

quantities of other elements, which may be detrimental to the accuracy of the<br />

indicator. In some cases, low melting point materials (lead, tin, sulfur,<br />

halogenated compounds) may be undesirable for the welding procedure.<br />

Most manufacturers can provide certification supported by analyses of typical<br />

batches. Documentation indicates which temperature ratings may contain<br />

contaminants that can be avoided by the user.<br />

In some critical applications (nuclear fabrication, aircraft assembly), actual<br />

chemical analysis of the specific lot number of the temperature indicators may<br />

be required. If the customer supplies a written certification requirement listing<br />

the compounds to be tested for, most manufacturers will send lot numbered<br />

samples for laboratory analysis. The customer is usually expected to pay lab<br />

charges for such specialized requirements.<br />

Charlie Chong/ Fion Zhang

Marking materials used on austenitic stainless steels typically have a certified<br />

analysis that meets the following specified maximum amounts of detrimental<br />

contaminants:<br />

1. inorganic halogen content less than 200 ppm by weight;<br />

2. halogen (inorganic and organic) content less than 1 percent by weight;<br />

3. sulfur content less than 1 percent by weight (measured in accordance with<br />

ASTM D129); and<br />

4. total content of low melting point metal (lead, bismuth, zinc, mercury,<br />

antimony and tin) less than 200 ppm by weight and no individual metal<br />

content greater than 50 ppm by weight.<br />

The certification typically indicates the methods and accuracy of analysis and<br />

the name of the testing laboratory.<br />

Charlie Chong/ Fion Zhang

6.3 Applications for Temperature Indicators<br />

Temperature indicators can be used for preheat temperature tests and in<br />

annealing and stress relieving procedures, hardfacing, overlaying for<br />

corrosion resistance, flame cutting, flame conditioning, heat treating, pipe<br />

bending, shearing of bar steel, straightening hardened parts, shrink fitting,<br />

brazing, soldering and nonferrous fabrication. The indicators can help find hot<br />

spots in insulation and engines, help monitor temperatures in curing and<br />

bonding operations and help check pyrometric calibration.<br />

Charlie Chong/ Fion Zhang

6.3.1 Tests of Railway Bearings<br />

Bearing breakdown can be detected by using fluorescent temperature<br />

indicating pellets as heat sensors for inboard journal boxes. The pellets are<br />

inserted in a specially fabricated stainless steel holder that contains two<br />

pellets. The holder is inserted into the hollow axle of each rail car with an<br />

insertion tool. The tool has a mechanical stop to ensure that the holder is<br />

located at a predetermined depth. This permits proper monitoring of journal<br />

box operating temperatures. Once a specified temperature is exceeded, in<br />

this case 100 °C (212 °F), the pellets melt and flow completely out of the<br />

holder. The fluorescent material is easy to detect and clearly indicates that<br />

excessive heat has been conducted from the bearing to the axle.<br />

Charlie Chong/ Fion Zhang

6.3.2 Verifying Oven Temperatures<br />

Technicians can determine if self cleaning ovens reach the proper cleaning<br />

temperature using pellets with pre-calibrated melting points at 450 °C (850<br />

°F). The pellets are placed on a flat piece of aluminum foil situated on the<br />

oven's center rack (see Fig. 65). The cleaning cycle is activated and as the<br />

temperature reaches 450 °C (850 °F), the pellets begin to melt. When the<br />

cleaning cycle is completed and the oven has cooled, the pellets are<br />

inspected—complete melting of the tablet verifies that the nominal cleaning<br />

temperature has been achieved.<br />

Charlie Chong/ Fion Zhang

FIGURE 65. Pellets used to verify oven temperatures over 450 °C (850 °C)<br />

Charlie Chong/ Fion Zhang

6.3.4 Process Control Applications<br />

A gas tight seal is needed to prevent leakage of combustion gases through<br />

the glass portion of a spark plug. To obtain optimum fusion properties, it is<br />

important to know and control the temperature inside the ceramic insulator<br />

and this can be done using a temperature indicating pellet. Sample insulators<br />

are loaded with pellets and processed with production parts. Information<br />

obtained from analyzing the samples is used to adjust furnace conveyor<br />

speed and temperature.<br />

Charlie Chong/ Fion Zhang

6.3.5 Monitoring Fabric Seam Temperature<br />

In the making of specialized cloth (protective clothing, aerostat balloons),<br />

seam integrity is an important manufacturing function. A good radiofrequency<br />

seal can be achieved on a given fabric substrate only within a specific<br />

temperature range, determined by the minimum temperature needed to<br />

ensure a complete seal and the maximum temperature possible before<br />

material degradation. Constant temperature control and verification are<br />

required. This can be achieved using temperature sensitive strips (one for the<br />

upper limit, one for the lower limit) applied to the sealing tape used in<br />

production. A visual test of each seam after sealing indicates whether the<br />

seam temperature was within the required range, allowing visual verification<br />

of conditions for all dielectric seams.<br />

Charlie Chong/ Fion Zhang

6.3.6 Precise Post forming Heat Control<br />

Temperature indicating materials are incorporated into<br />

many industrial applications where an indication is needed<br />

to show that a critical temperature has or has not been<br />

reached. A phase changing fusible liquid is used to indicate<br />

optimum postforming temperatures when bending decorative<br />

laminate for the contoured edges of countertops, desks,<br />

tables and other surfaces (see Fig. 66).<br />

Postforming is the process of bending a flat sheet of laminate<br />

around a radiused core material (particle board, plywood<br />

or fiber board). The process is typically done after controlled heating<br />

monitored with temperature sensitive liquids. Postforming can be a manual or<br />

mechanical operation. Hand postforming is used for unusual configurations or<br />

limited quantity production and mechanical postforming is used<br />

for high quantity production. Both methods have the need for a heat source,<br />

prepared cores, postforming grades of decorative laminate, pressuring guides<br />

and evenly applied pressure.<br />

Charlie Chong/ Fion Zhang

A core is prepared by first shaping the edges to be laminated.<br />

The core and laminate are evenly coated with a<br />

contact adhesive, preferably a spray. The laminate is positioned<br />

and registered with the core, allowing the laminate to<br />

overhang the radius. Postforming grades of decorative laminate<br />

are formable between temperatures of 156 and 163 °C<br />

(313 and 325 °F). A popular example of hand postforming is the 180 degree<br />

edge wrap. In this example, radiant heat is applied to the<br />

decorative surface of the laminate with the work supported<br />

over the heat. To determine the proper postforming temperature,<br />

the temperature indicating liquid is painted in stripes<br />

onto the laminate. When the liquid changes from a dry<br />

(matte) to a wet (melted) appearance, the assembly is wiped<br />

into the cavity of a fixture to form the 180 degree radius. The<br />

fixture is a U channel made by two boards attached to a base.<br />

The dimension of the U channel is the thickness of the core<br />

plus the thickness of the laminate, allowing about 0.5 mm<br />

(0.02 in.) clearance.<br />

Charlie Chong/ Fion Zhang

Another example of handforrning is known as a full wrap.<br />

In this application, the core is positioned over radiant heaters<br />

with temperature indicating stripes painted on the adhesive<br />

in the area of the radius. When the melt indicates forming<br />

temperature has been reached, the assembly is moved back<br />

onto a flat supporting surface. The wrapping action uses the<br />

flat surface as a pressure point.<br />

An example of mechanical postforming is the roll forming<br />

machine. Radiant heaters are located above an assembly<br />

supported by a moving carrier. When the forming temperature<br />

has been reached, slanted forming bars wipe the laminate<br />

over the radius. After the laminate has been formed, a<br />

succession of rollers maintains pressure until the assembly<br />

has cooled. In this application, temperature sensitive liquid<br />

is painted onto the laminate in order to verify that the dwell time under heat<br />

has been sufficient for reaching forming<br />

temperature.<br />

Charlie Chong/ Fion Zhang

FIGURE 66. Laminate postforming around a radiused core<br />

Charlie Chong/ Fion Zhang

6.3.7 Pipeline Coatings<br />

Epoxy powders are specially formulated to enhance corrosion<br />

proof resistance of utility pipe: that is, pipe usually buried<br />

underground, where it is subject to widely varying<br />

pipeline operating conditions. Intimately bonded to the<br />

pipe, the bonded epoxy is unaffected by widely varying soil<br />

compaction, moisture penetration, fungus attack, soil acids<br />

and chemical degradation. To achieve a long lasting bond of epoxy coating to<br />

metal pipe, the pipe must be preheated very carefully to the recommended<br />

preheat of 230 °C (450 °F). A spot on the pipe needs to be touched with the<br />

stick; its melting shows that the correct temperature for coating has been<br />

reached.<br />

Charlie Chong/ Fion Zhang

6.3.8 Preheating before Welding<br />

Heating to the proper temperature before welding lessens<br />

the danger of crack formation and shrinkage stresses in many<br />

metals. Hard zones near the weld are reduced and lessen the<br />

possibility of distortion. Preheating also helps diffuse hydrogen<br />

from steel and helps reduce the likelihood of subsequent<br />

hydrogen inclusions.<br />

The need for preheating increases with the mass of the<br />

material being welded. It is most useful for the thick, heavy<br />

weldments used in bridge construction, shipbuilding, pipelines<br />

and pressure vessels. Preheating is also recommended<br />

for (1) welding done at or below – 18 °C (0 °F); (2) when the<br />

electrode is a small diameter; (3) when the joined pieces are<br />

of different masses; (4) when the joined pieces are of complex<br />

cross section; and (5) for welding of high carbon or manganese<br />

steels.<br />

Charlie Chong/ Fion Zhang

The most common use for temperature indicators is the<br />

measurement of preheat, postheat and interpass temperatures<br />

for welding. In a typical application, the welder marks<br />

the test surface with an indicating stick of a specific temperature<br />

rating (see Fig. 67). When the mark changes phase<br />

(melts), the material has reached the correct temperature<br />

and is ready for welding. It is important for the user to<br />

understand that change of color has no significance; only the<br />

actual melting of the mark should be considered.<br />

Oxyacetylene equipment cannot be used for welding or<br />

cutting of high strength steels used in automotive components<br />

because too much heat can reduce their structural<br />

strength. However, in some instances an oxyacetylene torch<br />

may be used if the critical temperature of 760 °C (1,400 °F)<br />

for high strength steel is not exceeded.<br />

When preheat temperatures are 370 °C (700 °F) or when<br />

heating is prolonged, an indicating mark could evaporate or<br />

could be absorbed by the test material, Under these conditions,<br />

marks should be added periodically during heating.<br />

Charlie Chong/ Fion Zhang

When the rated temperature is reached, the stick leaves a liquid<br />

streak instead of a dry mark and welding can begin.<br />

To ensure accurate temperature indication with no override,<br />

two or more indicators can be used to alert the operator<br />

that the test object is approaching the correct temperature.<br />

When a range of recommended preheat temperatures is<br />

given, use of several indicators might be appropriate. For<br />

example, carbon-molybdenum steel should he preheated to<br />

between 95 and 205 °C (between 200 and 400 °F). A bundle<br />

of indicators with ratings at 95, 120, 150, 175 and 205 °C<br />

(200, 250, 300, 350 and 400 °F) might be useful for<br />

determining how much of the test object is within the preheat<br />

temperature range.<br />

Charlie Chong/ Fion Zhang

FIGURE 67. Temperature indicating stick<br />

Charlie Chong/ Fion Zhang

Preheating<br />

Charlie Chong/ Fion Zhang


7.0 General<br />

The information contained in this text is simplified and<br />

provided only for general instruction. Local health (OSHA)<br />

and environmental (EPA) authorities should be consulted<br />

about the proper use and disposal of chemical agents. For<br />

reasons of safety, all chemicals must he handled with care,<br />

particularly the concentrated chemicals used as aids to visual<br />

and optical tests.<br />

In visual nondestructive testing, chemical techniques are<br />

used to clean and enhance test object surfaces. Cleaning<br />

processes remove dirt, grease, oil, rust and mill scale. Contrast<br />

is enhanced by chemical etching.<br />

Charlie Chong/ Fion Zhang

Macroetching is the use of chemical solutions to attack<br />

material surfaces to improve the visibility of discontinuities<br />

for visual inspection at normal and low power magnifications.<br />

Caution is required in the use of these chemicals—the use<br />

of protective clothing and safety devices is imperative. Test<br />

object preparation and the choice of etchant must be appropriate<br />

for the inspection objectives. Once the desired etch is<br />

achieved, the metal surface must be flushed with water to<br />

avoid over etching.<br />

Charlie Chong/ Fion Zhang

7.1 Test Object Selection<br />

Figure 68 shows typical test objects removed from their<br />

service environment. Governing codes, standards or specifications<br />

may determine the number and location of visual<br />

tests. Specific areas may contain discontinuities from forming<br />

operations such as casting, rolling, forging or extruding.<br />

Weld tests may be full length or random spots and typically<br />

cover the weld metal, fusion line and heat-affected zone.<br />

The service of a component may also indicate problem areas<br />

requiring inspection. Location of the test site directly affects surface<br />

preparation. The test site may he prepared and nondestructively<br />

inspected in situ. Removal of a sample for laboratory examination<br />

is a destructive alternative test method that typically requires a repair weld.<br />

Charlie Chong/ Fion Zhang

FIGURE 68. Components removed from service for visual testing<br />

Charlie Chong/ Fion Zhang

7.2 Surface Preparation<br />

Preparation of the test object before etching may require<br />

only cleaning or a process including cleaning, grinding and<br />

fine polishing (improper grinding is shown in Fig. 69). The<br />

extent of these operations depends on the etchant, the material<br />

and the type of discontinuity being sought.<br />

7.2.1 Solvent Cleaning<br />

Solvent cleaning can be useful at two stages in test object<br />

preparation. An initial cleaning with a suitable solvent<br />

removes dirt, grease and oil and may make rust and mill scale<br />

easier to remove. One of the most effective cleaning solvents is a solution of<br />

detergent and water. However, if water is detrimental to the<br />

test object, organic solvents such as ethyl alcohol, acetone or<br />

naphthas have been used. These materials generally have<br />

low flash points and their use may be prohibited by safety<br />

regulations. Safety solvents such as the chlorinated hydrocarbons and high<br />

flash point naphthas may be required to meet safety standards.<br />

Charlie Chong/ Fion Zhang

FIGURE 69. Improper surface preparation; the grind marks mask indications and even a severe<br />

etchant does not give good test results<br />

Charlie Chong/ Fion Zhang

7.2.2 Removing Rust and Scale<br />

Rust and mill scale are normally removed by mechanical<br />

methods such as wire brushing or grinding. If appropriate<br />

for a particular test, the use of a severe etchant requires only<br />

the removal of loose rust and mill scale. Rust may also be<br />

removed chemically. Commercially available rust removers<br />

are generally inhibited mineral acid solutions and are not<br />

often used for test object preparation.<br />

Most surface tests require complete removal of rust and<br />

mill scale but a coarsely ground surface is often adequate<br />

preparation before etching.<br />

Grinding may be done manually or by belt, disk or surface<br />

grinding tools. Surface grinders are usually found only in<br />

machine shops. Hand grinding requires a hard flat surface<br />

to support the abrasive sheet. Coolant is needed during<br />

grinding and water is the preferred coolant but kerosene may<br />

be used if the test material is not compatible with water.<br />

Charlie Chong/ Fion Zhang

7.2.3 Grinding and Polishing<br />

Fine grinding and polishing are needed for visual tests of<br />

small structural details, welds and the effects of heat treatment.<br />

Finer grinding usually is done with 80 to 150 abrasive<br />

grit followed by 150 to 180 grit and finally 400 grit (an American<br />

indication of grit size, 400 being the finest). At each<br />

stage, marks from previous grinding must be completely removed. Changing<br />

the grinding direction between successive<br />

stages of the process aids the visibility of previous<br />

coarser grinding marks. Coolant is required for grinding and<br />

typical abrasives include emery, silicon carbide, aluminum<br />

oxide and diamond.<br />

Charlie Chong/ Fion Zhang

If the required finish cannot be achieved by fine grinding<br />

with 400 grit abrasive, the test surface must be polished. Polishing<br />

is generally done with a cloth-covered disk and abrasive<br />

particles suspended in paste or water. Common<br />

polishing media include aluminum oxide, magnesium oxide,<br />

chromium oxide, iron oxide and diamond with particle sizes<br />

ranging from 0.5 to 15 μm. During polishing, it is critical that all marks from<br />

the previous<br />

step he completely removed. If coarser marks do not<br />

clear, it may be necessary to repeat a previous step using<br />

lighter pressure before continuing. Failure to do so can yield<br />

false indications.<br />

Charlie Chong/ Fion Zhang

7.3 Etching<br />

7.3.1 Choice of Etchant<br />

The etchant, its strength, the material and the discontinuity<br />

all combine to determine surface finish requirements (see<br />

Table 15). Properly selected etchants chemically attack the<br />

test material and reveal welds (Fig. 70), pitting (Fig 71),<br />

grain boundaries, segregation, laps, seams, cracks aria heat<br />

affected zones. The indications are highlighted or contrasted<br />

with the surrounding base material.<br />

Charlie Chong/ Fion Zhang

FIGURE 70. Example of contrast revealing a weld in stainless steel<br />

Charlie Chong/ Fion Zhang

FIGURE 71. Effect of etching: (a) unetched component with shiny appearance at rolled area<br />

and (t)) pits are visible in the dulled area after etching with ammonium persulfate<br />

Charlie Chong/ Fion Zhang

7.3.2 Safety Precautions<br />

Etchants are solutions of acids, bases or salts in water or<br />

alcohol. Etchants for macroetching are water based. Etching<br />

solutions need to be fresh and the primary concerns during<br />

mixing are safety concentration and purity<br />

Safety precautions are necessary during the mixing and<br />

use of chemical etchants. Chemical fumes are potentially<br />

toxic and corrosive. Mixing, handling or using etchants<br />

should be done only in well ventilated areas, preferably in an<br />

exhaust or fume hood. Use of an exhaust hood is mandatory<br />

when mixing large quantities of etchants. Etching large<br />

areas requires the use of ventilation fans in an open area or<br />

use of an exhaust hood. Contact of etchants with skin, eyes or clothes should<br />

beavoided. When pouring, mixing or handling such chemicals, protective<br />

equipment and clothing should be used, including but not limited to glasses,<br />

face shields, gloves, apron or laboratory jacket. A face-and-eye wash fountain<br />

is recommended where chemicals and etchants are sorted and handled. A<br />

safety shower is recommended when large quantities of<br />

chemicals or etchants are in use.<br />

Charlie Chong/ Fion Zhang

Should contact occur, certain safety steps must be followed,<br />

depending on the kind of contact and the chemicals<br />

involved. Skin should be washed with soap and water.<br />

Chemical burns should have immediate medical attention.<br />

Eyes should be flushed at once with large amounts of water<br />

and immediate medical attention is mandatory. Hydrofluoric<br />

and fluorosilic acids cause painful burns and serious<br />

ulcers that are slow to heal. Immediately after exposure, the<br />

affected area must be flooded with water and emergency<br />

medical attention sought. Other materials that are especially harmful in<br />

contact with skin are concentrated nitric acid, sulfuric acid, chromic acid,<br />

30 and 50 percent hydrogen peroxide, sodium hydroxide, potassium<br />

hydroxide, bromine and anhydrous aluminum chloride, These materials also<br />

produce vapors that cause respiratory irritation and damage.<br />

Charlie Chong/ Fion Zhang

Etchant Safety<br />

Charlie Chong/ Fion Zhang

Etchant Safety<br />

Charlie Chong/ Fion Zhang

Etchant Safety<br />

Charlie Chong/ Fion Zhang

Etchant Safety<br />

Charlie Chong/ Fion Zhang

7.3.3 Containers<br />

Containers used with etchants must be rated for mixing,<br />

storing and handling of chemicals. Glass is resistant to most<br />

chemicals and is most often used for containment and stirring<br />

rods. Hydrofluoric acid, other fluorine based materials,<br />

strong alkali and strong phosphoric acids can attack glass,<br />

requiring the use of inert plastics.<br />

Keywords:<br />

Strong phosphoric acid attack glass<br />

Charlie Chong/ Fion Zhang

7.3.4 Generation of Heat<br />

Heat may be generated when chemicals are mixed<br />

together or added to water. Mixing chemicals must be done<br />

using accepted laboratory procedures and caution. Strong<br />

acids, alkalis or their concentrated solutions incorrectly<br />

add to water, alcohols or other solutions, cause violent<br />

chemical reactions. To be safe, never add water to concentrated<br />

acids or alkalis. In general, the addition of acidic materials to alkaline<br />

materials will generate heat. Sulfuric acid, sodium hydroxide<br />

or potassium hydroxide in any concentration generate large<br />

amounts of heat when mixed or diluted and an ice bath may<br />

be necessary to provide cooling. Three precautions in mixing<br />

can reduce or prevent a violent reaction:<br />

Keywords:<br />

To be safe, never add water to concentrated acids or alkalis.<br />

Charlie Chong/ Fion Zhang

1. add the acid or alkali to the water or a weaker solution;<br />

2. slowly introduce acids, alkali or salts to water or solution; and<br />

3. stir the solution continuously to prevent layering and a delayed violent<br />

reaction.<br />

Charlie Chong/ Fion Zhang

7.3.5 Chemical Purity<br />

Chemicals are available in various grades of purity ranging<br />

from technical to very pure reagent grades. For etchants, the<br />

technical grade is used unless a purer grade is specified. For<br />

macroetchants, the technical grade is generally adequate.<br />

Water is the solvent used for most macroetching solutions<br />

and water purity can affect the etchant. Potable tap water<br />

may contain some impurities that could affect the etchant.<br />

Distilled water has a significantly higher purity than tap<br />

water. For macroetchants using technical grade chemicals,<br />

potable tap water is usually acceptable. For etchants in<br />

which high purity is required, distilled water is<br />

recommended.<br />

Charlie Chong/ Fion Zhang

7.3.6 Disposal<br />

Before disposing of chemical solutions, check environmental<br />

regulations (federal, state and local) and safety<br />

department procedures. The steps listed here are used only<br />

if there are no other regulations for disposal. Spent etchants<br />

are discarded and must be discarded separately—mixing of<br />

etchant materials can produce violent chemical reactions.<br />

Using a chemical resistant drain under an exhaust hood,<br />

slowly pour the spent etchant while running a heavy flow of<br />

tap water down the drain. The drain is flushed with a large<br />

volume of water.<br />

Charlie Chong/ Fion Zhang

7.4 Using Etchants<br />

After proper surface preparation and safe mixing of<br />

etchants, the application of etchants to the test object may be<br />

done with immersion or swabbing. The technique is determined<br />

by the characteristics of the etchant being used.<br />

7.4.1 immersion<br />

During immersion, a test object is completely covered by<br />

an etchant contained in a safe and suitable material- glass can be used for<br />

most etchants except hydrofluoric acid, fluorine<br />

materials, strong alkali and strong phosphoric acid.<br />

A glass heat resistant dish on a hot plate may be used for<br />

heated solutions. The solution should be brought to temperature<br />

before the test object is immersed. Tongs or other handling<br />

tools are used and the test object is positioned so that<br />

the test surface is face up or vertical to allow gas to escape.<br />

The solution is gently agitated to keep fresh etchant in contact<br />

with the test object.<br />

Charlie Chong/ Fion Zhang

7.4.2 Swabbing<br />

Etching may also be done by swabbing the test surface<br />

with a cotton ball, cotton tipped wooden swab, bristled acid<br />

brush, medicine dropper or a glass rod. The cotton ball and<br />

the cotton tipped wooden swab generally are saturated with<br />

etchant and then rubbed over the test surface.<br />

Tongs and gloves should be used for protection and the<br />

etchant applicator must be inert to the etchant. For example,<br />

strong nitric acid and alkali solutions attack cotton and<br />

these etchants must be applied using a fine bristle acid<br />

brush. A glass or plastic medicine dropper may be used to<br />

place etchants on the test object surface and a suitable stirring<br />

rod can be used to rub the surface. The test object may<br />

be immersed in etchant and swabbed while in the solution.<br />

Charlie Chong/ Fion Zhang

7.4.3 Etching Time<br />

Etching time is determined by:<br />

1. the concentration of the etchant,<br />

2. the surface condition and temperature of the test object and<br />

3. the type of test material<br />

(see Tables 16 and 17). During etching, the material surface loses its bright<br />

appearance and the degree of dullness is used to determine<br />

when to stop etching. Approximate dwell times are given in<br />

the table procedures but experience is important as well.<br />

Charlie Chong/ Fion Zhang

7.4.4 Test Object Preservation<br />

Rinsing, drying, de-smutting and coating may be required<br />

for preservation of the test object. Rinsing removes the<br />

etchant by flushing the surface thoroughly under running<br />

water. Cold water rinsing usually produces better surface<br />

appearance than hot water rinsing. Hot water rinsing does<br />

aid in drying.<br />

If smutting is a problem, the test object can be scrubbed<br />

with a stiff bristled brush or dipped in a suitable de-smutting<br />

solution. The test object should be dried with warm dry air.<br />

Shop air may be used if it is filtered and dried. After visual<br />

inspection, the test surface may be coated with a clear acrylic<br />

or lacquer but such coatings must be removed before subsequent<br />

tests. If the component is returned to service, a photographic<br />

record of the macro-etched area should be made.<br />

Charlie Chong/ Fion Zhang

TABLE 16. Etchant characteristics and uses<br />

TABLE 16. Etchant characteristics and uses* (continued)<br />

TABLE 17. Etchants for welds<br />

See Text.<br />

Charlie Chong/ Fion Zhang

7.5 Conclusion<br />

<strong>Visual</strong> testing is performed in accordance with applicable<br />

codes, standards, specifications and procedures. Chemical<br />

aids enhance the contrast of discontinuities making them<br />

easier to interpret and evaluate. This enhancement is<br />

attained by macroetching- a controlled chemical processing<br />

of the surface. Macroetching gives the optimum<br />

results on a properly cleaned and prepared surface. Chemicals<br />

for etching must be mixed, stored, handled and applied<br />

in strict accordance with safety regulations.''''<br />

Charlie Chong/ Fion Zhang

Cellulose Replica<br />

Charlie Chong/ Fion Zhang

Cellulose Replica Sheets<br />

Charlie Chong/ Fion Zhang

Experts at Work<br />

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

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