ASNT Level III- Visual & Optical Testing

ASNT Level III- Visual & Optical Testing 

My Pre-exam Preparatory 

Self Study Notes Reading 4 Section 4B 

2014-August 

Charlie Chong/ Fion Zhang

For my coming ASNT Level III VT Examination 

2014-August 

Charlie Chong/ Fion Zhang 

http://www.cnoocengineering.com/en/single_news_content.aspx?news_id=12343

At works 


Reading 4 

ASNT Nondestructive Handbook Volume 8 

Visual & Optical testing- Section 4B 

For my coming ASNT Level III VT Examination 

2014-August 



Fion Zhang 

2014/August/15

SECTION 4 

BASIC AIDS AND ACCESSORIES FOR 

VISUAL TESTING 


SECTION 4: BASIC AIDS AND ACCESSORIES FOR VISUAL TESTING 

PART 1: BASIC VISUAL AIDS 

1.1 Effects of the Test Object 

PART 2: MAGNIFIERS 

2.1 Range of Characteristics 

2.2 Low Power Microscopes 

2.3 Medium Power Systems 

2.4 High Power Systems 


PART 3: BORESCOPES 

3.1 Fiber Optic Borescopes 

3.2 Rigid Borescopes 

3.3 Special Purpose Borescopes 

3.4 Typical Industrial Borescope Applications 

3.5 Borescope Optical Systems 

3.6 Borescope Construction 


PART 4: MACHINE VISION TECHNOLOGY 

4.1 Lighting Techniques 

4.2 Optical Filtering 

4.3 Image Sensors 

4.4 Image Processing 

4.5 Mathematical Morphology 

4.6 Image Segmentation 

4.7 Optical Feature Extraction for High Speed Optical Tests 

4.8 Conclusion 


PART 5: REPLICATION 

5.1 Cellulose Acetate Replication 

5.2 Silicon Rubber Replicas 



PART 6: TEMPERATURE INDICATING MATERIALS 

6.1 Other Temperature Indicators 

6.2 Certification of Temperature Indicators 

6.3 Applications for Temperature Indicators 


PART 7: CHEMICAL AIDS 

7.1 Test Object Selection 

7.2 Surface Preparation 

7.3 Etching 

7.4 Using Etchants 



PART 4: MACHINE VISION TECHNOLOGY 

1.0 General 

SKIP the whole part. 


PART 5: REPLICATION 

5.0 General 

Replication is a valuable tool for the analysis of fracture surfaces and 

microstructures and for documentation of corrosion damage and wear. There 

is also potential for uses of replication in other forms of surface testing. 

Replication is a method used for copying the topography of a surface that 

cannot be moved or one that would be damaged in transferal. A police officer 

making a plaster cast of a tire print at an accident scene or a scientist malting 

a cast of a fossilized footprint are common examples of replication. These 

replicas produce a negative topographic image of the subject known as a 

single stage replica. A positive replica made from the first cast to produce a 

duplicate of the original surface is called a second stage replica. Many 

replicating mediums are commercially available. The types commonly used in 

nondestructive testing typically fall into one of two categories: cellulose 

acetate replicas and silicone rubber replicas. Both have advantages and 

limitations but both can also provide valuable information without altering the 

test object. 


5.1 Cellulose Acetate Replication 

Acetate replicating material is used for surface cleaning, removal and 

evaluation of surface debris, fracture surface microanalysis and for 

microstructural evaluation. Single stage replicas are typically made, creating 

a negative image of the test surface. A schematic diagram of microstructural 

replication is shown in Fig. 56. 

5.1.1 Cleaning and Debris Analysis 

Fracture surfaces should be cleaned only when necessary. Cleaning is 

required when the test surface holds loose debris that could hinder analysis 

and that cannot be removed with a thy air blast. Cleaning debris from fracture 

surfaces is useful when the test object is the debris itself or the fracture 

surface. Debris removed from a fracture can be coated with carbon and 

analyzed using energy dispersive spectroscopy. This provides a 

semiquantitative analysis when a particular element is suspected of 

contributing to the fracture. 


Removal of loose surface particles is usually done by wetting a piece of 

acetate tape on one side with acetone, allowing a short period for softening 

and applying the wet side of the tape to the area of interest. Thicker tapes of 

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

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

procedure is normally repeated several times until a final tape removes no 

debris from the surface. 


5.1.2 Fracture Surface Analysis 

The topography of fracture surfaces can be replicated and analyzed using an 

optical microscope, scanning electron microscope or transmission electron 

microscope. The maximum useful magnification obtained using optical 

microscopes depends on the roughness of the fracture but seldom exceeds 

100 x . The scanning electron microscope has good depth of field at high 

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

transmission electron microscope has been used to document microstructural 

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

replica provides information regarding mode of failure and, in most instances, 

is sufficient for completion of this kind of analysis. 


An example of a replicated fracture surface is shown in Fig. 57. The 

transmission electron microscope is used in instances where information 

regarding dislocations and crystallographic planes is needed. Both single 

stage (negative) and second stage (positive) replicas can be used for failure 

analysis. Some scanning electron microscope manufacturers offer a reverse 

imaging module that provides positive images from a negative replica. This 

eliminates the need to think and interpret in reverse. This feature has also 

proven valuable for evaluating microstructures through replication. 

As with the removal of surface debris, it has been found that the thicker 

replicas provide better results, for the same reasons. The procedure for 

replication of fracture surfaces is identical to that for debris removal. On rough 

surfaces, however, difficulty may be encountered when trying to remove the 

replica. This can cause replication material to remain on the fracture surface 

but this can easily be removed with acetone. 


Replicas, in the as-stripped condition, typically do not exhibit the contrast 

needed for resolution of fine microscopic features such as fatigue striations. 

To improve contrast, shadowing or vapor deposition of a metal is performed. 

The metal is deposited at an acute angle to the replica surface and collects at 

different thicknesses at different areas depending on the surface topography. 

This produces a shadowing that allows greater resolution at higher 

magnifications. Shadowing with gold or other high atomic number metals 

enhances the electron beam interaction with the sample and greatly improves 

the image in the scanning electron microscope by reducing the signal-tonoise 

ratio. 


5.1.3 Microstructural Interpretation 

To date, the greatest advances in the use of acetate replicas 

for nondestructive testing have come from their use in microstructural 

testing and interpretation. Replication is an integral part of visual tests in the 

power generation industries as well as in refining, chemical processing and 

pulp and paper plants. Replication, in conjunction with microstructural 

analysis, is used to quantify microstrain over time and to predict the remaining 

useful life of a component. Future applications are not limited by material type. 

In industry, tests are carried out at preselected intervals to assess the 

structural integrity of components in their systems. These components can be 

pressure vessels, piping systems or rotating equipment. Typically these 

components are exposed to stresses or an environment that limits their 

service life. Replication is used to evaluate such systems and to provide data 

regarding their metallurgical condition. 


Microstructural replication is done in two steps: surface preparation followed 

by the replication procedure. Surface preparation involves progressive 

grinding and polishing until the test surface is relatively free of scratches 

(metallurgical quality). Depending on the material type and hardness, this 

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

compound as the final step. Electrolytic polishing can increase efficiency if 

many areas are being tested. Surfaces can be electropolished with a 320-400 

grit finish. The disadvantages of electropolishing are that (1) the equipment is 

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

and (3) pitting has been known to occur with some alloy systems containing 

large amounts of carbides. 

Next, the polished surface is etched to provide microstructural topographic 

contrast which may be necessary for evaluation. Etchants vary with material 

type and can be applied electrolytically, by swabbing or spraying the etchant 

onto the surface. With some materials, a combination of etch-polish etch 

intervals yields the most favorable results. 


To replicate the surface microstructure, an area is wetted with acetone and a 

piece of acetate tape is laid on the surface. The tape is drawn by capillary 

action to the metal surface, producing an accurate negative image of the 

surface microstructure. Thin acetate tape at 0.025 mm (0.001 in.) provides 

excellent results and gives the best resolution at high magnifications. Thicker 

tapes must be pressed onto the test surface and, depending on the expertise 

of the inspector, smearing can result. Thicker tapes are more costly and the 

resolution of microscopic detail does not match thinner tapes. Studies of 

carbide morphology and creep damage mechanisms have been performed at 

magnifications as high as 10,000 x with thin tape replicas. Before removal of 

the tape from the test object, the back is coated with paint to provide a 

reflective surface that enhances microscopic viewing. The replica is removed 

and can be stored for future analysis. 


If analysis with the scanning electron microscope is needed, replicas should 

be coated to prevent electron charging. This is accomplished by evaporating 

or sputter coating a thin conductive film onto the replica surface. Carbon, gold, 

gold-palladium and other metals are used for coating. There are differences in 

the sputtering yield from different elements and this should he remembered 

when choosing an element or when attempting to calculate the thickness of 

the coating. The main advantage of sputter coating over evaporation 

techniques is that it provides a continuous coating layer. Complete coating is 

accomplished without rotating or tilting the replica. With evaporation, only line 

of sight areas are coated and certain areas typically are coated more than 

others. 


Some examples of replicated microstructures, documented with both a 

scanning electron microscope and with conventional optical microscopy, are 

shown in Figs. 58 to 61. Replication is used for detection of high temperature 

creep damage, stress corrosion cracking, hydrogen cracking mechanisms, 

as well as the precipitation of carbides, nitrides and second phase 

precipitates such as sigma or gamma prime. Replication is also used for 

distinguishing fabrication discontinuities from operational discontinuities. 


Sputter deposition 

is a physical vapor deposition (PVD) method of thin film deposition by 

sputtering. This involves ejecting material from a "target" that is a source onto 

a "substrate" such as a silicon wafer. Resputtering is re-emission of the 

deposited material during the deposition process by ion or atom 

bombardment. Sputtered atoms ejected from the target have a wide energy 

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

(typically only a small fraction — order 1% — of the ejected particles are 

ionized) can ballistically fly from the target in straight lines and impact 

energetically on the substrates or vacuum chamber (causing resputtering). 

Alternatively, at higher gas pressures, the ions collide with the gas atoms that 

act as a moderator and move diffusively, reaching the substrates or vacuum 

chamber wall and condensing after undergoing a random walk. The entire 

range from high-energy ballistic impact to low-energy thermalized motion is 

accessible by changing the background gas pressure. The sputtering gas is 

often an inert gas such as argon. 


For efficient momentum transfer, the atomic weight of the sputtering gas 

should be close to the atomic weight of the target, so for sputtering light 

elements neon is preferable, while for heavy elements krypton or xenon are 

used. Reactive gases can also be used to sputter compounds. The 

compound can be formed on the target surface, in-flight or on the substrate 

depending on the process parameters. The availability of many parameters 

that control sputter deposition make it a complex process, but also allow 

experts a large degree of control over the growth and microstructure of the 

film. 


Sputtering 

http://en.wikipedia.org/wiki/Sputter_deposition 


Sputtering 

http://en.wikipedia.org/wiki/Sputter_deposition 


Sputtering 

http://clearmetalsinc.com/technology/ 


Brief Introduction to Coating Technology for Electron Microscopy 

Coating of samples is required in the field of electron microscopy to enable or 

improve the imaging of samples. Creating a conductive layer of metal on the 

sample inhibits charging, reduces thermal damage and improves the 

secondary electron signal required for topographic examination in the SEM. 

Fine carbon layers, being transparent to the electron beam but conductive, 

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

replicas to be imaged in the TEM. The coating technique used depends on 

the resolution and application 


http://www.leica-microsystems.com/cn/science-lab/coating-technology-for-electron-microscopy/

Sputter Deposition 


Sputter deposition 


5.1.4 Strain Replication 

The replication technique can be used to evaluate and quantify the 

occurrence of localized strain in materials exposed to elevated temperatures 

and stresses over time (materials susceptible to high temperature creep), 

Replication allows monitoring for accumulated strain before detectable 

microstructural changes occur. Strain replication involves inscribing a grid 

pattern onto a previously polished surface. A reference grid pattern is 

replicated using material with a shrinkage factor that has been quantified 

through analysis. This known shrinkage factor is included in future 

numerical analysis of strain. The grid is then coated to prevent surface 

oxidation during use. After a predetermined period of operation, the coating is 

removed and the area is again replicated. The grid intersection points on the 

two replicas are compared for dimensional changes and the changes are 

then correlated to units of strain. This technique does not yield absolute 

values of strain but does provide the change in strain calculated over time. 

This gives the operator information that can help approximate where the 

component is in its service life. 


Strain replication is especially useful for materials that do not exhibit creep 

void formation until late in their service life. As long as the strain calculations 

indicate a linear relationship between strain and time, the material is still said 

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

14). When the relationship deviates from linearity, the material has begun 

third stage or tertiary creep, where the strain rate can become unstable. 


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

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

removal 


FIGURE 57. Fracture surface documentation using replication shows fatigue striations on the 

surface at magnifications originally of (a) 2,000 x and (b) 10,000 x 


Fatigue striations 


FIGURE 58. Comparison of optical microscopy to scanning electron microscopy in the 

documentation of a replicated microstructure; evidence of creep damage is visible in the grain 

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

(b) scanning electron microscope image 


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

microscope; the microstructure consists of an austenitic matrix, precipitated nitrides and carbides; 

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

scanning electron microscope; grain boundary carbides, creep voids and particles believed to be 

nitrides can be observed in the matrix 


Creep Void 


FIGURE 60. Documentation of stress corrosion cracking found in the welds of an anhydrous 

ammonia sphere; 3 percent nital etch at 200 x original magnification 


FIGURE 61. Documentation of heat affected zone cracking in A516 grade 70 steel; cracking 

associated with a nonstress relieved repair weld; the presence of this repair weld was not known 

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

magnification 


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

Damage Parameter 

isolated cavities 

Oriented cavities 

Microcracks 

Macrocracks 

Action Required 

no action until next major 

scheduled maintenance outage 

replica test at specified intervals 

limited service until repair 

immediate repair 


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

under stress in a high temperature atmosphere; note that development of creep related voids in 

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

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

macrocracks (see Table 14) 

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


Cellulose Acetate Replication 

http://corrosionhelp.com/failures.htm 


Replication Microscopy Techniques for NDE 

Fig. 2 Schematic of the plastic replica technique 


http://www.asminternational.org/documents/10192/1850228/06070G_Sample.pdf/f08974f0-1eca-4072-a2e1-cf60d5ae7e5d


Figure 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 


http://www.asminternational.org/documents/10192/1850228/06070G_Sample.pdf/f08974f0-1eca-4072-a2e1-cf60d5ae7e5d











5.2 Silicone Rubber Replicas 

Silicone impression materials have been used extensively in medicine, 

dentistry and in the science of anthropology. In nondestructive testing, 

silicone materials are used as tools for documenting macroscopic and 

microscopic material detail. Quantitative measurements can be obtained for 

depth of pitting, wear, surface finish and fracture surface evaluation. Silicone 

material is made with varying viscosities, setting times and resolution 

capabilities. Compared to an acetate replica, the resolution characteristics of 

a silicone replica is limited. With a medium viscosity compound, fine features 

visible at 50 x can be resolved but difficulties are encountered at higher 

magnifications. With a low viscosity compound, slightly better resolution is 

obtained but curing times are long and not suited to field applications. The 

lower viscosity medium is also known to creep with time and is not 

recommended for applications where very accurate dimensional studies are 

needed. 


5.2.1 Use of Silicone Replicating Materials 

Silicone replicating materials are supplied in two parts: a base material and 

an accelerator. Although it is best to follow the recommended mixing ratios, 

these can be altered slightly to change the working time of the material. The 

two parts are mixed thoroughly and spread over the subject area. Additional 

material can he added to thicken the replica. Molding clay can also be used to 

build a dam around a replicated area. The dam supports the replica as its 

sets and allows thicker replicas to be made. Measurements of pit depth and 

surface finish can be obtained easily because of the silicone's ability to flow 

into crevices on the test object. To evaluate pit depth and surface finish, the 

replica is cut and the cross section is examined with a microscope or a 

macroscopic measuring device (a micrometer or an optical comparator). 


Wear can be determined in a similar manner by replicating and comparing a 

worn surface to an unworn surface (see Fig. 63). Fracture surfaces with rough 

contours can be easily replicated with silicone (taking an acetate replica of 

such surfaces is difficult). However, the resolution characteristics of a silicone 

replica are not as good as acetate replicas and this limits the amount of 

interpretation that can be performed. Macroscopic details such as chevron 

markings can be easily located with the silicone technique to determine crack 

propagation direction or to trace a fracture path visually to its origin. 


FIGURE 63. Silicon replicas used to determine wear variance on a failed pinion gear 


Brittle Failure Chevron Marking 


Brittle Failure Chevron Marking 



Cellulose acetate tape and silicone impression materials are commonly used 

for nondestructive visual tests of surface phenomena such as corrosion, wear, 

cracking and microstructures. Both types of replicating material have 

advantages and limitations but when used in the correct application, can 

provide valuable information. In terms of resolution, the silicone replica 

typically does not have the capability to copy fine detail above 50 x . The 

acetate replica can reveal detail up to 50,000 x on a transmission electron 

microscope. The acetate replica is limited, however, by the roughness of the 

topography it can copy. On rough fracture surfaces, difficulty is encountered 

in both applying and removing an acetate replica. The silicone material is not 

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

resolvable detail versus macroscopic features normally indicates whether 

acetate or silicone replicas are best for the application. 


PART 6: TEMPERATURE INDICATING MATERIALS 

6.0 General 

A temperature indicating stick (chalk or crayon) is typically made of materials 

with calibrated melting points and temperature measuring accuracies to ± 1 

percent. Indicators are available in closely spaced increments over a range 

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

marked with the stick. When the workpiece attains the predetermined melting 

point of the indicator mark, the mark instantly liquefies, notifying the observer 

that the workpiece has reached that temperature. 


Pre-marking with a stick is not practical under certain circumstances- when a 

heating period is prolonged a highly polished surface does not readily accept 

a mark or the marked material gradually absorbs the liquid phase of the 

indicator. In such instances, the operator frequently marks the workpiece with 

the stick. The desired temperature is noted when one ceases to make dry 

marks and begins to leave a liquid smear. A similar procedure can be 

employed to indicate temperature during a cooling cycle. But a melted mark, 

on cooling, will not solidify at the exact same temperature at which it melted, 

so solidification of a melted indicator mark cannot be relied on for temperature 

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

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

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

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


Temperature indicating sticks were developed in America by a metallurgist 

working on submarine hulls in the 1930s. At the time, preheat was measured 

with so-called melting point standards, granules of substances with known 

melting points used to calibrate heat sensing instruments. The engineer used 

the granules directly, spreading them on the preheated metal and using their 

melt as a signal to proceed with welding. 

The melting point granules were next formed into sticks held together with 

organic hinders. Different temperature ratings were added and some 

refinements have been made but the principle of indicators has remained 

unchanged. The sticks make physical contact with the heated test object, 

reach thermal equilibrium rapidly and do not conduct heat away from the test 

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

can usually be removed with water or alcohol. For ratings above 340 °C 

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

rated temperature and has become charred, abrasion may be needed for 

complete removal. 


6.1 Other Temperature Indicators 

In addition to the stick, temperature indicating pellets and liquids are available. 

The liquid indicator is brushed on before welding starts and is useful on highly 

polished surfaces or for making large marks viewed at a distance. Heat 

indicating pellets, about the size and shape of an aspirin, have greater mass 

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

use with large, heavy pieces requiring prolonged heating- applications where 

stick or lacquer marks could fade with time. 


Levels of 


FIGURE 64, Temperature indicating pellets 


6.2 Certification of Temperature Indicators 

Temperature indicating sticks are mixtures of organic and inorganic 

compounds. The purity of the source materials directly affects the accuracy of 

the predicted melting point. There is the possibility of contamination with trace 

quantities of other elements, which may be detrimental to the accuracy of the 

indicator. In some cases, low melting point materials (lead, tin, sulfur, 

halogenated compounds) may be undesirable for the welding procedure. 

Most manufacturers can provide certification supported by analyses of typical 

batches. Documentation indicates which temperature ratings may contain 

contaminants that can be avoided by the user. 

In some critical applications (nuclear fabrication, aircraft assembly), actual 

chemical analysis of the specific lot number of the temperature indicators may 

be required. If the customer supplies a written certification requirement listing 

the compounds to be tested for, most manufacturers will send lot numbered 

samples for laboratory analysis. The customer is usually expected to pay lab 

charges for such specialized requirements. 


Marking materials used on austenitic stainless steels typically have a certified 

analysis that meets the following specified maximum amounts of detrimental 

contaminants: 

1. inorganic halogen content less than 200 ppm by weight; 

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

3. sulfur content less than 1 percent by weight (measured in accordance with 

ASTM D129); and 

4. total content of low melting point metal (lead, bismuth, zinc, mercury, 

antimony and tin) less than 200 ppm by weight and no individual metal 

content greater than 50 ppm by weight. 

The certification typically indicates the methods and accuracy of analysis and 

the name of the testing laboratory. 


6.3 Applications for Temperature Indicators 

Temperature indicators can be used for preheat temperature tests and in 

annealing and stress relieving procedures, hardfacing, overlaying for 

corrosion resistance, flame cutting, flame conditioning, heat treating, pipe 

bending, shearing of bar steel, straightening hardened parts, shrink fitting, 

brazing, soldering and nonferrous fabrication. The indicators can help find hot 

spots in insulation and engines, help monitor temperatures in curing and 

bonding operations and help check pyrometric calibration. 


6.3.1 Tests of Railway Bearings 

Bearing breakdown can be detected by using fluorescent temperature 

indicating pellets as heat sensors for inboard journal boxes. The pellets are 

inserted in a specially fabricated stainless steel holder that contains two 

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

insertion tool. The tool has a mechanical stop to ensure that the holder is 

located at a predetermined depth. This permits proper monitoring of journal 

box operating temperatures. Once a specified temperature is exceeded, in 

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

holder. The fluorescent material is easy to detect and clearly indicates that 

excessive heat has been conducted from the bearing to the axle. 


6.3.2 Verifying Oven Temperatures 

Technicians can determine if self cleaning ovens reach the proper cleaning 

temperature using pellets with pre-calibrated melting points at 450 °C (850 

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

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

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

cleaning cycle is completed and the oven has cooled, the pellets are 

inspected—complete melting of the tablet verifies that the nominal cleaning 

temperature has been achieved. 


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


6.3.4 Process Control Applications 

A gas tight seal is needed to prevent leakage of combustion gases through 

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

important to know and control the temperature inside the ceramic insulator 

and this can be done using a temperature indicating pellet. Sample insulators 

are loaded with pellets and processed with production parts. Information 

obtained from analyzing the samples is used to adjust furnace conveyor 

speed and temperature. 


6.3.5 Monitoring Fabric Seam Temperature 

In the making of specialized cloth (protective clothing, aerostat balloons), 

seam integrity is an important manufacturing function. A good radiofrequency 

seal can be achieved on a given fabric substrate only within a specific 

temperature range, determined by the minimum temperature needed to 

ensure a complete seal and the maximum temperature possible before 

material degradation. Constant temperature control and verification are 

required. This can be achieved using temperature sensitive strips (one for the 

upper limit, one for the lower limit) applied to the sealing tape used in 

production. A visual test of each seam after sealing indicates whether the 

seam temperature was within the required range, allowing visual verification 

of conditions for all dielectric seams. 


6.3.6 Precise Post forming Heat Control 

Temperature indicating materials are incorporated into 

many industrial applications where an indication is needed 

to show that a critical temperature has or has not been 

reached. A phase changing fusible liquid is used to indicate 

optimum postforming temperatures when bending decorative 

laminate for the contoured edges of countertops, desks, 

tables and other surfaces (see Fig. 66). 

Postforming is the process of bending a flat sheet of laminate 

around a radiused core material (particle board, plywood 

or fiber board). The process is typically done after controlled heating 

monitored with temperature sensitive liquids. Postforming can be a manual or 

mechanical operation. Hand postforming is used for unusual configurations or 

limited quantity production and mechanical postforming is used 

for high quantity production. Both methods have the need for a heat source, 

prepared cores, postforming grades of decorative laminate, pressuring guides 

and evenly applied pressure. 


A core is prepared by first shaping the edges to be laminated. 

The core and laminate are evenly coated with a 

contact adhesive, preferably a spray. The laminate is positioned 

and registered with the core, allowing the laminate to 

overhang the radius. Postforming grades of decorative laminate 

are formable between temperatures of 156 and 163 °C 

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

edge wrap. In this example, radiant heat is applied to the 

decorative surface of the laminate with the work supported 

over the heat. To determine the proper postforming temperature, 

the temperature indicating liquid is painted in stripes 

onto the laminate. When the liquid changes from a dry 

(matte) to a wet (melted) appearance, the assembly is wiped 

into the cavity of a fixture to form the 180 degree radius. The 

fixture is a U channel made by two boards attached to a base. 

The dimension of the U channel is the thickness of the core 

plus the thickness of the laminate, allowing about 0.5 mm 

(0.02 in.) clearance. 


Another example of handforrning is known as a full wrap. 

In this application, the core is positioned over radiant heaters 

with temperature indicating stripes painted on the adhesive 

in the area of the radius. When the melt indicates forming 

temperature has been reached, the assembly is moved back 

onto a flat supporting surface. The wrapping action uses the 

flat surface as a pressure point. 

An example of mechanical postforming is the roll forming 

machine. Radiant heaters are located above an assembly 

supported by a moving carrier. When the forming temperature 

has been reached, slanted forming bars wipe the laminate 

over the radius. After the laminate has been formed, a 

succession of rollers maintains pressure until the assembly 

has cooled. In this application, temperature sensitive liquid 

is painted onto the laminate in order to verify that the dwell time under heat 

has been sufficient for reaching forming 

temperature. 


FIGURE 66. Laminate postforming around a radiused core 


6.3.7 Pipeline Coatings 

Epoxy powders are specially formulated to enhance corrosion 

proof resistance of utility pipe: that is, pipe usually buried 

underground, where it is subject to widely varying 

pipeline operating conditions. Intimately bonded to the 

pipe, the bonded epoxy is unaffected by widely varying soil 

compaction, moisture penetration, fungus attack, soil acids 

and chemical degradation. To achieve a long lasting bond of epoxy coating to 

metal pipe, the pipe must be preheated very carefully to the recommended 

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

stick; its melting shows that the correct temperature for coating has been 

reached. 


6.3.8 Preheating before Welding 

Heating to the proper temperature before welding lessens 

the danger of crack formation and shrinkage stresses in many 

metals. Hard zones near the weld are reduced and lessen the 

possibility of distortion. Preheating also helps diffuse hydrogen 

from steel and helps reduce the likelihood of subsequent 

hydrogen inclusions. 

The need for preheating increases with the mass of the 

material being welded. It is most useful for the thick, heavy 

weldments used in bridge construction, shipbuilding, pipelines 

and pressure vessels. Preheating is also recommended 

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

electrode is a small diameter; (3) when the joined pieces are 

of different masses; (4) when the joined pieces are of complex 

cross section; and (5) for welding of high carbon or manganese 

steels. 


The most common use for temperature indicators is the 

measurement of preheat, postheat and interpass temperatures 

for welding. In a typical application, the welder marks 

the test surface with an indicating stick of a specific temperature 

rating (see Fig. 67). When the mark changes phase 

(melts), the material has reached the correct temperature 

and is ready for welding. It is important for the user to 

understand that change of color has no significance; only the 

actual melting of the mark should be considered. 

Oxyacetylene equipment cannot be used for welding or 

cutting of high strength steels used in automotive components 

because too much heat can reduce their structural 

strength. However, in some instances an oxyacetylene torch 

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

for high strength steel is not exceeded. 

When preheat temperatures are 370 °C (700 °F) or when 

heating is prolonged, an indicating mark could evaporate or 

could be absorbed by the test material, Under these conditions, 

marks should be added periodically during heating. 


When the rated temperature is reached, the stick leaves a liquid 

streak instead of a dry mark and welding can begin. 

To ensure accurate temperature indication with no override, 

two or more indicators can be used to alert the operator 

that the test object is approaching the correct temperature. 

When a range of recommended preheat temperatures is 

given, use of several indicators might be appropriate. For 

example, carbon-molybdenum steel should he preheated to 

between 95 and 205 °C (between 200 and 400 °F). A bundle 

of indicators with ratings at 95, 120, 150, 175 and 205 °C 

(200, 250, 300, 350 and 400 °F) might be useful for 

determining how much of the test object is within the preheat 

temperature range. 


FIGURE 67. Temperature indicating stick 


Preheating 


PART 7: CHEMICAL AIDS 

7.0 General 

The information contained in this text is simplified and 

provided only for general instruction. Local health (OSHA) 

and environmental (EPA) authorities should be consulted 

about the proper use and disposal of chemical agents. For 

reasons of safety, all chemicals must he handled with care, 

particularly the concentrated chemicals used as aids to visual 

and optical tests. 

In visual nondestructive testing, chemical techniques are 

used to clean and enhance test object surfaces. Cleaning 

processes remove dirt, grease, oil, rust and mill scale. Contrast 

is enhanced by chemical etching. 


Macroetching is the use of chemical solutions to attack 

material surfaces to improve the visibility of discontinuities 

for visual inspection at normal and low power magnifications. 

Caution is required in the use of these chemicals—the use 

of protective clothing and safety devices is imperative. Test 

object preparation and the choice of etchant must be appropriate 

for the inspection objectives. Once the desired etch is 

achieved, the metal surface must be flushed with water to 

avoid over etching. 


7.1 Test Object Selection 

Figure 68 shows typical test objects removed from their 

service environment. Governing codes, standards or specifications 

may determine the number and location of visual 

tests. Specific areas may contain discontinuities from forming 

operations such as casting, rolling, forging or extruding. 

Weld tests may be full length or random spots and typically 

cover the weld metal, fusion line and heat-affected zone. 

The service of a component may also indicate problem areas 

requiring inspection. Location of the test site directly affects surface 

preparation. The test site may he prepared and nondestructively 

inspected in situ. Removal of a sample for laboratory examination 

is a destructive alternative test method that typically requires a repair weld. 


FIGURE 68. Components removed from service for visual testing 


7.2 Surface Preparation 

Preparation of the test object before etching may require 

only cleaning or a process including cleaning, grinding and 

fine polishing (improper grinding is shown in Fig. 69). The 

extent of these operations depends on the etchant, the material 

and the type of discontinuity being sought. 

7.2.1 Solvent Cleaning 

Solvent cleaning can be useful at two stages in test object 

preparation. An initial cleaning with a suitable solvent 

removes dirt, grease and oil and may make rust and mill scale 

easier to remove. One of the most effective cleaning solvents is a solution of 

detergent and water. However, if water is detrimental to the 

test object, organic solvents such as ethyl alcohol, acetone or 

naphthas have been used. These materials generally have 

low flash points and their use may be prohibited by safety 

regulations. Safety solvents such as the chlorinated hydrocarbons and high 

flash point naphthas may be required to meet safety standards. 


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

etchant does not give good test results 


7.2.2 Removing Rust and Scale 

Rust and mill scale are normally removed by mechanical 

methods such as wire brushing or grinding. If appropriate 

for a particular test, the use of a severe etchant requires only 

the removal of loose rust and mill scale. Rust may also be 

removed chemically. Commercially available rust removers 

are generally inhibited mineral acid solutions and are not 

often used for test object preparation. 

Most surface tests require complete removal of rust and 

mill scale but a coarsely ground surface is often adequate 

preparation before etching. 

Grinding may be done manually or by belt, disk or surface 

grinding tools. Surface grinders are usually found only in 

machine shops. Hand grinding requires a hard flat surface 

to support the abrasive sheet. Coolant is needed during 

grinding and water is the preferred coolant but kerosene may 

be used if the test material is not compatible with water. 


7.2.3 Grinding and Polishing 

Fine grinding and polishing are needed for visual tests of 

small structural details, welds and the effects of heat treatment. 

Finer grinding usually is done with 80 to 150 abrasive 

grit followed by 150 to 180 grit and finally 400 grit (an American 

indication of grit size, 400 being the finest). At each 

stage, marks from previous grinding must be completely removed. Changing 

the grinding direction between successive 

stages of the process aids the visibility of previous 

coarser grinding marks. Coolant is required for grinding and 

typical abrasives include emery, silicon carbide, aluminum 

oxide and diamond. 


If the required finish cannot be achieved by fine grinding 

with 400 grit abrasive, the test surface must be polished. Polishing 

is generally done with a cloth-covered disk and abrasive 

particles suspended in paste or water. Common 

polishing media include aluminum oxide, magnesium oxide, 

chromium oxide, iron oxide and diamond with particle sizes 

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

the previous 

step he completely removed. If coarser marks do not 

clear, it may be necessary to repeat a previous step using 

lighter pressure before continuing. Failure to do so can yield 

false indications. 


7.3 Etching 

7.3.1 Choice of Etchant 

The etchant, its strength, the material and the discontinuity 

all combine to determine surface finish requirements (see 

Table 15). Properly selected etchants chemically attack the 

test material and reveal welds (Fig. 70), pitting (Fig 71), 

grain boundaries, segregation, laps, seams, cracks aria heat 

affected zones. The indications are highlighted or contrasted 

with the surrounding base material. 


FIGURE 70. Example of contrast revealing a weld in stainless steel 


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

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


7.3.2 Safety Precautions 

Etchants are solutions of acids, bases or salts in water or 

alcohol. Etchants for macroetching are water based. Etching 

solutions need to be fresh and the primary concerns during 

mixing are safety concentration and purity 

Safety precautions are necessary during the mixing and 

use of chemical etchants. Chemical fumes are potentially 

toxic and corrosive. Mixing, handling or using etchants 

should be done only in well ventilated areas, preferably in an 

exhaust or fume hood. Use of an exhaust hood is mandatory 

when mixing large quantities of etchants. Etching large 

areas requires the use of ventilation fans in an open area or 

use of an exhaust hood. Contact of etchants with skin, eyes or clothes should 

beavoided. When pouring, mixing or handling such chemicals, protective 

equipment and clothing should be used, including but not limited to glasses, 

face shields, gloves, apron or laboratory jacket. A face-and-eye wash fountain 

is recommended where chemicals and etchants are sorted and handled. A 

safety shower is recommended when large quantities of 

chemicals or etchants are in use. 


Should contact occur, certain safety steps must be followed, 

depending on the kind of contact and the chemicals 

involved. Skin should be washed with soap and water. 

Chemical burns should have immediate medical attention. 

Eyes should be flushed at once with large amounts of water 

and immediate medical attention is mandatory. Hydrofluoric 

and fluorosilic acids cause painful burns and serious 

ulcers that are slow to heal. Immediately after exposure, the 

affected area must be flooded with water and emergency 

medical attention sought. Other materials that are especially harmful in 

contact with skin are concentrated nitric acid, sulfuric acid, chromic acid, 

30 and 50 percent hydrogen peroxide, sodium hydroxide, potassium 

hydroxide, bromine and anhydrous aluminum chloride, These materials also 

produce vapors that cause respiratory irritation and damage. 


Etchant Safety 








7.3.3 Containers 

Containers used with etchants must be rated for mixing, 

storing and handling of chemicals. Glass is resistant to most 

chemicals and is most often used for containment and stirring 

rods. Hydrofluoric acid, other fluorine based materials, 

strong alkali and strong phosphoric acids can attack glass, 

requiring the use of inert plastics. 

Keywords: 

Strong phosphoric acid attack glass 


7.3.4 Generation of Heat 

Heat may be generated when chemicals are mixed 

together or added to water. Mixing chemicals must be done 

using accepted laboratory procedures and caution. Strong 

acids, alkalis or their concentrated solutions incorrectly 

add to water, alcohols or other solutions, cause violent 

chemical reactions. To be safe, never add water to concentrated 

acids or alkalis. In general, the addition of acidic materials to alkaline 

materials will generate heat. Sulfuric acid, sodium hydroxide 

or potassium hydroxide in any concentration generate large 

amounts of heat when mixed or diluted and an ice bath may 

be necessary to provide cooling. Three precautions in mixing 

can reduce or prevent a violent reaction: 

Keywords: 

To be safe, never add water to concentrated acids or alkalis. 


1. add the acid or alkali to the water or a weaker solution; 

2. slowly introduce acids, alkali or salts to water or solution; and 

3. stir the solution continuously to prevent layering and a delayed violent 

reaction. 


7.3.5 Chemical Purity 

Chemicals are available in various grades of purity ranging 

from technical to very pure reagent grades. For etchants, the 

technical grade is used unless a purer grade is specified. For 

macroetchants, the technical grade is generally adequate. 

Water is the solvent used for most macroetching solutions 

and water purity can affect the etchant. Potable tap water 

may contain some impurities that could affect the etchant. 

Distilled water has a significantly higher purity than tap 

water. For macroetchants using technical grade chemicals, 

potable tap water is usually acceptable. For etchants in 

which high purity is required, distilled water is 

recommended. 


7.3.6 Disposal 

Before disposing of chemical solutions, check environmental 

regulations (federal, state and local) and safety 

department procedures. The steps listed here are used only 

if there are no other regulations for disposal. Spent etchants 

are discarded and must be discarded separately—mixing of 

etchant materials can produce violent chemical reactions. 

Using a chemical resistant drain under an exhaust hood, 

slowly pour the spent etchant while running a heavy flow of 

tap water down the drain. The drain is flushed with a large 

volume of water. 


7.4 Using Etchants 

After proper surface preparation and safe mixing of 

etchants, the application of etchants to the test object may be 

done with immersion or swabbing. The technique is determined 

by the characteristics of the etchant being used. 

7.4.1 immersion 

During immersion, a test object is completely covered by 

an etchant contained in a safe and suitable material- glass can be used for 

most etchants except hydrofluoric acid, fluorine 

materials, strong alkali and strong phosphoric acid. 

A glass heat resistant dish on a hot plate may be used for 

heated solutions. The solution should be brought to temperature 

before the test object is immersed. Tongs or other handling 

tools are used and the test object is positioned so that 

the test surface is face up or vertical to allow gas to escape. 

The solution is gently agitated to keep fresh etchant in contact 

with the test object. 


7.4.2 Swabbing 

Etching may also be done by swabbing the test surface 

with a cotton ball, cotton tipped wooden swab, bristled acid 

brush, medicine dropper or a glass rod. The cotton ball and 

the cotton tipped wooden swab generally are saturated with 

etchant and then rubbed over the test surface. 

Tongs and gloves should be used for protection and the 

etchant applicator must be inert to the etchant. For example, 

strong nitric acid and alkali solutions attack cotton and 

these etchants must be applied using a fine bristle acid 

brush. A glass or plastic medicine dropper may be used to 

place etchants on the test object surface and a suitable stirring 

rod can be used to rub the surface. The test object may 

be immersed in etchant and swabbed while in the solution. 


7.4.3 Etching Time 

Etching time is determined by: 

1. the concentration of the etchant, 

2. the surface condition and temperature of the test object and 

3. the type of test material 

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

appearance and the degree of dullness is used to determine 

when to stop etching. Approximate dwell times are given in 

the table procedures but experience is important as well. 


7.4.4 Test Object Preservation 

Rinsing, drying, de-smutting and coating may be required 

for preservation of the test object. Rinsing removes the 

etchant by flushing the surface thoroughly under running 

water. Cold water rinsing usually produces better surface 

appearance than hot water rinsing. Hot water rinsing does 

aid in drying. 

If smutting is a problem, the test object can be scrubbed 

with a stiff bristled brush or dipped in a suitable de-smutting 

solution. The test object should be dried with warm dry air. 

Shop air may be used if it is filtered and dried. After visual 

inspection, the test surface may be coated with a clear acrylic 

or lacquer but such coatings must be removed before subsequent 

tests. If the component is returned to service, a photographic 

record of the macro-etched area should be made. 


TABLE 16. Etchant characteristics and uses 

TABLE 16. Etchant characteristics and uses* (continued) 

TABLE 17. Etchants for welds 

See Text. 



Visual testing is performed in accordance with applicable 

codes, standards, specifications and procedures. Chemical 

aids enhance the contrast of discontinuities making them 

easier to interpret and evaluate. This enhancement is 

attained by macroetching- a controlled chemical processing 

of the surface. Macroetching gives the optimum 

results on a properly cleaned and prepared surface. Chemicals 

for etching must be mixed, stored, handled and applied 

in strict accordance with safety regulations.'''' 


Cellulose Replica 


Cellulose Replica Sheets 


Experts at Work

ASNT Level III- Visual & Optical Testing

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