vi self study exam preparatory note-part 4-section 4 b

Charlie Chong/ Fion Zhang

ASNT Level III- Visual & Optical TestingMy Pre-exam PreparatorySelf Study Notes Reading 4 Section 4B2014-August

For my coming ASNT Level III VT Examination2014-August

Charlie Chong/ Fion Zhanghttp://www.cnoocengineering.com/en/single_news_content.aspx?news_id=12343

At works


Reading 4ASNT Nondestructive Handbook Volume 8Visual & Optical testing- Section 4BFor my coming ASNT Level III VT Examination2014-August


Fion Zhang2014/August/15


SECTION 4BASIC AIDS AND ACCESSORIES FORVISUAL 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 Characteristics2.2 Low Power Microscopes2.3 Medium Power Systems2.4 High Power Systems


PART 3: BORESCOPES

3.1 Fiber Optic Borescopes3.2 Rigid Borescopes3.3 Special Purpose Borescopes3.4 Typical Industrial Borescope Applications3.5 Borescope Optical Systems3.6 Borescope Construction


PART 4: MACHINE VISION TECHNOLOGY

4.1 Lighting Techniques4.2 Optical Filtering4.3 Image Sensors4.4 Image Processing4.5 Mathematical Morphology4.6 Image Segmentation4.7 Optical Feature Extraction for High Speed Optical Tests4.8 Conclusion


PART 5: REPLICATION

5.1 Cellulose Acetate Replication5.2 Silicon Rubber Replicas5.3 Conclusion


PART 6: TEMPERATURE INDICATING MATERIALS

6.1 Other Temperature Indicators6.2 Certification of Temperature Indicators6.3 Applications for Temperature Indicators


PART 7: CHEMICAL AIDS

7.1 Test Object Selection7.2 Surface Preparation 7.3 Etching7.4 Using Etchants7.5 Conclusion


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 microstructuralreplication 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 of0.013 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 microstructuraldetails 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 alsoproven 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-to-noise ratio.


5.1.3 Microstructural Interpretation

To date, the greatest advances in the use of acetate replicasfor nondestructive testing have come from their use in microstructuraltesting 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, thiscan 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 etchintervals 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.) providesexcellent 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 theresolution 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 thanothers.


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 depositionis 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://clearmetalsinc.com/technology/

Brief Introduction to Coating Technology for Electron MicroscopyCoating 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 thedocumentation of a replicated microstructure; evidence of creep damage is visible in the grainboundaries; 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 opticalmicroscope; 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 benitrides can be observed in the matrix


Creep Void


FIGURE 60. Documentation of stress corrosion cracking found in the welds of an anhydrousammonia 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 Action Required

isolated cavities no action until next majorscheduled maintenance outage

Oriented cavities replica test at specified intervalsMicrocracks limited service until repairMacrocracks immediate repair


FIGURE 62. Creep damage curve showing the typical relationship of strain to time for a materialunder stress in a high temperature atmosphere; note that development of creep related voids inthis 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

Charlie Chong/ Fion Zhanghttp://www.asminternational.org/documents/10192/1850228/06070G_Sample.pdf/f08974f0-1eca-4072-a2e1-cf60d5ae7e5d

Replication Microscopy Techniques for NDE 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

Charlie Chong/ Fion Zhanghttp://www.asminternational.org/documents/10192/1850228/06070G_Sample.pdf/f08974f0-1eca-4072-a2e1-cf60d5ae7e5d

Replication Microscopy Techniques for NDE 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


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 featuresvisible 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


5.3 Conclusion

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 rangefrom 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 meltingpoint 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); and4. 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 intomany industrial applications where an indication is neededto show that a critical temperature has or has not beenreached. A phase changing fusible liquid is used to indicateoptimum postforming temperatures when bending decorativelaminate for the contoured edges of countertops, desks,tables and other surfaces (see Fig. 66).

Postforming is the process of bending a flat sheet of laminatearound a radiused core material (particle board, plywoodor 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 usedfor 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 acontact adhesive, preferably a spray. The laminate is positionedand registered with the core, allowing the laminate tooverhang the radius. Postforming grades of decorative laminateare formable between temperatures of 156 and 163 °C(313 and 325 °F). A popular example of hand postforming is the 180 degreeedge wrap. In this example, radiant heat is applied to thedecorative surface of the laminate with the work supportedover the heat. To determine the proper postforming temperature,the temperature indicating liquid is painted in stripesonto the laminate. When the liquid changes from a dry(matte) to a wet (melted) appearance, the assembly is wipedinto the cavity of a fixture to form the 180 degree radius. Thefixture is a U channel made by two boards attached to a base.The dimension of the U channel is the thickness of the coreplus 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 heaterswith temperature indicating stripes painted on the adhesivein the area of the radius. When the melt indicates formingtemperature has been reached, the assembly is moved backonto a flat supporting surface. The wrapping action uses theflat surface as a pressure point.

An example of mechanical postforming is the roll formingmachine. Radiant heaters are located above an assemblysupported by a moving carrier. When the forming temperaturehas been reached, slanted forming bars wipe the laminateover the radius. After the laminate has been formed, asuccession of rollers maintains pressure until the assemblyhas cooled. In this application, temperature sensitive liquidis painted onto the laminate in order to verify that the dwell time under heat has been sufficient for reaching formingtemperature.


FIGURE 66. Laminate postforming around a radiused core


6.3.7 Pipeline Coatings

Epoxy powders are specially formulated to enhance corrosionproof resistance of utility pipe: that is, pipe usually buriedunderground, where it is subject to widely varyingpipeline operating conditions. Intimately bonded to thepipe, the bonded epoxy is unaffected by widely varying soilcompaction, moisture penetration, fungus attack, soil acidsand chemical degradation. To achieve a long lasting bond of epoxy coating to metal pipe, the pipe must be preheated very carefully to the recommendedpreheat 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 lessensthe danger of crack formation and shrinkage stresses in manymetals. Hard zones near the weld are reduced and lessen thepossibility of distortion. Preheating also helps diffuse hydrogenfrom steel and helps reduce the likelihood of subsequenthydrogen inclusions.The need for preheating increases with the mass of thematerial being welded. It is most useful for the thick, heavyweldments used in bridge construction, shipbuilding, pipelinesand pressure vessels. Preheating is also recommendedfor (1) welding done at or below – 18 °C (0 °F); (2) when theelectrode is a small diameter; (3) when the joined pieces areof different masses; (4) when the joined pieces are of complexcross section; and (5) for welding of high carbon or manganesesteels.


The most common use for temperature indicators is themeasurement of preheat, postheat and interpass temperaturesfor welding. In a typical application, the welder marksthe test surface with an indicating stick of a specific temperaturerating (see Fig. 67). When the mark changes phase(melts), the material has reached the correct temperatureand is ready for welding. It is important for the user tounderstand that change of color has no significance; only theactual melting of the mark should be considered.Oxyacetylene equipment cannot be used for welding orcutting of high strength steels used in automotive componentsbecause too much heat can reduce their structuralstrength. However, in some instances an oxyacetylene torchmay 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 whenheating is prolonged, an indicating mark could evaporate orcould 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 liquidstreak 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 operatorthat the test object is approaching the correct temperature.When a range of recommended preheat temperatures isgiven, use of several indicators might be appropriate. Forexample, carbon-molybdenum steel should he preheated tobetween 95 and 205 °C (between 200 and 400 °F). A bundleof indicators with ratings at 95, 120, 150, 175 and 205 °C(200, 250, 300, 350 and 400 °F) might be useful fordetermining how much of the test object is within the preheattemperature range.


FIGURE 67. Temperature indicating stick


Preheating


PART 7: CHEMICAL AIDS

7.0 General

The information contained in this text is simplified andprovided only for general instruction. Local health (OSHA)and environmental (EPA) authorities should be consultedabout the proper use and disposal of chemical agents. Forreasons of safety, all chemicals must he handled with care,particularly the concentrated chemicals used as aids to visualand optical tests.In visual nondestructive testing, chemical techniques areused to clean and enhance test object surfaces. Cleaningprocesses remove dirt, grease, oil, rust and mill scale. Contrastis enhanced by chemical etching.


Macroetching is the use of chemical solutions to attackmaterial surfaces to improve the visibility of discontinuitiesfor visual inspection at normal and low power magnifications.Caution is required in the use of these chemicals—the useof protective clothing and safety devices is imperative. Testobject preparation and the choice of etchant must be appropriatefor the inspection objectives. Once the desired etch isachieved, the metal surface must be flushed with water toavoid over etching.


7.1 Test Object Selection

Figure 68 shows typical test objects removed from theirservice environment. Governing codes, standards or specificationsmay determine the number and location of visualtests. Specific areas may contain discontinuities from formingoperations such as casting, rolling, forging or extruding.Weld tests may be full length or random spots and typicallycover the weld metal, fusion line and heat-affected zone.The service of a component may also indicate problem areasrequiring inspection. Location of the test site directly affects surface preparation. The test site may he prepared and nondestructivelyinspected in situ. Removal of a sample for laboratory examinationis 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 requireonly cleaning or a process including cleaning, grinding andfine polishing (improper grinding is shown in Fig. 69). Theextent of these operations depends on the etchant, the materialand the type of discontinuity being sought.

7.2.1 Solvent Cleaning

Solvent cleaning can be useful at two stages in test objectpreparation. An initial cleaning with a suitable solventremoves dirt, grease and oil and may make rust and mill scaleeasier to remove. One of the most effective cleaning solvents is a solution ofdetergent and water. However, if water is detrimental to thetest object, organic solvents such as ethyl alcohol, acetone ornaphthas have been used. These materials generally havelow flash points and their use may be prohibited by safetyregulations. 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 severeetchant does not give good test results


7.2.2 Removing Rust and Scale

Rust and mill scale are normally removed by mechanicalmethods such as wire brushing or grinding. If appropriatefor a particular test, the use of a severe etchant requires onlythe removal of loose rust and mill scale. Rust may also beremoved chemically. Commercially available rust removersare generally inhibited mineral acid solutions and are notoften used for test object preparation.Most surface tests require complete removal of rust andmill scale but a coarsely ground surface is often adequatepreparation before etching.Grinding may be done manually or by belt, disk or surfacegrinding tools. Surface grinders are usually found only inmachine shops. Hand grinding requires a hard flat surfaceto support the abrasive sheet. Coolant is needed duringgrinding and water is the preferred coolant but kerosene maybe 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 ofsmall structural details, welds and the effects of heat treatment.Finer grinding usually is done with 80 to 150 abrasivegrit followed by 150 to 180 grit and finally 400 grit (an Americanindication of grit size, 400 being the finest). At eachstage, marks from previous grinding must be completely removed. Changing the grinding direction between successivestages of the process aids the visibility of previouscoarser grinding marks. Coolant is required for grinding andtypical abrasives include emery, silicon carbide, aluminumoxide and diamond.


If the required finish cannot be achieved by fine grindingwith 400 grit abrasive, the test surface must be polished. Polishingis generally done with a cloth-covered disk and abrasiveparticles suspended in paste or water. Commonpolishing media include aluminum oxide, magnesium oxide,chromium oxide, iron oxide and diamond with particle sizesranging from 0.5 to 15 μm. During polishing, it is critical that all marks from the previousstep he completely removed. If coarser marks do notclear, it may be necessary to repeat a previous step usinglighter pressure before continuing. Failure to do so can yieldfalse indications.


7.3 Etching

7.3.1 Choice of Etchant

The etchant, its strength, the material and the discontinuityall combine to determine surface finish requirements (seeTable 15). Properly selected etchants chemically attack thetest material and reveal welds (Fig. 70), pitting (Fig 71),grain boundaries, segregation, laps, seams, cracks aria heataffected zones. The indications are highlighted or contrastedwith 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 areaand (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 oralcohol. Etchants for macroetching are water based. Etchingsolutions need to be fresh and the primary concerns duringmixing are safety concentration and puritySafety precautions are necessary during the mixing anduse of chemical etchants. Chemical fumes are potentiallytoxic and corrosive. Mixing, handling or using etchantsshould be done only in well ventilated areas, preferably in anexhaust or fume hood. Use of an exhaust hood is mandatorywhen mixing large quantities of etchants. Etching largeareas requires the use of ventilation fans in an open area oruse 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. Asafety shower is recommended when large quantities ofchemicals or etchants are in use.


Should contact occur, certain safety steps must be followed,depending on the kind of contact and the chemicalsinvolved. 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 waterand immediate medical attention is mandatory. Hydrofluoricand fluorosilic acids cause painful burns and seriousulcers that are slow to heal. Immediately after exposure, theaffected area must be flooded with water and emergencymedical 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, potassiumhydroxide, 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 mostchemicals and is most often used for containment and stirringrods. 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 mixedtogether or added to water. Mixing chemicals must be doneusing accepted laboratory procedures and caution. Strongacids, alkalis or their concentrated solutions incorrectlyadd to water, alcohols or other solutions, cause violentchemical reactions. To be safe, never add water to concentratedacids or alkalis. In general, the addition of acidic materials to alkalinematerials will generate heat. Sulfuric acid, sodium hydroxideor potassium hydroxide in any concentration generate largeamounts of heat when mixed or diluted and an ice bath maybe necessary to provide cooling. Three precautions in mixingcan 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; and3. 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 rangingfrom technical to very pure reagent grades. For etchants, thetechnical grade is used unless a purer grade is specified. Formacroetchants, the technical grade is generally adequate.Water is the solvent used for most macroetching solutionsand water purity can affect the etchant. Potable tap watermay contain some impurities that could affect the etchant.Distilled water has a significantly higher purity than tapwater. For macroetchants using technical grade chemicals,potable tap water is usually acceptable. For etchants inwhich high purity is required, distilled water isrecommended.


7.3.6 Disposal

Before disposing of chemical solutions, check environmentalregulations (federal, state and local) and safetydepartment procedures. The steps listed here are used onlyif there are no other regulations for disposal. Spent etchantsare discarded and must be discarded separately—mixing ofetchant 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 oftap water down the drain. The drain is flushed with a largevolume of water.


7.4 Using Etchants

After proper surface preparation and safe mixing ofetchants, the application of etchants to the test object may bedone with immersion or swabbing. The technique is determinedby the characteristics of the etchant being used.

7.4.1 immersionDuring immersion, a test object is completely covered byan etchant contained in a safe and suitable material- glass can be used for most etchants except hydrofluoric acid, fluorinematerials, strong alkali and strong phosphoric acid.A glass heat resistant dish on a hot plate may be used forheated solutions. The solution should be brought to temperaturebefore the test object is immersed. Tongs or other handlingtools are used and the test object is positioned so thatthe test surface is face up or vertical to allow gas to escape.The solution is gently agitated to keep fresh etchant in contactwith the test object.


7.4.2 Swabbing

Etching may also be done by swabbing the test surfacewith a cotton ball, cotton tipped wooden swab, bristled acidbrush, medicine dropper or a glass rod. The cotton ball andthe cotton tipped wooden swab generally are saturated withetchant and then rubbed over the test surface.Tongs and gloves should be used for protection and theetchant applicator must be inert to the etchant. For example,strong nitric acid and alkali solutions attack cotton andthese etchants must be applied using a fine bristle acidbrush. A glass or plastic medicine dropper may be used toplace etchants on the test object surface and a suitable stirringrod can be used to rub the surface. The test object maybe 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 brightappearance and the degree of dullness is used to determinewhen to stop etching. Approximate dwell times are given inthe table procedures but experience is important as well.


7.4.4 Test Object Preservation

Rinsing, drying, de-smutting and coating may be requiredfor preservation of the test object. Rinsing removes theetchant by flushing the surface thoroughly under runningwater. Cold water rinsing usually produces better surfaceappearance than hot water rinsing. Hot water rinsing doesaid in drying.If smutting is a problem, the test object can be scrubbedwith a stiff bristled brush or dipped in a suitable de-smuttingsolution. The test object should be dried with warm dry air.Shop air may be used if it is filtered and dried. After visualinspection, the test surface may be coated with a clear acrylicor lacquer but such coatings must be removed before subsequenttests. If the component is returned to service, a photographicrecord of the macro-etched area should be made.


TABLE 16. Etchant characteristics and usesTABLE 16. Etchant characteristics and uses* (continued)TABLE 17. Etchants for welds

See Text.


7.5 Conclusion

Visual testing is performed in accordance with applicablecodes, standards, specifications and procedures. Chemicalaids enhance the contrast of discontinuities making themeasier to interpret and evaluate. This enhancement isattained by macroetching- a controlled chemical processingof the surface. Macroetching gives the optimumresults on a properly cleaned and prepared surface. Chemicalsfor etching must be mixed, stored, handled and appliedin strict accordance with safety regulations.''''


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Experts at Work


vi self study exam preparatory note-part 4-section 4 b

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