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Liquid Penetrant Method NDT Training Program The American Society for Nondestructive Testing LESSON ONE Liquid Penetrant Testing is a nondestructive means of locating surface discontinuities based on capillary action. In the liquid penetrant method, the liquid is applied to the surface of the specimen, and sufficient time is allowed for penetration of surface discontinuities. If the discontinuity is small or narrow, as in a crack or pinhole, capillarity assists the penetration. After sufficient time has passed for the penetrant to enter the discontinuity, the surface of the part is cleaned. Capillary action is again employed to act as a blotter to draw penetrant from the discontinuity. To insure visibility, the liquid penetrant contains either a colored dye easily seen in white light, or a fluorescent dye visible under black (ultraviolet) light. B. Time allowed for A. Penetrant applied to surface. penetrant to seep into the opening. C. Surface penetrant removed. D. Developer applied to draw penetrant out of opening. E. Specimen visually examined. F. Post-cleaning. Discontinuities that are subsurface in one stage of production could be open to the surface at another stage, such as after grinding or machining. Nonmetallic inclusions and porosity in the ingot may cause stringers, seams, forging laps, cold shuts, and the like as the billet or slab is processed in the manner shown on the next slide: Anything that could block the penetrant from entering the discontinuity must be removed. A list of contaminants that must be removed would include dirt, grease, rust, scale, acids, and even water. The cleaning solvent used must be volatile (readily vaporized) so that it easily evaporates out of the discontinuity and does not dilute the penetrant. Surface preparation by shot or sandblasting is not recommended. Discontinuities that were open to the surface may be closed by the shot or sandblasting. Historically, penetrant inspection was called the “Oil and Whiting Method,” as it used kerosene and a white powder for the inspection of railroad parts. However, in the past 40 years the process has been improved tremendously to the point where it is a reliable and accurate inspection technique. The liquid penetrants used in nondestructive testing can be categorized by the type of dye they contain. 1. Visible dye penetrants contain a colored (usually red) dye. 2. Fluorescent penetrants contain a fluorescent dye. 3. Dual sensitivity penetrants contain a combination of visible and fluorescent dyes. Penetrants can be further categorized by the processes used to remove the excess penetrant from the specimen. 1. Water-washable penetrants are either self- emulsifying or removable with plain water. 2. Post-emulsified penetrants require a separate emulsifier to make the penetrant water washable. 3. Solvent-removable penetrants must be removed with a solvent which is typical when using visible dye in pressurized spray cans. The flow chart below illustrates the processing sequence with visible dye and fluorescent penetrants. Dual sensitivity penetrants would follow a processing sequence similar t that shown below. The selection of the best process, as listed on pages 5 and 6, depends upon: 1. Sensitivity required. 2. Number of articles to be tested. 3. Surface condition of part being inspected. 4. Configuration of test specimen. 5. Availability of water, electricity, compressed air, suitable testing area, etc. Penetrant testing is successfully used on metals such as aluminum, magnesium, brass, copper, cast iron, stainless steel, titanium, and most other common alloys. It can also be used t test other materials, including ceramics, plastics, molded rubber, powdered metal products, or glass. Penetrant testing is limited by its inability to test materials with discontinuities that are not open to the surface or having an extremely porous surface. LESSON ONE QUIZ ANSWER KEY 1. FALSE 2. FALSE 3. FALSE 4. FALSE 5. FALSE 6. FALSE 7. FALSE 8. TRUE 9. TRUE 10. FALSE 11. TRUE 12. FALSE 13. TRUE 14. TRUE 15. FALSE 16. TRUE 17. TRUE 18. TRUE LESSON TWO This lesson discusses the equipment and material required to perform the various penetrant tests and the required pre- and post-test cleaning. Proper cleaning is essential to liquid penetrant testing for two reasons: 1. If the specimen is not clean and dry, penetrant testing is ineffective. 2. If all traces of penetrant materials are not removed after the test, they may have a harmful effect on the specimen. (Chlorine and sulfur may affect some alloys.) Immersion tanks and detergent solutions are common means of assuring that a specimen surface is both physically and chemically clean. Vapor degreasing is particularly effective in the removal of oil, grease, and similar organic contamination. However, certain alloys have an affinity for specific elements used in vapor degreasing and if exposed to them may become structurally damaged. Steam cleaning is particularly adaptable to the cleaning of large, unwieldy articles. Solvent cleaning may be used in immersion tanks or may be used in a wipe-on and wipe-off technique. Solvent cleaning is usually less effective than the previous methods of cleaning. Rust and surface scale can be removed by any good commercially available acid or alkaline rust remover following manufacturer’s directions. Paint removal is often done in dissolving-type hot-tank paint strippers, bond release, or solvent paint strippers. Etching is effective for use on articles that have been ground or machined. This process uses an acid or an alkaline solution to open up grinding burrs and remove metal from surface discontinuities. Surface cleaning processes to be avoided include blast (shot, sand, grit, or pressure), liquid honing, emery cloth, wire brushes and metal scrapers. These processes tend to close discontinuities by peening or cold working the surface of the specimen. Penetrant Test Equipment (Stationary) Stationary equipment used in liquid penetrant testing varies in size and is largely dependent upon the size of the test specimen. Depending on the type and process used, a stationary system could include the following: 1. Pre-cleaning station (usually in remote area) 2. Penetrant station (tank) 3. Drain station 4. Emulsification station (tank) 5. Rinse station (tank) 6. Developing station (tank) 7. Drying station (usually oven) 8. Inspection station (enclosed booth or table with proper lighting) 9. Post-cleaning station (usually in remote area) SEE PICTURE ON NEXT SLIDE Penetrant Test Equipment (Portable) Both visible and fluorescent dye penetrants are available in kits which can be used at a remote location or when testing a small portion of a large article. A visible dye penetrant kit usually contains: 1. Pressurized spray cans of cleaning or removal fluid. 2. Pressurized spray cans of visible dye penetrant. 3. Pressurized spray cans of non-aqueous developer. 4. Wiping cloths and brushes. A fluorescent dye penetrant kit usually contains: 1. A portable black light and transformer. 2. Pressurized spray cans of cleaning or removal fluid. 3. Pressurized spray cans of fluorescent dye penetrant. 4. Pressurized spray cans of non-aqueous developer. 5. Wiping cloths and brushes. Black light equipment is required in fluorescent penetrant testing since it supplies light of correct wavelength to cause the penetrant to fluoresce. A deep red-purple filter is used to pass only those wavelengths of light that will activate the fluorescent material. At least a five-minute heat-up time is required to reach the correct arc temperature when using mercury arc lamps. The black light emits a special light with wavelengths that fall between visible and ultraviolet. Provided that the filter is not broken or cracked, there is no danger of injury to the human eye. It is suggested that the filter be checked prior to each use. Penetrant testing materials can be used in a variety of combinations. Most materials are available in either pressurized spray cans or in bulk quantities. The flow chart below illustrates the different material combinations. However, care should always be taken to assure that manufacturers’ specifications or company procedures are closely followed. Penetrant Testing Materials Penetrant materials are often restricted to specific groups. The establishment groups of penetrant materials will use the following in a variety of combinations to obtain the best results: 1. Water-Washable penetrants – Contain an emulsifying agent which makes them easily removable by a water rinse or wash. This penetrant material can be obtained with either a visible or fluorescent dye. 2. Post-Emulsifiable penetrants – Are highly penetrating, oily visible or fluorescent penetrants which are not soluble in water. These penetrants must be treated with an emulsifier before they can be removed by a water rise or wash. 3. Solvent-Removable penetrants - Are oily penetrants that do not contain an emulsifying agent and are removable only by solvents specially designed for that purpose. 4. Emulsifiers – When applied to a penetrant-coated specimen make the resultant mixture removable by water rinse or wash. Emulsifiers have low penetrant characteristics and do not remove indications from the specimen surface. 5. Removers (Solvent) – Are designed to be used in conjunction with specific penetrants. Typical removers are available in bulk or pressurized spray containers. 6. Dry developers – Are a fluffy, absorbent white powder that is used in both fluorescent and visible dye penetrant tests. It functions to draw the penetrant indications to the surface thus making them visible. 7. Wet developers – Function similarly to dry developers except that they are a mixture of a developing powder and water. 8. Non-aqueous Wet developers – Differ from wet developers in that the developer powder is mixed with a rapid-drying liquid solvent. 9. Liquid Oxygen (LOX) Compatible Materials – Must be used when articles inspected are subjected to contact with either liquid or gaseous oxygen. These materials are specifically designed to be inert when in the presence of LOX. 10. Low Sulfur and Low Chlorine – Penetrant materials must be specifically designed to avoid the harmful effects caused on some nickel and titanium alloys by the sulfur and chlorine content. Safety Precautions In general, the materials used in penetrant inspection can be flammable and can cause skin irritations. In addition, the ultraviolet spectrum of light rays generated from the mercury arc lamp can cause sunburn and may be injurious to the eyes. However, if the proper filter for fluorescent dye inspection is used, the harmful rays will be filtered out. FIRE – Many penetrant materials are flammable. Safe precautions requires that penetrant materials used in open tanks have a flashpoint of greater than 120º F. SKIN IRRITATION – Skin irritation can be avoided by preventing unnecessary contact and by the use of gloves, aprons, and protective hand creams. AIR POLLUTION – The developing powders are considered nontoxic but excessive inhalation must be avoided. Exhaust fans should be installed in any confined area where dry developers or vapors from the penetrants are present. LESSON TWO QUIZ ANSWER KEY 1. FALSE 2. TRUE 3. FALSE 4. FALSE 5. FALSE 6. TRUE 7. FALSE 8. FALSE 9. FALSE 10. FALSE 11. FALSE 12. TRUE 13. TRUE 14. FALSE 15. TRUE 16. TRUE 17. TRUE LESSON THREE This lesson discusses surface preparation and penetrant application. Cleaning The effectiveness of liquid penetrant testing is based upon the ability of the penetrant to enter surface discontinuities. All paint, carbon, oil, varnish, oxide, plating, water, dirt, and similar coating must be removed before application of the penetrant. Liquid penetrant placed on the surface of a specimen does not merely seep into discontinuities, it is pulled into them by capillary action. This is the reason one can cover the under surface of an item with a penetrant and still have a valid test. The following are typical cleaning methods discussed earlier: 1. Detergent Cleaning 5. Rust and Surface Scale 2. Vapor Degreasing Removal 3. Steam Cleaning 6. Paint Removal 4. Ultrasonic Cleaning 7. Etching Application of Penetrants Almost any liquid could be considered a penetrant, but modern penetrants must have: 1. The ability to hold a dye material in suspension. 2. The ability to spread the dye evenly over the surface. 3. The ability to carry the dye into any discontinuity open to the surface. 4. The ability to bring up the dye as it is “coaxed” back to the surface. 5. The ability, when desired, to be easily removed. There are two types of dye used in modern penetrants: 1. Visible – A brightly colored dye that is highly visible under normal lighting conditions. This type of dye is normally called visible dye. 2. Fluorescent – An almost colorless dye which emits visible light rays when reviewed under black light. A dye with dual sensitivity contains both a visible dye for examination in white light and a fluorescent dye for a more sensitive evaluation of small discontinuities. Penetrants, either fluorescent or visible, can be applied by any one of the following means: 1. Spraying – Usually using a low pressure circulation pump or from pressurized spray cans. 2. Brushing – Usually applied with rags, cotton waste, or brushes. 3. Immersion – The entire part is dipped into a tank of penetrant. 4. Pouring – The penetrant is simply poured over the surface. Penetration (Dwell) Time The period of time during which the penetrant is permitted to remain on the specimen is a vital part of the test. Tight crackline discontinuities may require in excess of 30 minutes for penetration that will give an adequate indication. However, gross discontinuities may be suitably penetrated in 3 to 5 minutes. The temperature of the specimen and temperature of the penetrant can affect the required dwell time. Warming the specimen to 70º F or higher accelerates penetration and shortens dwell time. However, care should be taken not to overheat the specimen since too much heat may cause evaporation of the penetrant from the discontinuity. Dwell times are based on the assumption that the penetrant will remain wet on the part surface. Additional penetrant may be applied during dwell time. The penetrant manufacturer will provide suggested dwell times for the various penetrants that it produces. Penetrant Testing Processes We previously mentioned the two types of penetrants, visible and fluorescent. For both categories there is a further breakdown: 1. Water-Washable (visible or fluorescent) 2. Post-Emulsification (visible or fluorescent) 3. Solvent-Removable (visible or fluorescent) Water-Washable penetrants have a built-in emulsifier and the penetrant is easily removed by a water rinse. Care must be taken to insure that the spray volume and force does not wash penetrant out of the discontinuity. Water temperatures above 110º F are not recommended because this may speed up the evaporization of the penetrant. The sketch below represents the steps in a Water-Washable penetrant test. Water-Washable penetrants are usually preferred for use on articles with a rough surface or if they contain threads or keyways. The built-in emulsifier provides the best penetrant removal from blind holes and other hard-to-reach locations but has the disadvantage of poor reliability in detecting wide or shallow discontinuities. (See page 6 for advantages and disadvantages) Solvent-Removable penetrants – They have the advantage of portability and can be used outdoors without using heavy, complex equipment. They are excellent for many maintenance inspections and for checking portions of a larger structure. Penetrants is often applied from a pressurized spray can which makes the system very portable. After the specified dwell time, the excess penetrant is first removed by wiping with absorbent towels and then cleaned with towels dampened with solvent. Solvent is never applied directly to the specimen as it might wash out or dilute the penetrant in the discontinuity. (See page 6 for advantages and disadvantages of solvent- removable penetrants) Post-Emulsification penetrants require a two-step removal process. The emulsifier is usually applied by dipping or immersion. The amount of dwell time in the emulsifier is in the range of one to four minutes in accordance with manufacturers’ recommendations and the type of defects expected. The resultant emulsifier-penetrant mixture is removed by water rinse. (Emulsification will be covered in more detail in Lesson 4) 1. Water-Washable Advantages Easily washed with water Good for quantities of small specimens Good on rough surfaces Good on keyways and threads Good on wide range of discontinuities Fast, single step process Relatively inexpensive Available in oxygen compatible form Disadvantages Not reliable for detecting scratches and similar shallow surface discontinuities Not reliable on reruns of specimens Not reliable on anodized surfaces Acids and chromates affect sensitivity Easily over-washed Penetrant subject to water contamination 2. Solvent-Removable Advantages Portability No water required Good on anodized specimens Good for spot checking Specimens can be rerun Disadvantages Flammable materials Removal of excess surface penetrant is time consuming Materials cannot be used in open tanks Difficult to use on rough surfaces such as cast magnesium 3. Post-Emulsification Advantages High sensitivity for very fine discontinuities Good on wide shallow discontinuities Easily washed with water after emulsification Short penetration time Cannot be easily over washed Disadvantages Two step process Equipment required for emulsifier application Difficult to remove penetrant from threads, keyways, blind holes and rough surfaces Leak-through technique – The use of liquid penetrant in this method is suited for finding leaks in such articles as tanks, piping, tubing, and hollow castings. The sketch below illustrates the leak-through technique. Fixing and Recording Indications 1. Photographs – Typical black and white and color film are used with the self-developing films providing the most convenience. 2. Special Wax and Plastic Film Developers have been developed to absorb and fix the penetrant indication to form a permanent record. A. Strippable lacquers are sprayed in several coats over the indication and when dry can be “lifted” to provide a permanent record. B. Special “fixers” are sprayed over the indication and when dry are lifted with transparent tape. LESSON THREE QUIZ ANSWER KEY 1. FALSE 2. FALSE 3. FALSE 4. TRUE 5. FALSE 6. TRUE 7. FALSE 8. FALSE 9. TRUE 10. TRUE 11. FALSE 12. SPRAYING BRUSHING 13. DETERGENT, STEAM, VAPOR BREASING, RUST & SCALE 14. B LESSON FOUR This lesson discusses emulsification, penetrant removal, and developer application. Post-Emulsification Penetrant (P.E. Penetrant) This type of penetrant emulsification is accomplished by dipping the part in a chemical emulsifier prior to washing. The emulsifier will break down the penetrant and make it water soluble. The emulsifier can be applied by dipping or spraying, but not by brushing. The bristles of the brush may enter the discontinuity. The sketches below and on the next slide represents the steps in a Post-Emulsification Penetrant Test. If too short an emulsification time is used, not all penetrant will be removed, which will cloud over discontinuities. If too long a time is used, penetrant within the discontinuities will also become water soluble and be washed away with excess penetrant. The P.E. penetrant makes the water wash less critical. The sketches below compare the effects of water-washable and post- emulsification penetrants. (A) Water-Washable(B) Post-Emulsification Developer Application Some indications may be visible prior to the application of a developer but this step will ensure that all discontinuities are visible to the naked eye. Developing is accomplished when a highly absorbent powder is applied to the item being tested after excess penetrant is removed. The penetrant is actually drawn out of the discontinuity by the strong capillary action of the developer. As shown on the previous slide, the image of the discontinuity in the developer will actually be larger than the actual size of the discontinuity. There are two common types of developers in use today – wet and dry. Both use a white powder and the primary difference is in the method of application. 1. Wet Developers A. The non-aqueous developer is held in suspension in a solvent base and is usually supplied in pressurized cans. B. Another type of wet developer holds the white powder in suspension in a water base. This type of wet developer is generally used with water- washable or P.E. penetrants and is applied by dipping or spraying. After application, a short time is allowed for the water to evaporate leaving a thin layer of white powder. If an oven is used, the temperature should not exceed about 225º F, as this could evaporate penetrant in the discontinuity. 2. Dry Developers A dry developer is a fluffy white powder that is not carried in a liquid. Dry developers are applied directly to the article as a powder. This is usually done with slight air pressure or articles may be dipped into a container of dry developer. It is very necessary to have a dry surface prior to application of a dry powder. A wet surface will result in uneven layers of powder. Dry developer is usually used on fluorescent penetrants. Advantages of “Wet” Developers Better on smooth surfaces where the dry developer will not adhere. When a wide, shallow discontinuity is sought, a wet developer will leave a more even coat of developer. Advantages of “Dry” Developers On rough surfaces, and on sharp fillets, holes, and threaded articles, the wet developer tends to leave too much developer. LESSON FOUR QUIZ ANSWER KEY 1. TRUE 2. FALSE 3. TRUE 4. TRUE 5. TRUE 6. TRUE 7. FALSE 8. FALSE 9. TRUE 10. TRUE 11. FALSE 12. FALSE 13. FALSE 14. FALSE 15. TRUE 16. FALSE LESSON FIVE This lesson discusses the final steps in the penetrant process: inspection, evaluation, post-cleaning, and material control. Inspection Proper lighting should be the first consideration in the inspection of an article. 1. If a fluorescent dye penetrant is used, a room or booth with dim light and a black light with proper intensity are required. 2. If a visible dye penetrant is used, adequate normal lighting is necessary. Typical Indications in Penetrant Inspection All indications found with the liquid penetrant method will be surface discontinuities, but the indications may or may not affect the usefulness of the article. False Indications The most common source of false indications is poor washing. The operator can easily tell when a good rinse is obtained by using a black light during and after the fluorescent penetrant removal process. To avoid false indications, care should be taken so that no outside contamination such as the following occurs. 1. Penetrant on operator’s hands. 2. Contamination of developer. 3. Penetrant transferred to clean specimen from other indications. 4. Penetrant on inspection table. Nonrelevant Indications Nonrelevant indications are actual surface discontinuities that in most cases are there by design. They are caused by some feature of assembly such as articles that are press-fitted, keyed, splined, or riveted. Nonrelevant indications could also include loose scale or a rough surface on a forging or casting. True Indications (Some typical indications are shown in Chapter 6, pages 8-19 in the training handbook) True indications are those caused by surface discontinuities that have been interpreted as not being false or nonrelevant. True Indications could be divided into five basic categories: 1. Continuous line – This type of indication is often caused by cracks, cold shuts, forging laps, scratches, or die marks. 2. Intermittent line – These indications could be caused by any of the discontinuities mentioned above provided they were very tight or where the part had been peened, machined, or ground. 3. Round – Usually caused by porosity open to the surface. 4. Small dots – Tiny round indications caused by the porous nature of the specimen, coarse grain structure, or microshrinkage. 5. Diffused or Weak – These indications are difficult to interpret and often the part must be cleaned and retested. In many cases the diffused or weak indications turn out to be false indications caused by an improper penetrant procedure. Depth Determination of Penetrant Discontinuities The greater the depth of a discontinuity, the more penetrant it will hold and the larger and brighter the indication, Post-cleaning After the specimen has been inspected it is very important that it be thoroughly cleaned. Post-cleaning usually will involve the same types of cleaning operations were used in pre-cleaning. Quality Control of Penetrant Test Materials Aluminum Test Blocks – They provide a good means of evaluating general purpose penetrants. They should be used for comparisons only and not for absolute evaluations. These blocks are non-uniform, and the depth and width of cracks are uncontrolled. The size of the blocks may vary but are about 3/8 by 2 inches of 2024-T-3 aluminum. About 3 inches long. The blocks are heated and quenched to produce an overall crack pattern. They can be reused by reheating, quenching, and careful cleaning. Sensitivity Tests – While there have been no simple quantitative tests developed for measuring penetrant sensitivity. A simple comparative test is usually adequate. A small sample of the penetrant from the testing area is placed on one side of an aluminum test block, and a small sample of new penetrant on the other side. By visual observation it is determined if the old penetrant is contaminated to the point where it must be discarded. Meniscus Test – Provides a practical test for evaluating the dye concentration in thin-liquid films. The test utilizes a flat glass platen and a convex lens. When a drop of solution is placed between the lens and platen, a colorless or nonfluorescent spot is formed around the point of contact. The resultant contact angle simply indicates the ability of a liquid to wet a surface. The diameter of the remaining “spot” of colorless penetrant provides a measure of film thickness which can be used to compare the dye concentration of penetrants. Prolonged exposure of a film to ultraviolet light or accidental contamination by acids or alkalis will affect the dye concentration and therefore change the spot diameter. Ceramic Block Test – Consists of an unglazed ceramic disc which has thousands of micropores and cracks on its surface. A pencil mark is made on the block and a small amount of a test penetrant is placed on one side and a reference penetrant is applied to the other side. After the correct dwell time, the two penetrants are compared in a side-by-side visual examination. A reduction in the number or apparent brightness of pore indications should be observed when comparing a fresh and old batch of penetrant. Water Content Test – The ASTM standard D-95 describes a test where 100 ML of penetrant is placed in a boiling flask with a similar quantity of moisture-free xylene. The condensate is collected in a graduated tube to show percent of water by volume. If percent of water exceeds manufacturers’ recommendations, the penetrant is discarded. Viscosity Test – A viscometer is used to measure the viscosity in centistokes to determine if the penetrant is within the range recommended by the manufacturer. A typical standard is ASTM D-445. Fluorescent Penetrant Fade Test – This test involves the use of the aluminum test blocks in a side-by-side comparison test. The fluorescent penetrant is placed on both sides of the block and processed normally. One half of the block is then exposed to a standard black light for one hour, while the other side is covered with paper. The fluorescent brilliance of the two sides is observed and, if one side in noticeably less brilliant, the penetrant is discarded. Water Washability Test – This test evaluates the efficiency of the emulsifier by comparing two different percentage blends of penetrant and emulsifier. A special steel block is placed at a 75º angle and the two blends are allowed to flow separately down the block. After a five-minute waiting period, the block is washed and examined for traces of remaining penetrant. Developers – Dry Developers are simply visually inspected to see that they are not lumpy or caked instead of fluffy and light. Wet developers are often checked using a hydrometer to assure that the density of the powder in the vehicle is within the range recommended by the manufacturer. Filtered Particle Method of Penetrant Inspection – When testing a porous surface with filtered particles. The increased area created by a crack will “filter” the penetrant, leaving the particles on the surface. LESSON FIVE QUIZ ANSWER KEY 1. FALSE 2. TRUE 3. FALSE 4. TRUE 5. FALSE 6. TRUE 7. TRUE 8. TRUE 9. TRUE 10. FALSE 11. TRUE 12. FALSE 13. TRUE 14. FALSE 15. TRUE 16. TRUE 17. TRUE 18. FALSE LESSON SIX This lesson discusses types of discontinuities that can be evaluated with the liquid penetrant method. Discontinuities can be divided into three general categories: Inherent, Processing, and Service. 1. Inherent – They are usually related to discontinuities found in the molten metal. Inherent wrought discontinuities relate to the melting and solidification of the original ingot before it is formed into slabs, blooms, and billets. Inherent cast discontinuities relate to the melting and solidification of a cast article. Usually caused by inherent variables such as inadequate feeding, gating, excessive pouring temperature, and entrapped gases. 2. Processing Discontinuities – They are usually related to the various manufacturing processes such as machining, forming, extruding, rolling, welding, heat treating, and plating. 3. Service Discontinuities – They are related to the various service conditions such as stress corrosion, fatigue, and erosion. Remember, when you use the liquid penetrant method, you can find only those discontinuities which are open to the surface. However, during the manufacturing process, many discontinuities that were subsurface can be made open to the surface by machining, grinding, and the like. Remember that discontinuities are not necessarily defects. Any indication that is found by the inspector is called a discontinuity until it can be identified and evaluated as to the effect it will have on the service of the part. Listed on the next slide are some typical discontinuities that should be recognized when doing any type of nondestructive test. (A more detailed study of typical discontinuities can be obtained by reading programmed instruction handbook PI-4-1.) Discontinuities trapped in the ingot during the steel-making process may cause additional types of discontinuities as the steel is used in the manufacture of an article. There are three main types of discontinuities found in ingots. 1. Porosity – Caused by entrapped gas in the molten metal. 2. Nonmetallic Inclusions - Caused by impurities accidentally included in the molten metal. 3. Pipe – Caused by shrinkage at the center of the ingot as the molten metal solidifies. The “HOT TOP” is usually cropped off to remove most of the above discontinuities. When an ingot is further processed into slabs, blooms, and billets, it is possible for the discontinuities to change size and shape. As a billet is flattened and spread out, nonmetallic inclusions may cause a lamination. Pipe and porosity could also cause laminations in the same manner as shown below: As a billet is rolled into bar stock, nonmetallic inclusions are squeezed out into longer and thinner discontinuities called stringers. As a billet is rolled into round bar stock, surface irregularities may cause seams. Seams are caused by folding of metal due to improper rolling or by a crack in the billet as shown below: A seam could also occur when the billet is formed into a rectangular bar as shown below: Forging Discontinuities occur when metal is hammered or pressed into shape, usually while the metal is very hot. A forged part gains strength due to the grain flow taking the shape of the die, and the process is shown below: A forging lap is caused by folding of metal on the surface of the forging, usually when some of the forging metal is squeezed out between the two dies. A forging burst is a rupture caused by forging at improper temperatures. Bursts may be either internal or open to the surface as shown below. Casting discontinuities occur when molten metal is poured into a mold and allowed to solidify. A cold shut is caused when molten metal is poured over solidified metal as shown below: Hot Tears (shrinkage cracks) occur when there is unequal shrinkage between light and heavy sections as shown below: Shrinkage cavities are usually caused by lack of enough molten metal to fill the space created by shrinkage, similar to pipe in the ingot. Microshrinkage is usually many small subsurface holes that appear at the gate of the casting. Microshrinkage can also occur when the molten metal must flow from a thin section into a thicker section of a casting. Blow holes are small holes at the surface of the casting caused by gas which comes from the mold itself. Many molds are made of sand, and when molten metal comes into contact with the mold, the water in the sand is released as steam. Porosity is caused by entrapped gas. Porosity is usually subsurface but can occur on the surface depending on the design of the mold. Grinding Crack: Are a processing-type discontinuity caused by stresses which are built up from excess heat created between grinding wheel and metal. Grinding cracks will usually occur at right angles to the rotation of the grinding wheel. Heat treat cracks are often caused by the stresses built up during heating and cooling. Unequal cooling between light and heavy sections may cause heat treat cracks. Heat treat cracks have no specific direction and usually start at sharp corners which act as stress concentration points. Fatigue cracks are service-type discontinuities that are usually open to the surface where they start from concentration points. Fatigue cracks are possible only after the part is placed into service, but may be the result of porosity, inclusions, or other discontinuities in a highly stressed metal part. Welding Discontinuities – The following are types of “Processing Discontinuities.” LESSON SIX QUIZ ANSWER KEY 1. TRUE 2. TRUE 3. TRUE 4. TRUE 5. FALSE 6. FALSE 7. FALSE 8. FALSE 9. FALSE 10. FALSE 11. FALSE 12. FALSE 13. TRUE 14. TRUE 15. TRUE 16. TRUE 17. TRUE 18. FALSE LESSON SEVEN Identification and comparison of discontinuities that can be found with the liquid penetrant process. The student is asked to study the photographs and descriptions of typical discontinuities as printed in the PT Classroom Training Handbook (CT-6-2). Each of the specific discontinuities illustrated are divided into three general categories: Inherent, Processing, and Service. Cold Shut Page 7-10 Fillet Crack Page 7-12 Grinding Cracks Page 7-15 Heat-Affected Zone Cracking Page 7-19 Heat Treat Cracks Page 7-21 Shrinkage Cracks Page 7-24 Thread Cracks Page 7-26 Hydrogen Flake Page 7-31 Lack of Penetration Page 7-40 Laminations Page 7-42 Laps and Seams Page 7-44 Laps and Seams Page 7-46 Microshrinkage Page 7-49 Stress Corrosion Page 7-55 Hot Tears Page 7-63 Intergranular Corrosion Page 7-65 After reviewing the photographs and descriptions of typical discontinuities, it should be obvious that liquid penetrant inspection is not the best method in all cases. The selection of one testing method over another is based upon variables such as: 1. Type and origin of discontinuity. 2. Material manufacturing process. 3. Accessibility of article. 4. Level of acceptability desired. 5. Equipment available. 6. Cost. Training and Certification It is important that the technician and supervisor be qualified in the liquid penetrant method before the technique is used and test results evaluated. The American Society for Nondestructive Testing recommends the use of its document Recommended Practice No. SNT-TC-1A. This document provides the employer with the necessary guidelines to properly qualify and certify the NDT technician in all methods. To comply with this document the employer must establish a “written practice” which describes in detail how the technician will be trained, examined, and certified. The student is advised to study page 6 of the June 1975 edition of SNT-TC-1A to determine the recommended number of hours of classroom instruction and months of experience necessary to be certified as a liquid penetrant testing technician. Certification of NDT personnel is the responsibility of the employer and is usually at three levels. LEVEL I – is qualified to perform specific calibrations, specific tests, and specific evaluations according to written instructions. LEVEL II - Is qualified to set up and calibrate equipment and to interpret and evaluate results with respects to codes, standards, and specifications. Must be able to prepare written instructions and report test results. LEVEL III - Must be capable and responsible for establishing techniques, interpreting codes, and designating the test method and technique to be used. Must have a practical background in the technology and be familiar with other commonly used methods of NDT. The SNT-TC-1A Document recommends that the NDT Technician be examined in the following areas: A. General Examination B. Specific Examination C. Practical Examination ASNT provides a service to the industry by providing Level III Examinations in the general and practical areas. Because of the individual requirements of the many industries using NDT, the specific examination is still the responsibility of the employer. The following flow chart indicates the paths that can be taken to be certified according to the SNT-TC- 1A Document. LESSON SEVEN QUIZ ANSWER KEY 1. FALSE 2. FALSE 3. TRUE 4. FALSE 5. TRUE 6. TRUE 7. FALSE 8. FALSE 9. TRUE 10. TRUE Close • Summarize your points. • State your conclusion. Make it relevant to your audience. And, if applicable: • Describe options for future consideration. • Recommend a future strategy, plan and/or goal.
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