Industrii! Radiography - IAEA

iľin Jlfl-111 1 ľ-t i:ilii:)!*"' ' í T" ť í **'* v'liiilllfi -.v;:; Aul v - V ; : "tHb) .t- Ä.f-'igure i.I Four stages of liquid pcnelrant process(a) penetrant application and seepage into the discontinuity(b) removal of excess penetranl(c) application of developer, and(d) inspection for the presence of discontinuities1.1.3.1. General Procedure for Liquid Penetrant Inspection(a) Cleaning the surface to be examinedThere should be no material such as paint . plating or coatings of oxide or loose dirtcovering the surface. This is to prevent false indications and to expose hiddendiscontinuities Io the penetranl. Solid contaminants such as carbon, engine varnish, painisand similar materials should be removed by vapour blast, chemical dip or other acceptablemethods. Methods such as shot blasting, sand blasting, emery cloth, wire brushing or metalscrapping should not be used, especially for soft materials, since these cleaning methodswill cover up defects by cold working the surface. Contamination can occur due to thepresence of lubricants, protective oils, metal dust, polymerisation, oxidation, carbonaceousdeposits, protective paints etc. Various solvents have been developed by differentcompanies to remove them. Contamination due to inorganic corrosion products, heattreatment scale, operationally formed refractory oxides etc. is conveniently removed byabrasive blasting with glass beads etc. combined with a chemical cleaning. Which evermethod is employed the use of trichlorethylene vapour degreasing as a final stage isstrongly recommended.rt(b) Drying the surfaceIf. for any reason, separations are filled with liquid, they will prevent entry of penetraal,hence drying is an essential operation. It should be realised ihat although the surface mayseem dry. separations may still be filled with liquid. With "dismountablc cracks" used toevaluate penetrants. it is remarkable how long a liquid can stay in a small separation afterthe outer surface has become dry. The lesson is that improper drying may be worse thanno cleaning, because the remaining solvent may present a barrier to the pcnclrant loo. Ifpenetrant liquid does reach into the separation, it will be diluted by the solvent, and thisalso makes the treatment less effective.{cj Application of penetrantThe pcnetranl is applied with (he help of a brush or by spray or by dipping Ihe lest pieceinu a bath of the penetrant. After this a certain residence time is allowed for the penetranl10

lo seep into discontinuities. The residence time varies with Ihe temperature, the type ofpenclranl. the nature of the discontinuity and the material of the K\st specimen. It usuallyvaries between 5 and 30 minutes. In special cases it may be as long as one hour.(d) Removal of superfluous pcnelrantThe excess penetrant on the surface should be removed to obtain optimum contrast and loprevent misleading indications. The appropriate remover is usually recommended by themanufacturer of the penetrant. Some penetranls are water washable, others needapplication of an cmulsificr before they can be removed with water. The removal methodis to use a sponge or water spray. There arc special penetrant removers which areessentially solvents. It is most important that removal of the pcnctrsnl is restricted to :hesurface and that no pcnetranl is washed out of the flaws. This can easily happen when thecleaning is too rigorous. When the surface is smooth, washing can be less intensive than forrough surfaces; in the latter case there is a definite risk that penelrant may be washed outof small imperfections.A general criterion for the removal operation is that it must be fast and should beprolonged long enough to make the surface almost clean. It is better lo leave small tracesof penetrant on the surface than to carry out excessive cleaning. When removingfluorescent penetrants, the effect of the treatment should preferably be watched underblack light.(e) Drying the surfaceThe surface can be dried with a dry cloth or an air blower. Drying is generally needed toprepare the surface for the application of a powder developer, which would otherwise ;lotal wet places. It also decreases the adverse effect of insufficiently removed traces ofpenelrant. Here again excess should be avoided. Penctrani liquid left in flaws should notbe allowed to dry. This can happen when hot air is used for drying.(f) Application of developerDevelopers arc usually of two types, namely, dry or wet developer. Dry developer consistsof a dry, light coloured powdery material.It is applied to the surface after removal of excess penetranl and drying of the part. It canbe applied either by immersing the parts in a tank containing powder, or by brushiag it onwith a paini brush (usually not a desirable technique) or by blowing the powder onto thesurface of the part.Wet developer consists of a powdered material suspended in a suitable liquid such as wateror a volatile solvent. It is applied to the parts immediately following the water washingoperation.Developers should be such that th.-y provide a while coating that contrasts with thecoloured dye-pcnctranl, and draw the penetrant from the discontinuiiies lo the surface ofIhe developer film, thus revealing defects.The dry developers are applied generally with fluorescent penetrants. They are applied justprior to the visual inspection process. The wet developers J-C also generally used inconnection with fluorescent penetranls. They are applied after the washing operation andbefore the drying operation. The solvent based developers are generally used with thevisible dye-penetrants. T'cy are applied after cleaning off extra penetrant. A short timeshould be allowed for development of indications after the developer has been applied. Thistime should be approximately one half that allowed for penetration. Developer coating isremovd after inspection by water stream, spray no/zle. brush etc. The powderconcenUvition of the liquid developer should be carefully controlled lo obtain Ihe requiredthin and uni'orm layer over the surface.

(g) Observation and interpretation of indicationsAn indication in the developer will become visible after a certain lapse of time. Becauseall penetrant inspection methods rely upon the scciug of an indication by Ihc inspector, thelighting provided for this visual examination is extremely important. For best results,inspection for fluorescent indications should be done in a darkened area using Hack light,for the interpretation of indications, it is very important to observe their characteristicsat the very moment they appear. As soon as the flaws have bled out, the indications mayrun lo larger spots, depending on size and depth, and at this stage ii is difficult to derivecharacteristic information from the flaw.The extent to which observation of developing indications can be realised in practicedepends largely on the si/e and complexity of the s'irfacc to be examined as well as on thenumber of components lo be tested. A brief guide to the penetrant indications is givenhere. A crack usually shows up as a continuous line of penetrant indication. A cold shut onthe surface of a casting also appears as a contiguous line, generally a relatively narrow one.A forging lap may also cause a continuous line of penetrant indication. Rounded areas ofpenetrant indication signify gas holes or pin holes in castings.Deep crater cracks in welds frequently show up as rounded indications, Penetrantindications in the form of small dots result from a porous condition. These may denotesmall pin holes or excessively coarse grains in castings or may be caused by a shrinkagecavity. Sometimes a large area presents a diffused appearance. With fluorescent penctrants,the whole surface may glow feebly. With dye penetrants, the background may be pinkinstead of white. This diffused condition m:y result from very fine, widespread porosily,such as microshrinkage in magnesium. The depth of defects will be indicated by richnessof colour and speed of bleed out. The time required for an indication to develop isinversely pioportional lo the volume of the discontinuity.1.1A2 Penetrant Processes and EquipmentPenelrants are classified depending on whether the dye fluoresces under black light or ishighly contrasting under white light. A second major division of penctrants is determinedby the manner is which they can be removed from the surface. Some penctrants arc waterwashable and can be removed from the surface by washing with ordinary tap water. Otherpenctrants arc removed with special solvents. Some penctrants arc not in themselves waterwashable but can be made so by applying an cmulsificr as an extra step after penetrationis completed. During a short cmulsification period this emulsificr blends with the excesspenetrant on the surface of the part after which the mixture is easily removed with a waterspray.The Fluorescent Penelraut Water Washable Penetrant Process uses this method. Thefluorescent is used for greater visibility; can be easily washed with water; is good forquantities of small parts; is good on rough surfaccs;is good in kcyways and threads; is highspeed, economical of time and good for a wide range of defects.The Post Emulsification Fluorescent Process has fluorescence for greater visibility; hashighest sensitivity for very fine defects; can show wide shallow defects; is easily washedwith water after cmulsification; has a short penetration time; high production; especiallysatisfactory for chromáte surfaces.The Water Emulsifiablc Visible Pcnclrants Process has greater portability; requires noblack light; can be used on suspected local areas of large parts; aids in rework or repair;can be used on parts where water is not available; can be used where parts arc to berepaired in ordinary light. Best of all techniques on contaminated defects; sensitive loresidual acidity or alkalinity; high sensitivity to very fine defects.Fluorescent materials generally respond most actively to radiant energy of a wavelength ofapproximately 3( 5()Ä . This is just outside the visible range on the blue or violet side butnot sufficiently far removed to be in the chemically active or ultraviolet range. This is black12

light. Four possible sources of black light are incandescent lamps, metallic or carbon arcs,tubular "BL" fluorescent lamps and enclosed mercury vapour arc lamps. Mercury vapourarc lamps arc generally used. One of the advantages of this is that its light output can becontrolled by design and manufacture. At medium pressures, from 1 to 10 atmospheres, thelight output is about evenly distributed between the visible, black light and hard ultravioletranges. These medium pressure lamps are ordinarily used for inspection purposes. A redpurple glass is used to filter the light not desired. Factors such as the nature of inspectedsurface, extraneous white light entering the booth, the amount and location of fluorescentmaterials near the inspector and the speed with which inspection is to be carried out, havean effect on the black light intensity necessary at the inspected surface. The light level,once it is set for a practical job, should be maintained. Good eye-sight is also a requisite.1.1 .Í3. Areas of Application of Liquid PenelrantsLiquid penetrants can be used for the inspection of all types of materials such as ferrousand non-ferrous, conductors and non-conductors, magnetic and nonmagnetic and all sortsof alloys and plastics. Most common applications arc in castings, forgings and welds.1.1.3.4. Range and Limitations of Liquid PenclrantsAH imperfections which have an opening to the surface arc detectable no matter what theirorientation. Sub-surface defects which ire not open to the surface will not show up andconsequently will not interfere with the interpretation. No indications arc produced as aconsequence of differences in permeability (a weld in dissimilar steels, transition zonesetc.). There is no risk o.' surface damage which may occur, for example, during carelessmagnetisation with prods in the current flow method. The equipment is also low cost.Flaws may remain undetected by penetrant inspection if magnetic particle testing has beenpreviously used, because the residual iron oxide may fill or bridge the defect. Similarlyfluorescent penetrant will often fail to show discontinuities previously found by dyepenetrant because the dye reduces or even kills fluorescence. Reinspection should be donewith the same method. Surface condition may affect the indications.Surface openings may be closed due to dirt, scale, lubrication or polishing. Rough or porousareas may retain pcnctrant producing irrelevant indications. Deposits on the surface maydilute the pcnctrant, thus reducing its effectiveness.If all the surface pcnctrant is notcompletely removed in the washing or rinse operation following the penetration time, theunrcmoved penctranl will be visible. Such parts should be completely reprocessed.Degrcasing is recommended. Another condition which may create false indications is whereparts are press fitted to each other. The penetrant from the fit may bleed out and mask thetrue defect.Some of the precautions necessary for liquid penetrant inspection are briefly summarizedhere. Only one process should be used. Change of process is not advisable for reinspection.The lest materials should not be contaminated. Contamination leads to a loss of testsensitivity and reliability. Contamination of water with pcnelrants should be avoided. Wetdeveloper bath should be at the recommended concentration. The temperatures should notexceed certain limits depending on the materials used. The pcnctrant should not be heated.Avoid contact of pcnctrant with skin by wearing gloves. Keep penctrants off cloths. Checkfor traces of fluorescent penclrant on skin and clothes and inside gloves by examiningunder black light. Excessive amounts of dry pcnelrants should not be inhaled. Improperlyarranged black lights may cause some eye fatigue. The materials used with visible penetranlprocess are flammable and should not be stored or used near heat or fire. Do not smokewhile using them.1.1.4. Magnetic Particle TestingMagnetic particle testing is used for the testing of materials which can be easilymagnetised. This method is capable of detecting open-to-surfacc and just below the surfaceflaws. In this method the test specimen is first magnetised cither by using a permanent13

magnetic or an electromagnet or by passing electric current through or around thespecimen. The magnetic field thus introduced into the specimen is composed of magneticlines of force. Whenever there is a flaw which interrupts the flow of magnetic lines offorce, some of these lines must exit and re-enter the specimen. These points of exit and reentry form opposite magnetic poles and whenever minute magnetic particles are sprinkledonto the surface of the specimen, these particles arc attracted by these magnetic poles tocrcnte a visual indication approximating the size and shape of the flaw.Fig. 1.2(a,b) illustrates tbc basic principle of this method.Figure 1.2(a): The effect of defects on the flux flowCRACKDEEP LYINGFLAWFigure'ili'l! "FLUX FLOW LINES1.2(b): The effect of defects on the flux flowin a magnetised ring.1.1.4.1 Methods of MagnetisationElectric currents arc used to create or induce magnetic fields in magnetic materials. Severaltypes of magnetisation are in use for magnetic particle inspection Some of the types arcD.C magnetisation , half wave rectified current magnetisation and A.C. magnetisation. Allthese three types arc mainly used for magnetic particle inspection.Direct Current obtained from storage batteries was first believed to be the most desirablecurrent lo use, since it penetrates more deeply into lest specimens than any other14

current. The big disadvantage of the currcnl obtained from storage batteries is that(here is a specific limit to the magnitude and duration of current which can be drawn fromthe ballery before recharging. Battery maintenance ic costly and can become a source oftrouble. Battery current can be replaced by the current obtained through dry plate rectifiersfrom a.c. power lines. This has the advantage of permitting an almost unlimited supply ofDC.Half wave rectified current is the most effective current to use for detection of surface andsub-surface defects using dry magnetic particles. It gives mobility to magnetic particles andaids in the formation of indications.Alternating current is also used for detection of surface cracks like fatigue cracks. A.C.inspection units should be equipped with proper current controls. An advantage of usingA.C. is that the parts being inspected with thi* current can be easily demagnetised.Some of the commonly used methods of magnetising the test specimens of differentconfigurations are given below:(a) Circular magnetisationElectric current passing through any straight conductor such as wire or bar creates acircular magnetic field around that conductor.DEFECTFigure 1.3 : Flux flow in a bar carrying current.For inspection of axial cracks in a solid or hollow part, the part can be magnetisedcircularly. For a solid part the current is passed through the test piece and a circular fieldis developed inside and around the piece. In the case of hollow or tube like objects, acentral conductor is used to carry the magnetising current. The central conductor isalways a copper rod. The conductor is placed inside the hollow piece and current isthen applied to it. This induces a circular magnetic field on the inside and outsidesurfaces of the hollow piece as shown in Figure 1.4.(b) Longitudinal magnetisationParts can be magnetised longitudinally using a permanent magnet ( Figure 1.5 ) or by usingan electromagnet , Figure 1.6.Paris can also be magnetised longitudinally by the application of electric current. Whenelectric current is passed through a coil of several turns, a magnetic field is established15

CYLINDRICALTEST OBJECTFLAWSTHREADING BARCURRENT FLOWFlux flow in cylindrical specimens using threading bar.Figure 1.4 : Flux flow in cylindrical specimensusing threading bar.TEST OBJECTJ-I(Flux (lowlinesJTest objectiJIni jiCracVC11YOKE11';;líífi,""s-/Mild steel blockto complete palh"-"-- SNFigure 1.5 : Use of permanent magnet.CONDUCTORFigure 1.6 : ElectromagnetHELICALCOILFLUX LINESIN TEST OBJECTDEFECTSFigure 1.7 : Helical Coil161if;

lengthwise or longitudinally within the coil. The nature, and direction of this field is theresult of the field around the conductor which forms due to the number of turns in thecurrent carrying coil. A crack at right angles or tangential to this field can be revealed.Longitudinal magnetisation may be achieved by surrounding the test specimen with helicalcoils and passing current through them Figure 1.7(c) Magnetisation of irregular partsParts of irregular shape sometimes have to be tested. They can be tested using the magneticparticle inspection method. Local magnetisation is created by applying prods to the areato be tested. The area is magnetised circularly and any defect in the path of the magneticlines of force can be indicated as shown in Figure 1.8.The inspection of irregular parts by this method is time consuming since a.magnetisingcurrent has to be applied many times to achieve thorough inspection of a component.However the testing time is not a large problem, due to the quic

Manual for the Syllabi Contained in IAEA-TECDOC-628, "Training Guidelines in . in the Agency executed UNDP project in Asia and the Pacific (RAS/86/073) showed that . The mention of specific companies or of their products or brand names does not imply any endorsement or recommendation on the part of the IAEA. CONTENTS 1. GENERAL KNOWLEDGE 9