Module 10


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Module 10

  1. 1. Module 10 Visual inspection and Other NDE methods and Symbols. VISUAL INSPECTION (VT) Inspection check list shall help CWI organize the inspection effort and assure each specific task has been performed. CWI must review all documents relating to actual welding before starting the works. Some of the documents may provide ground rules for welding and inspection. Hold points are simply pre selected steps in fabrication/welding sequence when work must stop until CWI has a chance to review the work completed to that point. CWI may prepare a plan for performing inspection and recording and maintaining the result. The basis of inspection for welding is always WPS as it provides all conditions governing welding and inspection. If during any of the weld pass it is noticed that weld exhibits extensive convex weld build up it is essential to grind and bring the weld face to acceptable level. The excessive convex face can create a deep notch at toe and it can cause in complete or improper fusion during subsequent pass. The root pass / Root layer represents the most difficult welding condition especially in the case of an open root configuration. Inter pass cleaning must be thoroughly done, if not done properly it can lead to slag inclusion or incomplete fusion. If a weld requires a harsh treatment to remove the slag, it indicates that there is some problem with the welding process or technique. Inter pass temperature also must be monitored regularly. It also should be measured on best metal surface near the weld zone and not on the weld joint itself. While measuring a fillet weld, the weld size is determined by the size of largest isosceles triangle which can be totally contained within the weld’s cross section. 182/184—For a convex profile the leg and size dimensions are the same however a fillet weld exhibiting a concave profile will be sized based on throat dimensions. If the face is not very clearly showing concave or convex profile, the CWI shall check both leg and throat dimensions using both types of template to determine if weld size is sufficient. Shorter of the two legs size shall be governing size. If weld is convex, proper gage shall measure the leg dimensions. For concave fillet weld profile, proper gage shall be measuring throat dimension. Regardless of the shape of the template the size indicated will be related to the required size of the theoretical triangle which is inscribed in the existing fillet weld cross section. For each intermittent fillet weld dimensions, pitch and length shall be measured. For continuous groove or fillet welds, they are considered to be sufficiently long if they are filled to their required cross section for entire length of the shorter of the two members being joined. Visual inspection is limited to the discovery of surface irregularities. Hence it must be done at all stages of fabrication sequence to provide adequate coverage. 173/184 NON DESTRUCTIVE TESTING. Non destructive evaluation –Visual inspection is limited to discovery of surface irregularities. Hence it must be done at all phases of fabrication sequence for proper coverage of inspection. Essential elements of NDT. 1. Source of probing energy or medium. 2. Discontinuity must cause change or alteration of probing energy. 3. A means of detecting change. 4. A means of indicating change. 5. A means of observing or recording this indication for making interpretation. PENETRANT TESTING. (PT) Penetrant is applied to the duly cleaned test surface. Then the Penetrant is allowed to remain there for some time which is dwell time. If there
  2. 2. is any opening the Penetrant will be drawn into the opening by capillary action. Developer is subsequently applied which will draw remaining Penetrant from the discontinuity. As the resultant indication are shown in high contrast and magnify the presence of discontinuity potential indications are visible and fluorescent the visible dye (Red color ) is viewed under white light against a light background ,when observed under ultra violet (Black) light. Fluorescent Penetrant can be more sensitive test. 175/184--Penetrant removal- Water washable, Solvent removable or post emulsifiable. Water washable Penetrant contains an emulsifier to rinse off oily penetrant with low pressure water spray. Solvent removable Penetrant needs solvent to remove it from the surface. Post emulsifiable penetrants are removed by adding an emulsifier after dwell time. Penetrant can be applied by spraying or brushing. For small parts it can be done by dipping . Developer application is done either by a dry powder or powder suspended in a volatile liquid which readily evaporates. Leaving the powder or as a spray. A good technique is to apply the developer in several very thin layers. A thick layer of developer can mask very small indications. Large particle size and thick layer of developer will tend to decrease the sensitivity of the penetrant test. The PT can be used for checking even non porous materials (Non metal also) This can be used for checking joint between dissimilar metals and non ferrous metal also. LIMITATIONS It cannot detect sub surface discontinuity. It is a time consuming process. Due to rough surface appearance the result is always difficult task for the purpose of interpretation. 175/184—MAGNETIC PARTICLE TEST – It is primarily used to discover surface discontinuities in ferro magnetic materials. Sub surface discontinuities very near the surface can be detected but it is very difficult to interpret. Magnetic flux lines have their intensity reduced significantly as the length of air gap is increased i. e. where there is a crack the intensity is reduced. At the discontinuity or crack in steel bar, in the vicinity of the discontinuity, there are magnetic poles of opposite sign created on either side of the air gap present at the discontinuity. These poles of opposite signs have strong attractive forces between them and if the area is sprinkled with iron particles those iron particles will be attracted and held in place at the discontinuity. Hence in a test piece if it is magnetized and iron particles are sprinkled on the surface if discontinuities are present these particles will be attracted and held in place to provide a visual indication.. The magnetic particle testing is done with electro magnetic equipment. When magnetic field is oriented along the axis of the part, it is called LONGITUDINAL MAGNETISM, when the direction of magnetic field is perpendicular to the axis of the part it is called circular magnetism. When using a coiled electric conductor surrounding it to the part (This shall be called a coil shot) the magnetic field created is called longitudinal. With this magnetic field flaws lying perpendicular to the lines of force will be easily revealed. Those lying at 45 degree to the magnetic field also will be shown but flaws parallel to the induced magnetic field will not be revealed. Other types of the magnetic field are circular magnetism. Here the part to be tested becomes the electric conductor and the induced magnetic field tends to surround the part perpendicular to its axis. On a stationary testing machine this is called a head shot. In a circular magnetism longitudinal flaws will be revealed while those lying
  3. 3. transverse will not. In circular magnetic field magnetism is totally contained within ferro magnetic material, where as longitudinal magnetic field is induced in the part by electric conductor which surrounds it. In view of this circular magnetic field is more powerful, making circular magnetism more sensitive for a specified current. Both types of magnetic field can also be generated in a part using portable equipment. A longitudinal field results when yoke method is used. To produce a circular magnetic field the prod technique is used. AC & DC both can be used for introducing a magnetic field. Magnetic field created by AC is strongest at the surface of the test object. AC provides greater particle mobility on the surface of the part allowing particles to move about more freely which aids flaw detection even when the surface of the part may be rough or irregular. DC induces magnetic field having greater penetrating power and can be used to detect near surface discontinuities. Another type of electric current is referred to as half wave rectified AC and can be thought of combination AC and DC. Since this testing is most sensitive to discontinuities parallel to the lines of flux might not be detected. If the acute angle formed between the lines of flux and long axis of discontinuity is greater than 450 the discontinuity will form an indication, at angles less than 45 discontinuity might not be detected . Hence for better evaluation and to locate flaw lying in all direction, it is essential to apply magnetic field in two directions 90 apart. This procedure is not suitable for Austenitic SS Aluminum, Cu etc. Light weight AC yoke units are extremely portable and useful for inspection of objects too large to test otherwise. Less portable equipment include mechanisms for both head and coil shots. The stationary units include Demagnetization mechanisms. The iron particles used are very small and colored The particles are applied as dry powder with low velocity air stream or are flowed over the part as a suspension in inhibited water or light oil. Dry method is called DRY MAGNETIC PARTICLE TESTING & Oil or water use method is called wet magnetic particle testing. Wet fluorescent method has higher sensitivity. The advantage of MT is its low cost and fast speed and it is very good for surface discontinuities. The testing can be done through fine paint coating also. LIMITATIONS—It can be used for magnetic materials only. Most parts have to be demagnetized after testing. Very thick coating can mask detrimental indications. Demagnetization is done by AC either removing the part from magnetizing field slowly or reducing the induced magnetization current applied to the part to zero. Rough surface of casting or welding can make evaluation more difficult. Results of MT may be recorded by SKETCHING, PHOTOGRAPHING or by placing adhesive tape over the indication and then transferring the tape to a clear piece of white paper. 177/184 RADIOGRAPHIC TESTING (RT)—Areas of reduced thickness or lower density transmit more and therefore absorb less radiation. Areas of high radiation transmission, or low absorption appear as dark areas on the developed film. Areas of low radiation transmission, or high absorption appear as light areas on the developed film. The thinnest area of the object produces the darkest area on the film as more radiation is transmitted to the film . The thickest area of the object produces the lightest area on the film as more radiation is absorbed and less is transmitted. Metal with high density absorbs more and transmits less and hence light film. Lower energy, non particulate radiation, is in the form of Gamma radiation or X ray. Gamma rays are the result of the
  4. 4. decay of radioactive materials. Common radioactive sources include IRIDIUM- 192, CESIUM-137, and COBALT- 60. These sources are constantly emitting radiation and must be kept in a shielded storage container, referred as a GAMMA CAMERA, when not in use. These containers are made of lead and steel shielding. X- rays are man made, they are produced, when electrons traveling at high speed, collide with matter. The conversion of electrical energy to X- radiation is achieved in an evacuated (Vacuum) tube. The action of electron stream striking the target produces X rays. The radiation produced only when voltage is applied to the X-ray tube. Whether using Gamma rays or X- rays source, the test object is not radioactive following the test. Sub surface discontinuities which are readily detected by this RT method are those having different densities than the material being radiated. Voids such as porosity produce dark area of the film as they represent significant lose of material density. Material inclusions produce light areas on the film if the density is greater than that of base metal or test object. If inclusion density is less than metal it shows as a dark area on the film. However some electrode coating produces slag having a density very similar to the deposited weld metal and slag produced from them is found very difficult to find and interpret. The equipment for RT – 1. Source of radiation (X- ray machine or radioactive isotope) which produces Gamma radiation, Film, Film holder, Lead letters, Image quality indicator (IQI) , OR Penetrameter (Pennys) are used to verify resolution sensitivity. IQIs are hole type or wire type. Both these are specified as material type hole type shall have specific thickness and hole size where as wire type shall have wire diameter. Sensitivity is verified by the ability to detect a given difference in density due to penetrameter thickness and hole size or wire diameter. Hole penetrameter vary in thickness and hole diameter depending upon metal thickness being radiographed. #25 IQI indicates penetrameter thickness as 0.025”. The hole diameter and position are specified and are noted in terms of multipliers of individual IQI thickness. The largest hole in # 25 IQI is 0.100” and is called 4T ie 4 times IQI. These holes are used to verify film resolution sensitivity which is usually specified to be 2% of weld thickness. 1% sensitivity is difficult to attain. Radiation monitoring equipment and film processing equipment are also required. LIMITATIONS -- Health hazard. People need extensive training. It is expensive. Long duration training required. Access to both sides of object is essential. 179/184 ULTRASONIC TESTING. (UT)—This method of testing uses high frequency of sound wave well above the range of human hearing to measure the geometric and physical properties in material. Sound wave travel at different speed in different materials. Longitudinal sound wave travel about 1100FPS in air, 19000 feet per second in steel and 20,000 FPS in Aluminum. UT uses electric energy in the form of an applied voltage. This voltage is converted by a transducer to mechanical energy in the form of an applied voltage. This voltage is converted by a transducer to mechanical energy in the form of a sound wave. This conversion is referred as PIEZO ELECTRIC EFFECT. Quartz and Barium Titanate are examples of piezo electric materials of each type. Piezo electric material will produce mechanical change in dimension when excited with electric pulse. Similarly same material will also produce an electric pulse when acted upon mechanically. When Piezo electric crystal is squeezed and released suddenly it generates
  5. 5. spark which jumps across a gap to ignite gas. The UT machine transducer is given power supply following up a prescribed start up sequence and calibration procedure the base unit acts as an electric measuring devise. This machine will generate precise electronic pulse- transmitted through a co axial cable to the transducer placed in acoustic contact with test object. These pulses are very short duration and high frequency sound has ability to be directed precisely like light beams. When excited by electric pulses the transducer responds with mechanical vibration and creates a sound wave that is transmitted through the test object. Types of transducers.—1. Longitudinal waves or straight beam transducers- Used for measuring thickness of material or depth of discontinuity. 2. Shear wave or Angle beam transducers- used extensively for weld evaluation. The sound wave is sent into the part at an angle to allow the testing to be done without the need for removal of rough weld reinforcement removal. Types of testing. 1. Contact type and 2. Immersion type. In contact type the transducer is placed on the surface of the material. A liquid is applied between test object and transducer for improved contact. The liquid used is called Couplant.. In immerse type the item to be tested is immersed in water and sound wave transmitted from transducer and into the part through water. Contact testing is portable and immersion type is useful for production testing of small or irregular shapes. UT is used for surface and internal defect checking. This is most sensitive to plannar discontinuity especially those which are oriented perpendicular to sound beam. Lamination, crack, incomplete fusion, inclusion, and voids etc can be detected by UT. Thickness measurement can also be done by this. The UT equipment consists of electronics Instrument with either a CRT or digital display. Using a CRT the operator can find out the location, size and type of discontinuities. Inst. With digital display is more useful for thickness measurement. Couplants are grease, Oil, Glycerin, Water and Cellulose powder or corn starch mixed with water. Many transducers are mounted on plexi glass wedges which allow the sound to enter the test object at various angles for shear wave test. For flaw detection the calibration standard should contain a flaw such as side drilled hole, flat bottomed hole, or a groove. The flaw location must be accurately known. For angle beam testing used in weld testing IIW block is used which provides for beam exit and shear wave verification. If the standard calibration piece of same material is not available it can be done with other material with the info about difference in sound velocity. These substituted materials with correction curve can be used. The main advantage of UT is that it is considered to be truly a volumetric test. It provides length, lateral location and depth of flaw. UT can give info with only one side of material face available for testing. It can best detect critical planner discontinuity such as Crack, incomplete fusion. UT has deep penetration ability up to 200 “ in steel. It needs highly trend and experienced operator. Test object surface must be clean and fairly smooth. Reference standard must be available for actual location of flaw. 181/184 EDDY CURRENT TESTING (ET) If the coil carrying AC power near a metal specimen, eddy currents are induced into the metal by electro magnetic induction. The test is affected by the magnitude and direction of these induced eddy currents. When the test coil is calibrated to known standards the eddy current method can be used for various flaw detection and other measurements.
  6. 6. ET can be used to measure the thickness of thin section, electric conductivity, magnetic permeability, hardness and heat treatment conditions of objects to be tested. Even this can be used to measure thickness of non conductive coating on elec. Conductive test objects. This method can be used to detecting cracks, seams, laps, voids and inclusions near the test object surface. The equipment required is an electronic instrument with either a CRT or meter display and a coil probe consisting of one or more turns. The test coil can be a probe type or evaluating a surface, a cylindrical coil which surrounds a circular or tubular part or an inside dia. coil which is passed inside a tube or a hole. The advantage of this method is that it can be readily automated. The probe need not touch the object, no couplant required and process is expedient. As probe need not touch the object, even hot objects can be tested. It can be used for the inspection of any electrically conductive material, magnetic as well as non magnetic. It needs highly skilled operator requirement to calibrate the equipment and interpret the results. Its maxm. penetration is shallow 3/16 inch or less. Surface dirt or contamination which is elec. Conductive can affect test results. ET of magnetic material may need special probe and technique. Information like corrosion, cracking, pitting, etc from inside a tube can be obtained easily. 182/184-- NDE SYMBOLS – Type of test Acoustic Emission- AET, Eddy current—ET, Leak test—LT, Magnetic Particle – MT, Neutron Radiograph—NRT, Penetrant—PT, Proof test—PRT, Radiographic—RT, Ultrasonic—UT, Visual—VT. If it is not significant which side is to be tested, the test symbol can be centered on the reference line. A no. to the right of the test symbol refers to the length of weld to be tested. If no dimension to the right of the test symbol, it implies that the entire length of the joint is to be tested. End.