WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods PDF

„_ „ Ĩ WSSStudy Guide WC2.1 Sư ng An inidlativeof the . Intermediate Quality Control and Inspection Methods $cgvoess“e«ss2eseo _¬ Checking to determine that weld profiles are satisfactory Checking for signs of visible porosity Determining ïf there is excessive spatter Ensuring that slag has...

„_ „ Ĩ WSSStudy Guide WC2.1 Sư ng An inidlativeof the . Intermediate Quality Control and Inspection Methods $cgvoess“e«ss2eseo _¬ Checking to determine that weld profiles are satisfactory Checking for signs of visible porosity Determining ïf there is excessive spatter Ensuring that slag has been removed Checking for any signs of cracking Checking for any weld craters Examining for signs of excessive distortion (see Figure 29) Angular distortion - _~ G29] I l I I I Excessive distorton Other nondestructive examination methods can be used to supplement visual inspection and, ïn fact, are often required by the standard or specification. For example, a visual inspector can witness the root pass and subsequent backgouging or can call for radiographic inspection to ~_ look for slag or incomplete fusion after the weld is completed. The tools that are used for the traditional visual inspection of weld ¬ = surface discontinuities are most importantly good lighting, magnifiers, rulers, micrometers and optical comparators. Visual inspection is very commonly used in production because ït is perceived to require relatively little training and equipment. However, ït is very dependent on the trained inspectors. To facilitate visual inspection there are a number of commercial gauges available. The gauges can measure undercut, excess groove weld height, fillet weld leg length, theoretical fillet weld throat — and the permissible fillet weld tolerance for convexitv and concavitv. Use of any other nondestructive test does not relieve the supervisor or the welder of the responsibility for regular and continual visual inspection as a means of ensuring good workmanship as the job progresses. WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods 3.2 Liquid Penetrant Inspection (LPI or PT) Liquid penetrant inspection (LPI), also called penetrant testing (PT), is a versatile inspection method capable of locating cracks, porosity, laps and folds etc. that are open to the surface. The method is based on the ability of the penetrating liquid to be drawn into a discontinuitv. VWVhen the excess liquid is wiped clean from the surface of the object, the remaining the discontinuity can be drawn out with the aid of a blotting agent a developer. The contrasting colours of the penetrant wïll show up the developer, providing an indication of the discontinuity open to surface. liquid in called against the The basic steps involved in performing liquid penetrant inspection are shown in Figure 30: 1. Prepare the surface of the part to be inspected by cleaning and 2. 3. degreasing. Apply the penetrant to the surface of the part. Allow a period of time for the penetrant to be drawn into any discontinuities. 4. 5. 6. Remove the excess penetrant using a technique that will prevent removal of remaining penetrant held within the discontinuity. Apply a developer to the surface, enabling the blotting of the penetrant trapped in the discontinuities, thereby providing an indication of the location. Examine and evaluate the indication visually under appropriate viewing conditions. Clean the part and, if necessary, apply a corrosive prevention. Liquid penetrant inspection is simple and inexpensive. Page 37 | Copyright © 2015 CWB Group Industry Services WSS Study Guide WC2.1 „— _ An inlriaive of the . cwbgroup' . Intermediate Quality Control and Inspection Methods 1) Clean and dry surface 2) Application of penetrant 3) Dwell time 4) Removal of excess penetrant 5) Development 6)inspection "—< ấu FiG.20| The principles of liquid penetrant inspection _ One attraction of penetrant testing is that complex equipment is usually not required. Although permanent inspection stations with tanks and spray facilities are used for some applications, the process lends ïtself to portable applications such as localized inspection and spot testing. — mMost penetrant testing in the welding industry is carried out with simple sm portable kits comprised of the penetrant, developer and cleaner in handy aerosol cans (see Figure 31). Accessories commonly supplied with the cans are spray guns, wiping cloths and a wire brush. Fluorescent testing requires a black light. WSS Study Guide WC2.1 lạ cwbgroup Intermediate Quality Control and Inspection Methods mm Portgble inspection kit of penetront, developer and cleaner The two basic types of penetrant are: $ Visible dye: the penetrant is coloured to provide a contrast and viewing in this method is carried out under white light $ Fluorescent dye: viewing in this method is carried out under ultraviolet or black light The penetrant testing methods are reasonably quick to apply and obtain test results but are not as fast as magnetic particle testing. Fluorescent penetrant testing is a very effective method for detecting leaks in welded joints in tanks and containers. Detection of leaks by fluorescent penetrant through welded joints in tanks and containers is very effective. The inspector applies the penetrant on one side of the weld and after a suitable time the penetrant travels any leak path and Ìs revealed by either applying a developer or scanning the opposite side under black light conditions. This method is particularly suited to revealing micro-cracks, such as fatigue cracks, in most materials. WSS Study Guide WC2.1 in Intermediate Quality Control and Inspection Methods 3.3 Magnetic Particle Testing (MPI or MT) Magnetic particle inspection is a quick, effective way of detecting surface and near surface discontinuities in ferromagnetic materials such as steel. A crack or other discontinuity that lies perpendicular to the magnetic field (flux) induced into the part creates an additional set of magnetic poles. A fine, coloured, iron powder is dusted onto the surface under examination and tends to collect at these poles. Magnetic particle inspection is one of the best ways to detect fine cracking that comes to the surface. n Itis limited in its ability to detect isolated indications such as porosity. The magnetic field can be induced into the part in a number of ways depending upon the form and fñinish of the part. ~ The most common way to induce the field in the testing of welds is by the AC/DC voke method using alternating current or direct current as shown in Figure 32(a). With indirect magnetization, either a permanent or electromagnet is placed directly on the part or the part is placed inside an electromagnet. - Figure 32(b) shows a part placed inside a coil. a) Yoke b) Goil shot An electromagnet is used to The part is placed inside the produce a longitudinal field coil and a longitudinal field between the poles. Permanent magnetic yokes may also be used. is induced. FiG.22. r3 Page 40 Copyright © 2015 CWB Group Industry Services Two methods oƒ inducing ø magnetic flux in a part to be inspected WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods When direct current (DC) is used it is possible to detect surface discontinuities, as well as linear discontinuities that are near surface but do not break the surface of the weld. Alternating current (AC) is normally used where discontinuities break the surface of the weld of the weld under test. The legs of the yoke are positioned in sụch a manner that the magnetic field intersects the discontinuity. For this reason the legs should be placed in two positions: to detect linear discontinuities parallel to the direction of of the weld and linear discontinuities transverse to the direction of weld (see Figures 33 and 34). Power Switch Surface Cracks Magnetic Field WSS Study Guide WC2.1 — An inidotive of the. . cwbgroup . Intermediate Quality Control and Inspection Methods Ạ HN Indications in this _ - ^ - direction are detected EIG.24) AC yoke used to detect linear discontinuities The equipment shown in Figure 35 illustrates the use of prods to induce a magnetic field in the component. The use of prods is prohibited by certain codes due to the chance of causing arc strikes. lt Ìs now more common to find high current portable DC equipment used for coil wrap applications where the object under test is placed inside the coil of a power cable. —. Magnetizing Current Prod Type - MPI Magnetizing Current Test Piece Magnetic Field rà IMagnetic particle inspection using prods WSS Study Guide WC2.1 Ga NA L- cwbgroup Intermediate Quality Control and Inspection Methods Because the test is a magnetic method ït is not possible to examine materials that are nonmagnetic, such as aluminum, brass, bronze and austenitic stainless steels. The magnetic particles used as the detection medium are fine, iron powders of various colours (to provide a contrast against a given background). Another type of detecting media is a solution of iron powder and a suspension medium (fluid). There are two types: $ visible - visible suspensions are for use under regular lighting conditions. $ fluorescent - iron particles are coated with fluorescent material. Normal background lighting must be reduced to a level allowing the use of ultraviolet light (black light) to create the necessary intensity. The dry powder method is very flexible and more sensitive to subsurface discontinuities when used with direct current. The wet method is better at locating surface cracks and the wet fluorescent method is the most sensitive of the three methods since it can locate smaller discontinuities. 3.4 Radiographic Inspection (RT) Radiography is the most commonly used nondestructive method to detect subsurface volumetric discontinuities in welds. The radiographic method can be applied to most welded Joints but is most commonly applied to butt joints. As with all testing methods, radiographv has certain limitations. It is a method that simultaneously measures differences in the thickness and density of the material under examination. There must be enough difference to show up on the film image. There are two methods of radiography used in the inspection of welds: $ X-radiography $ Page 43 Cep | 15 CWB Group Industry Services Gamma radiography WSS Study Guide WC2.1 Ga G' 2 X-rays and gamma An n inidarlv initiative of the ° rays both have extremely short wavelengths and this enables them to penetrate solid objects that would block ordinary light. The two types of radiation affect photographic film in the same manner. lonizing radiation is dangerous and cannot be felt, seen, smelled or tasted. Whenever radiography is performed a safe zone is established and warning signs posted (see Figure 36). These barriers must never be crossed unless the radiographer in charge directs vou to do so. riG.36| High radiation area sign As x-rays or gamma rays are directed at the weld a certain amount will be absorbed by the structure of the metal and the remainder will pass through onto a film that is placed into position. The amount of radiation absorption depends upon the material type and thickness. Each material (steel, aluminum, copper, etc.) has a different density. When a weld has internal discontinuities such as slag or gas holes, more radiation wÏill reach the ñlm under these areas than in adjacent areas, which have no discontinuities and are therefore able to block more radiation. In this manner different amounts of radiation reach the ñlm. These differences in the amount of radiation passing through the weld appear on the developed film as light or dark shadows (see Figure 37). These differences are called changes ïn film density. The radiographic ñIlm is interpreted by evaluating the shape, density and location of the images created by voids and inclusions. ni WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods ¡. << „+ Anininariw nhu cwbgroup Radiator Source Radiation beam directed ›' down on weldment Void ¬+——Film nêm Principle of rgdiography The image of a discontinuity is affected by the radiographic technique. The geometric set up of the source in relation to the object being examined and the film affects the image created on the film. A penetrameter or image quality indicator (IQI) is an object of khown shape, size and geometric features used to assess the quality of the radiographic technique and the quality of the image. There are different types of penetrameters; some have wires of different diameters, others have holes or slots of specific dimensions machined into them (see Figure 38). The smaller the hole or wire visible in the radiograph, the better the resolution or ability to see discontinuities in the radiograph. The thickness of the hole-type penetrameter shall not be more than 2% of the thickness of the plate. In each penetrameter there are three holes of diameter equal to one, two and four times the penetrameter thickness, but in no case less than 0.010". WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods T Diameter 2T Diameter 4T Diameter Identifrcation number FiG.38. Hole- and wire-type penetrameters The radiographic inspection method is somewhat limited in ïts sensitivity to discontinuities that are perpendicular to the direction of the radiation beam. Discontinuities are revealed by differences in the darkness of the film. A tight crack or lamination may not cause a sufficient difference in darkness to be visible to the naked eye and will therefore go undetected. 3.5 Ultrasonic Inspection (UT) Ultrasonic inspection (UT) is used for detecting subsurface discontinuities and defects, such as tight cracks and ïs used in conjunction with other nondestructive testing methods. It may be used with other methods such as radiography when the depth of a discontinuity needs to be determined. The method uses transducers that are able to transform electrical energy into mechanical energy and vice versa. When a voltage is applied across a crystal, it can be made to vibrate and generate ultrasonic waves. Conversely, a mechanical vibration applied to a crystal will generate a small electrical charge. Thus the crystal can be used to both generate and detect ultrasonic waves. Vibrations are transmitted into the testpiece through a coupling fluid, usually a film of oil, called a couplant. In the pulse echo technique as shown in Figure 39, when the ultrasonic wave strikes a discontinuity in the testpiece it is reflected back to the origin and the transducer now acts as a receiver of the reflected energy. The signal is displayed on a screen or display. Page 46 Copyright © 2015 CWB Group Industry Services WSS Study ] Guide WC2.1 hẾG ; cwbgroup Intermediate Quality Control and Inspection Methods Transmissionpulse Flaw indication (a) Oscilloscope Crystal housing Crystal Indication of steel-to-air Iinterface (b) Electrical pulse out Electrical pulse return (a) Flaw In the steel (b) Steel-to-air (acoustic mismatch) Sound reflecis to crystal EiG.39i Page 47 Copyright © 2015 C! Principles oƒ ultrusonic inspection (acoustic mismatch) Sound reflects to crystal WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods Figure 40 shows a typical display presented on an oscilloscope. Signal strength is indicated by the height of the vertical deflection on the screen and the transmitted time is indicated by the horizontal deflection. By measuring the height of the vertical deflection, the size of the discontinuity can be determined. The depth of the discontinuitv from the surface can be determined by the horizontal distance. The echo from the discontinuity is between the front and rear deflections. Inilal pulse Volts Back surface reflection TỒN Flaw reflection Units of Time FG.40) Typical ultrasonic display A skilled operator is the key to successful ultrasonic inspection of welds. The operator must evaluate the presentation on the screen and interpret the signals based on a knowledge of the geometry of the connection and possible discontinuities. @ Page 48 Copyright © 2015 CWB Group Industry Services WSS Study Guide WC2.1 Intermediate Quality Control and Inspection Methods 3.6 Summary — Inspection Methods When nondestructive examination is specified, it should be integrated with visual inspection in the plan and not used ïn isolation. Good visual inspection is often needed to complement nondestructive examination and to assist in the interpretation of results. Various inspection methods may be specified, and each has its advantages, disadvantages and applications as shown in Table 2. Inspection Method s Simple, inexpensive ø Suitable for ferrous and nonferrous metals Liquid penetrant, PT ø Rapid results ø No electrical pDower required ® Simple, Inexpensive « Limited depth capability s Rapid results Magnetic particle, MT e Volumetric method e Provides permanent record e Large areas (e.g. complete pipe weld) can often be inspected Radiography. RT with one shot s Gamma ray requires no electrical power, portable s Volumetric method s lImmediate results s Portable s Suitable for a wide range of Ultrasonics, UT TABLE2) Page49 | Copyright © 2015 CWB Groip Industry S thicknesses s Sensitive Disadvantages Typical Applications ø Only detects discontinuities and defects open to the surface s Defecis easily hidden s Quick spot checks on welds e Detection of surface defects such 8sfalique cracks s Cannot be used on hot or cold parts surface or near surface discontinuities e Detection of surface defects such as cracks in steel welds ø Only suitable for ferromagnetic e Useful for testing partially completed materials s Electrical power required welds in thick s Potential safety hazard s Inspection of load carrying butt Joints to e Disrupts other work e Results not innrmediate verify general weld soundness through thickness s Only detects sections ø Some defects may be missed depending on beam angle ø Expensive, bulky equipment needed for X-ray ø Highly skilled operators required e Slow if applied manually to long weld lengths ø Skilled operators required s Thin material and region close to surface difficult to inspect e Detailed inspection of critical welds e Detailed measurement of defects s Very thick sections not suitable for radiography Advantages, disudvantgges and applications of NDT methods