Welding-inspectionfor soundness and efficiency:SOLUTIONS with Effective, Powerful AdviceWelding-inspection, visual inspection, radiographic inspection, magnetic particles inspection, penetrant inspection, ultrasonic inspection, leak testing, nondestructive inspections, useful links, inspection tips, how to interpret results, joining questions needing answers: these are some of the items developed in this Site page for the benefit of interested readers. (Sponsored Links) What is in here for me? In a previous page on destructive TESTING we discussed the importance of assessing the actual properties of real welds realized on test specimens. Here we are going to present to some extent the methods of nondestructive Welding-inspection that are specified to determine the soundness, quality and efficiency of welded structures. Welding-inspection is generally understood as non destructive. It is meant to expose superficial or internal discontinuities which cannot be found by regular visual inspection. Once exposed, they must be evaluated according to criteria based on the importance of the application. On the difference between defects and discontinuities see the page on WELDING DEFECTS. Important Announcementa rich collection of valuable information from expert Internet Sources, on Materials, Volume 1, and Metals Welding, Volume 2, is now available. See our New Page on Metals Knowledge Indications?... It should be understood that the Welding-inspection methods presented in the following are designed to find indications which have to be interpreted. Some indications may be irrelevant, and shall be ignored by the inspector. Other may point to discontinuities which have to be evaluated with reference to the imposed limits for acceptance. Only then they are considered acceptable or rejectable. What is the need for nondestructive Welding-inspection? It is part of engineering requirements and it is simple or complex, lenient or severe, inexpensive or costly, according to the requirements of the application, that is of service conditions, and to the projected consequences of a possible failure. An Article on Visual-inspection was published in the section on Contributions in the April 2004 issue of Practical Welding Letter. To read the Article click on PWL#008. An Article on Risk Assessment was published In Section 7 in Issue 29 of Practical Welding Letter for January 2006. To read the article click on PWL#029. An Article on Fatigue Monitoring was published in section 11 of the issue 30 of Practical Welding Letter. To see the article click on PWL#030. An Article on Risk Based Inspection was published in section 2 of the issue 57 of Practical Welding Letter for May 2008. To see the article click on PWL#057 An Article on Remaining Life Assessment was published (7) in Issue 56 of Practical Welding Letter for April 2008. Click on PWL#056 to read it. An Article on Acoustic Emission Inspection of Weldments was published (2) in Issue 62 of Practical Welding Letter for October 2008. An Article on Sinter Brazing and Resonant Acoustic Inspection was published (2) in Issue 75 of Practical Welding Letter for November 2009. To receive the Letter as it is published, please click on Subscription. A new page was added on Visual Weld Inspection. Click to see it. Inspectors... Who should perform Welding-inspection? Ideally the welder. At least visual inspection, usually with proper auxiliary lighting and some means of low power magnification, should be performed by the welder immediately after ending his/her job, even if a formal visual inspection is later assigned to an appointed inspector. Other non destructive Welding-inspection methods require special theoretical studies, formal training, serious examinations and accumulation of experience. Even if it not realistic to require expertise in all these methods from the welder's part, nevertheless learning to communicate with the inspectors on their findings will help his/her understanding of the problems and will clarify the need, if any, to improve the performance. Internal or hired inspection? If Welding-inspection could be performed by internal personnel, that would be the best solution, for shortening of communication lines, for speed of reaction. But is that possible? Can a welding shop budget in advance the cost of instructing workers, having them certified and procuring all the necessary equipment and accessories? Depending on purchase order requirements, Welding-inspection may be performed and the parts either approved or sent back for repair, (or rejected) either in house or by commercial laboratories acceptable to the purchaser and qualified by an external official agency to do inspection and evaluation work. Service van?... Transportation of heavy welded pieces to and from a Welding-inspection facility can be costly. What about hiring an external service at the welding place? They can have a service van available with all they need. One must insist in having a proof of inspectors certification by a known agency, and in obtaining signed test reports acceptable by the purchaser. Inspectors should be endowed with integrity, knowledge and experience, with common sense, and should not depend in the line of authority from the production manager in order to be independent in their judgement. In the following we shall introduce the interested reader to the principles of operation of the major Non Destructive Testing (NDT) methods used for Welding-inspection, as a matter of orientation. X-Rays... RADIOGRAPHIC Welding-inspection is performed by pointing a radiation source (an x-ray tube or a radioactive isotope) to the portion of the weldment to be inspected and by exposing for a certain time a radiographic film to the radiation on the underside (or the other side away from the source). An exposed and developed film contains information on the internal features of the weld, much as a medical radiograph is used to check the human body. In practice a radiographer should first develop by trial and error a Welding-inspection technique which takes into account the particular requirements of the job at hand. All the relevant parameters are then recorded on a Technique Sheet, which should be approved by a knowledgeable authority, who agrees that the technique employed (which includes "quality indicators" called penetrameters) is suitable to expose the discontinuities looked for. Due to the harmful nature of ionizing radiation (like x-rays or gamma rays) it is most important that the manipulation of ionizing sources be entrusted to trained and knowledgeable inspectors that will not endanger themselves or passers by. In modern practice there is an increasing tendency to substitute the sensitive element (film) with electronic sensors capable of collecting and of displaying x-ray images on computer screens. The next step on which researches are working is to develop acceptable methods of digital image processing, automatic analysing for feature extraction of indications, and defect evaluation through tools of pattern recognition. Interpretation... The radiographic Welding-inspection interpretation of the subtle cues appearing in the radiograph, requires understanding the basics of the process, learning the variables and their influence, knowing how to generate a meaningful film exposure and what are the possible expression on the film of internal features present in the weldment. In general the film registers local differences of absorption of radiation by the material, as a function of density and of thickness. So, very thin discontinuities parallel to the surface may go undetected. The normal radiograph is a flat reproduction of the sums of all the features present along the thickness of the welded structure, without indication of the position along the line of radiation: special (tomographic) techniques must be put in place if it is important to know where in the depth the defect is. General information and specific recommended practices can be found in:
Acceptance... Acceptance criteria for Welding-inspection are established by the designer with reference to standard images, called Reference Radiographs, identified in accepted Specifications by letters and numbers according to base material classes and to thicknesses and issued by ASTM and by other organizations.
The appearance of a radiograph has to be compared visually by the inspector with the limiting Reference Radiograph established by the designer in the instructions for Welding-inspection. If it appears equal to or better than the reference radiograph then it is accepted, otherwise it is rejected. In this case somebody with knowledge and experience has to decide if the weldment can be repaired and how. In the end a new radiograph has to be taken. NEW RESOURCES
Looking for more Online Reference Links? Hear here... ULTRASONIC Welding-inspection is based on the fact that acoustic waves of a frequency not heard by the human ear can propagate in materials, and be reflected in various ways by internal interfaces and by distant surfaces. The acoustic waves are generated by piezoelectric transducers which transform electrical vibrations into mechanical vibrations and vice-versa. By examining the traces of the signals of those reflections, in modern instruments on a computer screen, and by making reference to standard reflectors (normally flat bottom holes carefully machined on specimens of the same material) of given shape and size, the experienced inspector can conclude that if an echo is present where it should not be and if its trace is larger than that of comparison, then there is an indication of discontinuity that must be evaluated. Depending on the outcome of the evaluation, which may make use of additional techniques, the result of the inspection, either to accept or to reject, is decided. It should be emphasized that ultrasonic testing is capable of detecting thin interfaces normal to the line of propagation of the wave (that X-Rays cannot detect) so that both testing methods complement each other. Reference blocks... Accepted Reference Standard Blocks specially designed to be used for Welding-inspection by Ultrasonic Testing of weldments were established by the International Institute of Welding (IIW). They are called IIW Type 1 and IIW Type 2 and can be found in:
However specialized probes and setups can be developed for unique items, where the decision is reached using a "go/no-go" approach. This kind of instruments makes no assumptions on the operator, who need not have any previous experience in ultrasonics. Such instruments which are individually adapted and not universal, free the operator from all responsibility, once the setup and calibration procedure are established and implemented as directed by the manufacturer. The rejects have to be evaluated again later by a more knowledgeable and experienced inspector. Note: we personally know and recommend a specific small firm of ultrasonic instruments, specialized in particular solutions of difficult to reach inspection areas, and with a record of outstanding achievements for aerospace applications.If you need a special solution send us a sketch and let us know by e-mail. This firm will contact you and present you a solution to your problem. Click on the Contact Us Button at top left to send us your questions. An Article on Automated Ultrasonic Testing was published in the Issue 23 of Practical Welding Letter for July 2005. Click on PWL#023 to read it. An Article on UT TOFD (Ultrasonic Testing Time-of-Flight Diffraction), was published in section 7 of Issue 35 of Practical Welding Letter for July 2006. To read the article click on PWL#035. An Article on Phased Arrays of Ultrasonic Transducers was published in issue 45 of Practical Welding Letter for May 2007. For reading the article click on PWL#045. MPI?... MAGNETIC PARTICLES Welding-inspection is a most useful method applicable to magnetic materials only. It is based on the fact that any discontinuity present in the metal will generate a disturbance, like an interruption or distortion, in a magnetic field flux. Such a disturbance can attract and display a pattern of finely divided magnetic particles that will assemble on those distorted flux lines. The particles are made visible by some appropriate contrasting technique. The inspector has to interpret the indications formed by the particles and visually detected on the surface in order to sort out unacceptable defects, which may be present even at a certain depth. Various pieces of Welding-inspection equipment can generate and concentrate adequate magnetic fields of the proper direction and intensity on the welded part, to permit the detection of the defects sought for. The inspector knows that if the magnetic field is at ninety degrees from the direction of the discontinuity, then the chances of finding it are improved. The magnetic particles, either in a dry powder form or in a liquid suspension are applied on the part after magnetization. The surface is then scanned visually by the inspector to find suspicious concentrations of particles. Standard and Recommendations Documents are:
Penetrant... PENETRANT Welding-inspection is based on the fact that specially formulated liquids have the property of penetrating in tiny cracks or discontinuities open to the surface. When the part is further washed of all excessive liquid on its surface and when a type of absorbing material (called developer) is spread on it, the remaining liquid, including a visible dye, absorbed in the cracks seeps outside and spreads on the surface of the developer making itself visible to inspection. This type of inspection is used for all non magnetic materials like aluminum, magnesium, titanium, copper etc. and is most apt to detect the finest cracks. The penetrant liquid may contain instead a suspension of fluorescent particles which are visible under ultra-violet light (black light) for improved sensitivity: this method is then called Fluorescent Penetrant Inspection (FPI). Reference Documents are:
An Article on Validation was published (2) in Issue 54 of Practical Welding Letter for February 2008. It deals with new requirements intended to assure that all the conditions for serious and complete Nondestructive Testing are applied for equipment, methods, procedures and inspectors. To receive the Letter as it is published, please Subscribe. For fluid containers... LEAK TEST for Welding-inspection is done on all containers built to hold a liquid or a gas: many forms of tests are possible. One popular version consists in generating low air pressure within the container, and looking for bubbles when the container is immersed in water. This is similar to the normal leak test of tire inner tubes. NEW RESOURCES
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