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Weld-Macro:Understanding and LearningSOLUTIONS with Effective, Powerful AdviceWeld Macrographic Examination Weld-Macro as explained hereafter is a powerful tool for assessing weld quality and improving one's welding skills. Weld-Macro, or Weld Macrographic Examination, is a simplified metallographic technique readily available to anybody, that can help even the simplest welding shop to see the quality as it is produced, and to maintain or improve it as an essential tool to gain and keep satisfied customers. In our page on Welding Testing, two short paragraphs introduce the subject of Metallography, including Weld-Macro, and explain that if the services available from a metallurgical laboratory are out of reach for the welding shop, an important source of information contributing to quality improvement is simply neglected, unfortunately. Seeing inside your welds... If you operate a welding shop and do not test your welds, you may be losing important information. If you think that examining test pieces must be hard and difficult, you should consider again your position. We propose hereafter a simplified Weld-Macro procedure that may be just what you need...
Why Examine?... Management (yourself?) may decide that hiring external help for Weld-Macro testing is not a productive expense, and as such it is not really needed. They may consider unjustified the expense required to set up and operate a laboratory in house: our opinion is that this could be a costly mistake. It is so because all welding activities that could gain and profit from the assistance and help of a metallographic laboratory, including Weld-Macro and Micro sections would be deprived of a most useful tool. Such a laboratory should be equipped with all that is needed to examine properly prepared specimens under the microscope. Those running it should be adequately prepared to correctly interpret the different features related to welding, helping the welder to gain insight. Note - If you make arrangements with a metallurgical laboratory, you may be able to get in a short time your Weld-Macro photographs sent to you by e-mail on the Internet, with comments on the phone... Even a simple Weld-Macro examination may reveal features readily interpreted to permit improving welding performance. We recommend to establish in every shop a limited capability for Weld Macro examination of welded joint sections, to be developed later, if needed, into a fully functioning laboratory. For welding shop operations mainly concerned with steel, we propose to establish a priority to build a mini-lab., whose capabilities should at minimum permit to examine the following features in Weld-Macro cross sections of test pieces:
An Article on Macro Testing was published in Section 7 in the Issue No. 37 of Practical Welding Letter for September 2006. To see the article click on PWL#037. To read all Issues of PWL as they are published, please Subscribe and you will also get a bonus e-book on Practical Hardness Testing Made Simple.
What to do with the results? The information achieved from these Weld-Macro examinations, although partial and incomplete, helps nonetheless in correcting welding procedures with the purpose to improve results and obtain satisfactory weld performance. It is also a demonstration of commitment for quality, a powerful argument in any discussion with customers. Therefore if assuring quality is a recognized element in the welding shop operation, then one should evaluate the advantages achievable vs. the effort required. It should be noted that a single Weld-Macro section is not sufficient. For a test piece length of 150 mm (6"), at least four sections should be performed, preferably in the locations where a visual imperfection is visible externally on the weld bead. For a tube welded joint, at least four sections are required, at 90 degrees apart. Repeated Weld Macro examinations give also a measure of the consistency of the process performed. Note - If the welded materials are not steel, but rather aluminum or stainless, then it may be more difficult to prepare the speciments without metallographic equipment. Etchants should be suited to the materials used. Cross Sectioning Cutting the weld along a transverse plane for Weld-Macro can be performed by different means. One has to avoid heating that might alter the surface and introduce unrelated changes (called artifacts). Therefore flame and plasma cutting should not be considered, and also cutting with a dry abrasive disc, that introduces unwanted heating. Sawing however, if performed carefully with water cooling of the blade as necessary, is perfectly acceptable for Weld-Macro. It is recognized that heavy sections or hard materials are not readily cut by a mechanical saw. If these are the materials to study then one should be prepared to invest in a simple cutting machine that has means to clamp firmly the workpiece, and to section metal items with a water cooled abrasive disc. Different machines are available with features suitable for most diverse requirements defined by the largest dimensions accepted within the machine and by the power of the motor drive used to operate it. Generally an enclosure prevents cooling water to be sprinkled outside. Movements, either manual or mechanized, are given to the disc relative to the work to perform the Weld-Macro section. A provision is made for clamping long bars, to let them protrude outside the enclosure.
Mounting Small Weld-Macro sections must be embedded in a suitable handling device or in plastic media that permit the necessary manipulations as described hereafter. However sizeable chunks of metal can be held by hand without mounting. Small metallographic specimens that will be examined under a microscope must be prepared in such a way to be put in front of the objective lens. That is easier to do if the mounting is of the standard cylindrical shape. However for the low power enlargement lenses or microscopes that we propose to use for the above purposes with Weld-Macro examinations, any suitable arrangement or clamping device that will hold the specimens in front of the optical instrument may be adequate. Two types of mountings are used. A simple press, sometimes using a hand operated hydraulic jack, compresses in a cylinder the metallic sample embedded in phenolic or other thermosetting material. The cylinder is then heated to the temperature that will let the plastic flow. Upon cooling the specimen is extracted, ready to be further processed for Weld-Macro or Micro examination. The entire cycle may last between fifteen and thirty minutes. The other method involves the preparation of the metallic specimens within simple tubular rubber molds. Then a special formulation epoxy or other two parts mixed plastic material is poured into the molds on the pre-placed metallic specimens and let set, possibly overnight unless a low temperature heating furnace can be used to accelerate curing. The advantages of the first method are that the specimens are readily available, possibly two or three per hour per mounting press. However an initial outlay for the press is required, and a current one for consumables. In the second method the initial outlay is minimal, only for purchasing the rubber molds. The consumables are possibly more expensive and less user friendly because of bad smell. But the number of specimens that can be prepared at one time is limited only by the availability of molds and by the quantity of mixed epoxy that can be prepared at one time. However the time for setting may last overnight so that the specimens are not readily available for immediate processing, but once cured they can be further prepared all at once. Many fully equipped metallurgical laboratories have both method available, the selection being made on a case per case basis. But for our welding shop, at least at the start, we consider that mounting is not necessary unless the sections are very small. Grinding The raw cut has to be ground and polished. Special equipment is offered to grind the specimens for metallurgical examination. However assuming again that we have large sections for Weld-Macro, we could grind the cut surface on any metal grinder (not a hand grinder) capable of giving a flat surface, provided the operation can be performed while cooling the surface with a suitable liquid. The problem may be how to clamp the specimens for grinding, but a simple vise may do. It is important not to heat up the surface while grinding, as that might introduce unwanted changes in the weld to be examined. Polishing A series of polishing means, usually starting with abrasive papers, should then be used to obtain at best a mirror looking surface exhibiting the real structure undeformed by excessive local pressure or heat. It is true that for the most exact determination of high magnification metallography for all the finest features, the best obtainable polished surface should be obtained, and this would entail appropriation of metallurgical polishing machine and proper consumables. But if we wish to save on expenses we have to compromise on the quality of polishing, being satisfied with whatever finish can be achieved by the simplest means, by manually rubbing the metal specimen on a solid surface covered with progressively finer polishing papers, with or without water. Etching Serious defects like voids and porosity or cracks may be readily visible on the polished surface of the Weld-Macro without any further treatment. But for detecting the weld borders we need etching with a proper solution that attacks the metal in such a way that the specific features are made to stand out. The following reference is available for all information on Macro Etching:
ASTM E340-00e1 For our limited purpose as explained above we would propose a single etching solution suitable for common steels and able to reveal what we are looking for. WARNING: The solution called NITAL, containing from 3 to 5% of Nitric Acid in Methyl Alcohol, should be prepared by a chemist or a pharmacist or by a technician having experience in handling strong acids. It should be kept in a well marked glass or plastic bottle firmly plugged when not in use, with a label indicating the content and the date of preparation. WARNING: Keep clear of children. Not to be ingested. Harmful to eyes, mouth, skin. Use with care by dripping a few drops of the Nital solution on the polished Weld-Macro surface to be examined and spread with a wooden spatula, a small brush or a cotton wad wrapped on a stick. Do not touch with your bare hands! After a few seconds the attack is complete and the outline of the weld should be clearly visible. Rinse the specimen under running water and then dry it. If the section is not properly etched (under-etched) repeat the procedure for a further short while. If the section is too dark (over-etched), repeat the last polishing and etch again for a shorter time. Examination A low power lens or a simple binocular stereoscope should be used with proper lighting. With some trials and exercise it should not be too difficult to learn to perform the whole operation to permit examining the features of the Weld-Macro and to determine if it is acceptable. If different welding procedures are tested one after the other, it is recommended to number the test pieces and to record on a log-book the different conditions employed for welding. If it is desired to keep the etched specimens for a short while for further reference and comparison, one should use a clear lacquer spray to protect the surface, otherwise it will readily rust and the features sought for will not be visible any more (unless the whole polishing and etching procedure is repeated anew). Conclusion We recommend to implement the means for examining the quality of welds in cross section. If the material used is common steel and if the sections are not too thin and not too thick, it should be quite easy to set up a simple facility to see the weld section appearance as Weld-Macro. The study of the features observed will help improve the quality of the welds performed.
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Weld Macro Examination of test piece sections is an essential element for quality assessment and improvement. It may have important economic consequences. Setting up a simple test facility should be given high priority... -
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