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PWL#071- Comparing Standards, Tungsten for Ti-Tig, Filler Metals for Corr. Protection, Res. Stress July 01, 2009 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#071 - Comparing Standards, Tungsten for Titanium Tig, Filler Metals for protecting Steel Structures from Corrosion, Impact Test, Residual Stress Measurement, Electrical Discharge Cutting, High Frequency Resistance Welding and more... DON'T USE REPLY to send us your messages! Use Contact Us instead. This publication brings to the readers practical answers to welding problems in an informal setting designed to be helpful and informative. You are urged to pass-along this publication to your friends, if you like it, and if you want to help them. If you received this from a friend and if you like what you read, please subscribe free of charge and you will also receive a bonus book on Practical HARDNESS TESTING Made Simple. Note: References to articles or other documents are given here in one of two forms. If the links are "live" (usually underlined or otherwise highlighted) they are operated with a click of the mouse. If they are URL's (Uniform Resource Locator), which is the analogue of an address, they begin with "http://..." or "www.". These are not live and must be copied and pasted entirely into the browser (after having been selected with the mouse or otherwise). If they are long they may be displayed in two or more lines. In that case one has to care that the URL be copied completely in a single line without any space, and Enter.
1 - Introduction 2 - Article - Comparing Standards 3 - How to do it well: What Tungsten is used for Titanium Tig? 4 - Filler Metals for protecting Steel Structures from Corrosion 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - Impact Test 8 - Site Updating: Electrical Discharge, High Frequency Resistance Welding 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contributions: Residual Stress Measurement 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction Welcome to our Issue 71 of Practical Welding Letter. It is instructive to learn always something from certain Queries that are addressed to www.welding-advisers.com This time two queries aimed at sorting out the differences between accepted standards. The first article tries to answer to the question even if it does not supplies the list of differences sought by the inquirers. Then comes a straightforward answer to a very specific question that could have been satisfied also by looking a bit deeper in our Site Map. The content of a Welding Journal article is summarized where the author discusses the long time corrosion protection of steel structures by Thermal Sprayed filler metals, stressing the critical cleanliness conditions essential to successful operation. A short note on Impact Test presents the occasion to remind the differences between ductile and brittle failure, and how to test the conformance with precise requirements advanced by Codes to assure safe performance of structures at very low temperatures. A new and practical instrument for measurement of residual stresses is introduced. It is possibly capable of simplifying routine inspection of fabricated structures and bridges for checking the presence and amount of residual stresses, and possibly the effectiveness of stress relieving procedures. It may be worth to keep an eye on future developments and applications of this method because it seems much simpler than the available alternatives. The pages of this Month, added to the website, are about Electrical Discharge Cutting, sometimes a useful process to be kept in mind for its advantages, and also on High Frequency Resistance Welding, processes used for high speed welding of tubes and pipes roll-formed from flat strips. Other regular departments can be found at their usual place. Enjoy your reading. Use the Contact Us form to let us have your comments and feedback. (Don't use Replay). 2 - Article - Comparing Standards Two queries were recently received concerning the comparison of different standards. Any Company manufacturing similar items for different customers may occasionally be confronted with the question if the definite procedures that were approved as satisfying the requirements of a certain Code could be used for welding to a different Code. Being exposed to different Standards may enrich the individual experience of people and raise interesting questions. It must be acknowledged however that the documents themselves are the distilled experience of many professionals and follow the best engineering practice of different places. Therefore uniformity cannot be expected. While logical thinking might indeed support the common sense evidence suggesting sometimes the equivalence of different requirements, in practice it may be easier and more practical to submit a new set of specimens to testing according to the different Code. This is because the certifying agent may doubt his/her own authority in deciding such a sensitive matter, so that the request may be forwarded up the ladder to a higher authority or even to a committee to obtain a final decision. More often than not, the answer is going to be negative anyhow. The process may be long and the outcome far from being guaranteed. Practical people should strive to understand fully the requirements and how best to meet them, without caring too much if seemingly useless additional testing efforts are needed. Other than understanding how to behave in case a definite issue presents itself, there is not much point in comparing specific requirements, even if they address the same or a similar concern. It is out of place to compare a structural standard, with another, dedicated, say, to pressure vessels. See the page A similar question may arise for available materials, which may have similar but not identical specifications to those required. Here the problem is twofold. If the requirement is to change the material standard in drawings or engineering documents (to have it easier to procure and cheaper to purchase), it must be understood that this is an engineering decision that can be only taken by a responsible authorizing engineer. If the request (of purchasing or production) is to use a similar material, specified to a different standard from that demanded by the drawing, the substitution can be possible if each and every property of the actual supply can be checked against the requirements of the new document, and found acceptable. Readers having experience about this matter are invited to comment with a short note, to be sent using the form available at the Contact Us page. 3 - How to do it well: What Tungsten is used for Titanium Tig? As it is widely known, available tungsten electrodes used in Gas Tungsten Arc (or Tig) Welding can be made of a few different types of powder metal compacts. For Titanium welding the conventional thoriated Tungsten electrodes known as EWTh-1 and EWTh-2 are used (with 1 or 2%Thorium respectively), ground to a point. Welding-titanium is done with straight polarity direct current (tungsten electrode connected to the negative pole). See also Welding Titanium. 4 - Filler Metals for protecting Steel Structures from Corrosion A recent article published at page 46 of the Welding Journal of July 2009 explains that the corrosion protection provided for long periods by suitable alloys in different atmospheres depends on surface preparation, on application methods and on coating thickness. The protective materials considered are Aluminum, Zinc and an Alloy consisting of 85% Zinc and 15% Aluminum that provide barrier coating and cathodic protection if the layers are sufficiently dense and free from porosity. Zinc has greater chemical activity and provides greater sacrificial cathodic protection than Aluminum. Aluminum with lower chemical activity, adherent oxide film and higher temperature and abrasion-wear resistance provides longer term protection. The Thermal Spray processes employed for these applications are usually the well known twin-wire arc or the wire flame spray system. The arc between the two wires, fed at controlled speed through a special torch, is sustained by a low voltage direct high current. In the flame spray setup a single wire tip is molten by an oxyfuel flame. In both processes compressed air propels the molten metal drops onto the prepared substrate. The article explains that the quality of the substrate preparation is the most critical factor for the success of the application and refers to a few American standards that provide guidance on blast cleaning. On the Application technique the article explains that for minimizing internal stresses and spalling of the coating, thin layers should be deposited in several passes with crossing directions. The thermal spray gun should be kept perpendicular to the surface, at a standoff distance of 150-250 mm (6-10"). The article concludes with recommendations on Quality Assurance provisions for process approval, involving destructive tests on suitable panels, and with reference to a few pertinent specifications. Interested readers are urged to seek the original article from the above mentioned source. 5 - Online Press: recent Welding related Articles See my most recent Article on Explosive Welding Papers Navy Investigates Welds on Ships after Reports Falsified A Cost-Effective Material Solution for the Boiler Industry Joining News 6 - Terms and Definitions Reminder Ionic Bond is the primary force keeping together by electrostatic attraction ions of opposite charge. Manual Welding is any process performed by the skilled manipulation of suitable means (torch, gun or electrode holder) and consumables, by welders capable of coordinating the required actions to produce successful welds. Oscillation is the purposeful motion of the torch or electrode, generally perpendicularly to the welding progress direction, aimed at distributing adequately heat and filler metal. Roll Welding is a process that joins together a stack of sheets or plates that is fed through a cold rolling mill under sufficient pressure to produce solid state welding. See Roll Welding. Submerged Arc Welding is an arc welding process where the arc from one or more continuously fed bare filler metal wire and the base metal, is protected under a blanket of granulated flux. It is an energy intensive process with remarkable deposition rate, suitable for relatively thick butt joints or for cladding in flat position. See Submerged Arc Welding Tips Transformer Tap is a set of selectable connections to a transformer winding to vary the transformer turn ratio, to control secondary voltage and current. U-groove Weld is a type of groove weld whose section is similar to the letter U and needs less filler metal than the V-groove type. Welding Position describes the relative locations of joint members, weld pool, welding heat source and welding consumables during welding. Can be flat, horizontal, vertical, inclined or overhead. 7 - Article - Impact Test A recent query asked for information about the notched-bar Impact Test. Before dealing with the subject, one should clarify a few concepts relative to fracture. Fracture is described as the rupture or fragmentation of a solid body into two or more parts under the action of stress. Two stages are recognized as conducive to fracture: crack initiation and crack propagation. Depending on its aspect, fracture can be classified as ductile (if it is characterized by noticeable plastic deformation) or as brittle (if there is no or very little deformation). Fracture occurs in one or the other of the above types depending on the state of stress (if simple or tri-axial), on the rate of stress application, and on temperature. Because of this, it is not quite correct to talk of brittle or ductile materials. A material, showing a perfectly ductile fracture under certain circumstances, can behave in a dangerously brittle fashion in other cases (typically at low temperatures, in presence of stress raisers, or under a sudden impact). In general, engineering structures are designed to avoid sudden brittle failures. Deformations occurring before failure can alert of impending dangers and provide sufficient time to reduce the stresses and to save human lives. Standard notched-bar impact tests (Charpy or Izod) are usually performed according to well defined procedures, on specimens of given dimensions having a sharp notch of definite shape and size machined at a given location. The sharp notch provides a preferential location for crack initiation and, as a stress raiser, modifies the stress distribution. The specimen is tested by imparting a well defined blow designed to bend and fracture the metal. Usually a heavy swinging pendulum is released from a given height so that it strikes the specimen at definite speed and energy. The Charpy specimen has a square cross section and a notch at the center of its length. It is supported in horizontal position as a beam. The load is applied by the pendulum at the midspan of the beam on the side opposite from the notch. The Izod specimen is either circular or square in cross section and has a V notch near one end. The specimen is clamped vertically at the end near the notch and is struck by the pendulum at the other end. In both cases the blow is such that the bending of the breaking specimen opens up the V notch. Two results are sought from the test. The first is the energy absorbed by the specimen during fracturing, measured by the final height reached by the pendulum after having impacted and ruptured the specimen. This absorbed energy is a measure of toughness and is used as a tool to study temperature-dependent brittle-ductile transition. The second is the fracture aspect, where a certain measure of deformation can be observed, indicative of ductility (if present) or of brittleness (if absent). These results do not reveal a fundamental physical property but help in building transition temperature curves of given materials, describing the gradual passage from completely brittle fractures at low temperature, to acceptably ductile fractures at normal temperature. Adequate engineering design will care that operating stresses will not be unduly augmented by stress raisers due to faulty constructional concepts. Welding procedures and inspection methods will avoid discontinuities due to poor welding practices. Manufacturers of Welded structures likely to operate at low temperatures, may be requested by Codes and Standards to demonstrate that impact specimens obtained from welded test-pieces show the required minimum properties at given temperatures as a proof that the weldments will stand the service stresses even at low temperatures, without being subject to brittle failure. Similar considerations apply also to non metallic materials. 8 - Site Updating: Electrical Discharge, High Frequency Resistance Welding The Pages of this Month enrich the Welding Adviser Website with two somewhat specialized processes. The first Page, dedicated to Electrical Discharge Cutting, should be part of a larger subject called Electrical Discharge Machining. EDM concerns about manufacturing in general, and as such it has no direct interest for welders. However ED Cutting can be used, when appropriate, to give to individual items the right shape and dimensions for assembling them in fabricated weldments. It is not out of place therefore to request quotations for getting ready to weld items obtained with this technology from subcontractors with proper equipment and know-how. Depending on materials, shape, dimensions, tolerances and quantities, this cutting technology may or may not result less expensive than the alternatives. Click on Electrical Discharge Cutting to see the new page. The second Page deals with specialized Resistance Welding Processes, characterized by employing High Frequency Current to obtain continuous high speed welding of tubing and piping. The tubular shapes are obtained by cold roll-forming metal strips. Two variants are used for the power supply, either by induction or by contact. Each of the variants has specific advantages and limitations that dictate the most economic solution for given applications. The joint finally obtained is a modern sort of high quality forge welding. Click on High Frequency Resistance Welding to see the new page. Check for new pages in the Web Site Map or subscribe to the RSS feed using the instructions given in every page (www.welding-advisers.com) under the Navigation Bar. For finding what you may need, use the Google Search Box which appears in almost every page of the website. More often that not there is some reference to your subject either in the website or in the collection of Practical Welding Letters. 9 - Short Items Broaching is a machining process that cuts into solid metal with a tool called broach. This is a bar presenting teeth or a series of cutting edges on its surface. The cutting teeth are ordered in a succession where they increase in size and/or change in shape. The broach cuts small chips in a straight line leaving a profiled groove in the workpiece. Considerable power is needed to have the broach cut into the workpiece. The entire cut is made in single or multiple passes over the workpiece. Cold Rolling is a metalworking process in which metal is deformed by passing it through rollers at a temperature below its recrystallization temperature. It is used most often to decrease the thickness of plate and sheet metal and to enhance mechanical properties through cold working. Differential Thermal Analysis (DTA) is a method permitting to study internal processes occurring in a material subject to temperature changes. The material under study and an inert reference are made to undergo identical thermal cycles, while recording any temperature difference between sample and reference. By plotting differential temperatures against time, or against temperature, changes in the sample, either exothermic or endothermic, can be detected relative to the inert reference. Lost Foam Casting uses an expendable polystyrene pattern surrounded by unbonded sand. Upon casting the pattern is vaporized by the molten metal which then solidifies in the mold cavity determined by the expendable pattern. Permanent Magnet Material is a ferromagnetic alloy capable of being magnetized permanently because of its ability to retain induced magnetization and magnetic poles after removal of externally applied fields or an alloy with high coercive force. Thermal Fatigue is produced by cyclic stresses in a structure, resulting from the presence of temperature gradients that vary with time. 10 - Explorations: beyond the Welder The Fuel of the Future Is Grassoline Development of Cellulosic Biofuels (Video lecture) Tracking Poached Elephant Ivory Prepare for the Human Exploration of Mars Music and Learning 11 - Contributions: Residual Stress Measurement Reference was found online relative to a new technique useful for nondestructive measurement of residual stresses with a small, handheld instrument. The method involves analysis of vibrations induced in the body by a suitable impactor. In the downloadable page, a short description is provided of the instrument and of its applicability, unfortunately, for the time being, without examples or comparisons to alternative techniques. The nondestructive known methods for residual stress measurements are based on x-ray diffraction, that requires the use of dedicated instrumentation. Destructive methods need the application of strain gages upon the body where the stresses are to be measured, and then cutting or drilling the material for releasing the stresses, to be measured by the amounts revealed by strain measurements. If the new method above permits indeed to obtain significant results using the simple instrument described in the following link, then it may be useful for studies involving analysis of the effects of modified welding procedures on the deformations caused by residual stresses, and for assessing the effectiveness of stress relieving techniques. Find the document online at Stress Measurements. Readers having experience with these techniques are invited to share them with this audience. I am grateful to Mr. Ennio Curto, the promoter of the new technique, who kindly sent me the following Abstract, of a new article to be published later this year in the occasion of an NDT Conference. 57th Defense Working Group on Nondestructive Testing 17 - 19 November, 2009 NEW TECHNIQUE FOR RESIDUAL STRESS MEASUREMENT NDT by Ennio Curto. ABSTRACT This type of non-destructive testing method provides the possibility to measure residual stress and the effect of service loads with an impact device and a vibration measurement sensor. Internal stresses are to be considered as the following:
The new technique is able to measure the applied load and residual stress that are balanced on the surface of the material, and in a relatively large volume, at times even the same size as the entire structure. This stress is part of the metal’s elastic field and has a three axis spatial orientation. This new system for inspection of steel buildings and bridges is very simple to use, portable, and measures exact values of residual stresses due to welding and applied loads. After many years of research and tests about elastic field behavior of steel, it is now a very practical method to perform point to point inspection of buildings and bridge constructions. This new system makes steel bridges inspection very simple with a portable instrument and permits measuring exact values of residual stresses due to welding and applied loads. 12 - Testimonials On Mon Jun 01 23:59:24 2009, the following results were submitted
From: winamos emos (email removed for security) I mostly thank you for your rich work on the net [...]
13 - Correspondence: a few Comments 13.1 - If you run a blog or a website on welding, on artistic welding work or on a theme related to what is dealt with here, please let me know. I would be grateful to be allowed to publish an interview with you, where you will be able to introduce yourself and to explain your interests and achievements (even if you are mainly offline). The interview would bring to our readers the details of how to reach you and your online publications, possibly giving you the chance to make new friends and expand your reach. 13.2 - I am considering to send you by email a short Survey-Questionnaire to help me understand better the readers expectations and preferences. I will be grateful if you dedicate a few minutes to put in your answer and to send it back to me. 13.3 - From time to time some correspondent sends me the Contact Us form but leaves the comments blank. I interpret them as a kind of "Hello" but really I don't know what to think... 13.4 - Readers sometime ask for financial advice. That is outside of my competence. I could possibly point to sources of information, but would refrain from positive suggestions that might also result erroneous. I always admire those that take the risks and set up a shop, but I know that there are many dangers. 14 - Bulletin Board 14.1 - I would like to recommend that you enlarge your knowledge by taking up, for example, the following Course, titled On the same page click on "Try a free demo!" 14.2 - Essen Welding Fair (Schweissen & Schneiden) 14.3 - 25th ASM Heat Treating Society Conf. & Expo. 14.4 - Questions?
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