| Back to Back Issues Page | |
 |
|
|
PWL #029-Weld Deposition Rate, Preventing Distortion, Filler metal for SAW, Risk Assessment, TigWeld January 01, 2006 |
|
| We hope you will find this Letter interesting and useful. Let us know what you think of it. Weld Deposition Rate, Preventing Distortion, Filler metal for SAW, Risk Assessment, Tig Welding Tips, Developments in Underwater wet Welding and more...
This publication brings to the readers practical answers to welding problems in an informal setting designed to be helpful and informative. We actively seek feedback to make it ever more useful and up to date. We encourage you to comment and to contribute your experience, if you think it may be useful to your fellow readers.
You are urged to pass-along this publication to your friends, if you like it, and if you want to help them.
Date: January 2006 - Practical Welding Letter - Issue No. 29
1 - Introduction 2 - Article: Weld Deposition Rate 3 - How to do it well: Preventing Distortion 4 - Filler Metal for Submerged Arc Welding 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Risk Assessment 8 - Site Updating: Tig Welding Tips 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Developments in Underwater wet Welding 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction Although New Year's Eve is already gone when you see these lines, we present our best wishes for a happy and prosperous New Year to you, our readers, and your families. May all your constructive plans and ambitious wishes come true. You will find this new 29th issue of Practical Welding Letter when coming back to work with renewed energy and enthusiasm after festivities and vacation, hopefully curious about the subjects of this publication, and eager to benefit from the information provided here. The first article deals with Weld Deposition Rate, an important parameter of welding economics, equally remarkable for the largest industry and the smallest shop. How to increase it to the maximum for any given job should be given the highest priority in any manufacturing facility. In the Section on "How to do it well", a specific question that was raised gave us a push for treating a case of welding distortion. We understand the frustration linked to unsuccessful trials for overcoming an unacceptable situation. Maybe the example will help clarify the issue for other cases as well. Filler metals for Submerged Arc Welding may present a difficult selection problem because of the need to deal also with the choice of a proper flux. Official Specifications numbers are provided. The classification system code is introduced with the reference to the mechanical properties obtained from the weld metal. The next article reports on an advanced method permitting to plan the frequency of inspections as a function of the perceived Risk of failure of any component of a complex plant. It is based on continuing verifications according to Fitness-for-Service principles. This method substitutes an older program, rigidly based on periodic inspection, that did not take risks into account. The new method, being accepted by recent legislation, permits to save on maintenance programs and to assign limited resources (i.e. nondestructive testing capabilities) according to priorities. For the Site Update we introduce our new page on Tig Welding Tips that presents basic principles possibly forgotten, to ease the successful exploitation of this important welding process. Other articles on the same topic are referred to in Department 5, Online Press. In the Contributions department we refer on recent advancements in Underwater Wet Welding as presented in a recent article by the Welding Journal. Other Sections are in place as usual. We are open to your comments. Write us by e-mail, using our Form for Your Questions and Feedback. Please let your friends know of our Website, we believe we can be of help to anybody interested in welding.
2 - Article: Weld Deposition Rate Weld deposition rate is the weight (in kg or lbs) of weld metal deposited per unit of arc-on-time (usually one hour). The weight deposited is less than the weight of the filler metal used, because of various losses. The ratio of the weight of metal deposited in the weld to that of filler metal employed, expressed in percent, is called the deposition efficiency. One hour of welding job time includes, besides arc-on time, periods not used for welding, like operations performed before (cleaning, assembling, fixturing, tacking etc.) and after welding (slag and spatter removal, straightening if necessary etc.) and other periods of rest or of associated activities (providing tools or materials). In manual welding it is not uncommon that arc-on-time be between 1/3 and 1/2 of the time spent on the job. This is one of the reasons for the potential great advantage of robotic welding: increased deposition rate due to the disposition of all of the major distractions that usually reduce arc-on time. These concepts are most important for the economy of shop operation. Although simple and straightforward they are likely to be dismissed as not worth of being dealt with. It would be a mistake to ignore them because the first condition for possible improvement is an accurate assessment of the status of present operations. They should be looked at as tools to be used for control of daily operation and as rough elements of a database to be used for future new jobs cost estimates. One should admit that there is no standard value of deposition rate that all jobs with the same parameters should comply with. In fact the variation can be quite substantial, as reflected in tables and graphs published in handbooks. The best published information displays commonly accepted ranges, and should be considered as guidelines, to evaluate one's performance. To improve productivity, obviously arc-on-time should be maximized and all other periods spent in unproductive activities minimized. The best assessment would come, if possible, by actually weighing the welded parts and subtracting the weight of the same parts before welding, to determine the weight of the weld material deposited. If impractical the deposited weight should be estimated by calculating the average section welded per the length of weld beads deposited in an hour of arc-on-time. Handbooks display tables presenting calculated weld volume per unit length of typical joints. Any welding shop should know its own weld deposition rate for every job and should strive to increase it as long as other important characteristics like quality and distortion are not compromised. Different processes that operate at different parameters generate ranges of rates. Limitations like available equipment duty cycle or additional duties imposed upon the welders may limit the arc-on time and therefore reduce the shop productivity potential. Besides those cases when current limitations are set up on purpose, for example to reduce dilution of the base metal, or to control heat input for metallurgical reasons, current should be selected at the highest value compatible with electrode diameter, with warping control and with welder's comfort. In principle processes with the highest deposition rate should be preferred unless other reasons suggest selecting other processes. Mechanized setups, when applicable, permit to push the current to higher values. These could be uncomfortable for manual welding because of the substantial heat dissipated to the surroundings. Within the processes available in the shop, one should research which factors have the potential of being improved to increase deposition rate. One of the first is certainly joint preparation. By devoting attention and care to accurate fit up of joint edges one prevents the tendency of overwelding, a common if concealed source of poor productivity. Fillet welds are easily made larger than drawing requirement, mostly because of lazy carelessness. No useful purpose is achieved by that. By using simple gages frequently enough to measure the actual fillet face, one can assure the correct dimensions without squandering resources. When possible the flat position should be preferred because of its favorable influence on deposition rate. Occasionally this requires manipulation with lifting devices or with positioners. Selecting a higher size electrode, when acceptable for other considerations, should be checked because it would permit increase of current and of deposition rate. With processes using continuously fed electrodes (GMAW, FCAW and SAW), one should remember that increasing the electrode extension (also known as stick-out) permits to improve feed speed and deposition rate without unduly increasing the arc current. Upon determining that improved parameters produce higher deposition rate vs. the previous baseline, one should qualify the procedure by making sure that the production so obtained still satisfies all quality requirements.
3 - How to do it well: Preventing distortion Q: While welding a thin (1.5 mm = 0.060") element, standing upon a thick (8 mm = 5/16") flat bar of mild steel, by SMAW with one sided fillet weld, we got severe distortion: the flat bent in an upward bow, and the thin element would tend to bend instead of staying perpendicular. We tried clamping, preheating, cooling, and still end up with a bow. How can we prevent warping? A: The deformation is caused by residual tension (shrinkage) stresses along and across the weld. One way to reduce stresses and deformation is to weld a short length and immediately after that to peen the weld with a hammer to lengthen the weld bead by deforming it while still hot. Then proceed the same way by short not contiguous stretches, until the whole piece is welded. The purpose is to avoid the build up of substantial shrinkage stresses. The thin element could be positioned with a larger than 90 degrees angle, to have the welding process redress it. Clamping does not remove the stresses, unless stress relieving can be performed before removing the assembly from the fixture. Preheating reduces the rate of cooling after welding, with some effect on the magnitude of residual stresses only if the temperature is high enough. Cooling could help with intermittent or backstep fillet weld sequence.
4 - Filler Metal for Submerged Arc Welding Submerged Arc Welding (SAW) is a high deposition rate process used for welding and overlaying carbon steels, low alloy steels, stainless steels and nickel alloys. It performs best for relatively thick material, but only in flat or horizontal position. It cannot be used for aluminum, titanium or copper. Electrodes of three types are available: solid wire, cored wire and strip. Solid wire is used for standard steels. Cored wire includes in the core alloying elements that may be needed for matching special compositions. Strips are used for overlaying and hardfacing. A strip feeder is needed for using strips. Filler materials can be found in the following Standards:
For any job it is important to select both the filler metal electrode and the flux, because it is the correct combination that ensures acceptable properties. Actually the weld metal composition depends also on base metal and on parameters that control dilution. One should add that many other proprietary filler metals not yet standardized are in the market, possibly appropriate for special applications. Fluxes can be classified as fused or bonded, depending on manufacture, or as active (that include manganese or silicon as deoxidizers) or neutral depending on their action (or lack of it) on steel composition. An important control parameter is the ratio of flux-to-wire, by weight, which increases with voltage increase or current decrease and vice versa. Solid carbon steel electrodes are classified as low manganese (EL), up to 0.6 %Mn, medium (EM), over 0.8 and up to 1.4 %Mn, and High (EH), over 1.4 %Mn, with a few compositions specified for each class as indicated by a number and possibly a letter. AWS Standards above present a Table of Mandatory Classification Designators for Flux-Electrode Combinations that include minimum specified properties to be verified by mechanical testing on actual weld metal. The classification system code refers to the properties of the weld metal obtained. It includes first a designation for the flux indicating the properties it produces in as welded or post weld heat treated condition, and then the indication of the filler metal used in the specific flux-filler combination. The chemistry of the deposited weld metal is mainly determined by filler metal composition. Flux has much influence on the weldability of the selected combination. Occasionally, for overlay applications, one may need to reduce dilution to meet specific requirements, like composition limits or side bend test results free of cracks. This can be done either by reducing the current or by increasing the travel speed (or both). Current can be reduced by increasing, within limits, the electrode extension (stickout), without changing wire feed speed.
5 - Online Press: recent Welding related Articles From The Fabricator TIG Welding - an Overview From AWS In-Process Ultrasonic Resistance Weld Inspection From TWI
6 - Terms and Definitions Reminder Arc length is the distance between the tip of the welding electrode and the molten surface of the workpiece. It is strongly affected by voltage and by electrode manipulation. Brazement is an assembled work piece whose elements are joined by brazing. Collet is a mechanical clamping device used to locate and keep the position of the welding electrode in a welding torch. Dwell time in manual welding is the momentary pause at the sides of a welding bead when weaving the electrode. Plug weld is a weld filling a hole drilled in a component to join it to another element with which it makes contact. Seal weld is intended to provide a leakage free joint. Sieve analysis is a method used to quantify the particle size distribution of granular products or powders, by collecting and weighing the retained fraction of the product on each one of a series of screens arranged in order of decreasing mesh size. The results are expressed in percent weight per screen. Spliced joint consists of an additional member welded to two elements of a construction to connect them together.
7 - Article: Risk Assessment Power plants, oil refineries, major industrial process facilities operating at elevated temperature and/or pressure, or dealing with harmful substances, lifting and fairground equipment are examples of installations having the potential for catastrophic failures risking to endanger people and property. Therefore both builders and users are bound by appointed Authorities in different countries to submit to Statutory bodies the design, fabrication procedures and maintenance routines of those and similar structures in accordance with detailed code requirements and approval rules meant to reduce the risks of failures and consequent harm or damage. Welding in general, as the complex of technologies used to join metallic elements, is an important part of erection and building of large constructions, and is therefore in the focus for constant scrutiny for its possible weak points. The nature of the prescriptive provisions released to deal with safety issues was historically based on industrial experience with type of equipment rather than on specific risks. The rules of condition assessment according to the above method are quite rigid and restrictive, and require periodic inspection and maintenance to be performed in order to provide the required authorization for continuous use and operation of the structures. A number of limitations were identified with the said approach which, although it generally provided good records, did not address the subjects of cost of inspections and of impact of failures on the environment. In recent times a newer form of programmed maintenance has emerged, called Risk Based Inspection (RBI). This modified approach was made possible by changes in the law permitting the considerate usage of advances in material behavior understanding, in nondestructive inspection techniques and in Fitness-for-Service evaluations. RBI is a structured, cost effective procedure designed to optimize inspection planning. It is meant to provide plant integrity management by considering an integrated approach based on the sound principle that the more information is collected and evaluated, and its validity verified, on the status of a given installation, the less the associated risk of failure. The knowledge, based on operational and inspection records, permits to perform Fitness-for-Service calculations, enabling the planning of future inspections by setting priorities to keep in check mechanisms of deterioration which are known to take place. The American Society of Mechanical Engineers (ASME), the Nuclear Regulatory Commission (NRC), the American Petroleum Institute (API), and the European Commission through a series of European Health and Safety Directives, are among the bodies that promoted and developed Risk Assessment methodology and its applications. An introduction to the subject can be found in a report on Risk Assessment Inspection (186 pages) available online through TWI at
8 - Site Updating: Tig Welding Tips This month we added to our Website a page providing Tig Welding Tips. We feel that the plain explanation of basic facts and their interpretation may add to the knowledge of new or experienced welders or remind them known but long forgotten information. This may be important for welders, expert in other processes but being called to work on GTAW only occasionally. We plan to update the page itself if questions or comments from readers will show continuing interest in the subjects treated. Articles on the same topic from other sources are referred to in Department 5 above, Online Press. To find our Website new page click on Tig Welding Tips. To review all the website page Titles, old and new, click on the Site Map.
9 - Short Items 9.1 Beta Transus - In pure titanium, the alpha phase, of hexagonal close-packed crystalline structure, is stable from room temperature to about 882 0C (1620 0F). The beta phase has a body-centered cubic structure and, in pure titanium, is stable from that temperature up to the melting point of about 1688 0C (3040 0F). Certain alloying additions like aluminum and interstitials tend to stabilize the alpha phase. They extend the temperature range where alpha phase exists by raising the temperature at which the alloy will be transformed completely to the beta phase. In contrast with this behavior, alloying additions such as chromium, niobium, copper, iron, manganese, molybdenum, tantalum and vanadium extend the temperature range where beta phase exists (stabilize the beta phase) by reducing the temperature of transformation (from alpha to beta and vice versa). The transformation temperature from one crystalline structure to the other is known as the beta-transus temperature. The exact determination of this temperature for any given titanium alloy is found experimentally and is used to establish forging procedures, depending on the alloy and on the mechanical properties sought after heat treatment. 9.2 - Billet is a rolling or forging transformation product obtained from a bloom, the primary product forged from an ingot in standard steel production. In continuous casting billets are directly obtained from the melt. Billets are starting material for producing forged items or for rolling bars, profiles and seamless tubes. 9.3 - Laser drilling is a machining procedure that uses the energy concentrated in the coherent light beam of a laser to melt locally various materials to produce holes of definite dimensions and surface quality. The manipulation capabilities of laser beams and the speed of their movement permit to obtain irregular section holes with any angle to the surface. For certain applications the resulting surface recast layer must be removed by other means for better performance. 9.4 - The Metallic Bond is the force that holds metals together by a mechanism of attraction between the positive ions and the negative electron "sea". The sea analogy suggests that the mass of free valence electrodes are extremely mobile and permeate the whole volume. This is the chemical bond characteristic of metals, in which mobile valence electrons are shared among atoms in a usually stable crystalline structure. The strength of the metallic bond is derived primarily from the charges in the system. 9.5 - Single Crystal Castings were developed essentially as gas turbine airfoil castings. Originally the grain shape was equiaxed. Then a remarkable improvement was introduced, characterized by unidirectional solidification, called DS. Finally single crystals were produced with the purpose of disposing of grain boundaries, in order to eliminate weak locations in the structure, improving the creep resistance at elevated temperatures. The procedures involve investment casting molds undergoing solidification in a strongly directional temperature gradient, where only the most favorable solidification direction has been retained. 9.6 - Superalloys are complex materials, generally based on nickel or cobalt, designed with carefully balanced compositions. They respond to specific heat treatments by developing microstructural phases with characteristic distributions. They exhibit high mechanical properties, primarily remarkable creep resistance, at elevated temperatures, making them candidates for critical applications like gas turbines. Superalloys are not easily welded, except that limited repairs may be performed with special means and procedures if acceptable for their final use. 10 - Explorations: beyond the Welder From ASTM From DOE From IEI From ORNL Cats
11 - Contribution: Developments in Underwater Wet Welding Underwater wet welding for repairing large structures submerged in the sea, is often much more economic than the alternative of building a hyperbaric (high pressure inert gas) containment structure around the weld locations in order to perform welding in a dry environment. Unfortunately though, in the arc heat, sea water is decomposed into hydrogen and oxygen which risk to be promptly absorbed into the molten metal. Absorbed oxygen can cause loss of alloy elements and is expelled during cooling down into porosity pockets, while hydrogen risks to generate hydrogen assisted cracking and brittleness. The rapid cooling provided by water collapsing on the weld puddle upon arc extinction produces, in regular steels, depending on their composition and on that of filler metals, hard structures with low ductility and toughness. These problems have spurred dedicated research efforts to investigate which composition modifications would improve the mechanical properties of wet welds. Microstructural refinement intended to promote a particular (acicular) ferritic structure, hydrogen mitigation and porosity reduction were the main goals of several joint industry research programs in the past two decades. Compositional adjustment of consumables was determined to be the most important factor promoting quality of wet welds. Hydrogen reduction was obtained by including in covered electrodes high proportions of oxidizing agents capable of combining and sequestering hydrogen in the slag. The addition of titanium and boron in the flux coating of rutile grade electrodes was found to produce the favorable acicular ferrite microstructure, with improved mechanical properties. These improvements depend on the depth into the water, being influenced by the environmental pressure. This means that the coating has to be optimized for the prospected depth of wet welding work. Porosity was found to be reduced by the presence of ferromanganese, but then the amounts of titanium and boron must be reduced to low values, possibly in contrast with the above findings, therefore requiring accurate balancing of elements. Nickel additions were found to improve fracture toughness. Interested readers are urged to seek the original article on which this short note is based, at page 54 of the November 2005 issue of the Welding Journal. 12 - Testimonials 12.1 - We are delighted to announce that the Editors of the Welding Journal, the renowned American Welding Society publication, found it proper to introduce to their readers our website,
12.2 - From: ASOGAN K 'asogan@bhelrpt.co.in'
Thanks for your response sir. Asogan
13 - Correspondence: a few Comments Every month brings in a wide selection of letters from visitors of our Website with the most diverse questions on welding. Sometimes those are not sufficiently detailed, so that it is unclear what is really needed. In this case the short answer points to the fact that more details are needed in order to supply a pertinent response. Sometimes the request is most precise, pointing to a problem for a manufacturing facility, with a financial interest in the best solution for obtaining substantial gains. In this case I make it clear that although I use to provide no cost information to individuals, small shop owners and students, I am not ready to do the same for established companies. I add: "I am ready to provide my professional expertise to industrial companies willing to hire my consulting services. If you see the benefits of seeking technical advice, I would be happy to help you. I believe that solving technical problems by using qualified consultation saves much time and expenses, otherwise wasted in unsuccessful trials and rejects." This answer is usually sufficient to set apart the inquirers with a real interest from those just playing around. An agreement is easily found with serious people to proceed rapidly to the core of the welding issue. Anyway the occupation of correspondence with visitors from the entire globe is most interesting and rewarding for me, and possibly useful to those who receive the answers they looked for. 14 - Bulletin Board 14.1 - The premier event for brazing and soldering, the Third International Brazing & Soldering Conference, 14.2 - Three concurrent events: 14.3 - We recommend our Web Host, SiteSell.com for their competence, integrity and success in helping scores of regular people like you and me, with no previous special knowledge in the Internet, in building their own site for satisfaction and gain. To reach the Link Directory that may enlarge your horizons, click on
Best wishes for the New Year 2006 Copyright (c) 2005, 2006 by Elia E. Levi and welding-advisers.com,
|
|
| Back to Back Issues Page |