| Back to Back Issues Page | |||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||
|
PRACTICAL WELDING LETTER, Issue #002 -- How to Select Your GMA Welder October 01, 2003 |
|||||||||||||||||||||||||||||
| We hope you will find this Letter interesting and useful. Let us know what you think of it. Practical Issues, Creative Solutions
This publication brings to the readers practical answers -----------------------------------------------------------------------
You are urged to pass-along this publication to your ----------------------------------------------------------------------- Date: October 2003 - Practical Welding Letter - Issue No. 02
------------------------TABLE of CONTENTS---------------------------
==> 1 - Introduction
==> 2 - Article: How to select your GMA welder
==> 3 - How to do it well: Welding thin to thick Sections
==> 4 - Filler Metal: Stainless
==> 5 - In the presss: Recent Welding Articles
==> 6 - Terms and Definitions Reminder
==> 7 - Site Updating
==> 8 - Work in Progress
==> 9 - Readers' Contributions
==> 10 - Explorations: beyond the Welder
==> 11 - Correspondence: a few Comments
==> 12 - Bulletin Board
==>Unsubscribe link
-----------------------------------------------------------------------
Welcome to the present issue of Practical Welding Letter. We would like to help our readers to investigate the subjects near to their welding interests. In this issue we continue with a new article on Selection of GMA Welders, and introduce a summary on Stainless Steels Filler Metals. In the "How to do it well" section we address the problem of welding together thin and thick sections. We are encouraged by the continuing stream of subscribers who join our readership in increasing numbers in search of practical answers. So far the feedback from the readers has been quite meager, which makes our job harder in trying to address the hot topics for the largest possible audience. Readers are certainly busy with their daily occupations and do not find time for corresponding, but we believe that even a short note to us might be helpful in fine tuning our conversation. Therefore we urge all of you to share your thoughts with us. A number of recent publications in the field of welding in general, emphasize that the welders workforce is shrinking, while the requirements for expanded application of welding continue to grow. This is good news to young people selecting their new career for life. It is said that while a number of aging and expert welders retire, no immediate replacement force is available. Also the restructuring of many industries that reduced their specialized expert centers in order to cope with market pressure, means that less professionals are currently available for responding to the growth of industry at large. More and more companies strive to automate jobs that were done manually. In the voice of leaders in the industry, automation is the single most important growth sector in the welding industry. Therefore present and future welders will have to improve continuously their skills and their flexibility with automation requirements in mind. As we build up the routine of this publication we are still open to changes to suit the preferences of our readers: just let us know by writing to feedback@welding-advisers.com ----------------------------------------------------------------------- 2 - How to select Your GMA Welder
In this article we shall confront the choices one has when selecting the power supply for two popular processes: Gas Metal Arc Welding (GMAW) also known as Mig for Metal Inert Gas, and Flux Cored Arc Welding (FCAW). Both processes can use the same power supply. GMAW is claimed to be the easiest process to learn in that it requires less skill than other processes: this may be true, once the actual welding parameters have been properly selected by an expert welder. Alternating current is seldom used for GMAW. Direct Current Reverse Polarity (DCRP), with electrode positive is the usual selection, although Straight Polarity (DCSP), electrode negative, can be used when penetration must be kept to a minimum. Direct current is provided by machines of the motor generator type or of the transformer-rectifier type. While existing power supplies for GMAW may still use Constant Current output, new equipment is preferred with Constant Voltage, as better suited to the process, with some means of changing the volt-ampere curve. Changing the slope provides the function of arc stabilizer. This is required mainly when welding with fine filler at low voltage and current. The selection concerns the following alternatives. Motor-generator vs. transformer-rectifier. The selection is usually based on the energy supply at the location of welding. If there are no problems on the energy supply from the grid, the second solution is preferred, being more economical to purchase and to maintain. However if an unbalanced load on the grid is objectionable, then the motor-generator type may be better suited to the situation. The smaller power supplies are connected to one phase of the power grid, the larger ones are connected to three phase outlets. Maximum output current is the parameter governing the size of the power supply. The amount of current needed is closely related to thickness of base metal, filler size and wire feed rate, but also to welding position, joint shape and shielding gas selected. The torch, directing the weld to the joint, must be capable of bearing the maximum current. Small units are air cooled while larger torches depend on water cooling for temperature control. Different Tables give widely differing data, so that their usefulness is probably questionable. For mild steel a very rough indication can be seen in the following Table.
In practice one should establish the minimum and maximum thicknesses to be welded in a single pass, and then inquire with manufacturers which of their models would provide current ranges suitable for all sizes, from the thinnest to the thickest. If productivity is an issue, then duty cycle (the percentage of welding time vs. cooling time in any ten minutes period) is important. Manufacturers would probably encourage customers to select more versatile and expensive equipment which provides the capability of changing basic parameters. It must be stressed that for a generally fixed set of welding conditions in a repetitive job, this versatility may be useless and wasted. The more complex machines are those permitting continuous slope control, stepless inductance control, and a wide range of voltage control. Current, within the machine range, is determined by the filler wire feed rate. In more recent and less heavy inverter type power supplies the current can be shaped in pulses, usually between a maintenance current level and a peak current level, at a selected frequency: this level of control may be useful when welding thin material in that it permits reducing heat input to the minimum. However, an over-choice of parameters to set may confuse the issues to the point where good welds cannot be obtained, or may induce the welder to leave the machine settings fixed for any job. As already recommended the welder should be satisfied, before purchasing, that the equipment selected indeed corresponds to his/her needs. Gas Metal Arc Welding power supplies can be used also for Flux Cored Arc Welding, which uses the combustion products from the flux contained in the cored wire to shield the molten material, with or without the use of additional shielding gas. Wire Feed Attached to the power supply is the wire feeding unit, which can be an integral part of the welder or a separate unit: it is important to verify that it can accommodate the sizes and the materials needed, at the required feed rates, which establish the current drawn through the electrode wire. Different types of feeders exist, each one best adapted to a range of materials and sizes (push type, pull type and push-pull type) and built to suit the distance between the wire spool and the work. In Constant Feed Systems the rate of feed is selected before starting the welding operation. This is a most critical parameter that establishes the current drawn during welding, penetration and deposition rate: welding speed, either manually or mechanically controlled, should be determined in consequence. Variable speed wire-feed systems are used with constant current power supplies: depending on the volt-ampere characteristic curve of the power supply, the feed is self regulating: once the wire feed rate is set, any voltage deviation established by the arc length, results in a corresponding change in motor speed which tends to bring back the arc length and the voltage to the selected value. The GMAW torch must support the welding conditions and any of the possible parameters within the working range of the given power supply. Other accessories Shielding gas supply and cooling water control are also controlled through the power supply by instrumentation added to the unit. Conclusion The welder shopping for a power supply for GMAW is advised to learn the technique beforehand and to test thoroughly the equipment proposed before purchasing it, in order to make sure that it satisfies his/her requirements. ----------------------------------------------------------------------- 3. How to do it well: Welding thin to thick Sections Q: A thin tube has to be welded to a thick plate or bar: why is it so difficult to do so? A: The thick element absorbs a large quantity of heat before reaching melting temperature. On the contrary the thin tube melts almost immediately. Therefore to weld properly one has to change the configuration of the joint so that the difference in thickness be kept to a minimum. The bar or plate has to be machined so that at the joint location the thickness be comparable to that of the tube, or an intermediate transition element of proper shape and size must be welded between the two elements. Alternatively, if the joint shape permits it, one should consider brazing or friction welding.
----------------------------------------------------------------------- 4 - Selection of Stainless Consumables Bare stainless steel filler metals are covered in specification AWS A5.9. The selection of filler metal is based on considerations that take into account the special welding characteristics of stainless steels. These are introduced in some detail in the article on Stainless Steel Welding. Austenitic Stainless Steels Austenitic stainless steels excluding the free machining grades (303, 303(Se)) that should be considered unweldable, are more easily welded than ferritic and martensitic grades, and provide tough as welded joints. But because their coefficient of thermal expansion is about 50% greater and their thermal conductivity is one third that of carbon steel, their tendency to distortion is much greater. The processes of sensitization and of carbon precipitation after welding, with the consequence of corrosion susceptibility of these steels (and the usual methods for preventing them) have been discussed in the above page. Not always is the selection of filler metal equal or equivalent to the base metal the best choice available. Other important factors in filler selection are those which influence the tendency to microfissuring and underbead cracking. The formation of tiny hot cracks or microfissuring depends mainly on the composition of the deposited metal and on the resulting microstructure: a fully austenitic structure is more susceptible to cracking than a duplex structure, containing a certain proportion (3 - 5 %) of delta ferrite: for this reason filler metal manufacturers carefully balance the alloy content of their materials in order to promote the formation of the desired constituent. Other harmful conditions include the presence of stresses, of notches, of low ductility. Underbead cracking, concerning the heat affected zone of heavy sections after long service at elevated temperatures, is controlled mainly by reducing residual stresses. The prefix E designates electrodes and ER designates welding rods, but they are
omitted here.
A simplified list of stainless filler metal includes the following: Martensitic Stainless Steels Martensitic stainless steels harden upon welding. Special preheating procedures must be considered to prevent cracking by slowing the cooling rate. Ferritic Stainless Steels Ferritic Stainless Steels are less weldable than the austenitic grades, and present grain coarsening that reduces toughness. Precipitation Hardening Stainless Steels These steels obtain their optimum properties through a process of hardening by precipitation, following a solutioning treatment. The transformation products from elevated temperature are not hard, are not subject to cracking and do not
need preheating. Usually filler metals of comparable composition are used although austenitic fillers can be used if the development of full properties is not required. Conclusion The selection of filler metal for welding stainless steels has a remarkable influence on the outcome of the welding process. Other factors to consider are the application, the sections involved, internal and external constraints due to joint configuration, effects of heat treatments, and the suitability of the selected welding process to specific conditions and requirements.
Is this article helpful? Let us have your comments by e-mail.
Click here. ----------------------------------------------------------------------- 5 - In the press: Recent Welding Articles Here are References to articles on Welding Subjects. From AWS: ...Extending the life of
aging components ...Underwater welding in Nuclear Power Plants Notes - Unfortunately the original links are no longer supported by AWS: Readers may seek the articles from the Welding Journal, September 2003 Issue. From TWI: ...Shielding gases for arc welding and cutting From EPRI: ...New Filler metal Reduces Time, Improves Quality for dissimilar Metal Welds ----------------------------------------------------------------------- 6 - Terms and Definitions Reminder Do you remember what is... Depth of
fusion: Incomplete fusion: Root penetration: Joint penetration: Would you want other terms being presented in next issues? ----------------------------------------------------------------------- 7 - Site Updating We are glad to announce that a new page on Welding Tool Steel has been added to our Site (https://www.welding-advisers.com): it can be reached by clicking on Tool Steel Welding. The page on Cutting has been revised to include an updating on plasma cutting. This is to say that any time you go back to the Site you may find ----------------------------------------------------------------------- 8 - Work in Progress We want to pay attention to readers' comments and feedback, as they come in, in future issues of Practical Welding Letter and we will do our best to make it more helpful and interesting.
We plan to present an articles on Selection of Plasma Cutters, and to cover other processes, and new items as they come to our attention. ----------------------------------------------------------------------- 9 - Readers' Contributions We would welcome contributions of our readers with articles of general interest.
In particular, personal experiences may be interesting to a larger public. So you are invited to contribute with your thoughts and experience, by e-mail: click here. ----------------------------------------------------------------------- 10 - Explorations: beyond the Welder. We would like to recommend a few Sites of general interest that might take you over to new horizons, in unlimited Exploration trips, for discoveries, learning and pleasure. The most interesting Site of National Geographic can be browsed by clicking here.
Or see and learn on Wild Animals by clicking here. For anyone ready to learn more on how to build a business on the internet, we can heartily recommend the all-in-one Source that provides results. Click here. ----------------------------------------------------------------------- 11 - Correspondence: a few Comments A substantial number of young readers willing to undertake a career in welding wrote to us to obtain addresses and information. We welcome these inquiries and try to answer them to the best of our knowledge: we think it is important and we would like to help and to encourage them to find their way and build a rewarding career. If you know of some important hub of information useful to young people, we would be glad to hear on it and to post a note on that for the interested public. Most people address polite questions: they may not provide all the background and relevant data so that we have sometimes to guess what is the real problem, but in general we understand what one needs. Most people also thank for the answer, and this is highly appreciated. We are however set back, occasionally, from blunt impersonal questions, like: "What is...?" and from the total and complete absence of any sign of thanks. And also from unsigned messages. We can assure you that we are not going to investigate the personal identity or the preferences of our inquirers. We are not taking it too personally but we would be grateful for a small sign of consideration. We take pride in our advice and we would like to help. We are however disappointed and even frustrated if we get no reaction as to the usefulness of our comments. We are often left completely in the dark about the outcome of our efforts. A short note of follow-up would be most welcome. Keep on asking, we are all learning. ----------------------------------------------------------------------- 12 - Bulletin Board 12.1 - A few readers sent us attachments to their messages. We must again inform you all that, as a matter of policy and of self protection, we will not open attachments at all. It might be too dangerous. So you are invited to append your photo or sketch at the end of your message and avoid sending attachments.
12.2 - Occasionally our Hardness Book or our answers bounce back undelivered. We try to keep track of these instances, but we cannot assure complete attention. It seems that the ISPs of some readers were down for some time. If you do not get the answers, please ask again, it may not be our fault. We always try to give answers. 12.3 - We would like to let you know that we know and use an exceptional facility prepared to let anybody with no previous experience, build a successful Site on the Internet. We recommend it because it works. See for yourself. Click here.
-----------------------------------------------------------------------
Visit the Welding Advisers Site at ----------------------------------------------------------------------- Copyright © 2003, by Elia E. Levi and welding-advisers.com, all rights reserved
|
|||||||||||||||||||||||||||||
| Back to Back Issues Page | |||||||||||||||||||||||||||||