| Back to Back Issues Page | |
 |
|
|
PWL #030-GMAW Extensions, Cold Weather Welding, Braze Welding Filler, Damage Tolerance, Mig Welding February 01, 2006 |
|
| We hope you will find this Letter interesting and useful. Let us know what you think of it. GMAW Extensions, Cold Weather Welding, Braze Welding Filler, Damage Tolerance, Mig Welding Tips, Fatigue Monitoring 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: February 2006 - Practical Welding Letter - Issue No. 30
1 - Introduction 2 - Article: Process Extensions of GMAW 3 - How to do it well: Welding in cold Weather 4 - Filler Metal for Braze-Welding 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Damage Tolerance 8 - Site Updating: Mig Welding Tips 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Fatigue Monitoring 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction Welcome to this 30th issue of Practical Welding Letter. Looking back it seems that it was quite a long journey. We covered a long list of subjects which are still available and searchable to anybody interested in them. May we suggest that whenever looking for a specific subject, you might browse the titles in our site page reachable by clicking on Welding Topics (opens new page). We present in the first following article a few Process Extensions that were added to the basic Gas Metal Arc Welding to address special requirements for specific applications in a cost effective way. Next, answering a question from one of our correspondents, we provide guidelines for the problem of welding in Cold Weather. We found and report differences in severity, and it may be that other sources or Codes prescribe alternative requirements. For the Filler Metal department we show a list of copper base materials used for Braze Welding. Manufacturers may offer proprietary materials not yet standardized in official documents. The next article deals with Damage Tolerance, a design concept adopted in modern practice to provide a measure of the remaining useful life of any structure which may be endangered by slowly progressing hidden destructive attacks. It is important, we believe, to have an idea of what it means. We report hereafter in section 8 on our new Site page dealing with Mig Welding Tips, and we offer to update it following our readers' suggestions if useful additions will be proposed to our attention. A new fatigue monitoring device is introduced in the Contributions department 11, unfortunately without essential details, unavailable, that would extend our understanding. Interested readers should contact the source. The regular departments appear as usual for information. Our readers are invited to point our attention to subjects that they may need to investigate. Click to send us Your Questions and Feedback (opens new page). Would you be so kind to forward this publication to those of your friends and colleagues that may enjoy it? Thanks for trying.
2 - Article: Process Extensions of GMAW The Gas Metal Arc Welding (GMAW) process is quite popular because it offers users, as widely known, a large measure of versatility or adaptation to most different welding situations. As shown hereafter in section 8, a new page on this subject, called Mig Welding Tips, was added recently to our website. As it happens, three of the welding articles from the latest issue of the Welding Journal, related hereafter in section 5, deal also with this process. Additional applications that cannot be accommodated only by parameter selection, require somewhat modified equipment and techniques to achieve the results sought for. A short review of some of these process variations is presented hereafter. 2.1 - Gas Metal Arc Spot Welding This result can be obtained by applying some pressure to keep the elements in contact, on top of the upper part, with a modified GMAW torch, whose nozzle presents escape ports for excess gas, and triggering the arc. Melting will affect upper material, electrode and a portion of the bottom element, leaving behind a solidified GMA spot weld connecting the elements as required. If the upper element is quite thick, a hole may be drilled in it. The weld filling the hole and melting the surface of the bottom element will be called more properly a plug weld. 2.2 - Narrow Gap Gas Metal Arc Welding Precautions are used to assure that the electrode tip is positioned correctly. Oscillating or weaving the electrode by proper mechanical means moves the arc across the welding. Another technique involves a bent contact tip that induces in the electrode a sharp angle towards the faces. By twisting alternatively the contact tip to right and left, the arc is given a weaving movement. A different technique forms in the electrode a wave that persists past the contact tip. Feeding the waved electrode straight down in the gap oscillates the arc from one side to the other. A two intertwined wires twist electrode has the tip of each wire rotated as it is consumed, without additional weaving provisions. Finally for use in flat position only and for limited total thickness, a larger size electrode has been used, with contact tip held above the plate surface. A related process, "Narrow Gap Electroslag Welding for Bridges", that may have applications similar to those presented above can be found at Note that the main differences are that this process (NG-ESW) is done in vertical position, from bottom to top under a sheath of molten flux, while NG-GMAW is applied in flat position under gas shield. 2.3 - Multiple Wire Gas Metal Arc Welding When first introduced, the systems suffered from arc interactions. Later on improvements largely solved the problems. Travel speeds and weld deposition rates are remarkably increased. 2.4 - Laser assisted (Hybrid) Gas Metal Arc Welding 2.5 - Gas Metal Arc Braze Welding In the process variant presented here, a copper based electrode, like aluminum bronze or silicon bronze, is used. The main advantage is that joining is performed with less heat than that needed for welding. There is a color mismatch between filler and base, if used on steel. See further down in section 4 a note on filler metals for Braze Welding.
3 - How to do it well: Welding in cold Weather Q: What is the coldest ambient temperature allowed for welding carbon steel and stainless steel pipes and structure? A: Whenever ambient temperature causes water vapor condensation upon metals, it is recommended good practice to preheat before welding up to 120 C to make sure the joints are dry. A few authorities, specifying the conditions for welding of Bridges and similar Structures, put the coldest ambient temperature limit, below which welding is not allowed, at 0 0F or -18 0C. Other authorities, dealing with welding requirements for Piping and Pressure Vessels, put the coldest ambient temperature limit, below which welding is not allowed, at 32 0F or 0 0C. For ASME Codes and Standards, the minimum temperature for welding is generally specified at 50 0F or 10 0C. Minimum temperature and preheat requirements for welding on pressure retaining items are also referenced in the National Board Inspection Code (2004 Edition), Appendix B.
4 - Filler Metal for Braze-Welding Braze Welding is a process used for joining cast iron or steel whenever there is a need not to melt the base metal. The oxyacetylene flame is commonly employed with suitable fluxes and filler metals. See Braze Welding (opens new page). Fluxes are alkali fluorides and borax mixtures. They clean casting base metal and filler metal over its melting range. They may contain oxides that combine with graphite to improve wetting of cast iron. Fluxes are applied by dipping into the flux the heated filler metal, that is withdrawn covered with a flux layer, then moved to the joint. AWS A5.31-92(R2003) Copper alloy filler metal rods and electrodes are used, mainly variants of basic alloy 60Cu-40Zn, with added Tin, Iron, Manganese, Silicon and Nickel to improve flow and wetting properties, to lower melting temperature, to decrease volatilization of Zinc, to react with oxygen and to increase strength and hardness. AWS A5.27:85 - Copper & Copper Alloy Rods for Oxyfuel Gas Welding Note: This document, although not canceled officially, is no more included in the last (2004) AWS Catalogue. The main filler metal alloys are as follows:
See also : ANSI/AWS A5.7-84(R1991) ANSI/AWS A5.8/A5.8M:2004 ISO 3677:1992 BS EN 13347:2002
5 - Online Press: recent Welding related Articles From AWS Welding Journal, February 2006, (needs Adobe Acrobat Reader) Wire Feeding in GMAW Troubleshooting GMAW Note: Interested readers, non AWS members, not able to download the above, should look for the Magazine in a public Library or contact AWS to obtain a copy. From The Fabricator: Optimizing CO2 Laser Use: Part I
6 - Terms and Definitions Reminder Arc Force is the axial force developed by an arc plasma, responsible for the axial propulsion of molten metal drops in the direction pointed by the electrode. Butt Joint is a type of joint between abutting surfaces of two sheetmetal or plate members lying approximately in the same plane. Contact Tube is the device making a sliding electrical contact with a continuous electrode to provide energy for initiating and maintaining the arc. Deposition Efficiency in arc welding is the ratio of the deposited metal weight to that of the filler metal employed, expressed in percentage. Fillet Weld Size is the leg length (or lengths, if unequal) of the largest right triangle that can be inscribed within the fillet weld cross section. Procedure Qualification Record (PQR) is a document providing the actual welding parameters used to produce an acceptable test weld, and the Test Report showing the results of the tests performed to qualify a welding procedure specification (WPS). Root Opening is the gap or separation at the joint root between components to be welded together. Tack Welds are temporary welds of limited extent, intended to hold the elements to be welded in proper position and alignment until final welds are performed. In critical weldments, tack welds must be blended smoothly in the final weld without flaws, or else must be removed before final welding.
7 - Article: Damage Tolerance It might be objected that the subject proposed for this article is too remote from the interests of the average person involved in welding to the point of being off topic. We feel however that welding can be used to erect structures, which, although appearing strong and safe when new, happen to have a limited useful life. It may be important therefore to understand that the old principle of applying a generous safety factor as a necessary ingredient of design cannot be justified any more to guarantee the continuing safe operation of structures. Even standard fatigue analysis used up to a few decades ago, failed to consider the consequences of flaws either present or introduced while manufacturing, and the effects of their growth under variable loads. Damage Tolerance is a design concept developed primarily for aging aircraft structures, but later applied to quite different ones, like bridges and ships and finally to any construction subject to variable loading conditions. The criteria that originated for assessing residual useful life of aluminum assemblies were later successfully applied also to composite materials. While those that apply the said concept have a clear understanding of the object of their study, it appears that it is quite difficult to express a complete definition of Damage Tolerance accepted by all involved. One proposal describes Damage Tolerance as the ability of a constructional element or assembly to resist failure, that is to maintain adequate residual strength in a damaged condition, in the presence of cracks, for a specified usage period. Structures are designed so that they can tolerate the presence of cracks as long as they develop slowly. Designing for damage tolerance requires selecting materials that are inherently tough, damage resistant, finding out causes and types of damage, and learning how damage propagates. The designed structures must be able to operate even with a certain level of damage. Designers evaluate the amount of time required for a crack to grow from a size so small to be undetectable, to the point when the critical length is reached. At that stage cracks would proceed to failure by sudden catastrophic growth. The purpose is to permit detection by non destructive techniques. This is easier said than performed, the actual behavior depending on design, on loading conditions and on physical characteristic of materials. As it results impossible, at least at the present time, to predict the useful or the residual life of any structure with an acceptable degree of confidence, even when it was designed by applying Damage Tolerance criteria, the common practice is to establish time intervals for periodic inspection. Lately Fatigue Monitoring devices are being proposed for the purpose to ring an alarm bell before it is too late. See following section 11.
8 - Site Updating: Mig Welding Tips The Page of this Month brings a collection of essential information for exploiting successfully the popular Gas Metal Arc Welding process. It also present links to articles published in this monthly newsletter, proposing data for correct implementation of useful operating parameters. If our readers show interest and involvement, we would be glad to review the page periodically to provide additional insight and useful hints. To reach the page click on Mig Welding Tips (opens new page). To review all titles, old and new, see the Site Map (opens new page). Let us have your comments and requests by sending us Your Questions and Feedback (opens new page).
9 - Short Items Covalent Bonds result from the sharing of one or more pairs of bonding electrons between two atoms, producing a force holding molecules together. In Finite Element Modeling, a structure, as represented by a drawing, is divided into a large number of smaller parts, or "elements" by a procedure called mesh generation. Forces and displacements applied externally are distributed along and across the structure and sustained collectively by the elements. The structural problem is transformed into a finite set of equations that can be solved using a computer. The model, having specified element geometry, sizes and material properties, is then analyzed for stress and strain distribution. An example of a computer analysis performed on a complex welded structure is described in an Article on page 42 of the December 2005 issue of the Welding Journal. Hot Taps are branch connections or repair sleeves welded onto operating pipelines without interrupting service. If performed with proper consideration of all factors involved, including safety, it can be a very cost-effective procedure as it does not interferes with operation. The consideration of repair would usually present itself upon detection of a corroded or otherwise damaged area. Not always repairs are needed, as welding might worsen the situation. Hydrogen cracking danger must be addressed and taken into account. Welding procedures must be developed and qualified by separate testing also to prove to welders the essential safety of such an approach. An article on this subject, including rules of thumb and practical advice, was published on page 40 of the November 2005 issue of the Welding Journal. Nanotechnology is a term used to describe many types of research which exploit phenomena and structures that can only occur at the nanometer scale, where the characteristic dimensions are less than about 1,000 nanometers or 1 micrometer (1 nanometer = 1x10-9 meter). The properties of materials become markedly different when their size approaches that of a few hundreds or tens of atoms. Nanotechnology requires understanding and control of matter at dimensions of atoms and molecules, to manipulate them individually and place them in a pattern to produce a desired structure. For Nanotechnology in Europe, visit Physical Metallurgy is the science and technology dealing with the properties of metals and alloys, and of the effects of composition, microstructure, processing, and environment on their physical and mechanical characteristics. Upsetting is the cold or hot working of metal, so that the cross-sectional area of a portion or all of the stock is increased. It may be the first stage of a forging process. In controlled processes the new diameter may be specified to be about three times the original one.
10 - Explorations: beyond the Welder Stardust Climate and Storms Break Records in 2005 Damage Detectives Inspect Shuttle on Orbit What Is the Universe Made Of? How Far Can We Push Chemical Self-Assembly?
11 - Contribution: Fatigue Monitoring Readers may recall that in our PWL Issue 21 of May 2005 we announced a TWI development, a new sensor system designed to assist in structural health monitoring. Recently ASM International published in its e-newsletter a note on a different system designed to perform a similar task. Electrochemical fatigue sensor array detects active cracks An electrochemical fatigue system (EFS) has reportedly been designed to detect cracks in bridges by Material Technologies Inc., Los Angeles, Calif. The EFS system consists of a differential EFS sensor array, data collection hardware and software, and data interpretation software. The differential EFS sensor array consists of two individual EFS sensors, the measurement sensor and the reference sensor. During inspection, the measurement sensor (which is customizable in terms of size, shape, and three-dimensional geometry) is attached over the area to be inspected. The reference sensor is located near the measurement sensor, but in an area where the presence of an active crack is not likely. Comparison of the two sensor outputs is the basis for determining if active cracks are present at the measurement sensor location. The EFS has demonstrated its ability to find smaller active, growing fatigue cracks in bridges more effectively than any other nondestructive test method. It was utilized in preliminary tests last year in Ohio, California, Pennsylvania, and New York on six highway bridge structures. For more information: Robert M. Bernstein, Material Technologies Inc., 11661 San Vicente Blvd., Suite 707, Los Angeles. CA 90049; tel: 310/208-5589; fax: 310/473-3177; Web site: www.matech.com
12 - Testimonials From: Santonu Ghosh 'santonu.ghosh@gmail.com' Hi Elia, Thanks a lot for the prompt reply, this information certainly would help in my experiments. Actually I hit your web page through Google. Wish you a prosperous new year. With regards Santonu
From: "Lui, Kilun" 'Kilun.Lui@kocheng.com'" Elia: Thanks for your help. I appreciate your prompt response very much. Kilun
13 - Correspondence: a few Comments Reviewing past Correspondence, I find names of correspondents that asked already in the past: it is nice for me to be remembered as a possible source of answers, as I always try to provide information and links to valuable websites or publications. Sometimes the question is too general to be answered shortly so that a short response is out of question, a thorough treatment can be found in a Handbook or Encyclopedia. It also means that the inquirer has no idea of what he/she is asking. What I would really appreciate, and I almost never obtain, is a short note confirming that the advice, once applied, was found useful and solved the problem. Unfortunately it is apparently asking too much. So, if you really want to be nice, let me know if my help was instrumental for you to succeed.
14 - Bulletin Board 14.1 - The Argentinean Institute of Siderurgy (IAS), 14.2 - Shanghai Welding and Cutting Fair 2006 14.3 - Follow the lead of our admired and esteemed Site Host, SiteSell.
See you next time... Copyright (c) 2006 by Elia E. Levi and welding-advisers.com,
|
|
| Back to Back Issues Page |