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PWL #023-GMA Welding Parameters,Titanium Clad Steel,Brazing Ceramic Materials,Auto UltrasonicTesting July 01, 2005 |
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| 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 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 Date: July 2005 - Practical Welding Letter - Issue No. 23
---------------------------TABLE of CONTENTS------------------------
1 - Introduction 2 - Article: How to select your GMA Welding Parameters 3 - How to do it well: Welding Titanium Clad Steel 4 - Filler Metals for Brazing Ceramic Materials 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Automated Ultrasonic Testing 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Titanium Clad Steel 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
With more than 1800 subscribers, this Practical Welding Letter has now the potential of providing a vehicle for transmitting, to quite a substantial readership, information, news, contributions, experience, opinions and anything our readers might find interesting for others to know. We start this 23rd Issue, with a collection of practical tips and data assembled from the publications of Ed Craig, an authority in GMAW, who kindly gave us permission to quote from his work. Whoever is in need to know more on various aspects of the all important GMAW process, is urged to seek information at the source, given hereafter, to understand and learn. This article deals only with the short circuit transfer mode, to be considered for thin joints. Next time we will continue with the spray transfer mode, applicable to thicker material. Brazing of ceramic materials is not an easy endeavor. We show some of the work performed and some of the main routes for obtaining successful joints. References are given for interested readers. Automated Ultrasonic Testing (AUT), as applied to welded joints in pipeline works, has the potential to speed up joint approvals and to be safer than Radiographic Inspection. It needs however more experience and maintenance than the word "Automatic" might convey, so that concerns have been voiced as to the difficulty to find suitable inspector candidates to prepare for AUT. Only recently the technology of cladding steel with titanium, advanced to enable economic supply of material in the types and quantities needed for industrial applications. We report (in sect. 3) on a technique for joining and (in sect. 11) on some more information on the production problems and their solutions, both published in a recent article we quote from. Our Page of the Month update introduces the readers to Welding Standards and to the challenge of obtaining practical consensus on sometimes contradictory issues. The other departments show their content in their usual order. We welcome your input. Click to send us Your Questions and Feedback. May we ask that you forward this copy to a few of your friends, suggesting them to subscribe if they like what they read? Thanks.
2 - Article: How to select your GMA Welding Parameters (I) Here we show some information from books, articles and videos by Ed Craig of It is understood that the parameters are to be adjusted and balanced according to the particular conditions and preferences, depending on the results obtained. It is easier to do when one knows the optimum ranges and wherefrom to start. This first part deals with thin steel sheet from 0.5 to 3.0 mm (0.020 to 0.125"). As long as applications are for material less than 1/8" (3 mm) thick flat, horizontal, vertical up or vertical down, or where there is a gap to fill (root pass in pipe welding), short circuit transfer is the mode. Power supply: Traditional Gas Metal Arc Welding (GMAW) and Flux Cored Arc Welding (FCAW) use constant voltage (CV) power supplies. Pulsed GMAW may use CV or Constant Current (CC). Generators may use a CV Adaptor. If there is a slope selector, steep slope provides a limit to current output available during short circuit, and minimum current change with arc voltage variations. If an inductance knob is available, increasing inductance for short circuit metal transfer mode limits current surge at short, improves arc stability, reduces spatter and improves weld appearance. The power cable is connected to the positive terminal on the power supply and leads to the wire feeder. Ground cable is connected to the negative side. The common current range is 100 to 400 Amps. A traditional 450 Amps CV power source provides some moderate slope. Wire feeders: A permanent magnet motor runs cooler and responds faster than a shunt motor. Gas coverage is critical as the weld stops and starts. Essential features are therefore pre- and post-weld gas flow control, and gas purge switch to eliminate air from the gun. Burn back control eliminates the need to keep trimming the wire for correct stick out length, insuring minimum spatter as the weld starts. For amperage monitoring, an additional digital feed speed readout enables the welder to obtain consistent results. An important safety feature is a cold inch feed control, enabling feeding the wire without current flowing. GMAW Guns or Torches: Selection is critical in high production shops. Maintenance time is lost if the wrong type of nozzle is used: a heavy duty type may be required. Contacts are most important. They may be short or long, but should be suitable to the application. The gun itself should be designed for the duty cycle actually employed. Of the many factors to consider in selecting a gun, we list a few hereafter. With argon mixes using current up to 300 Amps and maximum arc on time (duty cycle) of 60%, a 400 Amps air cooled gun is recommended. If the current is up to 400 Amps, even if arc on time is less then 50%, a 600 Amps air cooled gun is recommended. When welding with argon mixes and current usage above 200 Amps but arc on time greater than 60%, a water cooled gun and a heavy duty gas nozzle are recommended. Short Circuit Mode: As already explained elsewhere, Gas Metal Arc Welding (GMAW) is a unique process capable of providing two different metal transfer modes. We now cover only the short circuit mode that, at low amperage with balanced parameters, will produce a stable arc. The number of shorts will be between 80 and 120 per second, ideal for thin sheet metal welding. At this rate the arc will give a steady, typical crackling sound, constant, smooth, rapid and consistent. It is an important clue to judge the process parameters. Listen to the sound: if it is a harsh, irregular high sound, then the voltage is too low. If the sound is soft and fluffy then the voltage is too high. The faster the wire is coming out of the gun, the more current we may draw. In short circuit transfer mode we may feed the wire at 100 to 350 inches per minute (2.5 to 8.9 m/min). This will deliver a short circuit current range of 70 to 200 amps. Wire feed speed: The following is a short description of the original Ed Craig's method for setting up wire feed speed, referring to an ideal clock dial. In the classic 10 turns potentiometric switch used to regulate wire feed, the 0 point (OFF) is at the 7 o'clock position. Turning the switch for the full 10 turns, brings the dial to the 5 o'clock position that delivers about 700 inches per minute (ipm) of wire. Every turn gives therefore approximately another 70 ipm (~1.8 m/min). To check the wire feed speed at a certain switch position, one can use the following method. One lets the wire run without current for exactly 10 seconds and one measures the length delivered. The measured number of inches or meters multiplied by six, gives the length per minute at that adjustment. When first operating a wire feeder whose characteristics are not known, if it is of the rotary switch type, it is recommended to run the above test for the mid position (12 o'clock). Even if it is of the digital type it would not hurt to check at least one setting. Let us now determine a few starting points. One should note that the correspondence between speed and amperage depends on the actual voltage, so that the values given are only approximate. When using a 0.030" (0.8 mm) wire, depending on the gas mix used, a practical wire feed speed would range from 70 to 420 ipm (1.8 to 10.7 m/min) that corresponds to 30 to 170 Amps. The optimum range however would be only 140 to 280 ipm (3.8 to 7.1 m/min) corresponding to 70 to 130 Amps. Therefore it is recommended to consider the 10 o'clock position, indicating 210 ipm (5.3 m/min) and 110 Amps, 17 V as a useful starting point. When using a 0.035" (0.9 mm) wire, speed could range again between 70 to 420 ipm (1.8 to 10.7 m/min) that now corresponds to 40 to 200 Amps. Optimum range would span between 140 to 280 ipm (3.8 to 7.1 m/min) corresponding to 100 to 170 Amps. Again considering the starting position at 10 o'clock would mean selecting 210 ipm (5.3 m/min) and 140 Amps, 17 V. For welding steel or stainless with welding wire of .035" (0.9 mm), 0.040" (1.0 mm) or 0.045" (1.1 mm), the optimum starting point would always be the 10 o'clock position, corresponding to about 210 ipm (5.3 m/min) (= 3 turns per 70 inches per minute per turn). If one needs more or less heat or weld one can increase or decrease the wire feed by one turn at a time (70 ipm ~ 1.8 m/min) and evaluate the results. It may be advisable to adjust at the same time also the voltage by increasing or decreasing 1 Volt at a time in correspondence with the number of turns changed in wire feed. As an example, one turn increase, with a 0.035" (0.9 mm) wire, will result in approximately 25 to 40 more Amps, depending on the voltage. Voltage: The next thing to adjust is Voltage. For short circuit transfer mode the voltage range is from 13 to 23 V. As the optimum range is between 16 to 18 V, one can start at 17 V. Strike an arc to read the meter and adjust the voltage as needed. The purpose is to obtain a spatter free weld with one simple setting. Excess voltage is the main cause of spatter in short circuit transfer. A typical 1/16" (1.6 mm) short circuit application, can be done with 0.035" (0.9 mm) wire, at 17-18 V, wire feed 10-11 o'clock (210-280 in/min ~ 5.3-7.1 m/min). Wire Stick Out: For short circuit welding, when welding with less than 150 amps, keep the contact end out of the nozzle about 1/16"-1/8" (1.6-3.2 mm). This allows a short wire stick out 1/4"-3/8" (6.4-9.6 mm) with low voltage, 13-15 V, ideal for low current application. When welding in the range 150-200 amps, recess the contact tip inside the nozzle about 1/8" (3.2 mm) and try to maintain about 3/8"-1/2" (9.6-13 mm) wire stick out (from the contact tip) that allows the use of 17-18 V. The goal in short circuit mode welding is to use the lowest possible voltage. Wire selection for Short Circuit: When welding only material less than 0.045" (1.2 mm) thick, use 0.030" (0.8 mm) wire. When welding sheet metal between 0.045" and 0.075" (1.2-1.9 mm) thick, use 0.035" (0.9 mm) wire. When welding sheet metal between 0.075" and 1/8" (0.125") (1.9-3.2 mm) thick, use 0.045" (1.2 mm) wire. With straight CO2 gas use high silicon E70S-6 wire. When welding plain, coated or galvanized steel use E70S-3. We may round up the basic starting parameters including an indicative theoretical Deposition Rate in the following Table 1. Note that the actual Deposition Rate is reduced by multiplying the tabulated one for the actual arc on time (duty cycle) percentage.
Gas selection: Material thickness, joint type and welding position are factors to be considered when selecting the gas. The thinner the sheet, the less heat is required. On metals 0.035"-0.040" (0.9-1.0 mm) thick or less, or when welding along an edge, one should consider 98% argon with 2% oxygen. This mix allows the use of lower voltage, 12 - 14 V. Reducing the proportion of reactive gas reduces the risk of burning through and minimizes damage to coatings on the outside surfaces, making welding easier. If welding material less than 0.060" (1.6 mm) thick, it is wise to use an argon mix which does not have more than 10% of reactive gas (CO2). For about 0.035"- 1/16" (0.9-1.6 mm), and if one never welds above 1/16" (1.6 mm) it would be appropriate to use 5 to 15% CO2, best 8 to 10% CO2 in the argon mix or three gas mixes. CO2 shielding gas produces a digging effect which reduces arc stability and generates spatter, displays a hotter arc than argon mixes, and increases burn through potential. The volt-ampere range is narrow and difficult to adjust to obtain optimum arc stability. With CO2, spatter increases with current, and more surface oxides must be removed before painting. For 0.080"-1/8" (2.0-3.2 mm) thick steel where increased heat is required, a larger proportion of reactive gas provides increased energy, higher penetration and additional weld fluidity. The standard argon mix includes 75% or 80% argon and 25% or 20% CO2 that would need 16-17 V to keep a stable arc, fine for an application of about 3 mm. The most practical all purpose gas mix for bulk supply is argon with 15-20% CO2. We plan to present an article on GMAW Spray mode parameter selection in the next issue of PWL.
3 - How to do it well: Welding Titanium Clad Steel Q: How is Titanium Clad Steel welded on itself? A: From the Article reported in Section 11 hereafter we quote: It has even been possible to make a welded joint by stripping back a minimum amount of titanium, fusion welding the steel substrate, covering the exposed steel with a weld deposited layer of silver and weld depositing titanium onto the silver. This gives a continuous and smooth joint profile on the titanium surface and an unbroken, continuous bond at the cladder/substrate interface."
4 - Filler Metals for Brazing of Ceramic Materials One must stress that Brazing of Ceramic Material is still an art that must be learned by trial and error with much dedication. One cannot just pick up a proven technology and apply it. One must develop the process from the start. While ceramic materials exist from the dawn of civilization and are admired as works of art, it is only in the last decades that structural or engineering ceramic materials were developed to perform in demanding applications. In fact the high hopes that were once held, only a few tens of years ago, of easy and fast transition to primary ceramic elements in internal combustion engines and in gas turbines, were premature and did not yet materialize. The essential drive for this search is the need to improve efficiency and increase component life. But much development work must still be invested to achieve some slow progress. The main characteristics that need improvement in structural ceramics are strength and fracture toughness. Ceramic materials are either monolithic or ceramic-matrix composites. Many families of materials are known, among them most developed are Alumina (Al2O3), Silicon Carbide (SiC) and Silicon Nitride (Si3N4). Wide differences in properties are found in ceramics from the same families, depending on the material of origin and on the manufacturing processes used. With material development, also the need was felt for suitable joining processes either to ceramics or to metals. Wetting and adherence are the main requirements for brazing, but this is exactly where the common brazing materials fail. Another problem derives from the substantial difference in the Coefficient of Thermal Expansion (CTE) between ceramics and metals (brazing filler). A few methods were developed to allow brazing of ceramics. One method consists in coating first the ceramic with a metal to be deposited by one of a series of methods (sputtering, vapor plating, thermal decomposition), and then brazing with regular filler wires. Another method is the molybdenum-manganese (Mo-Mn) process in which a slurry of mixed powders of these metals is painted on the ceramic, and then is fired in a furnace under a special hydrogen atmosphere. Glassy materials from the ceramic migrate to the surface where they bond the metallic powders to the ceramic surface. Brazing of the treated surface becomes possible. Manufacturers may offer their own active brazing filler metals, mostly patent protected, where special alloying additions (of titanium, hafnium, zirconium) promote wetting and adhesion onto untreated ceramics.
See for example: On this subject you may find a short overview by TWI, in the second Online Press reference, section 5 below. A very comprehensive report on this subject, with a wealth of References, is available at pages 948-960 of Vol.6 of the ASM HANDBOOK (1993).
5 - Online Press: recent Welding related Articles As already announced our Article on Fatigue Failures is visible at From TWI: Automation of fracture and fatigue assessment From AWS: From ASNT:
6 - Terms and Definitions Reminder Accelerated testing is performed on materials or assemblies to produce failures caused by the same mechanism as expected in field operation but in significantly shorter time by changing one or more of the controlling test parameters. May be used to rate different procedures. An artifact is a feature of artificial character, such as a scratch or a speck of dust on a metallographic specimen or some extraneous marking on an image or radiographic film, that can be erroneously interpreted as a real feature. Bauschinger effect is the phenomenon by which plastic deformation increases yield strength in the direction of plastic flow and decreases it in other directions. Burnishing is used for finish sizing and smooth finishing of machined or ground surfaces by displacement and compression, rather than removal, of minute surface irregularities with fixed or rotating tools. Fusion, as used in welding, is intended as the melting together of base metal surfaces, with or without filler metal, which results in coalescence. Any welding process that uses fusion of the base metal to make the weld is called fusion welding. Sputtering is a vacuum process used to deposit thin films on substrates. It is performed by applying a high voltage across a low-pressure gas producing the bombardment of a solid surface with a flux of energetic particles (ions) that results in the ejection of atoms for deposition where required. Transient Liquid Phase (TLP) bonding has the ability to produce a bond at a lower temperature than that at which it will be ultimately used. It is a process that employs a filler metal that produces a temporary (transient) liquid layer at the interface to be joined. Solidification occurs by diffusion at constant temperature. It is employed (among other uses) for joining Aluminum base Metal Matrix Composites, because the lower processing temperature is less harmful to properties. Wetting is a necessary condition for brazing, meaning the spreading, and sometimes absorption, of a fluid on or into a surface. It represents a condition in which the interface tension between a liquid and a solid is such that the contact angle is minimal. It allows a liquid filler metal or flux spread and adhere in a thin continuous layer on a solid base metal. Lack of wetting precludes successful brazing.
7 - Article: Automated Ultrasonic Testing for Girth Welds Automated ultrasonic testing (AUT) has been used for years in the inspection of critical forged blanks for the manufacture of rotating elements of turbine engines and other items. The systems, under comprehensive computer control, usually include a tank of water where the parts are immersed for inspection, precisely controlled linear movements of the transducers relative to the rotating part, so that the whole volume can be scanned. The ultrasonic instrument itself provides the electronic excitation of the transducers and the elaboration of the echo signals returned, for evaluation of discontinuities within the material. Various kinds of automatic signal processing and inspection permit record and identification of questionable signals, that must be further evaluated by certified inspectors. Testing sessions include the recording of all the test procedure for documentation. In the last few years much development work has been conducted to provide automatic ultrasonic testing capability to girth welding of pipes in the field. The drive for this movement away from the more traditional radiographic inspection is given by the potential gain in speed of testing and by the unmatched capability of sizing weld flaws. Ultrasonic testing (UT) is considered therefore a reliable and beneficial alternative to Radiographic Testing (RT) and is preferred for health and safety reasons as it does not make use of ionizing (harmful) radiation. Although already applied with success to a substantial list of pipeline construction projects, automated ultrasonic testing (AUT), which is becoming increasingly accepted, still lacks universally approved guidelines and standard procedures. Before being able to push the START button, the AUT inspectors have to set up complex and always new configuration layouts. They need to be creative and resourceful for imagining and implementing new ways around problems, because automated ultrasonic equipment is made up of many complicated components, which have to work together to produce useful data. Therefore at this stage of development the most important element is still the operator in automated ultrasonic systems because these require much maintenance and constant minor modifications. He/she should have a good background and experience of classic ultrasonic inspection but on top of that the willingness to learn, to improve, to try and to implement and adapt the knowledge to the work at hand. Flexibility, adaptability and a willingness to work long hours in the field in less than ideal working conditions are possible problems in the quest for suitable candidates. 8 - Site Updating The new Page of this Month concerns the subject of Welding Standards. Although the term seems self-explicatory, there are maybe some misunderstandings because documents written for different purposes may be described under the same name. Much effort is being made to promote International Standards, to ease and simplify global commerce and exchange. There are however many difficulties stemming from language, traditions and habits. Most of the documents are voluntary. Therefore a high measure of consensus is required before a draft is finally released. Different voluntary professional groups proposed in time quite a large number of Standards and currently continue to do so with revisions and new titles. Official agencies adopted certain documents to establish regulatory requirements. Other codes may be required by contract, or may be a precondition required by Insurance Companies in order to agree to take responsibility. On the same subject, one interesting contribution on Global Standards, is reported in the third Online Press announcement, section 5 above. Besides that, the whole Website is undergoing a continuous work of updating, correction of minor flaws and additions, on most of the pages. If you find anything needing revision or explanation, you are welcome to let us know. Click to send us Your Questions and Feedback. To read the new page click on Welding Standards. To review old and new subjects see the Site Map.
9 - Short Items 9.1 - Abradable Coatings applied by thermal spray are designed for gas path clearance control in gas turbine engines. Minimum clearance improves efficiency. Clearance is best established automatically by normal operation between moving and static elements. The actual seal surface is easily conformed by abrasion, while the rotating parts are not damaged. Several material compositions are available, suitable to the planned service temperature. 9.2 - Ceramic cores are used in the manufacturing of hollow investment cast parts to produce internal passages for air cooling and other purposes. The cores are designed to remain embedded in the metal parts after casting, and to be leached away in an autoclave process before finishing the cast parts. 9.3 - A pulsed Laser pyrometer that uses a patented technique, permits determining the target's emissivity of the heat radiating surface at the same time, location and wavelength that the infrared temperature measurement is being taken. The exact determination of emissivity is what allows precise and repeatable temperature measuring. Emissivity is calculated by measuring the fraction of output energy of the laser reflected by the surface. An article on this subject was published on page 28 of the May/June 2005 issue of Heat Treating Progress, a publication from ASM International. 9.4 - The operation of a Scanning Tunneling Microscope (STM) is based on the so-called tunneling current, which starts to flow when a sharp tip, formed by one single atom, approaches a conducting surface and slowly scans it across at a distance of only an atom's diameter. The tip is mounted on a piezoelectric tube, which allows tiny movements by applying a voltage at its electrodes. The electronics control the tip position in such a way that the tunneling current and the tip to surface distance is kept constant. This movement is recorded and can be displayed as an image of the surface topography. A profile of the surface is created, and from that a computer-generated contour map of the surface is produced. Under ideal circumstances, the individual atoms of a surface can be resolved and displayed. 9.5 - Sulfidation is a form of corrosion degrading the properties of items subjected to it at elevated temperature. The hot reaction of a metal or alloy with a sulfur-containing species produces a harmful sulfur compound on or beneath the surface of metals or alloys. The problem is quite serious for nickel base blades of gas turbine engines. The usual prevention is by means of a protective layer applied by a pack cementation and diffusion process containing chromium, aluminum and other elements. 9.6 - TWI developed a Crack Sensor, a device that measures the amount of fatigue suffered by a welded structure during service. The device comprises a steel shim that cracks at a rate that can be related to the fatigue damage being suffered by the structure. The associated electronics helps monitoring the crack progress in the probe. Designed to give advanced notice of potentially catastrophic fatigue failures, the device alerts maintenance personnel of structures and enables estimation of their residual life. See
10 - Explorations: beyond the Welder Declarable Substances The Mysteries of Mass The Trial of Galileo Mars rover escapes from sand dune Approaching Mars
11 - Contribution: Titanium Clad Steel A Report titled "A new titanium and zirconium clad plate: a breakthrough in cladding technology" has been recently published at page 114 in the issue 3/2005 of Welding and Cutting, the official journal of the European Welding Institutes (DVS, TWI and IS). Interested readers are urged to seek the original article. We summarize hereafter the main points. Roll-bonding is not successful for cladding titanium to steel. The only alternative, within strict limitations of thickness and sizes has been the application of explosion welding for further rolling. Even then the production of a brittle Ti/Fe intermetallic compound precludes rolling at over 750 0C. The new technology achieves the result of overcoming the inherent difficulties by producing first an intermediate slab. A critical step is the introduction of an interlayer niobium sheet between titanium and steel before the explosion weld. This slab can then be heated for rolling at temperatures higher than 1250 0C without disbonding. However there is a limit to the thicknesses of base and cladding materials that can be successfully explosive welded together. These dimensions can now be optimized for the process without worrying about the final thicknesses requirements. Roll bonding presents its own set of limitations relative to the final rolled product obtainable. The successfully explosion welded slab is now rolled a first time to obtain an extended area of reduced composite thickness. The next roll-bonding operation uses the intermediate product, now cut to proper size, as a transition interlayer element between prepared titanium and steel surfaces of selected thicknesses to obtain, after rolling, the final dimensions required. In this operation titanium is roll bonded to titanium on one side of the transition slab, and steel is roll bonded to steel on the other side. It is reported that the clad plates can be welded as required for fabricating vessels and that the technology can be extended to produce clad seamless pipes.
12 - Testimonials From: Biren 'biren@godrej.com' Dear Elia, Biren Desai
From: sridhar c 'advance_welding@yahoo.co.in' Thanks for sending such informative and easy to Regards, Sridhar.
13 - Correspondence: a few Comments With the full implementation of the new policy of e-mail management, on the part of our Internet Host, SiteSell, we were very much relieved to see almost complete elimination of spam, the unsolicited and unwanted junk mail. The principle is based on the slow build up of a "Whitelist" of authorized and recognized correspondents. The required step on the part of a new correspondent, is to write us using our special purpose Form. Click to send us Your Questions and Feedback. This is the best, recommended and guaranteed way to correspond with us and to become included in the Whitelist. Some bona-fide readers may still have in their records our old e-mail address. They should be warned that, if they use it, they might get their message bounced back. They should pay attention to instruction. They should just use the Form above to get through. Once they are included in the Whitelist they may just continue writing as they please.
14 - Bulletin Board 14.1 - ISEC 2005 - 4th International Surface Engineering Congress & Exhibition and 14.2 - 2005 ASM Heat Treating Society Conference & Exposition 14.3 - The Standard 14.4 - Our trusted Web Host, Sitesell, has released a new page you might be interested to visit. It is an invitation to consider starting a new Work from Home. Not a scam. Not a Get Rich Quick project. Real work with real results. If not now, maybe later. If not you, maybe someone near to you.
We are convinced that they are right. And they give full support as needed.
See you next time Copyright (c) 2005, by Elia E. Levi and welding-advisers.com, all rights reserved
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