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PRACTICAL WELDING LETTER, Issue #005 -- A Gift and a Bonus, Your Inverter, Hardfacing Materials December 31, 2003 |
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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 -----------------------------------------------------------------------
You are urged to pass-along this publication to your ----------------------------------------------------------------------- Date: January 2004 - Practical Welding Letter - Issue No. 05
------------------------TABLE of CONTENTS---------------------------
==> 1 - Introduction ==> 2 - Article: Your Inverter Power Supply ==> 3 - How to do it well: Fillet welding of rimmed steels ==> 4 - Filler Metal: Selection of Hardfacing Alloys ==> 5 - In the press: Recent Welding and related Articles ==> 6 - Terms and Definitions Reminder ==> 7 - Article: Receiving Inspection ==> 8 - Site Updating ==> 9 - Short Items ==>10 - Explorations: beyond the Welder ==>11 - Correspondence: a few Comments ==>12 - Bulletin Board ==>Unsubscribe link ---------------------------------------------------------------------------
1 - Introduction This Issue of Practical Welding Letter is now addressed to an audience of almost 500 Readers: it is a motive of pride to see subscriptions grow steadily from week to week, with no advertisement other that the invitations appearing in the Welding Advisers Site and our Readers spreading the word. Although we have no way of monitoring the diffusion efforts done by our Readers towards their friends and colleagues, we invite you to forward this very page to anybody you think might be interested in the information included. Thank you. We are always looking for new ways of providing useful and timely knowledge. We are helped, if you wish to collaborate, by your feedback but also by your questions, that cover a large and increasing range of subjects. We can assure you that we will study every feedback/request and that we will do our best to satisfy your curiosity and needs. In the present issue of Practical Welding Letter we introduce an Article on Inverters, to explain their most basic principles of operation and the rewarding fields of their use, while warning, as usual, against the incorrect perception that they might be the solution to any problem. We show a possibly overlooked aspect of fillet welding, indicating that it may be used to one's advantage with a certain class of steels. Selecting Filler Metals for Hardfacing may be difficult and confusing because every application has its own requirements and constraints. The information provided should help in asking the right questions, the first necessary step in trying to obtain significant answers. Related to this subject are Terms and Definitions presented in this issue to clarify the very conditions that Hardfacing is called to withstand. An Article on Receiving Inspection may provide some thoughts about a Quality activity that may seem superfluous, except that, if overlooked, it may be costly and painful when something goes wrong. The usual Sections, of course, are where you expect to find them... And for the Gift and Bonus we promised, when you finish reading this newsletter, find them in the Bulletin Board, which is Section 12 down towards the end of the publication... Your Feedback is always welcomed. Use our simple Form 1. Click here. ----------------------------------------------------------------------- 2 - Article: Your Inverter Power Supply Welding Power Supplies based on Inverter Technology were made possible by using Power switching Semiconductor devices called IGBT (Insulated Gate Bipolar Transistors, that are are voltage-controlled power transistors). The drive of the development was provided by the fundamental principles of transformer construction that permit to reduce the bulk of iron core and copper windings in direct proportion to the frequency of the primary voltage: the higher the frequency, the higher the possible reduction of the mass of the transformer for the same output. Therefore while a welding transformer of a given power for input frequency of 50 or 60 Hz is bulky and heavy, a welding transformer for input frequency of 20 to 100 kHz (kilo Hertz), for the same power, is small and light. In welding power supplies based on inverter technology, the input alternating voltage and current from the grid (at 50 or 60 Hz) are rectified to direct voltage and current. The rectified input is then passed through a power inverter that, by switching the circuit rapidly on and off, produces high frequency voltage and current of the order of thousands of Hz, typically 20 to 100 kHz. The high frequency voltage is reduced by a step down transformer to a value suitable for welding applications, while the corresponding current available is proportionally increased. In DC power supplies the current is again rectified and filtered to a smooth and constant output. The main advantage of inverters is their portability, because of their much reduced bulk and weight. Therefore they are ideal for frequently moved field work and for repair and maintenance. Another advantage derives from their energy consumption which is less than for regular transformers. The primary circuit of a transformer is always connected, even when not welding, so that a part of the energy drawn while idling goes into wasted non recoverable heat. In inverters energy is consumed only when actually welding. There may be some disagreement about the effective energy savings that can be realized but they are certainly more important for continuous operations. Older inverters supplied only DC, but later new functions and features were incorporated in these power supplies, providing pulsed DC, jumping continually between a lower level of current (background current), established to maintain the heat, and a peak current level that actually melts the metal. This feature permits a level of heat control unheard of before, permitting heat input to be reduced when needed, especially with aluminum (between 1 and 2.5 mm or 0.040" and 0.100") or for thin gauges in all metals. Pulse width, that influences drop size and arc cone width, and pulse frequency, that influences average amperage, heat input and arc length, can also be controlled. It is claimed that pulsed GMAW reduces spatter, increases weld deposition rate, lowers fumes and improves appearance. The arc provided by an inverter based AC power supply is so stable that it normally does not need superposed High Frequency except for starting, so that there is no Radio Frequency Interference. When AC features were finally introduced in inverter equipment, they permitted freedom from the rigid frequency from the grid, allowing to play with frequencies from 20 to 250 Hz. It resulted that the characteristics of the arc are changed so that for every application the most suitable AC frequency may be selected. A higher frequency produces a much stiffer arc that permits precise emplacement and deeper penetration, while lower frequency allows a softer arc with best cleaning properties producing a wide and smooth weld bead. This is also useful for depositing metal for rebuilding or for hardfacing. It was long known that the electrode positive portion of the AC cycle provides a cleaning action, disrupting surface oxides, while the negative portion provides for metal heating, fusion and penetration for a deeper, narrower bead. This was the case of the balanced wave, where 50% of the time the electrode was positive, and 50% was negative. But then it was found that also the time duration of each portion of a cycle could be manipulated, so that reducing the percentage of time with electrode positive well below 50% still provided acceptable cleaning action, while increasing the percentage of time at electrode negative increased heat input and weld deposition. But besides this advantage, a secondary gain was also achieved. In GTAW the Tungsten electrode is heated mostly during the electrode positive portion of the cycle. The introduction of the unbalanced cycle, reducing the time of electrode positive, permitted less heating of the same, allowing either the use of a smaller size electrode or maintaining its pointed, sharpened shape, without growing a drop at its end which promotes arc instability and interferes with proper welding practice. One more feature proposed by manufacturers, but only interesting when the power supply is frequently moved around, is the capability of certain modern inverter based machines to accept any one of a wide range of voltage inputs, with automatic adjustment freeing the welder from the concern of adapting the connections to the actual conditions. A basic welding power supply based on inverter technology may provide DC welding current up to about 200 amp for essential applications of SMAW and GTAW for steel. It is a unit of very light weight, that can be easily moved around and brought to the large pieces needing construction or repair. More elaborate SMAW-GTAW units, with AC/DC capability, usually with higher current output up to 300 amp, may provide pulsing of the direct current between a subsistence lower level and a peak current level: this feature may be handy for GTA Welding of thin sheets and tubing with low heat input. The maximum flexibility available is provided by an inverter based power supply that is designed to allow its usage for all manual processes, not only SMAW and GTAW, but, with a suitable wire feeder, also GMAW and FCAW. This universal power supply, offered for up to 400 amps, makes sense for any welder wanting to be able to select the most suitable process for the job at short notice, in any given situation: mostly for maintenance and repair work in a large facility with many different requirements. Obvious any application requires the process specific accessories. If it is determined that inverter technology is needed for one or more of the advantages listed above, then the suitable unit has to be selected taking into account the processes the unit will serve, and the maximum thickness that one needs to weld in one pass at the appropriate duty cycle (the actual percentage of arc on time, in any given ten minutes period). Summing up, the advantages offered by the implementation of inverter technology, must be evaluated against the higher cost not only of the unit, but also of its expensive repair that is a fact of life. --------------------------------------------------------------------------- 3 - How to do it well: Fillet welding of Rimmed Steels
Q: Why is Fillet welding preferred for Rimmed Steel? A: Rimmed Steel manufacturing processes provide a case or rim of very clean material free of defects. Conversely impurities tend to concentrate in the middle section of ingot or billet. This feature persists through the rolling process, so that plates of this kind tend to have their central core less clean than the superficial layers. This property provides and advantage when design calls for fillet welding, which does not penetrate to the center of the plate. --------------------------------------------------------------------------- 4 - Selection of Hardfacing Filler Materials Note - Hardfacing Material are used for providing working surfaces of implements or machines with improved properties making them suitable to resist the destructive actions of
forces acting on them. These harmful actions are best
described by Terms and Definitions grouped for your convenience in section 6 hereafter in this publication. A general introduction to the subject of Hardfacing is presented in the Welding Advisers Site: Click here. Here we intend to provide some more information that should be helpful to make an informed selection of the types of filler materials appropriate to a given situation. After having determined the general classes of materials most probably suitable to provide an acceptable solution, the inquirer should contact a few of the best known manufacturers and ask them to offer their products for a definite application. Then, with the information obtained, the total expense for a given job should be estimated for every one of the products, as indicated in the above page of the site, and again at the end of this article. The calculations permit to arrange the products in a list in the order of increasing total cost. Finally, based on previous experience and comparison with other known cases the final selection should be decided, trying to opt for the minimum total cost that ensures the longest and most satisfactory performance. It should be appreciated that more than one acceptable solution may be applicable to any given situation and that the actual operating conditions are the essential variables that govern the selection of the most suitable hardfacing products and processes. Different welding processes are suitable for hardfacing. The selection is usually based on availability, on the dilution obtained (which should be kept to a minimum) and on the deposition rate. AWS A5.13 and A5.21 provide classifications for limited classes of Filler materials. Most of the available alloys are provided by manufacurers under trade names. It is therefore important to investigate with the suppliers which materials they would help you select for your specific applications. And then nothing can take the determinant place of comprehensive comparative tests. The structure of the deposited metal usually consists in a basic metallic soft matrix providing support to a hard phase in the form of hard carbides, borides or intermetallics designed to resist abrasive wear and other surface damage. Matrix materials include low alloy steels, high alloy iron base alloys, white irons, cobalt or nickel alloys and, less commonly, copper alloys. One common classification of Hardfacing alloys is established
as follows: Iron is the least expensive matrix material, and it can be found in a great number of proprietary alloys. Due to the large variety of iron base alloys available for hardfacing applications, it has become customary to group them more by behavior under wear than by chemical composition. Pearlitic steels are low-alloy steels. They contain low carbon (<0.2% C) and low amounts of other alloying elements (up to 2% Cr), and are useful as buildup overlays, to rebuild parts back to size. This group of alloys has high impact resistance and low or medium hardness (in the range of 25 to 37 HRC), as well as excellent weldability. One typical alloy is designated E-Fe1. They are not designed to resist metal wear but to provide support for real hardfacing material. For building up Austenitic Manganese steel, which is highly resistant to impact and work hardens during usage, two types of filler manganese steel are used, containing also Nickel and Molybdenum: while Manganese is around 15% for both, Chromium may be on the low side (about 4 %)(EFeMn-C) for build up of machinery parts subjected to impact, or on the high side (about 15%)(EFeMn-Cr), used for buildup or welding to the same or other metals. In any case welding has to be performed with the least possible heat, while cooling by appropriate means the surrounding structures. For metal to metal wear resistance applications, martensitic steels similar to tool steels are employed, with due precautions during welding to avoid cracking: these, called also machinery hardfacing alloys, harden upon cooling from welding temperature and exhibit higher hardness although less impact resistance than the above. Caution! These overlays may be difficult or impossible to machine. Among various compositions offered on the market, EFe2 and EFe3 have been used successfully. AWS ER420 is quite popular, being also mildly corrosion resistant. White cast irons are used for metal-to-earth abrasion resistance. Main ingredients are Chromium, that can range between 6 and 35%, and Carbon usually between 2 and 6%. The low carbon alloys are preferred for moderate abrasion and impact. Higher carbon white irons are selected where impact is not an issue but where severe abrasion takes place. Additional elements that may be found are Silicon, Molybdenum and Manganese. Some of these alloys are specified as ERFeCr-A3, ERFeCr-A4(Mod) and ERFeCr-A2. Tungsten Carbides (and recently also carbides of Titanium, Vanadium, Chromium and other elements) are very hard particles of selected mesh size, that are embedded by the welding process into the weld pool of the matrix material. For best endurance, the carbide size should be smaller than the abrading particles size. It is important not to melt them in order to preserve their exceptional hardness. Therefore the oxyacetylene process may be preferred when applicable, as is a process involving pouring carbide powder through a funnel directly into the melt, skipping the passage through the arc. Tungsten carbides are important for sliding and earthmoving applications like plowshares and for rock crushing drill bits and in general for applications requiring maximum abrasion resistance under low or moderate impact. Besides the type of the carbides, their size and the welding process employed, also the volume fraction in the overlay is important for performance and for comparison in any given application. As heavy carbides tend to sink, it may be important to produce shallow melts. Of the nonferrous alloys, cobalt base hardfacing alloys are most versatile, resisting heat, maintaining hardness up to 815 0C (1500 0F) and oxidation up to 1090 0C (2000 0F), thermal shock, abrasion, erosion, galling, impact and wear. These carbide containing alloys depend on the amount of Carbon present for the volume fraction of carbides of different types and compositions: they present high hardness at room temperature and various degrees of abrasion resistance. Another type of cobalt alloys employs Laves Phases, which are hard intermetallic compounds of exceptional abrasion resistance: unfortunately it is quite difficult to deposit sound overlays without cracking except for small applications with correct preheating so that impact resistance and ductility result quite low. Therefore this type of hardfacing is preferentially deposited by metal spray. Cobalt alloys should never be used for hardfacing titanium base metal because of the production of brittle intermetallics. Nickel base hardfacing alloys contain iron, chromium, boron and carbon. The hard phases present are borides and carbides that exhibit excellent low stress resistance to abrasion, generally increasing with their volume fraction. Their resistance to galling in metal to metal wear is moderate and their corrosion resistance is lower than that of cobalt base alloys. Some of the nickel alloys are known by the designation ERNiCr-C, ERNiCr-B and Alloy 40. Carbide containing hardfacing Nickel alloys found applications in nuclear power industry as a substitute for Cobalt alloys because these are prone to become radioactive and to interfere with normal operation. Of the copper base two aluminum bronzes are used as hardfacing alloys on gears, cams and special dies. Two of these overlay alloys are known as ECuAl-B and ECuAl-D. Check list for describing the problem. A number of questions must be addressed when dealing with a problem of hardfacing, either if trying to solve it by research through available information or for submitting to a consultant or to a manufacturer to obtain their recommendations. It is recommended to try to characterize as much as possible
the problem at hand, by filling in most of the relevant items
of this check list: Description : The part is: Contact type: Surface damaging processes (primary and secondary): Welding process available: Unchangeable constraints: Special requirements: ----- Based on the conditions so defined, a knowledgeable consultant should be able to recommend a few products for the specific application. For each of the materials proposed he should present a proposal. Data Sheet including the following information: - Designation Cost Analysis Total cost analysis should be performed for each one of the proposed products and then listed in order of increasing total cost. An estimate and possibly a test should preferably be performed to evaluate the practical performance of all products. The items to include in the cost analysis are as follows: Definitions: Cost Items: - Material - Gases (when used) - Flux (when used) - Electric Energy cost (in case of Arc processes) - Labor cost - Overhead cost Total cost per unit weight of deposited hardfacing material = Sum of partial costs per unit weight of deposited hardfacing material for Material, Gases, Flux, Electricity, Labor and Overhead. Let us know your comments: send your Feedback. Use our simple Form 1. Click here. --------------------------------------------------------------------------- 5 - In the press: welding related Articles Following the Short Item on Passivation
presented in the last issue of PWL,
you may get a deeper look at the subject by reading
"How to passivate Stainless Steel Parts". Repair welding of complex structures is certainly
a major undertaking. See important remarks on
"Optimizing Repair Welding in oil Refineries"
by copying the following address to your browser As announced in a Short Item presented in this issue, a study describing Weldability and Corrosion Research on welding 316L can be found (in pdf. format) by clicking here. If you are interested in Failure Analysis and would
like to see a full single Issue of a Journal dedicated
to this topic, and downoad it your computer, then follow
these steps: One reader kindly sent the following link to
an Article concerning a litigation related to
welding. See it at --------------------------------------------------------------------------- 6 - Terms and Definitions Reminder Note: The following Terms are related to the Article on Hardfacing Filler Metals presented above in Section 4. Abrasion: Cavitation: Corrosion: Erosion: Galling: Heat damage: Impact: Oxidation: Wear: --------------------------------------------------------------------------- 7 - Article: Receiving Inspection All manufacturing facilities, independent of activity and size, should implement an established procedure for accepting materials. It is understood that this Quality related operation, although seemingly involving an outlay, may actually save untold expenses, anguish and time. An anedoctal report was released to the press a few years ago, when it was found that the main mirror for the Hubble Space Telescope did not undergo the thorough acceptance test it should have, probably because of time or budget pressure. This particular oversight was quite expensive, because, as soon at it was found that the mirror did not meet specification requirements, it was necessary to implement a costly repair procedure involving the addition of not anticipated lenses, and a special Space Mission had to be scheduled to have astronauts perform the repair. But even the simplest of the shops receiving the most common goods can be thrown into unnecessary turmoil by the mistaken delivery of substandard or unintended material. Although the strictest procedures were developed in the aircraft and space industry, where any minor oversight might have dire consequences, endangering people and missions, a minimum of control should be always applied because, in this imperfect world, mistakes do happen. It all starts with the material purchase order. The problem is not with what you write in the order: the problem is what you DO NOT WRITE. Better writing a loose and very comprehensive Specification than no Specification at all. If it is not included there, then for the supplier anything goes. Especially in these times of global marketing, anyone may have the temptation to look for less expensive materials. Nothing wrong with that, as long as it is you who make the decision, hopefully after satisfying yourself that your product is still as good and useful as it used to be. But suppose it is your supplier who gets a stock who-knows-wherefrom, and it may be even cheaper, and you may get a discount: how will you know that something is different? Only if you have the practice of an established Receiving Inspection procedure. You may ask for and obtain an original manufacturer's Certificate. As long as everything is OK you will just file those papers: but if you ever see something strange, or different, then you are going to ask a few questions. And you may decide to run a simple test. Is the hardness OK? Does it bend without cracking? Is the surface clean as it should be? It is easy to check the Heat Number: it should be stamped on the bar ends or roll-stamped on the sheets. Examine dimensions, tolerances, even weight. A difference in weight may point to a mix up of materials. Sometimes you may find color codes, or you may establish them for your facility, if the risk is real to take a material for another. Sometimes packing is most important, as for low hydrogen electrodes that should be kept dry until they are used. Or for rolls of welding wire, where cleanliness is essential. Is the description OK? Is it labeled correctly? Are there any suffixes of which you do not know the meaning? (Ask the supplier). If it is welding consumables, a functionality test is in order: Does it weld and flow as you are used to? Does it need different parameters? Does it produce cracks or porosity? It should be known that not all consumables are manufactured equal. This applies, for one thing, to Mig wires (for GMAW): it may be the consistency of chemical composition, it may be the standards of cleanliness, it may be the "throw and cast" (characteristics of wire spooling) that may influence how the wire is fed by the feeder. It is recommended that, even if you are satisfied with what you receive from a certain manufacturer, you test every new batch to be sure not to have surprises. And before moving to a different supplier you should make some practical tests to demonstrate that you will not get caught unprepared once deep into production. Some materials, like paints, rubber and other items, have to be kept at cool temperature, possibly in a refrigerator. This is also the case of brazing pastes with binders. Other goods have expiration dates, set by Standards at some agreed time after manufacturing: how will you know if the expiring date already passed when you receive the goods? From time to time you may need to actually test the composition of a material. While a full chemical analysis provides the answer, to be checked against the specification, there is a less expensive qualitative testing procedure, non destructive, called X-Ray Fluorescence, that can sort out alloys different from what required. The equipment may be expensive, but the service is usually inexpensive, and it is available from a lot of sources, mainly metal service centers, foundries and even junk yards, besides metallurgical laboratories. If you wish to refresh the information given in our Site, you can click on Material Identification. It should be understood, however, that this identification cannot take the place of a full certification from the material supplier, and that it may be dangerous to reissue for manufacture material put aside long ago and now without traceability. If the material responds to heat treatment, an easy test consists in performing a routine reference heat treatment and afterwards testing hardness: if different from what it should be, as long as the process parameters were correct, then most probably something is wrong with the material. Practically there is some paperwork involved. Every purchased item being used in Production should be identified in a central ledger or catalogue. And then any new delivery of each one of the items must be recorded with all the pertinent data, to make exceptions easier to sort out and to deal with. A good Receiving Inspection procedure, continued and consistent, is not an expense, it is a valued asset considered an essential element of any Quality Assurance program. ----------------------------------------------------------------------------- 8 - Site updating Our Welding Advisers Site is never static: it grows with the addition of items and of pages following questions or requests addressed to us by interested readers. This time we announce a new Section on the repair of cracks in cast iron bodies. We understood it is a hot topic by the many letters we received asking advice on how to repair a crack in an old engine block or in a Grand Piano frame, in a cover for a drain or in an old kitchen range. Now this Section is included in the page on Welding Cast Iron, available by clicking here. A brand new page on those difficult to weld Alloy Steels you
can now reach by clicking on Alloy Steel Welding We took the time to include in the existing pages of the Site the exact links to important articles published in previous issues of PWL, and so we plan to continue to do in the future. In this way we enlarge the scope of the information available to our readers, making additional knowledge easily reached. All pages links, including those concerning how this Site was
built, how to make a Site that sells, and what tools were used, are
best reached from the Site Map. Click on Site Map. Let us know on which subjects you would like to see a dedicated page in the Site. Use the Feedback Form 1. Click here. --------------------------------------------------------------------------- 9 - Short Items
Continuing as in the last issue, we will deal briefly with different subjects that were raised by readers as feedback on items of interest. 9.1 - Electrodes for GTAW (Tig) are made by Tungsten either pure or with addition of oxides of Cerium, Lanthanum, Zirconium or Thorium. AWS A5.12 specifies types, diameters and current (for argon). Electrodes should be used near their maximum current carrying capacity. If the current used is too low, the arc will wander on the electrode tip surface, producing arc instability. Cleanliness and smooth finish are most important. Effect of polarity: at any given current the electrode connected to the positive side (reverse polarity) is hotter than the same connected to the negative side (straight polarity): because of this, at any given current level, size is one higher when positive, than that used for negative connection. Although open circuit high voltage will minimize work contamination by electrode when touch starting the arc, it is better practice to use high frequency with no contact at all. Oxidized tungsten electrodes should never be used without cleaning them because they tend to contaminate the weld puddle. However, the oxides of special elements added on purpose into their composition (Thoria, Zirconia etc.) migrating to the surface enhance electron emission and reduce electrode temperature, improving arc starting and stability. The most common electrode is probably the 2% thoriated tungsten. However being thoria mildly radioactive, there has been a drive to develop other non radioactive materials. Pure and zirconiated tungsten electrodes are mostly used with AC. All the other types are used for DC. Although many different parameters can be studied on electrodes manufactured by different sources, it is only by testing that a welder can establish which type and size perform best for a given job: the elements to be evaluated in testing are generally the ease of arc starting, the quality of the weld beads obtained and the useful life of the electrode at a given current level. 9.2 - Orbital tube welding (OTW), is a mechanized system that permits to weld stationary tubes end to end or to tube sheets or to fittings. It consists generally in Weld Heads suitable for a certain range of tube diameters containing a special GTAW (Tig) torch which orbits around the joint. A weld schedule has to be established to ramp the current up from zero to the maximum appropriate for the thickness, and then, after some more than a full turn, to ramp it down back to zero. Depending on the requirements, special Heads can be used that allow multiple pass welding, as could be done manually. Welding is generally performed autogenously (without filler metal) but when needed a wire feeder can be added. For pressure resistant piping and for heavy joints, orbital welding is being done with special GMAW (Mig) or FCAW units, at much higher deposition rates. Power sources of different types can be employed, with the coordination being assured either manually or by microprocessor control. 9.3 - Heat Affected Zone Cracking is concentrated in the volume of base material immediately adjacent to the weld. Although the metal composition in the HAZ is not altered by the weld, the behavior of the Heat Affected Zone depends on the thermal cycles produced by the weld process as modified, if applicable, by additional heat input in the form of preheat or postheat. Furthermore, if hydrogen gas is evolved during welding, it can easily migrate to the hot HAZ and contribute to cracking. Therefore it is imperative to use only low hydrogen consumables, maintained in dry conditions or dried per manufacturer's instructions before use. In steels the susceptibility to cracking depends also upon the Carbon and alloy content of the base metal, usually expressed in the form of Carbon Equivalent (CE), on the thickness of the joint, on the level of mechanical restraint and on the heat input. Preheating can be applied to minimize cracking. 9.4 - Preheating is a technique involving the addition of heat before welding, usually at a limited temperature, for obtaining definite advantages when welding structures of hardenable steels or cast irons that can develop cracks if special precautions are not applied. Different materials and processes need suitable preheating procedures. The main functions of preheating are as follows:
9.5 - Tests for tubular Welding are of the most different types. The tests required to qualify welds employed for a particular structure depend upon the function, the stresses and applicable Standards and Codes. It is understood that the design of tubular elements and joints has to meet requirements of the Authority supervising construction. Here we shall review only the most common of the tests performed on welded joints, excluding those to be performed by welded tube manufacturers. The welder should always satisfy him/her/self that full penetration is achieved, as generally required. This is most easily done by cutting (with an abrasive disk) sections of test pieces to reveal the root of the weld: production welding should not be attempted until the tests are less than fully satisfactory. Other destructive tests for prototypes include crushing, tearing apart, opening up by force to reveal internal defects in the weld beads. On real production visual inspection is most important to assess completeness, uniformity, absence of visual defects (undercut, misalignment, incomplete weld etc.). Then, according to the function and importance of the job, non destructive tests may be required like magnetic particles (for magnetic materials) or penetrant inspection (for nonmagnetic ones) to detect cracks not visible to the naked eye. Ultrasonic and Radiographic testing permit to locate defects at the root of the welds. Hydrostatic or pneumatic pressure test are used to find leaks. Burst tests are used to find out the ultimate strength of a welded joint. 9.6 - Ferrite limitations in 316L are generally recommended to reduce the susceptibility to pitting corrosion in certain aggressive environments: a generally agreed upon maximum is as yet not available. However a minimum of delta-ferrite is usually prescribed to reduce the risks of hot cracking or microfissuring: this is established at about 3 to 5%. The solidification modes and the structure types present in the welds are determined mainly by chemical composition, as modified by filler metal and dilution. The phases are examined with reference to the Schaeffler's diagram, or more recently to a modified constitution diagram including also nitrogen effect. Other researchers study the resulting structures based on the ratio between Chromium equivalent and Nickel equivalent: one must remark however that calculations of equivalents by empiric formulas may be based on different assumptions. One such study describing weldability and corrosion research is listed in the section "In the press" in this publication. 9.7 - Grade x60 Pipe per API (American Petroleum Institute) Specification 5L has a minimum Yield Strength of 60 Ksi. For the root SMA Welding of this grade use only Low Hydrogen electrodes like E7018 with direct current reverse polarity (electrode positive). For fill up passes Flux Cored wire E71T-1, 1.2 mm (0.045") can be used. Preheat is always recommended even if not specified. For thickness 12 mm (1/2") or less, preheat at 38 0C (100 0F) is required. For thickness over that, preheat at 93 0C (200 0F). Minimum interpass temperature should be equal to preheat. Joint preparation, weld process and welding parameters should be selected according to tube thickness. 9.8 - Thin sheet welding, to be successful, starts with good design. Resistance welding is performed on overlapping sheets, and is most common on cars and consumer items. Two overlapping sheets or strips may also be fusion welded by melting the edge of each one to the surface of the other but other solutions are preferable. For fusion welding one should select joint configurations that minimize the possibility of burn through. Butt welding is not the easiest to weld because it may require a backup bar. In edge flange welds the sheets edges are presented to the welding torch when they are parallel and contacting each other: the sheets are bent as necessary at a distance from the weld as required. Most of these welds can be performed autogenously (without filler metal). Corner joints should be avoided, at least for the thinnest sheet metal. Good fit and adequate tack welding is most important. Low heat input manual processes like Oxyacetylene and GTAW are preferred, although mechanized and automated processes are used for thin formed tubes. 9.9 - Welding procedures are detailed instructions developed to ensure that welding is performed as required, for obtaining the designed conditions of performance, stability and durability, with repeatable and reliable confidence. Welding Procedures Specifications (WPS) are documents which must be established by any contractor undertaking to build structures supervised by regulating Authorities, as demanded by applicable Codes (like AWS D1.1 or ASME Section IX). The suitability of those procedures for the intended application is further verified by practical testing on welded specimens as required, and the results are presented in Procedure Qualification Records (PQR) that, when approved, clear the way to construction. An individual wishing to be accepted for performing welding on such work has to demonstrate his/her proficiency in operating the required equipment and in obtaining acceptable welding results as demonstrated by the performance of needed tests and as recorded in a valid Welder Performance Qualification (WPQ) document. 9.10 - Metal Transfer modes for Gas Metal Arc Welding (GMAW) are characterized by the different ways by which the tip of the continuous electrode wire is melted by the arc current. At low current levels metal transfer through the arc occurs in Short Circuit mode. The advancing wire approaching the base metal strucks an arc (due to high open circuit voltage) and a metal drop shorts the circuit estinguishing momentarily the arc. The molten drop detaches from the electrode opening the circuit and restarting the arc. It so happens that the drops form and detach in rapid succession. This mode is suitable when heat input must be minimal (i.e. for thin gages). At higher current levels the Globular Transfer mode takes place. The drop grows to a size larger than the electrode diameter, and then is dropped without shorting the electric circuit. Spray Transfer occurs at even higher current levels: a stream of droplets is directed with force against the workpiece. This mode gives high penetration, high weld deposition rate, and high heat input. Pulse Transfer mode, provided by certain power supplies, controls low heat input by releasing pulses of high current (during which metal transfer is of spray type) at a given pulse repetition rate, while keeping in between the arc at a lower level of background current, during which metal is not transferred. This mode is useful for welding thin gages. Maybe some of you have special experiences interesting and useful to fellow welders: you are urged to share them with all of us, as well as comments on what you find in PWL and what you would like to read. Readers are invited to inform us of their items of interest by a simple e-mail, (click here). If you prefer, use Form 1, which is a simple Feedback form, click here, or Form 2, which is a Survey, click here. --------------------------------------------------------------------------- 10 - Explorations: beyond the Welder All Heat Treating you may need is at the
Metal Treating Institute (MTI) at Heat Treating Progress journal Articles can be read and downloaded from The Naval Research Laboratory web Site can be reached at Centennial of Flight: see links at: --------------------------------------------------------------------------- 11 - Correspondence: a few Comments We are delighted to see a steady stream of questions addressed to us from all around the globe. We would like to provide significant answers but quite often we are prevented from doing so by lack of details pertaining to the specific situation. We have tried to improve on that by providing a Check List to let the inquirers know what is needed to get a complete picture. Not many readers use our Form 3. In a few cases where the questions were so general that they were not understandable, we simply address to Form 3 those who ask, to get at least an idea of what is the problem. You are invited to use it. Click here. It may be that when confronted with details they know nothing about, some readers realize that their questions are not relevant or at least not complete. Would you believe that, after only a few months of correspondence, the amount of letters exchanged is already in the hundreds? A few readers even bother to write that our answers actually helped them: we are very pleased and would like to help everybody. Please note that we would always like to know if our answers were significant in resolving a question or at least in starting a new line of thought or of testing, and if they brought you savings or gains. But if not yet much helpful, do not give up, you should persist and ask again, with more details of what you do and why the answer is not sufficient. And you are invited to spread the word that here at Welding Advisers somebody is spending time and efforts to supply useful answers to anybody who asks for... --------------------------------------------------------------------------- 12 - Bulletin Board 12.1 - With the first issue of Practical Welding Letter for the Year 2004, Welding Advisers would like to extend to all Readers and their Families the best wishes for a prosperous and happy New Year. 12.2 - A Gift is ready for you.
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This is real, important information: take some time to see it for yourself. -----------------------------------------------------------------------
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