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PWL, Issue #016-Thermal Fatigue, Repairing Holes in Al Panels, Filler for Alloy Steel, LHW, more... December 01, 2004 |
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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: December 2004 - Practical Welding Letter - Issue No. 16
-------------------------TABLE of CONTENTS-------------------------
1 - Introduction 2 - Article: Thermal Fatigue 3 - How to do it well: repairing Holes in Aluminum Panels 4 - Filler Metals for welding Alloy Steels 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Laser Hybrid Welding (LHW) 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: more on VSR 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This time we present in Practical Welding Letter some new subjects that we hope our readers will find interesting. Thermal Fatigue is a form of failure affecting welded and non welded parts operating at elevate temperature and subjected to cycles of sudden heating or cooling called thermal shocks. Before weld repairing such a cracked part one should be aware of what causes such a behavior. The Filler Metal section attempts to cover the main considerations that have to be taken into account for selecting proper electrodes and consumables for welding Alloy Steels. It is recognized that the types of these steels and their applications are too diversified to condense the recommendations in a few lines. It is hoped that the hints proposed here will motivate the interested readers to dig deeper to solve their specific problems. An unusual combination, now called Hybrid Welding, of two well known sources of heat for welding, namely Laser Beam and Gas Metal Arc, has been around for quite a few years, and success has been claimed in very different fields of application. It can be one more tool in your toolbox, if applicable to gain in performance and cost reduction. A kind reader found the time to put his thoughts in writing as comments on a previous article, a welcome Contribution to this publication. We urge again all readers to share their valuable experience with colleagues that can appreciate it. The Page of the Month is somewhat different from usual. It may be interesting, however, especially in the Gift Season... The rest of PWL should present by now no surprise, at least to those who have been reading this publication for some time. We will probably have soon to change contact us addresses, as the unsolicited junk mail reached recently such huge proportions to fill whatever inbox capacity available. If you find this reading interesting, please spread the voice to your acquaintances. Thanks.
2 - Article: Thermal Fatigue The gradual deterioration of a part and eventual appearance and progressive extension of cracks due to the consequences of alternating cycles of heating and cooling is called Thermal Fatigue. Although this kind of failure can develop also in any integral part, in welded assemblies the welding engineer is called to verify that the joints be located far from stress raisers, designed and performed to resist thermally generated failure. A thermal transient gradient due to sudden change of temperature provokes within a part that presents thickness differences, internal stresses caused by differential expansion or contraction of adjacent portions, which are found temporarily at different temperatures. These alternating stresses can cause thermal fatigue. A similar case, called Thermal Shock, refers to the consequences of sudden exposure of a part to a different temperature, both higher or lower. Here again if various portions of the part are of different thicknesses, they will display a thermal gradient due to the different time needed to reach equalized temperature. Thermal stresses thus arise, proportional to the thermal differential. Repetitive thermal shocks of sufficient importance may initiate and develop thermal fatigue cracks, even in so called heat resisting alloys. Gas turbines manufacturers know that certain turbine components are subjected to thermal cycles, under situations of thermal gradients and internal constraints. As these conditions can generate thermal fatigue failures, much thought is devoted to minimize thermal stresses and external and internal constraints. In the process of selecting the materials best adapted to the service conditions of any given engine, manufacturers developed special testing facilities intended to simulate as accurately as possible those conditions. A general testing instrument called the Gleeble, that can be employed for this kind of investigation, was described in the previous issue of this publication (PWL). Simpler and specialized testing devices were also manufactured consisting in clamping devices capable of sustaining thermally developed stresses within the specimens. The specimen is generally subjected to cycles of localized heating to high temperature, with a flame or by other means, followed by cooling with compressed air or other gas. In general the researched value is the number of cycles to the appearance of cracks or to failure. Sometimes a rotating hub with mounted turbine blades has the blades exposed to thermal cycling. In this setup centrifugal stresses are superposed to the thermal ones to render the stress state more realistically representative of actual life conditions in an operating turbine. The means researched for diminishing thermal fatigue are therefore those aimed at lowering thermal gradients and thermally generated stresses. Among them are sometimes thermally sprayed heat barrier materials and forced air cooling provisions, together with materials having low coefficient of thermal expansion. Welded joints should preferably be located between parts of the same section, to reduce as much as possible the development of thermal stresses in the weld.
3 - How to do it well: Repairing Holes in Aluminum Panels Q: How can we conduct annealing of 7075-T73 and 2024-T62, wrought and formed sheet respectively, welding some holes up and reheat treating? A: Forget it. The materials indicated are not weldable by fusion welding. Also heat treatment cannot be performed easily: even with most apt facilities, the deformations following re-solution heat treatment, quenching and aging would most probably cause scrapping of the parts. The holes should be enlarged and carefully machined to some simple geometric shape (circle, square with rounded corners, etc.). If both sheet sides are reachable one can prepare machined patches. The patches, of the same materials and condition, could be of double the sheet thickness. In the center the selected shape should be machined to stand out in relief, to fill the hole. This shape should be emerging from the rest of the patch, machined to present wide margins of the same thickness as that of the sheets. The patch so prepared and thoroughly cleaned has to be put in place from the back side, so that the hole is filled flush by the relief shape machined in the patch. The patch can be then resistance welded in place along the line running in the margins at mid distance between the raised shape and the patch border. If the patch has to be leak proof then seam or overlapping spot welding has to be performed, otherwise separate spot welds may be sufficient. If only the outer side is reachable then a simple sheet patch can cover up the hole and be adhesive joined in place.
4 - Filler Metals for welding Alloy Steels A few comments on selection of filler metals for welding Alloy Steels were offered in our page on that subject. (Click here for reading that page). In short one should note that the composition of the weld metal actually deposited is less important than the properties presented by the weldment as a whole. Due to the exceedingly vast variety of steels included under this designation only the most important factors influencing the filler metal selection will be discussed here. It is not uncommon that the welding efficiency that can be expected from a certain procedure be less than 100% so that the first concern is to satisfy adequately the strength requirements. The second major concern is to avoid cracking. Mild steel filler can be used when strength requirements are low or when the alloy steel has to be joined to mild steel. In any case, to control cracking susceptibility, the carbon content of the filler metal should not be higher than that of the base metal. If the obtainable strength level is acceptable, one should strive to produce a weld whose carbon content is about half of that of the base material, taking into account original compositions and prospected dilution. High-Strength Low-Alloy (HSLA) Structural Steels typically contain less than 0.2% C and less than 2% alloy. They are designed to meet specific mechanical property requirements given usually as minimum or range for Yield Strength and Ultimate Tensile Strength. Several ASTM and a few SAE Specifications, subdivided sometimes in Types or Classes, provide information on chemical composition and mechanical properties. Some elements may be added in different amounts and combinations to increase the level of atmospheric corrosion resistance. A pearlitic structure represents normally the as-rolled condition. Electrodes for Shielded Metal Arc Welding are designated by AWS classification with numbers whose first two (or three) figures indicate the minimum tensile strength expressed in thousand pounds per square inch (psi) of deposited metal in as welded condition. (E7016 refers to a strength level of 70,000 psi). ANSI/AWS A5.1 and A5.5 list different electrode classes and requirements. Electrodes having a tensile strength characterized by figures 70 or 80 are selected most of times for HSLA Steels. If designated impact properties need be assured at given low temperatures then electrodes having minimum guaranteed Charpy impact test values in those conditions must be selected from above Specifications, and procedure qualification tests have to be run to certify that requirements are satisfied. Dry, low hydrogen electrodes (last two digits 15, 16 or 18) are recommended for all grades. In practice the strength level of electrodes should be selected with an eye to the properties of base metal, but limiting its value to the minimum still satisfying overall requirements. Design engineers should remember that basic requirements for properties of high strength structural steels (yield strength in particular) decrease with increasing thickness. The filler metals to be used for Gas Metal Arc Welding (GMAW) are listed in ANSI/AWS A5.28 and are generally comparable with compositions given for electrodes as above. For filler metals to be used with Flux Cored Arc Welding (FCAW), the Specifications are ANSI/AWS A5.20 and A5.29. Consumables to be used with Submerged Arc Welding (SAW) are listed in ANSI/AWS A5.17 and A5.23. It must be noted that different metal/flux combinations may give different properties, so that it is impractical to propose guidance in selection: manufacturers can be questioned for their recommendations. Another class of Quenched and Tempered High Strength Alloy Steels with carbon content up to 0.25% exhibit carefully designed microstructures, to develop exceptional properties (high yield strength and high ductility with good toughness) through well controlled heat treatments, producing bainite and tempered martensite. To control hot cracking the sulfur content is usually kept quite low and the manganese to sulfur ratio is high, 30 to 1 up to 50 to 1. The selection of consumables is usually based on tensile strength, composition, and notch toughness of the weld metal. For some steels, more than one type of welding consumables may satisfy the fabrication requirements. These steels are designed to be welded with moderate or no preheat at all, and to be used in as-welded condition. For successful welding precise procedures have to be developed and followed rigorously. Preheat and interpass maintenance heating however may be required to prevent cracking. The suggested temperatures vary with type of steel and thickness. In general the minimum temperature should be preferred that permits to achieve the purpose of avoiding cracking. Exceeding the recommended maxima should be avoided. Preheat is used to prevent martensite formation by keeping the structure slightly above the Martensite formation starting temperature. Heat input should be monitored and limited: for certain types of steels, maximum acceptable heat input has been tabulated for different section thicknesses. It has to be noted that Post Weld Heat Treatment should be avoided if possible because it could even decrease properties. See a discussion on this subject in an article published in section 11 of issue # 09 of PWL. Click here. However, if there is danger of cracking either in the base metal or in the heat affected zone, then slow cooling and immediate treating should be considered. Either tempering or carefully selected and tested Post Weld Heat Treatment may be the solution required, especially for welds in thick steel sections. The purpose should be to reduce high as welded residual stresses and to improve toughness and damage tolerance.
The ultrahigh-strength alloy structural steels have elevated yield strength and tensile strength combined with good fracture toughness. Very low levels of impurities in the weld metal are mandatory to assure adequate properties. Therefore only vacuum melted filler metals are selected. To avoid contamination, GTAW and PAW are the preferred processes. Filler metals with lower carbon and alloying elements than base metal are preferred when possible, although the properties will result somewhat reduced.
5 - Online Press: recent Welding related Articles As already announced, our Article on Weld Repair is visible at: Strength and Spot Weld Size in Aluminum Controlled Atmospheres for Bright Brazing Bend Testing Welding Fume Health Hazards
6 - Terms and Definitions Reminder Cosmetic Weld Pass is a shallow depth weld pass performed on top of the previous pass with a smooth bead to enhance appearance. Flashback is the recession of the flame back into the mixing chamber of a gas torch, sometimes prevented through the use of a device called Flashback arrester. Mismatch is the error in positioning or misalignment of the surfaces of two joint elements that should have been in the same plane. Nugget is the volume of one weld (molten metal unit) in resistance welding. Reinforcement is the excess welding over and above the surface plane of the members of a butt joint. In a fillet weld it is the convex portion of the weld. Stack Cutting is the simultaneous cutting of several plates arranged in a stack one above the other. Undercut is a groove melted in the base metal near the toe (border) and left unfilled. Underfill is a surface condition of a groove joint where the weld or root face is lower than the surface plane of the members.
7 - Laser Hybrid Welding (LHW) Laser Beam Welding for its high power density produces deep and narrow weld profiles with keyhole technique, without addition of filler metal. Its capability of providing low distortion makes it attractive for industrial applications involving thick plates or pipes. Solid state Nd:YAG lasers are available up to power of 6 kW. Two or more laser sources may be combined when the power of just one is not sufficient for a certain job. The wavelength obtained with these sources permits the use of optical fibers to deliver the beam, facilitating its application for various geometries including pipe welds. CO2 lasers are needed when higher power is required. The microstructure available in certain steels, with autogenous welding, however, is unfavorable due to the rapid cooling rate, producing high hardness and low toughness. Gas Metal Arc process has a certain tolerance for the fitting clearance (root gap), provides the capability of adjusting weld composition through filler addition and is perceived as a low cost industrial process. It is slow however, requires large bevel angle and multiple passes if it has to join thick plates and causes wide distortions which is a problem when fitting preassebled panels for large constructions. In certain cases it is needed to flip the plates over for exposing the other face to welding: this handling may be an expensive move. By operating the two processes simultaneously on the same spot, the Hybrid, laser enhanced Gas Metal Arc Welding provides not only the sum of their capability, but a highly successful synergic combination that gives new latitude and improved economic results for specific industrial projects in different fields. In shipbuilding the hybrid technique has been used successfully with steel and titanium plates. In pipeline construction it is used for steels to meet current pipeline welding standards. In automotive construction it has been applied to aluminum frames. In the construction of vessels and containers to stainless steels and corrosion resistant alloys. Hybrid welding enables the combination of the advantages of conventional GMA welding, such as a good gap bridging ability and high weld efficiency, with those of laser beam welding as high welding depth and high welding speeds. Their respective disadvantages are minimized. The main advantages of the hybrid process are:
Although considered by now a demonstrated, mature and robust welding process, the hybrid version is not yet an off the shelf technique. For each potential industrial application a specific procedure with optimized hardware and control capability has to be developed and tested. A large body of research and experience is available with manufacturers and with research institutions so that it is worth exploring for its economic potential in many industrial new applications.
8 - Site Update The new Page of the Month this time is something unusual, maybe. We looked for Work of Art realized by Artists by means of Welding. We found a few references, we may find more in the future. We think that welders endowed with creative urge could find inspiration from some of the examples proposed. And express their potential. Pick up a gift idea... To see the new page click here. As usual old and recent pages are listed in the Site Map. Click here.
9 - Short Items ERRATA An error slipped inadvertently in the last line of the 9.3 Short Item published in the previous issue of Practical Welding Letter. An attentive and kind reader pointed this error to us. We present him here our thanks. Tungsten inclusions, from broken tips of electrodes falling into the weld, being more dense than surrounding material, block x-ray more completely. Therefore they leave white spots (not dark spots) in the exposed and developed film. Sorry. 9.1 - From Welding to Casting. The transition from a welded construction to a casting may need to be considered if economy of manufacturing appears favorable to the change. Much will depend on production quantities and requirements. It is not a straightforward process, though. It is more like a complex engineering project. The original welded design can be considered only as a starting point. The casting design has to take into account a completely different set of considerations and of constraints like flow of molten metal, filling capacity, gradual thickness transitions, reinforcing ribs, machining requirements etc. Also materials have to be changed to those readily available compositions for which casting experience has been collected. Mechanical properties are different from wrought and welded structures: available properties have to be checked with requirements and economy of manufacture. One or more Foundries can be called to propose their best design to substitute for the previously welded construction. The new design has to be checked for strength and stability. Then cost estimates have to be compared with the known costs of the previously welded production. If the conversion appears economically promising, some further cycles of revision and refining the design, including establishing mechanical properties specifications and non destructive testing requirements have to be performed before implementing the conversion. 9.2 - Leak Testing is intended to verify the performance of any given vessel, tubing or pipe system as to their containment capability, in the presence of a pressure differential between internal and external regions. The definition applies equally well to any conceivable medium, be it vacuum, compressed gas or vapor, hot or cold liquid etc. In principle this testing should detect the leak rate of the fluid passing through the walls in a time unit. The minimum detectable leak is a measure of the sensitivity of any given leak testing instrument. In practice in most of cases the requirement would be that no appreciable leak be detected by classical means like appearance of bubbles on the surface of a liquid in which the vessel under testing, containing pressurized air or gas, is immersed. Vacuum system require more specialized means to localize tiny leaks. But the mere existence of such leaks is easily detected by observing the pressure differential change occurring in a given time after the system has been sealed. 9.3 - Low Melting Point Metals can be used as a removable mandrel for bending tubes, especially thin wall ones. Certain metals, mostly based on Bismuth, have a quite low melting point, less then the temperature of boiling water. And they characteristically expand upon cooling. Several commercially available proprietary types were developed for these applications. Thin wall tubes tend to buckle or to flatten even when bent using properly designed bending devices. Filling of tubes by casting low melt metal, gives support to the internal surface, preventing the appearance of kinks or wrinkles and oval profile. After bending, the auxiliary low melting point metal is molten away by immersion in boiling water. The material is fully recyclable, it can be collected and reused. For final high temperature applications the bent tubes should be first decontaminated to remove tiny traces of low melting point metals, because they tend to attack the heat resisting alloys and cause their failure. 9.4 - Hot Isostatic Pressing (HIP) is a compacting technique, combining simultaneous heat and high pressure, used to eliminate voids from castings and weldings and to consolidate fine metal or ceramic powders into components approaching theoretical density. This results in improving their mechanical properties and ductility by eliminating potential failure paths and improves fatigue life. In the HIP unit a high temperature furnace is enclosed in a pressure vessel. Work pieces are heated and an inert gas, generally argon, applies to the parts uniform pressure. The temperature, pressure and process time are all controlled to achieve the optimum material properties. HIP is widely used in the casting industry to remove gas and shrinkage holes generated during the casting process. HIP closes and heals these pores. The rejection rate is reduced and the mechanical properties of the parts are more consistent. Casting alloys that are routinely treated by this process include nickel, cobalt, aluminum and titanium. Powder pre-forms are encapsulated in a sealed container, then consolidated by HIP directly into a near-net shape. Materials are cemented tungsten carbide, copper, nickel, cobalt. Ceramics and composite materials can be formed in this way. HIP is used also for the bonding of dissimilar material, consolidation of plasma coatings, improvement of welds, processing soft and hard magnetic material. 9.5 - A Hump Furnace is a continuous tunnel mesh belt type brazing furnace presenting a hump or an elevated section at about mid length. This section houses the hot zone, at higher elevation than load and unload stations. The purpose of the hump is to collect in the highest portion of the internal space the hydrogen gas used as a protective atmosphere for brazing stainless steel and heat resisting alloys. Hydrogen does not come in contact with air in order to prevent explosive mixtures. Inert gases like nitrogen or argon are used in the cold zones to push back and replace ambient air. This furnace is designed to work continuously for long periods. Special safety procedures must be followed at start up and for normal and emergency shut down. 9.6 - Safety with Machining Magnesium starts with education on the fire hazards presented while machining magnesium alloys. Chips and fines, with their high surface to volume ratios, easily ignite. If a fire starts on the machine, one must shovel cast iron chips from a barrel kept near the place to smother the fire. Special fire extinguishers suitable for magnesium fire can be used. Water should be never poured on magnesium fire, because it will actually increase the flames. Magnesium should preferably be machined dry and slowly, not to heat it up. However special lubricating fluids can be used. Chips should be disposed of in a safe manner. Nonflammable ventilated containers should be stored in isolated areas until disposal.
10 - Explorations: beyond the Welder Art and Science Solar Dish Engine Power Plant Cholesterol Recommendations Questioned Chromosaurs? All on Virology
11 - Contributions 11.1 - From: "Michael O'Brien" 'michael.obrien@ccipope.com.au' Dear Sir,
I believe that VSR should be adopted with caution. Michael O'Brien
Thank you Mr. O'Brien.
11.2 - A feedback question asked for comparing arc (GTAW) with (oxyacetylene) gas (welding) for thin gage welding. We should limit discussion to mild steel, because for aluminum and stainless steel oxyacetylene welding is not recommended. It seems mostly a question of manual skill and of personal preference, and the actual thickness limits where to pass from one process to the other are quite open. Some power supplies providing pulsed arc with frequency and polarity control may add current limiting capability and versatility to levels unthinkable of in regular equipment. Try both and see for yourself, you are the best judge.
12 - Testimonials To: feedback@welding-advisers.com
Hi Elia, Thanks for the information, it's very helpful.[...] Regards
From: John.Russell@alexandriava.gov
Thank you for the assistance. [...]
Thank you
13 - Correspondence: a few Comments We feel we should somewhat expand on the comment presented in the previous issue. It deals with a recurring question that often appears in the letters, asking what is the best process for welding certain given materials. This question is worth dealing with in some more detail for those readers who may think that any general question should deserve a general answer. Unfortunately it is not so. The question lacks definition, it is indeterminate. From the question side, one should know exactly the type of material, its dimensions and its condition, the design of the joint and the requirements the weldment is due to satisfy. Because it is the complex of all details that will suggest the processes to be considered and those that are excluded. One should also know which process is available in practice in the given shop for the particular job, considering equipment, consumables and experienced workforce. From the answer side one should appreciate that there is no such thing as "the best" process. The best is not defined. At the least one should define the best with respect to what?: is that strength? durability? reliability? quality? cost? time? safety? etc. As with almost all engineering projects, the decisions are weighted compromises that take into account the most important elements and constraints. We can however try to give an answer if we agree on the following definition: "the best" process is the one that in the particular situation fully described in the question, provides acceptable results, to the complete satisfaction of all engineering and design requirements, at the most economic cost. Then it will be feasible to search for the correct and comprehensive answer, but only after looking into all details and doing in depth comparison work of cost estimates among all the available welding processes that have the potential of providing acceptable realizations.
14 - Bulletin Board 14.1 - Our e-mail inbox is overwhelmed by junk mail. We are advised that when it is full we will lose all incoming messages over maximum capacity. Just now we are deleting hundreds of useless junk twice or three times a day. This is to inform you that if your message is not answered, it may have been lost: so send it again. The way out of this problem, as worked out by our Site Host, SiteSell.com is to change the structure of addressing so that the mail will, in a short time, be sent to us by filling special forms to be made available to our readers. Details will be published as soon as possible. 14.2 - The International Thermal Spray Conference and Exposition (ITSC 2005) is announced to be held in Basel, Switzerland, on May 2-4, 2005. For more information: www.dvs-ev.de/itsc2005 14.3 - If you have not visited yet, visit now the Downloads page (by clicking on the Downloads button in the NavBar of our site Being this the last issue of 2004, please accept our warmest wishes for a Merry Holiday Season and for a Prosperous and Happy New Year. See you next time in 2005.
Copyright (c) 2004, by Elia E. Levi and welding-advisers.com, all rights reserved |
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