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PWL #024-GMAW Parameters II,Alternatives to Welding,Brazing Superalloys,Welding in Space,Dew Point August 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: August 2005 - Practical Welding Letter - Issue No. 24
------------------------------TABLE of CONTENTS----------------------------
1 - Introduction 2 - Article: How to select your GMA Welding Parameters II 3 - How to do it well: Alternatives to Welding 4 - Filler Metals for Brazing Heat Resisting Alloys 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Welding in Space 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Dew Point Measurement 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This 24th issue marks two years of publication of Practical Welding Letter. We are proud we made it, and hope to continue. We would be glad to hear that you like it. As we are going to show you further down, in the Testimonials department, you will see the words of a regular reader who finds interest in our efforts and says just that, not to us, but to his pals in a welding forum. Many of them accepted his suggestion and subscribed to PWL. Thanks. We would also welcome any direct feedback or Contribution: we are sure many of you could teach us some important lessons from your experience. Just a short note could make this page more lively and interesting. Try to write!.. We wait for Your Questions and Feedback. Send them to us! We start this issue with the GMAW parameter selection for Spray Transfer Mode based, as the last issue article, on lessons from Ed Craig. We believe this information can be of some use to readers involved with this type of welding. For the Q&A section we took the hint from a reader's question who wanted to weld where it was neither recommended nor useful. One should be always open to consider alternatives, when their selection makes more sense. The next article deals with some special cases of brazing filler metals, mostly proprietary and not included in official classifications, that may solve problems for joining Superalloys, or Heat Resisting alloys. Then we present a few thoughts and experiences about welding in Space, a specialty that, although not for everyone, will conceivably be developed and extended in the future for practical and economic reasons. For our Website Update we introduce the readers to a new page on welding of thermoplastic materials, a specialization involving new materials and new joining methods. Maybe that some readers will be tempted to learn more on the subject, in view of expanding their business into new opportunities. The Contribution is derived from an article on Dew Point measuring instruments. The correct Dew Point is an important factor in operating furnace atmospheres for brazing or heat treating. Other departments show up as usual, proposing useful or interesting information.
2 - Article: How to select your GMA Welding Parameters (II) We continue to present in this issue information from Ed Craig of
www.weldreality.com In the last issue article we dealt with Short Circuit Transfer Mode. Now we are ready to tackle the Spray Transfer mode, which is characteristic of higher current regimes. In principle one should also consider an intermediate condition called Globular Transfer Mode. However it is commonly agreed that this one, named for the appearance of large drops at the end of the welding wire, should be avoided, if possible. One of the main reasons why this mode is not widely used is that, due to the fact that the big size drop it is acted upon by gravity, the practical welding position is limited to flat. Globular mode is also likely to produce spatter, so that welders would do well to prefer one of the other two instead, depending on the thickness and on the job. For Spray Transfer mode the norm is Constant Voltage power source, with positive lead connected to the electrode. Typically a three phase power supply for spray transfer mode applications should be capable of 400 to 450 Amp and it would be ideal for GMAW wire of 0.035 to 0.045 in (0.9 to 1.1 mm) and for FCAW wire of 0.045 to 1/16 in (1.1 to 1.6 mm). If the power supply offers the option to select between steep slope and flat slope, this last one is the selection suitable to the high current used for the spray mode. The same applies if it gives the choice of low current or high current. When working with current in excess of 300 Amp, also the torch and the nozzle should be selected for high performance, at least capable of 400 Amp. And the protective gear to be used should be suitable to provide a sufficient protection to the welder against heat and radiation. The shade of the darkened glass of the mask or helmet should be at least #11. For every weld wire size, the welder should first learn, or have ready for quick reference, the value of current marking the passage from short circuit to spray mode. These data establish firmly where one should work for any given welding job. One should note, however, that such a current value is influenced by the actual gas mix used, and by Wire Stick Out, and that it depends on voltage that should be selected as suitable for the job on hand. Therefore the values presented hereafter should be taken as representative, to be modified as needed as a consequence of the actual factors indicated above. The inquisitive welder could easily experiment for him/her/self to find the actual threshold of current for passing from Short Circuit to Spray Mode when using the gas mix and wire stick out used in practice for common work. If during such experiments with the minimum indicated current the globular transfer mode is detected, it would be better to increase the current until the transfer mode is definitely of the spray type.
Notes: The following Table II lists starting parameters for different thickness and jobs. Again it should be remembered that other factors, like type of gas mix and value of wire stick out, influence the actual outcome. Furthermore, when welding thicker metals with high feed speed and current, one should pause and consider if a larger wire size would not be a more economic selection. Larger size wire costs less per unit weight, runs at lower speed (which is good for the feeder) for the same current, and has the potential of providing higher deposition rate. All parameters must be adjusted accordingly.
Note that elevated amperages promote fluidity of the pool so that, when welding with elevated current values, flat or horizontal position should be employed. Making reference to the clock method explained in the previous article, it would be easy to remember that wire of 0.9 mm = 0.035 in. is welded in spray mode when the feed speed is adjusted from 1 o'clock (10.7 m/min = 420 ipm) to 5 o'clock (17.8 m/min = 700 ipm), and the majority of welds is done between 2 to 4 o'clock (12.4 to 16 m/min = 490 to 630 ipm). Wire 1.1 mm = 0.045 in. should have speed between 12 to 5 o'clock (8.9 to 16 m/min = 350 to 630 ipm) and most of welding is done between 1 and 2 o'clock (10.7 to 12.4 m/min = 420 to 490 ipm). Whenever the current is approaching the maximum limit, it is suggested to consider the next higher size wire. Ed Craig recommends to probe the limits to find optimum parameters. Correct voltage can be found by reducing it gradually from a high value, until the sound is just correct, and there is no spatter. Remember that if the voltage is too high, there is a quiet arc sound, a long arc gap, high puddle fluidity, intense heat on the welder and excessive smoke. The result is probably seen on the welds produced: excessive undercut, surface oxide, wide bead and possibly weld contamination. On the contrary if the voltage is too low there is a noisy crackling sound, high level of spatter and a reduced smoke level. The result is a narrow, convex bead. In the process of determining the best parameters for a job in view of establishing a written procedure to be used by all welders employed in the same job, one must prove, by performing macro sections of welded test pieces, that the metallurgical characteristics of the joint are correct as to penetration, side fusion, weld dimensions, bead aspect and absence of surface defects. In Spray mode, the contact tip is recessed inside the torch, to protect against burn back, and the wire stick out is from 0.5 to 0.75 inch (12 to 19 mm) and even more if necessary. Increasing the stick out length permits to increase the feed speed while limiting the current increase. Gas mix should be selected according to the work. Straight CO2 cannot be used because it provides unstable, erratic arc with much spatter. A general purpose mix would be 85% Argon and 15% CO2 suitable for a wide range of thickness and jobs. For welding of thicker plate, 12 mm = 1/2" and more, a higher energy gas mix would include 80-85%A - 20-15% CO2 For readers wishing to dig deeper in these subjects we suggest to look directly at Ed Craig's website, www.weldreality.com and to consider procuring and studying his training manuals and video films.
3 - How to do it well: Alternatives to Welding. Q: When and why should Alternatives to Welding be considered? A: Whenever Welding is difficult and expensive and if there is no essential need for the kind of metal continuity provided by welded joints. The main point is that welding processes in general are associated with elevated temperatures. In many situations the application of heat may have adverse effects like crack formation, hardness and strength changes, production of brittle phases, microstructural modifications affecting corrosion resistance properties, unacceptable deformations, alignment and fit problems, introduction of residual stresses. If the risks following the above effects entail the adoption of special procedures to counteract them, then welding becomes a much less attractive solution. Among possible alternatives one could consider the following. Brazing and Soldering that require less heat and may provide adequate properties to the joint. Otherwise Mechanical Fastening, structural Adhesive Bonding, surface modification if effective, elimination of joint by design change.
4 - Filler Metals for Brazing Heat Resisting Alloys Superalloys, or Heat Resisting Alloys are Iron base, Nickel base or Cobalt base Alloys that provide their most important characteristics, namely resistance to elevate service temperature, thanks to carefully balanced chemical compositions and to precisely tailored thermal treatments. Of these, the solid solution strengthened alloys cannot be hardened by heat treatments, and are therefore readily weldable, and brazeable with standard Nickel base brazing alloy filler metals. The precipitation hardenable materials are different. They can generate a secondary phase by selected heat sequences including first a solutioning treatment, and then a precipitation stage defined by holding the metal for specific times in given temperature ranges. It is the complex metallurgical structure developed by the said treatments that is responsible for their unusual properties at elevated temperatures. Other classes of exceptionally strong high temperature resisting metals include the mechanically alloyed (MA) materials and the oxide dispersion strengthened (ODS) metals. In most cases welding, if applied to the two classes of materials mentioned above, would destroy the delicate balance and the fine structure, and also the exceptional mechanical properties. Therefore welding is excluded as a viable joining process except in selected cases of weldable heat resisting alloys like Inconel 718. Brazing however is possible. The regular Nickel base brazing alloys, classified by AWS, may not be suitable at the elevated service temperatures. Proprietary compositions are available, not yet classified by AWS. An interesting class of filler metals for "diffusion braze" operates as follows. Some filler metals are characterized by melting point depressant additions like boron. After brazing, the boron tends to diffuse away from the joint. The result is that, by heating again the joint to the same brazing temperature, the filler metal does not melt any more, being by now depleted of boron. Other alloys may include phosphorus for similar purposes. Silicon would react with chromium and molybdenum to form intermetallic hard and brittle phases. Certain nickel base heat resisting precipitation hardenable superalloys contain substantial proportions of aluminum and of titanium that provide the precipitation phases. The oxides of these elements are almost impossible to reduce in controlled atmosphere of vacuum or hydrogen during brazing. Therefore the surfaces to be brazed must be pretreated by a thin nickel electroplated layer to allow wetting by the filler metal. For ODS and MA alloys the filler metals must have very high brazing temperature, over 1230 0C (2250 0F) but not exceed 1315 0C (2400 0F) which is the final heat treatment for these materials. The filler alloys would be based on nickel, cobalt, gold or palladium.
5 - Online Press: recent Welding related Articles From AWS Welding Journal Aluminum Weld HAZ Fundamentals The following article was notified by a kind reader From TWI (requires no cost registration) How one Shop benefited from Abrasive Waterjet Technology
6 - Terms and Definitions Reminder Amorphous is said of a solid lacking crystalline structure, with atoms disposed at random. Example: glass. Anodizing is the process of performing anodic oxidation, generally on aluminum alloys, to enhance resistance to further oxidation. Beach Marks, also called striations, are revealing signs left on a service fractured surface by the progressive advancement of a crack under variable stresses. Sometimes they have the aspect of irregular elliptical or circular rings, radiating from one or more origins . Buckling is a form of static instability and failure generally due to lateral bending associated with a long and slender column loaded in compression along its axis. Martempering is a hardening procedure performed on an austenitized material by quenching it at a temperature just above that of start of martensitic transformation, holding it there for a time sufficient to equalize temperature throughout the mass of the material, and then cooling in air. The main purpose is to control deformations while hardening. The treatment is followed by tempering. Residual Stresses are considered for weldments free of external forces. They arise in fusion welding because the weld metal contracts on cooling from the solidus to room temperature while constrained by the cold parts of the structure. Sniping means the beveling, or cutting to an angle, of the end of a member like a flat or a profile, to reduce the stress concentration at the weld. Strain hardening or work hardening is an increase in hardness and strength of a metal as a consequence of plastic deformation at a temperature lower than that of recrystallization.
7 - Article: Welding in Space With the continuing progress of present and future extraterrestrial missions, welding is considered a promising technological challenge for repair and for assembly of structures in space and on future lunar or satellite bases. For one thing, the repair of structures in orbit, if possible, should be much more economic than returning them to earth for fixing. The need to study and test applicability of earth based welding techniques was started a few decades ago, when the only zero gravity space available was provided by parabolic flight trajectories of specially equipped NASA KC-135 aircraft in free fall. Then other experimental testing followed in the Skylab and in the Russian Soyuz-6. For the purpose of current studies, space is characterized by micro gravity and by a high vacuum environment. Environmental Vacuum may assist and facilitate Electron Beam Welding. This was tested in the past and seems an advantageous process to use in space, but two problems must be taken care of. Electron beam striking a metal produces Therefore automatic or robotic solutions should be provided to minimize interaction with operators. Laser Beam welding can be used in space. Although laser can operate in air, vacuum is a natural protective environment for welding. Unfortunately laser is not energy efficient, and power supply available on board for welding purposes is very limited at present. Waste heat from the laser has to be dissipated and safety concern for operator have to be dealt with satisfactorily. Other problems connected with welding are the present lack of proven and certified non destructive testing method applicable in space. Therefore there is a high priority for the development of automated reliable welding processes that will require minimum of testing. Solidification behavior under microgravity was not yet investigated thoroughly. Even the shape of weld pool under microgravity is not known clearly. The tendency is to plan and test diagnostic procedures, robotic control systems, repairs with minimum of disassembly of engine parts. These are the challenges that will keep investigators busy for a long time.
8 - Site Updating The Page of the Month added this time to our website, introduces the manufacturing processes used for Welding of Plastics or, more exactly, of Thermoplastics, which may represent excellent alternatives to metals. Although possibly quite a specialized subject, learning it and practicing the methods, might usefully complement the activity of any shop serving certain industries. The materials are known as polyethylene (PE), polypropylene (PP), polyvinylchloride (PVC) and many others that are modifications or derivations therefrom useful to meet specific requirements. Depending on applications, thermoplastics can outlast metals for corrosion and abrasion resistance or containment of chemicals. Their intrinsic limitations however are service temperature and pressure which must meet material capabilities. Many different industries make good use of their properties. Forming and joining are of course essential factors in successful fabrication. It may be useful to open one's attention toward new fields. Click on Welding of Plastics for reading the page. Or see the Site Map to find old and new subjects. Send us Your Questions and Feedback.
9 - Short Items 9.1 - Arc Spot Weld is a local weld made to connect two overlapping members through a hole or slot made in one element to get access to the surface of the second. 9.2 - Atom Probe Microscope is a development of an older Field Ion Microscope. It operates by evaporating the specimen atoms from a sharp needle like extension toward a position sensitive detector, through a cycling high voltage pulse. The atoms (charged ions) are identified by determining their time of flight which depends upon their mass. This is essentially an atom by atom mass spectrography that permits to analyze composition at atomic scale. 9.3 - Glazing is a superficial remelting, using a laser or an electron beam, of a thin layer of material, to transform the surface microstructure of a specimen. Such a treatment may, in certain occasions, restore a surface damaged in service. 9.4 - Nickel Aluminide is a form of intermetallic material of composition Ni3Al. This material has a well ordered crystal structure, is hard and strong even at elevated temperatures and has long been considered potentially useful. It is less dense than superalloys and has good resistance to wear and deformation, with attractive fatigue resistance. The problem is brittleness, in common with ceramic materials. Studies are being conducted by various research teams, to find ways to provide useful toughness and ductility. In the process, experimental production runs were done to manufacture heat treatment support trays, furnace rolls and lightweight gas filters. 9.5 - Tungsten Carbide - Cobalt although not a new material, has known renewed interest and developments in recent times. It was long used in drill bits for oil exploration. To improve the characteristics of rupture and wear, modifications were finely tuned, generating new materials with enhanced toughness without reducing abrasion resistance. One such development is the production of high purity coarse grain tungsten carbide (WC) powders with highly improved properties. These are produced by a carburization process rather than by thermite processing, and provide higher toughness and better wear resistance. Other applications involve diamond enhanced inserts, which are made of a special multilayer coating on the surface of cemented carbides, and cellular structures of honeycomb shape, combining polycrystalline diamond material (wear resistant) with the tough WC-Co of the cell boundaries. 9.6 - Temper Bead Welding has long been known as a favorable multipass welding technique for carbon and alloy steels, that makes good use of the weld heat to temper the hard structures generated by previous passes. The incentive is to avoid the need to provide Post Weld Heat Treatment, that may be impossible for large structures locally repaired, or may be not recommended for its deleterious loss of toughness in certain vanadium steels. Based on solid understanding of the modifications of metallurgical characteristics depending on heating and cooling cycles, the recommendations involve calculation of heat input for every one of the multiple pass beads, and recording the parameters for the procedure qualification record (PQR). Actual testing of the test specimen is what proves that the procedure is effective in achieving the required properties. It is then quite simple to follow the rules for writing mandatory welding procedure specification (WPS) to control production. An extensive article on this subject can be found in the first item of Online Press, Department 5 above.
10 - Explorations: beyond the Welder Eastman Innovation Lab. Engines for Space Explorations ASM Education/Training Continuous Casting of Steel What Don't We Know?
11 - Contribution: Dew Point Measurement One of the most important parameters for controlling the composition of furnace atmospheres and for optimizing their operation is the Dew Point that provides a measure of water vapor concentrations. It is the temperature at which a gas mixture, at a given pressure, has to be cooled in order to reach the saturation point of water vapor. Any excess water will condense to liquid. Condensation Hygrometers employ an optical chilled mirror that is continuously cooled until dew or frost is detected. The temperature at which dew is first detected is by definition the Dew Point. While in the past such devices were manually operated and the point of dew appearance was judged by the operator, modern instruments are automatic and provide digital display. One such hygrometer employs a solid state thermoelectric heat pump, and a thermocouple to monitor the temperature of the mirror. A photodetector measures the infrared light from an emitter reflected by the mirror. As long as the mirror is dry, almost all the light is received. As soon as vapor condensates on the mirror, most of the light is scattered and the photodetector senses the change. A closed loop controller brings the amount of condensate to a steady state so that the dew point temperature can be measured with great precision. Full stabilization of temperature may take some time, especially for determining very low frost points. Metal Oxide Sensors rely on the fact that moisture absorption changes capacitance or impedance of metal oxides. Alumina (Al2O3) is a common material for such sensors. Alumina is deposited as a porous layer forming the dielectric of a condenser. Its properties change as a consequence of the water vapor absorbed, and can be readily measured. The instrument reacts immediately to an increase of humidity, but it may take some time until it is dry again, or in equilibrium at a drier point. Calibration is performed against a reference more precise chilled mirror hygrometer. The sensing element, in other instruments, is a thin film of polymer coated on a properly designed silicon substrate with embedded metal electrodes. The capacitance of the polymer changes in proportion to the surrounding relative humidity and is measured through compensating circuitry and calibration data stored in memory. Exact dew point measurement depends on total absence of contaminants. Therefore a correctly designed installation must include, besides the selected dew point meter, also a suitable sampling system capable of extracting, cooling and filtering atmosphere samples, without adding or removing water vapor. A comprehensive article that includes theory, examples and references on these subjects can be seen, starting at page 54, in the July 2005 issue of Heat Treating Progress, an ASM International publication.
12 - Testimonials http://www.hobartwelders.com/mboard/showthread.php?t=13909 "The Practical Welding Letter" is an informative email bulletin that provides info about welding and brazing processes. I've been getting it for almost 2 years and have enjoyed reading its welding articles, especially those about new and exotic welding processes. This month's edition has a great deal of information about GMAW. If you're not currently receiving it, it's available by free subscription, at: https://www.welding-advisers.com/subscription.html LarryL
From: Paul Smit (address withhold per author's specific request) Thanks for the (very quick) info Elia, Much appreciated. [...] Thanks again, and best regards, Paul Smit.
13 - Correspondence: a few Comments We are exposed sometimes to a strange correspondence to our website. Somebody asks a question, which is not clear enough: too many details are not specified. Therefore I ask to present the question with a more complete description of the case. I get no answer, no details, nothing. I can only suppose that the inquirer was not serious enough or was so far from understanding his/her own question to be induced to quit. Or somebody else comes back in a few months, requesting again what was asked long ago, but without additional information. I have nothing against working on the questions to assemble more details, it helps also the learning process. But I would appreciate a minimum of seriousness, otherwise it is only a waste of time. Please remember that all correspondence to us is screened against a Whitelist of approved correspondents. Therefore, at least whenever writing us for the first time, please use the form, by clicking on Your Questions and Feedback.
14 - Bulletin Board 14.1 - Corrosion Solutions Conference 2005 14.2 - ASNT Fall Conference and Quality Testing Show 2005 14.3 - Three-Day Course: "Modern Furnace Brazing" 14.4 - Click on Special Prize to review what business ordinary people can build with the proper tools.
See you next time Copyright (c) 2005, by Elia E. Levi and welding-advisers.com, all rights reserved |
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