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PWL #036 - Gravity Welding, Titanium to Cobalt Joints, Filler Metals for Tool Steels, Extensometers August 01, 2006 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. Gravity Welding, Titanium to Cobalt Joints, Filler Metals for Tool Steels, Extensometers, Stud Welding, Fiber Laser Girth Welding and more... This publication brings to the readers practical answers to welding problems in an informal setting designed to be helpful and informative. We actively seek feedback to make it ever more useful and up to date. We encourage you to comment and to contribute your experience, if you think it may be useful to your fellow readers. Click on Contact Us. You are urged to pass-along this publication to your friends, if you like it, and if you want to help them. If you received this from a friend and if you like what you read, please subscribe free of charge and you will also receive a bonus book on Practical HARDNESS TESTING Made Simple.
1 - Introduction 2 - Article: Gravity Welding 3 - How to do it well: Welding Titanium to Cobalt base alloy 4 - Filler Metal for welding Tool Steels 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Extensometers 8 - Site Updating: Stud Welding 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Fiber Laser Girth Welding 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This 36th issue of Practical Welding Letter for August 2006 opens with a description of an amazing form of SMAW simple automation, called Gravity Welding, developed in the 1960s by the Japanese for repetitive work in their shipyards. Although probably not much used these days, we got a specific question related to this process, demonstrating that the interest in it is still alive. For the department of "doing well" that hosted already many cases of dissimilar joints, we talk of the joint of a medical implant, this too originating from a reader's question. Welding of Tool Steels is a specialized technology. Critical is the selection of Filler Metals. Short of being able to present a complete list for every possible case, we give a few of the criteria to keep in mind when choosing them. Extensometers are delicate instruments that help in determining essential properties of metals like the Yield Strength. We describe briefly their principles and uses. The site updating is quite rich this time: besides the new Page of the Month dedicated to Stud Welding we propose also downloads from an important book on Underwater Welding, an essential reference source. For the Contributions department we talk of development efforts to bring to the field improved orbital laser welding for pipelines. It is probably a technology to keep an eye on, suitable for further successful applications. Other usual departments will be found at their place. We welcome your comments and feedback: use the Contact Us button from any page of the weldsite. Happy holidays to all. We too will take a few days off, we will advise with a note on the Website (in the Contact Us page) and Blog. 2 - Article: Gravity Welding The name Gravity Welding or Gravity-feed Welding designates a special variation of Shielded Metal Arc Welding using essentially a primitive and simple form of semi-automation. Gravity devices use the weight of electrode and holder to produce arc movement along the joint. The process, used to weld stiffeners to plates, is known to present specific advantages for one pass horizontal fillet welds where the length is limited to less than 3 meters (for longer welds other fully automated processes seem to be more economic). A single unskilled operator can attend to the performance of a few welds running at the same time, obtaining therefore increased weld deposition rate. It is recognized as a Japanese development of the early 1960s in shipyards, and has known successful use also in railroad car shops and barge yards, although its present application is probably limited. Nonetheless, these days we got a specific query from one of our readers that currently operates the process and could not obtain design size of fillet welds despite correct parameter development. In contact or drag-type electrodes like those used in this process, the arc is produced in a small cavity formed inside the flux covering, which takes care of arc length control without welder's intervention. Two characteristics are unique to gravity fed electrodes: they display a heavy covering, and are about 1 meter long. A tripod type holder gripping the electrode and capable of sliding down an inclined bar along the joint line, is placed by the welder at the weld start point where an arc is struck. From then on, gravity moves automatically the consumable end of the electrode along the weld, that progresses unhindered and unattended until the whole electrode is consumed. As soon as only a short stub remains, the mechanism throws it and its holder out and stops the process. Additional operations like slag removal and possibly straightening have to be carried out as usual by unskilled workers.
3 - How to do it well: Welding Titanium to Cobalt base alloy Q: How are welds performed between Cobalt base Alloy ASTM F75 and Titanium Grade 2 and how are the joints tested? A: Cobalt and Titanium cannot be successfully fusion welded to each other but they are currently joined by friction welding, provided that the shape is favorable for this type of process. Other solid state welding processes may also be suitable. Both Cobalt and Titanium are being used for medical implants and it is reasonable to research their joint properties to exploit their specific advantages for special applications. A recent paper dealt with "Investigations on the galvanic corrosion of multialloy total hip prostheses". An abstract can be read at The usual testing procedures that are used for any welding are also applicable here. In particular bend testing and tensile testing are standard. In case the joints to be tested are suspected of being brittle, they can be tested by notched bend test and by impact if required. Metallographic examination is usually conducted on sectioned, ground, polished and etched specimens. Readers interested in Materials and Processes for Medical Devices are advised that a periodic publication of the same name is issued by ASM International and included in their renowned Advanced Materials and Processes.
4 - Filler Metals for welding Tool Steels Although Tool Steels are, almost by definition, hard to weld due to their specific properties that make them so useful, (mainly hardness and hardenability), the need of welding occurs sometimes for the following reasons:
Welded tools may be made by using low cost steel for those elements that do not require exceptional properties. Tough steel may be used to provide support to hard and brittle tool steel. Arc welding, mostly with covered electrodes (SMAW), is the process usually applied, by considering the precautions presented in our page on Tool Steel Welding. Selection of Filler Metal has to take into account the chemical composition of the base materials, the metallurgical condition of the tool steels to weld (either annealed or hardened), the functional demands on the finished tool and the heat treatment to be applied. If the welded portion or hardface has to develop substantial hardness, either in the as-welded condition (with stress relieving after welding) or after heat treatment, the filler metal has to meet this requirement even if the composition may be different from the base material. Sections far from the working area may not need the hardness, therefore welding will be concerned essentially with avoiding cracking and providing ductility and toughness in the weld. Three are the main types of filler metal to select from. The first, mostly comprising proprietary brands not included in official classifications, provide hard deposits in as-welded or welded-and-stress-relieved condition. The filler metals correspond to the main tool steel types (water-, oil- and air-hardening, hot work or high speed) and should be selected to match the base metal. The second type, including low carbon and low alloy steel filler metals, are not required to meet the hardness of the tool steels, just to provide ductility and toughness to permit acceptable welds. The third type groups together non hardenable metals like nickel and non hardenable nickel alloys, austenitic stainless steels and copper-nickel alloys. These are useful in that their ductility permits to avoid cracking, and can be selected as a substrate for further hardfacing. Remember that for successful welding or repairing one needs knowing the type of base metal tool steel. In case it is not known it should be determined by chemical analysis, the most practical and inexpensive of which (although only qualitative) is called X-Ray Fluorescence (XRF) and is widely available with many metal service providers. Also the hardness should be known, tested as described in our book, available at no cost by Subscription, together with this monthly publication PWL. 5 - Online Press: recent Welding related Articles An article of mine, on Arc Welding Discontinuities Welding Tailored Blanks Cold Spray Technology Report Diffusion Bonding From TWI
6 - Terms and Definitions Reminder Arc Blow is the wandering of a welding electric arc under the influence of magnetic fields nearby. Arc Welding is a group of welding processes where the heating is produced by an electric arc struck between an electrode and the workpiece. Bevel consists in an angular edge preparation in a weld element, usually needed, in arc welding of thick members, to let the torch or the arc reach the weld root. Covered Electrode is a composite consumable metal wire surrounded by a sheath of materials designed to deliver, when heated by the weld arc, gaseous compounds and a floating slag protecting the weld from the harmful effects of the atmosphere, and possibly additional elements to modify the chemistry of the weld. Diffusion Bonding is a Solid-state diffusion process by which flat interfaces can be joined at an elevated temperature using an applied pressure for the needed time. This process permits the formation of bonds at the atomic level, as a result of local plastic deformation at elevated temperature and interdiffusion at the surface layers of the materials being joined. See the fourth article in section 5 above. Face Reinforcement is represented by additional material, over the bare minimum required by design, deposited on the bead from the side of welding. Inert Gas is a protective gas or mixture that does not react with molten or hot metals and that displaces harmful atmosphere. Argon and Helium are examples. Weld Pool is the local volume of molten metal generated and maintained at the point of welding before solidification, and moving along the joint during the process.
7 - Article: Extensometers Extensometers are precision devices used to measure local deformation or strain (change in length) of materials subjected to application or removal of force, causing a corresponding stress, in the tensile test performed on a definite specimen in a suitable testing machine. Normally uniaxial extensometers measure the strain in the direction of the applied force, either by being clamped on a test specimen, or from a visual distance for optical instruments. Exceptionally special extensometers may be used to measure at the same time strain in two orthogonal directions. Also by measuring displacements on two parallel faces of the specimen one can determine if bending is introduced. Clamping must be done in such a way that it does not change the specimen surface (by leaving a mark) and that the contact is fixed and secure during the whole test (no slipping admitted). Actually what is measured is the change of distance between two points, occurring because of the applied force: the base distance is called the gage length, and the strain, reported to the gage length, represents the average deformation. Depending on requirements, suitable extensometers are selected of the preferred gage length. This is quite exact until the specimen section remains uniform along the gage length. As soon as the specimen presents a neck or local restriction, the average strain is no more representative of the physical process occurring. The results gathered along the test are saved and displayed as strain values as a function of the corresponding stresses, in a table or in a graph called the Engineering Stress-Strain Curve. Most often extensometers are used for determining the engineering Yield Point of a material, defined by convention as the stress producing the permanent set established at 0.2 percent (other values may be selected but must be reported). The calculation is easily done on the graph by tracing a straight line parallel to the initial straight portion of the graph (representing the elastic stress-strain behavior, or strictly proportional) and at a distance of 0.2% along the abscissa, from the graph start point. The point of intersection of this line with the graph, measured in stress units (along the ordinate axis) is taken as the Engineering Yield Strength at 0.2% offset. Another characteristic defining Extensometers is the maximum strain they can measure. Normally for metals this is quite a limited value, but for elastic or plastic materials it may easily reach and surpass 100%. Currently Extensometers are based on (resistive) strain gages or on (inductive) Linear Variable Differential Transformers, both types supplying precise electric signals proportional to the measured strain. Optical or video extensometers (not suitable for measuring very small strains) are based on a camera monitoring the separation of two lines or dots traced on the test specimen. They cannot measure bend strains because they view the specimen from one side only. More advanced optical extensometers are based on optical interferometry. Special provisions that insulate the measuring device from heat, permit determination of strains when the specimen is tested at elevated temperatures within a furnace enclosure. From this short review one can see that the instruments to be employed must be selected according to the use. Furthermore their class of precision must be according to the governing specifications. Different classes exist. Documented periodic calibration is a requirement. Extensometers must be applicable to the material to be tested. The integrity and well functioning of extensometers must be maintained in all circumstances, even when the specimen breaks. Otherwise it must be possible to remove the extensometer from the test specimen without disturbing the test. The extensometer must accommodate the extension range required without losing accuracy and precision. ASTM E83-06 DIN EN ISO 9513 Metallic materials - Calibration of extensometers used in uniaxial testing 8 - Site Updating: This month we implemented a long overdue project: we obtained the permit to publish in our Website three short excerpts from the book It is widely acclaimed as an essential reference not only for divers-welders but for welders in general. To read the pages see the link further down this page in section 14.3. Besides the above presentations, we introduce in the Page of this Month a different welding process we did not deal with until now. It is a fast and economic process that permits to plan for lighter structures and that disposes of all additional machining operations needed when employing alternative design. It is worth knowing about, to implement it when useful. An article on a specialized application of Stud Welding, is visible at page 54 of the journal Welding and Cutting, a link to which is given in the last of the items presented in section 5 above. To see the new website page click on Stud Welding. To see the Site Map click on Site Map. To send us your comments and feedback click on Contact Us.
9 - Short Items 9.1 - Banded Structure consists of inhomogeneous distribution of alloying elements or phases arranged in alternating nearly parallel bands of different composition, typically aligned in the direction of primary hot working. They can usually be seen under a microscope on properly prepared specimens. 9.2 - Boriding also referred to as Boronizing is a thermochemical treatment, performed below the Ac1 temperature, involving the enrichment of the surface layer of an object with borides with the purpose of providing increased superficial hardness. 9.3 - Cemented Carbide is a solid body produced by pressing and sintering, with powder metallurgy techniques, a mixture of powders of metallic carbides, such as tungsten carbide, and a smaller amount of a metal, such as cobalt, as a binder. Used as cutting tools or as dies for wire drawing. 9.4 - Creep Test consists in a method of determining the extension of metal specimens, under a constant load at a given high temperature, involving the plotting of strain vs. time. The results are often expressed as creep rate, giving the elongation (in millimeters or inches) per hour on a standard gage length (e.g., 25 mm, or 1 in.). For practical purposes, specifications may establish the maximum strain acceptable in a given time under precise parameters. 9.5 - Dispersion Strengthening of a metal or alloy is obtained by incorporating in the metal volume, small size particles of a chemically stable nonmetallic phase that hinder dislocation movement and deformation at elevated temperature. These materials are particularly difficult to weld. 9.6 - Electroforming consists in making parts by electrodeposition or electroplating on a removable form, of metallic ions in an electrochemical bath. Typical is the fabrication of hollow metallic electromagnetic waveguides.
10 - Explorations: beyond the Welder Farnborough Airshow 2006 FIFA World Cup Ball Galvanized Rebar Resource Center Piaggio Fresco - Hydrogen Fuel Cell Scooter Juggling...Agggh!
11 - Contribution: Fiber Laser Girth Welding Several welding processes are currently used to perform field welding of pipelines. However severe demands are posed on quality and costs. New developments both in high power laser sources and in high power fiber laser systems have brought the technology near the point where it can be exploited in the field to weld pipelines. A system has been described providing the capability to perform automatic sensor controlled orbital welding of aligned and clamped pipe girth welding. In a demonstration, prequalification tests were conducted to check the properties of joints performed with the experimental equipment under laboratory conditions. It was established that the process is, in principle, capable of fulfilling all associated requirements, and in case of a 12 mm thick pipe, to perform adequate welds in one layer at 2.3 m/min speed. An article on this subject was published at page 30 in the July 2006 issue of the Welding Journal. Interested readers are urged to seek the original publication.
12 - Testimonials From: "arun mangla" (e-mail withdrawn for security) Respected Sir, Thanks for your reply and your valuable advice.[...] with regards
On Sat July 22 00:13:15 2006, the following was received [...] Thank You. 13 - Correspondence: a few Comments I was asked which welding machine would be appropriate for a gift to a new welder. With all due appreciation for the kindness of the thought, I think the welder should select by him/her/self the most suitable equipment capable of performing satisfactorily the kind of planned work. The same question could be posed for Welding Books. We present a long list of remarkable publications in our page on Welding Books and we are convinced that some of them provide invaluable reference information to all those wishing to build their own knowledge and experience. But the selection would depend on previous preparation and on actual interests of the individual concerned, so that it may be quite difficult to hit the mark. Our recommendation is, as usual, to chart a road map to improvement and specialization, and to follow it with persistence.
14 - Bulletin Board 14.1 - AWS Welding in Aircraft and Aerospace Conference 14.2 - The last PWL Mid July Bulletin contains important online Resources on Brazing, a list ready to be browsed and consulted for downloads and links. Keep it handy for future reference. In case you missed it, you can reach it now by clicking on PWL#035B. 14.3 - We are glad of having been authorized by David J. Keats, Author of the very important Book on Underwater Wet Welding, titled To read, bookmark, download or save the extract, click on The book was recommended by an AWS reviewer not only for underwater operations but for welding in general as an excellent and useful reference. 14.4 - Your own Boss for a part-time Job? Why should you (or family, friends) think of starting a new part-time job? Learn on a number of important advantages of the Internet over traditional business. It has been proven time and again that, with suitable assistance, anybody without previous experience can build, step by step, his/her own real business on the Net. The decision requires only commitment, common sense, readiness to learn and to work, perseverance and confidence. See our new page, based on our own experience, by clicking on 14.5 - Browse through our other unrelated new Website presenting a See you next time...
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