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PRACTICAL WELDING LETTER, Issue #014-- Heat Flow, Tool Steel Hardening, Filler Precious Metal,Induct October 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: October 2004 - Practical Welding Letter - Issue No. 14
-----------------TABLE of CONTENTS---------------
1 - Introduction 2 - Article: Heat Flow in Welding 3 - How to do it well: Heat Treating in a Bag 4 - Filler Precious Metals 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Induction Brazing 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Your Voice 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This fourteenth issue of Practical Welding Letter deals with different subjects that we consider pertinent and interesting enough for most of our readers. But there is nothing like a focused feedback to keep us on track, to please our curious readers and possibly help them solve their particular problems. Let us know what you would like to read about. Write us. Click here. Heat Flow in Welding may seem a theoretical subject. But it has quite practical consequences. Although a complete and precise scientific solution of complex mathematical formulae may not be needed in most practical cases, a qualitative description of what happens and why, can be most useful. The Question of this month is how to heat treat tool steel if a suitable protective atmosphere furnace is not available. Not a new solution but a practical one that may help. Precious metals may not be what you deal with, day in day out. However one can find a few interesting and possibly useful hints and tips, in case you have to repair an old family jewel... Induction Brazing was briefly described in PWL in the past. Now it is time to spend a few more words to explain the main features of the process and also to list advantages and limitations. Site Updating is full of activity this month, to make up for the previous month. This time three new Pages are offered on the Site, and one more will be released in a few days. As usual the Site Map is the easiest way to check updating. The other Departments present new information. Your Feedback is welcome: click here. 2 - Article: Heat Flow in Welding What are the factors affecting the transfer of heat from the intense heat source to the metal being melted and welded? In fusion welding the rapid heat excursions born by the metal as a consequence of the relative movement of the heat source, bring about physical state changes, metallurgical phase transformations, movement and thermal stresses. Listing and analyzing the physical processes involved can contribute to important insight necessary for technological progress. In particular the finished weld may contain imperfections, adverse or dangerous microstructure, excessive hardness and residual stresses. In the weld pool itself sudden and violent phenomena occur, causing local melting and turbulent fluid flow, driven by the conditions meeting in that limited space. The interactions establish the shape and volume of the liquid metal, the distribution of temperatures in the surroundings, the compositional changes occurring in the melt, the magnitude of thermal stresses and of deformations. The temperature variations, both in time and in space, bring about metallurgical phase changes and grain growth, dissolution or enlargement of inclusions, reactions with the surrounding medium with loss or gain of elements as well as of gases. Research on Heat Flow for different Welding Processes studies the relationships governing the distribution of temperature or heat cycle, in any point at any time, in a given specimen, due to the characteristics of the heat source and its movement. Direct acquisition of temperatures is difficult, even with special instrumentation, due to the rapid changes and huge differences at short distances. Therefore there is a need of describing the evolving of thermal gradients by applying theoretical study and quantitative calculations. There will be a description of the initial conditions and in general one will consider differently the transient situation (occurring at the start and stop positions) from that achieved in the steady state. For performing a mathematical (analytical) study, ideal assumptions are needed that permit writing general formulae whose solutions are valid within the limitations of the given frame of reference. These solutions give approximate indications for the real world and suggest which factors can influence determinate outcomes. It is fundamental to understand that only a part of the quantity of heat normally described as Heat Input, which is an expression of the energy expended in the process, is actually transferred to the liquid metal. Heat Input, or more properly Energy Input, which is generally expressed in energy units per unit of length, is used as an important factor determining the Cooling Rate, and therefore the metallurgical and mechanical properties of the weld. Absorbed Energy is the heat effectively accumulated by the workpiece, but much of it is transferred by conduction to far regions, contributing to the heating up of the part but detracting from the heat used for melting the metal. The difference is the Heat Loss dispersed from the source to the surroundings by radiation and convection. It is customary therefore to express as efficiency of the process the ratio of the absorbed energy to that delivered. In Arc Welding Processes the Arc Efficiency has been experimentally studied: wide differences were found. However a general indication has emerged: the lowest arc efficiency, that can be 0.20 to 0.50, pertains to GTAW (Gas Tungsten Arc Welding). Intermediate values, 0.8 to 0.9, are found for SMAW (Shielded Metal Arc Welding) and GMAW (Gas Metal Arc Welding). The highest results, 0.90 to 0.95, were obtained for SAW (Submerged Arc Welding). An experimental setup includes some form of calorimeter where the weld energy is transferred to running water whose temperature at the exit side is constantly monitored during the test for final calculation of arc efficiency. Another welding parameter studied, the Melting Efficiency, describes that fraction of net energy input that actually melts the metal: it is a function of both the nature of the heat source and of the heat transfer process that depends on parameters of the part being welded. The characteristics that one would like to determine for any welding process are essentially the maximum (peak) temperature in the metal, shape and size of the molten pool, depth and shape of penetration, amount of base metal dilution, isothermal distribution on both sides of the weld bead, and cooling rate. These data, together with some fundamental description of physical and metallurgical characteristics of the specimen studied, would help in determining the width of the Heat Affected Zone and of the metallurgical transformations brought about by welding. In Laser Beam Welding the absorption by the workpiece is affected by several factors such as the wavelength of the laser, the surface condition and the joint geometry. It appears that for shallow penetration welds, most of the energy is reflected back, so that the transfer of energy is quite inefficient. Coating with thin absorbing layers is reported as increasing the efficiency, but their influence on the soundness of welds must always be assessed before production. However for the so called "keyhole technique", used generally for thicker joints, the efficiency increases many times because of multiple internal reflections within the hole. Theoretical calculations of heat transfer in fusion welding are complex for the need of taking into account both conduction and convection phenomena. With certain simplifying assumptions equations can be written which attempt to describe heat deposition, heat transfer and heat dissipation to the surroundings. Powerful computers must be employed for calculating the solutions. Practical applications of such calculations could be used for determining the conditions of martensite formation in hardenable steels and the influence of preheating, which reduces the cooling rate, on the outcoming structures. This procedure, for any given hardenable steel, requires the determination of the critical cooling rate and the selection of the typical heat flow for cooling which depends on the thickness of the workpiece (two dimensional for thin plates and three dimensional for thick ones). Theoretical calculation of peak temperature in the solid could be used for determining peak temperatures in various locations within the Heat Affected Zone, for estimating the width of the HAZ and for evaluating the effects of preheat. In recent years the analytical solutions have been supplemented by numerical ones which have been made possible by exploiting high speed computers and finite elements treatment of heat flow, especially as ways were implemented to introduce detailed description of physical processes. However important the progress of these investigations in gaining insight of welding processes, it is admittedly still a research tool, far from real time welding use. Continuing progress in the understanding of physical processes involved in welding will contribute in the future to useful applications. 3 - How to do it well: Heat Treating in a Bag Q - Can occasional Tool Steel Heat Treatment be performed in Air Furnace? A - No. Heating High Carbon Tool Steel in Air Furnace will cause decarburization (loss of surface carbon) that will substantially reduce the surface hardness obtained upon quenching. Oxidation too is generally objectionable. In fact the higher the carbon content, the most significant the decarburization process will be, and tool steels generally contain high carbon. For occasional heat treatment of tool steels in an air furnace one can use air tight bags manufactured of stainless steel foil. These are made by multiple folds at the overlapping edges or by seam welding. The tool is introduced in the bag, most of the air is then removed by enveloping the tool as tightly as possible in the bag. A tiny hole must be left in the closed bag, usually by putting in the last fold a fine nail that is later withdrawn, for the residual hot air to find a way out. The bag will then protect the tool from oxidation and decarburization during heat treatment in the air furnace. 4 - Filler Metal for Precious Metals Precious metals have been used from the dawn of civilization as ornaments and jewelry. The use of Gold in jewelry is still one of the major sources of demand for fabricated gold. Brazing, matching filler to base materials as to color and surface condition, is used in the manufacture of jewelry. Biocompatibility, much earlier than the coining of this term, and ductility have promoted the use of precious metals in dentistry and medicine. Precious metals exhibit good thermal and electrical conductivity, and exceptional corrosion resistance which are required characteristics for electrical contacts materials with high-reliability. Besides of Gold and Silver, jewelry is made also of other precious metals like the Platinum group metals which are Platinum, Palladium, Iridium, Rhodium, Ruthenium, and Osmium. Pure and alloyed Platinum group metals have a wide range of industrial applications including catalysts, thermocouples, glass melting vessels and tools, laboratory implements, and medical devices. Dental materials developed over the years, may contain any of the precious metals in any possible combination. Interestingly many of the original precious metal contact, electrical and electronic materials were derived from existing dental and jewelry alloys. Brazing Filler metal used for jewelry and dentistry is found in a wealth of compositions. Generally Gold and Silver are their main components, with additional Copper, Nickel, Zinc, Cadmium or Tin. The exact composition influences color and properties. One of the requirements may be not to reduce the Gold content in final brazed jewels. Silver jewelry is generally brazed with silver base filler metals that contain copper and zinc. Older compositions containing Cadmium are progressively abandoned due to environmental awareness. Silver base and Gold base brazing filler metals are classified by AWS (the American Welding Society) as BAg and BAu alloys, but many proprietary composition lie outside classification. Platinum jewelry is generally brazed with binary alloys of Gold Platinum, Platinum Silver, or Palladium Silver. In industrial applications Platinum and Palladium are brazeable with pure Gold or by using one of the Platinum jewelry filler metals. Brazing techniques are not much different from those employed for Copper alloys. Manual flame torches are mostly used for one of a kind production, while furnaces are used for mass production of industrial applications. Fluxes suitable for Copper alloys are usually adequate for precious metal brazing. Their main characteristics are that they become active before reaching brazing temperature, remove films of oxide, exclude air and promote wetting of the base metal by the filler alloy. 5 - Online Press: recent Welding related Articles How Do We Prevent Hot Work Fires? Preventing welding-related fires. Inspecting Welds in Rail From TWI on Friction Stir Welding Process Variants. Corrosion Fatigue Performance of welded Risers 6 - Terms and Definitions Reminder Aluminizing is a corrosion protective coating of aluminum or aluminum alloy on steel, deposited by hot dipping, thermal spraying, with or without diffusion. More effective and costly than aluminum paint. Austenitic Manganese Steel is a wear-resistant material containing about 1.2 %C and 12 to 13 %Mn. It is characterized by high strength, high ductility and exceptional resistance to wear. Its hardness increases rapidly from impact or deformation. It is particularly useful for severe service that combines abrasion and heavy impact like earthmoving. Also known as Hadfield's steel. Barrel Finishing consists in improving the surface finish of metal parts by having them tumble in rotating equipment while mixed with abrasive particles either dry or suspended in a liquid. Buckling is a mode of failure of a structural element (i.e. a column), due to an unstable lateral deflection caused by compression. In metal forming, it can be any wavy condition of sheetmetal caused by compressive stresses. Eddy Current Testing is a nondestructive testing method in which eddy current flow is induced in the test object. Upon calibration with a known reference standard, changes in the flow caused by variations in the object (i.e. the presence of cracks) are sensed by a nearby coil permitting detection and visualization for measurement by suitable instrumentation. End Quench Hardenability test is a laboratory procedure known as the Jominy test (ASTM A255, BS 4437, ISO 642). Hardenability is defined and determined by heating a standard specimen to austenitizing (above the upper critical) temperature, placing the hot specimen in a fixture so that a stream of cold water from below impinges on one end. Different cooling rates are therefore determined along the specimen length. After cooling to room temperature is completed, parallel flats are ground on opposite sides of the bar and the hardness is measured near the surface of the specimen at regularly spaced intervals of 1/16" (1.6 mm) along its length. The data are given as Hardness values for each of a series of integer numbers expressing the distance from the end in 1/16" (1.6 mm) (1 is 1/16" far from the end, 4 is 4/16"=1/4" from the end etc.) or plotted as hardness versus distance from the quenched end. Notch Sensitivity represents a measure of how much a material is susceptible to fracture as a consequence of the presence of a stress concentration, such as a notch, a sudden change in cross section, a crack. Low notch sensitivity is characteristic of ductile materials, and high notch sensitivity is typical of brittle materials. Refractory Metals are those having an extremely high melting point and low vapor pressure, like Niobium, Tantalum, Molybdenum, Tungsten, and Rhenium. 7 - Article: Induction Brazing One interesting Brazing process, widely used for repetitive mass production of simple joints, is based on selective heating to brazing temperature of mating surfaces of components. Heating is the result of eddy currents induced into the workpiece and of resistance to flow of those high frequency alternating currents (I2R losses). An induction unit provides electrical energy to an induction coil or inductor that is designed for maximum efficiency. Only the surface of the components is heated, depending upon the "skin effect" of eddy currents that concentrate on a superficial layer, the thinner the higher the frequencies. The process can be very rapid, bringing the filler brazing metal to instant melting and the mating surfaces to correct heating for proper wetting. After completing the brazing cycle, and after full solidification, heat flows by conduction to other parts of the workpiece. Typical advantages of the process are:
Typical limitations are:
In certain cases induction brazing can be combined with heat treatment of suitable steel components, to be hardened and quenched in the same operation. The success of an induction brazing operation depends mostly on inductor design. It must be based on the configuration of the joint and on the geometry of the components, on the heating pattern sought, on the materials, on the power available and on the least time needed for brazing without overheating nearby areas. Therefore the assistance of experienced technologists familiar with the process may be essential for satisfactory implementation. Adequate advice can be probably provided by manufacturers of induction equipment with relevant experience. 8 - Site Updating 8.1 - We are glad to announce a new page on Welding Underwater, supplementing the previous one with additional information. We concentrated on providing some insight on compositional and structural changes that depend on depth in water. Furthermore we provide some information on recent developments in under water friction welding and on the trends, still not fully operational, for automatic equipment for joining pipes at great depths and for crack repair of structures. To read the new page click here. In the "old" page on Underwater welding, that can be reached by clicking here, we added a recommendation for a new book, "Underwater Wet Welding - a Welder's Mate -" which is a fully renewed edition of a previous book. It is full of practical information. A must for all divers serious about welding. 8.2 - A new page on Frequently Asked Questions on technical aspects of welding is now available by clicking on the new "FAQ" button appearing in the Navigation Bar on each page of the Site. If you would like to see new questions dealt with, just let us know, with your comments, by sending us your e-mail. Click here. 8.3 - For those of you having their own Site (if you are unhappy with its performance) or planning one (or even for those just playing with the idea) we have assembled a page of useful links to our Site Host and Service Provider, the well known and recommended SiteSell.com, that we believe can provide you with many and valuable benefits, even and especially if you are new to the Internet. The presentation of many, different, successful Sites, built using the SiteSell tools and help, together with other facts filled pages, is proof and demonstration of the happily conceived strategy, fully implemented with many improvements added and other still to come, and candidly offered to anyone for exploitation, without any withheld "secrets". To find, read and link over to their different pages, click here. 8.4 - For the very near future we plan a new page of valuable e-books offered as free Downloads, available again from the above Sitesell.com. The only problem is: "Are you going to read them?". If not, they will only clutter your hard disk. But if you are serious with your commitment to your own self improvement, then, according to the most diverse Testimonials, you should be able to grab sizeable benefits, by just applying their advice. As soon as the new page is Uploaded onto the Site, you will be able to read it by clicking on a new button, called "Downloads" (what else?), that you will find in the Navigation Bar of each page, in the top left margin. Enjoy and good luck. 9 - Short Items 9.1 - Superaustenitic Stainless Steels represent a special class of austenitic stainless steels that contain high levels of nickel, molybdenum, and, in some cases, nitrogen. These alloys are designed for severely corrosive environments. They provide improved resistance to Stress Corrosion Cracking, pitting, and Crevice Corrosion. These alloys are compositionally balanced to ensure fully austenitic microstructures. 9.2 - Superferritic Stainless Steels exhibit higher strengths when compared with the regular ferritic types. They are more highly alloyed and were developed to resist Crevice Corrosion and other forms of corrosion in marine applications. 9.3 - Preheat Temperature Indicators in the form of color coded crayons can be used to monitor preheat temperature for welding or thermal spraying. While less precise than contact pyrometers based on thermocouples, these are consumables which are intuitive in their use and adequate for not too demanding applications. 9.4 - Gamma Radiography is a non destructive test method using radioactive isotopes as the source of penetrating radiation for the examination of internal defects in heavy objects, through interpretation of developed, exposed radiographic films. Although useful for some ceramic applications such as penetrating thick, high
density, high atomic number materials, gamma radiography is limited by the high
energy of the radiation and/or the physical size of the source needed for usable
intensities of commonly employed isotopes. 9.5 - Neutron Radiography is complementary to other radiation test methods for applications dealing with low atomic weight materials such as hydrogen and light metals, barely distinguishable with x-ray. Its usefulness stems from the interaction of the neutrons with atomic nuclei instead of orbital electrons. A typical application is the control of defects in solid propellants for rocket engines. The disadvantages of neutron radiography are high cost, safety issues, and the need of fixed sources of neutron radiation. 9.6 - The Varestraint (variable restraint) Test is one of a series of fabrication weldability tests. It is the most common test used to evaluate the susceptibility of welded joints to hot cracking. The test utilizes external loading to impose controlled plastic deformation in a plate while a weld bead is being deposited on the long axis of the plate by a gas-tungsten arc torch. The specimen is mounted as a cantilever beam, and a pneumatically driven yoke is positioned to force the specimen downward when the welding arc reaches a predetermined position over a die block. The plate is bent to conform to the radius of the die block. This induces an augmented longitudinal strain on the welded surface of the specimen. By selecting the radius to which the plate is bent, the severity of deformation causing cracking can be determined. 10 - Explorations: beyond the Welder On the Telephone Inventor: A Guided Tour...to go with the Wind Puzzled by Black Holes? An atom-smasher on your desk? Online Field Guides. 11 - Contribution: Your Voice We invite again the professional readers who are ready to share their experience, to contribute here a short note on their particular field of interest. We are asked sometimes by students, at different academic levels, for information or opinions on particular aspects of their project or research. In due time most of those works will produce certain Conclusions. We would welcome a short notice on the research, on the work, on the results. Just let us know. We know that many of the readers, who asked for help, concluded successfully their projects. Why not sharing your doubts and successes by telling of your experience to other fellow readers? You are welcome. Click here. 12 - Testimonials From: "RANDY EMERY" 'remery1969@comcast.net'
My name is Randy Emery and I am a fulltime welder and a part-time welding instructor at our local college as well as an AWS CWI. I just wanted to let you know that I find the "Practical Welding Letter" very useful in both my jobs.[...]
From: "Shyam Bichu." 'shyambichu@sify.com' Dear Sirs, 3/09/04 Great job.I always get lots of technical inputs by reading this newsletter. Keep it up, and all the best.
With warm regards 13 - Correspondence: a few Comments A kind reader wrote that some articles from Practical Welding Letters could be used by him as instructional material if available in printer friendly format. We are flattered by the proposal but unfortunately we are not too familiar with the practice of manipulating text: we believe it should be not too difficult to copy and paste the articles in any word processor. The only request is that the source be always indicated with a reference to our Site, https://www.welding-advisers.com A recurring experience is that of industrial companies fishing for free technical advice. Although they would not dream of getting valuable advice for free in any field, they possibly think that the Internet is just for that. It is not. We are willing to continue to provide meaningful, free help to individuals running a hobby or a personal workshop, to students, to welders for their self improvement. We think that manufacturing companies should not apply for technical help, which may be economically meaningful, if they are not ready even to consider to pay for the advice sought. All questions are welcomed. But if we had to judge only from them, we would give quite often very low marks, because no sufficient detail is provided to give a meaningful answer. We request from all inquirers to think twice if their question is legitimate, complete, fully descriptive and understandable. 14 - Bulletin Board 14.1 - Having gained more experience, we are now in a position to offer our "Practical Hardness Testing made Simple" Book as truly downloadable from an address we are providing to our subscribers. Downloading is password protected. In the process we corrected a few minor errors (thanks to a kind reader who pointed them to us). 14.2 - Materials Solutions will feature more than 400 technical presentations Oct. 18-20 in Columbus, Ohio. Of particular interest is the Frontiers of Materials Science & Engineering Symposium: Bridging Science and Manufacturing. This symposium features nearly 50 presentations on topics ranging from Friction
Stir Processing and Rapid Prototyping to sessions on Nano, Electronic and
Optical Materials. For more information: 14.3 - Practical Experience and Applications in Vacuum Carburizing 14.4 - Watch out for new free books available for Download from our Site,
www.welding-advisers.com They all offer important benefits to those readers that will decide
that the advice provided is right and ready for them to implement.
Good luck. See you next time, take care. Copyright (c) 2004, by Elia E. Levi and welding-advisers.com, all rights reserved
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