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PRACTICAL WELDING LETTER, Issue #008 -- Ultrasonic Welding, Cast Iron Filler Metal, Stress Relieving March 31, 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 -----------------------------------------------------------------------
You are urged to pass-along this publication to your ----------------------------------------------------------------------- Date: April 2004 - Practical Welding Letter - Issue No. 08
------------------------TABLE of CONTENTS---------------------------
1 - Introduction 2 - Article: Ultrasonic Welding 3 - How to do it well: Use of Low Hydrogen Electrodes 4 - Filler Metal: Selection for Cast Iron 5 - Online Press: recent Welding and related Articles 6 - Terms and Definitions Reminder 7 - Article: Stress Relieving after Welding 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Visual Inspection 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board --------------------------------------------------------------------------- 1 - Introduction This issue of Practical Welding Letter is the first one to be distributed to almost 850 readers. We hope that the Subscribers read and enjoy its content. Shorter Letter A new page on Soldering has been added to the Site. Once mostly a manual process, it is by now a highly automated method economically providing reliable connections in Printed Circuit Boards. Ultrasonic Welding can be a preferable solution in certain tough situations. A short article explains what it is. We would like to remind why Low Hydrogen Electrodes and special procedures are essential requirements for welding high strength steels. A few suggestions are offered on how to select a suitable type of Filler Metal for Cast Iron. In Contributions, we present Visual Inspection, a subject proposed by a reader who was disappointed that it had not been dealt with thoroughly enough in the Site. Other sections follow as usual. We hope you will enjoy finding interesting matters and we welcome your feedback. Click here. --------------------------------------------------------------------- 2 - Article: Ultrasonic Welding Ultrasonic Welding is a process whereby mechanical energy, developed through the transformation of high frequency alternating current into mechanical vibration (by piezoelectric ceramic transducers) is transmitted through a metal tool called a horn (or sonotrode) to the interface of two overlapping surfaces to be joined, supported by a static suitable anvil, producing intimate contact and welded joint. There is a power supply which elevates the frequency of the electrical current from the grid, then the transducer that transforms electrical into mechanical energy, then a booster that modifies the shape and magnitude of vibrations an finally the horn that vibrates the material to be welded, while it is clamped unto the stationary anvil. This same principle is applied somewhat differently to plastic or to metallic materials. For plastics the vibrations under pressure are in the direction normal to the interface, the frequency is mostly 20 to 40 kHz (kilohertz or thousand cycles per second) and the amplitude of oscillations is in the range of 20 to 80 microns (thousandth of mm). This generates frictional heat at the interface, melting the materials together. Medical items are among the most demanding applications. Other uses are for Appliance, Automotive, Consumer, Electrical, Packaging and Toys. An annoying limitation is that large parts cannot be welded by this method. To get an idea of the different ease or difficulty of ultrasonic welding exhibited by various plastic families you can see an article at For metals the vibrations are parallel to the interface, the frequency is of the same order as above and amplitude is between 15 and 45 microns, while a certain normal clamping compressive force operates between the elements. An increase in temperature can be measured at the interface, well below the melting temperature of the metals. Surface oxides are disrupted and displaced. Welding is reached through a solid state process involving atomic movement and diffusion. Ultrasonic welding is applied in microelectronics, to join wires into packages and specifically for nonferrous metals used in electrical connections and automotive harnesses. Studies are currently being performed to develop suitable equipment and procedures to produce on industrial scale spot and or seam ultrasonic welds to substitute for resistance welding for aluminum alloys, for applications in automotive manufacture. The drive is the substantial economy in energy investment potentially achievable by the introduction of ultrasonic welding. Among other advantages, speed and ease of welding, practical real time quality control, high quality of welds.
----------------------------------------------------------------------- 3 - Doing it well: use of Low Hydrogen Electrodes Q: We are requested to weld with Low Hydrogen Electrodes. Why? A: Hydrogen gas is readily absorbed because of its high solubility in molten and hot steel, but it is rejected at lower temperatures as solubility decreases. Furthermore when dissociated in atomic form, hydrogen can diffuse in the Heat Affected Zone. Upon cooling down Hydrogen atoms combine to form molecules, a transformation producing a marked increase in pressure. Two kinds of defects can be generated by hydrogen in welds. Porosity is the presence of gas bubbles that weaken the structure, trapped in the solidifying molten metal. Cracks, even delayed long after the end of welding, are generated in weld or heat affected zone as a consequence of recombination of atoms to molecules and pressure raise. Hydrogen is particularly harmful in strong, hard, crack sensitive hardenable or structural steels, especially when high residual stresses are present or when design is rigid and most constrained. The use of low hydrogen electrodes, kept dry before use, with proper procedures including pre and postheating, is mandated by the need to provide measures apt to prevent the dangers outlined above. -------------------------------------------------------------------- 4 - Filler Metal: Selection for Cast Iron A general page on Welding Cast Iron is visible elsewhere in the Site (Click here). Different filler metals electrodes and rods are available for welding and for braze-welding Cast Iron. The selection as usual should be based on the material allowing for the least possible total cost of the application (Material + preparation + welding + finishing) that meets acceptable requirements of integrity and minimum properties. Here we propose a few guidelines for selection. However also the adopted procedure has a determining importance in the success of the operation. Maybe that a certain filler producing cracks if welded without adequate preheating or with too high heat input, will provide acceptable results with different parameters. For Shielded Metal Arc Welding, the following should be considered: Cast Iron Electrodes (ECI) must be used on preheated parts. Their use is usually limited to repairing small flaws. Steel Electrodes (ESt) are made of low carbon wire, with special covering designed to permit low amperage welding. Due to unavoidable dilution of carbon from cast iron, they develop a hard structure unless preheated to a high temperature or post weld annealed. Covered Nickel base electrodes are classified in the following: Pure Nickel (ENi-CI), Nickel-Iron (ENiFe-CI) with 55% Ni and Nickel-Copper (ENiCu). Due to the important presence of nickel in their composition they are relatively expensive. If phosphorus is high, nickel electrode welding is more susceptible to cracking than nickel-iron which are also less expensive. Copper base electrodes are either alloyed with tin (ECuSn-A, ECuSn-C) or with aluminum (ECuAl-A2). Their color match is poor and melt at lower temperature (providing effectively a sort of braze-welding) than cast iron. Deposits do not harden by carbon pick-up, yield to shrinkage stresses and are machinable. Copper-aluminum welds are stronger than copper-tin, and can be used for hardfacing on cast iron. Popular specifications are as follows: AWS A5.11 - NICKEL AND NICKEL ALLOY WELDING ELECTRODES FOR SHIELDED METAL ARC WELDING, SPECIFICATION FOR Cast iron electrodes and rods are given in: AWS A5.15 - SPECIFICATION FOR WELDING ELECTRODES AND RODS FOR CAST IRON Specifications represent experience of different groups, and a wide agreement reached among them. Specifications are therefore a frame of reference, not a rigid constraint unless called for in a binding document. When looking for a proper filler metal electrode for welding Cast Iron, one should remember that manufacturers experiment even out of Specification limits. Therefore they may come up with non conventional solutions which may be exactly appropriate for a particular problem. Their claims may be overly optimistic, but they cannot be dismissed without trying. Readers are urged to try new products not yet standardized if their problem looks them difficult. First ask (describe your problem) then try (with somewhat different procedures), finally decide. -------------------------------------------------------------------- 5 - Online Press: recent Welding related articles Friction Welding using Insert Metal Optimizing Projection Welding for Hermetic Sealing http://www.aws.org/wj/2004/03/042/ Flying High with Orbital Welding http://www.thefabricator.com/xp/Fabricator/Articles/Welding/Weld04/04web154.xml Order for Earthquake Resistance in the Court http://www.thefabricator.com/xp/Fabricator/Articles/Welding/Weld04/04web165.xml Resistance Welding of Sheet Metal - Guide to Best Practice http://www.twi.co.uk/j32k/protected/band_8/bprwsms01.html Soldering: Process Control http://www.twi.co.uk/j32k/protected/band_8/bpspcs01.html Low Porosity Laser Welds in Aerospace Aluminum Alloys http://www.twi.co.uk/j32k/protected/band_8/bpspcs01.html Note: Access to TWI Information is available trough free subscription. -------------------------------------------------------------------- 6 - Terms and Definitions Reminder Gray Cast Iron: a cast iron, ferritic or pearlitic depending on carbon and silicon content, characterized by a gray fracture surface due to the presence of flake graphite. Malleable Cast Iron: a cast iron made by long time annealing of white iron in which decarburization, graphitization or both take place to eliminate some or all of the cementite (iron carbide Fe3C). Carbon is in the form of clusters of finely divided graphite. If decarburization is the predominant reaction, the product will exhibit a light fracture surface. Otherwise, the fracture surface will be dark. Ductile Cast Iron: a cast iron that has been treated while molten with an element such as magnesium or cerium to give graphite as nodules or spherulites, which imparts a measurable degree of ductility to the cast metal. Also known as nodular cast iron, spherulitic graphite cast iron, and spheroidal graphite (SG) iron. White Cast Iron: a cast iron essentially free of graphite, most of the carbon content is present as separate grains of hard Fe3C (cementite). White iron exhibits a white, crystalline fracture surface along the iron carbide platelets. Flake Graphite: graphitic carbon, in the form of platelets, occurring in the microstructure of gray iron. Nodular graphite: graphite in nodular (rounded) form as opposed to flake form. Spheroidal graphite: graphite of spheroidal shape with a polycrystalline radial structure. This structure can be obtained, for example, by adding cerium or magnesium to the melt. Graphitic carbon: free carbon in cast iron. -------------------------------------------------------------------- 7 - Stress Relieving after Welding As explained in the page on Distortion (Click here), welding processes introduce residual stresses into weldments. The stresses are locked in the structure, and are in equilibrium. They may have introduced distortion in the manufactured item; inspection should check and be aware of that. Residual stresses cannot normally be seen, except for two instances: when the material is prone to stress corrosion cracking or when selective machining or other local disturbance causes stresses to rearrange, introducing further warping. One more chance should be considered: that external stresses, superposed to residual ones, exceed yield or ultimate strength limit and cause failure. Therefore it is good practice to provide for stress relieve right after welding, before any further finish operation. Thermal stress relieving consists in heating uniformly the welded structure, to allow for relaxation of peak stresses. Yield strength at high temperature is lower than at room temperature, so that existing residual stresses rearrange through plastic straining as they cannot exceed the yield limit. Residual stresses will still be there, but at a lower level and without excessive peaks. For mild steel it is customary to prescribe stress relieve at 650 0C for one hour per every 25 mm (one inch) of thickness. Hardenable carbon steels are often regulated by Codes that prescribe under which conditions this treatment is required. Austenitic Stainless Steels may suffer from stress relieving at 815 to 925 0C (1500 to 1700 0F) if they are susceptible to carbide precipitation and intergranular corrosion.
See Stainless Steel Welding Cast iron, Alloy steels and Tool steel can be stress relieved of most of their shrinkage stresses due to welding if their weld beads are mechanically peened with a proper rounded hammer or by shot peening as soon as deposited. They should then be allowed to cool slowly in a furnace or under insulating cover and followed by a regular stress relieve at their proper temperature. Stress relieve temperature for alloy steels should not exceed tempering temperature if properties are not to be adversely affected. Also due consideration should be given to the effect of post weld heat treatment on impact properties. Stress relieving of heat resisting materials is generally limited to those alloys that are not age hardenable, and temperatures are usually below those used for full annealing. Welded Aluminum Alloys should be stress relieved at low temperature especially when particularly thick or when part of complex weldments. In certain cases, as for brazing of phosphor bronze or nickel silvers, experience suggests to stress relieve the material before brazing. Stress relieving after welding of magnesium castings containing more than 1.5% Al is always required to prevent corrosion cracking in service. Titanium weldments are stress relieved after welding to prevent weld cracking and stress corrosion cracking in service. Grain growth should always be prevented. In certain cases testing is recommended for checking that stress relieving will not adversely affect fracture toughness, creep resistance or other properties. Whenever thermal treatments are not feasible but it is still important to check residual stresses, they may be reduced by mechanical means. Besides shot peening (a surface process), vibration stress relief uses sub-harmonic frequencies to reduce any residual stresses, by introducing mechanical energy into the work piece. Special equipment is designed to do just that. -------------------------------------------------------------------- 8 - Site update The Page of the Month we announce this time is on Soldering. It is not about a new process but it still has quite important applications in electronics. And due to new requirements regarding the environment, much research was dedicated to develop acceptable filler metals. Click here. Also we recently introduced on certain pages of the Site a Search feature, which should help those impatient readers that look for some specific information. It should also avoid the impression that the site promises something that is not delivered. For an overview of the subjects treated in the Site we think that the most useful and informative page should be the Site Map. Cross reference with articles published in previous issues of Practical Welding Letter has been added with the intent to provide in depth information to interested readers. -------------------------------------------------------------------- 9 - Short items 9.1 - Cryogenic Cutting is a relatively new and expensive process to cut through hard materials "like a knife through warm butter". An introductory article about that can be seen in the March 2004 issue of Scientific American (at page 14). The process consists in a supercooled nitrogen jet which emerges from special nozzles at high pressure and low temperature. It was originally developed in the 1990s by the Idaho National Engineering Laboratory as a nonthermal method to cut open barrels of spent nuclear fuels. 9.2 - Copper welding is considered more difficult then joining of other metals because of the high thermal conductivity that subtracts most of the heat from the molten pool. The lower melting temperature (relative to steel) eases somewhat the task which can be accomplished with or without filler metal. Arc welding is applicable to most copper alloys except leaded or free machining types. An acceptable alternative is sometimes offered by brazing. 9.3 - Gouging is the forming of a bevel or groove or the removal of deep defects in castings or weldments by an arc or gas process. Arc gouging removes metal by melting it with the heat of an arc struck between a carbon-graphite electrode and the base metal. A stream of compressed air blows the molten metal away. The process cuts almost any metal. A special oxyacetylene cutting torch with a proper nozzle can be used for gouging. 9.4 - Chromium-Molybdenum heat resistant steels contain from 0.5 to 9.0 % Cr and 0.5 to 1.0 % Mo, while carbon is usually below 0.20 %. Thin sheets and tubes can be welded with mild steel filler metal, as the dilution of alloying elements will not normally affect mechanical properties. When using filler metal of the same composition, of the low hydrogen type, however, it is recommended to use preheating and post heating cycles to minimize the risk of hard zones or even cracks. 9.5 - Graphitization is the gradual and progressive transformation of carbon from cementite into graphite, in steels during long service at elevated temperature above 425 0C (800 0F). Graphite formation occurs in a narrow region in the heat affected zone of a weld. Its consequence is an increased risk of brittle failure as occurred in steam power plants and refineries. Although certain special stress relieve treatments can improve graphitization resistance or recover partly transformed carbon, the best way out of the problem is the selection of heat resistant Chromium-Molybdenum steel for tubing and structures instead of plain carbon steel. 9.6 - Lap Joints are used more often for aluminum alloys than for any other metals. The efficiency of lap joints, meaning the percentage of strength as compared to base material, is from 60 to 80% depending on alloy and temper. Main advantages are ease and economy of preparation, with the concurrent benefit of no need for backing bar and root side inert gas supply, if compared with butt weld. For proper transmission of tensile stresses without bending, lap joint design should be balanced and symmetric. Some disadvantage may stem from higher difficulty of post weld inspection if absolute joint integrity must be assured. -------------------------------------------------------------------- 10 - Explorations: Beyond the welder Volunteers scan the Ground in Research Projects: Look at the activities of Idaho National Engineering Laboratory at http://www.inel.gov/env-techengineering/ For those who think one should be able to tell the difference between Science and Pseudoscience, the following may be enlightening: http://www.skeptic.com/Index.html For an unusual Challenge that confronts different teams striving to succeed in a difficult and novel task, see Excitement, adventure and success are what drive a private initiative determined to establish a new and exclusive primacy: the first solo, non-stop, non-refueled circumnavigation flight of the globe in 80 hours. See it all at: http://www.virginatlanticglobalflyer.com/index.html -------------------------------------------------------------------- 11 - Contributions - Visual Inspection A kind correspondent contributed the view that Visual Inspection as a subject has been neglected in our Site. As he may be right and as the topic is an important one, we take the opportunity of providing here our views in a more detailed article. Visual inspection includes all observations performed by the naked eye or by using a simple low power magnifying lens. Special accessories called Borescopes permit visual inspection of internal cavities. Essential to the successful implementation of Visual Inspection are good and strong illumination conditions, possibly with a portable high power lamp, avoiding direct glaring in the eyes. Formal specifications will request a minimum of light expressed in foot-candles or equivalent units, to be measured by a calibrated photometer, as a precondition to the performance of correct Visual Inspection. Associated with this requirement is that of minimum visual acuity of the visual inspector, as checked, certified and documented in writing, periodically, by an approved optometrist. Although quite simple and straightforward, Visual Inspection should not be dismissed as trivial: in general an ugly looking welded joint is also possibly not a good one. In principle visual inspection should be the object of a specific inspection sheet, stating clearly the items that need to be checked and the requirement limits. Either if set up by a fitter or by the welder him/her/self, the joint preparation is obviously most important to the success of the operation. Details that must be controlled include dimensions, alignment, surface condition and cleanliness, gap width, condition of equipment and consumables, temperature, availability of proper fixtures and of auxiliary tools. Welders should be instructed and requested to dedicate as much time and effort as needed to achieve correct joint preparation. There is no point in trying to cover up poor preparation with extra welding, be it a gap too large or a misaligned joint. Visual inspection is used to check dimensions of joint preparation before welding and of the welded joint after welding. Not only the size but also shape and contour must be within requirements. Appearance must be conforming to accepted standards regarding roughness, uniformity, absence of spatter and cleanliness. Visual inspection is intended to find and mark for repair obvious visual defects such as cracks, excess concavity or convexity, unfilled areas, undercut, misalignment, craters, unacceptable arc striking spots and any other objectionable visual features. With the current tendency of limiting the workforce to the bare minimum there may be a trend towards formally charging the welder with the task of performing visual inspection of his/her own production (informally it should always be done). Two aspects should be taken into account: for one thing welder's time devoted to inspection will be detracted from welding time. Second, with the best of intentions, welder's judgment cannot be free of bias. In any case even a simple visual inspection requires a good measure of training and experience, together with thoroughness, integrity and dedication. One should make sure that whoever is charged with this important inspection be endowed with all requisites. -------------------------------------------------------------------- 12 - Testimonials From: Richard Parkes Howdy Elia, Best Regards, Richard Parkes ------------------------------- From: Rhythman99@aol.com Welding Advisers, Thank you very much for the information that you have provided me.
Kevin Kram --------------------------------------------------------------------
13 - Correspondence: a few Comments People use the Net to seek information, to look for solutions. We believe our service to be of value, from the enthusiastic thanks we get from time to time as reported in Testimonials. For those considering how best to solve a problem, we would urge them to contact us, explaining the question as completely as possible. Obvious welding failures require attention. But more subtle difficulties may be hidden under low production rates or high costs. It may pay to question from time to time if what has been done for ages is indeed the best available. You are always welcome to let us have your feedback. Click here. -------------------------------------------------------------------- 14 - Bulletin Board 14.1 - Following our previous announcement of a new page in the Site on Adhesive Bonding (click here), we would like to propose a Database which should contain information and useful data. Unfortunately when we tried the link we obtained an error, possibly only temporary. The Site is at: 14.2 - We added a search window on some pages of our Site, to ease the search of particular items that our readers may wish to find. We hope that this added feature will improve the Site usability. Let us know if you appreciate it. 14.3 - We have been advised by our Site Host, SiteSell.com, that a new and enlarged Quick Tour of their own Site is now available to browsers. The presentation shows how SBI! works... and why it works. Take a short detour and let your imagination roam about. We can tell you they are serious people involved in serious business; we could not have done what we did without them. Click here and here. -----------------------------------------------------------------------
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