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PWL, Issue #019-Corrosion in Ferritic Stainless,Soldering Aluminum,Wet-Spot Welding,SS and Magnetism March 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: March 2005 - Practical Welding Letter - Issue No. 19
----------------------TABLE of CONTENTS-----------------------------
1 - Introduction 2 - Article: Corrosion in Ferritic Stainless Steels 3 - How to do it well: Carburize with Oxyacetylene Torch? 4 - Filler Metals: Soldering Alloys for Aluminum 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Wet-Spot Welding System 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Stainless and Magnetism 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This 19th issue of Practical Welding Letter presents new subjects and provides more details on information items that were dealt with before. We open with an Article on corrosion susceptibility of Ferritic Stainless Steels which some people wonder why should they be attacked at all by the environment. Welding has important influences to be taken into account. One of our readers had a bright idea: he asked if it would work. Unfortunately that is not the case. You can see both the question and the answer in section 3. While soldering is not the first process you would think of when confronted with joining aluminum, it is readily feasible and with a few advantages that may solve your other problems. It may be useful to know. The next featured Article was sent to us for publication by an innovative firm in the UK which caters mostly for Underwater Welding. The new self contained wet arc spot welding was designed to reduce the importance of manual skills in performing certain delicate applications. It should have the potential to be developed into a fully robotic operation. It may be useful also in dry applications. Of the Short Items we would like to point out the first, an ambitious research program dedicated to commercialize a new process, already demonstrated in the laboratory. It is about performing Electron Beam Welding in air within the protective environment of a plasma column. The potential for success seems huge. Other departments are in place as usual. For your feedback, most appreciated, we would ask you to write us by e-mail. Click here.
2 - Article: Corrosion in Ferritic Stainless Steels The susceptibility of Ferritic Stainless Steels to increased corrosion as a consequence of welding, pertains to the larger subject of Weldability and includes resistance to intergranular attack (IGA), Stress Corrosion Cracking (SCC) and general overall corrosion resistance. Ferritic Stainless steels, containing from 10.5 to 30% Cr with other alloying elements, in particular molybdenum, are noted for their excellent stress-corrosion cracking (SCC) resistance and good resistance to pitting and crevice corrosion in chloride environments. While these alloys have useful properties in the wrought condition, welding is known to reduce toughness, ductility and corrosion resistance because of grain growth and formation of martensite. Welding these commercial grades usually requires preheat and postweld heat treatment. To improve weldability, some of the standard ferritic grades have been modified to contain lower levels of chromium and carbon, with additions of ferrite stabilizers like aluminum. These alloys can be exposed to high temperatures and are suitable for diverse applications. Other modifications call for production of ultra pure alloys, limiting to the maximum the interstitial elements like Carbon and Nitrogen and adding tiny amounts of Titanium and Niobium. This is the most direct method of preventing intergranular attack in ferritic stainless steels. Ferritic stainless steel filler metals offer the advantages of having the same color and appearance, the same coefficient of thermal expansion, and essentially the same corrosion resistance as the base metal. However, austenitic stainless steel filler metal are often used to obtain more ductile weld metal in the as-welded condition. Certain temperature ranges promote the precipitation of undesirable intermetallic embrittling phases. In particular a typical embrittlement occurs at and around 475 0C (885 0F). Annealing treatments can alleviate the embrittled condition. Service exposure of welded ferritic alloys containing high levels of chromium and molybdenum should avoid these temperature ranges. These steels develop brittle behavior below room temperature: the ductile to brittle transition temperature (DBTT) depends on stainless type and thickness. Upon long time exposure to certain elevated temperature levels, varying with the types of ferritic stainless steels, a particularly troublesome sigma phase can develop which is both brittle and susceptible to corrosion. The mechanism for intergranular corrosion in ferritic stainless steels is due to Chromium compounds precipitating at grain boundaries and consequent chromium depletion in the grains immediately adjacent to the boundaries. This chromium content lowering increases corrosion rates in solutions used to evaluate intergranular corrosion. The sensitization of ferritic stainless steels occurs at higher temperatures than for the austenitic types, therefore the fusion zone and the weld itself are the most likely areas for intergranular corrosion.
3 - How to do it well: Carburize with Oxyacetylene Torch? Q: Is it practical to use an oxyacetylene torch to raise the carbon content of mild steel in an attempt to carburize the mild steel for case hardening by heating and water quenching? A: Unfortunately no. What can be done instead is "pack carburizing" in a stainless steel box. Proprietary products made for this purpose, contain besides carbon (in the form of charcoal and coke) also other important ingredients (carbonates of barium, calcium, sodium) called energizers. Parts are buried in the granular carburizing product, the box is closed with a cover and loaded in a furnace at about 850-900 0C (1560-1650 0F) for 4 hours or more, depending on the depth of case required. If parts can be removed quickly from the granules they may be then quenched immediately in water. Otherwise the box is slowly cooled in the furnace and then the parts are reheated for quenching.
4 - Filler Metals: Soldering Alloys for Aluminum Soldering for Aluminum Alloys can be considered when strength of joints is not an important issue. Advantages are minimum influence on properties of base metal, due to the low temperature, reduced distortion and easier removal of flux. Solderability depends on the ease of wetting the metal surface. Oxides present must be removed from the surface because they interfere with wetting. Oxides of non heat treatable alloys are removed by low temperature organic type fluxes. The heat treatable aluminum alloys, which present oxides that fluxes cannot remove, must be chemically or mechanically cleaned before soldering. Alloys containing Magnesium and/or Silicon over a certain level are difficult to solder. Commonly soldered wrought aluminum alloys are 1060, 1100, 1145, 3003, 5005, 6061, 7072, and 8112. Aluminum casting alloys are not solderable. Solders with high zinc content are used on wrought aluminum assemblies. Tin-lead solders are not recommended because of their poor corrosion resistance, besides their adverse influence on health. In the following table, derived from older sources, the present drive towards elimination of Lead containing Soldering Alloys because of health related problems, was not yet initiated. A revised source, the Aluminum Soldering Handbook of 2004 is now available from the Aluminum Association. of Soldering Alloys for Aluminum
5 - Online Press: recent Welding related Articles Our Article on Design for Welding will be online March 8. After that date
it will be visible in the Exclusive Articles box on Ultrasonic Spot Welding for Aluminum Electroslag Welding for fabricating Bridges Robotic Arc Welding Flowmetering technologies
6 - Terms and Definitions Reminder Alclad is a commercial designation of a composite wrought product (sheet or strip) that has an aluminum alloy core covered on one or both surfaces by a metallurgically roll bonded aluminum or aluminum alloy coating. The cladding protects the core against corrosion because it is electrochemically anodic to the core. Austempering is a special heat treatment for ferrous alloys in which a part is quenched from the austenitizing temperature at a rate fast enough to avoid formation of ferrite or pearlite. It is then held at a temperature just above Ms (the starting temperature for Martensitic transformation) until transformation to bainite is complete. Bridging in Casting is the premature solidification of metal across a mold section before the metal below or beyond solidifies. In Soldering it is an unintended solder connection between two or more conductors. It is also called a crossed joint or solder short. Chromizing is a surface treatment performed in pack, vapor, or salt baths at elevated temperature by the inward diffusion of chromium into the base metal. Surface alloying with chromium provides corrosion resistance to otherwise unprotected metals. Formability represents the relative ease with which a metal can be shaped without cracking through plastic deformation. Properties affecting formability of a metal include strength, ductility, and the amount of deformation required to cause fracture. Misrun in Casting is the incomplete filling of the mold due to low pouring temperatures, gas back pressure from inadequate venting of the mold, and inadequate gating. Quenching consists in the rapid cooling of metals (often steels) from a suitable elevated temperature. This generally is accomplished by immersion in water, oil, polymer solution, or brine (salt water including chlorides, carbonates, and cyanides), although forced air or inert gas are sometimes used. The salt addition improves the efficiency of water at the vapor phase or hot stage of the quenching process. Suitable quenching provides the required rate of cooling to generate the microstructure sought for, while avoiding objectionable results (distortion, cracking). Sensitization is the precipitation of chromium carbides, usually at grain boundaries in stainless steels. In the austenitic types it occurs on exposure to temperatures of about 540 to 845 0C (about 1000 to 1550 0F), leaving the grain boundaries depleted of chromium and therefore susceptible to preferential attack by a corroding medium. Welding is the most common cause of sensitization that leads to intergranular corrosion along two strips from both sides of a weld bead. Compare with the last paragraph in the article of section 2 above. For more information on this subject click on Stainless Steel Welding.
7 - Wet-Spot Welding System Note: the following Article was minimally edited from the one titled Speciality Welds Ltd. has developed solutions to the long-standing problems associated with underwater wet welding and in particular the skills required to produce high quality welds in poor and/or nil visibility. The new system, which has been named HammerheadTM in keeping with the company's ‘fish’ brand, addresses problems in obtaining high quality wet welds in nil visibility, without the need for experienced (skilled) welder-divers. By removing the individual welding skills from the operation, there’s no need for the diver-welder to control parameters that affect quality, such as travel speed, electrode angle, arc length, accurate deposition, etc. Because the operator no longer needs to control these parameters, it’s not essential to have good visibility. So, even in nil visibility conditions high quality repeatable welds can be produced time after time. How the System works In removing the skills necessary to carry out underwater wet welding, we have modified the fundamental approach to how ‘stick’ welding is carried out. Our system allows the operator a far more simplified role. How is all this achieved? In simple terms, by creating a spot/plug weld rather than having to deposit a fillet weld within a specified joint. By removing the need for a fillet weld deposit we have also simplified the joint configuration (simple lap joint) and all the preparation that goes with it, while also removing the need for extensive cleaning of the joint area, chipping off meters of slag prior to additional passes, etc. In fact there’s no need for additional passes as the process is designed as a ‘one-shot’ approach, i.e. one electrode produces one spot/plug weld. Other than the control system/electrodes, all other equipment is exactly as conventional ‘stick’ welding. The control unit is connected to the welding power source via the remote control facility and is powered by 110v supply. All welding leads pass through our 400 amps Piranha safety switch before going to the diver. The Role of the Control Unit The control system manages and manipulates the following;
The first high current setting allows the electrode to pierce through the materials, thereby, creating a hole through which both materials are joined together. The role of the timer is to limit the depth of this penetration, so as to avoid bursting through the base (back) material. After the first weld cycle is completed and depth of penetration achieved, the second, lower current is automatically initiated and it’s this current that fills the hole, creating a spot/plug weld that has penetrated both sections of material, creating a weld nugget. During the operation the diver or indeed a robot need only apply sufficient pressure to the electrode to push it through the material while welding. The guidelines shown below provide basic benchmark settings for selecting current and timer. The operator can then make any minor adjustments as are seen necessary to ensure adequate weld quality. Presently only one size of electrode is available, namely 3.2mm (1/8”) but as you can see this covers a wide range of material thickness.
Weld Strength Properties (Note: the original Article discusses here, neglecting any bending moment, the strength of one spot weld in shear, as a simple product of the circle (spot) area and the shear strength assumed as 4/5 of the Ultimate Tensile Strength). We see that the spot weld offers nearly twice the UTS capability for a fraction of the deposited weld metal. So when you consider the time saved in deposited weld metal, cleaning, preparation and ease of use, as well as being able to weld in nil visibility, the benefits are considerable. The weld offers strength improvements over normal ferritic steel electrodes and generally the heat-affected zone (HAZ) hardness is improved. The specially formulated electrodes have a 22.5Cr and 14.45Ni equivalent, thereby allowing for high percentage dilutions, of up to 50%. Conclusions The process clearly remains a manual welding operation and is not defect free. Trials were limited to flat bar/plate stock welded in the flat and vertical positions. However, the welding skills necessary for a diver to produce a weld have been removed, as have much of the material preparations and need for good visibility. Of course, there is a period in which the operator will need to familiarize himself with the process, but this can be achieved in a very short time. A good diver would be expected to produce an acceptable weld within a matter of minutes rather than hours, or days. The process does offer solutions to welding in poor visibility, without the need for skilled welders. So all in all, we believe this process offers a serious alternative to this long-standing problem. A full and detailed report has been published following the Smart Award trials and is available to download from the website at www.specialwelds.com The report covers the welding of wet and dry fillet welds and wet and dry spot welds, and offers a direct comparison for weld quality and strength, whilst providing for the welding variables used. It also shows the results provided by non-welders and welders alike. I would add that we have also discovered that the Hammerhead spot welding system is showing signs of being able to carry out dry welding very well. In particular, crack repairs in steel structures, due to the fact you only need access to one side of the material and that full penetrating welds are achieved, but also because there is no need for grinding and gouging out of material/weld. Simply place the electrode at the start of the crack and push the electrode through the wall thickness, thereby creating a nugget weld. These can then be joined up along the length of the crack. It saves a considerable amount of time and energy, not to mention costs. David J. Keats
400-amp safety switch/control unit.
8 - Site Updating This time the Page of the Month discusses setting up and managing a Welding Shop. Running a welding business is a job that requires much more than welding knowledge and experience but gives a new dimension of freedom and of satisfaction. Before jumping in a new venture the entrepreneur is urged to check all the subjects involved. To reach the new page click on Welding Shop. Readers are reminded that the best and updated page where all Site items are listed and linked is the Site Map. Write us your thoughts as you go through this Practical Welding Letter. For comments or feedback readers are urged to send us their messages. Click here.
9 - Short Items 9.1 - Plasma Window for EBW in air. An important development work that has great chance of winning a substantial market share of Electron Beam Welding applications in air was recently reported in the AWS Welding Journal for January 2005 (on page 20). It consists in enveloping in a plasma column a powerful welding electron beam as it exits from the high vacuum electron gun. Within the low density plasma "window" the electron beam can proceed without being dispersed by air molecules until it impinges on the target to deposit there its high density welding energy. It is a new "hybrid" technique (combining two unrelated principles) that when fully commercialized will bring considerable advantages and cost savings. 9.2 - Low pressure Vacuum Carburizing is a non-equilibrium case hardening process where advanced steels are first austenitized in a low vacuum dedicated furnace, directly carburized in hydrocarbon gas at partial pressure, then diffused to spread Carbon through the depth and finally quenched in gas or oil. Vacuum carburizing offers better uniformity and repeatability than traditional gas carburizing because of process control, improved mechanical properties due to reduced intergranular oxidation and shorter cycle times if, depending on the steel selected, higher process temperatures can be used without fear of grain coarsening. Typically the process includes the following steps:
9.3 - Ultrasonic Soldering permits disposing of tenacious oxides without the use of flux: this is a highly recommended feature because flux remnants are a contaminant that may be corrosive. Fluxless soldering can be very desirable especially with certain aluminum alloys, containing somewhat elevated levels of Magnesium or Silicon, that cannot be readily cleaned even by energetic fluxes. The process is usually applied when the self fixtured parts to be joined are immersed in a tank of molten solder. Applied ultrasonic energy generates cavitation and bubbles which, upon imploding, impinge on the metal surface creating a strong impact, which cleans the surfaces and destroys the oxide film permitting the solder to wet the metals and fill the joints. 9.4 - A Fuel Cell is a pollution free electrochemical device that combines hydrogen fuel and oxygen from the air to produce electricity, heat and water without combustion. Hydrogen is converted directly to electricity. The fuel cell, without any moving parts, is a quiet and reliable source of power that can operate at much higher efficiencies than internal combustion engines. The fuel cell is composed of an anode (an electrode from which electrons flow away in the external circuit), an electrolyte in between, and a cathode (the electrode toward which flow electrons from the external circuit). As hydrogen flows into the fuel cell anode, a catalyst on the anode helps to separate the gas into protons (positively charged hydrogen ions) and electrons. The electrolyte between the electrodes, which is a semi-permeable membrane, allows only the protons to pass through to the cathode side of the fuel cell. The electrons freed from the hydrogen molecules cannot pass through this electrolyte but flow through an external circuit as electric current that can power any electric load. As oxygen from air flows into the fuel cell cathode, another catalyst helps the oxygen, protons, and returning electrons back from the external circuit combine together to produce pure water and heat. Individual fuel cells can be then combined into a fuel cell stack. The total voltage is given by the number of fuel cells in the stack , while it is the surface area of each cell that determines the total current available. 9.5 - Inductors are critical items in Induction Heating for hardening, tempering, brazing: the success of induction applications depend on selection and design of suitable inductors, influenced by different factors. Heating can be generated within steel or cast iron parts by electromagnetic induction. When alternating current flows through an inductor or work coil, an alternating magnetic field is established within the coil. The rapidly alternating magnetic field induces eddy currents into the part to be heated. It is the resistance to the flow of induced eddy currents that causes heating by I2R losses. Inductors are coils, usually made of copper tubing cooled by circulating water, where a high frequency alternating current is made to flow. Heating pattern in the processed parts depends on the shape and number of turns of the coil, on frequency and on power input. Higher frequency concentrates the effect in a shallower layer. Short cycles are preferred to localize precisely the heating near the surface. Due to the complexity of the intervening factors it is highly recommended to rely on the advice of experts in the selection and design of proper coils for actual applications. 9.6 - Fire resistant liquid Quenchants. Quenching oils are available providing good and repeatable quenching characteristics to obtain consistent results in hardening steels day after day. Fire hazards however are a constant risk, the more serious the larger the quenching tanks. Oil surface can get ignited when a hot load from a furnace remains partially submerged for a short time, due to a mechanical failure of the hoisting device or because of human error. In case of fire that cannot be immediately extinguished, extensive damage to structures and injury to personnel may follow. The best measure against fire hazards would be the substitution of the quenching medium with a fireproof water mix. Water or brine as a quenchant would obviously be immune from fire dangers but its characteristics provide an unfavorable cooling pattern causing distortions or cracks in most parts of high hardenability steels. Organic polymers were developed that when mixed in various proportions with water provide a somewhat more sluggish cooling cycle than plain water giving adequate quenching results with full repeatability. If the proprietary products available prove their worth in practice in any given heat treatment shop with its own mix of steels and parts, it is highly recommended to eliminate fire hazards by switching gradually away from dangerous quenching oils.
10 - Explorations: beyond the Welder Cobalt Development Institute International Molybdenum Association Stainless Steel Information Center International Titanium Association Center for Heat Treating Excellence
11 - Contribution: Stainless and Magnetism The following question was received from one of our readers: "When welding 304/304L dual grade stainless structural tubing with 308L wire and using 982 (Argon 98%, Oxygen 2%) gas, why does the tube become magnetic or should it after welding? Does it change the physical properties of the metal in the heat affected zone? it seems that the closer you get to the HAZ it becomes more magnetic." Here is our answer, expanded to provide basic explanations: Austenitic stainless steel is essentially non magnetic when in annealed condition. It can gain slight ferromagnetic properties upon work hardening (tube drawing) because some of the austenitic structure transforms to martensite. This slight magnetism can be observed if tubing is not annealed after drawing. Welding will not increase magnetic properties, on the contrary it anneals the material reducing magnetism. However the compositions of most filler metals are adjusted by the manufacturers to produce weld deposits that have ferrite-containing microstructures. The amount of ferrite in the structure of the weld metal depends on the ratio or balance of ferrite-forming to austenite-forming elements. It is given by the ferrite number, an arbitrary, standardized value designating the ferrite content (of an austenitic stainless steel weld metal) that is determined by the magnetic test described in AWS A4.2. At least 3 or 4 FN (Ferrite Number) is needed in the as-deposited weld metal (of austenitic stainless steel) for effective suppression of hot cracking. Ferrite-containing weld metal may have certain disadvantages in a welded austenitic stainless steel. Ferrite is ferromagnetic, and the increased magnetic permeability of the weld metal may be objectionable in certain applications. When exposed to service at elevated temperature, the ferrite in some weld metals may transform to sigma phase and compromise mechanical properties and corrosion resistance. Physical properties are not changed, HAZ is essentially annealed. If strong ferromagnetism is detected, the material might be different from what it is supposed to be. In case of doubt, for identification, the material could be checked by X-Ray Fluorescence. Click here for Materials Identification.
12 - Testimonials From: "glenn webb" 'rodofgod@msn.com' Hi! Just thought I'd take this opportunity to thank you for an excellent service! Regards
From: "garth fort" 'garthfort@msn.com' Article very good for starters...
13 - Correspondence: a few Comments I am truly amazed and interested by the breadth of subjects treated in the correspondence I receive. I always admire people who start something new, like a horse farm in some need of fence weld repair. I believe that people new to welding should arrange to follow a formal course in a good welding school. The main gain from hands on experience is the exposure to actual practice of safety issues, besides the sort of familiarity one can get with materials and processes. And with people too. Readers ask sometimes for specific suggestions relative to the type of equipment they should purchase for their particular needs. This is a most difficult question. In order to provide a meaningful answer I should inquire in detail not only for the intended application but also for the previous experience of the inquirer. And even then any selection would exclude every other possible choice, that might be preferable when a certain level of skill is acquired. It is also a matter of personal preference, to find the specific piece of equipment that one loves to work with. Therefore even at risk of appearing unhelpful, I prefer generally to suggest that the inquirer get the opportunity to try for him/her/self different types of welding machines and accessories (before committing to a purchase) by one of the two following ways. Either by obtaining the permit of working for a short while in an existing welding shop (consider the additional advantage of getting also the comments of the people there). Or to check leasing or renting the hardware for a week or two. This will get the inquirer acquainted not only with the machinery but also with the people who are supposed to provide help and service.
I wish I could always help, but most of times I get no follow up, so that I do not know: it would be a nice reward for me to be informed that my advice actually solved one's problem, so please let me know. For your feedback, questions, comments etc., write us by e-mail, click here.
14 - Bulletin Board 14.1 - It seems that the World is full of Conferences, Conventions, Exhibitions. One of the most important is certainly the AWS Welding Show 2005, to be held at the Dallas Convention Center, 650 South Griffin Street, Dallas, Texas 75202 on April 26-28, 2005. Besides the long list of Exhibitors who will show their stuff on the Exposition Ground, a long list of featured Events are programmed. Professional lectures, Educational sessions, Conferences, Seminars and other activities. 14.2 - Why should we include, in a newsletter dedicated to Welding, the recommendation to our readers to visit a few pages of the Site Host we are proud of? It is because we believe that they have many benefits to offer to everyone, and because we are genuinely convinced that they are honest, conscientious and helpful. If not to the readers themselves than to one of their strictest family and friends. Our Website could not have been built without their help, and the education and tools they provide work just for anyone who has no previous experience of the Internet. We recommend that you take some time to visit any of their pages reachable from our Welding Advisers site or that you just link to a page called SiteSell Resources that may open new frontiers for you. We appreciate your involvement. Send us your feedback by e-mail, click here.
See you next time Copyright (c) 2005, by Elia E. Levi and welding-advisers.com, all rights reserved |
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