| Back to Back Issues Page | |
 |
|
|
PWL#052 - Cleaning for Brazing, Welding a Shaft, Filler Metals development, Abusive Grinding December 03, 2007 |
|
| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#052 - Cleaning for Brazing, Welding a Shaft, Filler Metals development, Metallurgy of Abusive Grinding, Welding Articles, Tandem GMAW, 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. 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: Cleaning for Brazing 3 - How to do it well: Welding a Shaft 4 - Filler Metals and other developments 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - Metallurgical Aspects of Abusive Grinding 8 - Site Updating: Welding Articles 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Tandem GMA Welding and Surfacing 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This new Issue, the 52nd of our Practical Welding Letter, opens with a reminder of a critical stage in brazing, namely cleaning for preparation. Even those who are not currently involved should remember its importance for successful processing. We then answer to a specific question from a reader, who attempted to modify an existing shaft by welding a new element. As the parts to be joined are carburized and hardened, it is unfortunately not a recommended procedure. If the welded part had to be investigated, the unsafe microstructures would be evident. But the originator seems unwilling to listen to the warning. Filler Metals Trends and Development were presented at the last FABTECH. It may be important to keep informed of advancements. Abusive grinding is a dangerous condition that can be produced in ground parts by careless machining. As it is not immediately evident by visual inspection, the dangers should be known, as well as the testing procedures to be used not only to reject affected parts but to keep the process under control. The Page of this Month provides a list of Articles written on welding and metallurgy related subjects by this Author in various occasions. As they appear mostly in the online professional press, they are easily reached by interested readers if the matter indicated in the titles is likely to interest them. Tandem Welding is a high productivity process capable of competing with other methods both for joining and for surfacing. Detailed cost analysis has to be performed in order to decide which process is the most economic to run. The proliferation of acceptable solutions demonstrates the vitality of the search for improvement. Other regular columns are as usual in place. We hope you enjoy this reading. Your comments are always welcome: but please do not use the "Reply" feature, because copying unnecessarily the whole newsletter clogs the inbox. Use instead the Contact Us form. 2 - Article: Cleaning for Brazing It is known that brazing depends of surface spread of molten filler metal and on capillary attraction to distribute it evenly and completely in the joints. Any surface condition that disturbs the wettability of the base metal by the filler metal is likely to interfere with the process and to cause flaws. One should remark that the flux performs only the specific action of removing oxides from the base metal surfaces but that it is unable to remove oil, grease, paint and other contaminants. Therefore the proper sequence should be always observed. Oil and grease have to be removed first, by immersion in proper solvents (liquid or vapor degreasers), then paint if present, then rust (by acid pickle or by clean mechanical means followed by rinsing and drying). Care should be used not to leave traces of cleaning media into crevices or blind spots. If compressed air is used to remove cleaners, it should be oil free and dry, as seen by blowing short puffs on white paper. A simple test for cleanliness involves the use of clean demineralized water. It is called the Water Break Test. If it does not wet the whole surface it could mean that cleanliness is not enough. Cleaned parts should be manipulated with clean gloves, not with bare hands, should be assembled in a hurry and promptly brazed. It is good practice to establish a maximum permitted time between cleaning and brazing. References: Introduction to Brazing of Aluminum Alloys. Understanding Brazing Fundamentals 3 - How to do it well: Welding a Shaft Q: I am trying to create a special longer input shaft for car manual 5 speed transmission by welding a newly created, heat treated SAE8620 shaft (HRC58-62) to the gear end section made from heat treated SAE4620 (HRC58-62). Shaft end is SAE 8620 HRC 58-62, (carburized) .025-.035" case depth, quenched in hot oil @ 350F and tempered @300F for one hour. Gear end is SAE 4620 HRC 58-62 with same heat treatment as shaft end. The parts are assembled with light press fit and welded. Samples have been preheated to 300F and TIG welded with 80SD2 filler metal, air cooled with no PWHT (Post Weld Heat Treatment). Using GTAW setup, I need recommendation for pre-weld heating, filler material, and post-weld cooling process. Must be careful not to create cracks or weak areas at weld and must not anneal weldment so as to reduce hardness of gear teeth. Is there a post weld non-destructive quality inspection process? I am modifying existing already hardened input shafts (gear end piece) by welding on new longer shaft end with different spline (26T vs 10T original). I had the shaft end hardened as needed for wear resistance for the clutch to slide along the splined area. A: Hardness of the two elements is way too high to be suitable for welding. Welding is here a recipe for trouble. If you were to cut a sample part through the welds already done and run a microhardness survey you would find that the surroundings of the weld have been annealed or softened, while the Heat Affected Zone bordering the
welds has hard, untempered martensite, which is dangerous in that it can develop cracks if subject to fatigue. In principle it is not good design practice. A non destructive testing could be found, but the proposed welding should not be approved for such a critical application. If you could first weld annealed materials and then carburize, harden, temper AND control Heat Treatment deformations, you might be able to produce a sound part. [Note: There may be an acceptable way out though, if the project is important enough to warrant paying for consultation] 4 - Filler Metals: Trends and Developments The most important welding event, FABTECH International and AWS Welding Show, has just taken place in Chicago the past month. The mass of information presented to the audience in only three days was enormous. Click on Fabtech Program. Those who attended were frequently confronted with tough choices because interesting lectures on unrelated subjects were delivered at the same time in different rooms. Some of the lecture summaries will be published in the coming months. A whole session was dedicated to Recent Developments in Consumables Design and produced eight papers of which meanwhile we can only see the titles in the Fabtech Program above. At least two of these announced the presentation of new tools to design innovative consumables. As already stressed in previous papers the use of predictive strategies is bound to grow in order to reduce developing time and expenses. Another lecture, delivered within the frame of the Hot Wire Welding and Cladding Conference, itself a very interesting and hot subject, presented a study titled Wire Surface Condition Impacts Hot Wire Weld Quality. It was found that the surface condition of welding wire 625 used (a nickel alloy), had remarkable influence on the quality and integrity of hot wire overlay deposits. In particular surface roughness, residual contaminants and wire cast were singled out for their damaging action if not controlled.
One modern development tool as mentioned above, Computational Thermodynamics, was used as reported in a specific case. The following paper, whose abstract only is available online, describes the current state-of-the-art and future trends in the development of welding consumables, directed by the strategy of providing higher efficiency, higher performance and "environmental improvement". Additional References: The Welding Industry and its Future. Cored Wire Trends in the American Market. 5 - Online Press: recent Welding related Articles From TWI: A Novel Method for joining dissimilar Materials The Use of Power Beams in Surface Modification Sorry! Both the links above were removed by the source TWI From ASNT: From Materials Characterization: A Sample Issue for
January 2007 is available. From Miller Electric: 6 - Terms and Definitions Reminder Flanged Joint can be any one of the basic joints (butt, corner, T-joint, lap, edge) if at least one of the elements has a flanged edge shape. Induction Brazing is a method of brazing where the heating is provided by electrical resistance of workpieces to the flow of induced currents. Joint is the location or volume where the joining process (welding, brazing etc.) has to take place or where it actually took place. Laser Acronym for Light Amplification (through) Stimulated Emission of Radiation. A device known by the same name, produces a beam of coherent (monochromatic) concentrated light capable of extremely high energy density useful for welding, cutting etc. Mechanical Bond (in thermal spray applications) is the adhesion of sprayed material by mechanical interlocking unto roughened surfaces of substrate. Oxidizing Flame is one capable of producing oxidation of metallic surfaces by virtue of excess oxygen in the combustible gas mixture. Sheet Separation (in resistance spot or seam welding) is the gap forming between the overlapping surfaces around the weld as a consequence of the electrode pressure. Surfacing is the deposition of a dissimilar material on a surface, to obtain special properties or dimensions by welding, brazing or thermal spraying. 7 - Article - Metallurgical Aspects of Abusive Grinding If you used to think that a ground surface has to be evaluated only by its smoothness and finishing level, you may be in for some surprise. We introduced briefly a method of grinding control (Barkhausen Noise Analysis - BNA) in a short note (6.2) in PWL#022. Here we deal with the metallurgical damages caused by abusive grinding. It is known that surface roughness refers to the arithmetical average of the deviation from a mean centerline. The measuring method is adopted by the American National Standards Institute, ANSI B 46.1 and is commonly expressed in micro inches or in microns. A smaller number is associated with a smoother surface. Grinding is a method of precision machining used to obtain accurate shapes and tight dimensional tolerances in metal objects by the mechanical action of irregularly shaped abrasive grains bonded to a grinding wheel. Grinding is an energy intensive material removal process. A large part of this energy is converted to heat. In grossly uncontrolled processes one can actually see a glowing (overheated) spot at the contact location between grinding wheel and work. Powerful cooling systems are usually in place, designed to avoid unduly high local surface heating. Nonetheless the risk exists of introducing high residual tensile stresses and even surface alteration defects like burns and cracks or local softening. Any such modification, even if not detectable by unaided visual inspection, can substantially reduce the resistance to fatigue failure. Several factors influence the actual grinding outcome: therefore all the parameters involved (coolant type and flow, grinding wheel type, linear speed and depth of feed) should be clearly specified for assuring constant processing. It is good practice, and indeed a requirement for the production of sensitive materials like hardened steels, and for delicate parts like gears and bearings, to perform certain nondestructive testing procedures aimed at correcting faulty grinding conditions and at repairing or rejecting faulty parts. For process control of normal (non stainless) steels the conventional method of detecting grinding flaws like rehardened spots and burns is called nital etch, using a mild solution (2-4%) of nitric acid (HNO3) in methyl alcohol. The procedure has to be formalized into specific instructions, and dedicated inspectors must be instructed and supervised. The chemical attack causes the appearance, on the steel surfaces affected by uncontrolled grinding heat, of specific stains that must be correctly interpreted. A white spot surrounded by a dark circle is indicative of rehardening, that is heating above the transformation temperature of the steel and self quenching: this is a most damaging condition likely to introduce cracks and let them grow to failure. Dark spots or burns indicate an uncontrolled heating above the correct tempering temperature, and a corresponding loss of hardness, dangerous on wear resistance surfaces. Additional tests to be used for process control may include microhardness exploration of the stained surfaces and X-Ray diffraction to determine the residual stresses present, besides BNA mentioned above. For mass production, grinding process control automated test methods may be preferred. References: Detection of Thermal Damage [...]. X-Ray Diffraction Stress Analysis as an NDE Technique. Surface Finishes 8 - Site Updating: Welding Articles The Page of this Month presents a collection of various Articles titles, written by this Author for specialized press, over a period of a few years, and mostly available online. Different subjects are dealt with, generally concerning welding and/or metallurgy. The page could be handy for readers wishing to explore the arguments exposed. It could also be a good start for digging deeper into the matter. The new page can be seen at Welding Articles. You can find new Website pages as they are written or updated by reviewing periodically the Welding Blog or the Site Map. You can still benefit from building your Encyclopedia Online by visiting the page Metals Knowledge. Let us have your comments and feedback. Click on Contact Us. Or, even better, accept our Invitation and participate in the Welding Talk. 9 - Short Items 9.1 - Grit Size is the nominal size of abrasive particles in an abrasive powder or medium or in a grinding wheel, corresponding to the number of openings per linear inch in a screen through which the particles can pass. The distribution of sizes in a given mass of powder is determined by the weight percentage remaining on each sieve of a series in a standard size progression. 9.2 - Heading is the upsetting or deformation by axial compression of wire, rod, or bar stock in dies, to form or forge parts that usually contain portions that are greater in cross-sectional area than the original wire, rod, or bar. 9.3 - Hooke's Law is a general rule applicable to solid materials, which states that stress is directly proportional to strain, below the proportional limit. The relationship is expressed as a constant, E equal to the ratio of stress to strain. E is the modulus of elasticity or Young's modulus. 9.4 - Nital Etching is a metallographic procedure consisting in applying a mild solution (2-4%) of nitric acid (HNO3) in methyl alcohol to metallic surfaces. It is used to etch metallographic polished specimens for viewing microstructures under the optical microscope. It is also used as a nondestructive test for abusive grinding on finished parts. 9.5 - Offset refers to the stress- strain diagram obtained while performing a tensile test with an extensometer mounted on the tested specimen. The segment along the strain coordinate, usually at a distance of 0.2%, is called offset. A parallel line to the initial portion of the obtained graph, drawn from the offset point, is used as a measure of the yield strength. This is defined on the stress coordinate at the intersection of the parallel line with the stress-strain curve. 9.6 - Reaming is an operation in which a previously drilled hole is sized and contoured accurately by using a rotary cutting tool (reamer) with one or more cutting elements (flutes or teeth, blades if reported). The principal support for the reamer during the cutting action is obtained from the workpiece itself. 10 - Explorations: beyond the Welder Hubble and the Mystery Comet Special Report: Climate Change GLAST - Gamma-ray Large Area Space Telescope Nobel Prizes 2007 Want to learn how to pull a tortoise out of a hat?
11 - Contribution: Tandem GMA Welding and Surfacing A continuing race is ongoing among different processes and their modifications or adaptations in search for ever increasing weld deposition rate and productivity. The relative advantages are promoted by recent progress in the design of power sources and of electronic controls, and by the experience gained in current applications. Each one of the improved processes may give its best performance for definite applications. As usual the "best" should be understood as the solution that provides all the quality requirements at the lowest production cost, all expenses included. Availability of equipment may overrule search results. Tandem gas-shielded metal-arc welding in the high productivity range is one of such processes suitable not only for welding of joints but also for weld surfacing. Its high deposition rate, application flexibility and good surface quality make it economically attractive to manufacture by surfacing of high-quality corrosion-resistant, temperature-resistant and wear-resistant components, mainly in cylindrical or in tubular form. Low dilution with the parent metal can be achieved, when advisable, by the selection of suitable parameters. Gas Metal Arc Welding is being extended beyond the classic and well known applications and is becoming a rewarding high performance method due to modern power source technology, availability of improved filler metals and increased experience with different shielding gases. By virtue of its improved deposition efficiency, high current Gas Metal Arc Welding can now be considered for new applications in direct competition to Submerged Arc Welding (SAW). References:
Tandem MIG Process for increased Production. Tandem MIG welding for improved Productivity. Using the Tandem Welding Process to your Advantage.
12 - Testimonials Cymmer, Thomas G. Dear Elia, I hope you can help and look forward to your reply. On Thu Nov 01 05:27:23 2007, the following results were submitted from the "Form 5" First Name: solomon Thank you. 13 - Correspondence: a few Comments Correspondence is always an interesting exercise, although sometime amusing, sometimes frustrating. Readers ask me if it is worthwhile to enter in a specific section of the market: obviously I cannot recommend any decision on financial matters. I might help by indicating how to check the matter more in depth. Other readers ask if I know where to perform specific welds of given materials in a certain geographic area. Although I may try to suggest how to find out what one needs, I must say I don't have all the answers. One correspondent needs certified welders. Can I provide candidates? Sorry, I cannot. Sometimes I am called in to settle a difference of opinions between a supplier and a contractor, for the benefit of the inspector who needs to know who is right. It is usually a challenging question as it is often times charged with economic implications (the one who should pay may look for pretexts). Or they may write: somebody suggested so and so. What do I think? Somebody else "saw a lot fine crack lines on the coupon section (of a weld). Is it norm or a weld defect?". [It may be painful and costly to admit, but cracks could never be considered normal]. Luckily seldom, abusive comments are thrown in because the information that was looked for was not found, this also happens, even if in fact it was available in another page. Sometimes, but to my dismay not often enough, a real challenge is proposed, a problem that it would be nice to investigate and to solve. And always a little thankful expression repays of many disappointments...
14 - Bulletin Board 14.1 - TMS 2008: Linking Science and Technology for Global Solutions 14.2 - SAE 2008 World Congress 14.3 - Start building your Encyclopedia Online! 14.4 - Comment on Welding Talk. 14.5 - Readers who administer Websites on subjects similar to those dealt with by this one, are invited to Contact Us to explore possibilities of Joint Ventures.
POWERED BY:
Click on this Logo NOW! Copyright (©) 2007, by Elia E. Levi and See you next time...
|
|
| Back to Back Issues Page |
