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PWL#037 - Preheating, Type 440C, Filler Metal for OAW, Macro Testing, OAW Tips, Thermal Spray August 30, 2006 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. Preheating, Type 440C, Filler Metal for OAW, Macro Testing, Oxyacetylene Welding Tips, Advancements in Thermal Spray This publication brings to the readers practical answers to welding problems in an informal setting designed to be helpful and informative. We actively seek feedback to make it ever more useful and up to date. We encourage you to comment and to contribute your experience, if you think it may be useful to your fellow readers. Click on Contact Us.
You are urged to pass-along this publication to your friends, if you like it, and if you want to help them. If you received this from a friend and if you like what you read, please subscribe free of charge and you will also receive a bonus book on Practical HARDNESS TESTING Made Simple.
1 - Introduction 2 - Article: Preheating 3 - How to do it well: welding hardened 440C 4 - Filler Metal for Oxyacetylene Welding 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Macro testing 8 - Site Updating: Oxyacetylene Welding Tips 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Advancements in Thermal Spray 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction Here we are with our 37th issue of Practical Welding Letter with new articles and information on a range of subjects meant to interest our readers. We start with a presentation of Preheating, a subject that may need some clarification, especially regarding its purposes. This originates from a letter from a reader who asked if he can preheat stainless pipes and valves. I answered that he sure can but then asked: "what for?". I got this answer: "The reason why we want to preheat the ss pipes is because they contain caustic soda and our client asked for heat treating the pipes before welding". I must admit that I still do not understand but, as there is no specific question and no details are given on heat treatment or welding process, I left the subject for the time being. Next we got again a question on welding hardened martensitic stainless steel type 440C. The short answer is: "Forget it!" because this material is unweldable. But there are other solutions. Although this answer already appears in our FAQ page, our reader obviously did not find it. (That is why we propose, in our Contact Us page, a short search before asking). The Filler Metal article connects to our new Page of the Month. Both deal with Oxyacetylene Welding. We think this process, although old, is still successful in many cases and should be always considered as an option when applicable. A list of AWS rod specifications follows the article. Macrographic examination (Macro test) is a most useful proof of weld quality. It is also a powerful training aid that shows in an easy, understandable way to the welder the truth on the welding performance achieved. We recommend either to set up in every shop a simplified facility to prepare macro specimens or to make arrangements with a metallurgical laboratory to the same effect, for development of welding procedures and for follow up. Some Thermal Spray advancements are presented in the Contribution section: the field is quite active with research and developments, and commercial versions are not far behind. Other departments appear where expected with references and links. We feel the information could be useful, don't you think so? Let us have your comments and feedback, use the Contact Us button from every page of our website.
2 - Article: Preheating The operation of heating metal to some predetermined temperature before engaging in actual welding is called preheating. The following presentation deals with carbon and low alloy steels, although the principles exposed are applicable also for high alloy and tool steels. The details and the modes may be different in various situations but in general the purpose is to influence the cooling behavior after welding so that shrinkage stresses will be lower (relative to welding without preheating) and cooling rate will be milder. More favorable metallographic transformation structures will develop, less hard and more ductile, to avoid cracking. In cold weather preheating to some low temperature may help prevent water vapor condensation that might interfere with welding and provide an unwanted source of dangerous hydrogen. It is known that carbon and alloy content in steels determine the microstructure developed upon cooling from elevated temperature. The maximum hardness of as-quenched (untempered) martensitic structures, both in the weld and in the heat affected zone (HAZ), is higher for higher carbon content. Whenever hard, untempered martensite is allowed to develop upon cooling from welding temperature, there is real danger of cracking in presence of shrinkage stresses, the more so in case hydrogen is present. In preheated structures, if dissolved hydrogen is developed during welding, it is more likely to evolve and migrate to the outside, instead of being trapped within the weld. The minimum preheating and interpass temperature to be assured to avoid cracking depends on the following factors:
Depending on the heat input of the actual welding process employed, there may be no need to continue preheating once welding is started, provided the structure is not allowed to cool down below the selected temperature. The preheat temperature should be applied and maintained for sufficient time to guarantee that only bainitic microstructures are formed upon slow cooling, allowing also for evolution of hydrogen. A lower temperature however may be sufficient to avoid cracking, in certain lower carbon steel, if the martensite is tempered and its hardness is lowered adequately. High preheating temperature may be uncomfortable for the welder and may develop an oxide layer on the joint likely to interfere with welding. Once welding is accomplished, the structure should be covered with insulating material or transferred to a suitable furnace for slow cooling down. It should be remarked that one may be able to reduce preheat temperature, which represents an additional cost, if one has some control on the following factors:
Once the procedure and the methods have been optimized all parameters should be included in the Welding Procedure Specification to ensure that the production process is under control. After the welding process is accomplished in the absence of cracks, the whole structure may be subjected to a complete hardening and tempering treatment if required, to develop the microstructures and the mechanical properties needed to withstand service stresses. A reference to suggested preheat temperature tables available from AWS was reported in a note in section 11 of Issue 10 of Practical Welding Letter for June 2004. Click on PWL#010. A reference to another Table published in ASM Handbook is included in the website page on Alloy Steel Welding
3 - How to do it well: welding 440C Q: We need to attach a .080" thick 1045 sheet stock part to a hardened part (RHC 58-62) that has a bearing race on it. The preferred material for this part is 440C. A corrosion resistant material is needed. The weld joint is a slip fit at this time with fillet welds on one side and seam welds on the other. Can Hardened 440C be welded successfully to a dissimilar material? A: No. Unfortunately 440C is unweldable. We already treated this matter in a different Question that you can see in our FAQ page. As the material 440C, a very hard stainless steel, cannot easily be replaced, one has to look for an alternative joining method. Given the slip fit of the assembly, it seems that the best solution would be adhesive joining. The adhesive and the clearance have to be selected with care, following adhesive manufacturers' recommendations.
4- Filler Metal for Oxyacetylene Welding Many oxyacetylene welding applications are performed autogenously, that is without addition of external material to the joint, especially with joint shapes that provide some material supply. However one should always consider the use of filler rods even if not needed continuously along the joint, for local addition if required. Filler rods are usually supplied straight and cut to length in well identified boxes. But occasionally one may find more easily reels of wire to cut and straighten as convenient. Discarded wire stubs can be welded to the rod for decreasing material waste. In principle one should use filler compositions closely matched to those of the base metal but this is not an absolute requirement. The problem may be more difficult in case one needs to repair some item without any knowledge of the base metal. In this case one should try to identify at least the class of the material by using the techniques explained in our page on Material Identification. Vendors and material suppliers may suggest where to perform the easy and inexpensive identification test called XRF (X-Ray Fluorescence test). If the unknown base material is unweldable with the technique used, no problem, you will easily find out: your weld will not succeed... Some problems may also arise when the base metal is plated, because the plating may interfere with welding, but in the general cases, considering only the most usual materials, one should have no problem. Filler rods should be kept always identified with the original manufacturer's markings, clean and protected from rusting or corrosion. Other than for mild steel, flux suitable to the materials welded should be procured, usually under brand name. The following Specifications List may be useful in looking for suitable filler materials for Oxyacetylene welding. ANSI/AWS A5.01-93(R1999) AWS A5.2-92 (R2001) ANSI/AWS A5.7-84(R1991) ANSI/AWS A5.8/A5.8M:2004 ANSI/AWS A5.9/A5.9M:2006 AWS A5.10/A5.10M-1999 ANSI/AWS A5.14/A5.14M:2005 ANSI/AWS A5.15-90R AWS A5.19-92 ANSI/AWS A5.21-2001 ANSI/AWS A5.28/A5.28M:2005 AWS A5.31-92(R2003)
5 - Online Press: recent Welding related Articles From the Wall Street Journal: From AWS: From the Fabricator From TWI:
6 - Terms and Definitions Reminder Bond Coat is a preliminary intermediate material deposited by thermal spray to improve adherence to the substrate of the following sprayed layers. Cone is the hottest part of an oxyacetylene flame from a welding torch, next to the orifice of the tip. Cylinder (gas cylinder) is a storing container for keeping and transporting gases to be used in a welding process. Faying Surfaces are the mating surfaces of two elements to be joined by having them make contact with each other. Inclined Position is that of a pipe or tube joint whose axis is approximately at 450 from the horizontal plane and fixed (non rotated) during welding. Mask is a temporary, removable, protective cover applied on a surface in order to keep it from being coated or plated during an application process. Oven Dryer is a heated enclosure designed to keep electrodes dry or to dry them if they absorbed humidity due to careless exposure to damp atmosphere. Preheat Temperature in welding and related processes, refers to an intermediate temperature of heating by external means for a short time immediately before welding, brazing, soldering, cutting, or thermal spraying with the purpose of lowering the cooling rate after welding in order to reduce shrinkage stresses and to control the microstructure obtained. 7 - Article: Macro testing The proof of the pudding, in this case of the weldment, consists in the visual examination of suitable sections, usually transversal, obtained by submitting the welded test piece to this destructive test. A lot of information is achieved by just observing the appearance of properly prepared macro test specimens. In order to obtain meaningful understanding of weld quality one has first to select significant sections, and then to remove the section without damaging (burning, deforming, destroying) its aspect. Therefore one looks first on the external surface of the weld bead for any sign of irregular shape or of visual defects, the reasoning being that the surface appearance might conceal a deeper flaw. If nondestructive testing was performed, one selects the section positioning for macro test in correspondence with the most severe indications. Only if the external appearance is quite even and acceptable one selects random or regularly spaced spots for cutting. If we prepare a series of sections, we should make a sketch or take a photo of the part beforehand, and establish by letters or numbers a clear correspondence between the sections and their emplacement. So if a defect is later found say in section three, we should be able to identify in which position it was in the test piece itself. Sectioning is done using a simple cutting machine that consists in a sturdy vise that keeps the part steady by gripping the two portions that will be sectioned in the process so that they will not move while being cut. The active element is an abrasive disc, driven by a powerful electric motor, and rapidly wearing out while cutting. The disc is to be replaced quite frequently. To avoid heating or burning of the test piece, the operation is conducted by flooding the place with cooling water, usually with an inhibitor to protect the machine parts from corrosion. The disc is moved gradually into the cut, either along an arc of circle (if the motor and disc assembly pivots around the shaft) or a straight line depending on the machine details. If the test piece or the part to be examined is too large to be easily gripped in the vise, its dimensions can be reduced to a proper size even by a flame, provided the selected portion remains quite far from the flame and is not altered by heat. Once the section is made one has to grind and to polish its surface. If the size is comfortable one can grip the part by hand. If it is too small to be held, it has to be set in a plastic mount. A metallurgical laboratory uses mechanized means to turn planar abrasive paper discs of progressively finer mesh on whose surface the item is ground and polished, always under a liquid lubricant and cooler. The art of preparing high quality metallographic specimens is beyond what is needed for the average welding shop. But with moderate effort one can set up simple means to get acceptable macros for examination under the naked eye or a low power microscope. A page giving some essential advice for setting up a simplified facility for the purpose of preparing macro etched specimens is available from our website at
Weld Macro. Otherwise it would be most useful to get the help of a metallurgical laboratory to set up a short inspection cycle turnover for the items to be examined. Although the appearance of cracks can be seen on polished samples, it is usually necessary to etch the surface with an acid to reveal the macro structure, the purpose of the test itself. For mild steel a solution of 2 to 5 % of Nitric acid in methyl alcohol is the standard etchant, briefly called Nital. The etching operation, although quite simple, may be dangerous if the acids are dealt with carelessly: but one can learn easily basic safety precautions. Only now, having in our hands either the etched specimen itself or a photo of its aspect sent to us by the metallurgical laboratory through e-mail, can we look at its appearance and learn on the quality of our process. The etching reveals the weld metal as different from the base metal so that we can see its borders. We have to imagine the shape and the contour of the original joint, as it was before welding, and we will immediately notice how deep the weld penetrated into the base metal. If the former joint shape with its straight lines is found, that means that the weld was deposited superficially and it will not be acceptable. The weld bead might have been deposited inadvertently near the joint instead of on it, to fill up all the joint space. Again, seeing is believing. Even your most stubborn welder will have to accept that his/her quality needs improvement. Also porosity, slag inclusions and unacceptable bead shape or size are immediately seen and understood in the macro section. With more training in examining these sections one will be able to find the borders of the Heat Affected Zone (HAZ) the region that was not melted but whose structure was altered by heat in such a way that it appears clearly against the unaffected background of the base metal. The macro test is therefore the most important teaching aid available to the instructor or shop supervisor, not only for the documentation requested in the certification process, but also for showing to the welders, in a way immediately understood, the quality of their work and their progressive improvement.
8 - Site Updating: Oxyacetylene Welding Tips Hoping to offer to our readers valuable information we are glad to present here the last addition to our list of previous six Welding Tips pages that you can reach from the Site Map. Our new Page of the Month deals with an old but still very popular and useful process. You can find it by clicking on Oxyacetylene Welding Tips As this equipment is found in almost every welding shop, Oxyacetylene Welding may be the first process to consider for joining thin mild steel or copper alloy sheet or tubing especially for single items or small production series. To send us your comments and feedback click on Contact Us.
9 - Short Items Anelasticity is a property of certain solids when strain is not a single-value function of stress in the low-stress range where no permanent set occurs and when internal friction is independent of amplitude. Bronze is a copper-rich copper-tin alloy with or without small proportions of other elements such as zinc and phosphorus. Also, certain other binary copper-base alloys, such as aluminum bronze (copper-aluminum), silicon bronze (copper-silicon), and beryllium bronze (copper-beryllium). Chromizing is a surface treatment performed at elevated temperature in pack, vapor, or salt baths, in which an alloy is formed by the inward diffusion of chromium into the base metal with the purpose of providing oxidation and corrosion resistance to otherwise unprotected materials. Cup and Cone fracture exhibits a mixed-mode aspect. It is often seen in tensile-test specimens of a ductile material, where the central portion undergoes plane-strain fracture and the surrounding region undergoes plane-stress fracture. It is called a cup-and-cone fracture because one of the mating fracture surfaces looks like a miniature cup. It has a central depressed flat face region surrounded by a shear lip or conical wall at about 450 from the specimen axis. The opposite fracture surface looks like a miniature truncated cone. Plastic Flow occurs when metals are stretched or compressed permanently without rupture as in forging and in certain forming operations. Radiography is a method of nondestructive inspection in which a test object is exposed to a beam of penetrating, ionizing radiation like x-rays or gamma rays and the resulting shadow image of the object is recorded on photographic film or on a digital photosensitive device (imager) placed behind the object, or displayed on a viewing screen or television monitor (real-time radiography). Internal discontinuities are detected by observing and interpreting variations in the image shadows and contrast caused by differences in thickness, density, or absorption within the test object.
10 - Explorations: beyond the Welder Sustainable Mobility Project: full Report (180 pages) Research that values the Planet Sudoku BLOG: Scientific American Observations Danish Schnapps
11 - Contribution: Advancements in Thermal Spray An article introducing the uses of Computational Fluid Dynamics (CFD) in the studies of performance of plasma spray nozzles explains how the complex analysis of behavior of the plasma plume within the nozzle brings to hardware improvements much faster than by the traditional way of trial and error. In particular the authors, associated with Sulzer Metco, describe how their analysis permitted to determine that a certain specific situation of plasma plume within a given divergent nozzle brought about flow separation with attendant turbulent region. The identification of the problem led quickly to a new design that shortening the nozzle divergent section reduced the Mach number and eliminated overexpansion, contributing to stabilize the plume and to reduce energy losses. The authors conclude that there is ample room for improvement in the design of plasma nozzles for thermal spray and that CFD will play an important part in new nozzle design. Interested readers are urged to look for the original article, titled "Better Performance of Plasma Thermal Spray" at page 65 of the quarterly supplement "International Thermal Spray & Surface Engineering" published in Advanced Materials and Processes, an ASM International publication, August 2006 issue. Another article, called "The Hybrid Spray System" is found at page 57 of the above publication. It describes studies aimed at combining two existing processes into a new hybrid spray system designed to exploit and improve on the advantages of both baseline methods. The processes in cause are the PS/HVOF (Plasma Spray/High Velocity Oxy-Fuel) and the twin arc spray process which although producing less dense coatings provides much higher deposition rates. The flexible design of the developmental hybrid gun can operate in different modes and combine them at will. By selecting suitable parameters, the average particles velocity, the temperature and the supporting gases, different regimes of operation are obtained and the results are readily compared.
12 - Testimonials From: Alex Labby (e-mail omitted for security) Thanks for the response, it was very helpful. [...] Alex Labby
From: Ganesan Nallathamby (e-mail omitted for security) Hi Elia Levi
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14 - Bulletin Board 14.1 - WARNING - Please be advised that an unscrupulous publisher has introduced a misleading URL very similar to that of our own website, just to disturb and confuse our readers and possibly to cause us damage. It is called If you happen to fall on a page devoid of any report, that gives you only a list of links with no content, while you were expecting the real information that is usually displayed in our pages, please remember to check if our address (URL) was printed correctly in your browser, otherwise you may remain disappointed of our service (if you wrongly think that it represents our website). To avoid this unpleasant experience please bookmark our correct URL 14.2 - We noticed that the RSS/Blog updates of the main Internet Actors (Google, MSN, Yahoo!) are not quick enough, in our opinion, to publish the last Blog version. We found even a delay of several days. To us it is annoying. If you see in the published update a lag of some days, may we suggest that you check it by yourself by copying and pasting in your browser the following address and by clicking on it. 14.3 - Materials Science & Technology 14.4 - In case you missed the previews of the important book See you next time... POWERED BY:
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