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PWL#054 - Validation, Cut Pie Weld, Filler Metal: Corrosion and Wear, PREN, FSW, R Stamp and more.. January 30, 2008 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#054 - Validation, Cut Pie Welding, Filler Metal for Corrosion and Wear, Pitting Resistance Equivalent Number, Friction Stir Welding, "R" Stamp 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: Validation 3 - How to do it well: Cut Pie Welding 4 - Filler Metals for Wear and Corrosion Application 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - PREN 8 - Site Updating: FSW 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: The "R" Stamp 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction As you may be expecting, this is the 54th issue of our Practical Welding Letter for February 2008. As it happens we have this time a few unusual items that hopefully will be of interest to our readers. We start with a subject, Validation, whose presentation was requested by a kind reader who must prepare to meet new unfamiliar requirements. We hope we'll help to dispel some of the fog. Then we discuss an uncommon weld design proposed by a reader. The discussion should clarify the issues. Regarding Filler Metals, a recent Article from the Welding Journal explains the reasoning behind development of new Cored Wires for improved Resistance to Wear and Corrosion. A short review may induce some readers to look for the source. The next Article deals with an Index of Pitting Corrosion Resistance, a useful number that may help to rate and select the most suitable material from a list, usually of Duplex Stainless Steels. Our Website, www.welding-advisers.com added this month a new page presenting essential features of Friction Stir Welding, a new process that in a few years enjoyed wide diffusion and successful applications in demanding design for different industries. The last Article explains the requirements for obtaining an "R" Stamp, that means the authorization to design and perform repairs and/or modifications of existing Boilers and Pressure Vessels in the United States. Similar requirements exist most probably in every other place. An important explanation was added by Dr. Timothy Volin to our Welding Talk page: readers are invited to read it, and to contribute from their experience. The rest of our columns are where they should be. We would be glad to hear if this publication meets your expectations and if you would like to see other improvement introduced to have it more interesting and helpful. Send it to your friends! Let us have your requests for new subjects and comments by using the Contact Us page.
2 - Article: Validation [Note: The following request was sent by a kind reader whom we thank for his feedback, answering to our inquiry on subjects of interest]. "Validations of specific welds, grouped welds (product range) welding processes, etc. It's a HOT topic in Medical related companies just now." Validation is the process of checking if something satisfies certain criteria. It should be used with precise reference to the discipline to which it is applied and it means different things in different situations. In fact Computers, Pharmaceuticals, Foreign Degrees, Food Production, Human Communication, besides Welding, are examples of activities requiring Validation for specific purposes. We suspect that the term Validation is not yet standardized so that different meanings can be found. Both the American Welding Society and ASM International returned no specific universally accepted definition for the term. In the context of manufacture employing welding, however, Validation is generally part of Quality Management Systems. Validation is required to prove the reliability of the Organization providing Goods or Services and it is considered an essential ingredient for assuring the integrity of materials, processes and manufactured items. Validation relates to meeting the requirements of an official Agency enforcing adherence to specific Codes or of an external Customer, as applicable to Non Destructive Testing (NDT) equipment, methods, procedures and inspectors. In fact it was found, often with much anguish and loss, that Non Destructive Testing may miss important clues to questionable indications, if applied nonchalantly by unscrupulous or inadequate Organizations. The Validation process is designed to demonstrate that the entire Inspection activity and indeed the whole Quality Assurance System meets continuously and repeatedly all requirements. The adequacy of NDT equipment, its current full operational order and calibrations, preparation and skill of personnel, the application of suitable methods and procedures, and the correct interpretation of results are all elements in need of Validation by an unbiased and reliable external agency. These Quality Assurance Requirements are over and above the standard requirements that the Company performing welding should have prepared its Quality Manual, and be ready to submit it for review, specifying fixed Procedures and using established written Instructions, acceptable to the Customer or to the Certifying Agency. In contrast, the checking procedure performed to assess the compliance to requirements internal to the Organization, is usually called Verification. We found in the open literature reference to Validation of a welding schedule, as a process whereby experienced welders were asked to determine the productivity of the baseline welding procedure. In Research work the term Validation is used for the purpose of determining if a model proposed for explaining a certain result or for simulation of a certain process can be used for predictions, as in the following sentence: "Thermal cycles measured with thermocouples embedded in specimens are employed to validate a numerical thermometallurgical model of an Electron Beam welding process." Validation of welding parameters is used to describe the assessment of a Welding Procedure Specification (WPS), generally in an experimental situation, as opposed to a production environment. Any manufacturing facility employing welding is requested to demonstrate to the Customer or to the appointed Authority that welding equipment is in good operating condition, being subject to scheduled maintenance, controls and periodic calibration. Validation of welding equipment according to See also Furthermore, in order to qualify for work regulated by Codes and Specifications, Welders engaged in the fabrication should be Certified for the process involved (including positions and thickness). Additionally, suitable WPS (Welding Procedure Specifications) should be established and approved using the required tests, PQR (Procedure Qualification Records) should be available for review, and Welding Inspection should be engaged in performing Surveillance duties. For further reading see: http://www.twi.co.uk/j32k/unprotected/band_1/nrndtvalid.html http://www.ndt-validation.com
3 - How to do it well: Cut Pie Welding Question: I have a situation where several plates are coming together at a point in the fashion of a cut pie. The plate thickness is 5/8" ABS (American Bureau of Shipping) Grade A steel. This design has all points of the pie being welded together and I know it will crystallize at the vertex, due to overheating. Where is it noted that this design should never occur, due to the crystallization of the material at all points of intersection? Answer: The design you propose looks to me problematic, not because of crystallization (which is not a metallurgically defined defect but means other things, see further down 9.2) but because it may be difficult to assure full weld penetration and because a triaxial state of stress (to be avoided) risks being formed in the place. It would be better, if possible, to stop the plates short of the vertex and butt weld each plate to those on its sides, with bevels as needed, leaving a circular hole in the center of the pie, to be covered if needed by a suitable independent cover or cap.
4 - Filler Metals for Wear and Corrosion Application To readers in search of information on flux cored filler metals specially developed for hardfacing applications to provide wear and corrosion resistance, we would like to point out a recent article. It is titled Specialty Cored Wires for Wear and Corrosion Applications and it is published in the February 2008 Issue of the Welding Journal at page 31. Cored wires are especially effective for hardfacing as the hard and brittle components are added as needed to the core, without interfering with the wire ductility. Development is easily performed by changing ingredients according to the research plan. Most of the cost effective solutions are iron based, and for low stress wear resistant systems the chromium carbide types are the most popular and economic. A few formulations were developed for meeting specific requirements, varying in composition and complexity. If corrosion is also involved stainless or nickel or cobalt alloys can be used as the base metal. The ability to achieve the desired deposit characteristics depends strongly on fine adjustment of matching the matrix with the filler. The developments reported in the article refer to three areas: modified chromium-carbide, crack free alloys and alloys resistant to corrosion-wear. Different carbide types may impose limitations as to the number of layers that can be deposited without unacceptable spalling. The simplest and more economic types are suitable for moderate wear with no or low impact. Improved wear resistance is provided by more complex carbides including also those of Molybdenum (Mo), Tungsten (W), Niobium (Nb) and Vanadium (V), but the deposits risk to be more brittle. Another type is characterized by having matrix microalloyed, designed to be more resilient than the simpler alloys. The test generally used to assess wear resistance is ASTM G65 Procedure A. ASTM G65-04 It should be stressed that in practice many forms of wear and abrasion are known and that in specific applications different coatings may rate differently than in standard tests because of complex abrasion mechanism not completely understood. One should also remark that behavior comparisons of most iron-based alloys is based on weight loss as the test rating criterion. Volume loss should be used instead, however, for comparing base metals having significantly different densities such as iron-, nickel-, and cobalt-based alloys. When cracks are not acceptable, there is need for crack resistant alloys. Typically martensitic steels have been used, but with improved wear resistance. Materials were derived also from known tool steel. Welding Position can be an issue because not all filler metals are weldable in all positions (as opposed to flat). To resist also corrosion, besides wear, nickel base filler metals have been developed, containing various proportions of hard tungsten carbides (WC) of different sizes. Low heat input has to be maintained to minimize the tendency of carbides to dissolve in the matrix, in order to assure sufficient retention of wear resisting properties. Special titanium and niobium carbides have been added to stainless steels to improve their wear resistance without compromising corrosion behavior. In conclusion there are now larger selections of different types of hardfacing flux cored filler metals available, developed by manipulating composition and form of carbides used for improving wear and corrosion resistance in demanding applications. Interested readers are urged to look for the above mentioned article. AWS Members can see all the articles online.
5 - Online Press: recent Welding related Articles Understanding flux-cored wires Looking for Rebates on Miller Equipment? From TWI: (may need no cost registration)
6 - Terms and Definitions Reminder Automatic Welding is performed by auto regulating equipment without operator intervention. Back Bead is that deposited on the back side of a welded joint. Backing Ring is a consumable insert filler metal ring providing support in the welding of pipes. Forge Welding is an ancient solid state process assuring coalescence of white-hot steel parts under mighty blows between hammer and anvil. Overmatching means the improvement of properties of weld metal relative to base metal, by judicious addition of suitable elements in the composition of filler metal. Puddle Welding is a nonstandard term for arc spot weld or plug weld, meaning welding of a member to another one at selected spots through prepared holes or just by fusing locally the upper member onto the other. Resistance Soldering is performed while the necessary heating is obtained by resistance to the passage of an electric current in a circuit including the workpiece. Skull is the material that remains after the low melting part of it separated by flowing away from a filler metal of wide melting range.
7 - Article - Pitting Resistance Equivalent Number The Duplex Stainless Steels were developed, as a Group, to provide desirable characteristics that were not available with the previously known types. The Duplex microstructures typically have approximately equal proportions of ferritic and of austenitic phases so that they combine the favorable properties of both types (austenitic and ferritic) offering significant advantages relative to single phase stainless steels. In particular they present strength much higher than that of Austenitic types, together with good resistance to Chloride Stress Corrosion Cracking and Pitting Corrosion Resistance. In comparison with Ferritic Stainless Steels, Duplex Stainless Steels have higher notch toughness, especially at low temperatures. Duplex stainless steels are usable in the range from -60 to 2500C (-75 to 4800F) for applications involving seawater or where resistance to acids and chloride solutions is important. The reason for limiting heat is that prolonged exposure to elevated temperatures promotes the precipitation of intermetallic phases (sigma, chi and eta phases) that reduce toughness and impair corrosion resistance. Weldability, although not comparable with that of the austenitic type, is still good, provided that specific welding practices are employed. A useful parameter, called Pitting Resistance Equivalent Number (PREN), was developed to describe their relative resistance to pitting corrosion. This parameter is calculated by a simple formula based on composition: Sometimes slight modifications of the formula are used, taking into account also Tungsten (W - if present). By calculating the parameter for different alloy compositions one can rate them relative to their pitting resistance. In practice the influence of welding in modifying the microstructure has to be taken into account, as well as the precipitation of intermetallic phases that can impair notch toughness and corrosion resistance. An objective determination of pitting resistance is performed by applying the ASTM G48 practice A procedure. ASTM G48-03 This experimental procedure takes into account all factors that influence actual resistance, like heat input and cooling rate, that contribute to modifications of microstructure, difficult to assess by traditional metallographic means. The procedure above is considered easy to perform and is credited with the capability of distinguishing between good and bad welds. Critical Pitting Temperature (CPT) is that at which pitting in ferric chloride solution is first observed. If the same test above is performed to both as received base metal and to a welded specimen with the same PREN, by checking the CPT one can quantify the reduction of weld metal properties. If one can find in a given case a difference of about 200C (360F), this represents the damage caused by that specific welding procedure on pitting resistance. For further reading see: Calculating Pitting Resistance Equivalent Number (PREN)
8 - Site Updating: Friction Stir Welding The Page of this Month deals with Friction Stir Welding, one of the most successfully applied new processes that, in less than two decades since its invention, has generated enormous interest in vastly diversified applications. Although not yet suitable for the average welding job shop, this solid state process can have solutions for difficult to weld materials or for special requirements. Being friendly to the environment, energetically efficient and economically attractive, it should be explored for more ambitious projects. See our new page at Friction Stir Welding. We were delighted to receive from Dr. Timothy E. Volin an instructive comment on the situation described in the Welding Talk page relative to the subject "Beware of 317LMN Castings!" Interested readers are urged to look down in the Comments to that page. You are invited to relate from your experience by adding new pages. Simply type in the above form and send. Did you meet recently challenging Welding Examination Questions? Send them in! We are building with your help a Volume to include the largest collection ever available for industrious students to learn on. Click on Examination Questions and add your own! To be alerted of new website pages consult periodically the Site Map and/or our Blog that you can see also by including it in your RSS reader (See under the NavBar in any page for subscribing).
Are you an Artist using welding or brazing for your Creations? Add photos of your artistic production to our page Welding Art and let everybody enjoy!
9 - Short Items 9.1 - Composite Material is obtained by a combination of two or more materials (matrix, reinforcing elements, fillers, and binder), each with significantly different properties. Mud reinforced by straw is an ancient composite material for building huts. Modern examples include carbon/epoxy, cermets and metal-matrix composites. 9.2 - Crystallization is: (1) The separation, usually from a liquid phase on cooling, of a solid crystalline phase. (2) The progressive process in which crystals are first nucleated (started) and then grown in size within a host medium that supplies their atoms. The host may be gas, liquid, or of another crystalline form. 9.3 - Endurance Limit is the maximum stress that a material can endure without failing for an extremely large number (exceeding 107) of fatigue cycles. 9.4 - Impregnation is sealing of porous castings with a medium capable of stopping leaks, or filling the pores of a sintered compact with a lubricant, or mixing particles of a nonmetallic substance (diamond) in a cemented carbide matrix, as in impregnated tools. 9.5 - Quarter Hard is a temper designation of nonferrous alloys and some ferrous alloys characterized by tensile strength between those of dead soft and half hard tempers. 9.6 - Rapid Solidification is cooling or quenching of liquid (molten) metal drops at rates that range from 104 to 108 0C/s.
10 - Explorations: beyond the Welder Boredom NASA's Moon Chariot In Action Making P/M Parts Adolf Martens and his Contributions to Materials Engineering The SBI! Video Tour 11 - Contribution: The "R" Stamp We saw recently a question on what the "R" Stamp is and how can it be obtained. Maybe the answer would be also of interest to some of our readers. ASME (American Society of Mechanical Engineers) has been recognized in the United States as the authority regarding design and construction of Boilers, Pressure Vessels and Piping (for industrial applications). Boiler repair in the United States is governed by the National Board of Boiler and Pressure Vessel Inspectors under a classification called Owners of Boilers etc., in need or modification or repair, are requested by law to entrust the work only to organizations authorized to do so, as demonstrated by the "R Stamp" in their hand. In order to be entitled to perform "R Stamp" welds and repairs, Organizations engaging in repair and/or alteration of boilers, pressure vessels, and other pressure retaining items are requested to obtain formal authorization from the National Board of Boiler and Pressure Vessel Inspectors, according to the requirements of their Inspection Code. The Organizations or weld shops and their welders are tested and certified as to the quality of their weld joints through a rigorous testing procedure. But even before that they need to comply with a number of Prerequisites (concerning Quality Assurance) as detailed in the official site: The National Board of Boiler and Pressure Vessel Inspectors
12 - Testimonials Name: Carmen Vertullo Thanks,
From: Craig McKenzie Elia, Regards,
13 - Correspondence: a few Comments 13.1 - I receive all sort of requests. Somebody offers quality gloves and protective aprons, thinking that I may store hard goods for selling. Sorry I do not. Another reader would invite me to review the workings of robotic cells that produce unacceptable distortion. I prefer to provide written assistance. I know it can be equally successful and much more cost effective. Inspectors request copyrighted standards (that I cannot provide). The strangest request of all? "I have a bottle marked HIG. It was here when we bought the building. What is it?". Sorry, I don't know. 13.2 - The most difficult questions are from those welders who would like to start a welding business. While I try to address them to whom I think may help, I would not take any responsibility in business initiatives. 13.3 - I enjoy receiving your requests. Not always can I satisfy all of the inquirers but I usually try to help. Keep on asking, it does not hurt. 14 - Bulletin Board 14.1 - West Coast Welding Shipbuilding Symposium 14.2 - Trends in Welding Research, 8th International Conference 14.3 - Send us your Requests for subjects you want dealt with. 14.4 - Forward this page to your friends (but do not Spam!),
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