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PWL#057 - Risk Based Inspection, Normalizing, Magnesium Joining, Cryogenic Processing, Brazing Metal April 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#057 - Risk Based Inspection, Normalizing Mild Steel, Filler Metal for Magnesium Joining, Cryogenic Processing, Brazing Titanium, Ceramic, Steel, Nickel, and more... DON'T USE REPLY to send your messages! Use Contact Us instead. 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: Risk Based Inspection 3 - How to do it well: Normalizing welded Mild Steel 4 - Filler Metals for Magnesium Joining 5 - Online Press: recent Welding related Articles and Video 6 - Terms and Definitions Reminder 7 - Article - Cryogenic Processing 8 - Site Updating: Brazing Titanium, Ceramic, Steel, Nickel 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Competence in welding 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This 57th issue of Practical Welding Letter for May 2008 opens with a hot subject. Risk Based Inspection is particularly important for those having responsibility for the safe and secure continuing operation of electricity producing aging integrated systems burning fossil energy. But it should interest anyone working with welded structures. It refers to the expanding mature technology involved in establishing inspection procedures based on assessment of the risks incumbent in operating facilities that deteriorate in time. The lessons learned in years of studying the problems provide now structured guidelines for establishing robust procedures. Then we comment on a question referring to Normalizing of welded mild steel. On Cryogenic Processing there are apparently discordant opinions relative to its applicability and usefulness. We report on what is known, and leave the discussion to those having acceptable experimental results to confirm their theories. Our Website was recently enriched by the addition of four new pages on Brazing of Titanium, Ceramics, Steel and Nickel. The Contribution relates a trend, outlined by TWI (The Welding Institute), requiring increasing demonstration of Competence in Welding not only for individual welders but likewise for organizations and manufacturing facilities engaged in producing welded structures. The purpose is to increase awareness of all factors contributing to welded quality and confidence in the professional and responsible attitude of providers. The other departments are found at their usual place. We would like to get your comments and feedback. Don't use Reply, use Contact Us instead.
2 - Article: Risk Based Inspection In Issue 29 of Practical Welding Letter for January 2006 (PWL#029), we published (7) a short introduction to Risk Assessment, a tool used to plan rationally and timely the inspections of industrial facilities having the potential of failure with consequent risks of dangers and damage. Although the document introduced(1) seems to address concerns limited to those having direct responsibilities for whole plants, it is recommended that all involved in welding have an idea of the implications of the quality of their work on the integrity of complex constructions. The Report aims to assist Duty Holders (owners) and regulators identify best practice for plant integrity management founded on Risk Based Inspection (RBI) by
Although the document was originally designed to cover requirements for pressure vessels and complete systems, the principles and practice of RBI within the report are also applicable to the management of other safety-related structures and equipment, like lifting and fairground equipment. The report views RBI as one of a range of measures within the wider process of plant integrity management. One of the main themes of the report is the amount of information that is known about an item of equipment and conversely the identification of where there is a lack of information. Information generated by the risk assessment process can be used to aid the judgements applied in deciding upon a suitable written scheme of examination in terms of the equipment to be inspected and the frequency and nature of examination, and to achieve a safe and suitable scheme that is not unduly restrictive. It is stressed that the advice given is not prescriptive, but is to be used as a guide to best practice, to be adapted to suit specific circumstances, and to be interpreted in terms of a goal setting safety regime. Owners have the option to manage the integrity of the plants and to plan inspection from assessment of risks of failure. They should be able to demonstrate to the appointed authorities (the regulator) that Risk Assessment and Inspection Planning are implemented in their plan in effective appropriate ways. Within this report it is interesting to note the definition of failure as an "unintentional release of stored energy" and that of probability of failure as the "mean frequency or rate" for "the specified failure event" being expected to occur in one year. "The consequence of failure (...) is the potential for harm". "The risk of failure combines the probability of failure with a measure of the consequences of that failure." Risk Based Inspection includes all forms of "examinations to determine the physical and metallurgical condition of equipment or structure in terms of fitness-for-service". In particular all concerned should be alert to the potential for degradation of materials and properties and to find ways to assess that deterioration. "Inspection is an initiator for actions such as the repair or replacement of deteriorating equipment, or a change to the operating conditions" with the purpose of reducing the risks connected with continued operation. Risk Based inspection, requiring a set of technical inputs and perspectives from different disciplines, is best undertaken by a team. The scope of work and the responsibility of the team must be clearly defined and obtain full support from management. All readers are invited to see the full document and to gain a deeper understanding of the requirements and potential advantages of applying the policies put forward as a way to confront the dangers and to pursue consistent plant integrity management. (1) - Report - Best Practice for Risk Based Inspection (186 pages) Note: - Readers having personal experience with the subject exposed in this note are invited to share some of it in the next issue of this publication.
3 - How to do it well: Normalizing welded Mild Steel Q: - We have been asked to normalize the welding on a unit that is made up of 4 types of mild steel. It is our understanding that normalizing is normal with 4130/4140 material for stress relieve and strength, but that it does nothing for mild steel. Can you verify this? Or do you have any documentation to verify it one way or the other? A: - Normalizing is a concept used sometimes quite loosely. If you mean heating mild steel above transformation point to austenite and cool in air, you only get stress relieving. The same result can be obtained at 650 deg C (1200 F) which is below transformation. Nothing will be gained at a higher temperature, only more scale to remove. If in the assembly also 4130/4140 are present, depending on their thickness (which reflects the rate of cooling in air) some strength will be gained. In order to avoid misunderstandings it is recommended to ask the customer for written instructions concerning heating temperature and time, and have him/her pay for the process performed. To learn more on Normalizing see the following article: 4 - Filler Metals for Magnesium Joining Two recent articles from The Welding Journal, March 2008, report on investigations performed to develop suitable filler alloys for joining of magnesium alloys at low temperatures. The first, at page 31, describes the difficulties being encountered when brazing magnesium, especially as a consequence of corrosion due to residual flux. Flux is essential to remove from the surface the oxide layers hindering wetting. The development effort was addressed to find conditions that would permit a fluxless process, in particular one suitable for ultrasonic soldering. An alloy of zinc, magnesium and aluminum was produced by continuous casting and then induction melt spun by rapid solidification in argon of molten drops onto a water cooled copper disc. The amorphous (non crystalline) material was then densified in tapes for use as filler material. Experimental soldering was performed to join different magnesium and aluminum alloys. Metallographic examinations showed good adhesion between elements. No details are given of this ultrasonic process except for the fact that soldering temperatures are about 350 0C. The authors announce that next efforts will be devoted to evaluate component related applications. The second article at page 38 reports on testing several experimental ternary alloys and a range of fluxes for brazing. The drive for this experimental work stems for the economic advantage of fabricating components from magnesium semifinished products, especially for automotive industry. In particular wetting behavior and clearance filling properties were examined and then mechanical testing of lap joints were performed. Given the quite low melting temperatures of the alloys studied, apparently below 450 0C and based on the standard definition of brazing, it seems that these processes should more properly be addressed to as soldering. Three alloys were identified as having potential for further research and development but quaternary alloy systems may need to be also investigated. Interested readers are urged to seek the original articles. Note: - Readers having personal experience with the subject exposed in this note are invited to share some of it in the next issue of this publication. 5 - Online Press: recent Welding related Articles and Video Modified GMAW for root passes Brazing and Soldering in The Welding Journal Laser brazing in the auto industry Recent advances in metal-ceramic brazing HTS-2000 Brazing Rods for Aluminum Repair (Video) Gas Welding using a Steel Coat Hanger (Video) Brazing with the Carbonizing Flame (Video) 6 - Terms and Definitions Reminder Brazer is a worker who performs manual or semiautomatic brazing. Brazing Operator is a person in charge of the operation of automatic or mechanized brazing equipment. Clad Brazing Sheet is a metal sheet covered (clad) from one or both sides with a layer of brazing filler alloy. Dross in thermal cutting is the amount of oxidized material remaining attached to the surface of the cut. Electrode Tip Life in resistance spot welding is expressed in the number of welds that can be done by a dressed electrode before redressing is necessary. Hold Time in resistance welding is the duration of force application on the electrode and workpiece after the last current pulse. Indentation is the visible depression, from the electrode pressed on the surface, remaining there after resistance welding was performed. Slot Weld is made by filling a slot or elongated hole (which may be open at one end) in one member to join it to another member. 7 - Article: Cryogenic Processing It is quite amazing to look for Cryogenic Processing in the Internet and to find only the most enthusiastic words of appraisal for this "magic" process and its successes in gaining longer wear life in hard tools... without a word on what it is in fact and why it is so helpful. A short note on what the process really is and why it accomplishes what it is claimed to do may be therefore more instructive. Tool steels are hardenable materials that respond to standard heat treatment involving heating to elevated temperature, beyond the so called transformation point, and then quenching rapidly to room temperature by immersion in oil or other media. By so doing the austenitic structure (displaying the characteristic FCC or face centered cubic crystallographic phase) is compelled to transform to a hard structure called martensite (of BCT or body centered tetragonal aspect). A final treatment called tempering is necessary to reduce internal stresses.
Steels with a relatively high level of Carbon are suitable to behave this way and to transform to hard, useful martensite. For maximum transformation to the hardest phase, quenching must be rapid. If heat removal is slow, other softer structures are formed instead of martensite. There is a limit to the rate of heat removal that can be achieved by immersion in oil, depending on the steel mass. In this case the steel may be hard on the surface and soft inside, sometimes but not always a favorable situation. It was observed that by alloying the steel with certain elements, hardenability, or the faculty to harden even under slow quenching, was improved, permitting to large sections of tool steel to harden even at the slow pace obtained when cooling in still air. The characteristics of austenite depend on its chemical composition. It appears that the addition of some Nickel and other elements to the steel favors the austenitic structure making it more stable. One of the consequences of promoting stabilized austenite with the favorable outcome presented above (improved hardenability), is that a sizeable fraction of austenite, a soft phase, is retained also at room temperature, decreasing the overall hardness of the material bulk.
It was found that by cooling the material to very low temperatures by so called "sub-zero treatment" or cryogenic processing, part of the retained austenite is forced to transform to martensite, thereby increasing the hardness. This transformation is essentially temperature dependent and not time dependent. In practice the treatment is performed by immersing the steel part in liquid nitrogen (at -196 0C or -321 0F) or in other suitable media. Tempering or double tempering must follow as usual. That in short is all the mystery around this process, or at least what common wisdom teaches us. It appears however that this subject is much more emotional for certain people as the following sobering comment would suggest. It was published in an article: "Its purported benefits have been embraced and promoted by some, and dismissed by others. While technical data and analysis has been generated by a variety of independent sources, it can generally be characterized most distinctly for its incompleteness. Simply put, there is not enough available published research to validate all of the claims of cryogenic treatment proponents, nor of its detractors." See also: Note: - Readers having personal experience with the subject exposed in this note are invited to share some of it in the next issue of this publication.
8 - Site Updating: Brazing Titanium, Ceramic, Steel, Nickel We are glad to be able to announce also for this month the issue of four new pages of our Website that you may reach by clicking on the links. Here they are: Brazing Titanium Brazing Ceramic Brazing Steel Brazing Nickel We hope that these pages may prove useful to readers looking for information, at least as the starting point for their query. To look for new pages as they are published see periodically the Site Map and/or the Site Blog, that you can see also by including it in your RSS reader, by taking the actions explained under the NavBar in any page. Visit also our New Page on Metals Knowledge for assembling at no cost your Encyclopedia Online, a rich collection of valuable information on Metals, from expert Internet sources. 9 - Short Items 9.1 - Density Ratio, also called theoretical density, is the ratio of the measured density of a powder compact to the absolute density (or mass per unit volume) of metal of the same composition, usually in percentage. 9.2 - Hot Shortness is the risk of certain metals to cleave or fracture along grain boundaries if stressed near their melting point. Typical of alloys, cooling after welding, that contain minute amounts of low-melting constituents like sulfur, segregated at grain boundaries. 9.3 - Scrap are discarded defective products unsuitable for use and metallic materials that may be reclaimed only through melting and refining. 9.4 - Transgranular Cracking (or transcrystalline) occurs through or across a crystal or grain, typically at lower than the equicohesive temperature (see PWL#044, 9.2) 9.5 - Ultrasonic Cleaning is typically done by immersion in a solvent or other cleaner solution agitated by ultrasonic waves. 9.6 - Vacuum Arc Remelting (VAR) is the remelting process of a consumable electrode inside a vacuum chamber in which heat is generated by an electric arc between the electrode and the molten tip of the solidifying ingot. The amount of gas dissolved in the metal is reduced by exposure to vacuum of the droplets of molten metal.
10 - Explorations: beyond the Welder Thixomolding Alloy for Ultrasupercritical Coal Fired Boilers Book by Metallurgists blames Rivets for Titanic Tragedy About Science Commons The Making of the Fittest Stereolythography (Video) People who built and love their job...
11 - Contribution: Competence in Welding The requirement of demonstration of welders competence by suitable examinations and testing with recording of test results has long been a practice in industry as established by well known standards. It appears that standards and industry specifications in Europe show a trend towards more stringent requirements in all areas of organizations involved in providing welded fabrications. A recent article by TWI reports on a current European movement to expand the verification of competence to all functions and capabilities of any industrial enterprise having an impact on welding performance. Although the requirements are being spelled in dedicated standards, for the time being the recommendations are presented as guidelines to manufacturing facilities to establish their own internal requirements and follow up. No external authority certification is demanded for the time being, although reference is made to qualification documents of the International Institute of Welding. It is evident that within a few years, once most of the operators will demonstrate of having complied voluntarily to the new requirements, these will be made mandatory, including the approval of an external Accredited Qualification body. Reference is made in the article to standards already approved and to some with a provisional status. It is the compliance with all the standards called for that should be addressed by industries willing to prepare for the time when they are required. In the TWI article there is a strong recommendation to all fabricators in the UK who seek such a certification to obtain it by using a third part certification body accredited in UK and Europe, for objectively demonstrating compliance over and above their own declarations. The subject of Education and Training is also presented. There are different organizational frames that include grouped professions susceptible of obtaining certification by recognized admistrative bodies (among them EWF - European Welder Federation and IIW - International Institute of Welding), to indicate competence in many aspects of welding at large. These certifications are instrumental for demonstrating compliance with the relevant standards. Although the article refers specifically to the UK and Europe, it is evident that in view of increasing global exchanges, the same or similar requirements will become commonplace all over the world. Managers should be alert to this trend in order to prepare in time to assure the competitive position of their organization in the global market. Interested readers are urged to look for the mentioned article and its various References. Note: - Readers having personal experience with the subject exposed in this note are invited to share some of it in the next issue of this publication. 12 - Testimonials From: CM.Venkat Thanks for your mail. I am surprised how this has happened. If it has happened inadvertently from my side, I apologize. I certainly look forward to your informative news letters, which are disseminated to my Departments. For your kind information, I am on the Board of The Indian Institute of Welding, and I certainly look forward to your Newsletters. [...] Thanks and Regards C M Venkat
From: Mick Santurio That was excellent!
13 - Correspondence: a few Comments 13.1 - Nothing new to notice, we discussed this more than once. People asking for different types of electrodes and their use (no material, no process indicated, no special purpose). An encyclopedia cannot be condensed in a short note. Readers should know that it is not the correct way to ask. 13.2 - Someone with important responsibility in an international Company asks for advice on a particular process: is Post Welding Heat Treatment (PWHT) really needed or could it be waived? Now it is clear that the Company has not only a resident metallurgist but probably a score of them, with laboratories and departments scattered all over the world. Why should such an official ask an external consultant? Only to go back to the person in charge and challenge his/her opinion with a different one, probably extorted without supplying all the facts and pertinent considerations. It is not fair and it is not productive to engage in such futilities. Please avoid submitting useless exercises. 13.3 - A reader copies a page from my website (on the subject of cost estimate) and then asks:"Which are the best factors to include on site welding?"
My impression is that the question is not clear to the inquirer himself. Exactly how to do that should be studied from manuals and handbooks, and some thorough homework should be dedicated to obtaining the results sought. Otherwise it is only hot air. 13.4 - I like questions, but please keep them real. Thank you.
14 - Bulletin Board 14.1 - Automatic Welding Conference 14.2 - AeroMat 2008 - 19th Aerospace Mtls & Processes Conf. & Expo 14.3 - This one may be more significant than your other efforts.
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