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PWL#061 - Cooling for HVOF, Welding 1018 to 4140, Underwater Electrodes, SCC, FSW Equipment, Weld Mo September 01, 2008 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#061 - Cryogenic Cooling for HVOF Thermal Spray, Welding 1018 to 4140, Underwater Electrodes, Stress Corrosion Cracking, FSW Equipment, Welding Molybdenum, Underwater Welders Pay and more... DON'T USE REPLY to send us 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. 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 - Cryogenic Nitrogen Cooling for HVOF Spraying 3 - How to do it well: Welding 1018 to 4140 4 - Filler Metals: Electrodes for Underwater Welding 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - Stress Corrosion Cracking 8 - Site Updating: FSW Equipment, Welding Molybdenum 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contributions: Underwater Welders Pay 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This 61th Issue of Practical Welding Letter brings some new information on a strong cooling effect that improves productivity in HVOF Spraying. It can definitely be an important innovation for those who use HVOF. The quoted article will refer readers to the commercial firm supplying the whole system. Answering a search for the problem of welding together mild steel to low alloy hardened steel, we elaborate on what happens when attempting such a weld. One should know beforehand if the results obtainable correspond to the needs. Filler metals for underwater welding are specialty electrodes developed by the commercial firms that supply them. There is no standardization on these electrodes. Practically every welder uses what meets requirements and suits him/her best. Stress Corrosion Cracking may occur suddenly making an appalling damage. One should be aware of the conditions causing this unfortunate outcome and check the fine details of the next project to see if there are any faint chances that this failure could happen. This month Website Update brings two quite unrelated subjects. One page shows needed updates to check how to reap the benefits of the new FSW technology in untested fields of application. The other page, written to satisfy a reader's query, provides information on Welding Molybdenum. See the links in section 8 further down this page. We were asked to supply data on take home pay for divers welders performing underwater work. It is quite difficult to find reliable data because the range of rewards is very wide and individual positions are based on so many factors that reaching a satisfactory answer is quite hard. Nevertheless we put together a few observations to provide an indication to curious inquirers. Other sections are available as usual. We welcome your feedback. Click on Contact Us.
2 - Article - Cryogenic Nitrogen Cooling for HVOF Spraying Thermal Spray processes provide useful coatings of a vast array of materials to protect the surfaces of engineering items from the effects of service environment, like corrosion, oxidation, wear etc. Although the Thermal Spray processed items are subjected to much less heat input than that produced by welding, the parts still undergo a quite remarkable heating that cannot be ignored as it affects distortion, hardness, nearby sensitive materials and so on. It is true that a recent development called Cold Spraying achieves remarkable coating properties for certain items without resorting to high heat, but it still cannot compete with more classic and proven thermal spraying methods. In particular HVOF (High Velocity Oxygen Fuel) Thermal Spray heats up the treated parts so much that special schedules must be set up to let the parts cool down before damage is caused. This process is the preferred one for the treatment of aircraft and of gas turbine engines parts because of the superior characteristics of the coating like density, hardness and adherence. A recent development, described in an article published by the Welding Journal, in the August 2008 issue at page 26, explains how a cooling jet of cryogenic nitrogen vapors has been successfully exploited to process HVOF. The cooling jet avoids liquid nitrogen droplets impinge on the surface. Such wetting would cause too steep a cooling gradient and harmful excessive internal stresses. The cooling action is described as effective and uniform, because the process is controlled by feedback procedures based on the continuous monitoring of the surface temperature and immediate adjustment of cooling jet modes to keep the part temperature constant at the value established by the operator. In comparison with the previous cooling method that used compressed air, excessive heating was reduced to shorter time so that productivity was increased by 50%. Additional savings were realized by avoiding the waste of expensive coating material that was previously directed away from the part during the cooling interval. Further savings are claimed because of the ease of removal of masking tapes made possible by the fact that they are not degraded by the high heat from which they are protected. Interested readers are urged to learn the details, including those of the commercial supplier, by seeking the original article indicated above.
3 - How to do it well: Welding 1018 to 4140 Question: How is 1018 welded to 4140? Answer: Welding 1018 to 4140 is possible although one should know more about the application, because the differences between the two steels may cause consequences one may not want to get. 1018 is a mild steel weldable without problems and not hardenable except by diffusion surface treatments like carbonitriding which need not concern us here. 4140 is a chromium molybdenum low alloy steel with medium carbon content (about 0.40%) that can be hardened by direct quenching and tempering. It can be welded but with special precautions like preheating, to slow down the cooling rate. Without preheating, fast cooling after welding generates martensite, a hard structure that risks to develop cracks near the weld. Welding a soft steel to a hard steel can be useful in certain cases, for example if a handle needs to be joined to a hard implement. One should understand that the handle will not become stronger, and that the implement, if it was heat treated to a high strength and hardness before welding, will become weaker near the weld, because of the heat. Two contrasting things will happen because of the localized heating of welding. On one hand all the volume heated, next to the weld, to a certain temperature up to the so called transformation temperature (say up to about 750 0C or 1400 0F) will become weaker due to annealing, effectively eliminating the higher properties due to heat treatment. But very near to the weld itself, the temperature will exceed the transformation temperature. The material structure will change into "austenite", hardenable upon cooling once the welding process is stopped. Not only will the material become very hard in a thin strip near the weld, it might also crack. Suppose that we can preheat with an oxyacetylene flame and that we want to weld by SMAW (or stick) with an electrode of almost any usual type. If we know what to expect, that our mild steel handle at the weld will be weaker than the hardened implement, and if this is OK for the application, we should heat with a flame, before welding, the 4140 implement where we want to place the weld. By so doing we avoid cracking. The full hardness of our implement will still be present in the part at some distance from the weld (say at 50 cm or 2", depending on thickness, on the size of the weld and on the amount of heat input). We should again heat the weld with the flame, to temper or soften any martensite that could have formed in the process (Local Stress Relief).
4 - Filler Metals: Electrodes for Underwater Welding Welding electrodes suitable for Shielded Metal Arc Welding (SMAW) underwater, are modified versions of those used in air. No specification exists for this kind of electrodes and generally they are known and purchased for their specific use according to commercial brand names, mainly according to welders' preference and ease of use. The most used are those for mild steel of the Carbon/Manganese type. It is known that the mechanical properties obtainable in underwater welding are rapidly worsening with increased depth, because of increased porosity being trapped in the weld. Also the development of Hydrogen Induced Cracking is a matter of concern in that certain acid types of coating have more capacity to absorb moisture that can dissociate in free hydrogen, being easily absorbed by the molten metal. Therefore for underwater welding rutile (titanium dioxide) base types of coatings are preferred, containing balanced compositions of ingredients for achieving special purposes. FeMn or Ti may be added to improve the absorption of oxygen and reduce porosity. Higher strength steels are usually welded underwater using austenitic stainless steel electrodes to minimize the occurrence of cracking, but additional precautions may be required to avoid cracking of the Heat Affected Zone. Depending on mechanical properties required and water depth specified, certain electrodes may prove the most suitable. Test are needed to qualify the proposed Welding Procedure Specifications and to accept the selected electrodes for the intended use. The outstanding difference between normal electrodes and those suitable for wet welding is that these are covered by a waterproof wax or plastic coating that will keep the flux coating dry until use or at least will minimize moisture penetration, without interfering with the arc and metal transfer. The waterproof coating of quality electrodes for underwater use must ensure moisture is prevented from reaching the flux coating. It should also burn away without leaving harmful deposits and without interfering with metal transfer. Research and development of flux ingredient proportions in the covering of wet welding electrodes promoted better understanding and progress in the control of hydrogen content, porosity, chemical composition, and microstructure of the weld metal. To get a taste of the trial and error methods employed, one could review the reports of the scientific efforts devoted to improve the performance of wet electrodes. See the following three reports downloadable from a page titled
For basic information on materials and wet welding techniques see also the
5 - Online Press: recent Welding related Articles From The Fabricator: From TWI: Fastening onto a Solution
6 - Terms and Definitions Reminder Edge Weld in different types of joints is such that the full thickness of the component parts is fused. Filler Metal is the additional material supplied, in a vast range of forms and shapes, for performing welding, brazing or soldering joint. Grip, a term normally associated with structural bolting, is defined in ASME B18.12 Glossary of Terms for Mechanical Fasteners as the thickness of material or parts that the fastener is designed to secure when assembled. Heat Input is the energy supplied in any given process to elevate the temperature and perform the action required. Infrared Brazing is a process that derives the heat needed for performing brazing, from the irradiation of infrared lamps. Local Stress Relief equalizes and reduces locally the level of residual stresses by affecting only a limited part of a structure. (See an example in section 3 above). Metal is an opaque lustrous elemental chemical substance that is a good conductor of heat and electricity and, when polished, a good reflector of light. Metals are mostly malleable and ductile and denser than the other elemental substances. Metallic atoms tend to lose electrons from the outer shells. Electric current is carried by free electrons, and decreases as temperature increases. Metal may mean also an alloy. Recicled Flux is that portion of unused flux that was recovered for reuse from a previous application of Submerged Arc Welding.
7 - Article - Stress Corrosion Cracking A brief introduction to this subject was presented (9.6) in the No. 25 Issue of Practical Welding Letter for September 2005. Why should this argument be of interest to welding professionals? All SCC Failures have in common the following three elements:
What may make the development of such a failure quite unexpected and catastrophic is its insidious progress characterized by substantial strength loss without appreciable external signs, so that a cursory inspection would miss its forewarning aspects. As Stress Corrosion Cracking can be unexpected, every effort must be done to scrutinize if in any specific case the conditions might be such as to promote the concurrent presence of the three elements above. Certain materials are susceptible to specific environmental attack. Therefore one should approach any new welding job with a routine check of the material and of the exposure conditions in service. As examples, consider the following combinations that are known to be dangerous and that should be avoided:
For a thorough screening more complete tables and information should be sought in the literature. A whole new class of stainless steels of duplex structure (partly austenitic and partly ferritic) were developed also for removing the danger of Stress Corrosion Cracking failures of regular austenitic stainless steels in presence of chlorides and stresses. Besides checking any tendency of selected materials to be subject to SCC, one should also reduce stresses left in the material by bending or
forming operations or generated by welding. As far as it is practical to operate on the process solutions, one should strive to remove critical environmental species such as hydroxides, chlorides, and oxygen. Design should also address and avoid stagnant volumes and crevices in heat exchangers where chloride and hydroxide might become concentrated. If the responsible people involved in the welding project have no previous experience of SCC, they would do well to look for pertinent advice and design review from professional corrosion experts. The following Standard is an important reference that addresses the issue of SCC in the alloys described for the uses indicated. ASTM B928/B928M-07
8 - Site Updating: FSW Equipment, Welding Molybdenum The Pages of this Month recently added to our Website refer to subjects that were recently called to our attention. Friction Stir Welding Equipment is by now the issue of concentrated experience accumulated in the hands of all those who applied the new process and of the manufacturers that built the machines capable of handling all their requests. The Welding Institute, willing to help with spreading the knowledge needed to introduce FSW in new applications is organizing intensive familiarization courses. It is an opportunity that those interested should not miss. To find our new page click on Friction Stir Welding Equipment (Opens a new Window). In response to specific queries that probably expressed the needs of
certain readers we prepared the following page on Welding Molybdenum (Opens a new Window). We hope that it will be useful and we would like to get your feedback. Readers are urged to use the Google Search window that appears in almost every page of our Website to locate specific information and links. Another method to find one's way in the maze of this Website is to look at the
Site Map. In case that everything else fails and if you still need not readily available information on welding and related subjects, please Contact Us and we will try to find it for you. 9 - Short Items 9.1 - Coefficient of Friction is the ratio (without dimensions) of the friction force (F) in the moving direction, between two bodies, to the perpendicular (normal) force (N) that keeps the bodies pressed together. 9.2 - Corrosion Resistance is the ability of a material to stand for definite length of time environmental actions of the media present in service without degradation or with a loss of material limited to some maximum level (measured in penetration or weight loss) or with appreciable change in properties. 9.3 - Ductile Fracture in metals is a mode of failure characterized by tearing, with visible gross plastic deformation and usually occurring with absorption of considerable energy. 9.4 - Emulsion is a stable dispersion of two immiscible liquids (like oil in water), generally by means of an emulsifying agent that has affinity for both. 9.5 - Permanent Mold made of metal, graphite, or ceramic (depending on the alloy being cast) is used again and again for the production of many castings of the same shape. 9.6 - Poisson's Ratio is the absolute value of the ratio of transverse (lateral) strain to the corresponding axial strain, in a standard uniaxial tensile test specimen, resulting from uniformly distributed axial stress below the proportional limit of the material.
10 - Explorations: beyond the Welder Visualization drives a Cerebral Arts Statesman Welding sparks Gigantic Blaze Addressing Concerns Over Information Spotlight in Metalcasting (video) The Exoskeleton (video)
11 - Contributions: Underwater Welders Pay From time to time a recurring query concerning the pay that is possible to get as an underwater welder is raised by readers searching our website. It is uncertain if a meaningful answer can be put together, because too many individually changing variables influence the makeup of the final take home pay. A recommended introductory page from an authoritative source (http://www.aws.org/education/plunge.html) puts the range of yearly pay between 15,000 and beyond 100,000 USD, probably too wide a bracket to be instructive. For North Sea operations an indicative value of 380 British Pounds per day for diving welder is quoted (Source: David Keats from www.specialwelds.com) for as long as the job lasts. There are probably wide variations around this value. For comparison surface welders pay is quoted (by the same source) as about 10 BP per hour with a maximum of 200 BP per day. We may try however to write down a few thoughts on this subject, hopefully as a guideline for whoever is interested in learning more on the argument. Nobody should think of the job of the underwater welder exclusively in terms of pay. Like with many other jobs, there should be a genuine preference on the part of the candidate for selecting just this kind of life. One has to remember that actually two crafts must be mastered by the candidate, diving and welding with a special set of skills, that the work may be physically wearing out, and that the age for retirement is much earlier than for other occupations. The long two-fold training needs also a non negligible investment, so that a firm commitment is needed to pursue this career at least for a certain time. This work may be demanding also because of other reasons. The locations are spread the world over, wherever there is metal under water to work upon, and may be changing at short notice, making normal family life quite difficult. Workers are recruited for given jobs and, except for remarkably skilled workers, there is generally no long time employment commitment on the part of employers. Welders therefore become self-employed workers who hire their skills trying to maximize the pay for reasonably long periods of the year. Thus the ability to bargain becomes a needed quality that not all welders may possess. Although bonuses are calculated on such parameters as the depth of the work to carry over, they tend to reward seniority and experience. Nothing prohibits diver-welders to advance in time to more rewarding and less exhausting jobs by specializing in supervising activities or managerial positions. Prospective diver-welders should possibly contact one or more vocational schools offering these programs and inquire what entry pay rates could be expected after graduation. They could also request from the schools the names of former pupils to ask them how they fare and which recommendations could they offer. If you have personal experience and happen to know more on this subject, you are welcome to share with your fellow readers useful information and recommendations, without disclosing any private data on yourself.
12 - Testimonials Date: 14 Jul 2008, 09:03:09 AM Great issue!
We live and die with fatigue concerns on a daily basis. Kindest Regards,
On Fri Jul 25 08:21:17 2008, the following results were submitted: Thanks big for the answer! Dmitriy.
13 - Correspondence: a few Comments 13.1 - Some readers would like to get ready to use procedures. Although there may be in the market such document for sale, I doubt they are easily found except by special order. Pre-qualified procedures are available from AWS. In general WPS should be developed in house to give more confidence and the opportunity to qualify them by testing. 13.2 - I cannot convince the readers that they should provide more details if they ask for a question. Otherwise it is impossible to understand what they really need. If certain details are not yet known or not decided, please say so. 13.3 - Readers requesting a change of address should use the line available at the end of this newsletter, and please don't send back the whole PWL just to use the reply address. Evidently many readers are not aware of my request.
14 - Bulletin Board 14.1 - MS&T '08 - Materials Science & Technology 14.2 - 34th Intl. Symp. For Testing & Failure Analysis
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