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PWL#112 - Welding Wrought PH Stainless Steels, Aligning a Long Shaft,Filler Metal for 17-4PH stainl. November 29, 2012 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#112 Welding Wrought Precipitation Hardening Stainless Steels: Martensitic PH Stainless Steels, Filler Metal for 17-4PH stainless steel, Productivity Improvements in GTAW with Penetration-Enhancing Compounds, Strength of Wrought Sheet Metal Stainless Spot Welds on overlapping test pieces, Welding Machines (R), Welding Plastics (R) and much more...
December 2012 - Practical Welding Letter - Issue No.112
The Mid November Issue of Practical Welding Letter, Bulletin 79, dealing with Fitness for Service was not distributed by e-mail but it is available at Bulletin 79 and from the Welding Resources Page.
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1 - Introduction 2 - Article - Welding Wrought Precipitation Hardening Stainless Steels - I 3 - How to do it well: Aligning a Long Shaft. 4 - Filler Metal for 17-4PH stainless steel 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Productivity Improvements in GTAW 8 - Site Updating: Welding Machines (R), Welding Plastics (R) 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contributions: Strength of Stainless Spot Welds 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
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1 - Introduction This 112th issue of Practical Welding Letter opens with a rather ambitious project: to give an overview on welding some special stainless steels, called PH (Precipitation Hardening). But for easing on myself and my readers, I divide this task in three parts, and publish here only the first one. The rest of it is due in the next two issues. Then I refer on an instructive experience I had while investigating the causes of repeated fatigue failures of a long shaft working in harsh conditions. Alignment should always be checked, to minimize possible causes for surprises. Further down this issue, when asked for a filler metal for a certain application I gave my advice not to weld at all. This happens sometimes. Discerning readers should be alerted to inquire on all problems likely to surface with any job, not just with one aspect. Productivity should be always one of the most important cares of any welding shop operation. Except that, probably, most supervisors are happy with what is known to work and fear to explore any innovation. They shun changes likely to introduce challenges, even when benefits should be expected. Anyhow an innovative GTAW method is described, based on serious and thorough research of EWI and other research institutes, claimed to provide substantial productivity gains. Interested readers could consider if their application might benefit from implementation trials. Two pages of the website were reviewed and updated, one on Welding Machines, the other on Welding Plastics. Many more pages are in need of being dealt with to remain useful, but time is a precious commodity. In the Contribution section, the scientific confirmation is presented of a conclusion published on these pages a long time ago, at that time without much evidence. It is comforting that the usefulness of the advice was not overthrown. The other usual sections can be found where expected. In particular the first of the Welding publications (section 5) should be of particular interest to those taking Safety issues at heart. It is hoped that most of readers will find some useful information at least in part of this publication. You can always use the Search function from almost every Welding Advisers page. You can also browse the Site Map or the Index Page to find references to the subjects you may look for. Titles of Articles from Practical Welding Letters can be found in the Welding Topics page. Links to the PWL complete list is available from the Index of Past Issues of PWL. The subjects treated in the Mid Month Bulletins are listed in the Welding Resources page. Please use the Contact Us form to send your comments, feedback or questions. Next issue is due out next year: until then, all the best. Let us hear from you.
2 - Article - Welding Wrought Precipitation Hardening Stainless Steels:
Of the five main classes or types of stainless steels mentioned in our Stainless Steel Welding page, the one identified by Precipitation Hardening presents a few characteristics that make it the preferred choice for demanding applications. It is not by chance therefore, that most of the applications of these materials are found in the aerospace field. It seems thus appropriate to dedicate some space to describe somewhat more in depth their properties, specifically in relation to welding. Precipitation Hardening (PH) Stainless Steels are further subdivided into three groups based on microstructure and on their hardening characteristics. These sub groups are called Martensitic PH, Semi-Austenitic PH, and Austenitic PH Stainless Steels. This article deals with the first subgroup. The other groups will be described in further articles. Martensitic PH steels, of which 17-4PH is a typical example, have a finely tuned chemical composition that makes them transform completely into martensitic microstructure upon cooling from a suitable solution treatment in what is technically the annealed (A) condition. In this A state the wrought material in sheet shape can still be formed with relative ease, and only then submitted to hardening. A single aging treatment at 480 to 620 °C (900 to 1150 °F) for 1 to 4 hours causes the precipitation out of solution of one or more of the elements copper, aluminum, titanium, niobium and molybdenum, remarkably increasing the mechanical properties achieved. Both the martensitic microstructure and the special phases precipitated out of the martensitic matrix contribute to build up the exceptional strength of these materials. Welding is preferably performed in the annealed condition. If the welds are of thick sections or highly restrained, highly overaged condition (590 to 620 °C = 1100 to 1150°F) is selected. Weldability is better than that of regular austenitic stainless steels due to the low carbon content. Therefore cracking is not common. Standard welding parameters as those for austenitic stainless steels can be used for arc welding of 17-4PH and the other metals indicated above. However ductility is much less, so that care must be exerted to avoid the presence of stress raisers like partial penetration welds. Should the need occur, it is recommended to weld the root pass with a ductile, low strength austenitic filler metal like 308L, (non hardenable), and then completing the weld with higher strength heat-treatable filler metal. Light gage materials can be welded autogenously (without filler metal). Only few of the filler metals for welding these base metals are included in current specifications. In case of need, narrow strips can be sheared from sheet metal of known composition, to be used as filler metals. The highest properties in the weldment can be obtained by solutioning after welding, rapidly cooling to room temperature, then aging as required and cooling. However solutioning may be objectionable if deformation is feared or if scaling (from air furnace) is not acceptable. In that case, by compromising on strength requirements, one could give up solutioning, and perform aging on as welded parts. However by so doing, the weak link would be the Heat Affected Zone (HAZ) where some parts will have become overaged during welding. In critical cases, qualification tests must be done to ascertain if the properties developed are acceptable for the application intended.
3 - How to do it well: Aligning a Long Shaft. It happened while working on a consultancy for an application involving a long shaft. Upon pondering on fatigue failures that had plagued its history, it dawned on me that something seemed missing from the design. The shaft was intended to transmit sufficient torque to perform a processing operation while rotating at low turning speed. It had been made by bar and tube sections of various length, with sturdy flanges welded to each end by double fillets. It was supported by two bronze bearings at its ends and by four more between its sections. The design did not include any element capable of mitigating the loads due to possible misalignment at assembly. Even by assuming that on assembling any pair of flanges the run out had been measured and minimized with a dial indicator, it is fair to assume that aligning had not been an issue. To assemble the long shaft, the flanges were bolted together in pairs, with opposite spring washers in between. In operation the shaft became heated to temperatures that were not measured systematically, worsening the alignment even more. No detailed metallurgical investigation could be performed on the failed elements: nevertheless the lack of alignment stood out as a major culprit to be addressed in any future design revision. Anyhow, given the mysterious origins that often fatigue failures seem to present, it could possibly help to consider, in any specific case investigated by our readers, if misalignment could be one of the causes. While researching the issue I stumbled upon two online references that could possibly help readers confronting similar problems: Update your shaft-alignment knowledge (13 pages) An Engineer Guide (192 pages) Readers are invited to contribute from their experience on this issue. 4 - Filler Metal for 17-4PH stainless steel Q: Welding motorcycle foot pegs. A: Unfortunately the best advice I can give to you is just don't do it. See also Welding Wrought Precipitation Hardening Stainless Steels, in section 2 above. 5 - Online Press: recent Welding related Articles Welding Safety Interactive Guide from Lincoln Electric Adhesive Bonding Produces Better Crash Test Results than Riveting or Welding in Cars Laser cladding replaces TIG for industrial gas turbines The blends that bind - Shielding gases for GMAW TWI Connect - Issue 180 - September/October 2012 6 - Terms and Definitions Reminder Arc Spraying is a thermal spraying process where an arc is struck between two consumable electrodes of surfacing materials to provide melting heat, while a compressed gas is used to atomize and propel the material onto the substrate. Convex Fillet Weld is a fillet weld showing a convex weld face. Direct Welding (resistance) is done with directly opposed electrodes transmitting pressure and current to the workpieces. Extension, relative to resistance welding, is the distance that the workpiece or electrode projects out from a welding die, clamp or holder. Flanged Butt Joint is a form of butt joint where at least one of the members has a flanged edge shape. Laser Beam Braze Welding, is a process variation where the heat source is a laser beam. Mechanically mixed flux for submerged arc welding is obtained by mixing two or more fluxes for special performance. Plug weld Size is the diameter of the plug weld in the plane of the faying surfaces.
7 - Article: Productivity Improvements in GTAW One of the techniques used for increasing GTAW productivity makes use of specially formulated Penetration-Enhancing Compounds available from different sources. The Edison Welding Institute (EWI) developed and patented compounds based on mixtures of oxides of silicon, titanium and chromium. The compound affects the surface tension of the molten weld pool and changes the fluid flow to inward direction contributing to increased and consistent penetration. Productivity is improved by achieving full penetration in simple square edged butt joints: less preparation is needed, while filler metal costs, heat input, distortion and welding time are also reduced. EWI recommends the use of the products with standard GTAW setup and similar procedures for mechanized and orbital welding with and without filler metal. Penetration increase up to 300% is claimed, and weld time reduction to 50%. Applications were developed for stainless steels, nickel base alloys, but also for carbon and low alloy steel, each material to be welded with a different compound. For welding stainless, EWI provides wire including the compound as a single product. It is recommended to maintain a short arc (1.2 mm = 0.050") to ensure maximum penetration, preferably with Arc Voltage Control (AVC). Among the Advantages, the following are listed:
Disadvantages are:
Further reading: EWI DeepTIG®: Innovation for GTAW Gas Tungsten Arc Welding with Penetration-Enhancing Compounds Distortion Control in Precision Weldment by Penetration-Enhanced GTAW Readers struggling with GTAW productivity problems may consider a few trials with introductory compound kits. Those with personal experience are invited to share it here with this readership. 8 - Site Updating: Welding Machines (R), Welding Plastics (R) The Pages of this Month are reviewed and updated issues, probably long overdue, of original expositions prepared a long time ago. Any serious checking of any page takes much time, which unfortunately has to be split among contrasting requirements. The first reviewed page is found at Welding Machines. It is recommended as an overview, requiring additional collecting of essential information on the type of equipment sought for any given production line. The second page, on Welding of Plastics shows now also AWS Specifications, much needed aids to implement correctly recommended practices and to communicate correctly with others in the same field. A large amount of information is freely available in the Welding Advisers website. One can review the Site Map and the Index Page to find what one looks for. One can also perform a search, by typing the requested terms in the box that appears in almost every page of the Welding Advisers website: that will include also the articles of the Practical Welding Letter, now at this 112th issue. Questions, comments and feedback are always welcomed. Don't use Reply, use the Contact Us form instead. 9 - Short Items 9.1 - Press-brake Forming is a metal forming process in which the workpiece is placed over an open die and pressed down into the die by a punch that is actuated by the ram of a press brake. The process is most widely used for the forming of relatively long, narrow parts that are not adaptable to press forming and for applications in which production quantities are too small to warrant the tooling cost for contour roll forming. 9.2 - River pattern is a term used in fractography to describe a characteristic pattern of cleavage steps running parallel to the local direction of crack propagation on the fracture surfaces of grains that have separated by cleavage. 9.3 - Surface Alterations are irregularities or changes on the surface of a material due to machining or grinding operations. They can be associated with metal removal practices include mechanical (plastic deformation, hardness variations, cracks, etc.), metallurgical (phase transformations, twinning, recrystallization, untempered or overtempered martensite), chemical (intergranular attack, embrittlement, and pitting), thermal (heat-affected zone, recast, or redeposited metal, and resolidified material), electrical (conductivity change or resistive heating) and Magnetic Particles Inspection (recast spots from unintended sparks at prod contact). 9.4 - Twin Bands are observed, by optical microstructure examination under a microscope, across a crystal grain, on a polished and etched section. Crystallographic orientations are found that have a mirror-image relationship to the orientation of the matrix grain. 9.5 - Vacuum Degassing is the use of vacuum techniques to remove dissolved gases from molten alloys. 9.6 - Web is a flat, thin portion of a forging, interconnecting ribs and bosses; a panel or wall that is generally parallel to the forging plane. A plate or thin portion between stiffening ribs or flanges.
10 - Explorations: beyond the Welder How I got to walk in the steps of Galileo? This Playground Is Made Almost Entirely from Air Shame Villagers about Toilets, Save a Child's Life Technically Art: Engineers Make Cameras, Then Hit the Pavement [Slide Show] Can the Burrowing Owl Population Rebound in North America? 11 - Contributions: Strength of Stainless Spot Welds In section 3 of Issue 11 of Practical Welding Letter from July 2004 it was suggested that good spot welds in stainless steel sheets will not break in the weld. Click on PWL#011 to see this note. Therefore the minimum strength reported for each spot weld in accepted Specifications has no real interest. When tested in tension in a standard overlapping specimen, good spot welds will rather fail in the material around the weld, by tearing a button in one half of the specimen and leaving a hole in the other half. To test this result one would need only the capability to pull the specimen in tension, even without any measuring instrument (to determine the maximum load at failure and to compare it with the minimum values in the Specification Tables). The conclusion was that this is a good result even if the tensile test load value at rupture is not known. Now, eight years later, we got at last the scientific confirmation of that unproved contention. One can read just that in a learned and thorough Research Paper published at page 303-s in the Research Supplement of the November 2012 issue of the Welding Journal under the title "Failure Mode Transition in AISI 304 Resistance Spot Welds". Here is an excerpt from the Paper Conclusions: 12 - Testimonials Date: 01 Nov 2012, 04:11:42 PM Name: Paul Ipolito
Name: Justin Doonerwind Elia Levi's Advice on [...] was most helpful Many thanks 13 - Correspondence: a few Comments This time I should just write: No Comment. One question was typically asked only for the suggestion of a filler metal, as if it were the most important issue of any welding job. In this case, as referred in section 4 above, my advice was not to weld at all, because of intractable complications arising with heat treatment. Besides that, this time I had no specially engaging questions, that I appreciate, for the occasion they give me to answer. Maybe people are busy with their worries or with the gift season. OK. Should anyone like to comment on this publication I would be glad to read your comments and to contribute my thoughts. 14 - Bulletin Board 14.1 - 8th Shipbuilding Conference
14.2 - Weld Cracking Conference
14.3 - Why I Love SBI... A Personal Journey Watch the following Video...
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