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PWL#055 - Heat Resistance, Zinc Fumes, Honeycomb brazing, Type IV Cracking, Joint Design, Laser Weld March 02, 2008 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#055 - Heat Resistance, Zinc Fumes, Honeycomb Brazing Filler Metal, Type IV Cracking, Adhesive and Brazed Joint Designs, Laser Welding for Low Porosity 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: Heat Resistance 3 - How to do it well: Fumes from welding Zinc coated Steel 4 - Filler Metals for Honeycomb Brazing 5 - Online Press: recent Welding related Articles and Video 6 - Terms and Definitions Reminder 7 - Article - Type IV Cracking 8 - Site Updating: Joint Design (Adhesive and Brazing) 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Low Porosity Laser Welding 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This new 55th issue of Practical Welding Letter for March 2008 opens with an Article on Heat Resistance of materials and on considerations for selection in view of demanding service conditions. We then answer to a reader's worry concerning zinc fumes evolving from spot welding of galvanized steel sheets. Breathing those fumes is not recommended. In the Filler Metals section we review a few sources providing information on Brazing of Honeycomb Structures. The subject is far from simple but can be mastered, if necessary, with focused efforts. We then introduce Type IV Cracking, a known and disturbing failure mode affecting the long service life of welds of power generating boilers intended for high performance and efficiency. Two new pages on joint design were added to our weldsite: one for Adhesive Bonding, the other for Brazing. The information is essential for realizing the benefits of these joining methods in practical applications. We then review an article on a procedure for Laser Welding in an aluminum alloy, developed especially to limit porosity to an acceptable level. This task required concerted efforts of a number of partners to obtain practical guidance for manufacturing procedures to be adopted for future production. The other departments are available as usual. Let us have your comments, feedback and contributions, using the Contact Us form. 2 - Article: Heat Resistance In a page of our website we dealt with Heat Resisting Alloys Welding. Here we want to present briefly the considerations one has to take into account when selecting materials for elevated temperature service. By Heat Resistance of metals one means in practice two different things. One is the resistance to the progressive propagation of oxidation, occurring in air at increasing rates as temperature goes up. Oxidation is the absorption of oxygen in the metal by chemical reactions that leave metal oxides in place of metal. It is known that rust, occurring at room temperature, has none of the mechanical properties of the steel it attacks. At higher temperatures different oxides, called scales, may be present, anyhow the metal integrity is gone. Alloying steel with sufficient amounts of chromium will produce on the surface an impervious chromium oxide layer that protects the material from further penetration of oxidation. The other meaning one must keep in mind is the influence of heating on mechanical properties of metals. Here two cases are considered. One refers to metal stressing at room temperature after exposure to high heat. The mechanical properties may or may not have been compromised by that. The other deals with actual stressing of the metal while hot. For demanding stressing conditions under heat only selected and expensive materials may be suitable. If weight is a limiting factor as in aerospace applications, one considers not the strength properties at temperature, but the specific strength under heat, meaning the ratio of strength to density. The reason why titanium alloys are generally selected for certain intermediate components of airborne gas turbine compressors is exactly because those alloys display the highest specific strength at intermediate service temperature from about 200 to 400 0C (400 to 750 0F) (when compared with aluminum alloys and with certain stainless steels). If weight is not a factor, i.e. for land based boilers, the least cost will be a determinant factor in selecting among equally heat resistant materials. Different materials may require different fabrication processing. There is increasing awareness among designers of the total life cost (which includes also maintenance) that may influence material selection. Two types of mechanical properties are considered for hot service. One is the short time resistance of the metal when progressively stressed to failure in tension while heated in a suitable test furnace. Specifications establish requirements of minimum mechanical properties (Tensile strength, Yield (0.2% offset) strength, Elongation and Reduction of Area) to be reached at given temperatures. The other requirement refers to the creep properties of heat resistant material, meaning the continuing gradual deformation under constant load at given temperatures. Creep strength establishes the maximum unit stress accepted in various parts of the structure, to minimize the rate of creep and therefore the permanent deformation allowed and, after the service life of the item is exhausted, its final failure. The accepted criterion for evaluating different Creep-resistant-steels is the stress that, if applied continuously on specified specimens at the given temperatures for one hundred thousand hours (corresponding to slightly more than eleven years), will cause in that time period an elongation
Two properties may be tested for acceptance of base material. One is the maximum creep rate (that is the strain or elongation per unit time under constant load) at given temperatures, generally as required by specification, unless implied by other conditions like the following one, expressed as the minimum time to rupture, again at given temperatures and load. The above are generally receiving inspection tests, that are established to assure that the actual material supplied meets the requirements that were at the base of the design for the service life conditions of the project. A specific case where long time creep compromises the resistance of weldments in heavy thickness welds for boilers, is briefly reviewed in section 7, further down this page. Se also our new website page on Creep Resistant Steels.
3 - How to do it well: Fumes from welding zinc coated steel Q: I need to know how to find the type of fumes which are created when spot welding steel sheets with zinc coating. A: You may have a look at Metal Fume Fever and at two other publications: Resistance Spot Welding of Zinc Coated Steels Handbook for Resistance Spot Welding Although not lethal, it seems better not to breath those fumes...
4 - Filler Metals for Honeycomb Brazing Honeycomb cores are artificially manufactured structures built from a vast range of materials, from paper to plastics, from ceramics to metals, for so many applications in different fields. The name is derived from real bees honeycomb made of wax, with which they share the form of hexagonal cells periodic pattern, repeated in two dimensions. The most commonly exploited advantage is that they permit to build panels having very high ratio of strength to stiffness and of stiffness to weight, for equal resistance to bending loads. Limiting this presentation to metals, panels are often built as sandwiches by brazing a suitable core to two face sheets. On core manufacturing technology see: For aerospace applications, that put a premium on light weight solutions, typically the following materials are used: Aluminum Alloys for low temperature structural applications. See: Introduction to Brazing of Aluminium Alloys ANSI/AWS C3.7:2005 Titanium Alloys for intermediate temperature applications as in gas turbine compressor section. A note on Filler Metal for Brazing Titanium Alloys was published in Issue 46 of Practical Welding Letter for May 2007. Click on PWL#046 to read it. A commercial company offers information on Filler Metals for Brazing of Titanium alloys in the following page: Two articles by A. Shapiro on Nickel Base Alloys for elevated temperature applications like turbine seals in gas turbine engines. Vacuum Brazing Honeycomb Seals Honeycomb Brazing Essentials for Successful Use as Turbine Seals Gas Turbine Seals 5 - Online Press: recent Welding related Articles and Video From The Fabricator, our Article on Creep Monitoring of Repair Welds in High Temperature Components To watch a Video on Friction Stir Welding How to weld a Frame Corner (Patriotic Video) How to remove a broken stud (Video) 6 - Terms and Definitions Reminder Arc Plasma is a column of gas ionized by passing through an electric arc and becoming therefore conductive. Capillary Action is the attractive force generated by closely placed surfaces on wetting liquids, essential for brazing. Continuous Weld extends continuously from one end of a joint to the other. Drop Through is the uncontrolled excess of molten metal, due to unskilled operation, protruding from the joint. Electrode Indentation in resistance welding is the depression remaining on the spot welded surface. Fillet Weld Leg is the distance in the weld section between the joint root (the innermost point) and the toe of the fillet (the farthest welded spot on the base metal). Open Root Joint an unwelded joint without backing or consumable insert. Reducing Flame a non neutral oxyfuel gas flame with excess fuel gas. 7 - Article - Type IV Cracking Type IV Cracking is a bothering problem that affects the efforts to produce higher thermodynamic efficiency (50%) Ultra Super Critical Steam Boilers for operating turbines of Fossil Power Plants at higher temperatures (650 0C = 1200 0F) and pressures (over 270 bar) with the additional advantage of significant reduction of carbon dioxide emissions. The classification of crack types occurring at or near welds, is based on a convention that describes weld related cracks according to their location. Type I cracks are those limited to the weld metal. Type II are those starting in the weld but extending into base metal. Type III are those found in the coarse grain section of the Heat Affected Zone. Type IV cracks are located in the fine grain region of the Heat Affected Zone. The crucial point is that this cracking severely impairs the usability of creep resistant ferritic steels whose life, as base metals, would be well within project expectations. As the problem of global warming is heavily dependent upon the technologies adopted for power generation from burning fossil fuels, the efforts to overcome these difficulties generated by present materials and processes are truly of international character, and are promoted by various initiatives. The several base materials considered for these applications are collectively known as 9-12% Chrome steels, and their development has known significant progress in the past decades. To control long term creep rupture strength, deeper understanding of the microstructural factors is vital if further improvements in creep rupture strength are required. But concerning the Type IV Cracking, although slow progress has been achieved relative to the positive influence of additions of balanced amounts of boron and nitrogen, cumulative evidence has shown that the weak link is the fine grained Heat Affected Zone where progressive damage may lead to cracking. In particular premature failures consisting in through cracks near welds of piping may lead to leaks of high temperature and pressure steam, a dangerous situation that imposes immediate shut down of the facility for high cost unscheduled repair and substantial economic loss. So much so that Type IV cracking is considered the most troublesome service failure in the power industry. The reduction of life attributed to weld Type IV cracking is evaluated at a factor of five for service conditions of advanced boiler design, compared with base metal life and is assumed to be the life limiting failure mechanism. In conclusion it is unsafe to select materials for long term creep resistance (for coal based power production plants designed to operate at high temperature and pressure), basing the selection on properties demonstrated only for the base metals. For further reading see: The third article reported above in section 5 is relevant to the problems exposed here. 8 - Site Updating: Adhesive and Brazing Joint Design This time we have introduced two new Pages of the Month to our website.
They can be found by clicking on Adhesive Joint Design and Brazing Joint Design. We hope these pages provide useful insight in the design of joints suitable for the processes indicated and they can be sought for if information is needed to exploit successfully the advantages of those processes. 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, under the NavBar in any page. See 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 - Creep describes the tendency of a solid material to deform permanently under stress and heat. It results after long term exposure to elevated temperature stress below the values of yield strength or ultimate strength of the material at room temperature. It is the time and temperature dependent strain or deformation that forms under stress. The creep strain happening at a decreasing rate is called primary creep; that at a constant rate, secondary creep; and that at an increasing rate up to final rupture, tertiary creep. 9.2 - Galvanized Steel has its surface coated with zinc using any of various processes as a protection against rust and corrosion. 9.3 - Interdendritic Corrosion is an attack that progresses preferentially along interdendritic paths. Dendritic (from Dendrite, a Greek word meaning tree) grains are crystals generally originating from slow solidification from a melt. This type of attack may result from local differences in composition, as found in alloy castings. 9.4 - Killed Steel is treated with a strong deoxidizing agent such as silicon or aluminum to reduce the oxygen content to such a level that solidification is calmed down (instead of bubbling) since no reaction occurs between carbon and oxygen. 9.5 - Scale is surface oxidation, appearing as partially adherent layers of corrosion products, left on metals by heating or casting in air. The formation of a thick layer of oxidation products on metals at high temperature is called scaling. 9.6 - Shear Stress is the stress or the component parallel to the plane on which the forces act. 10 - Explorations: beyond the Welder Cloud Gate by Anish Kapoor Greenpeace Leonardo's Last Supper in high Definition Hydrogen Technologies Retirement can mean freedom, enjoyment and extra income! 11 - Contribution: Low Porosity Laser Welding A recently published research report presents results of trials undertaken to test the feasibility of laser welding a specific aluminum alloy, selected for its potential usability in stress bearing aircraft fuselage components. Two conditions were imposed: the welded thickness was 12.7 mm (1/2"), and the maximum tolerated level of porosity in the weld was to be found within the limits of two stringent specifications (BS EN 13919-2 and AWS D17.1). Two processes were tested. One was straight autogenous welding (without additional filler metal) by Yb-fibre laser of 7 kW power, the other was a hybrid laser combination with GMAW arc. The material selected by the aircraft manufacturer was an unspecified 7000-series Al-Zn-Mg-Cu aluminum alloy of proprietary composition and temper. The research was carried out in the UK by a consortium of industry and academy with government support. Special attention was devoted to removing the oxide layer by dry machining of all surfaces just one hour before welding and using dry shielding gas. Welding speed had to be reduced to improve quality, and the size of the laser spot had to be optimized. The presence of porosity and its degree was assessed by radiography. The butt joint was without gap or bevel, welded by the keyhole technique. The positions tested were the horizontal and the vertical up. At the conditions tested there was no problem in obtaining welds in the tested alloy, acceptable visually and radiographically, and the vertical-up position gave consistently better results. Interested readers are urged to seek the original publication: 12 - Testimonials From: Dee Genna Thank you.
From: Zink, Jim Elia, Thanks for the quick reply. [...] Thanks Jim Zink 13 - Correspondence: a few Comments 13.1 - This much did not happen to me before, till now. Questions and Feedback : "I have 9 case studies with welding problems to answer
for the relevant course. (He must have collected the questions from a bunch of friends). He offered to pay, mind you. Of course I refused. What do you think? 13.2 - This comment is not new. Some of my answers bounce back, from time to time. I cannot know why, if there was an error in the e-mail address or if the Inbox was full. 13.3 - There are a few projects I was asked to suggest solutions for. What happened to them? Did my advice help? I would like to know. 14 - Bulletin Board 14.1 - I was asked if I can conduct Online welding inspection training.
If there is sufficient request I can prepare a course. Formal Certification is granted only by appointed official Authority. Please drop me a line, without obligation, to give me a feeling of the need.
Thanks. 14.2 - May I remind to all kind correspondents that promised in the past to send their Contributions to this newsletter, to "do it!" ? 14.3 - SAE 2008 World Congress April 14-17, 2008 - Detroit, Mich. USA http://www.sae.org/congress/ 14.4 - International Thermal Spray Conference & Expo (ITSC 2008) June 2-4, 2008 -Maastricht, Netherlands. http://www.dvs-ev.de/itsc2008/
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Click on this Logo NOW! Copyright (©) 2008, by Elia E. Levi and See you next time... |
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