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PWL#115 - Superferritic Stainless Steels, Braze Welding Galvanized Steel, Filler Metals for Brazing March 01, 2013 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#115 Superferritic Stainless Steels, Braze Welding Galvanized Steel, Filler Metals for Brazing Titanium to Steel, Laser Soldering, Low Hydrogen Filler Metal, Clinching (R), Cold Spray (R) and much more...
March 2013 - Practical Welding Letter - Issue No.115
The Mid February Issue of Practical Welding Letter, Bulletin 82, dealing with Resources on Cobalt Welding was not distributed by e-mail but it is available at Bulletin 82 and from the Welding Resources Page.
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1 - Introduction 2 - Article - Superferritic Stainless Steels 3 - How to do it well: Braze Welding Galvanized Steel 4 - Filler Metal for brazing Titanium to Steel 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Laser Soldering 8 - Site Updating: Clinching (R), Cold Spray (R) 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contributions: Low Hydrogen Filler Metals 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board (Sponsored Links)
1 - Introduction Continuing with the presentation of special Steels, this Issue, No. 115 of Practical Welding Letter, opens with an article on Superferritic Stainless. This is a special material which found extensive economic use for heat exchangers and condensers that use seawater as the cooling medium. It may help to know that certain precautions should be used for welding. Then a commercial copper base braze-welding filler metal is introduced, of which interesting applications were reported involving joining of galvanized steels. Curious readers may wish to test it for their applications. Brazing titanium to steel may not be quite common a task but one never knows which challenges can show up. It may be useful to remember that solutions are available, even if demanding special attention. Laser soldering is a specialized process having many advantages for suitable applications, mainly in the electronic industry. Recent feedback control helps to reduce quality concerns. Clinching and Cold Spray are two unrelated processes, whose pages have been revised and updated. The first is a mechanical joining process that has many economic applications in mass production runs of light gage sheet metal items. Cold Spray is a coating process that does not need high heat to provide good adhesion and surface modification for different material systems. Its technological success is out of doubt. See further down in sect. 8 and 14.2. Low Hydrogen Filler Metal appears to be a requirement that sometimes is not adequately specified. Understanding the issue might help in reducing confusion and producing acceptable structures. Both specifiers and users will gain by learning which values should be specified in which cases and, alternatively, if specifying a different property, like notch toughness is more appropriate. The other sections are where expected. 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 appear 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, providing links to Online Sources, are listed in the Welding Resources page. Please use the Contact Us form to send your comments, feedback or questions.
2 - Article - Superferritic Stainless Steels This class, called also third generation Group III ultra-high purity ferritic stainless steels, was developed under the pressure to find leaner and cheaper alloyed steels for definite applications. At first they were intended as an economical alternative to titanium grade 2 in seawater and high chloride applications. Later, as the focus of attention for seawater capable stainless steel had moved to super-austenitic (6% and 7% Mo alloys) and to super-duplex alloys, with good performance record, material raw material prices had driven the cost of these alloys to very high levels. Then superferritic steels were brought to renewed attention due to their potential capabilities. Ultra high Purity means very low levels of the interstitial elements Carbon and Nitrogen (less than 150 ppm = parts per million), but also Oxygen and Hydrogen must be severely restricted. Purity is considered important especially for higher chromium containing alloys, because of its influence on ductility, toughness and weldability. The special refining production processes needed (electron beam vacuum refining, vacuum induction melting (VIM), vacuum oxygen decarburization) were found exceedingly expensive. Therefore a compromise solution was sought with an intermediate level of purity, over 150 but less than 800 ppm, that was found achievable with cheaper remelting procedures like argon oxygen decarburization (AOD). Metal thickness though was limited to 3 mm (1/8 in.) for these materials, for assuring acceptable ductility and toughness. Anyway welding is not suitable for heavy section plates or forgings. These materials are identified by their Unified Steel Number, but are known by their Trade Mark name: E-BRITE, SEA CURE, AL 29-4 (three variants), SHOMAC® 30-2. Superferritic stainless steels are known for their high resistance to corrosion in oxidizing environments, and to chloride-induced stress-corrosion cracking (SCC). Therefore important applications are heat exchangers and piping systems for seawater, especially for power plant condensing applications. Studies determined that the superferritic alloys were the most cost-effective long-term choice, when compared with copper alloys, stainless steel, nickel based, and titanium alternatives. The composition of these steels assures that their microstructure is essentially ferrite at all solidus temperatures. Therefore neither preheating nor postheating are required or useful. However, upon heating at intermediate temperatures, their ductility is compromised by the precipitation of embrittling phases. In particular this dangerous condition developing in the range 370-550 °C (700-1020 °F) is called the 475 °C (885 °F) embrittlement. Welding requires exceptionally clean conditions. Filler metals are generally sheared from wrought sheets. Only argon and helium can be used as shielding gases, also on the backside. Hydrogen-induced cracking is a risk, therefore hydrogen sources must be carefully eliminated. Tack welding is most critical. Several large tack welds help avoiding cracking during handling for fabrication. Grain growth may have dangerous effects on the Ductile to Brittle Transition Temperature (DBTT), increasing it unduly. Full annealing and water quenching would be recommended for limiting the effects of embrittling, except that this might cause excessive oxidation (scaling) or distortion. Less drastic treatments can be used. 3 - How to do it well: Braze Welding Galvanized Steel Normally it is not recommended to resort to unique commercial materials for solving production problems. This is because a robust solution should permit the use of standard materials, well proven and used successfully, without the premium usually requested for trade marks. Occasionally however, a practical solution may be worth abandoning general principles, if the results are positive, reliable and economic. For this reason it is possibly useful to explore special filler metals, if the actual situation is disappointing. A published note was found at page 20 of the October 2009 issue of the Welding Journal, while researching suitable solutions for joining galvanized steels. Modern high strength galvanized steels for manufacturing crash proof cars, need reliable but quick joining processes, even when wide gaps are unavoidable in production. The commercial company, sensitive to market requests, researched the issue and developed copper base materials of two similar compositions. Applications were demonstrated by using gas metal arc weld-brazing and also by plasma and laser brazing. The experience gained by two manufacturers, at the time of publishing, justified the expectations and satisfied the engineering and manufacturing requirements. Readers facing similar challenges may wish to check the article referred to above, to determine if practical tests should be undertaken. 4 - Filler Metal for brazing Titanium to Steel A short note at page 26 of the February 2013 issue of the Welding Journal provides all one needs to know to perform successfully this dissimilar materials brazing. Basically one could use standard silver brazing alloys. The silver copper eutectic (AWS BAg-8) would be suitable for vacuum furnace brazing. The article explains which other alloys, including aluminum based fillers, are suitable, and introduces a new titanium flux called RL3 A16 that permits titanium brazing in air, using a gas torch or, preferably, induction brazing. Pre-coating or pre-plating with nickel and silver the titanium parts is recommended, to avoid titanium oxidation and to prevent the formation of brittle Ti-Fe intermetallics. If strength requirements exceed the levels achievable with these materials, the article suggests to change the joint design by mechanical securing of brazed joints by means of a threaded connection. Interested readers are urged to seek the original article referred to above. 5 - Online Press: recent Welding related Articles Flame and Weld Viewing Invert the GTAW equation Developing shop employees for the field Improved Corrosion and Wear Resistant Coatings by Cold Spray Connect - Issue 182 - January/February 2013 6 - Terms and Definitions Reminder Arc Welding Electrode is a part of the welding circuit through which the current flows, and that stops at the arc. Braze Interface in a brazed joint is the boundary between the brazing filler and the base metal. Contact Resistance, in resistance welding, is the resistance to electric flow between two surfaces of the workpiece or between electrode and workpiece. Deep Penetration Mode Welding is done with high energy welding (Laser beam or Electron beam) with the formation of a deep keyhole. Flange Weld Size is the thickness of the weld metal measured at the weld root. Gas Tungsten Arc Cutting is an arc cutting process in which metals are molten by the arc between a single tungsten electrode and the workpiece under shielding gas protection. Hot Crack is a fissure that develops during solidification from molten state. Inclined Position with Restriction Ring is the position of a non rotating pipe joint. The pipe axis is about 45 degrees from the horizontal plane and a restriction ring is located near the joint. 7 - Article: Laser Soldering A specialized application of a laser beam light source is being used successfully for component soldering. Using a well controlled laser beam, it is possible to deliver a carefully determined amount of energy to a well defined location for an exactly measured length of time. In the electronic industry, reliably performing one joint at a time can be a practical solution of certain production programs. The advantages are:
Limitations are due to:
Solid state laser (Nd:YAG)(Neodymium-doped Yttrium Aluminum Garnet), at wavelength of 1.06 micron, is less reflected from the target (than the carbon dioxide gas laser beam) and results in more efficient soldering. Two methods are in use. The first, simpler, is without any feedback whatsoever. It can be applied in highly automated production runs, where the elements submitted to soldering are essentially identical. Once the optimum parameters have been determined, they are going to be maintained. Uniform results are expected, as long as no occasional discrepancies are permitted to creep into the process. Variation should be kept at a practical minimum by avoiding changes in reflectivity in the solder or contamination by any possible cause. Also oxidation and handling could reduce uniformity. While the process can experience success, the entire lot must be individually inspected to assure that defective joints be repaired or removed. These limitations promoted the development of the intelligent version of the second method. Here feedback is obtained from an IR (Infrared) sensor that monitors the effective heat input to the individual joint. This sensor measures the thermal radiation of the joint as it rises and decays following laser heating being applied and withdrawn. This follow up permits recording the thermal signature of successful soldering. Any deviation from the correct heat injection permits instant feedback controlling the application time. In such a way constant heating is assured to all joints, resulting in successful defect free performance of every joint in the production run. 8 - Site Updating: The Pages of this Month are reviewed and updated issues, probably long overdue, of original expositions prepared a long time ago. The first reviewed page is found at Clinching which is a mechanical joining process suited to overlapping sheet metal parts, even of dissimilar materials. The process is proper for mass production of identical items, with specially made sets of tools. Properties can be tested to be sufficient for the applications involved. Quality is easily assured by regular maintenance of the equipment. The second page, on Cold Spray, describes this coating process, whose main advantages stem from the fact that heat is not needed to assure adherence to the substrate. Many other advantages, as opposed to issues referred to regular Thermal Spray, are listed in the above page. See further down in section 14.2 the announcement of a Conference on the subject, to be held in the near future. A large amount of information is freely available to anyone 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: the Results Page(s) will include also the articles of all issues of this Practical Welding Letter. Questions, comments and feedback are always welcomed. 9 - Short Items 9.1 - Nominal Stress is that calculated at a point on the net cross section without considering the stress concentration effects of geometric discontinuities, (such as holes, grooves, fillets, and so forth). 9.2 - Overstressing denotes, in fatigue testing, cycling at a stress level higher than that used at the end of the test. 9.3 - Pewter is a tin-base white metal containing antimony and copper, for making decorative object. Lead, used in the past, is excluded from modern pewter. 9.4 - Roughness is an appreciation of the predominant surface pattern considering relatively finely spaced surface irregularities. Also the microscopic peak-to-valley distances of surface protuberances and depressions. 9.5 - Satin Finish describes a diffusely reflecting surface finish on metals, lustrous but not mirror like. 9.6 - Tapping means producing internal threads with a cylindrical cutting tool having two or more peripheral cutting elements (teeth) shaped to cut threads of the desired size and form. By helicoidal combination of rotary and axial motion, the leading end of the tap cuts the thread while the tap is supported mainly by the thread it produces. It means also removing molten metal from a furnace. 10 - Explorations: beyond the Welder Large Hadron Collider Is Set to Halt for Upgrades Diamond-Based Quantum Devices Shrink MRI to Nanoscale Planet Earth Gets Ready for Its Close-Up Davos: The Future of Space Step into the Twilight Zone: Can Earthlings Adjust to a Longer Day on Mars? 11 - Contributions: Low Hydrogen Filler Metals Welding Drawings, list sometimes in the Notes, "low hydrogen electrodes" as a requirement. While such notes might suggest that the designer is concerned with reducing the danger of Hydrogen Induced Cracking (HIC), it must be stressed that the requirement is undefined and therefore open to misunderstandings. The official ANSI/AWS A3.0M/A3.0:2010 According to AWS A5.1/A5.1M:2012 Expressed in the alternative method described hereafter, this level corresponds to less than 16 mL/100g of diffusible hydrogen in the deposited weld metal. A more explicit method relative to hydrogen content is specified in This method requires measuring the amount of hydrogen gas that escapes from a test weld approximately 70 mm (3 in.) in length. It is recognized that this test is difficult to perform. It is probably made only by electrode manufacturers or by specialized laboratories. Diffusible hydrogen (H2) is expressed in milliliter of evolved gas per 100 grams of solid metal (mL/100g). From the results obtained by applying this method, an optional suffix convention has been developed for the SMAW electrode classification (e.g., H8, H4, and H2), for expressing the moisture content of electrode coverings at the time of manufacturing. The figure following H corresponds to the amount of diffusible H2 expressed in (mL/100g). For related subjects, readers may wish to see also the Hydrogen Embrittlement page and the Alloy Steel Welding page. If fracture toughness of the welded construction is the main designer's concerns, it would be more appropriate to establish the minimum Charpy V-notch (CVN) impact energy required, at any defined temperature, for suitable notched test pieces removed from weld metal. 12 - Testimonials From: Fraser Rock FRASER ROCK
On Mon Feb 18 01:27:26 2013, the following results were submitted from the "Form 5" on welding-advisers.com: First Name: Daniel Militaru 13 - Correspondence: a few Comments 13.1 - After trying for years to educate readers that incomplete questions cannot be answered without first completing essential details, I still receive letters like this: "Material for the rig wheel is cast Iron. That is the only information we have from our client. Please suggest us which electrode to be used and WPS if any?" Is it not self evident that this question cannot be answered? 13.2 - I received these requests: interested readers are asked to Contact Us for getting the link. "I need to find welders with experience in reactive metals. I am in need of tantalum welders." 14 - Bulletin Board 14.1 - GAWDA 2013 Spring Management Conf. 14.2 - North American Cold Spray 2013 14.3 - JOM-17, Int’l Conf. on Joining Materials. 14.4 - The Origin Of Webmaster
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