| Back to Back Issues Page | |
 |
|
|
PRACTICAL WELDING LETTER, Issue #009 -- Explosion Welding, Filler Metal Comparison, Furnace Brazing May 01, 2004 |
|
| We hope you will find this Letter interesting and useful. Let us know what you think of it. Practical Issues, Creative Solutions
This publication brings to the readers practical answers -----------------------------------------------------------------------
You are urged to pass-along this publication to your ----------------------------------------------------------------------- Date: May 2004 - Practical Welding Letter - Issue No. 09
------------------------TABLE of CONTENTS---------------------------
1 - Introduction 2 - Article: Explosive Welding 3 - How to do it well: Hardening Heavy Sections 4 - Filler Metal: Comparison Tables 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Furnace Brazing 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Post Weld Heat Treatment 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board ---------------------------------------------------------------------------
The list of Practical Welding Letter subscribers exceeded recently the mark A new page on Welding Cost Estimate has been recently added to the Site. Technically minded people may tend to shun this subject as
less interesting. To find your way in the Site we strongly suggest you click on the Site Map. If you can think of which new page you would like to see in the Site, just let us Explosive Welding is quite a shocking method of cladding, is not it? It is used If high hardness is needed from a Heavy Section bar of plain Where can one find information on Comparison of Filler metals Furnace Brazing can be the preferred solution for certain jobs, A few visitors looked for Post Weld Heat Treatment in the Site, and were Other regular items show up as usual. Let us know your feedback.
---------------------------------------------------------------------- -----
2 - Article: Explosive Welding Explosive welding or cladding (or bonding) is a solid state welding process(that is, no fusion or liquid state) that occurs with minimum heat input. The original mechanical properties of the two elements welded together are preserved, and no heat affected zone is present to reduce or impair them. Dissimilar metals can be welded, which could not be joined by other processes because of the formation of brittle intermetallic compounds. Explosive welding uses a progressive explosive detonation to force two metals to impact together at high speed and pressure to form a metallurgical bond. Usually there is a thick base plate providing the support, and a thin cladding sheet selected for special properties like corrosion, wear or heat resistance. The base is sustained by an anvil bed. The cladding sheet is precisely positioned at a certain stand-off distance and at an angle to the base, and covered with a plastic flexible explosive sheet. The explosive force brings the two metals progressively together at the collision front while expelling a jet of metal that forms just ahead. The jet cleans the surfaces from oxides and contaminants. The front's advancement must be lower than the speed of sound in the materials to avoid interference with shock waves. The weld line in cross section usually appears as a wave. A variation of the process permits cladding of tubes. Advantages:
tubular pieces. It is called Magnetic Pulse Welding. Speed, collision, impact, pressure are present, but the moving force is different: it comes from the production of powerful transient magnetic fields. For an article on this technique click here. ---------------------------------------------------------------------- ----- 3- How to do it well: Hardening Heavy Sections Q: A bar of high carbon steel (0.7-0.9 %C), of 100 mm dia. (~4"), quenched in oilfrom elevated temperature, failed to harden. Heated again and quenched in water, its hardness did not improve. Why? A: A heavy bar of plain high carbon steel cannot develop substantial hardness upon quenching because heat removal is too slow due to its substantial mass. Sluggish heat removal prevents martensite (the hard constituent) to form and permits less hard structures to appear, during the transformation from austenite. If ~60 HRC hardness is needed, one must switch to air hardening tool steel like SAE A1, where the composition allows slow cooling to produce full hardness. Do not forget tempering.
However, if only surface hardness is required (and the core may remain softer), ------------------------------------------------------------------- -------- 4 - Filler Metal Comparison Tables Filler Metals are the most important Consumables on which Quality and EconomicPerformance of welding jobs are based. It is recognised that the selection of the appropriate material may be confusing and difficult. The first selection can be done according to types and specifications, but then one must choose a specific commercial brand. A Correspondent asked for information on Tables describing commonly used electrodes and their parameters. Such publications are available for sale. To satisfy this request we prepared the following:
From AWS at Filler Metal Comparison Charts AWS Code FMC:2000. Listing more than 80 manufacturers, it is thought as one of the most comprehensive publications available. It has 483-pages in 8 1/2" x 11" format, softbound. Filler Metal Procurement Guidelines AWS Code A5.01 Also Filler Metal Specifications AWS Code: A5-ALL Suggested Filler Material for Welding Structural Steel AWS FMS D:Desk Chart or AWS FMS W: Wall Chart Users Guide to Filler Metals AWS UGFM Selecting Filler Metals: Low Hydrogen http://www.lincolnelectric.com/knowledge/articles/content/fillermetals.asp A list of proprietary Lincoln electrodes is available by clicking on the link in the above article. The title is: Product Comparison: Stick Electrodes - Mild and Low Alloy Steels Note: This publications lists only consumables from this specific manufacturer. From the same supplier, a Filler Metal Selector Guide is available at http://content.lincolnelectric.com/pdfs/products/literature/c150.pdf Other articles:
Article on Matching Strength criteria Article on selecting for Low Hydrogen http://www.jflf.org/pdfs/papers/keyconcepts5.pdf Commercial comparison of Flux Cored Wires http://www.praxair.com/praxair.nsf/d63afe71c771b0d785256519006c5ea1/fcb2fc5a 83170e3585256cc40055535a/$FILE/psg%208_6-14.pdf Commercial compilation with some general welding info http://www.casti.ca/books_ebooks/lite/BlueBookLite.pdf General information (Army) on Electrodes and Filler Metal http://www.machinist.org/army_welding/Ch8.htm A very large list of Welding Related links providing connection to a vast selection of suppliers of equipment and materials is available at http://groups.msn.com/WeldingandFabrication/weldingrelatedlinks.msnw Readers having personal experience with the lists above or other useful information are invited to let their fellow readers know of it, by writing us their recommendations. Click here. --------------- ------------------------------------------------------------ 5 - Online Press: recent Welding related Articles Pipe and Tube Weld Inspection:http://www.aws.org/itrends/2004/04/025/ Change in Penetrameters: http://www.aws.org/itrends/2004/04/017/ A Flowmeter Primer: http://www.asminternational.org/Template.cfm?Section= HeatTreatingProgress&template=MagazineDisplay.cfm&AbstractID=9182 The Trail to the Leading Edge (on the investigation on the loss of Columbia) http://www.memagazine.org/backissues/mar04/features/thetrail/thetrail.html Titanium: you can weld it! http://www.thefabricator.com/ArcWelding/ArcWelding_Article.cfm?ID=878 How good do piping system welds really need to be? http://www.sperkoengineering.com/html/articles/How%20good%20do%20Welds_.pdf --------------------------------------------------------- ------------------ 6- Terms and Definitions Reminder Annealing: a generic term denoting a treatment consisting of heating to and holding at a suitable temperature followed by cooling usually at a slow rate, used primarily to soften metallic materials, but also to simultaneously produce desired changes in other properties or in microstructure. The purpose of such changes may be improvement of machinability, ease of cold work, enhancement of mechanical or electrical properties, and stabilization of dimensions. Cold Welding: a solid-state process in which pressure is used at room temperature to produce coalescence of metals with substantial deformation at the weld. When happening unintentionally by chance between unlubricated surfaces it is a form of failure susceptible to bring much damage (i.e. seizing of bearings). Cooling Rate: the average slope of the time- temperature cooling curve of any item occurring over a specified time and temperature interval. In certain materials it must be controlled to obtain specific results. Explosion Welding: a solid-state welding process that produces coalescence by a controlled detonation of explosive, which causes the parts to move together at high velocity without significant heating. The resulting bond zone has a characteristic wavy appearance. Used mostly for cladding special metals unto common ones. (See 2 - Article above in this same issue of PWL). Peening: the mechanical working of metal by hammer blows or shot impingement to modify the condition of residual surface stresses in order to improve resistance to cracking and to fatigue failure. PWHT - Postweld Heat Treatment: any heat treatment that follows the welding operation for tempering, for stress relieving, or for providing a controlled rate of cooling to prevent formation of a hard or brittle structure and to improve mechanical properties. See an article further down in 11 - Contribution, in this issue of PWL. Recrystallization is the formation of a new, strain-free grain structure from that existing in cold-worked metal, usually accomplished by heating or the change from one crystal structure to another, as occurs on heating or cooling through a critical temperature. The time and temperature are selected so that, although recrystallization is complete, essentially no grain growth occurs.---------------------- -----------------------------------------------------
7 - Article: Furnace Brazing The subject of Brazing in general has been dealt with in a page in our Site, visible by clicking here. An article on Selection of Silver based brazing Filler alloys was presented in the November 2003 issue of Practical Welding Letter No. 03. Click here. In the present Article we intend to expand on the technology of Furnace Brazing. It can be a mass production continuous process or a batch one, performed in a suitable furnace heated at the proper temperature for the required time. The furnace is designed to be operating whether with a protective gas atmosphere or in vacuum as needed. Different types of protective atmospheres can be prepared and used to satisfy economically different requirements. One of the interesting applications of furnace brazing refers to the use of the step brazing technique (as explained in the above site page), where different joints are brazed in different cycles in a sequence, using filler metals of progressively lower melting ranges. In the past also air furnaces were used, but it seems that their use has been reduced or discontinued. In air, brazing results depend on flux availability and activity: no attempts are made to avoid oxidation of the base metal outside of the brazed joint. Four operations are performed: Cleaning, Assembling (possibly with some form of self retention by deformation to avoid the need of fixturing), Brazing in a Furnace and Cooling. Assembly for brazing should be carefully studied to retain the elements and the filler in place before and during brazing. The main advantage of furnace brazing is that many joints with preplaced filler metal, either in different items or in a single one, are brazed at the same time, generally under well controlled conditions. Therefore the quality of the joints is reliable once the inspection requirements are implemented and satisfied. When using copper filler metal for brazing carbon steels, the atmosphere can be made sufficiently reducing to eliminate the need for the use of a flux. If however low alloy steel contains 2 to 3% Chromium and other elements producing tenacious surface oxides, then a stronger reducing atmosphere is required like dissociated ammonia or dry hydrogen. Preassembly or jigging for furnace brazing is a labor intensive activity including preplacement of filler metal and possibly flux and/or stop-off. This work can be performed by diligent but otherwise unskilled workers. Four types of furnaces can be described, but they may be very different being usually adapted to the planned production. The batch type of Furnaces are loaded while cold, then heated. Brazed parts are removed when sufficiently cold to be handled easily. Continuous type furnaces can be of the conveyor type or of the push type. Either way the parts are usually placed on trays loaded from one end through double doors, to avoid external air entering into the heating chamber. Parts stay there for the required time and then advance, again through a baffle door, first in a water jacketed cooling chamber and finally to the outside through the exit door. A special type of continuous conveyor furnace is one with a mounting ramp, a horizontal heating chamber in the hump, and a ramp down: it is built for retaining hydrogen in its highest part. Dry Hydrogen is continuously supplied and exits at the partially closed doors, where it is burnt in air in a harmless way. Another type of furnaces employs a retort within which the required atmosphere is introduced and maintained. It is separate from the heating chamber, where it is introduced when needed and then removed at the end of the brazing cycle. All furnaces employing gases that form explosive mixtures with air, need implementation of precise safety procedures using non flammable gases for purging at start up and shut down, even in the event of power failure. A special atmosphere is that existing in a vacuum furnace. Clean parts are introduced and no flux is required. It is the preferred furnace for stainless steel or heat resisting alloys and in general for aerospace work. Electrical Heating elements are placed inside the furnace. Heat transfer is performed by radiation. This type of equipment is more complicate and requires constant maintenance. Cycles are longer and more expensive because of the need to evacuate the chamber before heating. In all other furnaces heating methods may be different, depending on the atmosphere required and on cost considerations. The protective atmospheres required depend on the base metal to be brazed, and on the need to protect it from oxidation and scaling, and to assist the correct flow of the filler metal and wetting of the surfaces. For steel base metal, one can use rich exothermic-base atmospheres, that usually promote wetting and maintain relatively bright surfaces on assemblies. Preferred filler metal is copper base. For a more reducing atmosphere that will not cause surface decarburization one can use a rich prepared Nitrogen-base atmosphere. It protects low carbon steel from oxidation. Endothermic-base atmospheres can have their carbon potential precisely tuned to the carbon content of the steel, to avoid carburization or decarburization, and give good protection from oxidation while helping brazing production. Dry inert gas atmosphere can be used in suitable retorts when special requirements are present, i.e. for titanium. Titanium brazing could be done also in a high vacuum. Material for fixtures and filler metal selection is critical, because of the reaction of titanium with many of the constituents to form undesirable intermetallic compounds. Aluminum alloys are preferably dip brazed in a molten bath, that provides buoyancy, uniform temperature and protection from oxidation. Furnace brazing can be used but requires very precise temperature control because the brazing temperature can be dangerously near to the solidus temperature of certain alloys. When dry inert gas is used instead of air, the amount of flux needed is drastically reduced. Special filler metals were specifically developed for fluxless vacuum brazing. Stainless steels can be brazed in vacuum, in dry hydrogen and in dissociated ammonia, usually with silver based filler metals. For copper alloys furnace brazing is used successfully as a mass production process, limiting the cost per joint. Both exothermic-base and endothermic-base atmospheres are used. In conclusion Furnace Brazing has the potential of providing an economic solution for joining in mass production projects, and its application should be investigated.-------------------------------------------- ------------------------------- 8 - Site update The Page of the Month published recently in the Site is on Welding Cost Estimate. We believe that the subject is an important one from a most practical economic aspect. Although at first it may seem a strange and unpleasant topic, it may be possible to introduce a routine for the regular recording of data that will in time ease interpretation of performance and comparison of different processes. Click here. We introduced in the Homepage a few notes explaining that, for industrial enterprises asking for advice, a certain fee shall be requested, proportional to the amount of work involved. We believe that practical solutions are worth in savings and gains many time their modest cost. It is the most usual thing that happens everywhere, that valuable solutions command a price. We offer however solid guarantees for the sceptics and for those who do not yet know us. Students and individuals welding around for hobby or personal achievement will continue to enjoy free consulting.---------------- ----------------------------------------------------------- 9 - Short Items 9.1 - Bimetals. Thermostatic bimetals are formed by an alloy with a high coefficient of thermal expansion clad to an alloy with a low coefficient of thermal expansion. When heated, the resulting bimetal will bend about its neutral axis because of the difference in CTE. Bimetals are used in various temperature-sensing devices and thermostats.9.2 - Boron steels. Boron is a strong carbide-and nitride-forming element and increases strength in quenched and tempered low-carbon steels through the formation of martensite and the precipitation strengthening of ferrite. Boron-containing killed carbon steels are available as low-cost replacements for the high-carbon and low-alloy steels used for sheet and strip. Boron increases hardenability when added to a fully deoxidized steel. Even an extraordinarily small amount of boron of the order of one thousandth of a percent has a powerful effect on hardenability. The effect is higher in low carbon steels. Nitrogen and deoxidizers may reduce boron hardenability effect as well as high temperature treatments. The low-carbon boron steels have better cold-forming characteristics and exceptional weldability. They can be heat treated to equivalent hardness and greater toughness for a wide variety of applications requiring strength and toughness at low temperatures.9.3 - Ion Nitriding is an extension of conventional nitriding processes. In low vacuum, high-voltage electrical energy is used to form a plasma through which nitrogen ions bombard and heat the workpiece, clean the surface, and provide active nitrogen that diffuse into the metal. Excellent dimensional control and retention of surface finish are achieved. Ion nitriding is done at temperatures lower than those conventionally employed. Control of white-layer composition and thickness enhances fatigue properties. Ion nitriding permits process automation and ensures repetitive metallurgical results. Treatment time is typically 2 to 4 h at 570 0C (1060 0F), the compound zone is pore-free with low surface roughness. The nitride case in the diffusion zone displays increased hardness and strength. Compressive stresses are also developed, as in other nitriding processes. Hardness profiles resulting from ion nitriding are similar to ammonia-gas nitriding, but near-surface hardness may be greater with ion nitriding because of lower processing temperature. 9.4 - Laser peening shock processing is used to develop surface compressive stresses that help in eliminating fatigue failures. With each pulse of the laser, an intense pressure shock wave is created that drives residual compressive stresses deep into the metal under the focused pulse. It is used in aerospace engine parts. 9.5 - Oxide Dispersion Strengthened Alloys. Mechanical alloying is a dry, high-energy ball-milling process for producing composite metallic powders with a controlled, fine microstructure obtained through impact and cold welding. Its widest use has been in the production of dispersion-strengthened nickel- and iron -base superalloys for service at temperatures of 1000 0C (1830 0F) and above. Mechanical alloying produces a material whose internal homogeneity is independent of starting powder particle size. The raw materials used for mechanically alloyed dispersion-strengthened superalloys are widely available commercially pure powders that have various particle sizes. These master alloys are relatively brittle when cast and easily crushed to powder. The powders must be consolidated and worked under conditions that develop coarse grains during a secondary recrystallization heat treatment. MA ODS alloys are now commercially available as bar, plate, sheet, tube, wire, shapes, and forgings. However, not all alloys are available in all product forms. 9.6 - Shape Memory Alloys (SMA) are a group of metallic materials that demonstrate the ability to return to some previously defined shape or size when subjected to the appropriate thermal procedure. Generally, these materials can be plastically deformed at some relatively low temperature, and upon exposure to some higher temperature will return to their shape prior to the deformation. Only those alloys that can recover substantial amounts of strain or that generate significant force upon changing shape are of commercial interest. NiTi alloys and copper-base alloys have been developed. The transformation, originated by heating, from one phase called martensitic to another called austenitic structure and its opposite, while cooling, is characterized by a measurable strain change, capable of performing a mechanical work. ------------------------------------------------------------------ --------- 10 - Explorations: Beyond the Welder Engineers without Borders - Internationalhttp://www.ewb-international.org/ All about Tin http://www.tintechnology.com/ A lot about Zinc http://www.interzinc.org/ International Magnesium Association http://www.intlmag.org/ To learn on all the Elements in the Periodic Table http://www.webelements.com/ ------------------------------------------------------------------- -------- 11 - Contribution: Post Weld Heat Treatment By reviewing the current literature available on the subject of Post Weld Heat Treatment (PWHT), one can see that recommendations are usually dependent upon specific alloys and filler metals involved, but also on thickness and restraint of welded joints. Furthermore it is reported that even accepted Codes are not unanimous in prescribing certain treatments, so that discrepancies can be found among them. There is a clear distinction between two thermal processes used after welding.
The mechanical properties of either the base metal or the weld metal can be adversely affected by post weld heat treatment and even intergranular cracking can occur in the coarse-grained region of the weld Heat Affected Zone (HAZ). The mill tempering temperature used by the steel producer should never be exceeded in the post weld heat treatment of quenched and tempered (Q&T) steels. The steel producer should be consulted on the recommended post weld heat treatment of a HSLA (High Strength, Low Alloy) Q&T structural steel. Post-weld heat treatment (PWHT) is often required to relieve residual stresses in austenitic stainless steel weldments, particularly in thick sections. Because the Coefficient of Thermal Expansion (CTE) and the elevated-temperature yield and creep strengths of austenitic materials are significantly greater than for ferritic materials, the magnitude of residual stresses is generally larger. Thus, if not reduced, they can cause severe distortion upon machining, and Stress Corrosion Cracking in certain environments. During welding of martensitic stainless steels, all the metal heated by welding to a temperature higher than the austenitization temperature is transformed, upon cooling to room temperature, to brittle, untempered martensite which is not fit for service in the as-welded condition. The functions of a PWHT are to temper the martensite in the weld metal and HAZ, in order to reduce the hardness and increase the toughness, and to decrease residual stresses associated with welding. The as-welded properties of the weld fusion zone and its response to PWHT can often be controlled by the appropriate choice of weld filler metal. However, the properties of the HAZ can be controlled only by welding process and process parameters, and by the use of a PWHT. It is therefore imperative to understand and control the properties of the weld HAZ for the correct use of welded martensitic stainless steels. Post weld heat treatments used for martensitic stainless steels involve tempering the weldment by heating at temperatures ranging from 650 to 750 0C (1200 to 1400 0F). The weldment should be held at temperature for a minimum of 1 hour per 25 mm (one inch) of weld thickness. In addition, a low-temperature PWHT at 300 to 400 0C (550 to 750 0F) is sometimes performed to ease the diffusion of hydrogen from the joint and decrease the chance of hydrogen-induced cold cracking. The 450 to 500 0C (840 to 930 0F) range should be avoided because it can compromise weld toughness.--------------------------------------------------------- ------------------ 12 - Testimonials From: Mark Clark "res219ng@verizon.net"To: feedback@welding-advisers.com Date: 25 Apr 2004, 08:06:58 PM Subject: Thank you! Well, your first letter answered many questions [...] Thanks again! Mark Clark ----------------------- From: "Yukiko" "huffy@surfline.ne.jp" To: support@welding-advisers.com Date: 26 Apr 2004, 04:34:32 PM Subject: Re: Side casing Elia, Thank you for the information. Mike Huffman From: "Yukiko" "huffy@surfline.ne.jp" To: support@welding-advisers.com Date: 27 Apr 2004, 09:11:13 AM Subject: Re: Side casing Elia, I did the welding today. The casing had a lot of cracks around the broken floor board bracket. I used 4043 rod and it worked well. I did not weld more than one inch at one time. I let it air cool for an hour each time. When I was done I did a die penetrant test and it looked good, the fourth time. Too many spider cracks so it took some time. Thank you for the help. Mike --------------------- ------------------------------------------------------
13 - Correspondence: a few Comments We regularly receive the most different questions. Sometimes we have only to point the inquirer to the proper page in the Site containing a much more detailed answer than that possible in a short letter. But there are also questions from middle Managers from established Companies that would like to get operational answers for free, to solve or to improve their process or their production, or to influence favorably their bottom line. To them we would say: do not dismiss the advice just because it may not be free. It is still a bargain. While we are confident that our contribution is likely to be a valuable piece of advice, capable of providing real savings and gains, we think we are entitled like anybody else to request a compensation for our work. Especially as the gain provided is usually worth many times the cost. And we are ready to provide rock hard guaranties. We welcome all questions, and we are ready to discuss procedures and guarantees for inquirers not qualifying for free answers. All other individual and hobby welders will continue to enjoy free welding advice. Write us by e-mail. Click here.-------------------------------- ------------------------------------------- 14 - Bulletin Board 14.1 - We would like to know if our readers with more practical needs would welcome the introduction of a few new online Courses on Welding, designed and prepared to be of real help to anybody willing to learn. Let us know:
------------------------------------------------------------------- -------- Copyright (c), 2004, by Elia E. Levi and welding-advisers. com, all rights reserved. |
|
| Back to Back Issues Page |