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PWL#044 - Vacuum Plasma Spray, Welding Aluminum Structures, Hydrogen Induced Cracking,Cost Reduction April 02, 2007 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#044 - Vacuum Plasma Spray, Welding Effects on Aluminum Structures, Hydrogen Induced Cracking, Welding Cost Reduction, Mechanical Fastening, Contribution, Comments 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.
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1 - Introduction 2 - Article: Vacuum Plasma Spray 3 - How to do it well: Welding Effects on Aluminum Structures 4 - Filler Metal for controlling Hydrogen Induced Cracking 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - Welding Cost Reduction 8 - Site Updating: Mechanical Fastening 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: more on Welders' Shortage 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This 44th issue of Practical Welding Letter opens with a short introduction to Vacuum Plasma Spray, a specialized coating process that enjoys widespread use in demanding aerospace applications. The article that follows, originated from an actual question sent by a correspondent, deals with Welding Effects on Aluminum Structures: correct design has to take into account that mechanical properties are affected by the heat of welding, as explained. Hydrogen Induced cracking is a major risk affecting the weldability of low alloy steels and in particular of those types exhibiting the highest strength levels. Consumables have to be selected cautiously to avoid increasing the dangers brought about by improper welding. Welding cost reduction is a major task affecting the whole industry, not only those segments afflicted by loss. One of the most important tools to be employed is a thorough analysis of the processes used, to be conducted with the aim of revealing the actual costs incurred, as a necessary preparation to the planning of adequate measures of cost control. The Page of this Month deals with Mechanical Fastening, alternative processes to welding: it is amazing to discover that in recent times much progress was achieved, especially for mass production with automated implementations, and that the subject is almost not covered by generally accepted standards. In the Contributions department we refer on additional initiatives (see PWL last issue) aimed at addressing the welders' shortage, as presented in three articles of the current (April 2007) issue of the Welding Journal. Other departments are presented at their usual place. Is this PWL useful to you, our reader? Please comment by e-mail. Don't miss..., at the bottom of this page, the 3 minutes
2 - Article: Vacuum Plasma Spray Inert gas is heated and ionized to a high temperature plasma by making it pass through a DC electric arc in a specially built torch or gun. The expanded gas exits the gun as a high speed jet dragging powder particles used for Plasma Spraying. While the particles in the plasma jet are heated to very high temperatures, sprayed items are not heated to more than about In Air Plasma Spray the powder particles in their flight from the gun to the substrate suffer some oxidation. To limit oxidation, and to improve the coatings quality, although at higher cost, plasma spraying can be performed in a controlled environment. That is done inside a sealed chamber, under vacuum (Vacuum Plasma Spray) or at low pressure (Low Pressure Plasma Spray) of an inert gas, to avoid the possibility of forming a glow discharge. This arrangement allows producing coatings of reactive materials without oxidation. The high impact velocities of the sprayed particles and the low oxidation proper to Vacuum Plasma Spraying produce coatings near their theoretical full density. As with all thermal spray processes, VPS is limited to line-of-sight. According to some estimations VPS is the highest value thermal spray coating process for the production of quality metallic (MCrAlY, where M denotes one or more metals like Fe, Co or Ni) coatings, despite its complicated instrumentation. Gas turbines, both for aerospace and for stationary applications, use vacuum plasma sprayed coatings to improve the surface properties of blades made of Cast Nickel Base Superalloys to enhance their resistance to high temperature oxidation and sulfidation. Antioxidation materials, mainly MCrAlY alloys and thermal barrier coatings, usually Yttria partially stabilized Zirconia, are the most used materials. Known applications of vacuum plasma spray process (VPS) were developed by NASA and Rocketdyne for liquid fuel rocket engines, including the Space Shuttle Main Engine. Other applications include thermal barrier coatings resistant to thermal shock for turbopump blades, nozzles and combustion chamber liners. Bond coatings were developed for cryogenic titanium components and wear resistant coatings and materials were used for various implementations. It was demonstrated that the VPS process is capable of producing dense, free standing forms with impressive mechanical properties. An overview of similar processes can be found in the website page on Thermal Spray
3 - How to do it well: Welding Effects on Aluminum Structures Note: The following is an actual question sent to us by a correspondent. Q: This application is structural. It is a large (42 feet in length) box cover structure. This structure is to have 2x6x1/8 inch wall rectangular aluminum chords running the full length of the box. This 2x6 tube only seems to come in 6063-T52. This aluminum box structure also has 6061-T6 sheet for the sides and top and also 2x2x1/8 inch tubes and angles that are 6061-T6. Problems: I cannot seem to find any allowable strength values for the 6063-T52 when it is welded - can you point me in the right direction? Also and more important is the 6063-T52 readily weldable to the 6061-T6 and is it an excellent weld as this application is structural in nature? Thank you for any help you can give me. A: In principle welding heat will destroy the mechanical properties of heat treated aluminum alloys and revert them to those of annealed condition. The Minimum Tensile Strength of Welded Aluminum Alloys with no post-weld heat treatment, is listed in Table 5.16 at page 232 of AWS Welding Handbook 9th Edition. For the above materials the following data are reported: 6063-T52: 17 ksi = 115 MPa It is true that some of the properties of heat treatable alloys like 6061 and 6063 could be recovered by accurate reheat treatment if correct filler metals were employed, but this is not a feasible option for a large structure. The welded solution is therefore applicable if the lower strength is considered adequate in design. The allowable load on the joint is established by the orientation of the heat affected zone relative to the stress direction and by its percentage relative to the whole section. The only way out, permitting to exploit the improved mechanical properties of the above products in their as received heat treated condition, would be to implement proper mechanical joints without welding. If it is decided to design suitable joining elements to be assembled later in the larger structure by mechanical fastening without welding, the joints themselves could be fabricated in heat treatable aluminum alloys by welding, and should be heat treated before mounting in the structure. The last warning is that one should beware from having aluminum making contact with other materials because of the danger of galvanic corrosion, except if proper insulation materials are employed.
4 - Filler Metal for controlling Hydrogen Induced Cracking Although the quality of consumables and their contribution to the amount of hydrogen likely to be absorbed in the weld is only one of the possible causes of damage that can be inflicted in susceptible materials, it has to be taken always into account. Hydrogen solubility in iron is quite elevated in its molten state, but it is reduced drastically in the solid state and decreases even more at lower temperatures. It is well known that hard and strong low alloy steels are most in danger of hydrogen induced cracking, also called cold-, delayed- or underbead-cracking. This is one of the most serious problems affecting weldability. This type of cracking occurs generally because of the presence of one or more of the following factors:
To avoid or reduce the damage likely to be caused in carelessly performed welds one should adopt a strategy aimed at taking care of various precautions simultaneously. These consist in:
The first precaution requires maximum cleaning of the joint before welding and use of consumable suitable for low hydrogen welding. These include electrodes and fluxes properly selected and maintained in good housekeeping storage conditions or dried in an oven per manufacturers' recommendation just before use. The maximum moisture permitted in low hydrogen electrode coatings is specified as 0.2%. Fluxes are known to retain moisture. Therefore they should be kept in sealed containers. In absence of fluxes the amount and type of residual lubricant necessary for correct feeding of solid wires should be closely controlled by using only specially made filler materials from dependable suppliers. The process that applies the minimum amount of hydrogen contamination from the consumables is the GMAW using inert shielding gas and solid wire. The problem posed by residual lubricant is of consequence for the highest strength steels unless high preheat temperature can be used. The second precaution suggests checking the feasibility of employing more ductile stainless or nickel base alloys filler materials when acceptable, so that the stress level in the joint be less than what it would be with regular low alloy steel filler metal. The third precaution consists in providing preheating or immediate transfer of the weldments to a furnace at such elevated temperature that absorbed hydrogen could find the conditions to diffuse outside of the metal. In practical cases there will be a need to find the suitable compromise among the different requirements to implement the most cost effective combination of approaches.
5 - Online Press: recent Welding related Articles From AWS From TWI Metallurgy makes or breaks tube fittings
6 - Terms and Definitions Reminder Bare Electrode is a metallic consumable filler material of suitable shape, wire or strip, which has no shielding flux cover. It may have residual lubricant or be plated by a metallic (copper) protective layer. Brazing Temperature has to be reached by the base metal of the parts to be brazed, for assuring capillary flow and wetting of the brazing filler material. It is slightly higher than its liquidus temperature. Electrode Pickup is the contamination of the external surface of resistance welding electrodes by the base metal or its coating during welding. Focal Spot of high energy electron or laser beams is the location of maximum energy concentration and least cross section area. Gun (or torch) is the manual or mechanical welding or cutting device that holds the electrode and directs the arc and the shielding gases to the welding or cutting location. It provides suitable electrical contact and proper (air or water) cooling. Horn in ultrasonic welding is the arm that transmits the mechanical energy to the materials to be welded resting on the anvil. In resistance welding it is an arm extension. Reverse Polarity is the connection of the electrode to the positive pole of an energy source of a welding power supply. It is also called DCEP (Direct Current Electrode Positive). Straight Polarity is the connection of the electrode to the negative pole of an energy source of a welding power supply. It is also called DCEN (Direct Current Electrode Negative).
7 - Article - Welding Cost Reduction While in large organization the pressure for Cost Reduction in activities concerning Welding Operations is likely to originate from Accountants, it is important to anyone involved to make up one's mind about efficiency improvement and cost savings. Click on Welding Economics to read an overview. When the pressure comes from outside it is likely to generate resistance because proud technical people don't like to have to justify their professional choices to people who may administer wealth but are perceived as not generating income. However, independent from any pressure, it is highly recommended to take the time and make the effort to know the real costs, at least of the most important welding processes used in the welding shop. Click on Welding Cost Estimate to read a page on this subject. As always common sense may be more important than formal accounting practice knowledge. The only precaution is to be consistent, referring every itemized entry to a constant reference unit (per project, per hour or per unit length). Every major process and welding line should be analyzed by breaking them down in checklists to record what is known and what should be ascertained, and especially which specific information should be gathered in an organized way. In particular, to be able to measure improvements, it is necessary to establish a realistic baseline of current costs, reporting the orderly accumulation of expenses, broken down in the single items involved. It may be important to establish from the start the objectives relative to the savings that management strives to obtain, possibly as a defense in a difficult economic situation or as a necessary step in view of reaching additional markets. It is also necessary to establish the time frame for achieving effective results, and the resources that can be made available for introducing new equipment or changes. This should include an analysis of the capabilities of the available workforce, and of the needed improvements to reach the goals. Due to the interplay of various inputs from different disciplines like design, engineering, manufacturing, maintenance, quality, purchasing, storing, accounting etc. it would be advisable to put together interdisciplinary teams capable of working together in view of reaching shared conclusions. If possible it would be highly rewarding to explore new ideas and run limited documented tests to help finding favorable still untried changes to perform the present jobs in more economic ways. An article discussing this subject referred to Resistance Welding was published at page 47 in the February 2007 issue of the Welding Journal. Interested readers are urged to seek it. Another article with reference to this subject can be found in the current (April 2007) issue of the Welding Journal, discussing Lean Welding Operations.
8 - Site Updating: Mechanical Fastening The Page of this Month deals with various Mechanical Fastening processes that can be used in place of welding or brazing if these heat based processes are harmful to material properties. Although many Mechanical Fastening processes were known and used already a long time ago, in recent times much research was devoted to different types of modern joining devices to develop systems suitable for automated applications. The driver, as usual, was the requirement of saving on assembling costs. As it sometimes happens, we find that this subject has pertinent reference to the article in section 3 in this issue of PWL, dealing with Welding Effects on Aluminum Structures and suggesting in the specific case discussed here to use mechanical fastening instead of welding to preserve mechanical properties. Another reference can be established with the article in section 7 of the previous issue of PWL, PWL#043, dealing with Joining incompatible Material Combinations. Click on Mechanical Fastening to see the new page. Website pages are regularly updated with new information when available. See the Site Map when looking for answers to your problems and keep an eye on the Welding Blog. Visit the Weld Resources if you have not yet seen them. They present an updated wealth of information readily available online. Let us have your comments and feedback by clicking on Contact Us.
9 - Short Items Duplex Stainless Steels are considered resistant to stress corrosion cracking. They consist of a mixture of fine-grained austenite and ferrite phases and display properties characteristic of both austenitic and ferritic stainless steels. Duplex stainless steels are in most cases tougher than ferritic stainless steels and their strength can be double that for austenitic stainless steels. Equicohesive Temperature is a narrow range where grains and grain boundaries have equal strength. This range depends also on strain rate and on purity of the metal. Above it the grain boundaries become weaker than the grains themselves, and therefore fracture occurs in intergranular way. Notch Sensitivity represents the extent to which the sensitivity of a material to fracture is increased by the presence of stress concentration, such as a notch, a sudden change in cross section, a crack, or a scratch. Low notch sensitivity occurs usually with ductile materials, and high notch sensitivity is found preferably in brittle materials. Relaxation as applied to mechanical systems means the release of a tension in a bolting or riveting element because of creep at elevated temperature. Due to creep relaxation, bolts at temperatures higher than about 50% of their melting point (in degrees K), must be regularly retightened to retain their design tension. Resilience is the ability of a material to absorb energy when deformed elastically and to return it upon release of the load. Thus resilient mechanical springs, that resist energy loads without undergoing permanent deformation, are made of materials having high yield strength and low elastic modulus. Rimmed steel is a low-carbon steel of such composition that evolves continuously carbon monoxide during ingot solidification. It results in a case or rim of metal almost free of voids. Sheets and strips made from rimmed steel ingots have very good surface quality.
10 - Explorations: beyond the Welder The Changing Earth An Archive of Human Languages Protecting Nature. Preserving Life Diversity - Women in Science: Building Better Balance Sealcoating at Asphalt Kingdom
11 - Contribution: more on Welders' Shortage Our readers may recall that in our last issue (PWL 43) we reported on an article by AWS President Gerald D. Uttrachi in the January 2007 Welding Journal, outlining some of the initiatives taken to confront the pressing problem of Welders' Shortage, that may become so critical in the near future to the point of crippling industrial initiatives and expansion. From the point of view of young people who just started to prepare themselves to this profession and for all those who think of welding as their possible career, this problem should represent a welcome opportunity. In fact being or becoming part of a limited community of requested and courted workers should provide, at least to the most endowed among them, nice chances of a successful and rewarding career. Bonuses, tuition reimbursement offers and salaries on par with those of newly graduated engineers are only a sample of the benefits now offered by industry to encourage and recruit new welders. It also appears that newly Certified Welders will enjoy guaranteed job placement. The fact that this issue of welders' shortage is real and acute is reinforced in the April issue of the Welding Journal where at least three articles address the same subject. The first article by a Professor at the University of Ohio illustrates a program designed to attract and interest of high school students to Engineering professions, by exposing them to an enriched curriculum of five core engineering concepts and subjects including welding, besides the traditional disciplines. The program enacted in 40 states is flexible and includes arguments that can be followed in depth by taking up and specializing in elective technologies among the most advanced new disciplines. The second article whose information was collected from various sources, illustrates the task assumed by Metalworking Trade Unions for training new welders, by supplying professional courses in newly established well equipped facilities. The locals of these unions offer apprenticeships in their training centers with programs that require three or four years of study combined with comprehensive training on the job. Many of the courses give credits usable for further education in academic institutions throughout the country. National supervising entities assist in promoting Welding Certification Programs. Similar to the Ironworkers Apprenticeship Programs are the Sheet Metal Workers Apprenticeship and the Plumbing and Pipe Fitting Industry Apprenticeships that offer welding training and certification. Trainees acquire most requested capabilities that assure their independence as certified workers and the industrial companies gain capable professionals to promote their performance and growth. Interested young readers who may wish to explore in depth the offers available in their area are urged to look for the original article at page 39 of the April Welding Journal, available in many technical libraries. Facing the shortage, not only Workers Unions but also Companies have now entered the field of training welders for their own needs and for those of their customers. To set up schools and welding training centers, companies are helped by manufacturers of welding equipment and by suppliers of consumables. The story of one such initiative in the energy industry is told in the third article. This example could be followed by other enterprises willing to grow and educate the future workforce to become self sufficient in the preparation of their welders by investing energy and resources. Worried readers may find practical hints on how to tackle the welders' shortage issue by looking through the said April 2007 issue of The Welding Journal.
12 - Testimonials From: Cookson, Jon Hi Mr. Levi,
Date: 15 Mar 2007, 12:44:06 AM
13 - Correspondence: a few Comments Readers may recall that we ran a note on Carbon Supersaturation, a case hardening treatment used to provide surface hardness to stainless steels, in our Issue 38 of PWL for October 2006. In the November 2006 Issue No. 39 a correspondent, Mr. John Nabors, expressed interest for the new process, and asked for new information, if and when available. Recently we received a message from another correspondent, Dr. Tim Volin, asking us to inform Mr. John Nabors "that the Kolsterizing process available from Bodycote Heat Treating achieves the same result and has been in use for 20 years." He also sent us the reference to an article of his, the last one reported in Section 5 above. Later on Mr. Nabors sent us a note stating that "We are currently evaluating the Kolsterising process. I'll report back to you when we have completed our evaluation." We are glad that our PWL helped in exchanging information. From the Bodycote website we report the following information: "Kolsterising ® is a casehardening process primarily for austenitic stainless steels. Wear resistance and resistance to galling is improved, while corrosion resistance remains unchanged. The process involves the diffusion of carbon into the work piece surface without the formation of chromium carbides. Post treatment hardness is between 70 and 74 HRc. Typical case depths offered are 22 or 33 microns." Bodycote International plc, the international metallurgical services group, announces the acquisition of Hardiff BV, based in Apeldoorn, the Netherlands. "Hardiff's founder, Professor B.H. Kolster, has developed a commercial process for the surface hardening of austenitic (corrosion resistant) stainless and similar alloys without impacting on the color, corrosion resistance or surface integrity of the material. The process has a wide range of applications in the automotive, foodstuffs, medical and aerospace industries and is known as "Kolsterising". "The new technology is ideally suited for the treatment of water resisting components made from austenitic stainless steel, duplex stainless steel and some nickel based alloys, such as Hastelloy and Inconel. Surface hardness of the order of 1000-1200 HV are achievable and the wear resistant "Kolsterised" layer retains good ductility." From the available information alone, the differences between this process and the one previously reported, developed at Case Western Reserve University with others, cannot be understood. If our readers know more on the comparison of both processes and care to tell us, we will publish the information in these pages. See: A commercial Brochure can be found at http://internet.bodycote.org/kolsterising/brochures/147-302_BODY_Kolst_RD_GB_FINR.pdf
A research article in French can be downloaded from
14 - Bulletin Board
14.1 - Joining Dissimilar Metals Conference 14.2 - 8th Int'l Conference on Brazing and Diffusion Bonding 14.3 - From SiteSell
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