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PWL, Issue #017-Robotic Welding, Quality of Spot Welds, Filler for Duplex SS, W. Quality Assurance January 02, 2005 |
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| 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 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
Date: January 2005 - Practical Welding Letter - Issue No. 17
--------------------------------TABLE of CONTENTS--------------------------------
1 - Introduction 2 - Article: Robotic Welding 3 - How to do it well: Improving Quality of Spot Welds 4 - Filler Metals for Duplex Stainless Steels 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Quality Assurance in Welding 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Welding Design Thoughts 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
After sending again to all our readers our best wishes for a happy and prosperous New Year we introduce hereafter the main subjects treated in this seventeenth issue of Practical Welding Letter. Robotic Welding is widely claimed as one of the most rapidly growing applied techniques due to its potential for exceptional productivity. It is also perceived as a means to make up for the increasing scarcity of skilled manual welders. It should be noted however that a higher education of different and more demanding talents will always be required, that should be cared for with proper training. A reader asked for details on selection of filler metals for Duplex Stainless Steels. We reported on the types used and on some of the criteria. The problems are different from those of austenitic Stainless Steels, so that some orientation is probably beneficial to those dealing with these materials. Then, answering to another real question, we briefly outline how the production quality of a resistance spot welding machine can be improved, if necessary to reduce the occurrence of failures. The advantages of upgrading have to be evaluated against costs. We invite other readers to ask their own well conceived questions because we believe that this publication should address practical welding problems. Write us by e-mail. Click here. The last featured article deals with Quality Assurance, a somewhat involved problem whose requirements may be missed by technical people essentially bothered by their daily problems of producing actual weldments. It should be noted that you may need to sell also your Quality Assurance System, to show that you understand what is more and more essential, and that you comply with requirements. For lack of external Contributions we propose once again our own. This time it is about an amazing way of thinking of Welding Design... The new Page of the Month for our Site is dedicated to Welding Books. We acknowledge that this Site, although crammed with know how, cannot replace a few really good manuals and handbooks that every serious person interested in welding should study and peruse. We would like to encourage everybody to believe that learning is the only way to progress. Other departments appear as usual. Let us know what you think and what you would prefer, as we would like to have an ongoing dialog with our readers. 2 - Article: Robotic Welding Advanced industrial welding automation applications use multi-axis servo-controlled manipulators called Welding Robots. Their versatility derives from the reprogrammable software that allows them to perform complex tasks, easily modified as needed to suit the requirements of new or modified parts. Robotics application represents a progress relative to dedicated fixed automated welding lines which, despite their high economic success, were built for an essentially unique configuration. They had to be extensively rebuilt or scrapped altogether for any substantial part change. Flexible automated welding robotic systems, although more expensive than fixed automated installations (which did not change much over the years), have in time steadily decreased in cost while their performance has substantially improved. First used for resistance welding, robots were progressively used also for fusion welding, generally with gas metal arc or flux cored arc processes, and recently also for laser welding. Jobs should not be simply transferred from manual to robotic processing. They should be redesigned to take into account and to profit from the particular requirements of the different production methods. For best results the production tolerances of the components and their fit up must be redesigned and kept consistent in time, because the repetitive program has minimum capacity of self-adjustment (unless it is provided by sophisticated and costly means). A major factor in the success of the process is the use of carefully designed and implemented fixtures, to position elements and components positively and consistently. Robots are described as rectilinear if their movements follow straight lines. They have usually three degrees of freedom and one more for the rotation of the wrist holding the welding clamp or the torch. Articulating robots are capable of movements duplicating those of a human arm. They are usually provided with six degrees of freedom permitting quite complex movements. Different manipulators vary in the maximum weight they can support as well as in the physical space envelope that they can cover. The welding space envelope varies with the type and construction of the robot The load capacity at the free end of their arm varies between 3 and 16 kg (6.6 and 35.3 lbs.), while additional essential welding accessories can be located in other places on the robot not to stress the arm more than needed. The repeatability of modern robots, that is their ability to return exactly at a programmed position, can be better than +0.1 mm (+0.004"), but has to be checked for the particular type of equipment considered. The accuracy of movement is described as the ability to move precisely along a path according to predetermined points. This characteristic may vary depending on the exact location within the working envelope. When the position of the welding source with respect to the seam is essential to guarantee production quality, special means of seam tracking are available to provide additional self correcting location capability. Reliability of robotic welding units is in principle very high, reportedly for a Mean Time Between Failures (MTBF) of 20,000 hours, and their useful lifetime is designed to last about 8 to 10 years. To exploit welding arc on time to the maximum, much higher than for manual welding, power supplies must be rated to 100% and selected to deliver much higher amperage than the maximum required. To achieve higher weld deposition rate, larger size filler metal can be used at higher current than with manual welding. Larger diameter wires typically cost less per unit weight, with still improved economy. Robots can easily sustain increased heat that would be unconfortable for manual welders working long hours on the same job. Due to their higher welding speed and minimum time between welds, robots have the potential of boosting productivity to very high levels, to provide remarkable return on investment generally planned to be achieved in less than three years. Robotic welding can save on consumables, by using consistently only the minimum amount of filler metal necessary for producing acceptable joints. The welding program schedule for a robot should be developed from the starting point of a good manual application technique, by improving gradually on the parameters that enhance the benefits of utilizing robotic capabilities without overlooking requirements and characteristics of the welding process. See further down Section 9.6. Torches designed for heavy duty robotic usage should be selected. Systems are available for quick neck change. Nozzle cleaning and contact tip replacement are then performed offline without stopping operation. Water based cooling systems are preferred. The robotic installation, either standalone or integrated in a welding cell, can work under hostile or dangerous conditions (to the human workforce), not being influenced by fumes, radiation, noise, heat or other hazards. During night shifts they can work in complete darkness if automatic loading and unloading are provided. Appropriate back up provisions for robots have to be implemented to confront either regular scheduled maintenance routines or unprogrammed breakdowns. The financial aspect of new implementations has to be studied carefully by comparing evaluations of different solutions. Adequate training and development time and facilities must be accounted for in any projected program. Medium to small companies increasingly accept introduction of robotic welding, now more affordable and cost-effective than ever, in a move intended to compensate for the decline in availability of experienced welders. The expertise necessary to successfully realize such a conversion however is not less important. It is probably different and more specialized. 3 - How to do it well: Improving Quality of Spot Welds Q: The customer reported that 5 parts out of thousands broke in the spot weld. Could the machine accidentally not send enough heat? A: Yes, it can happen that the machine, controlled only by preselected parameters, occasionally will not send a secondary current of such magnitude to develop in the joint enough heat, to produce a nugget of adequate dimensions and quality as required. In resistance spot welding, fluctuations in incoming power line voltage, electrical resistance variation due to changes in surface cleanliness of the materials, progressive deterioration of the electrode surface due to change of shape or of particles pick up, variation in electrode force from pneumatic cylinders and other factors may occasionally cause the heat generated to be less than expected. In order to obtain consistently correct results, one should be able to compensate for disturbances in real time. To provide for this capability closed loop adaptive controllers can be employed that evaluate the data supplied by suitable sensors so that the output is modified accordingly. The variables sensed are current, voltage, electrode force, electrode displacement, electrode speed and cooling water temperature. Time is controlled electronically so precisely that it is considered a fixed quantity that need not be monitored. If there is no economic justification to the use of adaptive controller one should be prepared to bear the consequences of a certain number of failures. One could also try to eliminate or repair defective welded parts by submitting them to non destructive inspection (typically by ultrasonics). Alternatively, if the part and the joint permit, one could consider to submit each welded part to a load test designed to break only the defective joints. 4 - Filler metals for Duplex Stainless Steel A short mention of Duplex Stainless Steels was presented in Section 9.1 of the issue No. 07 of PWL for March 2004 An accepted Specification, including compositions and requirements for these steels in plate and sheet shape is: ASTM A 240/A 240M The types are usually addressed in the following groups:
The reasons for selecting duplex stainless steel to substitute for regular austenitic stainless steels are their substantially higher mechanical properties and their outstanding chloride pitting corrosion and stress corrosion resistance. Welding problems of duplex stainless steels arise not from hot cracking susceptibility (as typical of austenitic stainless steels) but rather from the heat-affected zone (HAZ), like a loss of corrosion resistance and toughness, or of post-weld cracking. To avoid these problems, the welding procedure should be developed to control heat during the whole process, not just minimizing the heat input for any one pass. Willing to limit total time at temperature, it is generally better to complete a weld in fewer passes with relatively high heat input than in many passes of lower heat input. Cleaning before welding is most important to avoid contamination of the joint. All the regular practices used for stainless steel should be applied. For duplex stainless steels, a weld joint design must facilitate full penetration and avoid autogenous regions in the weld solidification as this would tend to form excessive ferrite in the weld. For consistent results ferrite amounts should be regularly monitored. Duplex stainless steels generate less important internal stresses than austenitic stainless steels, because of their higher thermal conductivity and lower coefficient of thermal expansion. Duplex Stainless Steels can be welded to other steels of the same group, to austenitic stainless steels and to carbon or low alloy steels. To weld duplex stainless steels to other duplex grades, duplex stainless filler metal is used, with higher nickel content than base material, like ER2209 and 25Cr-10Ni-4Mo-N. For welding duplex to austenitic stainless as well as to carbon steel, filler metal E309LMo/ER309LMo (electrodes or rods) is used. Preheating of duplex stainless steel is not recommended because it slows the cooling of the heat-affected zone. Low preheating may be used just to dispose of moisture. Postweld stress relief is not necessary or useful for duplex stainless steels. That normally used for carbon or low alloy steel is actually damaging to duplex steels and should never be used . The only post weld heat treatment to be considered for duplex stainless steels is full solution annealing followed by water quench. Weld procedure qualification should be always considered for demonstrating acceptable toughness and corrosion resistance by using appropriate tests. Those regularly used for stainless steels (hardness and bend tests) may not be informative enough as to the actual properties of welded joints in duplex stainless steels. 5 - Online Press: recent Welding related Articles New Gas Mixtures to the test Optimizing Mild Steel GMAW Realistic Goals for Robotic Welding Welding New Stainless Steels Ship Building and Repair 6 - Terms and Definitions Reminder Boxing is the continuation of a fillet weld around the corner of one of the elements. Corner joint is that on the convex corner along the edges between two sheet metal elements at right angle to each other. Macroetch test is an examination performed optically under low power magnification on a metallic surface (generally a transversal section) suitably ground, polished and etched to reveal weld boundaries, macrostructure and eventual presence of internal defects. Nontransferred arc is established between the electrode and the constricting internal surface of the plasma arc nozzle, while the workpiece is excluded from the electrical circuit. Robotic welding is a joining process performed automatically by the use of equipment under flexible automation computer control. Tie-in is the gradual junction of a newly laid weld bead on the underlaying material. Upset is the plastic deformation undergone by the materials in a pressure welding process. It is expressed by reporting the percentage reduction in length or other geometric change. Workpiece Lead is the electrical cable connecting the part undergoing welding (workpiece) and the current supply source. 7 - Article: Quality Assurance in Welding Quality Assurance has been defined as a systematic approach designed to assess the quality of products or services provided. It consists in planned actions necessary to provide sufficient confidence that given requirements for quality are consistently satisfied in order to meet performance requirements. Quality Assurance has developed in time into an organic discipline built to address routine good practice habits into a formal set of check list items. By adhering compulsively to such rules positively identified and described, any organization upgrades itself to a new level of continuous acceptable performance. Generally the conversion to an approved status by a Qualifying Agency implies a management and organization restructuring. More and more companies and organizations submit themselves, either spontaneously or because of external incentives, to rigidly established patterns of acceptable behavior and operation. It all begins with a Quality Assurance Requirement List that addresses items of concern that must be dealt with in a way acceptable to the Institution that administers the qualification procedures. The publication AWS WQAG: "Welding Quality Assurance Guideline for Fabricators" is an introductory outline developed to help small to medium fabricators in producing welding Quality Assurance systems to ensure integrity of weldments. The need to formalize quality requirements into a body of fixed procedures stems from higher expectations of successful performance and fitness for purpose of manufactured components for longer times, from expensive liability issues and from excessive costs of loss of service, repair or replacement in case of failure. Quality Assurance addresses such subjects as responsibility, establishment of demonstrated procedures and compliance with them, testing, inspecting and recording of retrievable acceptable results. In case of welding operations, within this frame the particular technological requirements are taken care of by considering the following items:
All quality requirements are then satisfied by rigidly conforming to exact instructions, by performing adequate checks at critical stages, by implementing with care the demonstrated procedures, and by conducting professional testing and inspections during and after fabrication. Implementation of Quality Assurance requirements, for all joining processes covered by code or specification, includes formal establishment of a set of documents or forms that are to be filled in with pertinent data and kept for the record. These include Welding Procedure Specifications (WPS), Procedure Qualification Records (PQR), Qualified Welding Procedure (QWP), Welder Performance Qualification (WPQ) and Welder Continuity Log (WCL). Qualification and Certification programs established by mandatory codes and standards intend to ensure that all welding related operations like design practices, manufacturing processes, fabrication techniques and inspection activities are performed by professional personnel who demonstrated capability to an acceptable proficiency level. 8 - Site Updating
The new Page of the Month for our Site is dedicated to Welding Books. Welding information, knowledge and experience is available in good manuals and handbooks that every serious person interested in welding should learn from. We would like to do our best to put the information on where to find necessary knowledge at the readers' fingertips for easy retrieval. The disadvantage is that purchasing all the needed books may not be an inexpensive decision. When possible, students should make use of Libraries before deciding what to buy. We are convinced though that only learning with dedication and persistence is the way to progress in this as well as in any other profession. To reach the page click here. 9 - Short Items 9.1 - Auto Darkening Welding Helmets: during arc or plasma welding and cutting procedures, the operator is exposed to dangerous ultraviolet and infrared (UV/IR) light radiation. Most common eye injuries from radiation are retinal burns and flash burns to the cornea. To prevent these, proper eye protection must be worn and used. Liquid crystal auto-darkening welding helmet were introduced in 1975. They consist of a multilayer lens that changes its optical properties when submitted to an electric field from a battery, as a consequence of changes in liquid crystals orientation. The lens of a welding mask is protected by a clear plastic against impingement by spatter, sparks and foreign objects. It is transparent before welding begins, leaving the workpiece in full view for preparation and adjustments. The instant the lens sensor detects welding arc light, the lens immediately darkens, providing full protection to the eyes. The light blocking delay is of the order of 1/10000 seconds, so that no flash is perceived by the welder. Shading requirements for safety of welding helmets are established by the following standards: In the USA, ANSI Z87.1 superseded by ASSE Z87.1 Some welding helmets are capable of accommodating a power respiratory equipment to provide protection from harmful fumes. 9.2 - The most important advantage of Fillet Welds, is the total absence of preparation requirements, excluding normal cleaning and degreasing that must be performed in any case. Fillets are welded in the concave corner presented by two elements meeting at right angle. If the workpiece is positionable, with each element surface laying at 45 degrees from the horizontal plane, the best welding position would be flat, possibly slightly slanted to climb in the direction of welding. Otherwise (with one element horizontal and the other vertical) welding is done along the corner in horizontal position. The size of a fillet weld is referred to the largest right triangle that can be inscribed in the weld cross section. For dimensioning, the weld legs are specified, measured along the surfaces from the corner to the toe (end of weld). For stress calculations the important dimension is the effective throat, that is the shortest distance from the root to the weld face. The throat times the fillet length is the area supporting the shear stresses parallel to the weld axis, which fillet welds are designed to sustain. It is important not to load a fillet weld with a moment that would rotate one of the elements around the root of the weld itself because of the stress raiser (notch) condition. Double smaller fillets, welded from both sides, are preferred to a large single one. Overwelding, or unduly exceeding the fillet size more than stated in the drawing is useless and uneconomic. 9.3 - The Gases selected for shielding GMA Welding, that is for displacing reactive elements in the atmosphere around the weld, have profound influence upon the process. A wide choice of a single gas and of gas mixes is offered in the market. It would be useful and economic to limit the selection only to those types that have the potential to improve performance in a measurable way in any set of applicable parameters. Their effectiveness should be proved and documented. Shielding gas composition has a strong influence on arc temperature distribution and a significant effect on weld pool shape. Arc physics relate essential welding variables (current, voltage, electrode gap, shielding gas) to arc temperature, current density distribution, and gas flow structure at the anode surface. Gases can be classified considering their weld energy and their influence on oxidation. The weld energy potential of a gas mix has much influence on weld geometry and on quality. It should be noted that metal thickness and size of gap are important factors in gas selection, together with stick out length (see hereafter 9.5). Short circuit transfer is a low current mode practical for welding steels less than 2.5 mm thick. Gas should be selected to provide the required energy for the application. Spray transfer is used for welding steels thicker than 2 mm with deeper penetration welds and with more dilution. Pulsed spray transfer is a low energy mode of spray transfer useful in certain cases for welding low thickness components. Limiting the presentation to mixes of argon with carbon dioxide (CO2) for steels we will note that:
The more the CO2, the higher the voltage required to provide energy for CO2 dissociation, resulting in good penetration and acceptable weld fusion profile. This contrasts with what is achieved with less CO2 or with O2. Mixing argon with O2 reduces weld energy as appropriate for thinner welds but could cause more oxidation. The oxygen potential of the shielding gas influences the amount of surface slag, the fume emission rate, the fluidity of the weld puddle, and the mechanical properties (both strength and toughness) of the weld metal. The importance of a correct assessment of oxygen potential depends on how the oxidation potential is linked to loss of silicon and manganese in the weld metal, to the weld metal oxygen content, and to the weld mechanical properties. In general, in the gas metal-arc welding of carbon and alloy steels, as the oxidation potential of the shielding gas increases, the toughness and the tensile strength of the weld deposit tend to decrease. Argon is used for copper and aluminum, except that mixes with up to 25% Helium are preferred to increase the arc energy. This is important for welding higher thickness components of these high thermal conductance metals. Gas manufacturing representatives present sometimes mixes of three gases as having distinct advantages over two component gas mixes. However, independent professionals question this assertion and insist that perfectly acceptable weldments can generally be obtained, with correct welding parameters, using regular two component mixes. It is recommended to obtain a practical indication of the threshold current over which spray transfer condition is obtained, by testing actual weld specimens. 9.4 - Scanning Acoustic Microscopy: the usefulness of ultrasonic signal processing for non destructive testing is brought to important new achievements essentially by using precision motion guidance electronics and advanced imaging software. Ultrasonic testing uses beams of high frequency sound waves to find discontinuities in a test object. The technique is based on the detection and analysis of sound reflections caused by acoustic impedance mismatch at internal interfaces representing joints between different materials or the presence of lack of continuity. New advanced technologies permit the presentation of C-scan images, that are maps of inspected surfaces indicating amplitude and location of electronic signals proportional to reflected sound waves, with a few microns resolution. By these means not only bond surfaces (by any type of joining) are detected but also internal reflections representing disbond, cracks, porosity, voids etc. Scanning Acoustic Microscopy is particularly helpful for examining microelectronic chip packages but also brazed or bonded joints not readily inspectable by other means. By using special signal analysis means that divide the depth (proportional to the time of flight of the signal trace) of the volume inspected, into different layers by use of so called gates, it is possible to locate the indications also along the line of sound propagation, providing what amounts to a computer based tomography or volume scanning. The intuitive images produced provide greater understanding of the real internal condition of the inspected hardware. 9.5 - The electrode extension called Stickout is the length of wire between the contact tip in the torch and the location of the arc at the wire end. The following is equally applicable to Gas Metal Arc Welding and to Submerged Arc Welding. It is in the hands of the welder to establish the best contact tip size and the stickout to the correct length for any given welding job. The importance of the appropriate stickout setup stems from the fact that with any given parameters setting this length influences the amount of current flowing through the arc. Arc current is determined by the wire feed speed selected, but increased electrode extension will decrease current at a given wire feed speed. Furthermore in GMAW a very short stickout submits the contact tip to the dangers of overheating and of spatter damage. Longer wire stickout protects the contact tip and prevents burn back problems and allows higher wire feed speed without the normal current increase than would be obtained with a regular stickout. 9.6 - Weld Process Control Program: This section is based on the plan proposed by Ed Craig in his book: It is a recommended necessary procedure to be used for obtaining the best performance when employing Robots, but we believe it is equally important for any job performed manually using the GMAW process, with due modifications. For a more deep insight into this subject the interested readers are urged to seek the complete information in the given reference.
A very important lesson can be learnt here from the proposal of a distinguished professional, endowed with long time first hand experience. It does not matter how good and authoritative are the suggested starting parameters for establishing an initial GMA Welding technique. A systematic approach is required, based on carefully planned practical trials on actual specimens, followed by destructive testing and by intelligent interpretation of the results. This is the only way to provide the necessary confidence in developing a robust, economic, profitable welding procedure for Robotic welding but also for manual GMAW applications. 10 - Explorations: beyond the Welder Carbon Nanotube Composites Sixteen Encyclopedias Online From Scientific American On Health and on Care Quake sped up Earth's Rotation 11 - Contribution: Design for Welding I recently happened to fall upon a query published on a well known professional forum online. The originator, probably a design engineer with experience who is currently engaged in an important construction project, asked where he could find joint detail examples for quite a long series of different structural element combinations. He seemed to overlook however the fact that there is more to Weld Design than joint detail. There is no question that the person is probably very competent and conversant with stress calculations, stability checks and good design principles and practice. He makes no claims however relative to his own welding experience or lack of it. An annoying question must nevertheless be raised: why should a person with experience in a certain field of engineering be totally unaware of implications from a long list of other factors connected to welding which directly affect the outcome of the resulting design? AWS Welding Handbook 9th Ed. Vol.1 dedicates its Chapter 5 to "Design for Welding". See:
Welding Science and Technology, Welding Handbook, 9th Ed., Vol. 1 Vol. 20 of the ASM INTERNATIONAL Metals Handbook, 9th edition describes the role of the Materials Engineer in the Design Process. The tasks involved include decisions on base materials, filler metals, processes, technical and economical issues, properties vs. performance issues and manufacturing aspects. See: The basic suggestion of recognized authorities that only a coordinated team study can cope with the design task in a comprehensive way is probably applied only in mass production or for aerospace realizations, as a practical way to share responsibilities. This approach, given the amount of knowledge involved in modern technology, should be considered for every important application. It is unfortunate that it is not so. The reasons are probably a certain narrow vision of management who are not thinking that expert advice would benefit their operation in more than one way. And contributes also possibly the attitude of the Chief Engineer who might fear that his standing be impaired if external help is sought. It is suggested that influential leaders with a vision consider to promote and to spread the design team concept in academic institutions and in industrial companies. 12 - Testimonials From: "Dalmat I.C." 'dalmat3@yahoo.ca'
Hi Elia, From: "Sarah Chandrasekar" 'sarah.chandrasekar@venturamfg.com'
Thank you very much for your advice. It was very helpful. Sarah Chandrasekar 13 - Correspondence: a few Comments Once more, reviewing the mail of the past month, I see that well formulated questions with sufficient description are the exceptions, more than the norm. Most of times people throw up general queries demonstrating that they have no idea of the matter they are seeking advice on, like the following: Even if you are a student with not much experience, you should understand that it is not serious on your part to formulate the question in these terms because any one of the subjects between commas could require a chapter in a handbook to be answered seriously. And that is exactly what I think you should do. Sit in the library and learn. These consideration bring to my mind the idea that possibly the error is on my part, as I should explain everywhere on the Site that questions should be qualified for me to consider answering. Maybe some of you could contribute your thinking? Let me know. Click here. 14 - Bulletin Board 14.1 - The 16th International Welding Fair will be held in Essen, Germany
between Sept. 12 and Sept. 17, 2005. Called also "SCHWEISSEN & SCHNEIDEN 2005" it is a periodic event occurring every four years. For details: 14.2 - Announced as the only event spanning the entire spectrum of materials science and engineering, Materials Science & Technology 2005, will be held Sept. 25-28, 2005 at the Lawrence Convention Center in Pittsburgh, Pa. The organizing societies are ASM International, American Ceramic Society, Association for Iron & Steel Technology, American Welding Society and The Minerals, Metals & Materials Society. Details at: 14.3 - Powered by SiteSell.com. Click here. See you next time
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