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PRACTICAL WELDING LETTER, Issue #006 -- New Gift, Your Robots, GMAW, FCAW Filler Materials
January 31, 2004
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Practical Issues, Creative Solutions
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Date: February 2004 - Practical Welding Letter - Issue No. 06
------------------------TABLE of CONTENTS---------------------------
==> 1 - Introduction
==> 2 - Article: Programming Your Mechanized or Robot welder
==> 3 - How to do it well: Furnace Hardening of Steels
==> 4 - Filler Metal: Selection of Electrodes for GMAW and FCAW
==> 5 - In the press: Recent Welding and related Articles
==> 6 - Terms and Definitions Reminder
==> 7 - Article: Heat Treatment of Carbon Steels
==> 8 - Site Updating
==> 9 - Short Items
==>10 - Explorations: beyond the Welder
==>11 - Correspondence: a few Comments
==>12 - Testimonials
==>13 - Bulletin Board
1 - Introduction
The present Issue of Practical Welding Letter is distributed free of charge to an increasing audience of Subscribers as a few readers add their name every day.
Some of the kindest readers bothered to the point of writing us that they were actually helped by what they read: we are very happy to hear this, as we would like to provide important and interesting information to all concerned. The section on Testimonials shows a few of these letters. Therefore we will gladly listen to your preferences and requests, when you take the time to inform us.
This time we can proudly announce we have added to the Site a new page on Welding Design. This subject, although most important for the successful implementation of weldments, seems to be somewhat neglected or taken for granted. It is unfortunate and unjustified that this be the case. You will find the link further down in the page.
Mechanized or robotic applications of Gas Metal Arc Welding and of Flux Cored Arc Welding are increasingly implemented because of their potential to improve productivity. It seems however that many applications run at grossly underrated capacity, mainly because of lack of sufficient understanding and expertise in the process. The short article we devote to the matter is intended to stimulate the curiosity and to urge readers to develop excellence and knowledge.
A quite common failure in heat treating can be easily avoided if only one knows how... A short note explains the problem and the solution.
Filler electrodes for Gas Metal Arc Welding and Flux Cored applications should be selected considering their special characteristics: while a complete reference is outside the scope of our review, the few hints provided may help for a first orientation.
An article on Heat Treatment of Carbon Steels is presented to refresh some important concepts often referred to when dealing with these inexpensive and useful materials.
Other usual sections will be found in their right place.
The high quality Gift we urge you to accept this time is loaded with benefits for yourself, your family and your friends. You can actually use it and gain. And you should profit of this valuable opportunity because of its high usability, despite its being offered at no cost to you. Grab it right now, see the reference in the forthcoming Bulletin Board.
And before you leave this page, write us a short feedback, click here.
The purpose of this short note is to point out the most important subjects only pertaining to the Gas Metal Arc Welding and Flux Cored Arc Welding used for mechanized applications, and to ask the questions that should be addressed when dealing with the problems of practical implementation.
In welding operations running in continuous and repetitive mode, although an experienced welder could take care of poor joint preparation or sudden irregularities that should not be present in the first place, there is no practical need that a welder's hand held the torch during actual welding,
If the joint is reasonably well prepared and consistent, the torch has only to be located in proper position relative to the weld, and held there at all times, while the relative movement of work and torch takes care of welding progression. The welder is then called a welding operator, whose job is to tend to the controls of the machine and to look after its performance.
The productivity potential of a well programmed mechanized or robot welding is much higher than that of a manual process. Upgrading to mechanized or robot welding originates mostly when Management studies ways to increase production or to reduce the required workforce. However there is more than production plans to actually realize the programs.
According to some estimates, GMAW, both manual and mechanized, is applied to the largest proportion of weldments, because of its many and important advantages over competing processes. Unfortunately it appears that attaching that torch to the manipulator of robots or of welding machines was often less than an overwhelming success. In many applications the benefits inherent in the process, namely consistent quality and high productivity were simply not reached. Worst, sometimes considerable manual repair of bad welding was required.
While the reasons for such an infortunate state of affairs may be hotly debated by all concerned, everyone should agree that an increase in diffusion and level of knowledge and of expertise in governing the process could only improve the results. In any case proper considerations of process requirements should start in the design stage.
GMAW is a complex Process that produces outstanding results when understood and correctly applied: in any given situation there are only certain ranges of parameters suitable for proper operation. If operators lacking basic education and training on the process "play" with commands in the hope of striking the best combination, only disappointement and costly rejects are bound to occur.
The following publication are suggested for reference:
In the following the main parameters are listed:
Power supply type and maximum amperage available:
In the vast majority of cases, for manual and mechanized welding alike, a regular power supply with constant voltage is recommended, capable of supplying at 100% duty cycle the maximum amperage ever to be drawn from the system. Manufacturers tend to propose advanced and expensive equipment with slope control or with pulsed arc capability.
Type of torch and cooling system:
Not all welding torches are alike. They must stand the heat, permit easy replacement of consumables (mainly the contact tip) and have acceptable durability. Water cooled torches may develop leaks, so that those air cooled by ambient or compressed air are preferred. For FCAW, special vacuum torches capable of disposing of excessive or objectionable fume production are available.
Gas or gas-mix of specific energy:
The energy available in the arc depends on the composition and on the physical properties of the gas-mix. This is because the gas is dissociated and ionized, and takes a prominent part in arc behavior. Voltage is to be set at a value suited to application, but will be adjusted later in the process.Notes:
1 - When employing plain CO2 one should use a filler like E70S-6 containing deoxidisers to counter the oxidizing tendency of carbon dioxide (CO2).
2 - Helium has increased thermal conductivity and requires higher voltages. The addition of Helium to Argon mix may be considered only for aluminum welding, if a higher energy gas is needed. One such case may be that of thick material, drawing heat too fast (because of high aluminum thermal conductivity), preventing sufficient penetration to be attained. But first one should try a smaller size wire at higher current density or at higher gas flow rate. Practical tests are always recommended.
Type of metal transfer:
Each mode is best suited to specific applications.
Filler Material type and size:
More on type selection in the article on Filler wires in this publication.
Most wire feed systems are of the constant feed type, where the speed is set in inches or meters per minute before welding. It is very important to establish the wire feed speed for every job. Either if the feeder includes a speed indicator or not, one should check the speed at any set point by measuring the length supplied in ten seconds: the speed is six times this length per minute. Feeders are usually of push type, where roll pairs draw the wire from the coil or spool and push it to the torch through a flexible tube. Essentially the feeder should be suitable for the materials and for the wire sizes to be used. Preventive maintenance is a must.
Wire feed Speed, controlling amperage:
Selection is based on base metal type and thickness, type of joint, position, metal transfer mode selected, penetration and dimensions of the deposited weld bead, stick-out (see next item), welding speed, quality required and testing employed.
Wire Stick-out is a most important control factor:
By increasing the length between the electric contact tip and the arc location (wire stick-out), the current is used also for preheating this length of wire (heat proportional to I2R) and less arc energy is required to melt the wire. The current increase following increased wire feed speed is less for a longer stick out than for a shorter one. The practical outcome is that higher feed speed can be safely employed for a long stick-out, improving welding results.Note: having selected the right stick-out for the application, one should preferably set the contact tip somewhat recessed from the nozzle end, to reduce eccessive heating of the same and burn-back of the wire.
Once the transfer mode and the feed speed are selected one should adjust the voltage setting to determine the arc length.It is recommended that before trying to program mechanized welding, a suitable parameter set be established for manual welding of the same application. Any procedure established for manual welding should be qualified by examining sections of test welds, to verify that penetration and quality are as needed.
Only then can further developments be attempted trying to achieve an even higher deposition rate, which is the most significant measure of productivity, by the use of mechanized or robot equipment. Particular attention has to be devoted to starts and stops of welding stretches, which may be sensitive to weld defects.
Once the welding parameters are set for an acceptable manual procedure it is much easier to program the mechanized or robot equipment to obtain the correct sequence.
To get the most from the machine one should consider ways to increase wire feed speed and current, weld speed and weld deposition rate. This may include experimenting with larger size electrodes (less expensive) capable of higher yield, but penetration must be strictly monitored. In general the highest weld deposition attainable may compromise quality and penetration and should therefore not be sought for. Even then there is ample margin to improve on deposition rate when upgrading from manual to mechanized welding.
For a successful mechanized or robot application, correct part set up and fixturing is most important. Consistence of positioning and of joint tolerance is essential to the favorable outcome of the operation. For best utilization of the robot or mechanized cell, down time is to be kept to a minimum, by implementing a conservative program of preventive maintenance, including contact tip changing.
Welding Procedure Qualification should be performed for the final programmed sequence, by documenting and retaining test pieces and other evidence of results. After that periodic quality control provisions have to be implemented during production welding to maintain acceptable results. The approved parameter set should be made available to all operators as the established procedure, with authorized limits of adjustment that should not be exceeded for any reason by unauthorized personnel.
To summarize these notes one should remember that the guarantee of success is not to be looked for in unduly complicated and sophisticated pieces of equipment or in aggressively advertised consumables, but in understanding the basic principles of the process and in a systematic approach to develop the appropriate conditions step by step by step by trial and testing .
Q: A medium carbon (0.35%C) steel bar 1" dia. (25 mm), did not give substantial hardness when quenched in water from an air furnace at 820 0C (1500 0F). Why not?
A: The problem is probably a consequence of carbon loss (decarburisation) due to the reaction of air in the furnace with the surface carbon of the bar; in effect, locally, one gets lower carbon steel, less capable of hardening when quenched. A protective atmosphere in a funace of different type would probably overcome the problem. Alternatively one could adopt an old trick, by putting the bar in an air furnace but in a stainless steel box full of charcoal or proprietary products, to counter the harmful influence of oxygen in air. The bar must then be withdrawn quickly for quenching as needed.
Gas Metal Arc Welding electrodes are classified in AWS A5.18 for mild steel and for low alloy steel. These are solid wires of controlled chemical composition, whose mechanical properties, when tested in all weld metal specimens, have to meet minimum specification requirements.
Solid wire are less expensive and should be considered first. Flux cored wires (which are more expensive) have the potential of higher weld deposition rate and possibly of easier use by less skilled welders. They are preferred for rusty or contaminated surfaces, for thick or out of position welds and when quality obtained with solid wires is not acceptable.
For GMAW with solid wire the two most used electrodes for carbon steel are E70S-3, (S stands for Solid) when the joint is clean, and E70S-6 for relatively more rusty surfaces, as the composition contains more deoxidizers (Manganese and Silicon).
Selection of applicable wire type may depend on many factors: it is reported that a general lack of agreement is found among users as to which type is most suitable for any given application. Therefore for any important job one should be prepared to test at least a few types and to check the results before a major purchase.
Flux Cored Wires are described in AWS A5.20 for mild steel. These wires combine the semplicity of operation of stick welding, where the protective atmosphere is generated around the weld by the combustion of flux covering the electrode and a protective slag is left on the weld bead, with the continuous operation of GMAW, where additional protective gas is supplied only if and when required.
When gas is not used the process is called Self Shielding FCAW. In this case the metal transfer is by short circuit or globular mode (the spray type would oxidize too much): out of position (i.e. vertical and overhead) welding is possible with certain small size wires. However weld quality would generally be inferior when compared with gas shielded flux cored filler metal.
The addition of gas, although introducing some complexity in the torch, extends the usability of the process to spray transfer mode and provides better quality welds. Although straight CO2 is sometimes used, better results can be achieved with gas-mix of argon + 15-25% CO2 for carbon steel.
Typically when welding with Flux Cored electrode one should be prepared to use a longer than usual stick-out length for the benefits it provides with these wires. For a description of stick-out and its influence check the article on mechanized welding in this publication.
In multipass welding the slag must be removed before depositing additional weld beads. Flux cored wire E70T-1 (T stands for Tubular) is best used with auxiliary gas. Joints should be reasonably free from oil, rust and scale if radiographic quality must be attained. This requirement is more strict than when welding with regular GMAW electrode. Also suitable when deep penetration is required and for single or multi-pass use in flat or horizontal position.
E70T-2 contains more deoxidizers, and is used for welding steel presenting more scale or surface contamination than can be tolerated by the above electrode. It is suitable for single pass welding in flat and horizontal position. Multiple passes are not recommended unless substantial dilution is obtained with base material.
E70T-5 is suitable for flat groove welds or for horizontal fillet welds, for single or multi-pass deposition, even with minimum surface preparation. Used when impact properties or better crack resistance are required.
The so called all position wire E71T-1, which is preferable for vertical and overhead position, would provide less appealing welds if used in flat or horizontal position. Vertical up is applicable to both plates and pipes. The diameter selected should provide the required quality and penetration with economic deposition rates.
Metal cored wires, containing metal besides flux, are offered and promoted by manufacturers, but do not offer tangible benefits when compared with the regular electrodes described above.
In particular one should be aware of the fact that electrodes of the same class produced by different manufacturers may behave quite differently in what concerns weldability and parameters.
Specifications limit the spooled wire helix and cast, which are checked by putting on a flat surface a length of wire cut from the spool. Helix is the measured raising of the end of a single strand. Cast is the diameter shown by a free standing length of wire. These parameters are particularly important when employing a long stick-out as the electrode may tend to wander in unexpected ways. When necessary a wire straightener should be employed at the feeder to correct for improper quality.
The surface of the wires must be consistently clean, not oxidized, and the original box or container shall remain unopened until used. It is good practice to inspect all incoming filler wires to control their acceptable condition (absence of rust, of excessive lubricant). Any remaining quantity of unused wire must be removed from the feeder and protected from moisture and dust, until ready to be used again.
Expanding on the subject presented here above on Filler Materials for GMAW and FCAW, you may find an article From The Fabricator, with a different point of view.
On "The future of Structural Welding" you will find the quite conservative view, that not much is going to change in the next twenty years. Is that right? Click here.
If you are interested in learning on "Hydrogen Related Failures" a documented article with examples will show you what is involved. Check the Welding Journal, January 2004 Issue.
If you are a proud Certified Welder, you may think that you are always covered. However you may discover that the question "Is that welder qualified?" may be asked also of yourself, and that the answer may not be straightforward.
A well presented reminder on "Inspecting with liquid penetrant" is offered to your attention: you may need that sometimes.
Flux cored electrode:
Metal cored wire:
7 - Heat Treatment of Carbon Steels
This section covers the fundamentals of Heat Treatment of Carbon Steels.
Carbon steels are iron base alloys whose principal alloying element is carbon. The amazing versatility of steels for the most diverse applications derives from the essential characteristic that with suitable composition and correct heat treatment, one can develop quality and properties best adapted to vastly different end product usages.
It should be kept in mind however that all Carbon Steels are susceptible to rust if left unprotected in a humid environment.
Low Carbon Steel.
To improve machinability, should this be an issue, one can heat in a furnace to 815 - 860 0C (1500 - 1600 0F) and then cool in air or in a suitable quenchant: the details of the process should be determined keeping in mind the exact composition, the purpose of the treatment, the mass of the material and the equipment available.
Medium Carbon Steels.
The structure obtained by heating at a relatively elevated temperature is called austenite and is characterised by a particular crystal structure of iron different from the one that was present at lower temperature (called ferrite). Upon rapidly cooling or quenching from high temperature, new crystals form of a different structure called martensite, presenting internal stresses which translate in high hardness and low ductility. This transformation induces an increase in specific volume of about 3%.
To obtain useful properties the internal stresses have to be mitigated by a further heating at lower temperatures, the second treatment being called tempering. In the usual jargon the succession of treatments performed to obtain the useful properties required is called "hardening and tempering" or "quenching and tempering".
Two major factors influence the outcome of the operations outlined above: mass effect and surface carbon depletion.
1 - Mass effect.
Quenching media include water, brine (a water solution of salt) and oil, used when one looks for a milder quenching action, or special polymers in water used to avoid the flammability of oil. Time delay in quenching will produce different, intermediate, weaker structures which may not be the ones sought for. As a rule of thumb one can assume that the highest hardness of medium carbon steels can be obtained in heat treatment, with a suitable quenching medium, up to a diameter or thickness of 12.5 mm (1/2"), but a useful hardness is achievable up to 25 mm (1").
2 - Surface carbon depletion.
Not only the surface cannot display the full hardness wanted but it will be subject to local tensile stresses (to oppose the compressive stresses developed in the deeper layers transformed to martensite): this is a dangerous condition regarding the ability to withstand fatigue.
High Carbon Steels.
Hard steels are also fragile, meaning by that their tendency to break if submitted to blows or shocks. They are more prone to decarburisation than steels containing less carbon and therefore their heat treatment is more difficult, requiring special means (like neutral gas atmospheres) to be performed successfully. And they are more sensitive to the rate of heating, especially if they present widely varying sections or sharp notches: if heating is applied too rapidly, internal stresses may develop, producing deformations or even cracks.
The versatility of Carbon Steel is very wide permitting the deployment of quite different properties to suit different needs. Even larger versatility is obtained by alloying Steels with various other elements to produce an extremely complex variety of useful types of materials: but this is another story.
As announced above in the Introduction, a new page on Welding Design was added in the Welding Advisers Site, since the previous issue of Practical Welding Letter. All involved in welding, and in particular those in a position to specify requirements, share the most heavy responsibility associated with assuring weldments performance.
Good Design is produced nowadays by cooperative and well informed Teams because the subjects involved extend over such wide fields that cannot be covered by a single person. It is recommended that designers look for suitable ways of practical realization of their intentions, by inquiring on the ways of doing things. The new page can be read by clicking here.
As usual we would like to recommend to use the Site Map to find the pages of interest when browsing in the Site.
Let us know what you need. Your feedback is most appreciated. Fill in our Form #1 and send it to us for prompt attention.
9.1 - Welding of Magnesium Castings.
A general overview of welding magnesium has been given elsewhere in the Site.
See Magnesium Welding.
In particular reference was made to the Gas Tungsten Arc Welding Process, which is generally the first choice. Here we will add a few details on the most used Filler Metals which are given in AWS A5.19 and are designated as AZ61A, AZ101A, AZ92A and EZ33A. The first is usually preferred because it is the less costly. However AZ101A can also be used successfully for repairing or joining castings. EZ33A is sometimes recommended for welding alloys HK31A and HZ32A.
9.2 - Spot Welding. Within the group of Resistance Welding processes, spot welding is one of the most useful and applied whenever overlapping joints of sheet metal are appropriate. For a short review see Resistance Welding.
With modern real time sensors and controls and with microprocessor based technology, a very high degree of confidence is achieved in quality and strength of every single spot. The process does not require skilled workers and can be fully automated like in car assembly lines. For certain heat treatable aluminum alloys it is the only process that can be used without destroying their high mechanical properties. The benefits of using spot welding where appropriate, designate it as one of the most economical welding solutions.
9.3 - Laser. The highest power density Laser Beams, which are essentially a concentrated form of light, find applications in welding for automated high productivity joining projects, where localized heat can easily melt almost any metal. It may reach quite far away surfaces that are in the line of sight.
Some suitable protective atmosphere should be deployed to prevent severe oxidation from taking place in the molten metal and its surroundings. Clean metal parts may need some coating to reduce the reflection of the beam on mirror shining surfaces. Metal type and size would dictate the power needed.
Wavelength should be adapted to the application. Safety issues impose most stringent requirements. Laser beam is used successfully for contour cutting of sheet metal and for piercing holes with or without auxiliary gases. In some applications the recast layer on the border may be objectionable and may require removal by mechanical means.
9.4 - Electron Beam Welding. A mature technology of high concentrated power density,(see Electron Beam Welding), normally requiring a vacuum enclosure, permits joining of surfaces in very special situations, with minimum heating of surrounding metal. It is therefore the preferred or unique solution in certain definite cases, and despite the elevate cost of equipment it may provide the only or the most economic solution for repairing or reclaiming costly items.
Magnetic masses may influence the path of the beam, which can be directed by magnetic lenses to follow any required contour. Although not suited for cutting sheet metal, it has been used for drilling thin holes in well defined positions. For high production of repetitive welds, special machines operating in partial vacuum have been developed and used successfully.
9.5 - Cryogenic Stainless Steel Pipe. The austenitic types of Stainless Steels retain their mechanical properties even at very low temperatures and are therefore the materials of choice when these charasteristics are required. As shown elsewhere (see Stainless Steel Welding), joining by different processes is not particularly difficult, provided all the usual precautions are implemented.
In particular the inside of the tubes to be welded must be purged locally with inert gas to prevent oxidation. Sometimes copper backing bars are used on the inside. Consumable ring inserts can be used to align the parts and to provide a better root joint.
9.6 - Welding Generators are power supplies that produce electrical power for welding. They are driven either by an electric motor or by an internal combustion engine. Engine driven generators are independent of power supply lines and are most suitable for field work.
The electrical characteristics of the supply side are similar to those of other accepted welders, namely providing mostly constant direct current and a drooping curve. Because of their moving parts they are generally more expensive than their transformer-rectifier counterparts and their maintenance is more costly. Most generators would also provide electricity for any other auxiliary need, besides welding.
9.7 - Welding Defects. A short review is available by clicking on Welding Defects. Most welded items will show welding defects, if the search is pushed deep enough. But only significant defects should be considered.
It must be remembered that the importance of any defect is related to the service of the welded structure in cause: therefore there is no such thing as a universally rejectable welding defect. It may well be that a condition harmful in certain conditions may be perfectly acceptable in others. Inspectors educated to inspect welds for aerospace applications may have a hard time to adapt to more lenient requirements suitable for less demanding uses.
Cracks should not be accepted most of times, because they can grow and compromise the integrity of the part. But also deformations or other visual defects should be considered. Rounded porosity, an internal condition to be determined by non destructive methods, may be accepted in certain cases if not excessive, as specified in well defined engineering requirements.
9.8 - Carbon Equivalent. This subject is dealt with in our page on Alloy Steel Welding. The numerical result obtained by using an accepted formula which makes use of the analysis of chemical composition of the alloy steel in question, indicates if special precautions like preheating and postheating should be put in place while welding for the purpose of avoiding cracking.
It should be noted that additional factors influence the outcome. The mass of the parts and the restraint, meaning the freedom to contract freely when cooling down, should be considered as even more important that chemical composition. Also the heat input, as provided by the specific process employed, and the filler metal type greatly affect the results. The conclusion is that one must use common sense and be careful: one should possibly make trials and tests to examine the most favorable working conditions.
9.9 - GMAW and FCAW for Pipes. The processes have very high potential of producing economically good quality welds of consistent complete fusion. The transition from previously used methods to gas shielded continuous electrode welding requires some understanding and training before one can obtain acceptable results. Experimenting with consumables and parameters is recommended, to find those that provide acceptable weld fusion as demonstrated by Weld Procedure Qualification.
Joint preparation and geometry (land and bevel angle) and root gap should be given consideration. Tack welding should be used. Whenever possible the rotating position 1G should be preferred. Gas composition, weld torch location, direction of turning and metal transfer mode (short circuit with low current, spray tipe with high current) are essential factors contributing to the success of the project. One should note that fill passes may require different parameters from root pass.
9.10 - Keeping Documents for Auditing. Audits are verification procedures adopted by Certifying Agencies or Customers to demonstrate that acceptable and valid work instructions are implemented as required while performing a given job. In general auditing is performed against written documents or standards, agreed upon between Purchaser and Contractor. Sometimes the Contractor is requested to prepare written procedures that reflect Customer's requirements.
Auditing is a step by step check of all the propositions listed in the binding document and the actual demonstration of performance of those actions in the field, generally in the form of dated and signed production and inspection documents. To provide for a smooth auditing, the Contractor has to organize and to keep the same in due order and traceability, for prompt exhibition to the Auditors. The task is not difficult but requires order, attention, discipline and firm support from Management.
10 - Explorations: Beyond the welder
The Nickel Development Institute can be reached at www.nidi.org/
The Copper Development Association is found at http://www.copper.org/
For reaching suppliers of materials and testing equipment check www.adinfo.cc
www.galvanizeit.org is the site of American Galvanizers Association
www.recycle-steel.org introduces to the Steel Recycling Institute.
11 - Correspondence: a few comments
The stream of questions is continuing. We interpret the arriving of questions as a proof of their usefulness and we believe that in certain cases we actually helped the inquirer. Only the minority of the readers however cares to express their thanks.
A few of these notes of appreciation we wish to present hereafter in the section named Testimonials. It is a great satisfaction for us to be sometimes informed that our efforts where usable and valuable.
It happens sometimes that the inquirers did not look for information in the proper page of our Site: in certain cases, when the question is quite general, the answer is already there. Some preparation could have refined the question to details more readily referred to the inquirer's problem.
Questions without sufficient details are the norm. Sometimes terms are used indifferently, to mean something different that only the author can imagine. When we can presume the intention, we usually propose it, with a note.
In this time of epidemic virus spreading we will refrain more then ever from opening attachments. Please use alternative solutions.
A few of the writers are students: they would need reference materials, only available through well endowed libraries. We wonder if some central research institute would be willing to cater to their special needs through online tutoring and long distance teaching.
Note: The following are messages I received from satisfied correspondents.
From: "David H. Fox"
Your welding advice proved useful as we implemented some of the changes in our design. Also the speed at which we received the advice was most helpful.
David H. Fox
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From: "David Doehler"
The help was beneficial.
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13.4 - Questions and Feedback are the main interface between readers and the Welding Advisers Site. You are urged to let us know how can we be of more help to solve your welding and related problems. Use our form, or use your own words.
13.5 - See you next time.
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