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Process Comparison

Q: A butt weld has to be performed on short lengths of sheet metal. It could be done by Plasma Arc Welding, by Electron Beam or by Laser Beam Welding: which process should be preferred?

A: Given the availability of different processes, once the quality requirements are satisfied, the most economic one should be selected. The economy of performance, expressed as cost per weld, has to be calculated by taking into account all the expenses for equipment, consumables, workforce, handling etc.

A first evaluation can be done by comparing advantages and disadvantages of each process in turn. If manual and automatic operation can be performed, the cost of both should be estimated.

(Note: See as a Guide our page on Welding Cost Estimate).

Plasma Arc Welding (PAW):

Advantages of PAW as compared to GTAW:

  • Higher energy concentration, higher heat
  • Improved arc stability, especially at low current
  • Greater arc length tolerance
  • Greater plasma and welding speed, shorter weld time
  • Tungsten contamination eliminated
  • Less skill required for manual welding
  • For larger thicknesses welding in one pass with Keyhole technique
  • Smaller weld volume, less filler metal than with GTAW.
  • Reduced rework and rejections.

Disadvantages:

  • Equipment more expensive than Gas Tungsten Arc welding but much less than EBW.
  • Short life of orifice body, requires replacement.
  • More welder's knowledge required
  • Higher rate of consumption of inert gas.

Electron Beam Welding (EBW)

Advantages:

  • High energy heat source, for deep penetration in thick narrow joints
  • Filler metal usually not required
  • Total heat input lower that for arc welding, limited deformation
  • Welding in vacuum, ideal for reactive metals
  • Difficult-to-weld materials can be joined.
  • Elevated welding speed
  • Beam shape, focus and path controllable by electric and magnetic lenses
  • Automatic beam tracker available
  • Permits solution to otherwise impossible procedures

Disadvantages:

  • Expensive equipment including vacuum chamber and pumping system
  • Beam sensitive to occasional magnetic fields
  • Unproductive pump down time required
  • Shielding against harmful by-product x-rays required
  • Precision set up required with special fixtures

Laser Beam Welding (LBW)

Advantages:

  • High power density heat source, for deep penetration in thick narrow joints
  • Welding performable in air (depending on materials)
  • Total heat input lower that for arc welding, limited deformation
  • Easily mechanized high processing speeds with very rapid stopping and starting
  • Micro welding possible, precise welds can be obtained.
  • Difficult-to-weld materials can be joined.
  • No electrode or filler materials are required.
  • Welds with little or no contamination can be produced.
  • The laser beam can also be time shared.
  • If a weld can be done by both EBW and LBW, (with limited power) the last one is more economic as vacuum system is not required
  • Hybrid systems available combining LBW and GMAW

Disadvantages:

  • Limitation on power available (affecting thickness) for solid state systems
  • Capital cost more expensive than power arc welding systems.
  • Even more expensive high power sources for welding thicker materials
  • Additional shielding provisions required for reactive metals
  • Safety concerns for operators' vision protection
  • Precise fit up critical
  • Low electrical conversion efficiency
  • The penetration is less than for EBW
  • The power at the workpiece will be significantly reduced due to reflection

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Carburize with Oxyacetylene Flame?

Q: Is it practical to use an oxyacetylene torch to raise the carbon content of mild steel in an attempt to carburize the mild steel for case hardening by heating and water quenching?

A: Unfortunately no. What can be done instead is "pack carburizing" in a stainless steel box. Proprietary products made for this purpose, contain besides carbon (in the form of charcoal and coke) also other important ingredients (carbonates of barium, calcium, sodium) called energizers.

Parts are buried in the granular carburizing product, the box is closed with a cover and loaded in a furnace at about 850-900 0C (1560-1650 0F) for 4 hours or more, depending on the depth of case required. If parts can be removed quickly from the granules they may be then quenched immediately in water. Otherwise the box is slowly cooled in the furnace and then the parts are reheated for quenching.

Top of Weld-FAQ-A

Fillet welding of Rimmed Steels

Q: Why is Fillet welding preferred for Rimmed Steel?

A: Rimmed Steel manufacturing processes provide a case or rim of very clean material free of defects. Conversely impurities tend to concentrate in the middle section of ingot or billet.

This feature persists through the rolling process, so that plates of this kind tend to have their central core less clean than the superficial layers. This property provides an advantage when design calls for fillet welding, which does not penetrate to the center of the plate.

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Manufacturing a Spherical Vessel

Q: What is the normal method of manufacturing a welded spherical pressure vessel?

A: Assuming quite a heavy wall spherical pressure vessel of large dimensions, the material and the thickness of the plates are to be selected depending upon service pressure and conditions.

The vessel will be manufactured by welding together prepared sectors. The dimensions of the sectors will depend upon the size of the press available in the workshop for hot forming the plates in special dies.

On the drawing table, (or on the computer screen) the sphere is divided into two hemispheres. Each of them is again divided in four, six or more sectors depending on their dimensions. Two caps shall be located at the poles.

One single sector has to be designed in detail: they will all be equal. Some material has to be added at the margins for precision cutting and chamfering after forming and stress relieving.

Depending on material, dimensions, process etc., the sectors have to be prepared for welding together once the joint details have been established. If clad material is used (i.e. a carbon steel with a thin layer of stainless steel) special welding procedures must be developed and tested.

Inlets, outlets, manholes and whatever passages are needed, are designed to be performed at the proper stage in the process.

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Use of Low Hydrogen Electrodes

Q: We are requested to weld with Low Hydrogen Electrodes. Why?

A: Hydrogen gas is readily absorbed because of its high solubility in molten and hot steel, but it is rejected at lower temperatures as solubility decreases. Furthermore when dissociated in atomic form, hydrogen can diffuse in the Heat Affected Zone.

A detailed discussion is included in the page on Weld Cracking: click on it.

Two kinds of defects can be generated by hydrogen in welds. Porosity is the presence of gas bubbles that weaken the structure, trapped in the solidifying molten metal. Cracks, even delayed long after the end of welding, are generated in weld or heat affected zone as a consequence of recombination of atoms to molecules and pressure raise.

Hydrogen is particularly harmful in strong, hard, crack sensitive hardenable or structural steels, especially when high residual stresses are present or when design is rigid and most constrained.

The use of low hydrogen electrodes, kept dry before use, with proper procedures including pre- and postheating, is mandated by the need to provide measures apt to prevent the dangers outlined above.

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Strength of Stainless Steel Spot Welds

Q: How to judge of the adequacy of Spot Welds in Stainless Steel Sheets?

A: Austenitic Stainless Steel sheets are easily spot welded. However, just by observing how many common household implements made of stainless steel fail sometimes in spot welds, one would think that it may be difficult to obtain adequate strength and to evaluate it.

Given standard single lap spot weld coupons, it appears that the minimum strength reported for each spot weld in accepted Specifications has no real interest, because good spot welds will not break in the weld. When tested in tension they will rather fail in the material around the weld, by tearing a button in one half of the specimen and leaving a hole in the other half.

This is a good result even if the tensile test value at rupture is not known exactly.

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Sorting unweldable Stainless Steel

Q: A batch of regular stainless steel fasteners to be welded on a sheet, was inadvertently mixed up with free-cutting stainless ones. As we are told that free-cutting stainless is not weldable, how can we sort out the weldable items?

A: Hopefully the batches, although unknown, are still separated: if this is the case you need only to examine representative specimens of batch A vs. batch B. It is quite straightforward to sort the types by microscopic examination, after grinding and polishing one surface of each one (a nondestructive process): the free-cutting material is peppered by sulfur particles readily standing out of the background. Alternatively you may try to weld: the non weldable will crack right away...

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Welding Thin Stainless Steel Sheets
[From PWL #32]

Q: Gas Tungsten Arc Welding thin sheets of austenitic stainless steel type 304L results in unacceptable distortion. What can be done to improve results?

A: Compared to carbon steels, austenitic stainless steels have higher thermal expansion and lower thermal conductivity: these are the main reasons contributing to unacceptable distortion.

GTAW is a proper process to weld such jobs but, due to the low current employed, manual operation may be difficult to control. Better results could be obtained by mechanized welding.

Other precautions include proper fixturing, pulsed current if possible and step welding. Good cleaning and preparation are always important.

Constant current, non pulsed power supplies of drooping-voltage characteristic are used with direct current straight polarity, electrode negative. Pulsed current may provide better weld control to avoid burn through.

Most widely used tungsten type is that with 2% Thoria (EWTh-2). High frequency should be used to start welding and to avoid contamination due to electrode contact with the weld pool. Argon is used as the shielding gas.

The conical electrode tip can be ground with different apex angles. A narrow angle of about 15 to 30 degrees tends to produce a relatively wide bead with shallow penetration. A larger angle of 60 to 75 degrees would give a narrow bead with increased penetration.

Thin gage stainless steel sheet should be properly clamped and aligned to avoid buckling. Fixturing of the abutting edges, with no gap for thickness up to about 1 mm (0.040"), is done with copper chill bars, usually nickel plated.

The backside chill bar includes a groove, placed under the joint, where argon can be provided to prevent backside oxidation. The two front side chill bars should be beveled to make room for the torch.

Good contact between stainless and copper bars helps in removing excessive heat. If the clamp down bars are very close to the line of welding and held with considerable pressure, a compressive force will act on the seam while welding, as lateral expansion is prevented. The upsetting force will reduce shrinkage stresses and distortion.

Tack welding should be provided at close intervals but with a proper sequence that will maintain alignment.

Pulsed current if available may be advantageous in reducing heat input. Current is pulsed at regular intervals between a background level and a peak level. A stable arc is more easily maintained.

In case of long seams, short welding stretches should be performed, at relatively far locations, taking care to let the joint cool down between welds. The start and stop of each weld and the tack welds should be ground to eliminate possible flaws in those places.

Implementing most of the above precautions should result in weldments of reduced and acceptable deformation.

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Stainless Steel Tanks for Water
[From PWL #79]

Q: After 7/8 years of service, some of my SS 304 tanks are corroded at HAZs & pinholes are observed in hydrotesting. I want to weld repair it. Please help.

A: Thank you for your question. Note that the letter does not mention if the tanks keep water or any other more corrosive liquid.

If the material is indeed SS 304, one should have known from the time of manufacturing that the failure is only a question of time. The problem is "sensitization" of 304 as explained in my page
http://www.welding-advisers.com/Welding-stainless.html

A more suitable selection would have been 304L or 321, if indeed the tanks are for water.
Now whatever repair you may do, it will last only so much time, because heating 304 in the interval between 600 and 900 0C (on both sides of the weld) will cause the base metal to become prone to corrosion. Sorry, there is not much to do by welding. Patching up using metal strips adhesive bonded on the leaks or other caulking solutions may be possible.
Next time ask before manufacturing.

Note: - The preferred filler metal for welding 304L is 308L. For welding 321 (which is "titanium stabilized"), the filler metal is 347 ("Niobium stabilized").

While not practical for welded tanks or sheet metal constructions, there is a way to eliminate the chromium carbide precipitation ("sensitization") by performing full solution treatment of 304 small implements (bars or tubes), at 900-1100 0C (1650-2010 0F) followed by rapid quenching in water. This process however must contend with problems of heavy oxide formation if not done in vacuum or protective atmosphere, and of distortions. Therefore it is not practical for large structures.

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Repairing Holes in Aluminum Panels

Q: How can we conduct annealing of 7075-T73 and 2024-T62, wrought and formed sheet respectively, welding some holes up and reheat treating?

A: Forget it. The materials indicated are not weldable by fusion welding. Also heat treatment cannot be performed easily: even with most apt facilities, the deformations following re-solution heat treatment, quenching and aging would most probably cause scrapping of the parts.

The holes should be enlarged and carefully machined to some simple geometric shape (circle, square with rounded corners, etc.). If both sheet sides are reachable one can prepare machined patches. The patches, of the same materials and condition, could be of double the sheet thickness. In the center the selected shape should be machined to stand out in relief, to fill the hole. This shape should be emerging from the rest of the patch, machined to present wide margins of the same thickness as that of the sheets.

The patch so prepared and thoroughly cleaned has to be put in place from the back side, so that the hole is filled flush by the relief shape machined in the patch. The patch can be then resistance welded in place along the line running in the margins at mid distance between the raised shape and the patch border. If the patch has to be leak proof then seam or overlapping spot welding has to be performed, otherwise separate spot welds may be sufficient.

If only the outer side is reachable then a simple sheet patch can cover up the hole and be adhesive joined in place.

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Welding leaded Brass
[From PWL#053 - Section 3]

Q: We are welding a cylindrical brass part(60Cu+37Zn+3Pb with approximatively 2% impurities) with stainless steel pipe, by oxyacetylene flame. The Brass part section is 3.5 mm thick where it is being welded and the rest of the part is 8.5 mm thick.
Some of the pieces are breaking on the 3.5 mm thickness side. Can you explain what is the reason?

A: The stainless steel pipe should be of the weldable type, not prone to sensitization, otherwise it could become easily corroded. See our page on Stainless Steel Welding.

The filler metal and the flux used were not specified but this is not the main thing.

However the fusion process selected is not suitable for the application, because lead (Pb), added for improving machinability, causes the brass to become hot-crack susceptible upon welding. Therefore this alloy should not be fusion welded.

If arrangements can be made to produce overlapping joints with small clearance, brazing could be performed instead of welding, with silver base alloys. See our page on Brazing.

Otherwise substitution of the said copper alloy with unleaded brass should be considered.

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Joining Copper to Steel
[From PWL#049 - Section 3]

Q: - We need to TIG weld 3/8" ODx.035" wall copper (SB-75) tubes to a carbon steel tube sheet. We will use Argon gas for shielding, 2% Thoriated Tungsten, would you recommend a low silver (5%) or a high silver (15%) filler metal or something altogether different. Would this be a Brazing (SB) or would this be a Tig (GTAW) process.

A: - Joints designed for welding have a shape different from those suitable for brazing. The selection is based upon the function of the assembly in service and on the ease of joining, strongly dependent upon the facilities available and on production quantities, which influence production costs.

Please note that ASME SB-75 includes the following materials:

ASME SB-75
Copper UNS No. Type of Copper
C10100 Oxygen-free electronic
C10200 Oxygen-free without residual deoxidants
C10300 Oxygen-free, extra low phosphorus
C10800 Oxygen-free, low phosphorus
C12000 Phosphorus deoxidized, low residual phosphorus
C12200 Phosphorus deoxidized, high residual phosphorus

If the joint design specifies butt welding of tube ends, one can Tig weld the tubes with Filler Metal:
ERCuAl-A2 or ERCu or ERCuNi-3 The arc is directed at the more conductive metal (copper).

If the tubes enter into the tube sheet for a suitable overlapping length, depending on service requirements, with clearance on the diameter of about 0.05-0.13 mm (0.002-005") at brazing temperature, one can select a silver based filler metal and a corresponding flux from quite a large list of available materials. For low production quantities an oxyacetylene flame would be adequate. For mass production furnace brazing in a controlled atmosphere would be probably more economic. Cleanliness is of the utmost importance.

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Welding of Brass Sheets

Q: How should brass sheets be welded together?

A: Although welding of brass sheets is feasible, one should explore the advantages of brazing instead. Brazing is performed at lower temperature with less deformation and, with adequate joint design, can develop considerable load carrying capacity.

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Brazing Fittings

Q: During torch brazing of a fitting onto a steel tube, it was found that the silver alloy filler covered the surface only in part, making the joint unacceptable. How can we improve?

A: First, the clearance between the elements should be correct, between 0.05 and 0.10 mm (0.002" and 0.004") on the side at brazing temperature.

Second, both surfaces must be absolutely clean from dirt, paint, rust and oil or grease even when a flux is used.

Third, rotating gently one element relative to the other for a quarter of a turn to the right (or to the left) and back while the braze is still liquid will most probably improve the results.

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Furnace Brazing of Tubular Joints

Q: What are recommended practices for furnace brazing of tubular joints?

A: Cleanliness is the single most important prerequisite for successful brazing. The use of suitable preforms, that is of prepared units of formed brazing alloy of definite shape and weight for a specific application, may contribute to improve quality and reduce costs.

The selection of the suitable filler metal should be adequate for joint and process requirements. An indication on usability of different silver brazing alloys was given in a note published in section 4 of Issue 003 of Practical Welding Letter for November 2003. Click on PWL#003.

Radial clearance of the capillary space should be calculated to be between 0.050 to 0.125 mm (0.002 to 0.005") at brazing temperature.

The brazing filler metal preform should be possibly located at the recessed side of the joint, so that the free side of it can be visually inspected for uninterrupted presence of the flown brazing alloy, indicative of an adequate brazed joint.

For designing Brazed joints with confidence one should learn the lessons of the following publication.

ANSI/AWS C3.3:2002
Recommended Practices for Design, Manufacture, and Inspection of Critical Brazed Components
American Welding Society, 01-Jan-2002
32 pages
Click to Order.

For an informative list of useful Brazing Resources available online, interested readers are reminded to consult again the Mid July 2006 Bulletin of the Practical Welding Letter at PWL#035B.

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Brazing in Steps

Q: A complex assembly to be furnace brazed requires complicated fixturing. How could the preparation be simplified?

A: By breaking down the assembly in a logical sequence one can divide the brazing operation in two or more simpler stages. One has to select appropriate filler metals in progressively descending order of brazing temperature.

In this way the subsequent furnace brazing temperatures do not impair previous brazements. A secondary gain to be considered is the longer time available for assembling after cleaning operations. This time is limited because harmful oxides form on metals even at room temperature, disturbing wetting and brazing.

Performing the brazement in steps allows preparation in shorter times, contributing to the brazing success. Even brazing repair, should it be necessary, is easier in partial assemblies.

Another way of simplifying fixtures consists in designing self fixturing (or self-jigging) provisions, that is built in temporary means (clamping, crimping, expanding, press fitting etc.) of keeping parts in place until brazing is completed. Brazing filler metal is usually preplaced in the joint or near to it. A capillary clearance at brazing temperature is always required.

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Leak Test

Q: X-Ray inspection of a repair weld on a pipe seems to indicate that porosity is OK, but pressurizing shows moisture around the weld edge. Where does it come from?

A: A suitable form of Leak Testing is recommended, depending on the application. It could be a simple bubble testing, with air or other gas pressurized inside, while the container is submerged under a liquid.

Or using Liquid Penetrant, either before or during hydrostatic tests, as a marker for detecting leaks.

More sensitive specific gas detectors could be applied in exceptional cases.

See Metals Handbook - ASM International Vol. 11 - 8th edition - Nondestructive Inspection and Quality Control
or Vol. 17 - 9th edition - Nondestructive Evaluation and Quality Control

See also our recent page on:
Leak Testing

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Class A Welding

Q: Which welding does Class A refer to?

ANSI/AWS D3.6M:2010
Underwater Welding Code
Edition: 5th
American Welding Society / 10-Sep-2010 / 144 pages
Click to Order.
presents four different Types or Classes:

Type A welds are characterized by requirements ensuring an underwater weld comparable to a surface weld. Usually dry (hyperbaric) underwater welds meet these requirements.

Type B welds refer to wet underwater welds. They define less critical applications with reduced ductility and increased porosity.

Type C welds have even less requirements than type B welds for applications without load bearing functions.

Type O welds present requirements of surface dry welds, as well as those of other Codes and Specifications.

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Hardening Heavy Sections

Q: A bar of high carbon steel (0.70-0.90 %C), of 100 mm dia. (~4"), quenched in oil from elevated temperature, failed to harden. Heated again and quenched in water, its hardness did not improve. Why?

A: A heavy bar of plain high carbon steel cannot develop substantial hardness upon quenching because heat removal is too slow due to its substantial mass.

Sluggish heat removal prevents martensite (the hard constituent) to form and permits less hard structures to appear, during the transformation from austenite. If ~60 HRC hardness is needed, one must switch to air hardening tool steel like SAE A1, where the composition allows slow cooling to produce full hardness. Do not forget tempering.

However, if only surface hardness is required (and the core may remain softer), one can still use plain high carbon steel, by localized intense heating through induction hardening (or possibly laser or flame hardening) and rapid quench.
Tempering is always required.

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Furnace Hardening of Steels

Q: A medium carbon (0.35 %C) steel bar 1" dia. (25 mm), did not develop 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 (decarburization) 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 furnace 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.

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Heat Treating Tool Steel in a Bag

Q: Can occasional Tool Steel Heat Treatment be performed in Air Furnace?

A: No. Heating High Carbon Tool Steel in Air Furnace will cause decarburization (loss of surface carbon) that will substantially reduce the surface hardness obtained upon quenching. Oxidation too is generally objectionable. In fact the higher the carbon content, the most significant the decarburization process will be, and tool steels generally contain high carbon.

For occasional heat treatment of tool steels in an air furnace one can use air tight bags manufactured of stainless steel foil. These are made by multiple folds at the overlapping edges or by seam welding. The tool is introduced in the bag, most of the air is then removed by enveloping the tool as tightly as possible in the bag. A tiny hole must be left in the closed bag, usually by putting in the last fold a fine nail that is later withdrawn, for the residual hot air to find a way out.

The bag will then protect the tool from oxidation and decarburization during heat treatment in the air furnace.

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Normalizing welded Mild Steel
[From PWL#057, Section 3]

Q: - We have been asked to normalize the welding on a unit that is made up of 4 types of mild steel. It is our understanding that normalizing is normal with 4130/4140 material for stress relieve and strength, but that it does nothing for mild steel. Can you verify this? Or do you have any documentation to verify it one way or the other?
Thank you.

A: - Normalizing is a concept used sometimes quite loosely. The official ASM description of the term Normalizing is: "Heating a ferrous alloy to a suitable temperature above the transformation range and then cooling in air to a temperature substantially below the transformation range."

If you mean heating mild steel above transformation point to austenite and cool in air, you only get stress relieving. The same result can be obtained at 650 deg C (1200 F) which is below transformation.

Nothing will be gained at a higher temperature, only more scale to remove. If in the assembly also 4130/4140 are present, depending on their thickness (which reflects the rate of cooling in air) some strength will be gained.

In order to avoid misunderstandings it is recommended to ask the customer for written instructions concerning heating temperature and time, and have him/her pay for the process performed.

To learn more on Normalizing see the following article:
The Importance of Normalizing
from Industrial Heating.

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Grinding a Plate and its Distortion

Q: A hardened steel plate was reasonably flat after heat treatment. However, after grinding from one side as needed, it distorted badly. What should be done to prevent deformation?

A: The hardened plate is subjected to internal stresses in equilibrium, (tension and compression) symmetrically distributed from both sides of the mid plane. By grinding from one side only, equilibrium is disrupted.

The remaining stresses rearrange in such a way that produces distortion. The remedy would be, if possible, to grind symmetrically from both sides.

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Preventing Distortion
[from PWL #66]

Q: I have a question regarding welding angular distortions in butt-welded stainless steel plates. I have done some experiments for plates with dimensions 150*200*2 mm, but something strange has happened. Some of my plates have distorted upward but the others have distorted downward. The welding conditions have been the same for all of them.

A: Distortion is the consequence of residual stresses set in by welding. Especially for manual welding, it is almost impossible to determine if welding conditions were or not exactly the same.

Taking into account that distortion will be always greater in stainless steel than in regular carbon steel, to reduce or prevent distortion in the simple setup described in the question above one should consider the following recommendations.

Use a joint configuration based on welding from both sides (X-joint), requiring less filler material and less heat input, instead of a simpler design welded from one side only (V-joint).

The recommended configuration above is also more symmetrical than the discarded one, and therefore it is likely to introduce more balanced residual tensile stresses.

Reduce as much as possible the root opening and, if applicable, the bevel angle, again to reduce filler material and heat input.

Restrain the plates in a heavy fixture and introduce compressive stresses by peening the joint with a hammer while the weld bead is still hot.

Preset the plates at an angle before welding (open like a book downward, more than needed to have the pages flat in the same plane). Residual stresses will pull the plates back in the same plane after cooling down.

Try to use block and back-step progressions to balance and reduce residual stresses and resulting distortions.

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Straightening a Warped Beam

Q: How can we straighten a warped beam?

A: You can use an oxyacetylene flame. The purpose is to introduce in the beam carefully planned tensile stresses to pull the beam straight. You know from our page on Welding Distortion that residual tensile stresses form because of local thermal expansion. The explanation is as follows.

Stresses are due to volume changes with heating and to decreasing yield strength at elevated temperature. Metal subject to thermal expansion while heated tends to be compressed by the surrounding cool structure. The heated volume has lower yield strength at high temperature, and then it is easily upset to shorter dimensions.

Upon cooling the same material tends to contract in all directions and is now stressed in tension by the attached cool structure which did not move appreciably in the process.

By now the yield strength is again higher, at lower temperature, so that the upset material cannot regain its original dimensions. The result is the development of residual internal tension stresses in the weld.

By selecting the convex portion of each bend and applying sufficient heat (in steel: to bright red) to a suitable amount of material, one can cause sufficient tensile stress to redress the beam. The careful selection of the location of heating is critical, and the amount of applied heat is what determines the success of the operation.

The practice can be repeated for other areas nearby until the result is acceptable. One should take care not to heat the whole beam too much because that interferes with the purpose of straightening.

What works for a beam works also for a surface presenting unwanted bulges. The principle is the same, to introduce two dimensional residual tensile stresses by heating up the convex portion of the deformed plate. A word of caution is necessary when dealing with stainless steel. One should beware of heating in the sensitization temperature interval. (See on this subject Stainless Steel Welding)

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Spot Welding Dissimilar Materials

Q: Can dissimilar materials be spot welded together?

A: The short answer is yes, but it may be neither simple nor advisable in certain cases if brittle structures are produced by the dissimilar metals mixing in the molten nugget.

In particular austenitic stainless steel and carbon steel are not usually spot welded together because the resulting nugget structure risks to be hard and brittle, although it could be studied and modified using the Schaeffler diagram and specially conceived heat treating cycles.

Materials having widely different properties require that a heat balance be achieved by compensation. The more conductive material, electrically and thermally, must be heated more as it provides less resistive heat, and the heat is lost more easily by conduction.

A common technique uses an electrode of smaller face diameter and higher resistivity facing the more conductive material, or by inserting a foil of poorly conductive material between them.

Concerning the number of sheets weldable with a single nugget, normal practice suggests not to exceed three layers, although four sheets are occasionally spot welded together. In any case the ratio of the thickest to the thinnest sheet should not exceed three.

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Projection Welding of Steel Nuts
[From PWL#086, Sect. 3]

Some of the most interesting articles explaining basic welding issues are found quite often in short notes intended to provide answers to practical questions proposed by readers. I would like to give here the summary of such a note full of significant details, published at page 16 of the September 2010 issue of the Welding Journal.

In his note Donald F. Maatz Jr., member of the AWS Detroit Section Executive Committee and, among others, also of the D8 and D8D Automotive Welding Committees, states that, for many different audiences, when it comes to problems regarding Resistance Welding, "projection welding always tops attendee's list of concerns".

The question in cause referred to the lack of published schedules establishing suggested parameters for the projection welding of steel nuts, to be used as a starting specification for the procurement of suitable equipment.

Having discussed the need of a suitable schedule for providing the necessary elements for the correct design of adequate equipment and tooling, the author acknowledges the current lack of a "robust set of welding schedule guidelines for the resistance welding of forged or coined projection weld fasteners".

It appears that the lacking data are part-specific and as such "not readily available to the welding community". The reason is that too many factors must be accounted for. Among them the materials of fastener and base metal, projection geometry, volume and number, substrate thickness, strength and coating.

The author then attempts to provide useful guidelines with the purpose to achieve tentative starting weld schedules to be refined with actual application trials. He suggests that the welding time should be less than what assumed from previous experience with sheet metal projection welding.

Then he remarks that the force required should be higher than what one may think, to assure proper contact and adequate forging pressure once the welding current has stopped.

He further notes that the welding current may need to be much higher than what presumed to be enough, and then the tooling must be able to carry that higher current and its controls must be suitable for fine tuning.

Finally he admonishes that the correct design of the fasteners projection plays an equally important role as the schedule for obtaining quality welds, and he plans to write a future article on this subject.

Interested readers are urged to seek the original article given above, where they can find also how to contact this instructive expert author.

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Welding together Different Carbon Steels
[From PWL #45]

Q: I wish to weld different materials, ASTM A-36 and AISI 1045. The tensile strength is different, AISI A-36 Ts= 40-55 kg/sq.mm and AISI 1045 Ts= min 58 kg/sq.mm.
What kind of filler metal will match both materials? Do we need preheating?

A: It is not the strength level but the carbon content of AISI 1045 that may make problems. Obviously the heat of welding will reduce the strength locally, but if the cooling conditions are such that martensitic structure develops in the heat affected zone then there is risk of cracking. Therefore you should use low carbon filler metal to dilute the carbon content of the weld and, depending on the mass involved and the process selected, you may need preheating, at least locally, to reduce the cooling rate (to avoid hard and brittle martensite) and stress relieving after welding (to temper hard microstructures and to reduce residual stresses).

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Welding Aluminum to Stainless
[From PWL #26]

Q: How is Aluminum welded to Stainless Steel?

A: It must be realized that fusion welding is generally not suitable for welding together dissimilar materials like aluminum and stainless steels. That is because of widely different melting temperatures, no mutual solubility in molten state, and because of differences in thermal conductivity and in thermal expansion that cause stresses and cracks.

During welding, low temperature melting phases and several brittle intermetallic phases are generated that compromise the integrity of the weld. Also not every aluminum type and not every stainless steel type can be considered for being joined together.

However a highly localized fusion welding process of elevated power density like Electron Beam Welding in vacuum may be sometimes used, provided that a third transition metal, compatible with both base metals, is used in between. In the specific case Silver might be used as a transition element, or to bridge the gap.

Solid state welding is applicable in certain combinations, providing acceptable joints can be realized that meet requirements. One of the most used of these processes is friction welding. Cleaning of the surfaces is of the utmost importance because contaminants entrapped in the joint risk to undermine its properties.

For joining large parts a suitable transition hybrid element (part of which is aluminum, the other part being stainless) can be prepared, welded by friction. The ends of the transition element can then be welded to the main structure parts by more conventional procedures between similar base metals.

Besides that, if alternative solution can be considered, brazing or adhesive bonding, if appropriate, are applicable.

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Welding Steel to Aluminum
[From PWL #40]

Q: How is Steel welded to Aluminum?

A: A similar question was formulated involving Stainless Steel.

One should remember that the same obstacles already mentioned there hinder the proper formation of successful fusion welding of such dissimilar metals. These are: widely different melting temperatures, no mutual solubility in molten state, discrepancy in thermal conductivity and in thermal expansion that cause stresses and cracks.

Furthermore during fusion welding, but also during heating to some low temperature like 200 0C (400 0F) melting phases and several brittle intermetallic phases are generated that compromise the integrity of the weld.

If confronted with a similar problem, short of selecting a different material for one of the components, so that the combination be more favorable, one should explore which alternative process is suitable for the application.

Solid state processes like Explosion-, Friction-, Magnetic Pulse-, Ultrasonic-welding, Roll bonding and High Temperature Diffusion joining avoid fusion by definition. Obviously not all of them can be suitable for a given application, because of the specific limitations of each one of them.

Other than those, High Energy like Electron- and Laser-beam welding could sometimes be applied as they are able to concentrate their energy in a very tiny spot limiting their influence in heat, location and time duration.

Finally, if the joint configuration can be adapted to process requirements, brazing, soldering or adhesive bonding might provide a suitable solution.

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Welding Copper to Stainless
[From PWL #27]

Q: Can Copper be welded to Austenitic Stainless Steel?

A: Copper is readily welded to austenitic stainless steels with the Gas Tungsten Arc Welding (GTAW) by using suitable filler metals like ERCuAl-A2 or ERCu-Ni3. Welding is usually limited to thin sections, less than 3mm (0.13 in).

Buttering is not needed, especially for thin sections. The heat has to be addressed to the higher conductivity metal (Copper).
Preheating at 540 0C (1000 0F) may be used to reduce thermal stresses on the finished weldment.

For Gas Metal Arc Welding, preferred for heavier thicknesses, the filler metals are the same as above, but one may wish to perform buttering by braze welding the stainless side to reduce dilution.

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Welding Copper to Stainless (B)
[From PWL #56]

Q: I would like to have information on how to effectively weld stainless to copper. In particular the pieces have cylindrical shapes. The 2 parts to be joined are cylindrical (equal dimensions) ranging from 1 to 10 mm. Materials are copper and stainless (different grades).

A: It would be inappropriate to give a general answer. Depending on the actual shape of the joints, on the exact materials and on the application there are possibly a few techniques suitable to do the job. If you mean end to end joining I would start by trying friction welding: except for the smallest sizes where there might be some difficulty, for the others it should be OK.

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Welding Copper to Stainless (C)
[From PWL #79 (4)]

In certain cases, see the report in PWL#079, cracks can be formed by Copper Contamination Cracking, a known cause of failure due to penetration of molten copper into the grain boundaries. Therefore it is suggested to perform preliminary trials to avoid the conditions leading to CCC.

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Welding Copper to Aluminum
[From PWL #64]

Q: How can one weld Copper to Aluminum?

A: Copper and Aluminum are incompatible materials that cannot be fusion welded together. They can however be welded by solid state processes that do not heat the materials to melting temperatures.

Among these processes are friction welding, friction stir welding, magnetic pulse welding, ultrasonic welding, cold welding, explosion welding.

After having prepared bimetal transition parts, welded by a suitable solid state process, one can proceed with regular welding or brazing of additional components by matching the materials by type (copper to the copper side of the transition element, aluminum to the aluminum side).

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GMAW with straight CO2 gas
[From PWL #76]

Q: - I see a lot of GMAW being used around the place but they are using a straight CO2 gas. In my experience, we always used a CO2/argon mix gas which not only produced a better weld, had less spatter also.

There also was some discussions with my American colleague who mentioned in the States they will not allow the use of GMAW with only CO2 (carbon dioxide) Gas. Apparently there were some failures resulting from this process.

Is there anything to back this theory up and have you heard anything similar?

A: - The use of CO2 is not forbidden as far as I know, so that failures, if there were any, may be due to other causes too. Gas selection is probably also a question of cost and of ease of supply, especially in developing countries.

You are right about spatter and quality, but if Welding Procedure Specifications (WPS) were approved according to requirements of applicable Codes there is nothing wrong in using straight CO2.

A thorough presentation and discussion of Shielding Gases for GMAW can be found at page 64 of ASM Handbook Volume 6 that is a fundamental reference book for anyone involved in welding.

See Welding Books.

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Welding Aluminum Bronze to Mild Steel
[From PWL #41]

Q: - Can we weld Aluminum Bronze to Mild Steel A36? If yes what should be the electrodes?
Note: This is an actual question sent to us by one of our readers.

A: - Yes, Aluminum Bronzes are weldable to carbon steels using SMAW (Shielded Metal Arc Welding), GTAW (Gas Tungsten Arc Welding)(with Alternating Current stabilized by High Frequency) and GMAW (Gas Metal Arc Welding)(with Direct Current Electrode Positive).

One should note that aluminum in these bronzes forms tenacious oxides that must be removed before welding. This is probably the most pressing concern that should worry whoever considers to perform this welding. Shielding gas, or fluxing by electrode cover are used for preventing their formation while welding.

For the first process, electrodes ECuAl-A2 can be used with preheat from 150 to 200 0C (300 to 390 0F) for sections thicker than 6 mm (1/4").

ANSI/AWS A5.6/A5.6M:2008
Specification for Copper and Copper-Alloy Electrodes for Shielded Metal Arc Welding
Edition: 9th
American Welding Society, 06-Nov-2007, 38 pages
Click to Order.

For the other processes above, rods or wires ERCuAl-A2 are used. For repair welding of aluminum bronze casting with highly stressed cross sections, ERCuAl-A3 may be preferred because it has less tendency to crack.

ANSI/AWS A5.7/A5.7M-2007
Specification for Copper and Copper-Alloy Bare Welding Rods and Electrodes
American Welding Society, 12-Apr-2007, 32 pages
Click to Order.

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