how to relieve stress in welding

8/4/2019 How to Relieve Stress in Welding

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How to relieve stress in weldingBack to basics on stress relief and reducing distortion

ByTim Heston

November 2, 2009

Relieving residual stress through welding technique as well as temperature control can greatlyreduce weld distortion.

It's a shame arc welding works so well. It's proven, cost-effective. For many applications, nothingcomes close, at least not yet. Why is it a shame? Because at the microlevel arc welding inducessome serious stress, thanks to dramatic temperature changes measured in thousands of degrees.

The welding gun deposits filler metal that becomes molten and expands from its previously cool stateas wire or rod. Immediately after being deposited and subsequent fusion between the base and weldmetal, the metal cools quickly. The high-yield-strength weld filler metal contracts, or shrinks, pullingthe lower-yield base metal with it. Clamped tight, the metal may stay in place until after welding, butthis doesn't make the contracting force go away. The cooled weld metal still wants to shrink. When

the metal is unclamped, the weld metal pulls at the base metal, and the weld distorts. The degree towhich this occurs depends on the weld joint geometry, part design, and material grade and thickness.Generally, the higher the metal's carbon content, and the more restrained a joint is, the greater thestress.

Of course, the metallurgical picture is much more complicated, but that's the basic idea.

Industry has numerous ways to reduce such weld stress. Any method must accomplish at least one oftwo things: control temperature and refine the welding procedure, both of which counteract thoseunavoidable forces that come from fusing two metals together with an electric arc.

For this month's "How To" feature, The FABRICATOR spoke with three experts. For heating andwelding technique, we spoke with Carl Smith, longtime quality manager and welding technician atKanawha Manufacturing Co. We also spoke with two experts about some nontraditional stress relief

technologies: Tom Hebel, vice president of Bonal Technologies, and Bill Kashin, territory manager forBolttech Mannings.

1. Refine the Welding Procedure

Setup; electrode selection; along with weld type (fillet, groove, butt, etc.), size, and orientation allaffect how a weld joint reacts to stress.

Prebending or presetting. The base metal can be set up in such a way to compensate for weldshrinkage. For example, when two workpieces are preset with one end of the joint together and thefar end of the joint slightly apart, the cooling weld metal pulls the two workpieces until, by the end ofthe weld, the joint is in the proper orientation.

Balance the weld. Double-sided welds, such as double V-groove joints, balance induced stresses andoften result in an assembly that's more stable. "This is especially true on thicker material," said Smith."Two half-inch welds on either side of a 1-inch plate balances the weld and minimizes distortion."
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Backstepping. Backstepping is a bit like moonwalking with a welding gun. You start several inchesfrom the beginning of the joint and weld back to the edge; then go farther up the joint and weld backto where you initially struck your previous arc; then go farther up the joint and again weld back to theprevious welded segment; and so on until the joint is complete. This counteracts shrinkage byfocusing the initial stresses away from the workpiece edges.

Intermittent welding. When intermittent or stitch welding meets the design requirements, it not onlyhelps reduce distortion, but also uses less weld metal.

Consumables. In wire welding, "you can make a 0.035-inch wire lay down just as much as a 0.045-inch wire," Smith said. "You can just crank the wire feed speed." He added that lower heat inputrequired to melt the smaller wire outweighs any heat reduction benefit that might occur with a fastertravel speed using a larger wire.

Weld metal: More isn't better. Codes spell out specific weld size requirements, including the maximumallowable height of the bead above the plate. The trick is to lay just enough weld metal to create thestrongest jointand no more. A highly convex bead doesn't make a weld stronger, but it doesincrease shrinkage forces, because more high-tensile weld metal is pulling on the base metal as theweld cools.

Here, technique factors in. "A multipass weld with stringer beads will create less distortion than aweave bead," Smith said.

The stringer bead technique generally allows faster travel speeds, which lowers heat input. Each passof the gun lays down less weld metal, which in turn helps control the weld size better.

Welders usually weave only as a last resort, Smith said. "The cover pass on a weave bead can lookbetter than a stringer bead, but if a welder knows what he's doing and places his stringer beadproperly, he can make it look just as good as a weave bead."

Exceptions abound, of course. Pipeline welders often weave downhill, but the beveled opening in apipeline is usually much smaller than on conventional plate. And "round pieces do not distort nearly asbadly as flat pieces anyway," Smith said.

Still, when it comes to controlling distortion, stringer beads usually are best. "Each bead has its own

level of stress," Smith explained. "The wider the bead, the more stress you're going to put into theweld, so you're going to have more 'pull,' more distortion than a smaller bead."

Fit-up: Small root is best. Solidifying weld metal pulls the base metal, and that effect is exacerbatedwith an excessively wide root opening, especially in large weldments and in areas of poor fit-up."Some situations don't work with a tight root," Smith said, "but usually, with today's welding machines,you can get by with a 1/16-inch root opening" in many applications.

Weld from most restrained to least restrained area. This follows similar principles to that ofprebending and presetting, Smith said. Consider a frame with a crosspiece going down the center.The crosspiece, surrounded by the frame, is the most restrained of all pieces in the assembly. So thiscrosspiece should be welded first. The centerpiece, if welded first, is less restricted by thesurrounding metal and has freedom to move and expel residual stress before you go on to weld the

frame.2. Control Temperature

Preheating, maintaining temperature between weld passes (interpass temperature), and postweldheat treating (PWHT) work toward one goal: to control changes in heat levels. The more control youhave over heat, the more you can counteract stress, and the less chance there is for weld distortion,especially in highly restrained joints. When you slow the cooling rate, you reduce shrinkage stressesand provide more time for hydrogen to dissipate, reducing the chance for under-bead cracking.

Material factors. Predicting necessary minimum preheats, interpass temperature, and PWHT dependson the application and how restrained the joint in question is. Specific material properties affect howdrastically metal will distort. These include the coefficient of thermal expansion (how much the metalexpands when heated), thermal conductivity (how fast it dissipates heat), yield strength, and modulus

of elasticity (material stiffness).


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As a starting point, refer to the AWS D 1.1 structural welding code, Welding Handbook, guidelinespublished by the steelmaker, and other sources for recommended minimum preheat and interpasstemperatures for specific alloys. Generally, higher carbon content equates to higher minimum preheatand interpass temperatures.

Most preheating, interpass heating, and PWHT do not require maintaining a precise temperature, aslong as you maintain a minimum temperature. There are exceptions, though, including quenched andtempered steels. These come to the welding station already heat-treated by the steelmaker, sopreheating at a too-high temperature can destroy the material properties; in other words, quenchedand tempered steel will no longer be tempered. "For instance," Smith said, "the ASTM A514 and A517alloys should never be preheated to more than 150 degrees F above the recommended [minimum]preheat."

Stainless steels can be particularly touchy. "We keep interpass temperatures below 350 degrees F,"Smith explained. "We use distilled water in a spray can. Water on carbon steel causes it to crack. Butit has no effect on stainless steel, as long as you use distilled water, which doesn't have any chlorinein it." Stainless's nickel and chromium content make the metal particularly sensitive to distortion,because the elements don't dissipate heat quickly.

As a rule, metals that dissipate heat quickly require higher preheats. Heat-treatable aluminum alloys

can be preheated to 300 to 400 degrees F as an extra precaution against cracking and, mostimportant, to dissipate hydrogen. Aluminum oxide on the base and weld metals attracts moisture,which introduces hydrogen (the H in H2O). Because aluminum dissipates heat rapidly, hydrogenbecomes trapped as the weld metal quickly cools. The slow cooling created thanks to the preheatgives time for that hydrogen to bake out of the weld. "This is why a welder may often say he's 'boilingthe water' out of the material," Smith said.

High-alloy materials such as chrome-moly also dissipate heat quickly and generally require highpreheat temperatures. Preheating even the tack welds often is best practice, Smith said. Cracks canstart in the tack and "come right through the weld and all the way to the top." He added that certainchrome-moly applications require preheats of about 400 degrees F and a postweld holdingtemperature of about 600 degrees F prior to stress relieving.

Copper, which dissipates heat extremely quickly, requires a very high preheat "just to allow the

welding filler metal to flow into the joint and form a good bond," Smith said. Copper more than 1 in.thick may require preheats up to 1,200 degrees F. (See Streamline Stress Relief section for ways toapply such high preheats directly to the workpiece, without an oven.)

Coffee break effects: Keep it hot. Imagine you preheat a joint with a torch, weld a few feet, stop, takea short break, and then resume without picking up the preheat torch and heating the joint area again.To minimize distortion, you should pick up the preheat torch again to bring that material back up to therequired interpass temperature. "You need to maintain the interpass temperature throughout theweld," Smith explained, adding that heat cycling is especially dangerous with chrome-moly andquenched and tempered materials.

Torch preheating. When preheating with a torch, "we recommend 6 inches on either side of the weld"for large workpieces, Smith said, adding that the width of the applied preheat and specific method

used depends on the workpiece material and geometry.Torch styles vary, but Smith's welders use a multiflame torch with a swirl tip and propylene gas. "Thepropylene gas is not as highly concentrated as acetylene," he said, "and we don't want to concentratethe heat while we're preheating."

PWHT doesn't replace preheat. Postweld heat treatment and preheat complement each other,explained Smith, but they don't replace one another. It's true that in some cases localized preheat canserve as a PWHT substitute when moving the workpiece to an oven for PWHT isn't practical (thinkoffshore oil rigs). PWHT doesn't function as a preheat substitute because it does nothing to reducethe stresses that occur just after you strike an arc on cold, unpreheated base metal. By the timePWHT is applied, it's too late to correct the problem.

3. Streamline Stress Relief

"Over the years welders have perfected techniques to relieve stress and minimize distortion:preheating in an oven or with a torch, using heat blankets, and when necessary sending parts to an


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oven for postweld heat treatment. Note one common thread among all these methods: time. Butcertain technologies take alternative approaches that streamline the operation and even improve weldquality.

Various alternatives are available, including induction-heating methods. Here, we discuss two options:resistance heating and vibration.

Resistance heat control. A resistance heating pad incorporates resistance heating elements that canraise the workpiece temperature to the appropriate level before, during, and after welding, to complywith standard preheat, interpass, and PWHT practices (seeFigure 1andFigure 2). The padincorporates interlocking beads woven together using a high-resistance wire. The unit can heat up to1,850 degrees F. (Smith's company has used this technology to preheat thick copper plate to morethan 1,000 degrees F.)

A temperature controller uses a system of thermocouples spot welded to the part to read the actualmetal temperature, which is monitored throughout the operation. Welders don't have to usetemperature crayons to measure the preheat temperature. The pad also doesn't have to be removedduring welding.

As Bill Kashin of Bolttech Mannings explained, "Say you're welding two pieces of pipe together, and

the code says you need to preheat it to 400 degrees F. You would attach the thermocouple, attachthe heating pad, put insulation on to protect yourself, and raise the temperature up to 400 degrees F.When the heater gets to that temperature, it will cycle on and off to hold that temperature until you'refinished welding."

Readings from the machine also can be saved as a record of the part's temperature before, during,and after welding, helpful for code-level or insurance-related work, such as repair jobs at powerplants.

The pads are designed to wrap around the workpiece, with a piece of removable insulation over thejoint. For preheat, the entire workpiece is covered. You then remove the insulation from the weld jointarea and start welding. When you take a break, you put the insulation back over the joint to helpmaintain the preheat temperature. The heater pads can then be added to the weld area for stressrelief, eliminating the need to transfer the part to a furnace for PWHT.

Vibratory stress relief. Another technique uses something that doesn't seem to be related, but it is:vibration (seeFigure 3).

"Heat is vibration, according to physics," said Tom Hebel of Bonal Technologies. The more somethingis heated, the faster its molecules vibrate. "We induce a vibration into the part, and the part respondsas if it has the same internal action when the part is heated up for heat treatment. It's a cool process,but internally, there's movement."

If you vibrate metal at a certain frequency during welding, it complements the weld heat that vibratesthe molten metal at the molecular level. It's roughly analogous to shaking a can of dissimilar-shapedbeads or a vibratory bowl feeder in a stamping operation, which gets everything to settle and "packdown." The vibration level, Hebel said, is very specific: in the lower, or sub-harmonic, portion of theharmonic curve, just before the amplitude quickly rises and reaches the part's natural resonance.

The device induces vibration into the workpiece and monitors the workpiece's reaction. The morevibration that's put into the part, the more it will absorbup to a point. "At a certain point anyadditional energy will cause the workpiece to throw off the energy," he said.

The trick, Hebel explained, is to induce a vibration frequency that's at a specific point below itsresonance point. It's here that the vibration has the greatest dampening effect, at which point itneutralizes the stress induced by the weld's heat.

Most commonly, the vibratory device is applied after welding to relieve stress, essentially replacingPWHT. But it also can be applied during welding to improve weld quality through grain refinement andstress reduction. In fact, applying the right vibration during welding can eliminate the need for PWHTcompletely, unless tempering of the heat-affected zone is required.

"When you weld you induce thermal stress," Hebel said. "So when you weld-condition [using sub-harmonic vibration during welding], you're eliminating the effect of thermal stress as it's induced. So
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after welding, if the effects of thermal stress aren't there, why send the part to a furnace for stressrelieving""

In certain applications, Hebel said, it can replace low-temperature preheating requirements, between250 and 300 degrees F. "Because of the accelerated motion in the base material, the weldment'thinks' it's preheated." Usually, though, the vibratory weld conditioning complements existing preheatprocedures to increase weld quality.

Hebel compares a large steel part with welding-induced stress to an out-of-tune instrument. Afterwelding, temperature drops sharply. At this point within and around the heat-affected zone, the part'snatural harmonic curve shifts slightly, "out of tune" with the rest of the assembly. Counteracting thateffect with induced vibration during and after welding relieves stress as evidenced by the harmoniccurve moving back "in tune" with the rest of the assembly.

A review of common nondestructive testsAssessing each process, its tools, advantages, and disadvantages

ByMark Willcox,George Downes

June 13, 2006

Five types of nondestructive testing are common for tube and pipe weld inspection, and each hasadvantages and disadvantages that may make one more suitable than another for your

inspections.

Nondestructive testing is one quality control functionand complements other, long-established methods.

By definition, nondestructive testing is the testing ofmaterials for surface or internal flaws or metallurgicalcondition without interfering in any way with theintegrity of the material or its suitability for service.

The technique can be applied on a sampling basis forindividual investigation or may be used for 100 percentchecking of material in a production quality controlsystem.

Five nondestructive testing methods are most common, and each has advantages and disadvantagesthat will determine whether it is suitable for your particular testing application. These techniques are:

1. Radiography

2. Magnetic particle crack detection

3. Dye penetrant testing

4. Ultrasonic flaw detection

5. Eddy current and electromagnetic testing

Radiography

Basics. Radiographic testing can detect internal defects in ferrous and nonferrous metals. X-rays,generated electrically, and gamma rays emitted from radioactive isotopes penetrate radiation that is

absorbed by the material they pass through. The greater the material thickness, the greater the ray
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absorption. These rays help form a latent image that can be developed and fixed in a similar way tonormal photographic film.

Tools. Various radiographic and photographic accessories are necessary, including radiationmonitors, film markers, image quality indicators, and darkroom equipment. Radiographic film andprocessing chemicals also are required.

Advantages. In radiographic testing, information is presented pictorially. A permanent record isprovided, which can be viewed at a time and place distant from the test. This type of testing is usefulfor thin sections and is suitable for any material. Sensitivity is declared on each film.

Disadvantages. Radiography is not suitable for several types of testing situations. For example,radiography is inappropriate for surface defects and for automation, unless the system incorporatesfluoroscopy with an image intensifier or other electronic aids. Radiography generally can't cope withthick sections, and the testing itself can pose a possible health hazard. Film processing and viewingfacilities are necessary, as is an exposure compound. With this method, the beam needs to bedirected accurately for 2-D defects. Also, radiographic testing does not indicate the depth of a defectbelow the surface.

Magnetic Particle Inspection

Basics. Magnetic particle inspection can detect surface and near-surface discontinuities in magneticmaterial, mainly ferritic steel and iron. The principle is to generate magnetic flux in the article to beexamined, with the flux lines running along the surface at right angles to the suspected defect. Wherethe flux lines approach a discontinuity, they will stray out into the air at the mouth of the crack. Thecrack edge becomes magnetic attractive poles, north and south. These have the power to attractfinely divided particles of magnetic material, such as iron fillings. Usually these particles are an ironoxide 20 to 30 microns in size. They are suspended in a liquid that provides mobility for them on thesurface of the test piece, assisting their migration to the crack edges. However, in some instancesthey can be applied in a dry powder form.

Tools. Basically, magnetic crack detection equipment takes two forms. First, for test pieces that arepart of a large structure, or for pipes and heavy castings, for example, that can't be moved easily, theequipment takes the form of just a power pack to generate a high current. For factory applications on

smaller, more manageable test pieces, bench-type equipment

with a power pack, an indicating inksystem that recirculates the fluid, and facilities to grip the workpiece and apply the current flow ormagnetic flux flow in a methodical, controlled manner generally is preferred.

Advantages. Magnetic particle inspection generally is simple to operate and apply. This testing isquantitative, and it can be automated, apart from viewing. However, modern developments inautomatic defect recognition can be used in parts with simple geometries, such as billets and bars. Inthis case, a special camera captures the defect indication image and processes it for further displayand action.

Disadvantages. This type of nondestructive testing is restricted to ferromagnetic materials, as well asto surface or near-surface flaws. Magnetic particle inspection is not fail-safe; lack of indication canmean that no defects exist, or that the process wasn't carried out properly.

Dye Penetrant Testing

Basics. Dye penetrant testing is used frequently to detect surface-breaking flaws in nonferromagneticmaterials. The part to be tested must be cleaned chemically, usually by vapor phase, to remove alltraces of foreign material, grease, dirt, and other contaminants from the surface, generally, but alsofrom within the cracks. Next, the penetrant, which is a fine, thin oil usually dyed bright red or ultravioletfluorescent, is applied and allowed to remain in contact with the surface for about 15 minutes.Capillary action draws the penetrant into the crack during this period. The surplus penetrant on thesurface then is removed completely, and a thin coating of powdered chalk is applied. After theappropriate development time, the chalk draws the dye out of the crack to form a visual indication,magnified in width, in contrast to the background.

Tools. Various substances can be used and may be applied in many ways, from simple applicationwith aerosol spray cans to more sophisticated means, such as dipping in large tanks on an automaticbasis. More sophisticated methods require tanks, spraying, and drying equipment.


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Advantages. A quantitative analysis, dye penetrant testing is simple to do and is a good way todetect surface-breaking cracks in nonferrous metals. It's suitable for automatic testing, but with thesame limitations that apply to automatic defect recognition in magnetic particle inspection.

Disadvantages. Dye penetrant testing is restricted to surface-breaking defects only. It is lesssensitive than some other methods and uses a considerable amount of consumables.

Ultrasonic Flaw Detection

Basics. This technique detects internal and surface (particularly distant-surface)defects in sound-conducting materials. A short pulse of ultrasound is generated by means of an electric charge appliedto a piezoelectric crystal, which vibrates for a very short period at a frequency related to the thicknessof the crystal. This pulse takes a finite time to travel through the material to the interface and to bereflected back to the probe. Probing all faces of a test piece reveals the 3-D defect, measures itsdepth, and determines its size.

Tools. Modern ultrasonic flaw detectors are fully solid-state, can be battery-powered, and generallyare built to withstand work site conditions. The process can be automated and now is used in manyfoundries.

Advantages. Ultrasonic flaw detection can be used to test thickness and length up to 30 feet. This

type of testing can determine defect position, size, and type. It's a portable type of testing that offersextreme sensitivity when required and can be fully automated. Access to only one side is necessaryfor testing, and no consumables are used.

Disadvantages. No permanent record is available unless one of the more sophisticated test resultsand data collection systems is used. The operator can decide whether or not the test piece isdefective while the test is in progress. Test indications require interpretation, except for digital wallthickness gauges. A considerable degree of skill is necessary to get the most information from thetest. Finally, very thin sections can be difficult to test with this method.

Eddy Current Testing

Basics. The eddy current technique can detect surface or subsurface flaws and measure conductivityand coating thickness. This testing is sensitive to a test piece's material conductivity, permeability,and dimensions. For surface testing for cracks in single or complex-shaped components, coils with asingle ferrite-cored winding normally are used. The probe is placed on the component and "balanced"by use of the electronic unit controls. As the probe is scanned across the surface of the component,cracks are detected.

Tools. Most eddy current electronics have a phase display that allows the operator to identify defectconditions. Some units can inspect a product simultaneously at two or more different test frequencies.These units allow specific, unwanted effects to be electronically canceled to give improved defectdetection. Most automated systems are for components with simple geometries.

Advantages. Suitable for automation, eddy current testing can determine a range of conditions of theconducting material, such as defects, composition, hardness, conductivity, and permeability.Information can be provided in simple terms, often go or no-go. Phase display electronic units can be

used to obtain greater product information. Compact, portable testing units are available, and this typeof testing doesn't require consumables, except for probes, which sometimes can be repaired. Thistechnique is flexible because of the many probes and test frequencies that can be used for differentapplications.

Disadvantages. Many parameters can affect the eddy current responses. This means that the signalfrom a desired material characteristic (for example, a crack) may be masked by an unwantedparameter, such as hardness change. Careful probe and electronics selection is necessary in someapplications. Also, tests generally are restricted to surface-breaking conditions and slightly subsurfaceflaws.


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Weld inspection before you weldUsing procedure qualification testing to standardize welding processes

ByPaul Cameron

April 11, 2006

Although it takes effort and time, procedure qualification testing can help you standardize yourwelding procedures and know what to expect when it comes to the quality of your manufacturedparts.

While patrolling a shop floor playing "parameter police," awelding inspector may commonly hear questions like "Why can'tI run my machine above XXX wire feed speed?" or "XX volts?"

Welding parameters aren't guidelines merely plucked out of thinair; they are developed and determined after much trial anderror. By standardizing the welding procedures you use tomanufacture your products, you'll have a model that everyonecan turn to for quality assurance.

Procedure Qualification Options

Procedure qualification can be performed in one of three mainways:

1. Prequalified Joint Procedures. As the name suggests,prequalified procedures have been tested in advance.Although they're convenient to follow, requirementsstill must be met. For example, one requirement thatoften is overlooked or misunderstood is that theprocedure must be written. Just pointing to the "goodbook" isn't nearly enough. Written requirements arelaid out clearly in the applicable code or specification.

For this type of procedure qualification, the American Welding Society (AWS) hasdetermined that, within a given set of circumstances, additional testing is not required.

2. Prototype Testing. Although initially economical, prototype testing can be limiting becauseonly those conditions that are tested can be qualified. Any changes require additionaltesting, which can change the economics of prototype testing significantly.

For example, off-road, agriculture, and construction equipment manufacturers often qualifya process through "push" testingbuilding a structure, documenting the entire fabricationprocess (joint by joint), and submitting the structure to several destructive tests. When thestructure survives the test requirements, the procedure is qualified. As the component goesinto production, all conditions used in the initial test must be maintained during fabrication.

Significant changes in production can require additional testing.

3. Procedure Qualification Testing. Procedure qualification testing initially can be costly andtime-consuming, but it can be used to develop standard weld procedures that cover alljoints, consumables, and positions (conditions) used in production.

Procedure qualification testing is a test or series of tests that are performed, documented ona procedure qualification report (PQR), and then turned into a weld procedure specification(WPS) or a series of them.

Procedure Qualification Testing: The Basics

In procedure qualification testing, you may find it helpful to try to complete all testing using readilyavailable resources. Completing testing on one groove weld typically qualifies all groove types and

fillets.

A welder conducts a GMAWfillet weld test.
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Changes in essential variables, however, often require additional testing. For example, in the off-road,agriculture, and construction equipment manufacturing industry, customers often require fillet welds toobtain penetration beyond the root, typically 1.5 millimeters. Many codes and standards requirepenetration to the root, "... but not necessarily beyond ... ." These same books also may saysomething like "... joint penetration ... beyond the root ... determined from a significant number ofcross-sectioned samples ... ." With an additional customer requirement such as this, you may need to

complete both groove and fillet weld testing when creating a standard WPS.

Standard weld procedure testing requires the following samples:

One test plate for each position

One test plate for each process

One test plate per wire type and diameter

But if your customer requires additional fillet weld testing, you must complete the following:

One test plate per position (as per standard weld procedure)

Two test plates per fillet size (one single pass, one multipass)

More Customer Requirements: An Example

Crenlo LLC, a cab and canopy manufacturer based in Rochester, Minn., supplies to off-road,agriculture, and construction equipment manufacturers. The company welds grooves in the flat (1G)and horizontal (2G) positions, and fillets in the flat (1F), horizontal (2F), and overhead (4F) positions.The shop runs on a bulk 90 percent argon/10 percent CO2 mixture and typically uses a 0.035-inch-diameter ER70S-6 filler metal. After consulting a wire manufacturer and other industry professionals,Crenlo engineers determined that a 550-in.-per-minute (IPM) wire feed speed (WFS) and 27 voltsshould yield the best results.

Let's say you want to develop a weld procedure for gas metal arc welding (GMAW) high-strength, low-alloy (HSLA) carbon steel. You need to develop this procedure for both groove and fillet welds.

First you should know what a test sample will look like and how many samples you're going to need.

Figure 1

For the groove welds, use one test sample for the flat (1G) position and one for the horizontal (2G)position (see Figure 1).

Figure 2

For fillet welds, you'll need samples in the flat (1F), horizontal (2F), and overhead (4F) positions. You'llalso need a sample for each weld size, and on that sample you'll weld a single-pass fillet on one side


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and a multipass fillet (if applicable) on the other. In this case, you'll want to qualify 5- through 13-mmfillets (see Figure 2).

So that's a total of two grooves and 24 fillets26 tests. What if you want to qualify a higher WFS?According to most codes and standards, 550 IPM qualifies in the range between 495 and 605 IPM. Ifyou want to run a WFS at 650 IPM or more, you'll need to do another 26 test samples. The sameapplies if you want to use a 0.045-in.-dia. wire, a cored wire, or change another variable

Standard weld procedures are fairly labor-intensive, but the finished WPS will be out of the way andready to be applied across the board whenever you encountersimilar essential variables.

Performing Qualifications

In procedure qualification testing, you'll encounter two main typesof qualifications: fillet weld and groove weld.

Fillet Weld Qualification. Fillet weld qualification is prettystraightforward (see Figure 3). For each weld size, make a single-pass fillet weld on one side of the test plate and a multi-pass filleton the opposite side (see lead photo).

Then simply cut and etch the sample as required in your code orstandard and document the results with a digital camera. Usecaution when dealing with etching solutions. Strictly follow theinstructions and safety requirements laid out by your code or standard.

Groove Weld Qualification. As previously mentioned, it may be beneficial to use materials readilyavailable in your shop for your testing. For example, if your shop uses a lot of tubing, you can usesections of 4-in. by 4-in. by 3/8-in. tubing for your groove testing (see Figure 4). The tubes, laid sideby side, can create a good flare V groove, and you won't need to bevel the plates or fabricateseparate backing (see Figure 5).

If you use tubing, watch where you place the tubing's weldedseam. You don't want it to be located in such a way that it will

influence your bend and pull tests. If you place the seamsface-to-face, you'll be sure of their location and know that you'llcut them out later.

After you've tacked your samples together, always, always,always mark each coupon with a steel stamp. For example,you can use a two-digit number for the sample and 1 through 6for the individual coupon, increasing in number in the directionof the weld (23-1, 23-2, 23-3, and so on) (see Figure 6). Thesewill be cut from the finished weld later.

Next collect and log all required data per pass, by sample:preheat and interpass temperatures, WFS, voltage, travel

speed, electrode stick-out, everything. This system will helpyou if you're left with a pile of bent and broken coupons andhave to figure out what went wrong. That's no time to wonder ifyou have the right coupon.

Figure 3A T joint is used for fillet

welding.

Figure 4This 4-in. by 4-in. by 3/8-in.tubing is used to weld a flare

V-groove test plate in theoverhead (4G) position.


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Once you finish welding, perform the first required test: visualexamination. All test samples must meet visual acceptancecriteria first. If any test sample (fillet or groove) is not visuallyacceptable, you must discard it. Figure out why it wasn't visuallyacceptable, correct your process, and make another testsample. Never continue testing on a sample that isn't

considered visually acceptable.

Your code or standard may require radiographic evaluation. Ifso, perform this next. Remove all portions of the tube that don'tmake up the test sample, clean them up, and ship them off (seeFigure 7). If you use tubing, this can require a lot of whittling hopefully you're good with an oxyfuel torch. As with visualevaluation, if your test samples don't pass radiographicevaluation, don't take additional action on the test sample.Figure out what went wrong and make a new test plate.

Figure 6Mark each test sample's coupons

with a steel stamp so you can traceeach coupon back to its

corresponding test.

Figure 7This test sample has been

removed from the tubing and isready to be cut with a saw.

After you successfully complete these tests, you'll need to start cutting your individual coupons. Yourcode or specification will dictate exact coupon size and location. Typically, you'll need four couponsfor bending and two for tensile testing. Next, move on to etch testing.

Bend and tensile testing comes next. Bend testing equipment is fairly inexpensive and often can befabricated in-house; documentation is on the market for the equipment's dimensional requirements.Tensile testing equipment isn't as economical, but many companies can perform this type of testing.

Finally, collect and document all test data on a procedure qualification report (PQR) and develop yourWPS per your code or standard. Educate your inspectors, supervisors, and welders about the allowedparameters and the need to stay within them.

Although many steps are necessary to complete procedure qualification testing, this process will helpensure that your WPS meets the requirements of the codes and standards you create product to andwill serve your company and customers for years to come.

Figure 5This 4-in. by 4-in. by 3/8-in.

tubing is used to weld a flareV-groove test plate in theoverhead (4G) position.


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What welding inspectors should know about welding codesand standardsWhat they are, when they're used, and how they're developed

ByTony Anderson

January 24, 2002

Many aspects of welded component design and fabrication are governed by documents knownas codes and standards.

Many aspects of welded component design and fabrication are governed by documents known ascodes and standards. End users or purchasers often specify these documents in a contractualagreement to control the characteristics of the welded component that may affect its servicerequirements. Manufacturers also use them to assist in the development and implementation of theirwelding quality systems.

Many end users of welded components have developed and issued specifications that address theirown requirements. However, national interest in areas such as public safety and reliability haspromoted the development of welding codes and standards that command industrywide or nationalrecognition.

For example, national engineering and technical societies have developed numerous committees thatcontinue to evaluate the needs of industry and develop new welding codes and standards. Themembers of these committees are technical experts and represent all interested parties, such asmanufacturers, end users, inspection authorities, and government agencies. After a committeecompletes a new or revised document, it usually is reviewed and approved by a review committeeand published in the name of the applicable engineering society if it is accepted.

Legislative bodies or federal regulating agencies sometimes adopt documents that have significantinfluence on public health and safety. In those jurisdictions, such documents become law and oftenare referred to as codes or regulations.

Welding inspectors should know which codes and standards are applicable within their jurisdiction,understand the requirements of the relevant documents, and perform their inspections accordingly.

Sources of Welding Codes and Standards

Following are some of the more popular sources of welding codes and standards in the U.S.

American Welding Society (AWS). AWS publishes many documents addressing welding use andquality control. These documents include such general subjects as welding definitions and symbols,classification of filler metals, qualification and testing, welding processes, welding applications, andsafety.

American Society of MechanicalEngineers (ASME). This society is responsible for the development ofthe Boiler and Pressure Vessel Code, which contains 11 sections and covers the design,construction, and inspection of boilers and pressure vessels. ASME also produces the Code for

Pressure Piping, which consists of seven sections, each prescribing the minimum requirements forthe design, materials, fabrication, erection, testing, and inspection of a particular type of pipingsystem.

American Petroleum Institute (API). This institute publishes many documents relating to petroleumproduction, a number of which include welding requirements. The most well-known is possibly APIStd 1104Standard for Welding Pipelines and Related Facilities.

Typical Welding Code and Standard Content

The specific content and requirements of a welding code or standard can vary in detail, but they havesome basic elements in common.

Scope and General Requirements. This usually is found at the beginning of the document and

typically describes the type and extent of welding fabrication for which the document should be used.It also may explain limitations for the document's use.
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Design. If the document provides a section for design, it may contain minimum requirements for thedesign of specific welded connections, or it may refer the user to a secondary source of information.

Qualification. This section typically outlines the requirements for qualification testing of weldingprocedure specifications, as well as the requirements for qualifying welders. It may provide theessential variables, which typically have change limitations of each variable that govern the extent ofqualification. Essential variables typically include:

The welding process

Base metal type and thickness

Filler metal type

Electrical parameters

Joint design

Welding position

This section of the document also may provide the qualification testing requirements, which usuallyare divided into welding procedure and welder performance. Typically, it provides the types and sizesof test samples to be welded and prepared for testing, the testing methods to be used, and theminimum acceptance criteria to be used for evaluating the test samples.

Fabrication. This section, when included in the document, typically discusses fabrication methods orworkmanship standards. It may contain information and requirements for base materials, weldingconsumables, shielding gas quality, and heat treatment.

Inspection. This section of the document usually addresses the welding inspector's qualificationrequirements and responsibilities, acceptance criteria for weld discontinuities, and the requirementsfor nondestructive testing procedures.

Opportunities to Improve Weld Quality and Reliability

Welding fabricators often use welding codes and standards to achieve process control to meet therequirements of ISO 9000 and other quality management systems. Often the major elements ofprocess control specified by quality management systems are the same elements addressed inwelding codes and standards:

Production procedures must be documented. For welding, this is the welding procedurespecification.

Criteria for workmanship must be stipulated in the clearest practical manner. For welding, thismay be the code or standard acceptance criteria.

Personnel must be qualified. This may be addressed by the welder performance qualification.

Regardless of a fabricator's overall quality system, selection of the appropriate welding codes andstandards can help improve welding quality and reliability even more.

Welders turn to induction heating for preheating, stressrelieving

ByMike Roth

November 15, 2001

This article discusses using induction heating for preheating and (postheating) stress relief of

welds. It focuses on what this technology is, how it works, and how it can be used in an industrial
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setting. This article also gives several real-life examples of how the technology has been used inactual applications.

Although many industries have used inductionheating for decades (see Sidebar), it is anewcomer to industrial and construction

applications involving welding. Some companieswith welding-intensive operations now useinduction heating for preheating before weldingand stress relieving (i.e., postheating) afterwelding.

Induction Heating: How It Works

Induction heating systems use noncontactheating. They induce heat electromagneticallyrather than using a heating element in contactwith a part to conduct heat, as does resistance heating. Induction heating acts more like a microwaveoven the appliance remains cool while the food cooks from within.

In an industrial example of induction heating, heat is induced in the part by placing it in a high-frequency magnetic field. The magnetic field creates eddy currents inside the part, exciting the part'smolecules and generating heat. Because heating occurs slightly below the metal surface, no heat iswasted.

Induction heating's similarity to resistance heating is that conduction is required to heat through thesection or part. The only difference is the source of heat and the temperatures of the tool. Theinduction process heats within the part, and the resistance process heats on the surface of the part.The depth of heating depends on the frequency. High-frequency (e.g., 50 kHz) heats close to thesurface, while low-frequency (e.g., 60 Hz) penetrates deeper into the part, placing the heating sourceup to 3 mm deep, which allows heating of thicker parts. The induction coil does not heat up becausethe conductor is large for the current being carried. In other words, the coil does not need to heat upto heat the workpiece.

Induction Heating System Components

Induction heating systems can be air- or liquid-cooled, depending on application requirements. A keycomponent common to both systems is the induction coil used to generate heat within the part.

Air-cooled System. A typical air-cooled system consists of a power source (5 kW or 25 kW),induction blanket, and associated cables. The induction blanket consists of an induction coilsurrounded by insulation and sewn into a high-temperature, replaceable Kevlar sleeve.

This type of induction system can include a controller to monitor and automatically controltemperature. A system not equipped with a controller requires the use of a temperature indicator. Thesystem also could include a remote on-off switch. Air-cooled systems can be used for applications upto 400 degrees F, designating it as a preheat-only system.

Liquid-cooled System. Because liquid cools more efficiently than air, this type of induction heatingsystem is suitable for applications requiring higher temperatures, such as high-temperaturepreheating and stress relieving. The principal differences from an air-cooled system are the additionof a water cooler and the use of a flexible, liquid-cooled hose that houses the induction coil. Liquid-cooled systems also generally use a temperature controller and built-in temperature recorder,particularly important components in stress-relieving applications.

The typical stress-relieving procedure requires a step to 600 to 800 degrees F, followed by a ramp orcontrolled temperature rise to a soak temperature of approximately 1,250 degrees. After a hold time,the part is control-cooled to between 600 and 800 degrees. The temperature recorder collects data onthe part's actual temperature profile based on a thermocouple input, a quality assurance requirementfor stress-relieving applications. The type of work and the applicable code determine the actualprocedure.


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Induction Heating's Benefits

Induction heating offers numerous benefits, including good heat uniformityand quality, reduced cycle time, and long-lasting consumables. Inductionheating is also safe, reliable, easy to use, power-efficient, and versatile.

Uniformity and Quality. Induction heating is not particularly sensitive to

coil placement or spacing. Generally, the coils should be spaced evenlyand centered on the weld joint. On systems so equipped, a temperaturecontroller can establish the power requirement in an analog fashion,providing just enough power to maintain the temperature profile. Thepower source provides power during the entire process.

Cycle Time. The induction method of preheating and stress relievingprovides relatively quick time-to-temperature. On thicker applications, suchas high-pressure steam lines, induction heating can slash two hours fromcycle time. It is possible to reduce cycle time from the control temperatureto soak temperature.

Consumables. The insulation used in induction heating is easy to attach

to workpieces and can be reused many times. In addition, induction coilsare robust and do not require fragile wire or ceramic materials. Also,because the induction coils and connectors do not operate at hightemperatures, they are not subject to degradation.

Ease of Use. A major benefit of induction preheating and stress relieving is its simplicity. Insulationand cables are simple to install, usually taking less than 15 minutes. In some cases, how to use theinduction equipment can be taught in one day.

Power Efficiency. The inverter power source is 92 percent efficient, a critical advantage in an era ofskyrocketing energy costs. Additionally, the induction heating process is more than 80 percentefficient. Regarding power input, the induction process requires only a 40-amp line for 25 kW ofpower.

Safety. Preheating and stress relieving through the induction method is worker-friendly. Inductionheating does not require hot heating elements and connectors. Very little airborne particulate isassociated with the insulation blankets, and the insulation itself is not exposed to temperatures higherthan 1,800 degrees, which can cause insulation to break down into dust that workers may inhale.

Reliability. One of the most important factors impacting productivity in stress relieving is anuninterrupted cycle. In most instances cycle interruption means the heat treat will need to be rerun,which is significant when a thermal cycle can take a day to complete. The induction heating systemcomponents make cycle interruptions unlikely. The cabling for induction is simple, making it less likelyto fail. Also, no contactors are used to control the heat input to the part.

Versatility. In addition to using induction heating systems to preheat and stress relieve pipe, usershave adapted the process for weldolets, elbows, valves, and other parts. One of the aspects ofinduction heating that makes it attractive for complex shapes is the ability to adjust the coils during the

heating process to accommodate unique parts and heat sinks. The operator can start the process,determine the effects of the heating process in real time, and modify the coil position to change theresult. The induction cables can be moved withoutwaiting for air cooling at the end of the cycle.


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Induction Heating in Welding Applications

This technology has proved itself on a number ofprojects, including oil and gas pipelines, heavyequipment construction, and maintenance and repairof mining equipment.

Oil Pipeline. A North American oil pipelinemaintenance operation needed to heat pipe beforewelding encirclement repair sleeves or STOPPLE fittings to the pipeline's 48-in. girth. While workerscould make many repairs without having to stop oil flow or drain it from the pipe, the presence of thecrude itself hampered welding efficiency because the flowing oil absorbed the heat. Propane torchesrequired constant interruption of welding to maintain heat, and resistance heating while providingcontinuous heat often could not meet required weld temperatures.

The maintenance company turned to induction heating as a solution (see Figure 1). Workers usedtwo 25-kW systems with parallel blankets to obtain a preheat temperature of 125 degrees onencirclement sleeve repairs. As a result, they reduced cycle time from eight to 12 hours to four hoursper girth weld.

Preheating for a STOPPLE fitting (a T junction with valve) repair was even more challenging becauseof the fitting's greater wall thickness. With induction heating, however, the company used four 25-kWsystems with a paralleled blanket setup. They used two systems on each side of the T. One systemwas used on the main line to preheat the oil, and the second was used to preheat the T at thecircumferential weld joint. The preheat temperature was 125 degrees. This reduced the weld timefrom 12 to 18 hours to seven hours per girth weld.

Natural Gas Pipeline. A natural gas pipelineconstruction project entailed building a 36-in.-diameter, 0.633-in.-thick pipeline from Alberta,Canada, to Chicago. On one stretch of this pipeline,the welding contractor used two 25-kW powersources mounted on a tractor with the inductionblankets attached to booms for speed and

convenience. The power sources preheated bothsides of the pipe joint. Critical to this process werespeed and reliable temperature control. As alloycontent increases in materials to reduce weight andweld time, and to increase part life, controllingpreheat temperatures becomes more critical. Thisinduction heating application it required less thanthree minutes to obtain the 250-degree preheattemperature.

Heavy Equipment. A heavy equipment manufactureroften welded adapter teeth onto its loader bucketedges. The tack-welded assembly had been moved

back and forth to a large furnace, requiring thewelding operator to wait while the part was reheatedrepeatedly. The manufacturer opted to try inductionheating to preheat the assembly to preventmovement of the product (see Figure 2).

The material was 4 in. thick with a high required preheat temperature because of alloy content.Customized induction blankets were developed to meet the application requirements. The insulationand coil design provided the added benefit of shielding the operator from the part's radiant heat.Overall, operations were considerably more efficient, reducing welding time and maintainingtemperature throughout the welding process.

Mining Equipment. A mine had been experiencing cold-cracking problems and preheatinginefficiency using propane heaters in its repair operations of mining equipment. Welding operatorshad to remove a conventional insulating blanket from the thick part frequently to apply heat and keepthe part at the correct temperature.

Figure 1:Induction heating maintains the

desired preheat temperaturethroughout the welding process whileseveral welders work to complete the

girth welds.

Figure 2:The induction preheat blanket

maintains the temperature of thebucket edge during the attachment ofteeth.


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The mine opted to try induction heating using flat, air-cooled blankets to preheat the parts before welding.The induction process applied heat to the partquickly. It also could be used continuously during thewelding process. Weld repair time was reduced by 50percent. In addition, the power source was equipped

with a temperature controller to keep the part at thetarget temperature. This almost eliminated reworkcaused by cold cracking. The company reported anannual savings of $80,000.

Power Plant.A power plant builder was constructing a natural gaspower facility in California. Boilermakers andpipefitters had been experiencing construction delaysdue to the preheating and stress-relieving methodsthey were employing on the plant's steam lines. Thecompany brought in induction heating technology inan attempt to increase efficiency, particularly for work

on medium to large steam lines, as these pieces takethe most heat-treating time required on a job site.

On a typical 16-in. weldolet with a 2-in. wallthickness, induction heating was able to shave twohours off the time-to-temperature (600 degrees) andanother hour to reach soak temperature (600 degrees to 1,350 degrees) for stress relieving. Thesimplicity of wrapping the induction blankets around complex shapes further reduced the time toperform the heat treat (see Figure 3). It took the fitters 15 minutes to wrap a joint that previously hadrequired two workers two hours to prepare.

Figure 3:The simplicity of wrapping the

induction blankets around complexshapes, such as at this natural gas

power plant, can reduce heat-treatingtime.

how to relieve stress in welding

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