November 2, 2009
Relieving residual stress through welding technique as well as temperature control can greatly reduce weld distortion.
It's a shame arc welding works so well. It's proven, cost-effective. For many applications, nothing comes close, at least not yet. Why is it a shame? Because at the microlevel arc welding induces some 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 state as wire or rod. Immediately after being deposited and subsequent fusion between the base and weld metal, the metal cools quickly. The high-yield-strength weld filler metal contracts, or shrinks, pulling the lower-yield base metal with it. Clamped tight, the metal may stay in place until after welding, but this 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 to which 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 the stress.
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 of two things: control temperature and refine the welding procedure, both of which counteract those unavoidable 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 and welding technique, we spoke with Carl Smith, longtime quality manager and welding technician at Kanawha 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 for Bolttech Mannings.
Setup; electrode selection; along with weld type (fillet, groove, butt, etc.), size, and orientation all affect how a weld joint reacts to stress.
Prebending or presetting. The base metal can be set up in such a way to compensate for weld shrinkage. For example, when two workpieces are preset with one end of the joint together and the far end of the joint slightly apart, the cooling weld metal pulls the two workpieces until, by the end of the weld, the joint is in the proper orientation.
Balance the weld. Double-sided welds, such as double V-groove joints, balance induced stresses and often 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."
Backstepping. Backstepping is a bit like moonwalking with a welding gun. You start several inches from the beginning of the joint and weld back to the edge; then go farther up the joint and weld back to where you initially struck your previous arc; then go farther up the joint and again weld back to the previous welded segment; and so on until the joint is complete. This counteracts shrinkage by focusing the initial stresses away from the workpiece edges.
Intermittent welding. When intermittent or stitch welding meets the design requirements, it not only helps 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 input required to melt the smaller wire outweighs any heat reduction benefit that might occur with a faster travel speed using a larger wire.
Weld metal: More isn't better. Codes spell out specific weld size requirements, including the maximum allowable height of the bead above the plate. The trick is to lay just enough weld metal to create the strongest joint—and no more. A highly convex bead doesn't make a weld stronger, but it does increase shrinkage forces, because more high-tensile weld metal is pulling on the base metal as the weld cools.
Here, technique factors in. "A multipass weld with stringer beads will create less distortion than a weave bead," Smith said.
The stringer bead technique generally allows faster travel speeds, which lowers heat input. Each pass of 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 look better than a stringer bead, but if a welder knows what he's doing and places his stringer bead properly, 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 a pipeline is usually much smaller than on conventional plate. And "round pieces do not distort nearly as badly 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 the weld, 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 exacerbated with 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 of prebending 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 this crosspiece should be welded first. The centerpiece, if welded first, is less restricted by the surrounding metal and has freedom to move and expel residual stress before you go on to weld the frame.
Preheating, maintaining temperature between weld passes (interpass temperature), and postweld heat treating (PWHT) work toward one goal: to control changes in heat levels. The more control you have 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 stresses and provide more time for hydrogen to dissipate, reducing the chance for under-bead cracking.
Material factors. Predicting necessary minimum preheats, interpass temperature, and PWHT depends on the application and how restrained the joint in question is. Specific material properties affect how drastically metal will distort. These include the coefficient of thermal expansion (how much the metal expands when heated), thermal conductivity (how fast it dissipates heat), yield strength, and modulus of elasticity (material stiffness).
As a starting point, refer to the AWS D 1.1 structural welding code, Welding Handbook, guidelines published by the steelmaker, and other sources for recommended minimum preheat and interpass temperatures for specific alloys. Generally, higher carbon content equates to higher minimum preheat and interpass temperatures.
Most preheating, interpass heating, and PWHT do not require maintaining a precise temperature, as long as you maintain a minimum temperature. There are exceptions, though, including quenched and tempered steels. These come to the welding station already heat-treated by the steelmaker, so preheating at a too-high temperature can destroy the material properties; in other words, quenched and tempered steel will no longer be tempered. "For instance," Smith said, "the ASTM A514 and A517 alloys 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. But it has no effect on stainless steel, as long as you use distilled water, which doesn't have any chlorine in 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, most important, 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, hydrogen becomes trapped as the weld metal quickly cools. The slow cooling created thanks to the preheat gives time for that hydrogen to bake out of the weld. "This is why a welder may often say he's 'boiling the water' out of the material," Smith said.
High-alloy materials such as chrome-moly also dissipate heat quickly and generally require high preheat temperatures. Preheating even the tack welds often is best practice, Smith said. Cracks can start in the tack and "come right through the weld and all the way to the top." He added that certain chrome-moly applications require preheats of about 400 degrees F and a postweld holding temperature 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 to apply 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, take a 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 the required interpass temperature. "You need to maintain the interpass temperature throughout the weld," Smith explained, adding that heat cycling is especially dangerous with chrome-moly and quenched 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. "The propylene gas is not as highly concentrated as acetylene," he said, "and we don't want to concentrate the 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 can serve as a PWHT substitute when moving the workpiece to an oven for PWHT isn't practical (think offshore oil rigs). PWHT doesn't function as a preheat substitute because it does nothing to reduce the stresses that occur just after you strike an arc on cold, unpreheated base metal. By the time PWHT is applied, it's too late to correct the problem.
"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 oven for postweld heat treatment. Note one common thread among all these methods: time. But certain technologies take alternative approaches that streamline the operation and even improve weld quality.
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 can raise the workpiece temperature to the appropriate level before, during, and after welding, to comply with standard preheat, interpass, and PWHT practices (see Figure 1 and Figure 2). The pad incorporates interlocking beads woven together using a high-resistance wire. The unit can heat up to 1,850 degrees F. (Smith's company has used this technology to preheat thick copper plate to more than 1,000 degrees F.)
A temperature controller uses a system of thermocouples spot welded to the part to read the actual metal temperature, which is monitored throughout the operation. Welders don't have to use temperature crayons to measure the preheat temperature. The pad also doesn't have to be removed during 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, attach the 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're finished 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 power plants.
The pads are designed to wrap around the workpiece, with a piece of removable insulation over the joint. For preheat, the entire workpiece is covered. You then remove the insulation from the weld joint area and start welding. When you take a break, you put the insulation back over the joint to help maintain the preheat temperature. The heater pads can then be added to the weld area for stress relief, 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 (see Figure 3).
"Heat is vibration, according to physics," said Tom Hebel of Bonal Technologies. The more something is heated, the faster its molecules vibrate. "We induce a vibration into the part, and the part responds as 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 vibrates the molten metal at the molecular level. It's roughly analogous to shaking a can of dissimilar-shaped beads or a vibratory bowl feeder in a stamping operation, which gets everything to settle and "pack down." The vibration level, Hebel said, is very specific: in the lower, or sub-harmonic, portion of the harmonic 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 more vibration that's put into the part, the more it will absorb—up to a point. "At a certain point any additional 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 its resonance point. It's here that the vibration has the greatest dampening effect, at which point it neutralizes the stress induced by the weld's heat.
Most commonly, the vibratory device is applied after welding to relieve stress, essentially replacing PWHT. But it also can be applied during welding to improve weld quality through grain refinement and stress reduction. In fact, applying the right vibration during welding can eliminate the need for PWHT completely, 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 after welding, if the effects of thermal stress aren't there, why send the part to a furnace for stress relieving""
In certain applications, Hebel said, it can replace low-temperature preheating requirements, between 250 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 preheat procedures to increase weld quality.
Hebel compares a large steel part with welding-induced stress to an out-of-tune instrument. After welding, temperature drops sharply. At this point within and around the heat-affected zone, the part's natural harmonic curve shifts slightly, "out of tune" with the rest of the assembly. Counteracting that effect with induced vibration during and after welding relieves stress as evidenced by the harmonic curve moving back "in tune" with the rest of the assembly.
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