The root causes of weld defects
Poor procedures, design lead to welding problems on the floor
Bad welds can be traced back to poor workmanship, poor design, or a little of both.
Weld defects happen for a host of reasons. Air may creep into the weldment to cause porosity. The wrong amount of heat can cause cracking. Bad welding technique can cause undercuts or incomplete penetrations of one kind or another.
Various factors contribute to weld problems, but many lead back to the same place. In fact, most causes of weld defects can be traced back to two general areas: first, a combination of poor instruction and workmanship; second, poor weld design and/or material choice.
Better Procedures, Better Workmanship
A welder may produce poor work for many reasons, including poor instruction. This can be a result of bad or incomplete advice from a welding teacher or experienced welder, either from a technical school or in a company training program. But often it's the result of an incomplete, unspecific welding procedure specification (WPS).
Luckily, the WPS is far easier to correct than bad training.
A lot of defects could be prevented from the get-go if the WPS delved into specifics. In a way, a good WPS should help a welder become better at his craft with every job. The more detail a WPS gives, the less chance there is for error at the welding station.
Hand a welder a standard weld procedure, and it tells him the volts and amps. If he's using gas metal arc welding, it will tell him whether to do a string or weave bead. It will show the contact-tip-to-work distance; whether to make multiple or single passes; what shielding gas to use, the mix, and a range for the flow; and the type of wire to use. It also shows the weld position and if weld travel is uphill or downhill (see Figure 1).
A good welding procedure is supposed to tell the welder how to make the weld. That's straightforward enough. But there's more to it than volts, amps, the wire, and shielding gas. Welders need to know how to handle the gun and at what angle, the difference in the amperage and voltage settings when traveling uphill versus downhill, as well as if pre- or post-heat treatment is necessary. The standard form may say to weld overhead, uphill, or downhill, but it doesn't tell exactly what the welder should do to accomplish the weld at hand.
All this won't fit on the standard WPS form, of course, but the welding supervisor is free to add detailed notes describing exactly how that weld should be performed (see Figures 2 and 3).
Many students go through welding training programs without extensive (if any) knowledge about how to read and write a good WPS, but it's a vital part of becoming a good welder. What good grammar is to a writer, a clear WPS is to a welder.
An experienced welder should make a list of all the important variables, factors he himself might take for granted after years of welding—depth of passes, any peening necessary, weld sequence, and so on—to make sure the WPS includes these details when needed.
For instance, there is a huge difference between welding overhead, uphill, and downhill, and the WPS should include procedures that describe these differences. When welding overhead, a welder should increase his travel speed. If the welder doesn't, he'll find out his mistake the hard way, with the weld falling apart above him. For uphill (vertical-up), the amps and volts decrease. And for downhill, the travel speed increases naturally, so that requires an increase in wire feed speed.
In the horizontal position, the welder needs to fill the top edge of the weld to avoid underfilling the joint, with undercut on the top and overlap on the bottom. The weld pool has a tendency to fall down, and there's no way to refill it, because the welder is fighting gravity (slowing travel speed won't help like it does when welding vertical-up or vertical-down).
Although the standard WPS gives a gas flow range, more specific information can help. A gas flow that is too high can cause turbulence in the weld pool, creating spatter. If it's too low, porosity forms from the gas not completely shielding the pool from the atmosphere. For outside, open-air welding, 40 cubic feet per hour (CFH) is a safe bet to protect against the elements; inside welding may require between 25 and 30. But anything higher than 50 CFH is likely to cause trouble, inside or outside. Turbulence in the weld pool will draw in atmospheric elements and cause problems.
For gas tungsten arc welding (GTAW), the standard WPS specifies cup size, but for GMAW it may not specify the nozzle diameter. The higher the amperage, the larger the nozzle. As a rule, anything over 200 amps requires a welder go up to a 5⁄8-inch nozzle; anything less than 200 amps, a 3⁄8-in. nozzle can be used.
Distortion is a natural consequence of the hard weld metal cooling next to a softer base metal. Consider A36 carbon steel plate with a 36,000-PSI yield strength; an ER70S6 wire has a yield of 58,000 to 60,000 PSI. When the weld cools, it immediately reaches its full yield strength. So, put 60,000 PSI against 36,000 PSI, and guess which one wins? The weld metal's high yield strength creates a strong joint, but it also causes distortion.
Many factors cause this distortion, and the WPS should describe the techniques to prevent it. Consider improper weld sequence. The weld metal constantly works against the softer base metal, but if a welder lays down beads in specific sequences, the distortion can be minimized—and the WPS can spell out these sequences. Specifying where to start and end each pass can minimize the chance for that distortion. Certain programs, such as WeldCAD, allow you to draw the weld and identify the weld sequence and number the passes; you can then export the file into the WPS's word processing file (see Figure 1).
Preheat also can play a big role in minimizing distortion. Most welders preheat everything for high-carbon material. For mild steel, some say metal less than 1 in. thick should not require preheating. But certain metallurgical charts show that the structure can become seriously affected in the heat-affected zone (HAZ) starting at about ¾ in.
It depends on how much restraint is in the weld area. Consider a design that requires four weld beads placed in tic-tac-toe fashion. These intersecting welds create stresses going in every direction (called multiaxial stresses), and weld preheating would help relieve some of the stress. If a metal requires a 300-degree-F preheat, a welder should maintain the heat until the weld is completed, and use a blanket to control the cooling rate. The more a part is preheated, the slower it will cool, which leads to less distortion.
A standard WPS might say to preheat to a certain temperature, but it doesn't say where to preheat or precisely how. Additional illustrations after the standard WPS form can speak volumes. One way to do this is by lifting out the section requiring preheat from the WeldCAD illustration and placing the section in detail on a separate page. Detailed labels will describe how to preheat the section, with arrows pointing to each highly restrained area. Even a trained welder may overlook these details, but if they are in the WPS, he's less likely to miss them.
A good WPS acts as a guide for the weld inspector too. If the WPS's instructions go against welding fundamentals, he should know immediately what to look for in the weld. If the WPS for a wire process specifies too much helium as a shielding gas, for instance, the inspector can look for a weld profile that's too flat. If the WPS gives incorrect amperage, or if the specified voltage is too high, the inspector should look for undercut in the weld. Too much voltage also produces excess spatter. And if the wire feed speed is too high, the inspector will most likely see overlap in the weld profile, because too much wire is going into the weld pool.
Designing out Defects
Sometimes weld defects occur less because of welder error or poor instruction and more because the design itself makes the welder's job difficult. Designers experienced in structural welding know to avoid certain elements when they can, but problems can arise from less experienced design engineers.
As an example, a 1-in. plate standing vertically, with two plates, each 2 in. thick, parallel to each other on either side of that 1-in. plate. Joining the assembly requires four single-bevel groove welds, creating a highly restrained, stressed area (especially for materials like A36 carbon steel, well-known for lamellar tearing). This design is one among many that, no matter how talented the welder, may result in weld defects or outright failures, because there is so much inherent stress.
As a rule, designs shouldn't have highly restrained welds where they don't need to be. The previous example, for instance, could be redesigned so that the 2-in. plate would run through the 1-in. plate. It would still require bevel groove welds on the ends of the 1-in. plates, but the design would involve significantly less stress and make the welder's job easier. And a welding supervisor shouldn't hesitate to call the designer to suggest changes, ideally before the welder strikes an arc.
If a welder can't reach a weld easily, there's a better chance his resulting welds will have defects. Say a worker must crouch underneath a structure and weld at an odd angle overhead to reach a thick-gauge, double-groove joint. Rotating fixtures could help for smaller parts, or the seam could be beveled so that the welder could approach it from the top and produce a 100-percent-penetration weld from one side. Of course, if the designer could eliminate or move the joint to where the welder could access it from both sides, even better.
Another common problem involves overwelding. Inexperienced designers may see a T joint with 1-in. plate and, to maintain tensile strength for the assembly, call for a 1-in.-deep fillet weld on either side. This creates various problems. Because the fillet weld is more than 3⁄8 in., it will require multiple passes. This wastes time and weld metal, which can cost a company dearly in the long run. It also exerts unnecessary stress, which can lead to weld cracking; using so much weld metal can cause overlap as well. At worst, defects can even cause the vertical member of the T to pull away from the base plate.
Good weld design is a bit counterintuitive, because in most cases more weld metal does not make a better weld. For this T-joint application, welding reference materials may recommend certain weld sizes: In agreement with AWS D1.1, 5⁄16-in. fillets are common on 1-in. plate, for instance. But ideally, a designer should consider each weld joint and determine the minimum weld size to satisfy strength and safety requirements—and not go above it. Again, more weld metal usually does not increase the weld's strength. In reality, the minimum amount of weld metal may produce the safest, most economical, and strongest joint possible.
At the same time, designers should avoid fillet welds and use groove welds whenever possible. Not only do groove welds exert less stress, determining weld size is straightforward; the plate thickness is the weld thickness.
Design mishaps include process selection. Some companies, even the largest ones, use welding procedures first written years ago, and because of that they often specify less-than-optimal processes. A procedure may require shielded metal arc welding (SMAW, or stick) when the welder could do a much better job using another process. SMAW has its place for certain applications, but it does produce slag, which can get trapped in the weld and cause defects. Moreover, it applies a lot of heat and has an average 2-in. stub loss (the length of unusable electrode left after welding).
Usually a welder will use less consumables and less heat with a wire process like GMAW or flux-cored arc welding (FCAW). For most applications 1 in. thick and less, pulsed gas metal arc welding (GMAW-P) works best at minimizing distortion as well as increasing penetration. For the thickest materials, submerged arc welding (SAW) works best.
Finally, material selection is especially critical during the design phase. Selecting hard-to-weld material leads to more defects. Designers should know material weldability and use hard-to-weld material only when there is no other choice.
Say a designer is making a shaft and calls for 1045 high-carbon steel, which will add tensile strength and rigidity to the shaft, but has no chemical additives that will help the welder. What about 4330 or 4340 material? These contain nickel, which eases welding, makes the material more ductile, and adds toughness.
It's also true that nickel-bearing weld metal can make 1045 base metal easier to weld. But considering material prices, switching base metal may make more economic sense (depending on the amount of metal the job requires). The price difference between material like 1045 and nickel-bearing metal like 4340 may be only 5 to 7 percent. Meanwhile, a high-nickel filler may be significantly more expensive than filler metal without nickel.
Preventing Future Defects
Like any quality problems in manufacturing, weld defects rarely if ever have just one cause. Bad welding may result from a combination of poor workmanship, a poor WPS and welder instruction, and bad design.
The trick is for weld inspectors to recognize how each of these causes contributes to a weld problem and work together with the design engineers and shop floor welders to prevent as many defects as they can before welding begins.
Friday, December 26, 2008
The root causes of weld defects
Posted by BR at 10:21 AM
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