Evaluating joint design, welding processes for edge preparation
April 11, 2005
While beveling is known as a common procedure used to shape the edges of thick plates or pipes for welding, not everybody knows how to make the process cost-efficient in the overall welding operation.
It's first important to know why and how beveling is used, and the key is realizing that joint design is process-dependent. In other words, you can't use a certain joint configuration from a book for just any process.
Changing your welding process will affect the impact beveling has on your total operation. But before you consider a change—for instance, from shielded metal arc welding (SMAW) to gas metal arc welding (GMAW) or flux-cored arc welding (FCAW) or submerged arc welding (SAW)—you should ask yourself if the joint preparation you are using will be adequate and economical for the new welding process.
Electron beam welding (EBW) doesn't require beveling or filler metal even for the thickest plates. If for a specific case plasma arc welding (PAW) is considered, you should check to see if welding with this keyhole procedure will make beveling superfluous.
Ideally, for each job, you should produce a cost estimate to determine the most economical welding process. In reality, though, you probably use only two or three processes in your shop, so you may be limited by your equipment and personnel, as well as by your consumables and time schedules.
If you already have tons of beveled material on hand, your decision already is made. You aren't going to scrap that material even if it's not the most economic edge preparation for the new process you intend to use from now on.
No matter how you arrive at a decision, it's important to evaluate the joint design first to find out if it should stay the same or if you should change it.
Now you can look into whether you should consider beveling.
Bevel angles and root openings are essential for adequate weld penetration in thick plates or pipes. The joint design must specify all the relevant dimensions and angles. The joint design and root opening specification are both part of the weld procedure.
Up to a certain thickness, depending on the process and heat input available, edges may be welded with no preparation.
Whereas joint design details, such as size, length, and relative orientation, are based on formal stress analysis, the specifications for beveling are based on practical considerations, such as volume to be filled and ease of fabrication.
Common bevel shapes—single (welded from one side) and double (welded from both sides)—are used when square butt joints don't allow full penetration because of excessive thickness. They are called bevel groove and J groove if beveling is performed on the edge of only one of the two plates. V grooves and U grooves require edge preparation of both plates.
Double-groove weld joints—joints that require welding from both sides of plates or for pipes from both outside and inside—require less filler metal than single-groove joints, in which a groove weld is made from one side only. Double grooves typically are used for thick plates or pipes whenever both sides of the joint are accessible.
To decide whether to weld from one or both sides, weigh the filler metal savings for double-groove joints against the need to manipulate the plates to get the other side up if welding is performed in the flat position. It's important to note, however, that even if most of a weld is performed from one side, the need to grind the root from the opposite side of the weldment probably will require you to handle and reposition the workpiece.
But the option of welding from one side or from both may not be available when one side isn't accessible; for example, the inside of a pipe of limited diameter. The root pass of butt joints often is grooved by grinding from the other side, for eliminating defects and assuring full penetration when welding from both sides.
You always should be aware that establishing weld sequences may have unexpected results. For example, say you have to grind the root pass from the other side. If you aren't advised otherwise, you may start to weld the root pass from the outside of a pipe. Then you have to stop welding and start grinding. At this point, you will see that grinding from the inside of the pipe is anything but easy because your accessibility is limited. So, when grinding, be sure to present the side where this operation can be performed with the least amount of hindrance. For pipes, this typically is the exterior.
Besides reducing the quantity and cost of filler metal needed, double-groove welding introduces less heat and deformation. A double-joint design may allow you to alternate weld passes on each side of the joint, which further reduces distortion but increases workpiece handling.
Bevels sometimes are prepared on the edges of transverse elements to be joined to main structures by fillet welding to produce a more favorable configuration for stressing if fatigue is involved.
To calculate the volume of filler metal needed per unit length for many given joint shapes and dimensions, consult the American Welding Society Welding Handbook. Be sure to check your calculation with real weld setups to establish the weld deposition rate (in units of deposited metal weight per hour) for the welding machine.
After choosing a joint configuration, select your beveling method. The process you choose should be the most economical one that results in acceptable quality.
"Three optional techniques for beveling: Understanding the advantages and drawbacks of each" is an article that describes available mechanical beveling methods and equipment. It was published in the January/February 2004 issue of Practical Welding Today and is online at www.thefabricator.com.
Waterjet cutting with or without abrasives may be suitable for certain steels because it doesn't produce any adverse metallurgical transformation.
Traditional flame beveling methods are oxyfuel cutting with oxyacetylene, or another combustible gas, or gasoline, although this method is limited in what materials can be beveled.
Usually these methods introduce some surface oxidation and decarburization that, in most cases, may be of no consequence. The edges then are immediately ready for welding.
In steels with more than 35 percent carbon, preheating is required to prevent cracking. All medium-carbon steels should be preheated if you need to machine the gas-cut edges.
Gas-cutting higher-carbon (more than 45 percent) and hardenable alloy steels at room temperature may produce a thin layer of hard, brittle material on the cut surface that may cause cracks from cooling stresses.
You can alleviate hardening problems and residual stress formation by preheating before and annealing after oxyfuel cutting.
Oxyfuel cutting is limited by the materials it can cut. In particular, oxyfuel cutting isn't suitable for aluminum and stainless steels because they don't burn readily. Plasma cutting and water-shielded plasma cutting can overcome materials' limitations, but their use may be limited by material thickness. Cutting speed is usually fast, and better cut quality is possible.
Bevel cutting for weld preparation is an important application of plasma arc cutting. The intense heat of the process is characterized by higher power density. It's suitable for all types of beveling at high efficiency because the job is done quickly, in a short amount of time, at a higher speed. The high speeds possible with plasma arc cutting result in relatively low heat input to the workpiece. Less heat per unit length of the cut is introduced, and it doesn't spread so widely. Heat-affected zones (HAZ) therefore are narrow.
All of these welding-derived methods, as opposed to mechanical processes, produce a HAZ perhaps with cracks. Be sure to assess the situation before you decide if you can weld without mechanical conditioning of the cut edges.