Multipass welding is typically used to add layers of weld to thick metal joints to increase the strength of critical welds for applications such as pressure vessels, oil and gas midstream pipeline, shipbuilding and heavy construction. These welds often undergo X-ray or ultrasound testing, since weld failure could be catastrophic. In this type of welding, each pass must be cleaned before the next pass is laid down to ensure there are no inclusions or issues that would cause a weld to fail inspection. Choosing the proper abrasives for cleaning and grinding multipass welds can help optimize efficiency and reduce costs. Let’s review the types of abrasives and some best practices to optimize results.
WHEN ARE MULTIPASS WELDS USED?
The thickness of the base material and the strength requirements for the application are two key factors that determine if multiple passes are necessary. Multipass welding can also be used to control heat input when there is a need to avoid warping or deforming the base material. Multipass welds are often used when welding steel, but they can offer benefits for stainless steel and aluminum base materials as well. The number of passes necessary will vary depending on the material type and thickness. For example, consider welding two pieces of pipe together for transportation of gas or oil: The first pass, called the root or bead pass, is critical because it connects the two pieces of material and acts as the foundation for the remaining passes. The second pass, called the hot pass, pushes the bead into the base material to help ensure full penetration. The hot pass is followed by a varying number of filler passes, determined by the material thickness. The filler passes help fill any gaps, since these weld joints are often in a V-shape that gets wider as the joint is filled. The final pass is called the cap pass, and it’s typically laid slightly above the base material.
CHOOSING ABRASIVES FOR MULTIPASS WELDS
Selecting the proper abrasive product for cleaning and grinding multipass welds depends on the specific weld pass being worked on and the desired outcomes. If possible, try different abrasive products to see what works best for each pass of the application. Again, referring to the example of pipeline welding, grinding the weld after the root pass in a U-shape removes wagon tracks, minimizes bug holes and provides a smooth surface for laying down the hot pass and gaining full penetration. During multipass welding, the operator often doesn’t have time to wait for the first pass to cool before grinding it. This makes it important to use a grinding wheel that is specifically designed for use on hot multipass welds. The hot, fill and cap passes are typically brushed for cleaning, with little to no grinding. Grinding these passes does sometimes happen when it’s necessary to grind down the weld stops and starts or to remove inclusions. A common sequence for cleaning multipass welds is using a grinding wheel on the root pass, followed by a stringer bead brush or encapsulated brush for the remaining passes.
- Grinding wheels: Sometimes called combination wheels, 1/8 in grinding wheels are often used to clean the root pass. When selecting a grinding wheel, consider if it’s necessary to perform any other tasks beyond vertical grinding. If there is a need to grind bevels, face lands or do any operation other than grind on the edge, look for a wheel that’s designed with extra reinforcement so it can be used at an angle. Some 1/8 in grinding wheels have two layers of fiberglass, allowing them to be used for vertical grinding only. Others have three layers that hold the product together and allow it to be used for more than vertical grinding. It’s also important to choose a wheel specifically designed for multipass welding and grinding hot welds. Running a standard grinding wheel over a hot root pass can result in wheel loading and eventual glazing. In response, operators may modify or cut a chip into the wheel, which is a safety hazard. Using a grinding wheel designed for multipass welding such as our Tiger® Pipeliner wheel reduces chatter and glazing while offering superior cut and life – without the need to chip the wheel.
- Power wheel brushes: Stringer bead wheels are often the next step after a grinding wheel, as they are good for getting into tighter areas and still provide aggressive cleaning. The hot pass in pipeline welding tends to be lower in the bevel, which can be a tight space. While some operators may continue to use the stringer bead wheel to clean all of the remaining fill and cap passes – even as the bevel gets wider toward the top and requires a lot of back and forth movement with the smaller stringer bead brush – there are other options that provide better efficiency. Filler pass brushes are wider than stringer bead wheels and can get the cleaning done faster on the wider parts of the bevel.
- Encapsulated brushes: Also called buffing or rubber buffing brushes, these tools are more aggressive than stringer bead brushes and are also very good for cleaning smaller spaces. Operators may heat up the encapsulation or try to hit an encapsulated brush with a file to break it apart slightly, but that’s not a recommended practice. The encapsulation on these brushes is designed to wear down with use. Be aware when using them that an encapsulated brush feels a lot stiffer than a stringer bead, so a bit more pressure can be used for optimal performance without the fear of long wire breakage.
- Flap discs: These abrasive tools are typically used to grind and finish in one step, but they have other uses in multipass applications as well. In pipeline applications, flap discs are typically used when a defect must be repaired or pits on a bevel need to be smoothed. A flap disc will shine up the surface to expose the defect for repair. Flap discs might also be used if the welds must be ground down for a specific finish requirement or aesthetic reasons.
TIPS TO OPTIMIZE MULTIPASS WELD CLEANING
There are some key best practices that can help operators control the abrasive product and keep it in the bevel to ensure full cleaning action and avoid problems like slag or inclusions that could result in failed welds:
- Good setup. Proper setup helps ensure a consistent gap between the two pieces being welded. A 1/8 in gap is commonly used for many multipass welds, including most pipeline applications.
- Proper angle and consistent pressure. There may be a tendency to dig in too much and push too hard when using an abrasive product for weld cleaning. It’s important to use moderate pressure and let the wheel or brush do the work. With grinding wheels, long, even strokes help maintain consistent pressure. When using wire brushes, pushing too hard can result in long wire breakage. Use lighter pressure and let the tips of the wires do the work. Wire brushes are used vertically – rather than at an angle like grinding wheels can be used – so be sure to have proper orientation to the workpiece for the product type being used. Consistent movements and pressure are key no matter the product type.
- Visual inspection after each pass. Whether it’s the first pass or the last, it’s important to check each weld pass once grinding or cleaning is complete. This helps ensure the abrasive product is getting into the sides of the weld and not leaving wagon tracks or black tracks. Sometimes the operator must work the brush back and forth to make sure all portions of the weld are being hit.
Optimizing abrasive performance when cleaning and grinding multipass welds requires careful product selection to best match the weld pass and the desired outcomes. It’s also important to always keep the tool guard in place to maximize operator safety.
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