By: Mark Riley and Ron Silver, sourced from The Welding Journal
Proper cleaning plays a key role in a successful welding operation. It impacts quality because a material surface with dirt or impurities can lead to inclusions or issues such as porosity. Surface cleaning and preparation can also impact the appearance of the final weld as well as overall operation costs for rework and labor. Some jobs may require the use of an abrasive product, or can be done with a cloth or chemical solvent.
Interpass and postweld cleaning are also important steps in the process, depending on the application and the requirements for the finished weld. While the same rules don’t apply to every job, there are some general best practices to follow when cleaning welds.
How to Choose?
There are many options available for weld cleaning. It’s important to keep these factors in mind when making a choice.
If there are surface finish requirements or aesthetic demands for the weld (e.g., if it needs to have a mirrored finish), these can impact how to clean. There may be multiple products required during postweld cleaning to achieve the desired finish. Start with an abrasive with the heaviest grit or grain allowed by the application, and then move to a finer grit as the job progresses. As an example, a welding operator could start with a 40-grit flap disc to clean a weld and end the job with an 80-grit flap disc for fine finishing, especially if the material will be painted or powder coated.
Typically, the smoother the completed weld, the better. When welding stainless steel, a wet cloth can be used for postweld cleaning of the weld surface and to remove free iron. This also helps the passivation process, which is the treating or coating of a metal (usually with acids or pickling paste) to reduce the chemical reactivity of its surface. When weld cleaning, save time and money by using as few steps as possible. Using a product with a finer grit will be slower, so it’s a balancing act between productivity and surface finish requirements.
Different abrasives are suited to different types of material. When welding stainless steel or aluminum, for example, it’s important to thoroughly clean it prior to starting because these materials are not as forgiving to dirt and debris during the welding process. Take care when using heavier-grit abrasives on these materials. A 24- or 40-grit flap disc may be too aggressive for aluminum, bronze, or other nonferrous metals. A good rule of thumb is the finer the grit, the better.
Often, 70% isopropyl alcohol can be used for cleaning both filler rods and base materials prior to welding. Look for tools designed for use with stainless steel, such as stainless steel brushes or grinding wheels, to avoid introducing contamination into the weld during interpass cleaning. These wheels can also be used on other materials that are susceptible to contamination such as aluminum, brass, or copper.
To avoid introducing dirt and debris into the weld, cleaning before and during welding is typically necessary regardless of material type, but some materials require extra attention.
Some welding applications may have code requirements for weld inspection. Weld specifications may determine how many inclusions the finished weld can have, for example. These are important factors to consider when choosing the right products for cleaning welds.
Common Tools for Cleaning
Common options for surface and weld cleaning include bonded abrasives/grinding wheels, coated abrasives/flap discs, and wire brushes and wheels. The choice depends on the requirements of the application and welding operator preference. Abrasive products and wire brushes differ in their performance and purpose. Abrasive products are designed to remove base material, whereas wire brushes are not.
If there are slag inclusions or porosity in the weld, a grinding wheel can be used for interpass cleaning to remove some of the weld material in addition to removing the inclusion. These products are often used to clean mild steel and for sloppier welds that may have a lot of slag or spatter because the wheels will remove more material faster. Grinding wheels rely on a combination of the grain type, grain size, and bonding agents (resins and additive fillers) to determine their performance.
Because bonded abrasives are generally more aggressive and remove material faster, they require a higher level of operator skill to prevent damage, gouging, or undercutting. Application needs determine the thickness of the wheel to be used — the heavier the material to be removed, the thicker the wheel necessary. Coated abrasives/flap discs use the same grain types as those found in bonded abrasives, but the grains are bonded to a backing cloth rather than molded and pressed into a hard grinding wheel. This cloth is layered to form a flap disc, a design that gives flap discs a softer feel. Flap discs can be used on stainless steel (though be sure to use a finer grit) or on mild or carbon steel for slight material removal in preweld cleaning as well as for blending and finishing the surface postweld. This makes them a good choice when the finished material needs to be painted, primed, or powder coated. Be mindful of the direction of spin when using a flap disc. Make sure it’s spinning and throwing the sparks and debris away from the base material and weld — and not back across them — to avoid contamination.
Wire brushes and wheels are a good option for interpass or postweld cleaning when it is necessary to remove spatter and other contaminants. If the material has a lot of mill scale, rust, or heat discoloration to remove (without removing a lot of material), wire brushes also work well for precleaning. When choosing a power brush, there are several knot styles, wire gauges, and trim length options. By changing one or more of these characteristics, the welding operator can fine-tune brush performance for a specific application.
For example, stringer bead brushes have narrower knots twisted from base to tip, making them better suited to penetrate tighter spaces like corners, fillets, and root pass welds during cleaning. Remember, the tips of wire brushes do the work, so using appropriate pressure is key to performance and efficiency. Using excessive pressure causes the wires to flex and bend, which can hinder cleaning and cause wire breakage that reduces brush life.
Best Practices for Success
In addition to proper abrasive selection and cleaning techniques, there are some practices to consider. Looking at only the initial purchase cost may be tempting, but lower-quality, less expensive abrasives, brushes, or wheels may not provide enough aggressiveness for the job, or they may have a much shorter product life. It may be best to invest more for a product that cleans better or lasts longer because it can significantly impact overall operation costs and productivity.
Another best practice is to avoid overspeeding. Choose an appropriately sized product for the operating tool, and always use the manufacturer’s recommended tool guard. Be sure the maximum safe rpm marked on the wire brush or abrasive wheel is greater than or equal to the maximum operating rpm on the tool. A 41 ⁄2-in. grinder and a 41 ⁄2-in. wheel may both be rated for 13,000 rpm, which means they are safe to use together. However, removing the guard on that tool to use a larger wheel can cause safety and performance issues because the larger product is rated at a much lower rpm. The product isn’t designed to run that fast, making it more susceptible to breakage, which increases the potential for injury and shortens product life.
Proper Cleaning Yields Success
Clean materials and welds are critical in any welding operation. Remember, however, there is no one-size-fitsall solution for choosing an abrasive, brush or, wheel for preweld, interpass, and postweld cleaning. Always assess the priorities and requirements for the job. The right choice can help generate efficiency, quality, and cost savings in the long term