Finishing Steel Weld Seams

January 30, 2020



Photos courtesy of Norton | Saint-Gobain Abrasives
Transforming a component from as-welded to final requires forethought and adherence to needed steps to ensure an ideal finish. Following the correct finishing sequence, and using the correct techniques and abrasive products, will deliver optimal results.

Prior to welding, particularly when working with carbon steel, clean the workpiece to remove any mill scale formed from the hot rolling manufacturing process. Mill scale can interfere with weld quality.

Finishing Carbon Versus Stainless Steel

The decision over finishing requirements links strongly to the application of the finished piece and the base material itself. Note that not all welds require seam removal in order to achieve a functional piece. This is especially true for carbon steel—it’s almost always painted, or the seam otherwise may not be visible. Examples include undersea piping or when the metal serves a structural function and hides behind a panel. Note, too, that unfinished welds are inherently stronger than finished welds due to the material removal involved in grinding them down.

In finishing carbon steel, a fairly simple process, the majority of cases involve preparing the steel only where paint will be applied. A rough and well-scratched surface helps the paint adhere better to the metal than a highly finished surface. In fact, for powder coating applications, a coarse-grained two-step weld removal probably is sufficient.

Stainless steel, intrinsically stronger than carbon steel, generally is of much thinner gauge. This feature has implications for grinding, which will be discussed later. The weld finish for stainless steel also depends on the application. For instance, a highly refined finish may be used for aesthetic purposes, whereas an elevator wall panel or hand rail requires a functional No.4 finish to disguise the visibility of fingerprints and scratches.


Flap discs typically get the nod for stainless steel finishing.

Finishers working with both materials should keep them in separate areas of the shop―as well as any abrasives used―to avoid cross-contamination, especially when transitioning from carbon to stainless steel finishing.

Initial Weld Grinding

Weld finishing for both carbon and stainless steel involves the same initial stage: removal of excess stock from the weld itself, with the aim to grind the joint down to a level and continuous surface with the rest of the parent metal. To achieve this initial stock removal, should the surface finish be deemed nonessential, the metal worker might choose to use a simple grinding wheel with an angle grinder.

Although a grinding wheel can be used for stock removal on both materials, a high level of skill and experience is necessary to achieve a passable quality result on stainless steel. Often, a finisher may choose a flap disc when working with stainless steel (see below). Should a grinding wheel be chosen, care must be taken in adopting the correct grinding angle to avoid gouging and undercutting.

Grinding wheels should be the choice for carbon steel, as they will remove the weld seam quickly and any scratches should not be much of an issue. As for the grinding angle, aim for the range of 5 to 35 deg. to horizontal, depending on the grinding wheel used. Apply consistent pressure in both forward and backward motion to achieve an even finish.

If employing a grinding wheel on stainless steel, use a stainless-suitable product―identified on the disc blotter as nonferrous and appropriate for inox. Choose a medium grit over a coarse option for stainless steel, as any scratches imparted are difficult to blend at a later stage, especially when looking to create a refined finish. Another reason to choose medium grit: Working with thin-gauge stainless steel risks creation of noticeable flat spots; particularly on tubular pieces.

Grinding-wheel selection involves a multitude of potential options. As always, the grain size, type of grain and the bonding agent will define how the product performs and feels. Determine appropriate needs before proceeding.

Using Flap Discs

Flap discs represent a popular choice when weld finishing both stainless steel and carbon steel as they hold key advantages over the standard grinding wheel. Flap discs provide long life, superior operator comfort and control (the user typically gains more room for error), lower noise generation, and high surface-finish quality.

Opting for a coarse 40 grit in a flap disc will remove a carbon steel weld seam quickly, readying the workpiece for any additional conditioning prior to painting. Flap discs typically get the nod for stainless steel where a high-quality, more refined surface finish is required. An 80-grit flap disc, the Norton Blaze R980P model for instance, is an ideal choice, but less-experienced operators should opt for 120 grit.

Conditioning and Blending

Should a carbon steel workpiece be destined for powder coating, only one more blending stage is required to complete the grinding process, using a coarse prep disc, such as the coarse-grade Norton Vortex Rapid Prep. A powder coat, thick enough to mask the appearance of any remaining scratches created by a coarse-gritted abrasive, readily adheres to the surface of scratched carbon steel. A paint layer thinner than a powder coat may require further refinement of scratches to avoid showing through the final layer. Here, a medium-grit conditioning pad is recommended. At this point, the carbon steel workpiece should be ready for painting.

On the other hand, whether requiring a highly refined finish or a No. 4 finish, stainless steel certainly needs further conditioning and blending, as the initial scratch would look unsightly on a stainless steel surface. To blend this out, use a medium-grit disc. For further refinement, follow up with a softer blending disc.

The decision over which finish to choose for a stainless steel workpiece depends entirely on the final product’s end use. An abrasive supplier can provide details and options for achieving needed final finish. MF

Information for this article supplied by Norton | St. Gobain Abrasives, Worcester, MA; 800/551-4413.

Industry-Related Terms: Abrasive, Carbon Steel, Coat, Gauge, Grinding, Grit, Layer, Model, Point, Powder Coating, Scale, Stainless Steel, Surface
View Glossary of Metalforming Terms

 

See also: Saint Gobain Abrasives, Norton Abrasives

Technologies: Finishing

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