How to grind, blend and finish a weld seam

As the processes involved and required finishes are often different and are normally driven by your material type and its inherent finish requirements, this article will look at both carbon steel and stainless steel.

Before welding even takes place, particularly with carbon steel, the workpiece should be cleaned to remove any mill scale formed from the hot rolling manufacturing process - mill scale can interfere with the quality of the weld.

We would recommend our Norton Blaze Rapid Strip disc as the ideal product to remove the scale in this instance.

Carbon Steel

Stainless Steel

MIG (Metal Inert Gas) welding

• Also known as Gas Metal Arc Welding (GMAW). 
• A low cost yet highly productive method of welding, which can be used on all common metals and alloys.
• The process uses an inert shielding gas and a semi-automatic wire feed, which acts as the filling material.
• Generally considered to be the best option for heavier or thicker items but will produce a fat weld seam.
• We would suggest MIG welding on carbon steel.

TIG (Tungsten Inert Gas) welding

• Can be referred to Gas Tungsten Arc Welding (GTAW). 
• Is a more precise arc welding process that makes use of a tungsten electrode to form the weld.
• TIG welding is generally recognised to be a more difficult skill to master.
• Is much slower than MIG welding.
• The weld seam is neater, smaller and the most common weld use on stainless steel.

Carbon Steel

Stainless Steel

• Carbon steel is almost always painted.
• Not all welds need to be removed for the metal to be functional, such as carbon steel in applications where the seam is not going to be visible. For example, in undersea piping or when the metal is of structural importance and being hidden behind a panel.
• Unfinished welds are inherently stronger than finished welds due to the material removal involved in grinding them down.
• With carbon steel, finishing a weld is a fairly simple process.
• The steel usually needs preparing only to the extent where paint can be applied. A rough and well scratched surface will actually help the paint adhere better to the metal than if it were highly finished to a shine.
• Where powder coating is going to be used to colour the metal, a coarse grained 2-step weld removal is probably going to be sufficient.

• Stainless steel is inherently stronger than its carbon steel counterpart
• When it comes to use, it generally has a much thinner gauge, which has implications for grinding.
• Stainless steel has a multitude of commercial applications and the weld finish selected is also intrinsically linked to this application.
• For instance, a highly refined finish (which this article will help you achieve) may be used for its aesthetics, whereas an elevator wall panel or hand rail must have a No.4 finish to disguise the visibility of fingerprint and scratches; thus a more functional finish.

The first stage of weld finishing is the same; the removal of excess stock from the weld itself. The aim here is to grind the joint down to a level and continuous surface with the rest of the parent metal.

To achieve this initial stock removal and if the surface finish is not essential, the metal worker might choose to use a simple grinding wheel with an angle grinder.

Using a grinding wheel on a weld

Although a grinding wheel can be used for stock removal on both materials, it would take a high level of skill and experience to achieve a passable quality result on stainless steel.

Pitfalls such as gouging and undercutting can be experienced, so care must be taken to ensure the right angle is adopted when grinding.

VIEW PORTABLE GRINDING WHEELS

Carbon Steel

Stainless Steel

• Grinding wheels should be the choice for carbon steel. 
• They will remove the weld seam quickly and since the appearance of scratches are not such an issue compared to stainless steel, are ideal for this operation.
• Typically, in the spectrum of 5 to 35 degrees to the horizontal (depending on the grinding wheel used) should be the target.
• Applying consistent pressure in both the forward and backward motion is essential to achieve an even finish.
• With a self-sharpening ceramic grain, Norton Quantum3 grinding wheels are made to bite and make light work of stock removal.

• Using grinding wheels on stainless steel requires a good level of skill and experience to achieve a suitable finish.
• Many people instead opt for a fibre disc or flap disc at this stage (which we will come onto).
• If you are working with a grinding wheel, you must use a stainless steel suitable product; this is identified by the disc blotter as non-ferrous (iron-free) and appropriate for inox.
• Use a medium grit over a coarse option for the following reasons: 1. The scratches imparted will be very difficult to blend at a later stage, especially if you are looking to create a refined finish. 2. Stainless steel has a thin gage and you are at risk of creating noticeable flat spots; particularly on tubular shaped pieces.

Using a flap disc on a weld

Flap discs are an ever-popular choice when it comes to weld finishing on both stainless steel and carbon steel and it’s not difficult to see why as they hold key advantages over the standard grinding wheel.

Flap discs are the perfect choice due to their long product life, far superior comfort & operator control (the user is typically afforded more room for error), lower noise generated, and the quality of the surface finish are all significant reasons for choosing these abrasives.

VIEW FLAP DISCS

Carbon Steel

Stainless Steel

Opting for a P40 coarse grit in a flap disc will remove that carbon steel weld seam in no time at all and ready the workpiece for further conditioning (if required) before it is eventually painted.

Metal workers would typically opt for a flap disc where a higher quality, more refined surface finish is required, as such a flap disc (or fibre disc) is the star performer for stainless steel.

Our Norton X-Treme R860 in grit P80 would be ideal at this first stage of stock removal, we recommend that less experienced operators should opt for P120.

Carbon Steel

Stainless Steel

• If you are powder coating your carbon steel, the grinding process is nearly over for you.
• Just one more blending stage is required with the Norton Rapid Prep Vortex in Coarse grade.
• Powder painting is thick enough to mask the appearance of the remaining scratch created by the coarse gritted abrasive and it readily adheres to the surface of scratched carbon steel.
• If the paint layer is thinner than Powder Coat, then you may need to refine the scratches a little further so that they don’t show through the final layer.
• Here, we would recommend a medium grit conditioning pad such as the Norton Vortex Rapid Blend Medium. 

• Whether you are looking for a highly refined finish or a number 4 finish, stainless steel would certainly need further conditioning and blending.
• The initial scratch would look unsightly on a stainless steel surface, to blend this out, we would once again recommend the Norton Vortex Rapid Blend medium grade.
• The Vortex disc performs best in the 5000-6000 RPM range, this non-woven abrasive imparts a consistent, smear-free finish to the metal.
• If this is still not fine enough then follow with Norton Rapid Blend 2SF - the soft material combined with fine silicon carbide grain imparts a smooth bright finish to the surface.
• There are a number of options available in the Norton Industrial Catalogue