You have just welded your sections of metal together and now your newly formed workpiece requires a seamless finish. Here is our guide to removing and finishing your 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.
MIG (Metal Inert Gas) welding
Also known as Gas Metal Arc Welding (GMAW) is 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. MIG welding is 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) and 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 and is much slower than MIG welding. The weld seam is neater, smaller and the most common weld use on stainless steel.
The difference between finishing Carbon Steel and Stainless Steel
The decision over what kind of finish you are looking to achieve is linked strongly to the application of the finished piece and the base material itself (carbon steel is almost always painted).
It should be noted that although this article is primarily looking at how to remove weld seams, not all welds need to be removed for the metal to be functional.
This is particularly the case for 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.
After all, 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. In the majority of cases, the steel 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.
In fact, 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 and when it comes to use, it generally has a much thinner gage.
This feature has implications for grinding which will be discussed as we come onto initial stock removal. 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.
It should be noted that if you are working with both materials, they should be kept separate in the finishing section of your workshop to avoid cross contamination; especially when going from carbon steel to stainless steel.
The last thing you want to do is impart some of the carbon steel stock on the stainless steel workpiece. Be sure to keep any abrasives you are using separate also.
Initial weld grinding
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.
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 and 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.
As previously alluded to, 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.
There are a multitude of potential options out there when selecting a grinding wheel. As always, the grain size, type of grain and the bonding agent will define how the product performs and feels, so make sure you are clear on what you need before you proceed!
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.
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 Quantum flap disc in grit P80 would be ideal at this first stage of stock removal, we recommend that less experienced operators should opt for P120.
Conditioning and blending a welded joint
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 you need to 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, details of which can be found in our new 2019 Norton Industrial Catalogue and as demonstrated by Paul Gray in the video.
At this stage, the welded seam between fused metal pieces should be almost gone. For carbon steel, the surface will now be ready to be painted or used.
For stainless steel, further steps are required for the finer finishes, that’s why the information below will only be relevant to stainless steel.
The decision over which finish to choose for your stainless steel workpiece depends entirely on what the final product is going to be used for.
Time to shine…
A bright and homogenous finish can be achieved relatively easily and quickly.
To blend the remaining surface imperfections and scratches left on your stainless steel by the previous processes, we recommend using Norton’s Rapid Blend NEX-2SF. The fine grade silicon carbide will give the metal a really impressive bright finish.
It’s best used at a 10-15° angle and needs to be moved slowly across the metal, using just the weight of the angle grinder for pressure. Also try NEX-3SF spec for more durability when flexibility and comfortability is less of a concern.
For best results and to avoid burn use at 6000 to 7000 RPM.
This finish should leave the seamless shining surface you are looking for, but if you want an even glossier looking shine, Norton’s Rapid Polish felt disc is perfect.
Number 4 finish
For balustrade or hand railings, a number 4 finish might be specified. The number 4 finish is a common but specialist stainless steel finish that cannot be achieved by a rotary abrasive such as a disc.
Its linear appearance can only be achieved with a belt or wheel. In this case, the previous step is replaced by using a P80 - P120 grit abrasive belt, followed by a Rapid Prep medium grade non-woven belt, before finally being finished off by a very fine non-woven belt.
The intention here is to remove only a small amount of the surface metal without impacting significantly on the overall thickness. It is important to ensure that the sanding follows only one direction if a linear effect is required.
It should be noted that on flat surfaces we would need a Satinex machine with belts on a pump sleeve or interleaved Satinex wheels (p80 coated flaps with a coarse grade non-woven), and for tubes, abrasives belts on a tube finishing machine.
Going a stage beyond the number 4 finish to produce a clean, smooth, and seamless shine to the welded piece, a finer grade abrasive with a very low cut rate is needed.
For more help and information…
More information on any of the Norton Abrasives products described in this article, please refer to our new 2019 Norton Industrial Catalogue.
This PDF download details all of our industrial products alongside some great tips and helpful advice. We would also like to point you in the direction of our Youtube channel which is full to the brim with handy ‘how-to’ videos and product demonstrations related to metal fabrication.
As always, our Norton Experts are also on-hand to answer any questions you may have, just contact us using our web form.