Hardly a new challenge, dealing with mill scale has always been an inherent part of structural steel fabrication and so has its removal – a necessary but time-consuming requirement. This makes it a common problem for general fabrication as well as in construction, bridge making, shipbuilding and pressure vessel manufacturing, to name a few.
Basically, mill scale is a layer of iron oxide that forms on hot-rolled steel as it cools. If it is not removed, it compromises the base material and interferes with the adhesion of paint and coatings.
For welding, fabricators must remove mill scale from the weld area to expose clean base material and ensure weld quality. For shops working under structural welding codes, proper weld preparation is more than a best practice, it is a requirement. The oxide layer must be removed.
“Any structural fabricator doing any bridge work, structural fabrication or civil construction is adhering to a welding code,” says Justin Stauffer, district sales manager, Weiler Abrasives. “An ASME pressure vessel shop is going to adhere to a welding code. Whether it’s AWS D1.1, D1.8, D1.5 or any of the structural codes, they all specifically state that mill scale has to be removed before the welding process.”
Stauffer has a 33-year background in the welding and cutting in the fabrication industry. And about 19 years of his career have been spent as a certified welding inspector.
The oxide layer that results from the hot-roll process is extremely thin (less than 1 mm) but also extremely hard and brittle – much harder than the carbon steel base material. The longer the material sits around, the harder the layer becomes. It is a lot more difficult to remove the mill scale layer from plate that has been sitting out in the yard for two years than from steel that is maybe a month old that came from the mill.
That hardness is precisely what creates problems during welding.
“Because that oxide layer is extremely hard and contains a lot of carbon, it doesn’t melt at the same temperature as the steel itself,” Stauffer says. “During inspection, whether it’s radiographic or ultrasonic inspection, it’ll show up as an inclusion and cause porosity and other issues for the structural steel, which could lead to a failure in the field.”
Traditional removal
On large beams and plate, mill scale is often removed through blasting systems or chemical methods before painting or coating to ensure proper adhesion and prevent corrosion beneath the surface. For the welding, operators typically remove mill scale only in the immediate weld area before the part is put through full removal, “so that they can get a nice, even wetting action on the top of the weld to the base plate itself,” Stauffer explains.
For the welding prep, a right-angle grinder paired with an abrasive wheel or disc is the typical choice. Bonded abrasives, such as standard grinding wheels, can remove mill scale, but they are designed for stock removal and often used at a 35-degree angle.
“Therefore, if the operator is using a little too much pressure on the wheel, they can remove that oxide layer quickly, but they can also start to remove some of that base material,” Stauffer says. “Excessive material removal can lead to undercut conditions, and after it goes through inspection, the welder will have to come back and repair that spot.”
Coated abrasives, such as flap discs, offer more versatility and are available in a range of grit sizes, but they can still be quite aggressive, depending on the grit, and end up removing base material. The discs do offer better operator control because they can be operated at shallower angles. However, the hardness of the mill scale wears out a coated abrasive disc very quickly.
Stauffer says he wants readers to know mill scale cannot be removed with a wire brush.
“The only thing a wire brush does to the mill scale is make it shiny,” he says. “It’s still going to be on the base material. It looks great, but that’s just shiny mill scale. Please use an appropriate wheel or disc.”
New to the market
Recognizing that many structural fabricators wanted a tool dedicated solely to mill scale removal, Weiler Abrasives developed the Tiger mill scale wheel. The new wheel specifically fits the structural steel market. It is designed to be less aggressive as far as removing material than a standard grinding wheel.
“And that really is the whole purpose of this wheel – just to remove mill scale without digging into the base material,” Stauffer says. “It features a distinctive circular studded shape design on the bottom that allows it to be aggressive enough to remove mill scale, but not too aggressive as to dig into the base plate. It’s really a nice hybrid in a bonded wheel that provides enough cutting action to remove mill scale while reducing any effect on the underlying base material.”
The Tiger wheel also has a Type 29 curved shape that ensures uniform surface contact while the hub design ensures the tool guard does not interfere at the proper angle. The result is a wheel that removes mill scale efficiently while preserving the integrity of the base material.
The wheel is intended only for mill scale removal rather than general grinding or weld cleanup. Once the removal is complete, operators can switch to the abrasive for the next stage of the process.
Proper technique
While choosing the right product, i.e., the Tiger wheel, is the first step, following best practices can help operators achieve the most efficient results when removing mill scale.
“Operators don’t need to apply any additional pressure,” Stauffer says. “They should just let the wheel do the work for them and move it in a nice, easy motion across the area that needs mill scale removed. It doesn’t need a lot of pressure to work, and with the circular studded design shape, it doesn’t want to dig deep into the base material.”
Because the Tiger wheel wears differently than a conventional grinding wheel, operators should monitor the condition of the bottom of the wheel.
“Once all the circular studs on the bottom are pretty much flush with the wheel itself, it’s time to replace the wheel,” Stauffer says. “Unlike a standard grinding wheel, most of this wheel isn’t going to be used from the edge back so the operator won’t see it start to shrink on them. But they’ll start to notice it’s taking more time to remove the mill scale and they’re having to use more pressure.”
Standard abrasive safety practices also apply. As with any bonded abrasive, operators should inspect and replace any damaged wheels before use and follow established safety procedures for grinder operation.
Customer connection
Dedicated abrasives for mill scale removal have been limited. Weiler’s development of a purpose-built mill scale wheel reflects feedback from fabricators seeking a faster, more efficient way of preparing structural steel for welding.
“This was the result of the voice of the customer more than anything else,” Stauffer says. “Our customers kept telling us, ‘We just want a tool to remove mill scale before welding.’ There was a need in the marketplace and we’re providing a solution.”
Removing mill scale properly promotes better welding. For shops focused on code compliance, weld quality and minimizing rework, a specialized abrasive specific to mill scale removal can help them achieve their goals.
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