Removing Mill Scale Is Easier With the Right Abrasive Product

By Tony Hufford

Mill scale, that stubborn layer of iron oxide that forms on hot-rolled steel, is a common headache in metal fabrication and manufacturing applications.

Leaving mill scale on your base material can compromise adhesion for paint or coatings and weaken welds. Therefore, removing mill scale is a necessary evil — and a time-consuming process — for most fabrication and manufacturing operations.

Using abrasive products is an effective way to remove mill scale in many cases. Find out how to choose the right product for the job and get tips for making quick work of mill scale removal.

What is Mill Scale?

Mill scale is a type of iron oxide that forms on the surface of carbon steel during the hot-rolling process in the steel mill, which is where it gets its name. The high temperatures used in the rolling process induce oxidation of the iron and harden it on the surface of the steel. After hot rolling the steel gets quenched — a process to strengthen and harden it that involves heating the material and then rapidly cooling it. This captures the mill scale properties along the outer surface of the steel.

Mill scale is commonly found on steel plates and sheets because those pieces are frequently hot rolled in a mill. This makes it a common problem for steel workpieces used in general fabrication, as well as in structural, shipbuilding and tank applications.

One easy way to tell if a piece has mill scale is the presence of rust. Mill scale is a natural inhibitor that prevents rust from forming, so if a piece of steel has heavy rust, that means it does not have mill scale. However, if a piece of hot-rolled steel has been sitting outside for a long time but has no rust, it likely has a heavy mill scale layer that will be difficult to remove.

Challenges of Mill Scale

While the mill scale layer is usually thin, it is typically not thin enough that it can be removed by hand without a tool. This is partly because the mill scale layer is extremely hard — much harder than the carbon steel base material.

You can choose to weld through mill scale, but this is difficult to do properly. When mill scale isn’t removed before welding, weld failure can result. Even in applications that don’t require welding, the presence of mill scale can cause difficulties because it prevents paint or coatings from adhering well to the steel.

As a result, most operations — especially those for which weld quality is critical — will take the time to remove mill scale rather than try to weld through it.

Options for Removing Mill Scale

Larger operations like shipyards may use a shot blast machine to remove mill scale, but this method requires expensive equipment so it’s not a viable option for many applications. Others may try to remove mill scale with chemicals, which is a messy solution that introduces acids, resulting in environmental and safety concerns.

For most fabrication and manufacturing operations, a right-angle grinder paired with an abrasive wheel or disc is an efficient and cost-effective option. Because the extra labor time and downtime spent removing mill scale can add up, choosing the right abrasive product for the application is key. Mill scale removal is a non-value-added process, so it’s important to do it as efficiently as possible.

There are three main product categories for removing mill scale:

• Coated abrasives, such as ceramic flap discs or resin fiber discs, are the choice for a majority of operations. These products provide better operator control since they can be operated in a flat orientation against the workpiece and they provide lighter material removal, which helps avoid damage to the base material. However, the hardness of the mill scale layer wears out a coated abrasive disc quickly — often in just a few minutes depending on the mill scale’s thickness and hardness.
• Bonded abrasives, such as ¼” or 1/8” grinding wheels, provide aggressive removal, which is good for taking off the hard mill scale layer. But facilities must be careful to remove only the mill scale and not remove or gouge the base material since grinding wheels are optimally used at a 35° angle to the workpiece. However, grinding wheels provide much better life when tackling mill scale compared to coated abrasives, so it’s a trade-off that operations must consider when choosing the right product.
• Mill scale wheel: A new product from Weiler Abrasives is designed specifically to remove mill scale. It delivers the benefits of a coated abrasive while offering the durability and product life of a bonded abrasive. The new wheel is slightly curved and is textured to provide aggressive mill scale removal. It also offers a larger contact area and is designed to be run at a flatter angle, like a flap disc or resin fiber disc, so there is less risk of gouging the workpiece or removing too much base material.

Tips for Removing Mill Scale

While choosing the right product is a critical first step, several other best practices can help you get the most efficient results when removing mill scale.

1. Test various abrasive options: Not all mill scale is the same. Sometimes it is a lighter layer that can be easily removed with a flap disc or resin fiber disc. But in some cases, it is a thicker layer that is more stubborn. Operations always want the fastest solution possible, but what’s right for one application may not be right for another. It’s best to try a variety of solutions for removing mill scale to find out what works most efficiently for your specific application.
2. Don’t use too much pressure: Using a coated abrasive to remove mill scale will quickly dull that product and result in a worn-out grain. This can lead to operators pushing harder on the grinder to improve performance — increasing fatigue. It’s recommended to use the right abrasive with the proper amount of pressure. Don’t press too hard: Let the tool do the work.
3. Watch the material removal: Only remove the necessary amount of mill scale to promote quality welds — and do it in a way that doesn’t remove base material. This can especially be a struggle when the wrong abrasive product is used. An operation may be removing more material than necessary to complete a good weld. Using a product that allows operators to work at a flatter angle helps reduce the chances of gouging the workpiece or removing too much material. Bonded wheels are used at a 45° angle, whereas coated abrasives or the new mill scale wheel from Weiler Abrasives can be operated at a flat orientation to the workpiece.

Removing Mill Scale Efficiently

Selecting the appropriate abrasive product for mill scale removal depends on the severity of the mill scale, the desired surface finish and the application workpiece. By understanding these factors, you can choose the most effective and efficient method to achieve a clean and weld-ready surface. New wheel technologies can also help deliver the best of both worlds when it comes to choosing between bonded or coated abrasive products.  

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