What is Mill Scale And How To Remove It

Mill scale is something that metal workers and welders should be aware of. The remaining mill scale from production can develop in all varieties of steel. It must be eliminated before working with the steel because its presence can cause corrosion and impair a weld. This article will define mill scale and go over a few methods for getting rid of it.

MILL SCALE: WHAT IS IT?

Hot-rolled steel develops a flaky surface known as mill scale or simply scale. Iron(II) oxide (FeO), iron(III) oxide (Fe2O3), and iron(II,III) oxide (Fe3O4) make up its composition.

Mill scaling is present in steel plates, sheets, and profiles. It appears in rolling mills during the rolling of hot iron or steel billets. Steel will come into contact with oxygen during heating, conditioning, and rolling; this oxygen will oxidize the steel's surface, leaving it flaky—mill size.

It appears bluish-black and has a thickness of roughly 0.004 inches. It may have a flaky or powdery consistency. Moreover, the thickness of the mill scale is not uniform

At first, mill scaling serves as a safeguard. It sticks to the surface and keeps the steel from rusting from the air. This defense, though, is just momentary. Because of its uneven thickness and texture, the mill scale easily chips.

Because mixed iron oxides are less reactive than steel, they are used as cathodes. Any cracks in the coating will accelerate steel corrosion on the exposed surface.

The air, moisture, and degrading iron oxides all contribute to the rapid oxidation that takes place. High porosity levels caused by excess oxygen focus stress on the remaining metal.

Mill scale cannot be avoided unless steel is hot-rolled in an inert atmosphere.

It will be challenging to process the steel because of the mill scale. The paint you use will flake off when the scale is removed. Moreover, it becomes bothersome when welding.

MILL SCALE AND WELDING

Welding procedures suffer from the effects of mill scaling. Although some welding methods are effective, the scale may adversely affect the characteristics of steel.

ARC STABILITY AND PUDDLE FLUIDITY

Before welding, mill scale removal must be done. The scale functions as an electrical and thermal insulation. This behavior leads to arc instability and limits the fluidity of puddles.

Steel naturally conducts heat and electricity. The puddle and arc will be smaller because the mill scale acts as an insulator.

Your bead will become convex and have undercuts as the puddle becomes less fluid.

When metal and earth are covered in scale, the current and voltage traveling through them will drop and become unstable. This facet generates arc instability.

It will keep you from initiating the arc and result in sparks, splatter, and shallow penetration.

IMPURITIES 

Oxygen and other contaminants are present in the scale and can seep into the steel. Oxygen ions are released into the weld metal as the mill scale degrades. The porosity will rise due to the oxygen, weakening the joint. Concentrated tension is placed on the weld metal's non-porous areas.

Additional contaminants in the mill scale might produce non-metallic inclusions in the weld metal. These impurities will affect the material qualities of the steel, albeit the exact consequences will depend on the final alloy.

Modified characteristics could alter the steel's capabilities and render it inappropriate for the intended use.

INSUFFICIENT FUSION

It can also prevent fusion, allowing the metal component to disintegrate naturally in the absence of outside pressure. The toes will experience uneven weld metal deposition, which will prevent fusion. The decreased fluidity resulting from an out-of-position weld will prevent fusing.

You might not be able to give the metals enough heat because the mill scale melts at a temperature more significant than that of steel. The steel component won't ignite as a result.

SURFACE IMPERFECTIONS 

Surface flaws like porosity and undercuts result from mill scale chips. Because of these flaws, the surface will get more concentrated with oxygen from the surrounding moisture and air.

Steel will rust rapidly due to the breakdown of the protective barrier since it is highly reactive to oxygen.

METHODS FOR ELIMINATING MILL SCALE

Scale damages your metal part negatively. You've come to the correct site if you're looking for information on how to remove the mill scale.

Since fresh mill scaling is firmly attached to the surface, it will be difficult to remove. The majority of mill-scale removal methods will agitate it on the surface.

1. ANGLE GRINDER

The mill scale can be eliminated by using an angle grinder. Be careful not to use sanding wheels or discs with ceramic, zirconia oxide, or aluminum oxide in them. These will soon become jammed. Choose one made of silicon carbide instead, as it won't clog as quickly.

A silicon carbide disc designed for concrete, stone, or metal can be used. Remember that you should not use the ones made of metal but rather the ones intended for stone or concrete on the mill scale

Before using stripper discs, remove any oil from the workpiece to further prevent clogging.

You could use wheels, stripping discs, or nylon brushes if your scaling is not too severe. There will be no harm done to the surface beneath the scale by these abrasive products.

But nylon won't function as well as a wire wheel. Thick layers of mill scale can be removed with wire wheels. Remember that using the wire should only be done in extreme circumstances because it will harm the surface beneath the scale.

2. SHOT BLASTING 

Metal producers are aware that shot blasting may clean, polish, and strengthen the metal. You can also use shot blasting to remove the mill scale if you already use it to treat the steel's surface.

Any contaminants on the surface will be removed by shot blasting, keeping it sufficiently clean for a protective coating to be applied.

Shot blasting is fast and economical, but it requires tools like a conveyor, swing table, tumble blaster, and monorail.

It might not be worth the bother if you do not intend to paint or treat the surface. To remove the mill scale, shot blasting takes longer than using an angle grinder.

3. CLEANING WITH FLAMES 

A hot oxyacetylene flame is passed over the steel surface during the flame-cleaning process. It prepares the steel for processing by removing mill scale and corrosion.

Paint, burrs, faulty welds, flaws, and surface blemishes can all be removed using flame cleaning. You should only use this method for mill scale removal if you have more than just mill scaling to repair because it can be expensive.

Additionally, do not flame clean a thin workpiece. This technique may cause irreversible warping of the metal.

4. ACIDS 

It is difficult to predict when the chemical process will end, though. A protracted reaction can harm the steel itself, which could result in further corrosion and diminished mechanical qualities.

You must add another material to halt the chemical reaction. You risk damaging your part if you don't. If you do not allow the reaction to continue for a long enough period, you might have to repeat the process

Furthermore, the removal of chemicals may cause issues for people, animals, and the environment. It emits poisonous vapors and can burn your skin. Acid can harm aquatic life if it comes into contact with water.

If you can wait a little while, a vinegar bath is another safer choice. Removing mill scale from steel is frequently a pretty simple operation that involves soaking the steel in distilled white vinegar for up to 24 hours. But it works best to do this using small pieces of steel that will fit within a vinegar-filled container.

ALTERNATIVES FOR WELDING WITHOUT REMOVING MILL SCALE

GAS-SHIELD WELDING 

The mill scale should ideally be eliminated before welding. Removal procedures, however, raise the price. You can attempt welding through the mill scale if money is tight.

If you operate in a production setting, gas-shielded welding is a great choice. Deoxidizers, which eliminate scale during welding, are frequently included with solid wires. High processing rates are possible without sacrificing quality.

Solid wires, however, have the potential to cause arc instability and splatter

A flux-cored wire's high deoxidizer content and slag system allow it to weld through thick scaling. Wires with a metal core can increase output and remove mild to thick deoxidizer scaling.

Remember that wires with metal and flux cores are more expensive than those without. However, they can fill gaps, boost travel speeds, lessen splatter, and increase productivity.

SUBMERGED ARC WELDING 

For large items, wire-fed submerged arc welding (SAW) is the method of choice. A torch that travels along the weld joint passes through the wire. Granular flux is required for this process.

Throughout operation, the arc is invisible because it is immersed in the granular flux. The flux, wire, and base material will melt under this arc, forming a pool of welded metal. The molten flux will deoxidize the surface after melting and provide a shielding environment for the metal.

SAW fluxes may be neutral or active. Neutral ones are less successful in welding through mill scale than active ones. Neutral fluxes, however, offer superior mechanical characteristics.

SELF-SHIELDED  WELDING 

You can utilize flux-cored welding (FCAW-S) or self- shielded metal arc welding (SMAW) when welding outside.

Because the slag system and deoxidizers on its electrodes break down the scale, SMAW is able to weld through the mill scale. Although most electrodes function on thin to moderate scaling, you can obtain specific stick electrodes to deal with thick scaling.

Though less efficient than gas-shield wires, FCAW-S wires can nevertheless be helpful if gas cylinders cannot be transported to the site. These feature scale-cutting deoxidizers and a slag system.