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Better material surface, better laser cutting

Study on laser cutting hot-rolled steel quantifies material quality’s importance

Editor’s Note: This article is based on “How Different Steel Surfaces Affect Laser Cutting Speed and Quality: A Quantified Case Study,” presented in Las Vegas at FABTECH®, Nov. 16-18, 2016, www.fabtechexpo.com, by Steve Pugh, director of laser engineering, and Danny Lerman, director of marketing, Steel Warehouse, www.steelwarehouse.com. Pugh travels the country helping fabricators with their thermal cutting operations, including both plasma and laser cutting.

When we talk about precision sheet metal fabrication, cold-rolled steel usually enters the conversation, and for good reason. A cold-rolled steel sheet is essentially hot-rolled steel that has undergone further processing, usually involving compression between rollers. Hot-rolled steel sheet comes from a heated slab that is flattened and run through rollers to achieve its final dimensions, then cooled after processing. This locks the microstructure into place by preventing diffusion, but it also creates a less controlled final shape than its cold-rolled counterpart. The process of holding hot-rolled steel at extremely high temperatures also creates a scaly surface.

Hot-rolled steel has built this country. Buildings are made of it, as is much of the heavy equipment that makes those buildings and the trains that carry the raw material. More and more of these applications are calling for greater precision, which is why more hot-rolled plates are ending up on cutting beds under a high-power laser cutting beam. The one hurdle when laser cutting hot-rolled steel is the material’s surface quality. These challenges were the impetus behind a recent study Steel Warehouse conducted to test the effects of hot-rolled steel surfaces on laser cutting.

The study revealed that throughout the thickness range of hot-rolled material, surface quality has the greatest impact on cutting performance. Get the material surface right, and dialing in the remaining laser cutting parameters becomes much easier.

The Results

Overall, lighter-gauge stock offers more flexibility of laser adjustment, while thicker stock offers a smaller process-parameter window to obtain a good cut edge. The smaller the window, the more challenging it is to dial in laser cutting parameters for optimal cutting.

Any operator who has pushed a machine to its material-thickness limits knows this all too well, and it makes sense intuitively. Machine variables abound: gas flow, laser power, focal point, kerf width setting, and gating or pulsing frequency. (Heavier materials tend to respond well to a lower gating frequency, which puts more heat into the kerf at the same laser power level.)

But these variables can be pushed only so far; at some point, the only variable that can be improved upon in a significant way is the material quality. The study basically quantified this experience, but it also showed that, at any thickness, material surface quality has an outsized effect on cut quality.

What defines “quality” material for laser cutting? It must be flat, of course, but it also must be smooth—and the study quantified this assumption. For apples-to-apples comparison, cutting parameters were kept at the machine’s factory settings; cutting speed was the only parameter that was changed between runs.

The study tested three varieties of hot-rolled material. It first cut straight-from-the-mill, hot-rolled material, scale jacket and all—commonly called hot-rolled black (HR-black). The study also tested hot-rolled pickled and oiled (HRP&O) material as well as blasted material, including hot-rolled blasted (HR-blasted) sheet and blasted plate.

Compared to HR-black and blasted, HRP&O exhibited higher average cutting speeds in both thin and thick material.

Different grades and thicknesses, between 0.05 and 1 inch, were cut using more than 1,500 production runs. Blasted material had the narrowest cutting-speed window, while HRP&O offered the widest window. The data also showed that higher cutting speeds could be achieved using HRP&O.

Flaky Scale and Cut Quality

The study’s results make sense when you consider laser cutting fundamentals. After it’s pickled and oiled, HRP&O has a smooth, clean surface that allows the laser to produce a much more consistent cut.

A good edge comes from the interplay between the basic cutting parameters, including assist gas pressure and the beam’s focal position, the cut’s hottest point. The location of the focus point, be it just above, at the surface, or within the material, determines the depth of focus. The more consistent the depth of focus, the better chance your cutting parameters will produce consistent results.

Herein lies the hot-rolled steel laser-cutting challenge. On a microscopic level, HR-black’s scaly surface has valleys and pits, and these vary the laser’s depth of focus. This continuously changes heat level in the cut, which can throw off other cutting parameters.

As the laser removes parts of the scale jacket, flakes can fall into the melt pool, which in turn can cause blowouts and striations in areas of the cut. If you see a part edge that’s smooth and then suddenly riddled with deep striations for a short distance before immediately becoming smooth again, chances are that flakes of scale got into the cut and caused a blowout. Carried by the assist gas, these flakes become “cutting tools” that gouge the plate edge.

Heavy scale jackets aren’t necessarily bad, especially on heavy plate, as long as the jacket is consistent (which is why some material suppliers will prep HR-black by buffing the surface). A scale jacket helps distribute the beam’s heat along the surface of the material. The problem is that heavy scale jackets often are not entirely adhered in all locations of the material surface. They can be loose and flaky, which doesn’t allow the heat from the beam to distribute evenly and dissipate as uniformly. This makes the cutting process harder to control.

This loose, flaky jacket also can create something that provides perhaps the greatest hurdle to cutting hot-rolled material: an air gap between the scale jacket and the material surface. It’s almost as if the laser beam now must cut two surfaces: the top of the scale jacket and the base material. After the beam hits the scale and travels through that air gap to the base plate surface, its temperature (energy) drops.

Flaking scale makes the problem worse. As noted previously, the flakes can detach and fall into the melt pool. The assist gas can cause the debris to blow through the kerf (blowout), or can even raise up more scale, allowing heat to go under the scale jacket, leading to the spread of uncontrolled heat.

Watch the Plume

This is all happening on a microscopic level. Although operators can’t see it happening with the naked eye, they can find evidence of it happening as they look at the plume of sparks emerging from under the material.

A consistently conical plume is a good sign that the laser is producing a consistent cut edge. If the plume underneath goes off to one side or trails behind the beam, there’s a good chance something (like flaky scale) is causing problems with heat distribution. Sparks may be bouncing on the leading edge of the kerf simply because it’s not melting all the way through, thanks to the loss of heat. The bouncing can get so bad that material may start to blow back up through the cut path. Eventually the laser can lose its cut and start to weld. At this point, operators can stop cutting and restart to regain the cut path. Of course, all this hinders productivity in a big way.

HRP&O ran at a higher percentage of the machine’s factory-set (stock) speed than HR-black or blasted material.

Pits and Valleys From Blasting

The blasting used to produce blasted material produces small microcraters on the material surface, which again spurs laser cutting problems. An inconsistent surface creates an inconsistent beam focus, which creates inconsistent cut quality.

Those microcraters can sometimes make laser cutting more of a challenge than cutting regular, scaly, hot-rolled plate. Oil can help fill in those micro-craters to make a more consistent surface.

Pickling and Scale Jackets

Pickling the hot-rolled steel removes flaky scale, so the surface is constant throughout the entire plate. Note that though pickling does remove the loose, flaky scale, it does not remove the entire scale jacket. A minute scale jacket remains. You can’t see it, but it helps laser cutting. That’s because, again, the scale distributes heat. The top layers of the scale jacket comprise three iron oxides: FeO, Fe2O3, and Fe3O4.

Note that the makeup of the scale jacket can vary depending on whether the material comes from a basic oxygen furnace mill (an integrated mill) or an electric arc furnace melting mill. The latter uses recycled material that leaves trace byproducts, like residual copper, in the scale. Steel from ore doesn’t have these impurities.

The low melting temperature of these impurities can actually help heat distribution and aid laser cutting in some instances. Specifically, these byproducts get into the cut pool and keep it hotter as the laser moves through. This all depends on the amount of impurities in the material, of course. Too much will hinder more than it helps the cutting process.

Straight pickled hot-rolled material (without oil) wasn’t tested, but it’s still applicable in many applications. The pickling creates a completely clean surface, though, again, with a minute scale jacket remaining. It’s good for some applications in which an oiled surface isn’t acceptable to meet specific requirements for welding, painting, or elsewhere. Still, as with anything in fabrication, straight pickling has tradeoffs. Condensation on straight pickled stock, particularly when sitting in plants in the humid South, can lead to rust. This again leads to an inconsistent material surface and a challenging laser cutting situation. In many cases, it can make sense to use such material with a dry oil substance called dry lube that protects against rust without the thin oil coating. Dry lube’s protection doesn’t last as long, however, so it shouldn’t sit in raw stock for long before cutting.

Surface Conditions Really Matter

After cutting thousands of sheets and plates, the study quantified what many experienced operators have assumed for years: material surface has a much greater effect on edge quality than any other cutting condition.

It’s why brushing HR-black with oil before laser cutting makes sense. Some metal suppliers use buffers to dress the surface and then oil it. This can help matters some, but it doesn’t remove all the loose scale and air gas as pickling does. It goes back to giving the beam a consistent surface: no imperfections, no pits and valleys, no loose scale, nothing to hinder a constant focus point and consistent depth of focus.

Yes, cutting parameters may be different depending on the shop climate and various other factors. This of course includes proper preventive maintenance on the machine. Bad optics will give you a bad cut no matter how good your material is. But if you give a well-maintained laser cutting machine consistent material with a flat and smooth surface, simply using the machine’s original cutting parameters from the factory can get you many steps closer to cutting perfection.