Whenever we discuss precision sheet metal fabrication, cold-rolled steel typically comes into the discussion, and even for good cause. A cold-rolled steel sheet is hot-rolled steel that has gone through further processing, normally associating compression between rollers. A hot-rolled steel sheet originates from a heated slab that is flattened and runs through rollers to accomplish its ultimate dimensions, then cooled after processing. This locks the microstructure into position by avoiding diffusion, it also produces a less controlled final shape as compared to its cold-rolled version. The entire process of holding hot-rolled steel at very high temperatures also produces a scaly surface.
Hot-rolled steel has produced this country. Buildings are made from it, as is a lot of the heavy equipment which makes those buildings and the trains that carry the raw material. Many of those applications are calling for greater precision, which explains why more hot-rolled plates are winding up on cutting beds under a high-power laser cutting beam. The one difficulty when laser cutting hot-rolled steel is the material’s surface quality. These obstacles were the impetus behind research conducted recently Steel Warehouse conducted to check the results of hot-rolled steel surfaces on laser cutting.
The analysis has shown that through the entire thickness range of hot-rolled material, surface quality has got the greatest effect on cutting performance. Obtain the material surface right, and dialing in the remaining laser cutting parameters becomes much simpler.
In general, the lighter-gauge stock provides more versatility of laser adjustment, while thicker stock provides a smaller process-parameter window to acquire a good cut edge. Smaller the window, the greater challenging it is to dial in laser cutting parameters for optimal cutting.
Any operator that has pushed a piece of equipment to the material-thickness limits knows all of this too well, and it seems sensible intuitively. Machine variables abound gas flow, laser power, center point, kerf width setting, and gating or pulsing frequency. (Heavier materials often respond well to some lower gating frequency, which puts more heat into the kerf in the same laser power level.)
However these variables could be pushed only up to now; sooner or later, the only real variable which can be enhanced substantially is the fabric quality. The analysis quantified this experience, it also demonstrated that, at any thickness, material surface quality comes with an outsized impact on cut quality.
What defines “quality” material for laser cutting? It should be flat, obviously, it also should be smooth as well as the study quantified this assumption. For an apples-to-apples comparison, cutting parameters were kept in the machine’s factory settings; cutting speed was the only real parameter which was changed between runs.
The analysis tested three types of hot-rolled material. It first cut straight-from-the-mill, hot-rolled material, scale jacket and all of-commonly called hot-rolled black (HR-black). The analysis also tested hot-rolled pickled and oiled (HRP&O) material in addition to blasted material, including hot-rolled blasted (HR-blasted) sheet and blasted plate.
When compared with HR-black and blasted, HRP&O exhibited higher average cutting speeds both in thin and thick material.
Various grades and thicknesses, between .05 and 1 inch, were cut using a lot more than 1,500 production runs. Blasted material had the thinnest cutting-speed window, while HRP&O provided the widest window. The information also demonstrated that higher cutting speeds might be achieved using HRP&O.
Flaky Scale and Cut Quality
The study’s results seem sensible considering laser cutting fundamentals. After it’s pickled and oiled, HRP&O features a smooth, clean surface that enables the laser to generate a lot more consistent cut.
A great edge originates from the interplay involving the basic cutting parameters, including assist gas pressure as well as the beam’s focal position, the cut’s hottest point. The position of the focus point, whether it is just above, in the surface, or inside the material, determines the depth of focus. The greater consistent the depth of focus, the greater the chance your cutting parameters will produce consistent results.
Herein lies the-rolled steel laser-cutting challenge. On the microscopic level, HR-black’s scaly surface has valleys and pits, which vary the laser’s depth of focus. This continuously changes the heat level within the cut, which could throw off other cutting parameters.
Because the laser removes areas of the scale jacket, flakes can fall under the melt pool, which may cause blowouts and striations in regions of the cut. If you notice a part edge that’s smooth and after that suddenly full of deep striations for any short distance before immediately becoming smooth again, most likely flakes of scale got into the cut and caused a blowout. Carried from the assist gas, these flakes become “cutting tools” that gouge the plate edge.
Heavy scale jackets aren’t necessarily bad, particularly on a heavy plate, so long as the jacket is consistent (which explains why some material suppliers will prep HR-black by buffing the top). A scale jacket helps distribute the beam’s heat across the top of the material. However, heavy scale jackets often usually are not entirely adhered to in every location from the material surface. They may be loose and flaky, which doesn’t enable the heat from your beam to distribute evenly and dissipate as uniformly. This will make the cutting process harder to manage.
This loose, flaky jacket can also create something which provides probably the greatest hurdle to cutting hot-rolled material: an aura gap involving the scale jacket as well as the material surface. It’s near as when the laser beam now must cut two surfaces: the top of the scale jacket as well as the base material. Following the beam hits the scale and travels through that air gap towards the base plate surface, its temperature (energy) drops.
The flaking scale helps make the problem worse. As noted previously, the flakes can detach and fall under the melt pool. The assist gas may cause the debris to blow with the kerf (blowout), or may even raise more scale, allowing heat to visit under the scale jacket, resulting in the spread of uncontrolled heat.
Watch the Plume
This is all happening on the microscopic level. Although operators can’t look at it happening using the human eye, they could find proof of it happening because they consider the plume of sparks emerging from underneath the material.
A consistently conical plume is a great sign the laser is creating a consistent cut edge. When the plume underneath goes off and away to one side or trails behind the beam, there’s a high probability something (like flaky scale) is bringing about issues with heat distribution. Sparks might be bouncing around the leading side of the kerf mainly because it’s not melting right through, because of the loss in heat. The bouncing could get so bad that the material may begin to blow back up with the cut path. Eventually, the laser can lose its cut and begin to weld. At this time, operators can stop cutting and restart to regain the cut path. All of this hinders productivity greatly.
HRP&O ran in a higher portion of the machine’s factory-set (stock) speed than HR-black or blasted material.
Pits and Valleys from Blasting
The blasting utilized to produce blasted material produces small micro craters around the material surface, which again spurs laser cutting problems. An inconsistent surface creates an inconsistent beam focus, which creates inconsistent cut quality.
Those micro craters can occasionally make laser cutting much more challenging than cutting regular, scaly, hot-rolled plate. The oil will help complete those micro-craters to create a more consistent surface.
Pickling and Scale Jackets
Pickling the-rolled steel removes flaky scale, therefore the surface is constant through the entire plate. Remember that though pickling does take away the loose, flaky scale, it will not take away the entire scale jacket. One minute scale jacket remains. You can’t look at it, however, it helps laser cutting. That’s because, again, the scale distributes heat. The very best layers from the scale jacket comprise three iron oxides: FeO, Fe2O3, and Fe3O4.
Remember that the makeup from the scale jacket can differ based on whether or not the material originates from a simple oxygen furnace mill (a built-in mill) or perhaps an electric arc furnace melting mill. The second uses recycled material that leaves trace byproducts, like residual copper, within the scale. Steel from ore doesn’t have these impurities.
The reduced melting temperature of those impurities can help heat distribution and aid laser cutting in some circumstances. Specifically, these byproducts enter into the cut pool whilst keeping it hotter because the laser moves through. All of this depends upon the number of impurities within the material. Excessive will hinder a lot more than it will help the cutting process.
Straight pickled hot-rolled material (without oil) wasn’t tested, but it’s still applicable in lots of applications. The pickling produces a completely clean surface, though, again, having a minute scale jacket remaining. It’s great for some applications where an oiled surface isn’t acceptable to satisfy specific requirements for welding, painting, or elsewhere. Still, just like anything in fabrication, straight pickling has tradeoffs. Condensation on straight pickled stock, specifically when relaxing in plants within the humid South, can result in rust. This again results in an inconsistent material surface along with a challenging laser cutting situation. Oftentimes, it will make sense to make use of such material having a dry oil substance called dry lube that protects against rust with an oil coating. Dry lube’s protection doesn’t last for as long, however, therefore it shouldn’t sit in raw stock for very long before cutting.
Surface Conditions Matter
After cutting 1000s of sheets and plates, the analysis quantified what many experienced operators have assumed for a long time: material surface features a much greater impact on edge quality than every other cutting condition.
It’s why brushing HR-black with oil before laser cutting is sensible. Some metal suppliers use buffers to wear the top and after that oil it. It will help matters some, however, it doesn’t remove all of the loose scale and air gas as pickling does. It is returning to giving the beam a regular surface: no imperfections, no pits and valleys, no loose scale, absolutely nothing to hinder a continuing focus point and consistent depth of focus.
Yes, cutting parameters might be different based on the shop climate as well as other factors. This includes proper preventive maintenance around the machine. Bad optics will provide you with a poor cut regardless of how good your material is. However, if you provide a properly-maintained laser cutting machine consistent material having a flat and fine surface, simply utilizing the machine’s original cutting parameters from your factory can get you several steps nearer to cutting perfection.