Give It to Me
Flatness of the steel is simply the amount of space between the plate and a perfectly flat surface. It’s usually measured by placing the sheet on a flat surface and determining the distance from the surface to the highest point of the sheet, then subtracting the thickness of the material. Some people may just measure from the flat surface to the point where the steel starts. ASTM has set industry standards for flatness that all companies are required to follow. What happens if a piece of steel comes out and it’s not within the industry standard flatness tolerance? Well, there are a few different ways to flatten it out to come into tolerance. These include press breaking, roller leveling, and heat treatment. In this article, we’re going to look into each of these approaches, talk about realistic expectations to have when you receive your steel, and explain tolerance standards.
A press brake is a machine tool that utilizes dies by placing the metal between a matching set of dies to get the desired shape. The top portion is mobile
while the bottom die is stationary. The press brake uses a V-shaped die to do this to give enough room to bend the steel. If it was a flat surface,
the steel would just bounce back to the bend. The steel will be placed in the press brake where a bend or bow is, the top press is lowered just enough
to bend the steel beyond the yield point. When it's released, the bend will be flattened out.
Roller leveling is essentially a bending process. The steel goes through a series of alternating bends, which are created by going through upper and lower
sets of leveling rollers. The rolls are able to be individually set for precision. The metal moves through like a wave, where the entry has the biggest
wave and smallest is at the exit point. The steel comes out flat because of the alternating bends and decrease of bend intensity.
This is not recommended since the steel gets heated above the tempering point. The extreme heat changes the properties of the steel, which you want to avoid if you aren’t specifically trying to. After the steel is heated to such a high degree, it needs to be cooled evenly through the quenching process. A cooling medium (water, polymer, or oil) will be used to create the desired grain structure of the steel in place by spraying the steel from the top and bottom. If the quenching portion is done evenly, the plate will be flat. If for some reason the quench is uneven, the plate may have a slight bend or wave to it. If the plate isn’t within tolerance, the heat treat process isn’t as precise as it could be. In that instance, it is best to use a secondary flattening technique.
While I would love to be able to tell you to expect all of the steel you order to be perfectly flat, that just isn’t realistic. It’s not a naturally flat material, but there are ways to flatten it out. Industry-wide standards exist, but that doesn’t mean they will match the tolerance that you require. Most of the time, people don’t take that into consideration to account for additional costs to reach their tolerances needed.
Another thing that tends to be overlooked is what happens to the flattened steel when it’s cut. Your pieces may not remain as flat as the plate. That is because when steel is flattened, stresses are caught inside the metal. When the metal is cut by saw, plasma, laser, waterjet, oxyfuel, or the steel is welded, those stresses are released. That can result in the cut pieces being less flat than the steel plate. Those pieces can sometimes be flattened after to reach the tolerances needed.
Tolerances are different for every plate, depending on length, width, thickness, and type of steel. We talked about how ASTM created standards to follow, but companies are also able to set their own tolerances beyond that. For example, here at Clifton, we strive for ½ of the ASTM A6 standard.
Below is an example of part of the ASTM A6 commercial standards for heat treated plate.
|Specified Thickness||Permissible variations from A Flat Surface for Specified Widths, In.|
|in inches||to 36, Excl.||36 to 48, Excl.||48 to 60, Excl.||60 to 72, Excl.|
|to 1/4, excl.||13/16||1-1/8||1-3/8||1-7/8|
|1/4 to 3.8, excl.||3/4||15/16||1-1/8||1-3/8|
|3/8 to 1/2, excl.||3/4||7/8||15/16||1-1/8|