Some misconceptions exist among engineers and buyers about the “strength” of “mill angle iron” versus “roll formed angle” given essentially the same parameters — gage, leg length, and type of material. The purpose of this article is to assist engineers in the decision-making process when they are faced with the choice of which one to use.
“Structural” shapes are hot mill angle iron products produced by rolling steel while in the heated “plastic” state. These are commonly produced in two chemistries: ASTM A36 and ASTM A588.
The A36 is a low carbon steel similar to the 1008 commonly available as commercial quality cold rolled steel. The A588 is a high strength low alloy material usually sold in sheet form as Cor-Ten.
Dimensionally, the differences between a hot mill product and a cold rolled product are as follows:
- The outside corners of a hot mill product will usually be square corners, whereas the cold roll formed product will have radius corners which are a product of the inside radius (usually 1T or greater) plus material thickness and stretch.
- The web of channels and the legs of channels and angles will be tapered with a hot mill product, whereas the cold formed product is a uniform thickness.
- The outside leg edges of a hot mill product will be rounded whereas the cold formed product will be square due to the fact that the edges are slit from coil.
There is a significant advantage to using cold finished product when there is to be subsequent fabrication such as holes, notches, and miters, etc. The flatness and consistent thickness inherent in the cold formed product facilitates the accurate location of fabricated features difficult to achieve with a hot mill product. These features can often be imparted to the material in the “flat” state prior to forming using a pre-notch process that allows roll formers to combine operations that otherwise would have to be done as discrete secondary operations using a hot rolled process if, depending on the secondary operation in question, it can be done at all. The rigidity of the section may be assumed to be similar for a given gage, using the median measured thickness of the hot mill product as that of the cold formed product.
In summary, gage for gage and dimension for dimension, the cold roll formed process offers the user similar strength and rigidity, but more flexibility than the hot rolled process making roll formed angle, for example, preferable to hot rolled angle, which is commonly referred to as “angle iron”. Please contact firstname.lastname@example.org for more information about this subject if you are considering the use of angle iron or hot rolled (structural) angle.