Our last post, Steel Grades and Specifications Explained (Part 1), was an overview of the standards organizations that have developed grades to classify various steels by their composition and physical properties. We also shared ANSI MH16.1: 2012, Specification For The Design, Testing And Utilization Of Industrial Steel Storage Racks, sections 1.2 (Materials) and 1.3 (Applicable Design Specifications) to show how the ANSI specification defers to the standards set forth by several of these organizations, namely, the ASTM, AISI, and AISC.
Here, in part 2, we ‘re going to take a look at the broader steel categories, as well as some of the types of classifications. We ‘ll also take a further look at the sub-categories in the carbon steel category, as most pallet rack on the market today is manufactured from carbon steel.
All steel is composed of iron and carbon. It is the amount of carbon, plus the additional alloys, that determine the properties of each grade. In steelmaking, impurities such as nitrogen, silicon, phosphorus, sulfur and excess carbon are removed from the raw iron, and alloying elements such as manganese, nickel, chromium and vanadium are added to produce different grades of steel.
According to the American Iron & Steel Institute (AISI), steel can be broadly categorized into four basic groups based on the chemical compositions:
Carbon Steel – the major alloying element is carbon, from 0.1-1.5 percent)
Alloy Steel has had small amounts of one or more alloying elements (other than carbon) added.
Stainless Steel generally contains between 10-20 percent chromium as the main alloying element. It’s valued for its high corrosion resistance.
Tool Steel – a term used for a variety of high-hardness, abrasion resistant steels.
Steel can also be classified by several different factors:
Composition: carbon range, alloy, stainless.
Production method: continuous cast, electric furnace, etc.
Finishing method used: cold rolled, hot rolled, cold drawn (cold finished), etc.
Form or shape: bar, rod, tube, pipe, plate, sheet, structural, etc.
De-oxidation process (oxygen removed from steelmaking process): killed & semi-killed steel, etc.
Microstructure: ferritic, pearlitic, martensitic, etc.
Physical strength (per ASTM standards).
Heat treatment: annealed, quenched & tempered, etc.
Quality: commercial quality, drawing quality, pressure vessel quality, etc.
Carbon Steel Range
While all steel contains carbon and iron, the category of “carbon steel is steel in which the main interstitial alloying constituent is carbon in the range of 0.12-2.0 percent. The American Iron and Steel Institute (AISI) considers steel to be carbon steel when:
- No minimum content is specified or required for chromium, cobalt, molybdenum, nickel, niobium, titanium, tungsten, vanadium or zirconium, or any other element that might be added for a desired alloying effect.
- The specified minimum for copper does not exceed 0.40 percent.
- The maximum content specified for any of the following elements does not exceed the these percentages: manganese 1.65, silicon 0.60, copper 0.60.
NOTE: To confuse matters even more, the term “carbon steel” is also used in reference to steel which is not stainless. When used that way, carbon steel may include the alloy steels. For the purposes of this post, we ‘ll be discussing “carbon steel as defined by AISI above.
Generally speaking, as the carbon content goes up, strength increases, but machinability and weldability decrease. As the carbon percentage content rises, steel can become harder and stronger through heat treating; however, it becomes less ductile. (Ductility is a solid material’s ability to deform under tensile stress.)
Carbon steels are further categorized into these four groups, depending on their carbon content:
Low Carbon Steels (or Mild Steels) contain up to 0.3 percent carbon and is one of the largest groups of carbon steel, covering a great diversity of shapes; from flat sheet to structural beam.
Medium Carbon Steels contain 0.3 – 0.60 percent carbon. Increased carbon means increased hardness and tensile strength, decreased ductility, and more difficult machining. These steels are stronger than low carbon steel, but they are more difficult to form, weld and cut.
High Carbon Steels contain 0.60 to 0.75 percent carbon and are more challenging to weld.
Very High Carbon Steels contain up to 1.5 percent carbon and are used for hard steel products such as metal-cutting tools and truck springs. They are very difficult to cut, bend and weld.
While there are steels that have up to 2 percent carbon content, they are the exception. Most steel has less than 0.35 percent carbon.
Stay tuned for our next post – we ‘ll be discussing tensile strength and yield strength of carbon steel and how those numbers affect your pallet rack’s quality and strength.
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