Steel Terminology - Words of Steel

The choices for construction of the modern race car spans a myriad of materials, from the most common and obtainable forms of steels to exotic alloys of aluminum and titanium. The material may be in the form of tubes, sheetmetal, various parts carved out of billets, and complex structures of carbon fiber. All of them are selected with one singular purpose: to build a winning race car.

Due to rules governing the sport, the use of carbon-fiber components is somewhat rare in the construction of the modern stock car with the exception of seats, the base of a HANS Device, and some of the more expensive helmets. Steel, in its various alloys, is the material of choice for the construction of race cars. This applies to go-karts, Midgets, Sprint Cars, Street Stocks, Late Models, and Nextel Cup cars.

The term steel is broadly applied to a variety of materials that fall into the ferrous metal nomenclature. The material we call steel takes many forms. It is very difficult to look at a section of steel and determine strength or suitability for use in a race car--you need to see much deeper than is possible with the naked eye. An examination of the component parts that make up the steel from a molecular perspective will determine the characteristics of a particular alloy of steel.

So the word steel can mean many different things, and you will need some further definition to really understand the true meaning and application. What follows is a list of words and phrases that define steel in terms that might be beneficial and meaningful. While all of these terms may or may not be used in our discussion, review them for possible future reference.

The Terms

Alloy: A mixture at the atomic level of two or more metals, one replacing or occupying interstitial positions between the atoms of the other (i.e., well-mixed, homogenous). For example, brass is an alloy of copper and zinc.

Steel: A generic term that has come to mean an alloy of iron with small amounts of carbon. mechanical properties can vary over a wide range based on the materials alloyed (e.g., copper, carbon, magnesium, or titanium).

Anneal: To soften metal. For ferrous metals (i.e., those containing iron--this definition works for our particular application), the process involves heating the steel to its critical temperature, then slowly cooling.

Normalize: Heating above the re-crystallization phase, followed by a cooling process at a controlled pace. The normalizing process often takes place in open air at room temperature. Normalized metal generally has greater strength than metal that has been annealed. Most metal fabricators prefer to work with metal in this state.

Billet: A solid, semifinished round or square product that has been hot-worked by forging, rolling, or extrusion. The definition applies to both ferrous and nonferrous metal.

Carbon Steel: Steel containing small percentages of the elements carbon, silicon, phosphorus, sulfur, and manganese, in addition to iron.

Ferrous: Metals that consist primarily of iron.

Cold Rolling: Forming process in which metal is rolled or drawn through dies at room temperature. This produces a product with multiple advantages over hot-rolled steel, such as tighter tolerances, improved finish, and straightness.

Ductility: Refers to the amount of deformation a material experiences before complete structural failure. The greater the ductility, the more abuse the metal will take prior to failure. This is not to be confused with strength.

Elasticity: Ability of a material to regain its original shape after it is distorted.

Heat Treatment: A combination of heating and/or cooling operations applied to a metal or alloy while in its solid state in order to obtain desired conditions or properties relating to hardness, ductility, or elasticity.

Hot Rolling: A metallurgical process in which the metal is passed through a pair of rolls and the temperature of the metal is above its recrystallization temperature.

Mild Steel: Steel with carbon content from 0.15 to 0.25 percent.

Chrome-Moly Steel: High-strength, high-stress steel. Also known as 4130, this steel is often called chrome-moly because of the chromium and molybdenum concentration, which is around .80-1.10 for chromium and .15-.25 for molybdenum. This alloy of steel has a very high tensile strength in non-annealed conditions around 95,000-110,000 psi. With proper heat treatment the strength numbers can soar to as high as 225,000 psi. A very common choice for race-car construction, costs are at least double over mild steel.

Stress Relieving: The process of reducing residual stresses in material by heating to a defined temperature and time. This treatment is utilized to relieve stresses induced by casting, quenching, normalizing, machining, cold-working, or welding.

Pickling: Chemical or electrochemical process for the removal of oxides from the surface of metals.

Passivation: The changing of the chemically active surface of a metal to a much less reactive state.

Temper: A process to harden metal by reheating and cooling in oil.

Tensile Strength: The ultimate strength of a material subjected to tensile loading.

Works Harden: The hardening of metal as a result of pressure or bending.

Yield: The amount of stress just above the elastic limit when a substance begins to be permanently changed in shape (i.e., deformed).

TIG-Welding: Also known as gas tungsten arc-welding (GTAW), the legal name was first patented by C.L. Coffin in 1890. Tungsten inert gas (TIG) is the more common nomenclature. In the '20s, H.M. Hobart further developed the process and used helium as a shielding gas, which is where the term Heliarc originates. This process was much better for welding exotics such as magnesium, stainless, aluminum, and plain steel. The gas shield used in 99 percent of race car fabrication is argon, not helium.

MIG-Welding: Patents for Gas Metal Arc Welding (GMAT), or Metal Inert Gas (MIG), as we commonly refer to this type of welder, were filed in 1924. A similar patent was filed by General Electric in the mid-'30s. The basic premise for this type of weld is that the arc is shielded by an inert gas--e.g., carbon dioxide or a mixture of argon and carbon dioxide--and the filler rod is fed into the weld puddle automatically, thus preventing oxidation of the joint.

For the most part, the materials we can use to construct our cars are designated by the various sanctioning bodies. This makes the decision and design process much easier. Essentially, we are rule-bound to use steel in the construction of race cars, with the exception of open wheel and sports cars of the highest formulas. They use more exotic and expensive materials such as carbon-fiber for chassis construction, but even these cars have many component parts fabricated from steel.

The steel used to build race cars falls into two distinct categories: low-carbon-steel and high-carbon-steel alloys. Low-carbon steel is also referred to as mild steel, or steel that has less than 0.15 percent carbon. High-carbon steel has carbon levels over 0.28 percent. While the chemical makeup of steels is an interesting topic, it does not fit into this discourse. For the purpose of this discussion, these basic designations are adequate.

Within the race car realm, the majority of steel used in the construction of a chassis is made from steel tubes, which may be round-, square-, or even oval-shaped. For the most part, tubes are manufactured in two ways. The manufacturer rolls the steel flat into plates, then bends or rolls it into the desired shape. The ends are butt-welded into the desired configuration.

The other method is extrusion. The metal is drawn over an appropriately sized mandrel--a process known as drawn-over-mandrel (DOM) tubing. This process yields a product that is more uniform in size from tube to tube. Also, both the inside and outside are smooth, without the seam from the weld process that is present on the tubes that are butt-welded together. Generally, welded-seam tubing is available only in the less-expensive and lower-strength steels.