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Written by G.L. Huyett Marketing Department on 10/07/2020 with 0 comments

Carbon and Alloy Steels: Part One

CARBON STEELS

Carbon steel is steel that has properties made up mostly of the element carbon, and relies upon carbon content for its structure. The hardest carbon structure in the world is a diamond, which is 100% carbon. Carbon is present in all steel and is the principal hardening element that determines the level of hardness or strength attainable by quenching. It raises tensile strength, hardness, and resistance to wear and abrasion as the carbon content of steel is increased. However, it can also lower ductility, toughness, and machinability.

Cold‑drawn carbon steel is typically numbered with the prefix "10" in the AISI numbering system, followed by two numbers that represent the nominal percentage of carbon in the product (up to 1.00%). For example, 1018 has 0.18% carbon, while 1045 has 0.45%.

Carbon adds hardness to the material improving its wearability. For carbon contents above 0.30%, the product may be direct hardened ("through hardened"). Carbon steel below this level typically requires carburizing when heat treated. During carburization, carbon molecules are introduced so that a hardened "skin" develops on the surface, creating a "case." This is where the concept of case hardening is found.

Carbon is maximized under 1.00% of steel because higher levels cause the material to become brittle. The higher the carbon content, the more difficult carbon steel is to machine.

In the same way that salt lowers the temperature at which water changes phases, carbon lowers the temperature at which iron changes phases.

Four-Digit AISI Alloy Numbering System

ALLOY STEELS

Alloy steels are derivatives of carbon steels where elements are added or deleted to yield certain properties. Typically these properties include machinability, wearability, and strength. An iron‑based mixture is considered to be an alloy steel when manganese is greater than 0.165%, silicon over 0.5%, copper above 0.6%, or other minimum quantities of alloying elements such as chromium, nickel, molybdenum, or tungsten are present.

Iron alloys are the most common ferrous alloy. Steel is a solid solution of iron and carbon. The carbon is dissolved in the iron – iron is the solvent and carbon is the solute.

Steel, like water, can go through phase changes. With water, the phases are solid, liquid, and gas. With carbon steel the phases are liquid, austenite, and ferrite. If salt is added to water, the transition temperature between phases is shifted. This is why salt is a common ice melt compound. Salt will lower the transition temperature from solid to liquid, but raises the transition temperature from liquid to gas. When carbon is added to iron, the temperatures are altered in the same way. The more carbon that is added (to a point), the lower the temperature at which the phase change will occur. Carbon also creates new phases that don't exist in iron by itself. Pearlite is a mixture of cementite (Fe3C) plus ferrite. The most carbon that can be dissolved in austenite is 0.80%. This is called "eutectic." Other alloys can be described as being eutectic alloys. These alloys have the maximum amount of the alloying elements that can be dissolved into the parent material. It is important to note that the thermal history of a piece also plays a role in its phase development.

For most low‑alloy and carbon steels the more carbon you add (above 0.20%), the more pearlite you get, up to 0.80%. Above 0.80%, you get carbides. If a steel is less than 0.20% carbon, all you can get is ferrite. If a steel has 0.40% carbon, you get pearlite and ferrite. If a steel has 0.90% carbon, you get pearlite and carbides. See illustration below.

To know the chemistry of a steel by knowing its grade, remember the following rules: plain carbon steels are 10xx grades. 10 is plain carbon, and the next two numbers are the carbon content. All 10 grades also have managanese, phosphorus, and silicon. The last two numbers of ALL grades designate the carbon content. If a grade is 12L14 or 10B21, the L means it contains lead for machinability, and the B means it has boron for increased hardenability. If you know the chemistry of the alloy, you will know its hardness, strengths, and if a thermal treatment will work at all.

Steel Phases

Common Carbon Steels and Steel Alloys

One of the more common alloys is 1144, a carbon steel in which alloying elements enhance machining. StressProof™, a product of LaSalle Steel, is an example of 1144 alloy with good machinability and hardenability features that possesses high strength and can be through hardened.

Chrome alloy steels, such as 4130, 4140, and 4340 are so named because chromium content is high (around 1.00%), and is the primary alloying element. Chrome alloy steels begin with a "40" prefix and end in two numbers that account for the nominal percentage of carbon. For example, 4140 has 0.40% of carbon and 1.00% chromium.

Nickel alloy steels substitute nickel in place of roughly half of the standard chromium content for chrome alloys. For example, whereas 4140 has 0.00% nickel and 1.00% chromium, 8630 has 0.60% nickel and 0.50% chromium. These alloys begin with an "80" prefix.

It is difficult to make mechanical comparisons between chrome alloys and nickel alloys as they are similar but unique to a grade. Nickel alloys can be drawn to a more precise finish size and, therefore, are more common in end use steels such as keystock.


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