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Basic iron-carbon diagram applications

By Udo O.J. Huff, M.ED.

The short journey from heat treatment to hardening steel

January 2012 - Knowing the fundamentals of heat treating steel, which last month’s column introduced, gives steel workers the basic tools to manipulate the metal’s structure for a desired application. Changing the structure of steel and steel alloys by heating the material up to 1,333.4 degrees Fahrenheit (723 degrees Celsius) is key to achieving different steel characteristics. This temperature, with its corresponding carbon levels and structure transformations, is known as the iron-carbon phase.

With the core knowledge gained from the iron-carbon phase, one can investigate different types of structures derived from heat treating steel.

In a pure pearlite lattice with 0.83 percent carbon, all grains are transformed into austenite structures at 1,333.4 degrees Fahrenheit (723 degrees Celsius).

If the carbon content is less than 0.83 percent, only the pearlite portion is transformed into austenite. In this state, the ferrite portion remains largely in the form of iron crystals in the structure. This results in a mixture of austenite and ferrite. If the carbon content is higher than 0.83 percent, hard iron carbide shells form around the crystals. This structure becomes a mixture of austenite and cementite. It’s important in the heat-treating process to consider the GSE-Line of the iron-carbon diagram. At temperatures above this line, all grains are transformed into austenite. In this temperature range, steel consists of a uniform structure of mixed crystals of Gamma iron. The steel becomes corrosion resistant, soft and non-magnetic.

Reverse transformation
What does reverse transformation mean during slow and rapid cooling?

Slow cooling: The original grain structure recurs after slow cooling. This happens in cooling phases, withholding to restructure, and then continues in cooling. Technically, at the GSK-Line, the face-centered crystals are transformed back into body-centered crystals.

Whether these are ferrite grains, pearlite or cementite grains depends on the carbon content. Below 1333.4 degrees Fahrenheit (723 degrees Celsius), all carbon atoms are once again freed from the face-centered lattice of the austenite grain structure.

Rapid cooling: On the other side, in rapid cooling, the pearlite grain formation is suppressed. Even though the lattice form changes from face-centered to body-centered, carbon atoms are held in the positions they already have occupied in the austenite structure.

Because the body-centered alpha-lattice is smaller than the face-centered Gamma lattice, the crystals become distorted and stressed by the carbon forces. The result of the lattice stress is a hard, brittle and acicular (needle-like) structure known as martensite. The result is a hardened steel.

Hardening: This also is known as austenitizing: for example, heating the steel to hardening temperature with subsequent rapid cooling. With this method, attaining maximum hardness depends on the carbon content.

The hardening process consists of red-heating and rapid cooling. When the steel is heated to temperatures above the GSK-Line, a uniform austenite structure is obtained (austenitizing). The steel then is cooled very rapidly, which results in a transformation of the grain structure when retaining the carbon in the lattice (martensite formation).

The hardness of the steel depends on the carbon content and whether sufficiently hard and brittle martensite grains can form. Ferrite grains are soft, pearlite grains with 0.83 percent carbon, are medium-hard and are not very malleable, while cementite grains are very hard. FFJ

Udo O.J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.

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