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Heat treatment

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

Know the basics of how temperature and carbon levels affect steel alloys

November/December 2011 - When working with steel and steel alloys, the goal is to change the size, form and shape for an end use. A skilled journeyperson in the metal trades will apply one or more machining methods to achieve the desired form, such as drilling, turning, milling and grinding as well as cutting by acetylene torch, laser and waterjet.

Aside from machining, one can change steel’s properties through heat-treatment processes using different welding types: gas, electric, MIG/MAG, TIG and laser. The application of high temperatures in the cutting or welding zone will change steel’s grain component structure.

Conventional machining does not generate significant heat to initiate any crystalline changes, but in any welding application, heat is the “dividing” factor with a great influence on the crystalline structure of steel and steel alloys. Steel is structurally crystalline in the solid state.

All iron alloys, except for pig iron and cast iron, are known as steel. The material properties of steel are determined largely by the carbon content. Generally, the higher the carbon content, the more brittle the steel. Heat will have a greater influence on steel’s structure if it contains more carbon.

Structural changes
When steel is heated to 1,333.4 degrees Fahrenheit (723 degrees Celsius), the following structural components, or iron-alloy phases, may form ferrite and cementite. The elementary particles of pure iron consist of cubic body-centered crystals, which each consist of nine ions and free electrons, and they arrange themselves in a lattice-like structure. Ferrite grains form when these pure iron crystals combine. When combined with carbon, these iron ferrite grains become iron carbide crystals (Fe3C). The iron carbide technically is known as cementite. Ferrite and cementite form a mixture known as pearlite, which often appears in laminar (or layered) form. (See Figure 1)

Throughout the lattice structure of the ferrite crystals, the mixture of ferrite and cementite—pearlite—appears as carbon content increases. With 0.83 percent carbon content, all ferrite crystals form a mixture with cementite crystals. This mixture is known as eutectoid, which has the lowest conversion temperature of all iron-carbon alloys. In the solid state, it shows a fine lamellar, uniform structure, similar to the hard layers of a pearl shell. The structure itself is known as pearlite.

Steel structures containing more than 0.83 percent carbon have extra iron carbide crystals, which cannot combine with iron crystals to form the pearlite grains. These are deposited in the form of shells around the pearlite grains and are known as carbide shells. As carbon content increases, these iron carbon shells become progressively thicker and reach the maximum thickness of 2.06 percent carbon.

The resulting steel structures occur when steel reaches 1,333.4 degrees Fahrenheit (723 degrees Celsius):

Hypoeutectoid steel (ferrite and pearlite) with less than 0.83 percent carbon.

Eutectoid steel (pearlite) with 0.83 percent carbon. 

Hypereutectoid steel (pearlite and cementite) with more than 0.83 percent carbon.

When heating a steel alloy more than 1,333.4 degrees Fahrenheit (723 degrees Celsius), a change begins to take place in its grain structure. The carbon atoms dissociate themselves from the iron carbide crystals and distribute themselves uniformly among all the crystals.

At the same time, a curious and interesting phenomenon occurs: the shape of the lattice changes. It becomes face-centered cubic with 14 iron ions. The structure of this solid solution is known as austenite or gamma phase iron. If the steel is heated further, the austenite crystal grains become progressively larger. (See Figure 2)

Knowing these factors, a journeyperson can achieve desired characteristics such as a certain ductility and tensile strength. This basic description of the attitude of the steel alloy in relation to applied temperature and levels of carbon are the foundation for understanding the fundamentals of heat treating steel. 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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