Whether you are through hardening steel, annealing a cold-worked metal, or solution treating an aluminum alloy, the heat treating temperature is critical for obtaining the desired microstructure, and therefore, the desired metal properties.  Microstructure refers to such things as the metallurgical phases present in a metal and the grain size.

Using a temperature that is too hot can result in a metallurgical transformation that proceeds too quickly or the formation of undesired phases.  Using a temperature that is too low can result in incomplete metallurgical transformations, cold worked metals that do not soften sufficiently, or insufficient stress relief.

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Steel heat treating

For example, during the through hardening heat treatment of a carbon steel, the steel is heated to transform all the ferrite and cementite to austenite and then quenched to form martensite.  If the steel is not heated to a high enough temperature, then there is the risk that all the ferrite and cementite does not transform to austenite.  If this occurs, then when the steel is quenched, the remaining ferrite and cementite will be present along with the martensite.  These ferrite and pearlite remnants can weaken the steel.

Annealing

Another example is cold-rolled sheet metal that is annealed to improve its ductility, and reduce its strength and hardness.  If the annealing temperature is too high, then excessive grain growth will occur.  This will result in the metal having lower strength and hardness than intended.  Also, if the metal is to be formed, there is the risk of orange peel, a cosmetic defect in heavily formed metals with grains that are too large.

Why temperature might be too high or low

So, why might a heat treater use a heat treating temperature that is too high or too low?  To save money, to save time, or just sloppy.  To reduce energy costs a heat treater might try to run its furnaces at the low end of the required temperature range.  However, normal temperature variations throughout a load and normal composition variations within the metal can result in the temperature being too low to cause the desired metallurgical transformations.

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To save time, a heat treater might operate a furnace at the high end of the specified temperature range to try to move the metallurgical transformations along as fast as possible.  Again, with normal temperature and composition variations, the temperature may end up being too high, resulting in excessive or undesired changes in the metal’s microstructure.

As for a sloppy heat treater, who knows what you will get from batch to batch of metal stock or components.

To learn more about the effects of temperature control on steel microstructure and properties, take our Metallurgy of Steel Heat Treating course or read Practical Heat Treating by J.L. Dorsett and H.E. Boyer or Steels: Processing, Structure,and Performance  by George Krauss.  Also, the two courses mentioned in the introduction above will discuss the effects of temperature control on precipitation strengthening and annealing cold-worked metals.

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In this article, I’m going to take a step back to consider the basic science of metallurgy.  On a microscopic level, there are many things going on inside of a metal.  Metals consist of numerous microscopic structures that have a direct and large influence on the properties of metals.  Through composition, mechanical treatment, and thermal treatment these microscopic structures can be modified to impart specific properties.  Whether the desired structures, and resulting properties, are obtained in a completed component or joint between components depends on the knowledge and skill of designers and manufacturers.

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One set of major structures within a metal are the crystal lattice, grains, and phases.  The crystal lattice is the arrangement of the atoms within the metal.  Grains are individual crystals within a metal.  Figure 1 shows grains in a brass alloy.  Phases are different combinations of the elements present in an alloy.  Figure 2 shows pearlite in steel.  The light colored material is the ferrite phase, which is comprised of iron with a little bit of carbon mixed in.  The dark colored phase is cementite, which is comprised of the compound Fe₃C.  It is also referred to as iron carbide.  The properties of a metal are affected by the size of the grains and the phases present.

Grains in brass

Pearlite in steel

Defects in the metal crystal lattice make it possible to form alloys and deform metals with the metals cracking.  These defects are not the same as manufacturing defects such as voids, inclusions, seams, and cracks.  Instead, without crystal lattice defects we would only have pure, brittle metals.

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Various mechanical (e.g. cold rolling) and thermal (e.g. through hardening and precipitation strengthening) processes take advantage of these crystal defects in order to bring about modification of the grains and phases present in a metal, to obtain the desired properties.

Also, the number of crystal defects in the metal can be modified to obtain desired properties.  For example, cold rolling results in an increase in the number of dislocations in a metal, resulting in increased strength.  The, annealing a cold-rolled metal results in a reduction in the number of dislocations and modification of the grains, resulting in a decrease in the metal strength.

A common representation of the relationship between properties, composition, microscopic structures, and manufacturing defects is shown in the image below.  When the effects of the manufacturing processes on the microscopic structures are properly understood, it is possible to consistently produce metal components and joints that have the desired properties.  Essentially, the people in charge of the manufacturing processes are responsible for making sure that during the processes the atoms in metals move to where they need to be.  And designers are responsible for specifying where the atoms should be.

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For more information about the microscopic structures in metals take our online, on-demand Principles of Metallurgy course or read Metallurgy for the Non-Metallurgist, A.C Reardon, editor or Materials Science and Engineering, W.D. Callister.