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.
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.
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.
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.
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.