During our March 19, 2015 metallurgy question and answer webinar I answered the following question: What is needed to meet specs for solution treated and aged 7175 aluminum? This video recording shows my reply.
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.
The strength of metals is improved by impeding the motion of dislocations through the metals. One approach to achieving this improvement is to form a uniform distribution of closely spaced sub-micron sized particles throughout an alloy. The particles, which are called precipitates, impede dislocation motion through the alloy. Not every alloy can be precipitation strengthened. Alloys that can be precipitation strengthened include Al-Cu, Al-Mg-Si, Cu-Be, and 17-8 PH steel. The figure shows precipitates in a Al-Cu alloy.
The particles are formed by precipitation, which involves a series of heat treatment steps. The first step is solution heat treatment. This involves heating the alloy up to a temperature that results in the atoms of the alloying element being dissolved within the solid structure formed by the array of atoms of the main element. For Al-Cu alloys, the copper atoms dissolve into the array of aluminum atoms. The dissolved structure is then retained at ambient temperatures by cooling the alloy rapidly, such as by water quenching.
After cooling, precipitates are formed either by natural aging or artificial aging. With natural aging, the precipitates form at room temperature. With artificial aging, the precipitates form when an alloy is heated to a temperature lower than the solution heat treatment temperature. Only certain alloys will undergo natural aging. The other alloys must be artificially aged.
Regardless of the aging process, as the precipitation process proceeds the precipitates go through a series of stages, with changes in the size, form, and composition of the precipitates. The particular stage of the precipitates has a direct influence on the strength of the alloy. For artificially aged alloys, this is controlled by the aging temperature and time. At any particular aging temperature, there is an aging time at which the alloy will reach its maximum strength. This maximum strength corresponds to a specific stage of the form and composition of the precipitates. Aging for a time that is too short or too long will result in less than maximum alloy strength.
For artificially aged alloys, the aging temperature affects the maximum strength that can be obtained, and the time required to reach maximum strength. For naturally aged alloys, the strength increases over time. The time required to reach maximum strength depends on the alloy.
Finally, precipitation strengthening can be combined with cold-working to give even greater alloy strength.
More information about the metallurgy of precipitation strengthening and precipitation strengthening heat treatment is in our Precipitation Strengthening course. Also, Heat Treatment: Structure and Properties of Nonferrous Alloys by C. R. Brooks, Precipitation Hardening by J.W. Martin, and ASM Handbook, Volume 4: Heat Treating discusses precipitation strengthening.