Tempering and annealing are both methods of heat treating metal. The purpose of heat treating is to intentionally alter the physical (and chemical) properties of metal in order to prepare it for manufacturing. Heat treatments can affect many properties of a metal product, including strength, hardness, formability, ductility, malleability, and machinability.
Annealing & Tempering: Heat Treatments for Metal Products
In general, three variables impact the heat treatment of a metal:
- The temperature to which it is heated
- The length of time it is kept at that temperature
- The method or rate of cooling
Depending on the type of metal undergoing heat treatment and the desired result, these variables will shift. But regardless, effective heat treatment requires exact control over all three.
Although there are numerous techniques for heat treating metal, I’ll be covering just two of them today: annealing and tempering.
The Annealing Process
How Does Annealing Work?
When a metal is annealed, it is heated to the ideal temperature and then kept at that temperature for a predetermined length of time. The cooling rate for annealing is on the slow end of the spectrum. For instance, if you were cooking something in the oven and instead of removing it at the end of the cook time you allowed it to cool inside the oven, that’s similar to annealing.
The primary reasons a metal product undergoes annealing is to reduce its hardness/make it softer. That way, it can be machined more easily. Additionally, some metals are annealed in order to increase electrical conductivity.
The Three Stages of Annealing
The annealing process includes three distinct stages:
- Recovery
- Recrystallization
- Grain Growth
Let's take a closer look at what happens during each stage.
Annealing Stage 1: Recovery
The first stage in the annealing process is recovery. During recovery, metal is heated. The common heating device used is a furnace — gas, electric, and induction furnaces can be used to anneal metal. The goal of the recovery stage is to heat the metal to a temperature where its internal structures relax.
Annealing Stage 2: Recrystallization
The second stage in the annealing process is recrystallization. During recrystallization, the metal is heated even further. The sweet spot is to reach a temperature that's greater than the metals recrystallization temperature without surpassing its melting temperature.
When material is held in this sweet spot, new grains can begin to develop within the metal's internal structure without the formation of preexisting stresses.
Annealing Stage 3: Grain Growth
The third stage in the annealing process is grain growth. During this final stage, those grains that began to develop during stage two are able to fully develop. By controlling the cooling process and maintaining a specific rate, the grain growth will yield a more ductile, less hard material.
The Tempering Process
How Does Tempering Work?
Tempering is a heat treatment that improves the toughness of hard, brittle steels so that they will hold up during processing. Tempering requires that the metal is heated to a temperature below what’s called the lower critical temperature — depending on the alloy, this temperature can range from 400-1,300˚F.
Once a steel's lower critical temperature is achieved, that temperature is maintained for a predetermined amount of time. It is the type of steel being processed that determines what the critical temperature is and how long the material needs to be kept at that temperature.
The other variables that impact tempering include the length of time a metal is held at its critical temperature and the rate of cooling. Both of these factors will impact the mechanical properties of the material.
The Visual Impact of Tempering
Tempering is also known to affect the colors that appear on the surface of the steel. These colors, which can range from a light yellow to shades of blue, are used to indicate the final properties of the steel. For example, our tempered spring steels – 1075 and 1095 – are both blue, but due to distinct differences in carbon contact, their shade of blue and other physical properties differ. You can view an in-depth comparison here.
