Understanding the Difference Between Annealing and Tempering
When it comes to annealing and tempering, there are often different explanations for both, but it is important to understand the difference when it comes to the production of metal and what each means to a metal’s properties. Often people will refer to a material as tempered when it has increased hardness and has not been annealed yet to relieve its properties. Material can also be tempered through cold rolling and not heat treatment. We will primarily be discussing the effects with stainless steel in this article but most of the concepts apply across a broad range of metals.
Tempering metal is the process of increasing the hardness of a metal in order to increase the toughness. This can be done through both heat treatment or cold rolling. Ulbrich does this through cold rolling and also performs intermediate and final annealing operations so that is what will be focused on in this article. The properties between different metals and alloys obviously differs when it comes to annealing and tempering but the premise and conclusions we can draw will remain the same. Stainless steel and carbon steel are both popular metal choices for both processes, but we will be focusing primarily on the annealing and/or tempering of stainless steel strip and stainless steel wire. For a general overview on the processes, please check out this article about rolling, annealing and slitting at Ulbrich.
Stainless Steel Heat Treatment
The stainless steel material process begins with material being melted and hot rolled down to thicknesses before being sent to a precision re-roller, such as Ulbrich. From here the material is rolled down very thin gauges (thicknesses) with tight tolerances. To understand how annealing plays into these processes let’s look at an example. There is a 301 Stainless Steel alloy 1/8 inch thick master coil Ulbrich has received from a melt mill. A company in the area would like material that is .0045 inches thick. This would be a 96.4% change in reduction, and the metal cannot make the dramatic change in thickness from .125 inches (1/8) to .0045 inches in one shot, it must be reduced in stages. Therefore, the material is work hardened to a reduced thickness at which it can no longer take any more reduction without breaking. From here the stainless steel is annealed. This resets the properties, allowing the material thickness to be rolled to an even thinner gauge, and the process is repeated until the 96.4% reduction from the original thickness is achieved. Resetting the properties in our terminology just implies the material has been changed from tempered to annealed through the annealing process.
When is annealing the best process
Annealing allows the stainless steel’s properties, such as tensile strength, yield strength, hardness, elasticity, or elongation to reset and prepare them for another rolling operation. Often, annealing is the final step of manufacturer if a part maker such as a company deep drawing material orders the material in the annealed form. Annealed material is typically softer and easier to work with for operations with tooling setups that may break or deform easily. When material is ordered to DQ (deep draw quality), this is just a softer annealed form in which the tensile strength and hardness are more reduced than the standard annealed form. As one might venture to guess, DQ stainless steel is better suited for a deep draw operation at the next tier of manufacturing. As with in between rolling operations, annealing also relieves properties for manufacturing operations in between different tiers of manufacturers, such as re-rollers and part makers.
What is Tempering Metal
While annealing is the resetting of a material such as stainless steel’s properties, tempering is the act of getting that steel to a certain hardness or other properties. Tempering can be done through heat treatment in some cases, but with most of Ulbrich’s alloy selection it is done by rolling to a certain thickness with a percentage reduction in mind. Typical tempers achieved in Ulbrich’s rolling processes include Quarter Hard (QH), Half Hard (HH), Three Quarter Hard (3H), and Full Hard (FH). There is also extra full hard and other increments that can be targeted as well but all of these are considered tempered because they have not been stress relieved via the annealing process. Different tempers are appropriate for different applications and Ulbrich’s metallurgy staff is always happy to listen and help make recommendations on this.
The heat treatment process, as mentioned before, is another way to temper a material. Heat treating involves running material through a specialized tempering temperature in order to get that hardness and tensile strength to a certain range or point. This differs from the annealing process because the annealing temperature is chosen with softening the material in mind, not hardening it. With all of these operations, it is paramount to consider the critical temperature and cooling rate specific to the alloy you are working with.
When to temper steel instead of annealing it
The specific market and application of a material are critical when deciding the temper of the raw material needed, but it is important to consider the tools that will be handling the material. Different types of temper will react with different tools differently and in certain cases, it will be preferable to have a softer temper or soft annealed material that may take a little longer to process but won’t cause machine issues that result in downtime. Specific applications are also important too of course. If an automotive part is being stamped and a range of desired mechanical properties are required, then tempering a material may be required. As previously discussed, this is sometimes done with heat treating the material but in Ulbrich’s case, hardening is usually done with the cold rolling process. On the other hand, if you have a medical part that requires a deep draw, we will want to put this through an annealing process before shipping. The stress relieving that comes from annealing (sometimes referred to as soft annealing) will make the material easier to work after going through the hardening process of cold rolling.
Selecting Annealed or Tempered Material
Choosing between annealed and tempered material depends entirely on your manufacturing processes and application of the parts being produced. Ulbrich has a full team of engineers, product managers and metallurgists willing to work with you on selecting the appropriate temper for your project and making sure roadblocks and challenges throughout the way are more easily overcome. Through our partnership we can ensure the correct alloy and temper are selected by trialing material and then locking in a dedicated process routing for the future. As always, feel free to contact us if there are any further questions.