Annealing vs. Stress Relieving Heat Treatment: What’s the Difference?

Annealing and stress relieving are heat treatment processes that are both used as an intermediate step in the manufacturing process to enhance the dimensional stability and machinability of steel. While both processes reduce residual stresses inside metal parts, they have differing objectives. In this article, we’ll cover the basics of (and differences between) annealing and stress relief and share a few examples of how they are applied to some common materials.

What Is Annealing?

Annealing is three-step heat treating process that softens metal, relieves stress, and increases ductility. The process alters the physical properties of the part and is often used to return the material to its original state, removing hardness that is created as a result of working processes such as stamping, drawing, or other cold forming methods.

Annealing Process Steps

• Recovery. The first step in the annealing process, recovery restores the physical properties of the metal. Heating the material in a furnace reduces dislocations in the part structure, which is what relieves residual stress and enhances ductility. 
• Recrystallization. This step corrects the plastic deformation that occurs as a result of cold working manufacturing processes. By heating the material above its recrystallization temperature, new grains are formed which take the place of deformed or elongated grains. This decreases the hardness of the material and further increases its ductility.
• Grain Growth. During this phase, the new grains that begin to form during the recrystallization stage become fully formed. The rate at which this takes place is determined by the temperature of the process.

How Is Stress Relief Different from Annealing?

Unlike annealing, stress relief does not change the chemical or mechanical properties of the material, its objective is to reduce the stress. 

Depending on the desired outcome and the material, annealed parts will be heated to temperatures ranging from just below or well above the critical austenitic temperature. Annealing is also not typically performed on parts that have been hardened through heat treating, as it is used to mitigate hardness that remains in parts as a result of the manufacturing process. There are three types of annealing processes:

• Full annealing means heating the metal to a temperature where it becomes fully austenitized. It is typically conducted on some stainless steels and superalloys, in addition to other materials.
• Inter-critical annealing is carried out in within a temperature range: above the material’s lower critical temperature (where austenite starts to form) and below the upper critical temperature (where austenite is fully formed). Inter-critical annealing is performed on carbon steels, alloy steels and engineering alloys.
• Sub-crticial annealing fully remains below the critical temperature where a material begins to form austenite. It can also be conducted on carbon steels, alloy steel and engineering alloys, and is usually conducted as an intermediate step between fabrication steps.

Stress relief, however, is performed on both through-hardened and non-heat-treated steel parts. The temperature of the process depends on whether the part has been through hardened, the desired final characteristics of the part, and the specific material. For parts that have not been previously heat treated, it is best to use as high of a temperature as possible without affecting the final properties of the part. For through hardened parts, stress relief is typically done at 50°F below the tempering temperature. Here are some examples:

• For heat treated H13 steel that was tempered at 1,100°F, air stress relief can be performed at a temperature up to 1,050°F to relieve the residual stress formed as a result of heat treatment. Another common stress relieving application for this material is performed on high-pressure die casting dies as a maintenance process to relieve the stresses that form during the die’s use.
• A 4140 steel part tempered at 700°F can only stress relieve up to 650°F, otherwise you risk losing the properties imparted by the heat treatment.
• As a general rule for non-heat-treated steel, 1,250°F is a good temperature for stress relieving.
• For nonferrous materials such as aluminum, deep cryogenics are often used for stress relief instead of heat treatment.

Which Process is Right for Your Application?

Our metallurgy experts can help you evaluate the effectiveness of your heat treatment specifications if you’re unsure whether annealing or stress relieving is the right process for your parts. 

Why Anneal or Stress Relieve with Paulo?

You rely on your heat treater to produce consistent results, job after job. We deliver greater consistency than other commercial heat treaters due to our precision process controls and PICS system, and we demonstrate the accuracy of your process with data and testing. If you’re looking for a partner who can improve your heat treatment results, then let’s start a conversation.