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What is stress relieving in metals?
Stress in metals can be introduced by the smelting process, by fabrication methods such as rolling, by manufacturing operations like machining, stamping, and fineblanking, and even by heat treatment processes.
All of these operations cause movement in the microstructure of the part, resulting in crystal dislocations that require stress relief to correct.
Stress relieving can be conducted at various times during the manufacturing process or throughout the part’s life. For parts requiring tight tolerances, stress relief is often performed prior to finish machining to create more predictable and repeatable results in the finished part’s geometry. Stress relief can also be performed prior to heat treatment to minimize distortion in the resulting parts. Additionally, for tooling like high pressure die casting dies, stress relief can be performed periodically as part of ongoing maintenance.
Benefits of Stress Relieving
If stresses aren’t relieved by thermal processing, then parts are more likely to become distorted while they are in application. The greater dimensional stability imparted by stress relieving helps components hold their tolerances when they are in use. Application stress by hot service temperatures and high impact loads can compound the residual stresses inside a part if they are not relieved via heat treatment prior to the part entering service.
Components that have tight tolerances, especially those we encounter in the aerospace, automotive, and medical markets, rely on stress relieving to make precision machining possible. Eliminating the crystal dislocations inside the part’s structure makes fine cutting operations more consistent and predictable.
Longer Part Life
Stress relieved parts are less likely to crack or deform while they are in their application, allowing them to last longer.
The Stress Relieving Heat Treatment Process
For most carbon steel and cast iron parts, stress-relief heat treating is the uniform heating of a structure to a temperature below the austenitic transformation range. After holding the parts at the stress relieving temperature, we take them through controlled cooling. Stress can be reintroduced if parts are allowed to cool too quickly. This is especially critical for larger components that naturally cool at a faster rate on the outside when compared to the core of the part.
Types of Stress Relieving Heat Treatment
Depending on your desired results, there are two main methods of stress relieving to choose from to process your metal parts.
Vacuum Stress Relief
If bright, clean parts are of concern for your application, stress relieving in a vacuum furnace (offered in our Kansas City, St. Louis, Nashville, and Cleveland divisions) may be your best choice. The lack of atmosphere in the furnace prevents scale from forming during heat treatment. Also, if your parts require temperatures in excess of 1,650°F, these ranges can only be achieved in vacuum equipment. For ultra-high precision components, we often see stress relieving specifications call for temperatures as high as 2,050°F.
Air Stress Relief
Air stress relief is conducted in a tempering furnace. When stress relieving parts that have not been heat treated, processing at the highest possible temperature will yield the best results. For through hardened parts, the maximum stress relieving temperature is typically 50°F below the tempering temperature; for example, for H13 tool steel that is tempered at 1,100°F, we will process it at up to 1,050°F to relieve residual stress that formed during manufacturing or heat treatment.
Deep Cryogenics for Aluminum
Aluminum parts are typically stress relieved by subjecting them to extremely low temperatures through cryogenics. As temperatures decrease toward absolute zero, the crystals in the metal structure become smaller, which makes dislocations mobile and decreases stress inside the part.
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