ASTM A516 is a widely used specification for pressure vessel steel plates. It encompasses various grades, including Grade 55, Grade 60, Grade 65, and Grade 70.
These grades are known for their excellent weldability, high strength, and improved notch toughness. To further optimize the performance and reliability of ASTM A516 steel, stress relieving plays a crucial role.
In this blog post, we will explore the stress relieving process for ASTM A516 Grade 55, 60, 65, and 70, its benefits, and its impact on the material’s properties.
Understanding Stress Relieving
Stress relieving is a heat treatment process used to reduce residual stresses that may have accumulated during fabrication, welding, or forming operations.
The process involves heating the steel to a specific temperature range, typically between 550°C and 650°C (1022°F – 1202°F), and holding it at that temperature for a specific period. The most common temperature range is 1100-1200°F with a minimum holding time of 1 hrs (reference: API 582).
The steel is then slowly cooled in a controlled manner. Stress relieving helps to minimize the risk of distortion, cracking, and stress-related failures in pressure vessel components made from ASTM A516 steel.
Benefits of Stress Relieving
- Residual Stress Reduction: Stress relieving effectively reduces the residual stresses that can develop in ASTM A516 steel during manufacturing processes. By eliminating or minimizing these stresses, the material’s dimensional stability is improved, reducing the risk of deformation and cracking during service.
- Improved Mechanical Properties: Stress relieving can lead to improved mechanical properties of ASTM A516 steel. It helps to enhance the material’s ductility, toughness, and impact resistance, making it more capable of withstanding high-pressure environments and sudden load changes.
- Weldment Integrity: Stress relieving is particularly beneficial for welded components made from ASTM A516 steel. The process helps to alleviate the residual stresses that can arise from welding, reducing the susceptibility to stress corrosion cracking and promoting the integrity of the weldment.
- Enhanced Service Life: By relieving residual stresses, stress relieving extends the service life of pressure vessel components. It helps to prevent premature failure and ensures the long-term structural integrity and reliability of ASTM A516 steel under demanding operating conditions.
Stress Relieving Process for ASTM A516 Grade 55, 60, 65, and 70
The stress relieving process for ASTM A516 steel involves the following steps:
- Heating: Preheat the steel to a temperature range of 550°C to 650°C (1022°F – 1202°F) in a furnace or other suitable heating equipment. Ensure uniform heating throughout the steel plate to achieve consistent stress relief.
- Soaking: Hold the steel at the specified temperature for a period of time, typically between one to two hours, depending on the plate thickness. This allows the heat to penetrate the material and relieve the internal stresses.
- Controlled Cooling: After the soaking period, slowly cool the steel in a controlled manner. This can be done by turning off the heat source and allowing the furnace to cool naturally or using furnace controls for controlled cooling rates. Avoid rapid cooling to prevent the formation of additional stresses.
Can ASTM A516 be hardended by heat treatment?
No, ASTM A516 steel is not intended to be hardened by heat treatment. ASTM A516 is a specification for carbon steel plates used in pressure vessel applications, specifically for moderate- and lower-temperature service. The steel is typically supplied in the normalized condition, meaning it has undergone a heat treatment process called normalizing to refine its grain structure and improve its mechanical properties.
Normalizing involves heating the steel to a temperature above its critical transformation temperature, typically around 900°C to 950°C (1652°F – 1742°F), and then cooling it in still air. This process helps to achieve uniformity in the microstructure and relieve any residual stresses that may have formed during manufacturing processes.
While normalizing enhances the mechanical properties of ASTM A516 steel, it does not result in significant hardness increase. The steel’s strength and toughness are primarily governed by its chemical composition and the normalizing process, rather than through hardness achieved by heat treatment.
If higher strength or hardness is required for specific applications, alternative steel grades with different chemical compositions and heat treatment capabilities should be considered. These alternative grades may offer the potential for hardening through processes such as quenching and tempering or through the use of alloying elements that promote martensitic transformation.
Conclusion
Stress relieving is a vital heat treatment process for ASTM A516 Grade 55, 60, 65, and 70 steel. By reducing residual stresses, stress relieving improves the material’s dimensional stability, mechanical properties, and weldment integrity.
It enhances the steel’s resistance to stress corrosion cracking and prolongs its service life in pressure vessel applications. Properly executed stress relieving procedures ensure the optimal performance and reliability of ASTM A516 steel, contributing to the safety and efficiency of pressure vessels in various industries.