In compression fitting tee manufacturing, controlling residual stress after heat treatment is critical to ensuring equipment safety and reliability. Residual stress stems from uneven material deformation or thermal gradients during the manufacturing process. If not effectively controlled, it can lead to deformation, cracking, or even failure of the vessel. The ASME code imposes strict requirements on heat treatment processes. Its core goal is to relax and equalize residual stresses through temperature control and process optimization.
During heat treatment, temperature gradients are the primary factor causing residual stress. When a material is heated, the temperature difference between the surface and interior causes uneven expansion, resulting in tensile stress on the surface and compressive stress inside. The opposite occurs during cooling. The ASME code emphasizes the need to precisely control the heating rate, holding time, and cooling method to minimize the impact of temperature gradients on internal stresses. For example, preheating minimizes the temperature difference between the weld zone and the entire material by uniformizing the material temperature, thereby reducing stress caused by differential thermal expansion. This method is particularly suitable for brittle materials such as high-carbon steel and cast iron.
Optimizing the quenching process is another important means of controlling residual stress. During quenching, rapid cooling of the material surface generates tensile stresses, while slower cooling within the material generates compressive stresses. This combined stress can lead to plastic deformation. The ASME code recommends selecting an appropriate cooling medium based on the material composition and performance requirements.
For example, water cooling is effective but can easily induce large temperature gradients, leading to residual stress concentration. Oil cooling is slower, promoting uniform temperature distribution and is suitable for steels with good hardenability or complex workpieces. Furthermore, the quenching temperature must be selected to balance the completeness of the phase transformation with the control of temperature gradients to avoid stress increase or substandard performance due to excessively high or low temperatures.
Tempering and stabilization are key steps in controlling residual stress after heat treatment. Tempering involves heating the material to a temperature below Ac1 and holding it at that temperature. This allows the residual stresses generated during quenching to be gradually released through plastic deformation and creep, while also improving the material's toughness and dimensional stability. The ASME code states that the tempering temperature and holding time must be set to ensure sufficient stress release while avoiding excessive softening of the microstructure or grain enlargement. Stabilization treatment, through low-temperature treatment or multiple tempering cycles, further releases residual stresses and stabilizes retained austenite, improving the material's resistance to stress corrosion.
Mechanical methods, as a supplement to heat treatment, are also widely used in compression fitting tee manufacturing. Vibration aging (VSA) applies high-frequency vibrations to induce lattice slip within the material, achieving stress relaxation. This method is low-cost and energy-efficient, and is suitable for adjusting residual stresses caused by the forming process in castings, weldments, and other components. Hammering uses external forces to induce plastic deformation in the material, offsetting the original stress distribution. It is often used for post-weld stress relief in thin plates or small components. However, the application of mechanical methods must be combined with material properties and engineering requirements to avoid the induction of new stress concentrations due to improper operation.
Process optimization is a key strategy for controlling residual stress at the source. The ASME code emphasizes that improving welding processes and sequences can significantly reduce the potential for stress generation. For example, using low-current, short-arc welding combined with segmented back-off welding can reduce the heat-affected zone. Prioritizing butt welds with significant shrinkage, followed by fillet welds, balances shrinkage forces and minimizes deformation. Furthermore, hydrogen control technology, through the use of low-hydrogen electrodes, rigorous flux drying, and post-weld heating to remove hydrogen, can reduce the risk of hydrogen-induced cracking and further enhance the vessel's resistance to stress corrosion.
In compression fitting tee manufacturing, residual stress control requires a multi-layered, multi-dimensional control system that integrates heat treatment, mechanical methods, and process optimization. By strictly adhering to ASME specifications, scientifically selecting heat treatment parameters, rationally applying mechanical methods, and continuously optimizing the manufacturing process, residual stress can be effectively reduced, ensuring the long-term safe operation of pressure vessels under complex operating conditions.
