What are the special requirements for welding P122 steel pipe?
Welding P122 steel pipe requires extremely high standards and requires a strictly controlled process. First, a matching, specialized welding consumable must be selected, typically with an alloy composition similar to or containing more austenite-forming elements than the base metal. Sufficient preheating is essential before welding, typically between 200-300°C, to prevent cold cracking. A high interpass temperature is required during welding. Immediate post-weld heat treatment, primarily high-temperature tempering, is required to eliminate residual weld stresses and improve the properties of the heat-affected zone (HAZ).
What are the main challenges in welding P122?
The main challenge in welding P122 is its high tendency to harden due to its high alloy content. This easily forms a hard and brittle martensitic structure in the HAZ, which can cause cold cracking. Another challenge is the "softening" of the HAZ-the tempered zone is weaker than the base metal, potentially creating a weak link in the joint. Furthermore, due to the presence of chromium, the heat-affected zone (HAZ) may undergo precipitation phase changes during welding thermal cycles, affecting long-term performance. Therefore, welding requires experienced welders to perform under very strict process specifications.
Why is post-weld heat treatment (PWHT) crucial for P122 welded joints?
Post-weld heat treatment (PWHT) is mandatory and crucial for P122 welded joints. Its main objectives are fourfold: First, it eliminates the significant residual stresses generated during welding, preventing stress corrosion cracking and delayed cracking. Second, it transforms the uneven weld and HAZ microstructures into uniform tempered martensite, thereby restoring toughness and ductility. Third, it promotes appropriate carbide precipitation and stabilizes the microstructure. Fourth, it improves the properties of the HAZ's "softening zone," maximizing its strength close to that of the parent material, ensuring the overall high-temperature strength of the joint.
Can P122 steel pipe be cold worked?
Cold working (such as cold bending) of P122 steel pipe is extremely difficult and not recommended. Due to its high strength and hardness, cold working requires enormous forces and results in significant work hardening, which can easily lead to cracking or excessive residual stresses in the pipe. If cold working is necessary, it is usually limited to minimal deformation, and a complete stress relief annealing treatment is required after processing. In practical applications, hot induction bending (hot induction bending) is a more common and safer method for P122 pipe.
What precautions should be taken when hot bending P122 pipe?
When hot bending P122 pipe, an induction heating coil is used to locally heat the pipe to a specific hot working temperature range (usually below the normalizing temperature, such as 900-950°C). Throughout the bending process, the heating temperature and bending speed must be precisely controlled to avoid overheating and grain coarsening. After bending, the elbow must be treated as a "weld" and immediately undergo the same complete heat treatment (normalizing + tempering) as the parent pipe to restore the microstructure and properties altered by the hot working, ensuring that it has the same mechanical properties and high-temperature creep strength as the straight pipe section.
