The purpose of preheating before welding and heating after welding of American standard seamless steel pipes is to reduce the cooling rate of welds and prevent the formation of hardened structures and cold cracks in the joints.
The preheating temperature before welding is generally 100-200℃, and the post-welding heat treatment is not a heat treatment, but also a slow cooling measure. The post-heating temperature is 200-300℃, some are slow cooling, and some are hydrogen elimination treatment. A certain post-heating temperature will cause hydrogen to diffuse out of the weld, and the 16Mn seamless pipe will not gather and cause cracks. The post-heat preservation time is determined according to the thickness of the American standard seamless steel pipe, generally not less than 0.5 hours.
There are many post-weld heat treatments for American standard seamless steel pipes, which are mainly divided into four types:
1. The post-weld heat treatment is at a lower phase transition temperature, such as stress relief annealing, the temperature is generally between 600-700, and the main purpose is to eliminate welding residual stress.
2. The post-weld heat treatment is higher than the upper phase transition temperature, such as normalizing, the temperature is between 950 and 1150, which refines the grains and improves the mechanical properties of the material; the stainless steel is solid solution and stabilized, and the temperature is about 1050, which improves the American standard seamless Corrosion resistance of steel pipe. Especially the ability to resist intergranular corrosion. Another example is quenching. Different quenching processes can achieve different effects and improve the wear resistance and hardness of American standard seamless steel pipes.
3. The heat treatment is higher than the upper phase transition temperature and then lower than the lower phase transition temperature. For example, the upper and lower transition temperature between 4 kinds of post-weld heat treatment, such as normalizing and tempering, quenching and tempering.
4. Between 750-900, some materials are effectively strengthened by recrystallization annealing.
Tips:ASTM A53 covers seamless and welded steel pipe with nominal wall thickness. The surface condition is usually black and hot-dipped galvanized. ASTM A53 is produced mainly for pressure and mechanical applications, and is also used for transport of steam, water, gas line pipes.