This chapter analyzes the main factors affecting on-line welding quality from five aspects: environmental requirements, stainless steel pipe body requirements and pre-welding processing, and introduces foreign advanced online welding technology, equipment and software. The main conclusions are as follows:
(1) For on-line welding environment requirements, foreign countries have stipulated the allowable ambient temperature for welding. For stainless steel pipe body requirements, the higher the steel grade, the higher the carbon equivalent, and the worse the weldability. For stainless steel pipe diameter and wall thickness, the higher the steel grade, the stronger the hydrogen-induced cracking sensitivity. The requirements for preheating and post-welding treatment are relatively high, and for the medium in the pipeline, the flow rate of the medium has a great influence on the cooling rate. The higher the flow rate, the greater the cooling rate of the weld. To avoid hydrogen-induced cracking, the required heat input level is high, but not too high. For pre-welding treatment, surface treatment and preheating must be carried out before welding, first consider induction heating. The welding process generally uses low-hydrogen electrodes as welding materials, and thick stainless steel pipes use large-diameter electrodes. Pay attention to the storage and transportation of the electrodes. For avoiding burn-through and hydrogen-induced input cracks, the input heat range can be estimated according to the Battelle thermal analysis model. When the internal pressure of the stainless steel pipe is 5 MPa and the large flow rate is 3 m/s, the recommended heat input range for X65 steel on-line welding is 0.76 kJ/mm to 0.86 kJ/mm, X70 steel is 0.94 kJ/mm to 1.03 kJ/mm, and X80 steel is 1.06 kJ/mm to 1.13 kJ/mm. The welding sequence, the basic principle of fillet welding is that the welding must be carried out in the direction away from the stainless steel pipe. After welding, pay attention to the heat treatment temperature and holding time. For thick stainless steel pipes, increase the holding time appropriately. For residual stress, it is related to the residual stress distribution and strength of X65. Stress depth and welding frequency, while X70 stainless steel pipe does not have this relationship. The key influencers mentioned above should be benchmarked, see Chapter 3 for details.
(2) Three items are listed for advanced foreign online welding technologies: advanced welding methods, temperature control measures for the inner wall of stainless steel pipes, and heat input measurement. Advanced equipment and software include ACRS automatic welding system and PRCI hot opening. Welding thermal analysis software and RHS rapid heating device can introduce foreign advanced technology, equipment and software to improve domestic online welding quality.
