Welding characteristics of P91 steel
① Since P91 is a medium alloy steel, it has a relatively high tendency to cold cracking. The welding crack rate is 100% without preheating. When preheating at 200-250℃, the cold cracking can be avoided.
② During the welding process, it is the primary task to strictly control the interlayer temperature of the weldment and keep it at the preheating temperature or higher. Secondly, it is necessary to pay great attention to the time interval from the interlayer temperature cooling to the start of post-weld heat treatment.
③ The control requirements for welding input heat are relatively high. Practical experience has shown that the impact toughness can be greatly improved by using a smaller electrode diameter, a relatively low interlayer temperature, and a smaller welding line energy.
④ The welded joints are all unstable structures with high hardness in the post-weld state, and corresponding heat treatment must be performed after welding.

Welding construction process
Based on a qualified process assessment, the relevant welding operation instructions were compiled promptly based on the process assessment report. The contents of the P91 steel welding operation manual are as follows:

Preparation before welding
1. Selection of welding materials: welding wire ER90S-B9/welding rod E9015-B9
2. Groove processing: The groove is machined, the groove should be smooth and uniform, and the size should meet the requirements. Careful inspection is required before matching. If unqualified, it must be polished with a grinder or other tools until it is qualified. Clean the rust, oil, and other dirt within 15-20mm on both sides of the inner and outer walls of the groove until the metallic luster is exposed.
3. Matching: Special matching fixtures should be used, and forced matching is not allowed. The inner wall should be flush when matching. If there is misalignment, the misalignment should not exceed 10% of the wall thickness and should not exceed 1mm. Before matching, carefully check whether the base material at the groove has defects. If there are any, they should be treated separately. The welding materials, welding processes, and selected welder technical conditions for spot welding should be the same as those for formal welding. When spot welding small-diameter thin-walled pipes, the gap is 2-3mm, and 2-3 points can be directly fixed in the groove. When spot welding large-diameter pipes, the gap is 3-5 mm, and the spot welding can be fixed with a "positioning block".

Preheating before welding
Use computer temperature control equipment to track and preheat the weld. Thermocouples are arranged symmetrically. Thermocouples should have good contact with pipe fittings. The preheating temperature of argon arc welding is 150-200℃, and the preheating temperature of arc welding is 200-250℃. The preheating width is calculated from the center of the groove by no less than 3 times the pipe wall thickness on each side.

Base layer welding
Use WS-400 welding machine, direct current positive connection. When the preheating temperature reaches the specified temperature and is uniform, the large diameter pipe is welded symmetrically by two people, and the first and second layer welds are both welded by argon arc welding. When welding the base layer, use the internal wire feeding method, pay attention to good root fusion, and control the thickness within the range of 2.8-3.2mm. To prevent the root weld metal from oxidizing, argon protection should be filled in the pipe.

Argon filling protection can refer to the following requirements:
a. The argon filling protection range is based on the center of the groove. At 200-300 mm on each side, soluble paper or other soluble materials are glued with high-temperature-resistant tape to make a sealed air chamber.
b. Use an "inflating needle" to insert from the groove gap or "defect detection hole" for argon filling. At the beginning, the argon gas flow rate is 10-25L/min on the back, 4-8 L/min on the back of the second pass, and 8-12 L/min on the welding gun.
c. Put a lit lighter or match near the gap of the weld. When the flame goes out, it means that the internal air has been emptied and argon arc welding can be performed.

Filling and capping layer welding with welding rod
After argon arc welding is completed, the preheating temperature is raised to 200-250℃, and arc welding is started; DC reverse connection method and two-people symmetrical welding are adopted. Because the molten iron in the molten pool has a high viscosity and poor fluidity when welding P91 steel, and the welding specification is small, it is easy to have defects such as slag inclusion and interlayer infusion. Therefore, to avoid large defects and ensure the comprehensive mechanical properties of the welded joint, multi-layer multi-pass, and small specification welding must be adopted. The thickness of the weld layer should be as thin as possible, and the thickness of each weld should not exceed 4mm; during swing welding, the swing amplitude of the welding rod should not exceed 4 times the diameter of the welding rod. Each weld must be cleaned, especially the joints and both sides of the weld. After the welding of the weld surface is completed, the welder is required to clean and self-check immediately. If the surface is found to be poorly formed, it is immediately repaired. It is strictly forbidden to repair the weld directly after the weld is cooled.

Post-weld heat treatment
When the welded joint cannot be heat treated in time, it should be immediately heat treated at a heating temperature of 300-350℃ and a constant temperature time of 2 hours after welding. Post-weld heat treatment uses multi-channel output and multi-point temperature measurement, and the heater and the thermocouple correspond one-to-one. The upper and lower temperatures of the weld joint are always consistent so that the heating and cooling speed of the heat-treated weld joint is ≤150℃/h. When the temperature drops to 300℃, it can be cooled to room temperature without control. The heating temperature is 750-770℃, and the constant temperature time is 5min/mm, but it must not be less than 4 hours.

Weld quality inspection
The weld quality is inspected according to the acceptance standards for Class I welds, and the inspection content and requirements are as follows:
1. Appearance: According to DL/T869-2004 "Welding Technical Regulations for Thermal Power Plants", the quality standards for the appearance of Class I welds are accepted;
2. Nondestructive inspection: According to JB4730, 100% ultrasonic (UT) and magnetic particle (MT) flaw detection is performed on the welds, and Class I is qualified;
3. Hardness: After heat treatment, 100% hardness measurement is performed, and the hardness value is less than 350HB;
4. Spectrum: 100% spectral analysis and review of the weld metal alloy composition.

Weld repair
1. When the appearance inspection of the welded joint does not meet the standards. For minor cases, it should be polished and welded, and for serious cases, it should be cut off and re-welded.
2. For welded joints that fail the nondestructive inspection, their defects can be welded, but they must be carried out according to the normal welding process or welding repair process regulations based on confirming that the defects have been eliminated.
3. For welded joints that are repaired by welding, the same weld should not be repaired more than twice, otherwise it should be cut off and re-welded.
4. Welded joints that are repaired by welding must be re-heat-treated and non-destructively inspected.

Quality monitoring
Strengthen the control of the welding process of P91 steel. Due to the poor welding performance of P91 steel, poor control of any link in the welding process will lead to fatal defects such as poor mechanical properties and cracks in the welded joints. First of all, all welders must be trained and obtain the qualification certificate for P91 steel before taking up their posts. Secondly, strict control is carried out in terms of measures, materials, equipment, matching, welding, heat treatment parameters, surface quality of welded joints, non-destructive testing, and repair of defective welds.