SA335-P91 Alloy Steel Pipe

• Overview

The alloy content of SA335-P91 steel is wCr=9%, wMo=1%, wV=0.2%, wNb=0.08%, wN=0.05%. It is a martensitic heat-resistant steel. The metallographic microstructure is low carbon tempered martensitic steel. body. Due to the adoption of grain refining measures such as micro-alloy controlled rolling technology, it becomes fine-grained steel, which not only helps to improve the impact toughness of the steel, but also is extremely conducive to improving the high-temperature creep strength of the steel.
SA335-P91 is a martensitic fine-grained steel, which makes the main problem of welding P91 steel different from other low-alloy heat-resistant steels. The weak link of the welded joint is not in the fusion zone, but in the weld metal, which is mainly manifested by the decrease in toughness of the weld metal. , The weld has high hardness.

• Process principle

(1) Since SA335-P91 steel is fine-grained steel, if the interlayer temperature is too high during the welding process, t8/5 will increase, causing the grains to grow and lose the original strength and toughness of the steel. On-site welding is impossible It is normalized, so the interlayer temperature must be strictly controlled during the welding process to prevent grain growth.
(2) The heating width, constant temperature temperature, constant temperature time, thermal insulation width, and thermal insulation thickness of heat treatment are the main factors affecting the toughness of the weld. Appropriately increasing the heating width, thermal insulation width, thermal insulation thickness, and extending the constant temperature time will help to increase the martensitic strength. The degree of tempering of the bulk structure improves the toughness of the weld.

• Welding process

(1) Double-layer argon arc welding is used for bottom welding, and multi-layer and multi-pass welding process is used for other layers. Choose a φ3.2mm electrode, and the single layer thickness is ≤3mm. During the welding process, the welding current and welding speed must be well controlled. Due to the relationship between speed, by increasing the welding speed, reducing the thickness of the weld bead, and adopting the operating method of wide swing fast thin layer welding.
(2) During welding, technicians use a far-infrared temperature measuring gun to measure the interlayer temperature of each layer of weld (the interlayer temperature is the temperature 10~20mm in front of the molten pool, expressed as the highest value). The interlayer temperature is strictly controlled at Below 300℃. When the far-infrared thermometer gun shows that the temperature exceeds 300°C, stop welding immediately and continue welding when the temperature drops to 230°C.
After each layer of welding is completed, technicians use vernier calipers to measure the thickness of the weld bead. The maximum thickness is ≤3mm. It is strictly forbidden to form a fillet weld between the groove and the weld bead.

• Welding precautions

The welding current is selected according to the characteristics of the electrode. For electrodes with transition coatings, a smaller current can be used to melt the electrodes, which can reduce heat input. The disadvantage is that the melting point of tungsten in the coating is high, which can easily cause tungsten to be caught in the weld. In short, no matter what kind of welding rod is used, the fluidity of the molten iron must be ensured and the molten pool must be clear. In particular, the root of the groove must ensure good fusion. On this basis, try to use small specifications for operations.

• Heat treatment process

Post-weld heat treatment uses a heat treatment machine model DKPC-12360-12, with crawler-type ceramic resistors for heating and thermocouples tied and fixed. Use K-type armored thermocouples, matching compensation wires and automatic temperature recorders, and use aluminum silicate insulation cotton.
(1) Preheating before welding and interlayer temperature control: Electric heating is used for preheating, and 4 thermocouples are used to control the temperature. The temperature control points are 3 o'clock, 6 o'clock, 9 o'clock, and 12 o'clock. The end of the thermocouple is 20mm away from the edge of the weld groove, and the preheating temperature is 150°C. When the temperature is reached, the temperature is kept constant for 0.5h before starting welding to maintain temperature balance and improve the weldability of the base metal. During electrode arc welding, the temperature rises to 230°C, the over-temperature alarm is set to 260°C, and the interpass temperature is required to be 200~300°C. The heat treater monitors the temperature and heats it up immediately if the temperature is too low. If it is too high, the welding should be stopped immediately. , continue welding when it returns to 230°C, and use a heat treatment machine to track and control the interlayer temperature during the entire welding process.
(2) Post-weld heat treatment
First, during heat treatment, the thermocouple must ensure good contact with the weld. The hot end of the thermocouple is generally placed on the first weld near the edge of the groove. It must be tied firmly with 20# iron wire to prevent thermal expansion at constant temperature. loose.
Second, when installing the heater, the welding nodules, welding slag, and spatter on the surface of the weldment should be cleaned so that the heater and the surface of the weldment are close to each other. After the heater is installed, tie it with 20# iron wire to prevent the heating element from expanding at high temperatures.
Third, increase the thickness and width of heat-treated insulation, with the insulation thickness being 100mm.
Fourth, for elbows, tees, or welds located close to valves and cylinders, in addition to crawler heaters, rope heaters should also be used for auxiliary heating. Use a rope heater to wrap around the location where the heating plate and the workpiece cannot make good contact.
Fifth, the heat treatment parameters of P91 pipeline are shown in the attached table. Reasonably increasing the constant temperature time, heating width, etc. will help increase the toughness of the weld. However, the reduction in weld hardness cannot be overly dependent on the increase in constant temperature time and heating width, otherwise it will cause the base metal to soften. It should be done with precise temperature control and packaging. Find solutions verbally.
Sixth, heat treatment process: after welding, the temperature is first lowered to 110°C and kept at a constant temperature for 60 minutes to fully transform martensite, and then the temperature is raised for heat treatment. All post-weld heat treatment uses "far infrared" electric heating.