All-position welding process of stainless steel thick-walled pipe

 

All-position welding of austenitic stainless steel 1Cr18Ni9Ti thick-walled pipes commonly used in thermal power plants is analyzed.

 

1 Weldability analysis

1.1 1Cr18Ni9Ti stainless steel Ф133mm×11mm large pipe horizontally fixed all-position butt joint, the welding difficulty is high, the quality of the welded joint is very high, the inner surface is required to be well formed, the protrusion is moderate, and the concave is not concave. After welding, PT and RT inspections are required.

 

1.2 The thermal expansion rate and electrical conductivity of 1Cr18Ni9Ti stainless steel are quite different from those of carbon steel and low alloy steel, and the molten pool has poor fluidity and poor forming, especially in all-position welding.

 

2 Welding method and preparation before welding

2.1 Welding method

The material is 1Cr18Ni9Ti, the size of the pipe fittings is Ф133mm×11 mm, and it adopts manual tungsten argon arc welding for bottoming, CO2 gas shielded welding for filling and cover welding, and vertical and horizontal fixed all-position welding.

2.2 Preparation before welding

2.2.1 Clean the oil and dirt, and grind the groove surface and the surrounding 10 mm to give a metallic luster.

2.2.2 Check whether the water, electricity and gas circuits are unblocked, and the equipment and accessories should be in good condition.

2.2.3 Assemble according to the size, the tack welding is fixed by the rib plate, and the tack welding in the groove can also be used, but the quality of the tack welding must be paid attention to.

 

3 TIG welding process

3.1 Welding parameters

Use Wce-20 tungsten electrode with a diameter of Ф2.5 mm, tungsten electrode extension length of 4-6 mm, no preheating, nozzle diameter of Ф12 mm, use of TCS-308L welding wire with a diameter of Ф2.5 mm, welding current 80-90A, arc voltage 12～14V, argon flow rate 9～12L/min, Ar purity 99.99%.

3.2 Operation method

3.2.1 The horizontal fixed weld of the pipe butt joint is all-position welding. Therefore, the welding is difficult. In order to prevent the internal welding seam from being concave in the overhead welding, the bottom layer is welded with the wire filling in the overhead welding part, and the wire filling method in the vertical and flat welding parts.

3.2.2 Before starting the arc, the tube should be filled with argon gas to replace the air in the tube before welding. During the welding process, the welding wire should not be in contact with the tungsten electrode or directly penetrate into the arc column area of ​​the arc, otherwise it will cause tungsten inclusion in the welding seam and damage to the arc. Stable, the end of the welding wire should not be pulled out of the protection zone to avoid oxidation and affect the quality.

3.2.3 No matter where the welding position is, the tungsten electrode should be perpendicular to the axis of the tube, so that the size of the molten pool can be better controlled, and the nozzle can evenly protect the molten pool from being oxidized.

3.2.4 When welding, the distance between the end of the tungsten end and the weldment is about 2 mm. The welding wire should be sent to the front end of the molten pool along the groove along the tangent point of the pipe, and the welding wire should be melted by the high temperature of the molten pool. After the arc is ignited, preheat one end of the groove, and immediately send the first drop of welding wire to melt the metal after the metal is melted. Then the arc swings to the other end of the groove, and feed the second drop of welding wire to melt the metal, so that the two drops of molten iron are connected to form a weld. The foundation of the seam, and then the arc swings laterally, stopping on both sides for a while, and the welding wire is evenly and intermittently fed into the molten pool for forward welding.

3.2.5 Do not disturb the argon gas flow during the wire filling process. When stopping the arc, pay attention to the argon gas to protect the molten pool and prevent the weld from being oxidized. In the half circle after welding, the arc melts the overhead welding part of the first half circle. When the molten hole appears, the welding wire is fed. The first two drops can be used to give more spot welding wire to avoid the concave joint. After that, the welding is performed normally.

3.2.6 Grind the end into a slope shape. When welding to the slope, stop the wire feeding, use the arc to melt the slope into a molten hole, and finally close the mouth. Note that the flow rate of the internal shielding gas should be reduced to 3 L/min when the second half of the welding cycle is left, to prevent the welding seam from being concave due to excessive air pressure.

 

3.3 Causes and prevention of common defects

 

3.3.1 Incomplete penetration: The welding current is small, the root gap is small, the welding speed is too fast, the angle of the welding torch is abnormal, etc., all of which are prone to defects of incomplete penetration. The root gap must not be less than 3.5 mm. Proper welding current and correct adjustment of the welding torch angle can avoid incomplete penetration.

 

3.3.2 Severe oxidation: During bottom welding, the argon filling device in the tube fails to play a good protective role, and the back of the weld will be oxidized; during the welding process, the protection of the molten pool and the end of the welding wire is poor, or there are oxidized impurities on the surface of the welding wire. will be severely oxidized. The argon filling device should be as close as possible to the tube, and no gap should be left. The gap of the tube should be pasted with high temperature resistant tin oil paper to avoid oxidation of the weld.

 

3.3.3 Slag inclusion and tungsten inclusion: During the welding process, if the end of the welding wire leaves the argon protection zone during the high temperature process, it is oxidized in the air. In the molten pool, it is judged as slag inclusion in the fracture test; if the length of the tungsten electrode is too large, the action of the welding torch will be unstable, and the welding will not be terminated after the tungsten electrode and the welding wire or the tungsten electrode and the molten pool collide, resulting in clamping. Tungsten. Because the tube is round, the angle of the welding torch and wire feeding must be changed at any time, so the method must be stable and accurate, so as to avoid the phenomenon of slag inclusion and tungsten inclusion.

3.3.4 Concave: The assembly gap is small, and the welding torch swings greatly during the welding process, so that the arc heat cannot be concentrated at the root, resulting in the concave phenomenon that the back weld is lower than the surface of the test piece. The arc heat should be concentrated on the root as much as possible, and more spot welding wire should be given to the overhead welding part to avoid concave.

4 CO2 gas shielded welding welding process

4.1 Operation method

4.1.1 Before welding, pay attention to the nozzle, whether the contact tip is clean, whether the gas flow is appropriate, clean the surface of the bottom layer, and control the temperature between layers.

 

4.1.2 Due to the use of CO2 gas shielded welding for filling and cover layer, the length of the wire extension has a great influence on the stability of the welding process. As a result, the welding process is unstable, the metal spatter is serious, the welding seam is poorly formed, and the protection of the molten pool is not good; if the extension length of the welding wire is too short, the welding current increases, the distance between the nozzle and the workpiece is shortened, the welding vision is unclear, and the welding At the same time, if the extension of the welding wire is too short, the nozzle will be overheated, causing the spatter to stick to or block the nozzle, thereby affecting the gas flow.

4.1.3 When welding, the angle of the welding torch should be perpendicular to the axis of the pipe. Because the pipe is round, the angle of the welding torch should be changed at any time, so as to ensure the quality of the welding seam and avoid the phenomenon of pores and slag inclusions in the welding seam. When welding, use a small crescent swing, stay on both sides to stabilize the arc, and the middle speed is slightly faster, so as to avoid the welding seam bulging and unevenness; the upper and lower joints should cross the center line by 5-10 mm, and the back When half-circle filling and cover-up welding joints, the first half-circle arc-starting welding position can be ground with a gentle slope, so that the second half-circle joints will not be defective; Welding of the cover layer. When capping, it should pause at the edge of the groove to ensure better fusion of the molten pool and the groove. During the welding process, the swing amplitude and frequency of the welding torch should be adapted to ensure the surface size and edge of the welding seam of the capping layer. Fused neatly.

 

The development of steel for power station requires a long period of time. Practices at home and abroad have proved that 12Cr1MoV, 2.25Cr-Mo, TP304, TP347 and other steels have good technological performance and reliable operation. However, in order to increase the steam temperature and pressure, since the 1960s, countries have devoted themselves to developing steel grades with temperatures higher than 580 °C and lower than 650 °C. It can be said that the successful development of T91/P91 steel is a breakthrough in the field of power station steel for nearly 30 years. my country began to introduce and use this steel in 1987. For more than 10 years, some units have basically mastered the welding process of T91/P91 steel. At the same time, they have also carried out research on the welding of dissimilar steels between T91 and steel 102, 12Cr1MoV, and TP304. The operation reliability of superheaters and high temperature reheaters made of steel 102 replaced by T91 is significantly improved. The thickness of the wall of the steam pipe made of P91 can be reduced exponentially. The reduction of the wall thickness reduces the weight of the component, reduces the structural stress and thermal stress, and also reduces the manufacturing cost and construction difficulty.

 

T91/P91, T92/P92, P23/T23, T122/P122 are all tempered martensitic steels used in the quenched and tempered state, and they are all developed under the same idea, and they have similar basic characteristics. New steel grades are significantly less susceptible to weld cracks due to reduced carbon and impurity elements. Since the use of this type of steel can reduce the wall thickness of the components exponentially, the difficulty of welding to obtain a complete and crack-free joint is also greatly reduced compared to steel 102, T9, X20, etc. Nonetheless, the significant deterioration of joint properties is the main difficulty in welding this type of steel.

From the basic characteristics of this type of steel can be imagined:

1) Because the deposited metal does not have the opportunity of controlled rolling and deformation heat treatment, it is impossible for the grains to be refined, and because the Nb and V in the deposited metal are difficult to form fine C, N compounds during the solidification and cooling process Precipitation, the toughness of the weld will be far less than the base metal.
2) The performance of the base metal in excellent supply condition is subject to the high temperature cycle of welding, and the HAZ performance of the base metal will be significantly deteriorated
3) The degree of this deterioration is exacerbated with the increase of welding heat input. The practice of welding T91/P91 steel has proved these assumptions.

 

With this process, all our welded products are well produced and selled with 100% satisfication.