Due to the good high temperature comprehensive performance of P91 steel, it has been widely used in the main steam pipeline of Large Power Stations in China in the past ten years. This paper describes the welding characteristics, welding process, welding quality inspection, welding quality monitoring and development trend of P91 pipeline.
In the construction of many large-scale thermal power station units in China in the past ten years, A335P91 (referred to as P91 steel) steel is widely used in the main steam pipeline. The steel is a new martensitic heat-resistant steel developed in the late 1970s and early 1980s in the United States, equivalent to the domestic 9Cr-1MoV steel, the steel compared with domestic steel, with high temperature strength, good oxidation resistance and creep resistance and relatively high thermal conductivity and low thermal expansion rate, etc., to fill the gap between ferritic steel (such as P22) and austenitic steel, so that the weldment has a small cross-sectional size, effectively reducing the weldment wall thickness, reducing the amount of material used, reducing the thermal stress of the pipeline, Reduces the risk of thermal fatigue cracks.
Huaneng a certain area of the power plant phase II project #3, #4炉扩建的2× 300MW coal-fired units, designed by Zhongnan Electric Power Design Institute, the boiler is produced by Dongfang Boiler (Group) Co., Ltd., the model is DG1025/17.4-II.12, the steam turbine is produced by Harbin Steam Turbine Co., Ltd., the type is subcritical one-intermediate reheat, single-shaft double-cylinder double-cylinder steam condensing steam turbine. The design temperature of the main steam main, branch pipe and high side valve front pipe is 545 °C, and the pipe specifications are Φ364×41, Φ273×29 and Φ216×23, respectively, and the material is A335P91.
P91 steel in the past in China’s thermal power station application is not very common, the welding process evaluation work of each construction unit and the characteristics of welding, heat treatment need to be explored in the construction process, and the pipeline assembly sequence also plays a key role in welding quality. In order to ensure that the welding process and welding quality of P91 steel pipes meet the requirements, the welding process assessment must be made according to the welding characteristics of P91 steel, and the basis for on-site installation and welding construction is used according to the assessment report, and this process requirement is strictly implemented during on-site construction.
The chemical composition of steel
The chemical composition of A335-P91 steel is as follows:
As can be seen from the above table, this steel belongs to the medium alloy heat-resistant steel.
Welding characteristics of P91 steel
(1) Because P91 is a medium alloy steel, it has a fairly high tendency to cold crack, welding cracks up to 100% without preheating, and cold cracks can be avoided when preheating 200 ~ 250 °C.
(2) Strictly control the interlayer temperature of the weldment during the welding process, so that it is kept at the preheating temperature or higher temperature is the primary task. Secondly, great attention should be paid to the time interval from the inter-layer temperature cooling to the start of post-weld heat treatment.
(3) The control requirements of welding input heat are relatively high, and practical experience has proved that the impact toughness can be greatly improved by using a smaller welding rod diameter, a relatively low interlayer temperature and a smaller welding line energy.
(4) The welded joints are unstable tissues with high hardness after welding, and must be treated accordingly after welding.
Choice of welding method
P91 steel has a high tendency to harden, is very sensitive to welding cold cracks, and can be welded with tungsten argon arc bottoming and low hydrogen welding rod filling and cover surface.
Welding process specifications, parameters
Table 1 Welding process specifications and parameters of each layer
|layers||Welding method||Welding materials||Welding parameters|
|model||Specification||polarity||Current (A)||Voltage (V)|
|1 to 2||GTAW||ER90S-B9||f2.5||DC positive connection||90～110||10～14|
|3 to 6||SMAW||E9015-B9||f3.2||DC reverse||100～120||20～24|
|7 floors and above||SMAW||E9015-B9||f4.0||DC reverse||135～150||20～24|
|cover layer||SMAW||E9015-B9||f3.2||DC reverse||100～120||20～24|
Preheating method/temperature: tracking electric heating/150-200°C (GTAW), 200-250°C (SMAW).
Shielding gas: argon, purity ≥99.99%, argon flow: 8-12L/min for welding torch, 10-25L/min for the first pass on the back, and 3-8L/min for the second pass.
Interlayer temperature: <250℃ (using far-infrared measuring instrument to control temperature).
Post-heat treatment: After welding, the temperature is raised to 300-350℃, and the temperature is kept for 2h.
Heat treatment: the temperature is 750℃-770℃, the holding time is 5min/mm, and not less than 4h; the post-weld heat treatment lifting temperature is less than 150℃/h.
According to the above process, the Φ273×30 specimens with one piece fixed horizontally and one piece vertically fixed were welded. After inspection and inspection, they all met the requirements of the specification.
Welding construction process
On the basis of qualified process qualification and according to the process qualification report, relevant welding operation instructions were compiled in time. The contents of the P91 steel welding operation instructions are as follows:
Preparation before welding
1. Selection of welding materials: welding wire ER90S-B9 electrode E9015-B9
2. Groove processing The grooves are machined, the grooves should be smooth and uniform, and the dimensions meet the requirements. Careful inspection is required before the counterparts. Those found to be unqualified must be ground to pass with tools such as a grinder. 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. A special pair of fixtures should be used for the counterpart, and no forcible alignment is allowed. The inner wall should be flush with the counterpart. If there is a misalignment, the misalignment should not exceed 10% of the wall thickness and not greater than 1mm; Carefully check whether the base metal at the groove is defective, and if so, it should be dealt with separately; the welding materials, welding process and selected welder technical conditions for spot welding should be the same as those used for formal welding; during spot welding of small-diameter thin-walled pipes, the gap should be 2 to 3 mm, 2 to 3 points can be directly fixed in the groove. When spot welding large-diameter pipes, the gap is 3 to 5 mm, and the spot welding can be fixed by “positioning blocks”.
Preheat before welding
Computer temperature control equipment is used to track and preheat the welding junction. The thermocouples are arranged symmetrically. The thermocouple and the pipe fittings should be in good contact. The preheat width is calculated from not less than 3 times the pipe wall thickness on each side of the groove center.
Using WS-400 welding machine, DC 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 layers of welds are welded by argon arc welding. When bottom welding, use the internal wire feeding method, pay attention to the good fusion of the root, and control the thickness within the range of 2.8 ~ 3.2mm. In order to prevent oxidation of the root weld metal, the tube should be filled with argon for protection.
Argon filling protection can refer to the following requirements:
a . The protection range of argon filling is based on the center of the groove, 200 ~ 300 mm on each side , with soluble paper or other soluble materials, and fastened with high temperature resistant tape to make a sealed air chamber.
b. Use the “inflating needle” to insert argon from the groove gap or the “flaw detection hole”. At the beginning, the argon gas flow is 10~25L/min on the back side, 4~8 L/min on the back side of the second pass, and 8 L/min on the back side of the welding torch. ~12 L/min.
c. Use an ignited lighter or match to place it near the gap between the welds. When the flame is extinguished, it means that the internal air has been evacuated, and argon arc welding can be performed.
Electrode Arc Welding Filling, Covering Welding
After the argon arc welding is completed, the preheating temperature is raised to 200 ~ 250 ℃, and the arc welding is started; the DC reverse connection method is adopted, and two people are symmetrically welded. When P91 steel is welded, the molten pool of molten iron has high viscosity, poor fluidity, and small welding specifications, so defects such as slag inclusion and lack of fusion between layers are prone to occur. Therefore, in order to avoid large defects and ensure the comprehensive mechanical properties of welded joints, it is necessary to Multi-layer, multi-pass, small-scale welding is used, and the thickness of the welding layer is as thin as possible, and the thickness of each welding bead does not exceed 4mm; during swing welding, the swing of the electrode does not exceed 4 times the diameter of the electrode, and each layer of welding bead must be cleaned, especially Pay attention to cleaning the joints and both sides of the weld bead. After the welding of the surface of the weld is completed, the welder is required to perform cleaning and self-inspection immediately. If it is found that the appearance is not good, repair welding immediately. It is strictly forbidden to directly repair the welding after the weld is cooled.
post weld heat treatment
When the welded joints cannot be heat treated in time, post heat treatment with a heating temperature of 300-350°C and a constant temperature time of 2 hours should be done immediately after welding.
The post-weld heat treatment adopts multi-channel output and multi-point temperature measurement, and the heater and the thermocouple correspond one by one, and the upper and lower temperatures of the welding joint are always consistent, so that the heating and cooling speed of the heat treatment welding joint should be ≤150℃/h. When the temperature is lowered to 300°C, it can be cooled to room temperature without control. The heating temperature is 750～770℃, and the constant temperature time is 5min/mm, but at least 4 hours.
Weld quality inspection
The weld quality shall be inspected according to the acceptance criteria for Class I welds. The inspection contents and requirements are as follows:
1. Appearance, according to DL/T869-2004 “Technical Specifications for Welding of Thermal Power Plants”, the quality standard for the appearance of Class I welds;
2. Non-destructive inspection, 100% ultrasonic (UT) and magnetic particle (MT) flaw detection of the weld according to JB4730, and the I grade is qualified;
3. Hardness, after heat treatment, do 100% hardness measurement, the hardness value is less than 350HB;
4. Spectrum, 100% spectral analysis and review of the weld metal alloy composition.
1. When the visual inspection of the welded joint does not meet the standard. If it is light, it should be polished and welded, and if it is serious, it should be cut off and re-welded.
2. Defects of welded joints that fail the non-destructive inspection can be repaired by welding, but it must be carried out according to the normal welding process or welding repair process on the basis of confirming that the defects have been completely eliminated.
3. For the welded joints that are repaired and repaired, generally the same welding joint should not be more than twice, otherwise it should be cut off and re-welded.
4. The welded joints that have been repaired by welding must be re-processed by post-weld heat treatment and non-destructive inspection.
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 poor mechanical properties of welded joints and fatal defects such as cracks. First of all, all welders must undergo training and obtain the qualification certificate of P91 steel before taking up their jobs. Second, from the measures, materials, equipment, counterparts, welding, heat treatment parameters, surface quality of welded joints, non-destructive testing, and repair of defective welds Strict control in all aspects.
Through the selection of the above construction techniques, as well as strict technical and quality management measures, the appearance quality and joint surface hardness of the main steam pipeline welds of the second phase project of Huaneng power plant in the area of #3 and #4 all meet the standard requirements, and the non-destructive testing is qualified once rate reached 100%. In addition, judging from the hydrostatic test of the main steam pipeline of the power plant and the situation since it was put into operation, the quality of the welds is normal. Therefore, the above welding process is effective for P91 steel pipes. In addition, the use of P91 pipes for the main steam pipes of thermal power plants has higher cost performance than the commonly used P22 pipes in the past.