Field welding and heat treatment process of 2cr1mov tee

In a thermal power project in Shanghai, due to the deviation of the nozzle of the manufacturer, the connecting pipe of the main pipe at the inlet of the reheater can only be replaced on site, and the material is 12Cr1MoV. The final welding and heat treatment processes are as follows: groove form: single V shape 50 – 60 °; Welding method: manual argon arc welding GTAW backing, manual arc welding SMAW filling cover; Welding material: GTAW: welding wire tig-r31, diameter 2.5mm; SMAW: electrode r317, diameter 3.2mm; Welding parameters: GTAW: welding current 110-120a, arc welding voltage 14-15v, SMAW: welding current 100-120a, arc welding voltage 23-25v; Preheating before welding: GTAW: 150 ℃; SMAW: 200 ℃, post weld heat treatment: holding temperature: 740 ℃, holding time: 1H, temperature rise and fall speed less than or equal to 150 ℃ / h; After welding, it is qualified through UT and MT inspection, and after heat treatment, it is qualified through hardness and MT inspection.

In a thermal power project in Shanghai, the connecting pipe of the main pipe at the inlet of the reheater has a 5mm offset at the nozzle of the manufacturer, and the cumulative error is about 6mm, resulting in a 10mm misalignment between the pipe seat and the module. The pipe joint needs to be replaced on site. The alloy content of 12Cr1MoV is relatively low and the weldability is good, which is mainly due to the large hardening tendency after welding, so it is necessary to prevent cold cracks. Under the high temperature and high pressure environment of power plant operation, the received alternating impact load leads to brittle cracks in the service process, which endangers the safety and reliability of power plant operation. Considering that the on-site welding conditions and equipment conditions are not as perfect as those of the manufacturer, it is particularly important to formulate reasonable and feasible welding and heat treatment construction schemes and processes.

Analysis of welding performance

Chemical composition analysis of 12Cr1MoV

After testing, the main alloy content of the 12Cr1MoV pipeline is shown in Table 1.1, which meets the specified 12Cr1MoV alloy content. The main alloying elements added to the alloy steel are Cr, Mo and a small amount of v. the existence of these alloying elements makes the weld and heat affected zone have the tendency of hardening.
Table 1.1 chemical composition of site 12Cr1MoV pipeline (wt%)

Chemical composition C Cr Mo V Mn Si
Content 0.12 1.1 0.32 0.23 0.56 0.31

Preparation of welding and post weld heat treatment process

The preparation of welding and post weld heat treatment process is based on weldability analysis, DL / 869-2012 technical specification for welding of thermal power plant and DL / 819-2010 technical specification for welding heat treatment of thermal power plant.

Groove preparation and assembly requirements

The blanking of weldment and preparation of groove shall adopt the method of mechanical processing, and the base metal in the groove and within 20mm of the edge shall be free of crack, double skin, groove slope damage, burr and other defects. The groove size shall meet the requirements of the drawing. Before assembly, the oil, paint, scale and rust on the groove surface and nearby base metal weld leg size + 10mm shall be cleaned. The welded junction is fillet weld, and the size of pipe joint is Φ 273 × 12mm, according to DL / 869-2012 technical specification for welding of thermal power plant, the groove form and size recommended during weldment assembly can be seen that the groove should be single V-shaped 50 ~ 60 °.

Welding process

Welding method: in order to ensure the welding quality, the manual argon arc welding GTAW backing and manual arc welding SMAW filling cover are finally determined.
Welding materials: the welding materials are mainly based on the chemical composition of 12Cr1MoV and the mechanical properties at room temperature, and the electrodes r317 and F3 are selected 2mm, welding wire tig-r31, F2 5mm;
Welding process parameters: GTAW: welding current 110-120a, arc welding voltage 13-15v, SMAW: welding current 100-120a, arc welding voltage 23-25v.

Heat treatment process parameters

  • 1) Preheating: the preheating temperature of 12Cr1MoV is 200 ℃ ~ 300 ℃. Considering that the ambient temperature during construction is about 30 ℃, 200 ℃ is selected for preheating.
  • 2) Post weld heat treatment: the insulation temperature of 12Cr1MoV is 720 ~ 750 ℃, because the heater only heats from the outer wall during heating, there is a temperature difference between the inner and outer walls during heat treatment. In order to ensure the quality of heat treatment, a higher insulation temperature of 740 ℃ is selected. At the same time, in order to reduce the temperature difference and temperature stress during heating, a smaller heating speed of 150 ℃ / h is selected, The holding time shall be 1 h according to its nominal thickness.

Site construction

1) Determine the pipe joint to be replaced and mark it. Remove the pipe joint, grind the welding groove of the main pipe, and conduct PT inspection. The groove and edge main pipe are free of defects after inspection. The processed pipe joint is a single V-shaped 50 ~ 60 ° groove, and the blunt edge of the groove is processed to 1 ~ 1.5mm. Before welding, remove oil, water, rust and other sundries in the area to be welded and the area adjacent to the metal surface for at least 20mm, and polish to expose the metal luster. Conduct welding according to the established welding process and heat treatment process. Multi layer and multi pass welding shall be adopted. The welded joints shall be staggered, and the interlayer temperature shall be ≤ 300 ℃. Pay attention to the arc starting and arc stopping quality of the weld bead. The arc pit shall be filled during arc stopping, and the weld transition shall be smooth. After the appearance inspection by the welding quality inspector, there are no cracks, incomplete fusion, incomplete penetration, pores, slag inclusion and other defects, and the size is qualified. Then, the spectrum, UT and MT inspection are carried out, and the re inspection meets the requirements.

20220221121338 69663 - Field welding and heat treatment process of 2cr1mov tee
Fig. 3.1 binding diagram of thermal insulation cotton

20220221121750 37212 - Field welding and heat treatment process of 2cr1mov tee
Figure 3.2 schematic diagram of thermocouple distribution
After passing the inspection, the post weld heat treatment process shall be strictly implemented for post weld heat treatment. Clean the spatter and welding slag on the surface of the heater and welding parts before installing the thermocouple. As shown in Figure 3.1, the field heater binding diagram is shown. Due to the rapid heat dissipation of tee welded junction, in order to ensure the heat treatment effect, the pile head adopts 18.5M rope heater with 10kW power, and the main pipe adopts two 530mm rope heaters with 10kW power × 450mm LCD crawler heater is symmetrically bound as shown in the figure for auxiliary heating. The distribution of thermocouples is shown in Figure 3.2. Temperature control thermocouple 1 and temperature control thermocouple 2 respectively control the LCD crawler heater on the main pipe, and temperature control thermocouple 3 controls the rope heater. The monitoring thermocouple is located at the highest part of the main pipe to prevent local overheating. The pipe joint and main pipe shall be wrapped with aluminum silicate refractory fiber insulation cotton in a full wrapping way to reduce the heat dissipation rate during heat treatment. The thickness of insulation material shall not be less than 50mm (Note: the temperature rise and fall rate shall not be controlled below 300 ℃). The post weld heat treatment adopts the automatic balance recorder to record the heat treatment process, the model is EI100-06, and the paper feeding rate is 2 grids / hour.
MT inspection and hardness inspection after heat treatment. First, carry out hardness inspection, and the inspection data are shown in Table 3.1.
DL / 869-2012 technical specification for welding of thermal power plants stipulates that the hardness of welded joints of the same kind of steel after heat treatment shall not exceed the Brinell hardness value of base metal plus 100hbw, and shall not exceed the following provisions: the total alloy content shall be less than or equal to 3%, the Brinell hardness value shall not be greater than 270hbw, the total alloy content shall be less than 10% and not less than 3%, and the Brinell hardness value shall not be greater than 300hbw. It can be seen from the table that the test results meet the requirements. Finally, through MT test, the test result is qualified.
Table 3.1 hardness of post weld heat treatment

Style number Test value (HB)
a b c d e Average value
Base metal 1 (pipe joint) 170 161 172 162 167 166
Weld line 240 238 245 232 231 237
Base metal 2 (main pipe) 160 164 162 161 163 162

Conclusion

Tig-r31 welding wire is used for manual argon arc welding GTAW during backing in 12Cr1MoV tee welding. The welding current is 120a and the welding voltage is 15V; When filling the cover, r317 electrode is used for manual arc welding SMAW, with welding current of 100A and arc welding voltage of 25V; Preheating process: preheat at 150 ℃ when priming and 200 ℃ when filling the cover; Post weld heat treatment: the temperature rise and fall speed is 150 ℃ / h, the holding temperature is 740 ℃, and the holding time is 1 h. The developed process can meet the requirements of on-site construction and relevant standards. After on-site construction, all tests are completely qualified.

Source: China Stainless Steel Tee Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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