Operation Instruction for Heat Treatment of Large Diameter Carbon Steel Pipe

1. Scope of construction

XX power plant 1×135MW project, drop pipe, collector butt material are carbon steel, specifications are: φ426×40, φ325×35 and φ273×40.
This operation guideline is applicable to the preheating and post-welding heat treatment construction of carbon steel large diameter pipe welds, as well as to the heat treatment of its reworked welds.
The heating method is far infrared heating, computer temperature control instrument control temperature, automatic temperature recorder record.
Post-weld heat treatment using high-temperature tempering process.

2. Preparation basis

  • Hangzhou boiler manufacturer’s drawings.
  • Welding Technology Regulations for Thermal Power Plants” DL/T869-2004.
  • Welding project list.
  • Company welding process assessment report.

3. Construction preparation

3.1 Materials
The insulation material is aluminum silicate needle punch blanket and asbestos cloth, 16# iron wire.
3.2 Construction machinery
Selected DWK-A computer temperature control cabinet – 180KW, working input voltage 380V, output voltage 220V. need to be calibrated and qualified and within the validity period before use.
Armored thermocouple, need to be calibrated and qualified and within the validity period before use.
A number of digital display thermometer pen, mainly used for welding preheating and welding process temperature monitoring comparison.
The secondary power supply line uses 16mm2 copper flexible wire, thermocouple connection line all use compensation wire.
Far infrared heater type according to the shape of the welded parts, space location and other circumstances to choose. Among the specifications for φ426×40 pipe, processing heater specifications for 1320×450, power for 30KW; preheating heater specifications for 1320×200, power for 20KW. specifications for φ325×35 pipe, processing heater specifications for 1000×400, power for 20KW; preheating heater specifications for 1000×200, power for 20KW. For the pipe of φ273×40, the specification of heater is 400×400 and the power is 30KW; the specification of heater for preheating is 800×200 and the power is 20KW.
Other working instruments: vise, screwdriver, electric pen, school line light, flashlight, insulating tape, marker, etc.
3.3 Technical data

  • Heat treatment process curve diagram;
  • Technical briefing record.

4. Construction process

4.1 Construction preparation
Heat treatment personnel shall be trained and qualified, and be licensed to work.

Heat treatment parameters are selected in the following table:

Specifications Material Constant temperature time (min) Lifting/cooling speed (℃/h) Preheating width of each side (mm) Total width of treatment heating (mm)
φ426×40 20G20G/20MnMo 120 155 200 450
φ325×35 20G20G/20MnMo 90 175 200 400
φ273×40 20G 120 155 200 400

Electric welding before preheating temperature of 100-200 ℃ (ambient temperature is lower than 5 ℃, preheating temperature selected upper limit; higher than 5 ℃, select the lower limit), post-welding heat treatment temperature of 650 ℃. Among them, the material is 20G/20MnMo weld, preheating temperature before welding is 150-200 ℃.

Construction sites should have measures to prevent rain and snow, and scaffolding should be solid and reliable when working at height.
According to the specifications of the weld mouth prepare the appropriate size of insulation cotton and wrap it with asbestos cloth, sewn neatly to facilitate the wrapping of the weld mouth.
4.2 Preheating before welding
Check whether the temperature control equipment is intact before construction, and if there is any failure or abnormality, it should be dealt with in time.
Thermocouple two points symmetrically arranged at a distance of 20mm from the bevel, the hot end is in close contact with the workpiece to ensure accurate temperature measurement.
Use preheating special heaters are tied on both sides of the weld, the tying should be firm, and the heating piece should be close to the wall of the tube. The heater should be 15mm from the edge of the bevel to facilitate welding. Preheating width from the center of the weld, each side of the heating width value, preheating temperature and lifting temperature speed value is shown in the table above.
When wrapped with aluminosilicate material (the outside must be wrapped with asbestos cloth), each side is widened by 50-100mm than the heater.
Connect the heater to the temperature control cabinet with a power lead, and connect the lead of each heater to the corresponding output socket of the temperature control equipment according to the division of the temperature control point.
Heater power lines are all connected, should be checked in detail for short-circuit phenomenon, the heater leads, thermocouple compensation wire is not touching the heating body, whether the connection method is correct, to confirm that there is no error before sending power heating.
When the temperature rises to the required preheating temperature, and the temperature is basically uniform (thermocouple reading values of the two measurement points deviation of not more than 10 ℃), that is, notify the welder to start welding.
4.3 Post-welding heat treatment
Thermocouple using two points symmetrically arranged on both sides of the weld edge of the welded pipe (horizontal pipe should be arranged up and down).
In the heat treatment, will be matched with the pipe specifications of the heater unfolded around the weld mouth a week, with 16 # wire tied tightly, the heater and the pipe wall tightly, should not be loose.
The heating width of heat treatment, counting from the center of the weld, each side shall not be less than the value specified in the table above.
First two layers of aluminum silicate acrylic blanket insulation cotton wrap, and then asbestos cloth wrap the outermost layer, wrap the weld should be neat and beautiful, insulation width per side than the heater widened 100-150mm. for the horizontal fixed position of the weld, insulation thickness of the lower part should be slightly thicker than the upper part.
Temperature rises to 650 ℃, and the temperature is basically uniform, the temperature difference is less than 50 ℃ when starting to calculate the constant temperature time. The cooling process, such as cooling speed is too slow (compared with the set value), to take appropriate measures, can reduce the thickness of insulation cotton to improve the cooling speed. Temperature down to below 300 ℃ no longer control the cooling rate, the power should be turned off, so that the natural cooling of welded joints.
Heat treatment process does not allow the pipeline inside the wind through the hall, otherwise should take measures to seal.
Weld mouth in the heat treatment process shall not be subjected to stress, buttressing and welding with a fixed device should be dismantled only after processing.
The whole process of heat treatment should be carried out continuously and should not be interrupted. In the event of a sudden power failure, should be waiting for the resumption of power supply, according to the normal rate of temperature re-heating; in the event of heater burnout, should be slowly cooled down, the temperature drops to below 300 ℃, re-wrapped, re-powered for processing.

5. Control points

Heat treatment staff should do a good job of recording the work every 10 minutes, while monitoring the automatic process curve recording chart for abnormalities.
The whole process of heat treatment should be marked with the operator code steel seal after the end of the process.

6. Quality standard

Heat treatment process, heat treatment process curve record chart abnormalities, commissioned to carry out hardness inspection.
After heat treatment, the hardness of the welded joint does not exceed the Brinell hardness of the parent material plus 100.

7. Safety technical measures

Heat treatment personnel must comply with the provisions of various safety and civilized construction.
Before sending power, check the power line for short-circuit phenomenon, the workpiece should be well grounded.
It is strictly forbidden to plug and unplug the power connector during the operation of the equipment.
Safety belt should be tied when working at height.
The construction personnel should wear good work clothes and asbestos gloves when touching the high temperature workpiece.

8. Personnel organization

  • Construction person in charge.
  • Technical person in charge.
  • Safety person in charge.
  • Quality person in charge.
  • Heat treatment personnel: 3-4 people.

9. Civilized construction process requirements

Warning signs should be hung in the construction process.
Heat treatment line direction should be rule, forbidden to pull, pull out indiscriminately.
Insulation cotton should be stacked neatly, and should be recycled in time when not in use.
Construction should be cleaned up construction site, shall not leave insulation cotton residue, wire and other debris.


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