1. Surface quality
1). Cracks, folds, folds, scabs, and delaminations are not allowed on the inner and outer surfaces of the internal threaded seamless steel pipes for high-pressure boilers. These defects should be completely removed, and the actual wall thickness at the removal should not be less than the minimum allowable wall thickness deviation. value, the clear should have a smooth transition.
2)The allowable depth of the straight track should not be greater than 4% of the minimum wall thickness, and the maximum depth of the straight track should not be greater than 0.2mm. Other local defects not exceeding the minimum wall thickness are permitted.
3) The iron oxide scale on the inner and outer surfaces of the internally threaded seamless steel pipe for high pressure boilers should be removed, and the oxide thin layer that does not hinder the inspection is allowed to exist.
2. Ultrasonic inspection
Internally threaded seamless steel pipes for high-pressure boilers shall be ultrasonically inspected one by one in accordance with the provisions of GB/T 5777. Ultrasonic inspection can be performed before or after the internal thread is formed.
For the ultrasonic inspection before the forming of the female threaded pipe, the longitudinal groove depth grade on the surface of the comparative sample pipe shall be C5. The ultrasonic flaw detection inspection of the internally threaded pipe after forming, the depth grade and position of the groove of the comparative sample pipe can be determined through negotiation between the supplier and the buyer.
3. The microstructure of finished internal threaded pipes with grades of 20G, 20MnG and 25MnG should be ferrite and pearlite. The microstructure of finished internally threaded pipes with grades of 12CrMoG and 15CrMoG should be ferrite plus pearlite (including granular bainite), and there should be no incomplete transformation products (such as yellow lump horses) between the critical transformation temperatures AC1~AC3. body).
The actual grain size grade of the finished internally threaded pipe should not be less than grade 4, and the difference between the maximum and minimum grain sizes on the two test pieces should not be greater than grade 3.
The depth of the full decarburization layer of the finished internal threaded pipe with a nominal outer diameter of not more than 76.2mm should meet the following requirements: the outer surface is not greater than 0.3mm, the inner surface is not greater than 0.4mm, and the sum of the two is not greater than 0.6mm.
The thread parameters of the internally threaded pipe shall be taken from each batch to be cut and measured, and the other dimensions and shapes of the internally threaded pipe shall be measured one by one with a measuring tool that meets the accuracy requirements.
The quality of the inner and outer surfaces of the internally threaded pipe shall be visually inspected one by one under sufficient lighting conditions.
Other inspection items, test methods and sampling methods of the internally threaded pipe shall comply with the provisions in Table 7.
testing regulations
Inspection and acceptance
The inspection and acceptance of internally threaded seamless steel pipes for high pressure boilers shall be carried out by the technical supervision department of the supplier.
Group batch rules: Internal thread pipes are inspected and accepted in batches. Each batch shall consist of the same grade, the same heat number, the same specification and the same heat treatment system of internally threaded pipes. The quantity of each batch of internally threaded pipes shall be no more than 200 pieces.
Sampling quantity: The sampling quantity of each batch of internal threaded pipes for each performance inspection shall comply with the provisions in Table 7.
Recheck and Judgment Rules
When the fully decarburized layer of the internal thread seamless steel pipe for high-pressure boilers fails to pass the inspection, double samples shall be taken for re-inspection. The remaining internal threaded pipes are allowed to be inspected one by one or the batch of steel pipes shall be scrapped, and re-heat treatment is not allowed.
The re-inspection and judgment rules of the impact test shall comply with the provisions of GB/T 17505.