Q1: How to standardize the low-temperature impact test of Q345D steel pipe?
The specimen is strictly processed according to GB/T 229 standard, with a size of 55×10×10mm with V-notch. The test temperature is controlled at -20℃±2℃, and liquid nitrogen + ethanol mixed medium is used for cooling. The insulation time is ≥5 minutes to ensure that the specimen reaches the test temperature as a whole. Take 3 specimens in each group, and the single value is ≥34J and the average value is ≥47J to be qualified. The fracture morphology requires a fiber rate of ≥70%, and a crystalline fracture needs to be retested.
Q2: What are the key areas of concern for non-destructive testing of Q345D steel pipe?
The base material area focuses on the detection of delamination defects (UT sensitivity is set to Φ1.6mm flat bottom hole). TOFD+UT dual detection is used in the weld area to ensure the detection of unfused and crack defects. The heat-affected zone needs to be subjected to magnetic particle testing (MT), and it needs to be polished to Ra6.3μm before testing. 100% penetration testing (PT) is required for the 25mm range of the pipe end to check for damage during transportation and loading and unloading. For steel pipes with a thickness of ＞50mm, ultrasonic diffraction time-of-flight method (TOFD) testing is required.
Q3: How to accurately evaluate the grain size of Q345D steel pipes?
The sample preparation is in accordance with GB/T 6394 standard, using picric acid + detergent corrosion solution. It is required to observe under a 100x microscope, and the actual grain size is ≥8 (average diameter ≤0.022mm). If the grain size of a local area is found to be ≤7, an additional impact test is required to verify the performance. The test report must be accompanied by a 100x metallographic photo and a comparison with the standard rating chart. For important components, the grain shape coefficient must also be determined to evaluate the uniformity of the structure.
Q4: Why is residual stress testing required? In what cases must testing be performed?
Thick-walled pipes (≥50mm) and cold-formed pipes must be subjected to residual stress testing using the X-ray diffraction method. The surface residual compressive stress is required to be ≥-200MPa, and the tensile stress is required to be ≤0.3 times the yield strength. When the test value exceeds the standard, overall stress relief annealing (550-600℃×2h) is required. For components subjected to alternating loads, residual stress control is more stringent. The test report must include stress magnitude, distribution and treatment recommendations.
Q5: What quality certification documents must be provided when Q345D steel pipes leave the factory?
The material certificate must include chemical composition and -20℃ impact energy measured data. A third-party test report (such as SGS, TUV) proves that the low-temperature performance meets the standard. Normalizing heat treatment curve record, showing temperature control. Complete non-destructive testing report (UT/MT/RT test results). Declaration of conformity (EN 10204 3.1 or 3.2 type certificate) to ensure traceability.
