Answer: The inner surface of the pressure vessel is in contact with the medium. Surface cracks are most likely to occur, so the inner surface should be inspected first. Generally, pressure vessels are mainly inspected macroscopically, and the welds on the inner surface (including the near-seam area) should be inspected with the naked eye or a 5-10x magnifying glass. However, for steel or working conditions that are prone to cracking, it is not limited to macroscopic inspection, but surface flaw detection of not less than 20% of the weld length should be performed. This is because:
(1) Material strength grade σb＞540 Mpa. This material has a high strength grade, a high hardening tendency, poor weldability, and a high possibility of cracking. In addition, pressure vessels made of this steel are also prone to cracking during use.
(2) Cr-Mo steel pressure vessels. Cr-Mo steel pressure vessels are pearlite heat-resistant steel pressure vessels. The main tendencies during welding are hardening tendency, cold cracking tendency, and reheating cracking tendency. Therefore, cracks are easily generated during welding.
(3) Austenitic stainless steel cladding layer. The base material of the cladding layer is mostly carbon steel or low alloy steel, and the surface layer is clad with stainless steel electrodes. Due to the large difference in physical properties such as thermal expansion coefficient of the two metals, cracks are easily generated during cladding.
(4) Pressure vessels with stress corrosion tendency in the medium. When the pressure vessel has stress corrosion conditions, that is, there is a corrosive environment, tensile stress in the structural parts, and the material and corrosion are matched, stress corrosion may occur, accompanied by stress corrosion cracks.
(5) Suspicious welds. Mainly refers to the suspicious parts of the macroscopic inspection, it is necessary to further inspect with surface flaw detection.
When the above welds are sampled, if cracks are found, the flaw detection ratio should be increased. If cracks are found again, not only the inner surface should be 100% surface tested, but also the outer surface should be checked. The cracked parts of the inner surface are spot-checked for the corresponding outer surface parts.
