| The 1500m3 9% nickel steel spheres are important equipment in the 450,000t/a ethylene plant of BYPC. By metallographic examination, standard corrosion test, two-direction tensile test and the field environment corrosion test, the nature and the formatting mechanism of the OK69.45 weld surface defects of 9% nickel steel spheres have been investigated. The feasibility repairing the defects with A557 electrode has been researched. Meanwhile, the technological requirements repairing welding the defects have been pointed out.The results of test indicate:1. The nature of the OK69.45 weld surface defects of the spheres is intergranular stress corrosion cracking(IGSCC). The formatting mechanism of IGSCC is: the corrosion electric potential at grain boundary has been greatly dropped by the precipitation of rich Cr carbide at the grain boundary resulting from the high C content of OK69.45 weld; the strain concentration at the grain boundary has been caused by the slip in the grain under the action of welding residual stress, which results in the breaking of passivation film; the IGSCC has been finally caused by the solution of the Cr poor zone under the field environment with high Cl" content.2. The IGSCC has not been caused in the A557 weld under the same environment, because of the precipitation of less carbide due to the less C content, and the less strain concentration at the grain boundary due to without slip in the grain under the action of welding residual stress. It is feasible to repair the defects with A557 electrode.3. The original line of fusion must be ground away within the depth of grinding while grinding defects, for preventing the cold cracking tendency from increasing. Two layers of the A557 weld metal should be welded at least, so that the better ability to resist IGSCC of the weld surface can be ensured.After one year the spheres repaired have been used, any overproof defect has not been found by examination.
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