Abstract: In the running process of the reactor in an MTO plant, cracks appeared in the top seal head welds, resulting in product gas leakage. During the subsequent downtime overhaul, the reactor was inspected andanother non-through crack flaw was found nearby the same leakage crack welding seam. By taking a sample from the non-through crack, a professional inspection company was entrusted to conduct macroscopic inspection of cracks and microscopic analysis of chemical composition, ferrite content, hardness, metallurgy, fracture, radiographic energy spectrum and so forth. The causes of cracks were analyzed and discussed in terms of reactor design, manufacturing, installation, and service conditions. The results indicated that the cracks had the characteristics of intergranular cracking; the high temperature creep generated by the features of austenitic stainless steel welding and high temperature conditions and the structure instability in the hemi-ellipsoid head stress distribution were the internal causes of the cracking. The temperature difference stress instantaneous peak due to insulation failure of the top seal head was the external cause of the cracking.

Key words: MTO plant, top head of reactor, austenitic stainless steel, weld metal cracking, fault analysis