Incident of weld cracking, leakage, and fire involving TP347 material furnace tubes

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The new equipment installed at the factory has been in operation for a year now; it has undergone several shutdowns for repairs, and to this day there is still one equipment issue that hasn’t been resolved, so it is operating under suboptimal conditions. A fire broke out in the first two months due to cracks in the welds of the furnace tubes, which caused carbon at 590°C to be ejected and catch fire instantly; fortunately, it was detected in time and extinguished. Under normal conditions, carbon gas at 590°C and 10 kPa flows inside this furnace tube; the crack occurs at the root weld of the socket weld in the branch pipe. Significant stress exists at this location under hot conditions, but according to the design calculations, the strength of the normal weld is sufficient to meet the stress requirements. The maintenance plan for that area involves welding repairs to the welds and modifying the furnace tube supports to reduce stress. However, cracks appeared again after it was in use for some time...... The repair welding method used at that time was direct low-current manual arc welding, without any heat treatment; upon inspection after welding, pits were found on the surface of the weld, indicating that the fusion quality was not very good. Information on TP347 material indicates that TIG welding is generally used for welding, and post-weld heat treatment at temperatures above 800°C or solution treatment is carried out to eliminate intergranular chromium-depleted zones and prevent intergranular corrosion. Is there a problem with the welding process used to repair cracks in that weld area? Perhaps the strength of the welds formed as a result of those repairs is insufficient, which leads to further cracking once operation resumes?