In-service inspection for assessment of weld degradation in SS 310S reduction retort

Austenitic stainless steel AISI 310S is used as the material of construction for Reduction retorts. SS 310S is selected for its ease of fabrication, corrosion and elevated temperature (around 850ÚC) properties. Matching welding consumables (E 310/ER 310) are used for welding of longitudinal and circumferential joints during fabrication of these retorts. The high temperatures applied during both welding and service, significantly affect the microstructure and properties of the weldment. During operation, these retorts are subjected to highly corrosive chloride environment in addition to high temperature. The middle zone operating temperature is 600ÚC to 700Ú C and undergoes inter granular stress corrosion attack (IGSCC) from inside and oxidation on outside surface. The bottom zone operating temperature is 800ÚC to 850ÚC and undergoes IGSCC and weld degradation. The retort is operated at an internal pressure of 5 psig and carries a tensile load of around 6 tons at 850ÚC. Hence these reduction retorts are subjected to in-service inspection (visual inspection and radiography of hot zone welds) after 40 runs and thereafter at regular frequencies to assess the healthiness of the weld joints. During In-service inspection (ISI), in retorts which were welded with ER 310 filler wire, many cracks were observed in the weld between shell and dished end. Inspection of welds during service was carried out using digital radiography by using a panoramic X-ray exposure. In order to find out the cause for the weld degradation and linear defects formation and propagation, failure analysis of the retorts was taken up. The observations in the failure analysis were correlated with the fabrication history, operating data and in-service inspection data. With this the mechanisms of weld degradation were identified and recommendations were made to improve the life of the retorts. The use of E 310 Mo welding consumables, modification of process conditions and fabrication procedures resulted in improvement of the life of the reduction retort. (author)