Through macroanalysis, chemical composition analysis, metallographic observation, and SEM, the leakage reasons for joint of tube of header and coil tube of coal economizer was analyzed. The results show that the alloy element compositions of welded tubes and conversion strength both met the standard of GB 5310—2008. The grains at fusion zone and overheated zone were coarse and even formed widmanstaten structure which made the performance of the zone weaken. Leakage from the joint of tube of header and coil tube of coal economizer resulted in the micro cracks in the weakened region of fusion and overheated zone under the thermal stress. Though throughwall crack was not formed after micro cracks initiation, micro cracks resulted in crevice corrosion and the crevice corrosion accelerated crack propagation from the inside outwords, and eventually caused the leakage.

Counting and Photoshop pixel method were used to measure ferrite volume percentage of ER2209 duplex stainless steel layering, the result shows that ferrite volume percentage of layering respectively is 50.1% and 56.5% by counting and Photoshop pixel method, the values are in between 30% and 60%, the outcome meets to phase ration requirements. Standard deviation of ferrite volume percentage φα is 2.06% by Photoshop pixel method, but that is 4.89% by counting, compared with two methods, Photoshop pixel method is better to measure ferrite volume percentage of duplex stainless steel layering.