Abstract: Leakage occurred on the 304L stainless steel heat-exchanging pipes accounted 1/3 of the total after import gas furnaces used for a period of service. cracking properties and reasons were identified by observing buming loss, scaling crack, fracture morphology of the heat-exchanging pipe were observed and their microhardness and chemical composition were tested. The results show that the cracking mode of the heat-exchanging pipe was brittle cracking caused by stress corrosion. The basic reason of cracking for heat-changing pipe was a lot of scale deposits comprising acid corrosion elements in the pipe, which caused by the poor water quality or excessive water. This leakage could be prevented by lessening the hardness of water and reducing sucfide, chloridion and reasonable circulation and water replenishing.