THE STRESS CORROSION CRACKING OF AUSTENITIC STAINLESS STEEL HEAT EXCHANGE TUBES—THREE CASES STUDY

摘要

In this paper, three leakage failure cases of heat exchange tubes have been introduced. The reasons of the leakage for austenitic stainless steel tubes and overlay welding layer on the tube sheet have been analyzed. Through the investigation of the operation process and histories of the equipment, and after chemical compositions analysis of tube material and corrosion products, metallographic test of specimens with cracks, and fracture surface scan with Scanning Electron Microscope (SEM), the cracking reason and mode are described as the Stress Corrosion Cracking (SCC) of austenitic stainless steel. This kind of cracking in three cases was induced by the micro chloride in the high temperature water (or steam). Moreover, sulfide and dissolved oxygen also reduced the threshold value of chloride concentration and enhanced the corrosion rate for SCC. The cracking mode of Case A and B are transgranular; and Case C is intergranular. It indicates that for this kind of in-service heat exchanger, the operators should not only control the chloride concentration in feed water, but also the sulfide and dissolved oxygen in the future. The austenitic stainless steel tubes (China steel types-lCrl8Ni9Ti and 0Crl8Nil0Ti, equal to Type 304 and Type 321 according to ASME code) used in this cases are not fit to this condition. Thus, for the new heat exchanger design,the tube material should be changed into austenitic-ferritic (duplex phase) steel, such as 2205 Series, which has an excellent performance for SCC resistance in the high temperature water (or steam) with chloride.