Cracking of duplex stainless steel due to dissolved hydrogen
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INTRODUCTION
D U P L E X stainless steels contain both ferrite and austenite and combine the toughness of austenite with ferrite's resistance to chloride stress corrosion cracking (SCC). Because of their superior corrosion and stress corrosion resistance, duplex stainless steels have replaced austenitic stainless steels in many applications. These alloys are useful in a highly oxidizing environment, chloride environments, and environments containing H2S. Duplex stainless steels, therefore, become candidate materials for use in chemical plants, in the pulp and paper industry, in oil and gas production, in marine environments, and in heat exchangers. [~] Because of the use of duplex stainless steels in environments containing hydrogen and the possibility for dissolving hydrogen, as in oil and gas production, effects of dissolved hydrogen should be carefully investigated. Little work has been done on hydrogen embrittlement, except as a small part of broader investigations of the application of duplex alloys in corrosion or stress-corrosion environments. ~2"3~ Recently, the effects of hydrogen on duplex stainless steels have received more attention. Using X-ray diffraction, Cohen e t a l . i41 found that after cathodic charging, changes occurred in the lattice parameters of both phases in duplex stainless steels. They suggested that because of the different hydrogen diffusivities and solubilities in the two J.-H. HUANG, Professor, is with the Department of Nuclear Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China. C.J. A L T S T E T T E R , Professor, is with the Department of Materials Science and Engineering, University of Illinois at U r b a n a - C h a m p a i g n , Urbana, IL 61801. Manuscript submitted February 3, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
phases, hydrogen outgassing would occur faster in ferrite than in austenite, and the lattice parameter of ferrite may return to equilibrium faster than in austenite. As a result, a tensile stress state in the ferrite/austenite interface could be created. One of the most damaging aspects of hydrogen in structural materials is delayed failure. Perng and Altstette~ 5i studied the cracking kinetics of Ferralium 255 duplex stainless steel in hydrogen gas. They found that this duplex stainless steel was embrittled by hydrogen gas at ambient pressure and temperature. The subcritical crack growth (SCG) velocity increased as the temperature increased from 0 ~ to 100 ~ in 108 kPa hydrogen gas or as the hydrogen pressure increased from 54 to 216 kPa at 25 ~ The SCG results for duplex alloys, plus those for deformed AISI 301 alloy containing a mixture of c~' and y ' , were then compared with cracking behavior for stable austenitic and for ferritic stainless steels. Perng and Altstetter confirmed the hypothesis that a higher rate of supply and accumulation of hydrogen in the region of the crack tip allowed a higher cracking velocity. Since austenite and ferrite have different hydrogen diffusivities and solubilities, it is difficu
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