Effect of nitrogen content on the environmentally-assisted cracking susceptibility of duplex stainless steels
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IT is well known that duplex stainless steels (DSSs) are generally more resistant to stress corrosion cracking (SCC) than single-phase austenitic grade stainless steels in chloride containing solutions.[1,2,3] The resistance to SCC may be improved by increasing the percentage of ferrite phase in DSS. Because the mechanical and electrochemical behaviors of austenite (␥ ) phase and ferrite (␣ ) phase in DSS are different, the mechanisms for environmentally assisted crack initiation and propagation become more complex. The crack initiation of DSS in chloride-containing environments can be assisted by pitting corrosion[4,5,6] or selective dissolution of the constituent phases.[4,7] Fourie and Robinson[8] have observed that the electrochemical potential of the austenite in DSS in 1 M H2SO4 ⫹ 0.5 M NaCl solution at 60 ⬚C was 20 mV nobler than that of the ferrite phase. Therefore, austenite phase was cathodically protected by ferrite phase. On the contrary, Sridhar and Kolts[9] found that preferential dissolution of austenite occurred in low nitrogen containing DSS in all environments studied. They also found that in high nitrogen containing DSS, corrosion occurred preferentially in austenite in sulfuric and phosphoric acids, while the ferrite was prone to dissolution in hydrochloric acid and oxidizing chloride solutions. Obviously, the relative resistance to dissolution between austenite and ferrite is still widely debated. In general, alloy compositions of DSSs have a great influence on phase transformation, mechanical properties and corrosion behavior.[10] Nitrogen is one of the most important alloying elements in DSSs. The addition of nitrogen into SSs has become popular because of several beneficial effects therein. Nitrogen is known to be a strong austenite stabilizer. Its presence can lead to solid solution strengthening[11,12,13]
and lowering of the stacking fault energy,[14,15] which consequently causes an increase in the strength of SSs. Most importantly, the alloying of nitrogen into SSs can greatly improve passivation behavior and increase pitting corrosion resistance[11,16–19] of SSs in aqueous environments. The partitioning of nitrogen in the constituent phases, namely, austenite and ferrite, may alter the localized corrosion and cracking behavior of DSS. In this investigation, therefore, the effect of nitrogen content on the environmentally-assisted cracking behavior in chloride solutions is investigated.
II. EXPERIMENTAL METHOD The 22 pct Cr DSSs with a nitrogen content ranging from 0.103 to 0.195 wt pct were used with their chemical compositions close to that of conventional 2205 DSS (0.174 N), as listed in Table I. The hot-rolled DSS plates provided by China Steel Corporation were first melted in a 250-kg vacuum furnace and cast into 160 ⫻ 160 mm square ingots. Then, these ingots were reheated at 1250 ⬚C for 90 minutes and hot rolled into 13 mm-thick plates. In the subsequent tests, all DSSs were solution annealed at 1100 ⬚C for 30 minutes followed by water quenching. Metallographical examinat
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