SynopsisThe effects o f cold rolling, heat treatment and volume fraction of ferrite on stress corrosion cracking (SCC) behavior of duplex stainless steel have been studied with means of U-bend method in boiling 42 % MgC12 solution. The main results obtained are as follows:(1) SCC susceptibility of the solution treated material is small at about 50 a, but it has a tendency to be increased with either way of increase or decrease of a content from about 50 % a. Solution treatment temperature resulting in abrupt increase of SCC susceptibility shifts to higher temperature with the decrease of a content.(2) SCC susceptibility is increased by grain growth of a and r that occurred with extension of holding time at solution temperature, and otherwise, by cold work in the case of the materials having much volume fraction of a.(3) On aging at 475 °C, the hardness is increased by spinodal decomposition of a. The crack resistance increases remarkably with the progress of aging in the case of materials having much a, and corresponds to age hardening behavior.(4) In the case of heating at 700 °C, SCC behavior of aged materials has not been made clear by U-bend method because of r embrittlement.
The relation between MnS inclusions and corrosion behavior of stainless steels such as SUS 304 and SUS 303 (AISI) containing 0.004-0.25% S has been investgated in a refreshing drink, NaCl+ C6H8O6, NaClO, and HCl solutions by using mainly EPMA, X-ray diffraction analysis and atomic absorption spectrophotometer. Specimens containing more than 0.008% S rusted easily in 100ppm NaClO solution at room temperature for 24hrs, and colored black in 10% NaCl+1% C6H8O6 solution under the same condition. The solutions used were degenerated and at the same time generated a strong smell of H2S. These tendencies became more pronounced with an increase in S content. In 10-1,000ppm HCl, 1-30% NaCl+1% C6H8O6 solutions and in a refreshing drink, the amount of Fe and Cr ions increased with an increase in S content of steel. It was found that the preferential dissolution of MnS inclution activated the localized corrosion of steels containing more than 0.004% S, and that increasing amount of MnS accelerated the active corrosion. The corrosion potential of the specimens in the 10% NaCl+1% C6H8O6 solution was nearly proportional to the S content. Results of EPMA and X-ray diffraction analysis revealed that the main cause for the blackening of specimen surface after immersion in the solutions was the redeposition of some kinds of amorophous C and S.
The e.ffects q[ cold drawing and heat treatment on the stress corrosion cracking (See) behavior of stainless steel having two phases of a and r were studied in boiling 42 % MgC12 solution. I t was found that the effect 0/ cold drawing on see susceptibility was remarkable, and when applied stress WIlS 8.3, 25 kg jmm 2 or 38% of Y.S. , the maximum see susceptibility was observed in the materials drawn at 15% and 60 % after solution treatment. The threshold stress on the see was observed in an underformed material, but it was not observed in those which were subjected to cold drawing. The restraining e.ffect of a on lhe propagation of cracks was not observed in all the malerials, on the contrary, the cracks were propagated in a preferentially in the 60 % cold drawn material. On aging at 475°e , the change in hardness showed a two-stage hardening behavior. At the second stage of the hardening (aging for 100 h or longer) the see susceptibility of the cold-drawn material was decreased and did not fail even after 10" min. This phenomenon mtry be interpreted by assuming the two phases separation of a-phase. On aging at 700 0 e from 5 to 17 h or more, the see susceptibility was excessively increased, and at the same time the transformation of a-+ r+a occurred. This transformation has a detrimental e.ffect on the see susceptibility because a volume fraction of a was varied, and furthermore a promoted local corrosion and embrittlement of the materials.
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