The electrochemical dissolution of MnS inclusions and pit initiation processes on type 303 stainless steel were investigated by an electrochemical microcell technique. In 1 M Na 2 SO 4 , the electrochemical dissolution of MnS inclusions started at 0.1-0.3 V vs Ag/AgCl͑3.33 M KCl͒, which was followed by the initiation of a hemispherical and smooth wall pit at MnS/matrix boundary. It was suggested that the dissolution products of MnS inclusions induced the formation of a salt film, which caused localized electropolishing. In 0.1 M NaCl, the onset potential of electrochemical dissolution of MnS inclusions was 0.3-0.4 V and the stable pit growth occurred at a potential of around 0.5 V. Field-emission scanning electron microscopy observations revealed that many metastable pits with a diameter of approximately 1 m were formed at MnS/matrix boundary on the specimen, of which the polarization measurement was stopped immediately after stable pit initiation. From a morphological point of view, the metastable pits were found to be very similar to flat-walled etch pits. The dissolution products of MnS inclusions and chloride ions would attack the metal surface newly exposed by dissolution of the inclusions. It is proposed that the chemistry of dissolved species from MnS inclusions have a substantial influence on pit initiation at MnS inclusions.

Synopsis : Toughness, especially ductile-to-brittle transition temperature (DBTT) such as fracture appearance transition temperature (FATT) obtained from Charpy impact test, is one of the most important properties of steels to assure material reliability. One of determining factors of DBTT in ferrite-pearlitic and martensitic steels is well known as the effective grain size (d EFF) on cleavage fracture surfaces. However, in the steels with intermediate stage transformation microstructures (Zw), relationship between DBTT and d EFF has not been clarified because of the complicated microstructures. Meanwhile, absorbed energies of Charpy impact test has been standardised for steel applications. The present study aims to determine the relationship between DBTT and d EFF , and the relationship between upper shelf energy (USE) and plastic properties in a quenched and tempered low carbon high strength steel which has a Zw microstructure consisting of granular bainitic ferrite and quasipolygonal ferrite. Size distribution of d EFF was measured on the cleavage fracture surfaces revealed by Charpy impact testing at 77 K, and correlation between d EFF and the microstructure was examined. It was found that FATT is inversely proportional to ln (d EFF −1/2) with a slope close to that approximated for ferrite-pearlitic steels. Investigation of the correspondence between cracks and EBSD grain boundaries revealed that d EFF apparently agrees with the grain size of bainitic ferrite enclosed by large angle grain boundaries with misorientation over 15 degrees. Moreover, USE has been proportional to total plastic works until fracture of tensile test. These results indicated that the absorbed energies at any temperatures could be computable.