Microscopical Characterisation of Martensite Formation in a Metastable Austenitic Stainless Steel

Abstract
Stainless steels have widespread applications due to their good corrosion resistance, but for certain types of large naval constructions, other requirements are imposed such as high strength and toughness, and modified magnetic charactristics. 302 type metastable austenitic stainless steel after solution treatment possesses a fully austenitic microstructure. However, its tensile strength in the solution treated condition, is low. Consequently, cold rolling following solution treatment is applied and results in the strain induced transformation to α'-martensite within the austenitic matrix and enhancement of its tensile strength. Following cold rolling a mixed martensitic-retained austenitic microstructure is obtained. In addition, when ferromagnetic α'-martensite is introduced to an otherwise fully paramagnetic matrix, the magnetic response of the steel changes. The magnetic hysteresis loop for the cold rolled 302 type stainless steel was recorded, under static field, together with the hysteresis loop obtained for an 817M40 type steel (reference material). It is observed that the 302 type stainless steel shows much lower maximum induction and remanence values, without any increase in the coercivity. TEM examination revealed in greater detail the fine elements of the microstructure of the material. Hexagonal α'-martensite forms initially and then at a later stage of deformation α'-martensite forms. This behaviour is typical of 302 type stainless steel deformed below its M_d temperature. In addition to conventional electron microscopical investigation, magnetic domain imaging has been performed using the low magnification mode of the TEM. Magnetic domains then appear as lines of dark and light contrasted areas ; when the sample is imaged in the out of focus condition. These domain boundaries are present only in the α'-martensite regions and are normally oriented at an angle to the long axis of the martensite lath. It has been identified that significant reductions in the maximum magnetic induction and remanence values can be achieved when a 302 type stainless steel is substituted for the conventional types of low alloy medium carbon steel. These changes can be attributed to the paramagnetic austenite fraction and the magnetic domain structure existing within the α'-martensite areas (ferromagnetic phase). There was no magnetic domain structure within the paramagnetic α'-martensite region.