Separation of Static Recrystallization and Reverse Transformation of Deformation-induced Martensite in an Austenitic Stainless Steel by Calorimetric Measurements

Abstract

During plastic deformation of metals and alloys a small fraction, between 1 and 5%, of the energy employed in cold working, is stored inside the material in the form of defects, mainly dislocations. This stored energy derived from cold working is the driving force for recrystallization and is released during annealing. Despite recrystallization presents one of the smallest driving forces amongst the solid state transformations, for highly deformed metals, measuring stored energy due to deformation can be assessed by calorimetric methods. For austenitic stainless steels (ASSs), depending on steel composition and processing variables, plastic deformation apart from introducing crystal defects, can also cause the appearance of deformation-induced martensites, DIM, which has an important influence on the strain-hardening coefficient and on the formability. Two types of martensite may occur in the ASSs, namely: a -(bcc, ferromagnetic) and e-(hcp, paramagnetic). Some researches show that for low deformation levels the amount of e-martensite is predominant in relation to a . When the quantity of a grows continuously with the degree of deformation, the amount of e-martensite goes through a maximum and lowers thereafter, suggesting the following transformation sequence: g→e→a . Reversion of a -martensite of the DIM in the ASS is less studied than its formation. A good judgment about the thermal stabilities of the eand a martensites, as well as the deformation bands introduced by cold working, can be obtained from the work of Singh. He detected that during annealing (with duration of 1 h) of a cold rolled AISI 304, e-martensite remained stable up to 200°C, that a was stable up to 400°C, whereas deformation bands stayed in nonrecrystallized regions up to 800°C. Guy and co-authors studied the reversion of a induced by cooling and by deforming in two different steels (18%Cr–8%Ni and 18%Cr–12%Ni). In both cases, after 2 min at 600°C, practically all a -martensite reverted to austenite. Martins and co-authors studied reversion of a -martensite in the 304 and 304L steels and concluded that the reversion temperature was little sensitive to the type of steel, strain and the effect of precipitation treatments introduced prior to cold working. For the 12 studied conditions (2 steels 2 pretreatments and 3 strain levels), the temperature (annealing for 1 h) for 50% a (T50%RM) reversion was always within the 550 20°C range. Tavares and co-authors studied reversion of a -martensite and observed that the T50%RM depended only slightly on strain and heating rate. The temperatures, measured for 3 levels of strain and 3 heating rates, stayed between 549 and 573°C. Analysis of the above mentioned works shows that reversion of the strain induced martensites in the ASS occurs at temperatures much lower than the recrystallization temperature, despite the complete reversion of a -martensite, may reach temperatures of the order of 750°C. The main objective of this work is to verify if it is possible to separate, with the help of calorimetry, the phenomena of reversion of deformation-induced martensite and recrystallization in a AISI 304 cold worked austenitic stainless steel.