چكيده به لاتين
Abstract:
In this research, effect of friction stir process (FSP) on microstructure and mechanical properties of Fe-24Ni-0.3C and Fe-24Ni-0.2C steel has been investigated. For this purpose, the FSP was carried out on sheets with thickness of 1 mm using a tungsten carbide tool with rotational speeds of 600 and 950 rpm and traverse speeds of 50, 100 and 150 mm / min. Microstructure was analyzed using scanning electron microscope equipped with electron backscattered diffraction (EBSD) detector and optical microscope. The results showed that the stirring zone of both alloys involved from ultrafine grain of austenite due to the continuous dynamic recrystallization (CDRX) phenomenon. It should be notified that deformation induced martensite transformation is not occurred in the studied steels during the FSP.
Mechanical properties of Fe-24Ni-0.3C alloy showed that the tensile samples were broken down from the stirring zone and the yield stresses of the FSPed samples increased comparing to the starting materials. However, due to the elimination of TRIP effect in the FSPed samples, the fracture strength of the samples was reduced compared to the starting material. Hardness diagram showed that in Fe-24Ni-0.3C steel before tensile test, the hardness increases in the stirring zone, which has a direct relation with the transverse velocity of the tool. However, after the tensile test due to the effect of TRIP, the hardness results are reversed.
In Fe-24Ni-0.2C steel, the microstructure of the stirring zone is completely austenitic, while the microstructure of the base metal of this steel includes austenite and martensitic. Also, the tensile samples were broken down from the base metal region and the study of the tensile properties with the second tensile sample taken from the stirring zone showed that the tensile strength along with the formability in the tensile samples were cut from the stirring zone increased compared to the base metal. Hardness results for Fe-24Ni-0.2C steel showed that the hardness of the stirring zone in samples with rotational speed of 600 rpm is higher than that of the base metal, while the hardness of the samples having a rotational speed of 950 rpm is lower than that of the base metal which is due to the difference in the size of the grains at two different speeds of the tool rotation.
Keywords: friction stir process, TRIP, EBSD, continuous dynamic recrystallization, microstructure, mechanical properties
