Relation between the Metallographic Structure and Mechanical Properties of a Bimetallic Steel Bar after Fire

Bimetallic steel bar (BSB) consisting of S30408 stainless steel (cladding layer) and HRB400 carbon steel bar (substrate) has outstanding durability, indicating its suitability for RC structures in corrosive environments. The effects of fire on the mechanical properties of BSB in relation to its metallographic structure (MS) were investigated experimentally by exposing BSB specimens to different elevated temperatures and cooling methods. When the exposure temperature (ET) was lower than the austenite transformation temperature, the changes in the MS were relatively small. When the ET was higher than 700 degrees C, lamellar pearlite and lath martensite were formed in the substrate of BSB specimens with cooling in air (CIA) and cooling in water (CIW), respectively, which led to changes in the mechanical properties. The hardness of the BSB specimens also was affected by the ET and cooling method. There always was a yield plateau in the stress-strain curve of the BSB specimens with CIA. When the ET ranged from 700 degrees C to 900 degrees C, the formation of granular pearlite improved the homogeneity of cementite in ferrite, which enhanced the ductility. For BSB specimens with CIW, when the ETs were 800 degrees C and 900 degrees C, the yield plateau in the stress-strain curve disappeared because of the lath martensite in the substrate. Ductile dimples were observed in the scanning electron microscope images of the BSB specimens with CIA. However, for BSB specimens exposed to 900 degrees C with CIW, the ductile dimples were almost negligible.