316 HTM stainless steel reinforcing bars as a green material

Abstract

Stainless steel reinforcing bars, which are known as green material, have been adopted in construction industry recently because of their excellent corrosion resistance on chloride-containing environment and their long lifetime when compared to carbon steel. Although the initial cost of using stainless steel rebar is higher than the one with carbon steel, the building structure can last for many decades with minimum maintenance. Besides, they can provide high strength and good durability among the building construction; hence, they are treated as one of the most potential building materials in construction industry. However, the chance of fire accident in buildings may affect the building structure and the strength of the reinforcing bars during and after the accident.

This project aims for investigating the mechanical behavior of austenitic stainless steel 316HTM with Grade 650 reinforcing bars after the fire accident. Since the mechanical properties play an important role for the structure of building, this project is intended to reveal the relevant mechanical issues after fire accident simulation so as to prevent the construction failure of buildings. The 316HTM stainless steel used in this study has a single phase structure with a high tensile strength behavior. By simulating the condition of fire accident, three annealing conditions, which were annealed at 700°C (for 2 hours) and 950°C (one was for 2 hours and another one was for 24 hours), were designed. Then, they were air-cooled in order to simulate the real cases of fire accidents. Tensile test and hardness test were carried out for evaluating the changes after the fire accident simulation. Results revealed that both of the tensile strength and hardness dropped with increasing annealing temperature. It was due to the three stages of annealing (recovery, recrystallization and grain growth) took place during the annealing process. Investigation on microstructure also helps to explain the effect of fire on different specimens. The optical microscope and scanning electron microscope have revealed some of the microscopic characteristics of 316HTM stainless steel after the fire accident simulation. It was suspected to have carbide precipitates and inclusions at the specimen which annealed at 700°C (2 hours). Inclusions may restrict the grain growth process and result in higher mechanical properties for the materials. Grains enlarged at 950°C (2 and 24 hours) were also noted, indicating the specimens were less effective in dislocation restriction; hence, the hardening effect was weakened.