Alireza keyhan khadivi - MA student of energy conversion of shahid madani azarbaijan university,
Ali sharifi - MA student of energy conversion of shahid madani azarbaijan university,
s.Faramarz ranjbar - Assistant professor of mechanical engineering department of Tabriz university,

Due to its earthquake resistant properties, most building structures are constructed with reinforced concrete on a steel structure. As the temperature of steel increases especially during a fire, it will reduce the strength of a structure. Since the properties of steel, such as the Young’s modulus and yielding strength, drop rapidly with increasing temperature, during a fire the load capacity of such structures will decline dramatically. However, it is important to evaluate the structural performance of steel frames after a fire for reuse and reconstruction due to the high cost of reconstructive building. This study is concerned with the development of nondestructive testing techniques to investigate the structural behavior of steel subjected to fire. The results of ultrasonic wave speed measurement show that heating up to 1000 _C and cooling by water produces a large change in wave speed. Under metallurgical microscope observation, the texture could be seen to have changed into bainite and martensite. In the study, the properties of the structure became hard and brittle due to phase change

Fire- Nondestructive testing techniques- Ultrasonic wave speed- Steel structural behavior- Metallurgical microscope