In light of the weakness of steel structures against fire and the welding process in steel structures connections, realizing the post-fire behavior of the welding after experiencing different temperature levels is vital in modeling and analyzing a structure. In this study, the behavior of butt-welded connections after experiencing different temperature levels has been investigated. The number of 168 specimens and different St-37 steel plate thicknesses (6, 8, 10, 12, 15, 20, 25, and 30 mm) are heated under seven temperature levels of 25, 100, 250, 400, 500, 700, and 900°C, and after the cooling phase, tensile tests are implemented to determine the stress–strain diagram. By examining the resulting stress–strain diagrams, the samples mechanical properties, including yield stress, ultimate stress, and elasticity modulus, are evaluated after different temperatures. The experimental results reveal that the ultimate strength, yield strength, and elasticity modulus of samples are restored to an average of about 82, 79, and 82%, respectively, after being exposed to 900°C and the subsequent cooling phase. Additionally, penetration welds have shown acceptable performance in post-fire conditions, and the failure mode of the majority of samples is the rupture of steel plates.
