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The purpose of this investigation is to study the effect of postweld heat treatment (PWHT) on impact toughness with various temperatures at heat affected zone (HAZ) of 3.5%Cr steel by gas tungsten arc welding (GTAW) process with AWS ER90S-B3 Cr-Mo steel filler metal. The tested samples are divided into two conditions; No PWHT and PWHT at 690๐C for 1 hour. The results indicated that the microstructure of the weld metal and HAZ of No PWHT samples contained bainite and martensite, respectively. However, those of PWHT specimens transformed to ferrite and tempered martensite with lower hardness. From charpy impact test at HAZ regions, the lower the tested temperature (from 25๐C to -80๐C), the lower the impact energy of the HAZ (such as from 104 joules to 6 joules in case of the PWHT samples), which meant the impact toughness of HAZ decreased. The HAZ of the PWHT specimen exhibited lower hardness and better impact toughness compared to the No PWHT sample. This is due to this region contained tempered martensite instead of martensite structure resulting from the improvement of metallurgical and mechanical properties of the weldment from postweld heat treatment.