ICONE11-36477 Optimum Condition of Thermal Cycle for Temper Bead Weld Repair of SQV2A Pressure Vessel Steel : Fundamental Investigation in Gleeble Simulation Test

Abstract

Temper bead welding technique is one of the most important repair welding methods for large structures for which post weld heat treatment (PWHT) is difficult to perform. In this study the optimum condition of weld thermal cycles to carry out temper bead weld repair for pressure vessel steel SQV2A are investigated taking interest in the improvement of the characteristics of coarse grained heat affected zone (CGHAZ) re-heated to the temperature between 670℃ (Ac1 temperature) and 837℃ (Ac3 temperature). Thermal/mechanical simulator Gleeble 1500 is employed to give specimens repeated thermal cycles. Improvability of the Charpy absorbed energy and the hardness for the thermal cycles is discussed. The absorbed energy of the CGHAZ improved to about 70% of that of as-received steel and the hardness of the CGHAZ decreased to lower than 350HV1 when the peak temperature of the succeeding thermal cycle reached to the temperature from 600℃ to Ac1. The absorbed energy of the CGHAZ degraded to about 25% of that of as-received steel and the hardness of the CGHAZ was still higher than 350HV1 when the peak temperature of the succeeding thermal cycle reached to the temperature from Ac1 to Ac3. In order to improve the absorbed energy to about 70% of that of as-received steel but to decrease the hardness to lower than 350HV1,additional one more thermal cycle ; that is, triple thermal cycle is required. The absorbed energy of the CGHAZ improved to more than 70% of that of as-received steel but the hardness of the CGHAZ was still higher than 350HV1 when the peak temperature of the succeeding thermal cycle was higher than Ac3. The absorbed energy improved to almost the same as that of as-received steel but the hardness was still higher than 350HV1 even after the following thermal cycle, the peak temperature of which is less than Ac1. To reduce the hardness to lower than 350HV1,fourth thermal cycles are required. The recommended temper bead thermal cycles were suggested that in the second thermal cycle the peak temperature Tp2 should be selected as lower than Ac1 but near Ac1. In this case triple thermal cycles temper bead process are enough to improve the characteristics of CGHAZ. When the other peak temperatures (that is, higher than Ac1) in the succeeding thermal cycle are applied to CGHAZ, quadruple or more thermal cycle temper bead process should be applied.