13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Evaluation of Creep Damage and Fracture in High Cr Steel Welds Masaaki Tabuchi1,* 1 National Institute for Materials Science, Tsukuba Science City 305-0047, Japan * Corresponding author: TABUCHI.Masaaki@nims.go.jp Abstract Creep strength of high Cr ferritic steel welds decreases than base metals due to Type-IV creep damages formed in the heat affected zone (HAZ) during long-term use at elevated temperatures. In the present study, microstructural changes and damage evolution behaviors in HAZ during creep were investigated for the ASME Gr.91 and Gr.122 steel weld joints. Creep voids formed at an early stage of life and coalesced to form a macro crack at 0.8 of life for the Gr.91 steel weld. On the other hand, for the high strengthened Gr.122 steel weld, small amounts of Type-IV creep voids formed at 0.5 of life, increased slightly until 0.9 of life and rapid crack growth occurred after that. KAM and grain boundary length of fine-grained HAZ obtained by EBSD decreased and saturated till 0.2 of life in the Gr.91 steel weld, whereas they decreased after 0.5 of life in the Gr.122 steel weld. It was found that the recovery of dislocation structures in HAZ was completed at early stage of life for the Gr.91 steel weld, whereas it occurred after recrystallization at the later stage of life for the Gr.122 steel weld. These differences of microstructural changes was considered to relate to the differences of Type-IV creep damage behavior; early initiation of creep voids at 0.2 of life in the Gr.91 steel weld and later damage evolution after 0.5 of life in the Gr.122 steel weld. From these results the methods for remaining life assessment of high Cr steel welds were discussed. Keywords High Cr ferritic steel, Welded joint, Creep, Type-IV creep damage, EBSD 1. Introduction In order to improve the efficiency of power generation, the pressure and temperature conditions of steam in thermal plant have been continuously increased. In 1990s, the high Cr ferritic heat resisting steels were applied to the boiler components in 600 ˚C class ultra super critical (USC) thermal power plants in Japan. The base metals of these steels with tempered martensite structures have excellent high temperature strength; however, fine-grained structures without lath-martensite are formed in the heat-affected zone (HAZ) during weld thermal-cycle and the creep strength of welds decreases than the base metals. The Type-IV failure along the inter-critical and fine-grained HAZ of weld joints is caused through the nucleation and growth of creep voids and cracks during long-term services at high temperatures [1-8]. Recently, it was recommended to take the weld strength reduction factor (WSRF) into account for the high temperature design using high Cr steels [9, 10]. It is important to understand the microstructural changes and damage evolutions in HAZ during creep for the remaining life assessment of weld components. In the present paper, aiming to elucidate the Type-IV failure mechanisms of Gr.91 and Gr.122 steel welds, we have evaluated the Type-IV creep damage evolutions and microstructural degradations through interrupting creep tests of the large-scale welded joint specimens. Remaining life assessment methods of high Cr steel welds were discussed based on the experimental results. 2. Experimental procedure The materials investigated are the Gr.91 steel (9Cr-1Mo-VNb steel) plate with a thickness of 25mm and the Gr.122 steel (11Cr-0.4Mo-2W-CuVNb steel) plate with a thickness of 30mm. The plates were welded by gas tungsten arc (GTA) welding using a double U groove. After welding, a post-weld heat treatment (PWHT) was conducted at 745 ˚C for 60 min for the Gr.91 steel weld and for 75 min for the Gr.122 steel weld.
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