FRACTURE TOUGHNESS OF WELDED JOINTS OF X100 HIGH-STRENGTH PIPELINE STEEL

Fracture toughness of base metal, weld seam and heat-affected-zone (HAZ) in X100 high-strength pipeline steel welded joints was investigated by three-point crack tip opening displacement (CTOD) test. Microstructure and inclusion near fracture zones were observed by means of SEM and TEM. The results indicated that fracture toughness of X100 high-strength pipeline steel welded joints was greatly influenced by test temperature. At the same temperature, the numerical values of apparent crack initiation delta(0.05), conditional crack initiation delta(0.2) and delta(0.2BL) of base metal are higher than that of weld seam and HAZ, and low temperature fracture toughness of base metal is better than those of weld seam and HAZ. With temperature decreasing, the fracture toughness of base metal, weld seam and HAZ decreased. The microstructure of near fracture zones of base metal specimen was composed of granular bainite (GB), a small quasi polygonal ferrite (QF) and lath bainite ferrite (BF), and the fine and equally dispersed M-A structure distributed on the grain boundary. The microstructure of near fracture zones of weld seam specimen was composed of acicular ferrite (AF), and the form of M-A constituents shows diversity, sharp-angled clearly. The microstructure of near fracture zones of coarse grain HAZ was composed of GB and parallel LBF, and the square, wedged and bar M-A constituents distributed on the interior of grain, grain boundary and lath boundary. The poor fracture toughness of weld seam and HAZ specimen results from large size and cusp type M-A structure. While the higher distribution of inclusion in weld seam makes fracture toughness worse.