Effects of Niobium on the Mechanical Properties and Corrosion Behavior of Simulated Weld HAZ of HSLA Steel
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NTRODUCTION
WITH the extensive exploitation of ocean resources, steel used in ocean engineering has been designed for high strength, toughness, and corrosion resistance.[1] In general, microalloying elements such as Nb, V, or Ti are widely added to produce steel equipped with high strength, resistance to brittle fracture, and good weldability.[2] Niobium can readily form NbC, which has high hardness, and a high melting point, and is very effective in restraining the growth austenite grains and inhibiting austenite recrystallization.[3] NbC precipitates also improve the strength and elongation of the weld metal and the resistance of hydrogen-induced delayed fracturing.[4–6]
ZIHAO WANG, JUNSHENG WU, JIA LI, XIAOGUANG WU, and YUNHUA HUANG are with the Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, China. Contact e-mail: [email protected] XIAOGANG LI is with Corrosion and Protection Center, University of Science and Technology Beijing and also with the the Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, Zhejiang, China. Zihao Wang and Junsheng Wu have contributed equally to this work. Manuscript submitted April 21, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The heat-affected zone (HAZ) of a weld joint in high-strength low-alloy (HSLA) steel is generally categorized into five distinct regions[7]: the base metal (BM), the tempered zone (TZ), the intercritical HAZ (ICHAZ), the fine-grained HAZ (FGHAZ), and the coarse-grained HAZ (CGHAZ). In regard to mechanical properties, the CGHAZ, which has coarse grains, and the ICHAZ, which has an uneven microstructure, have the lowest toughness among the weld joint regions. Moreover, the HAZ corrodes more easily than the BM,[8] and the passive current density produced from the HAZ is found to be higher than that of the BM in a NaHCO3/Na2CO3 solution.[9] Aside from the above, the differences in composition and microstructure between the HAZ and the BM can create local electrochemical potential variations, leading to galvanic corrosion.[10] The effects of Nb microalloying on the mechanical properties and microstructure of steel have been widely studied. Previous reports revealed that Nb improves properties by refining the grains, and making the microstructure of base steel and weld joints more uniform, etc.[11–15] Meanwhile, a large amount of research on the electrochemical corrosion behavior in HAZ of weld joints were reported.[8,9] However, except for our relative work,[16,17] the roles of Nb in the corrosion behavior of HSLA steels, particularly for each part of the weld HAZ, were rarely reported.
and Ac1 and Ac3 for the Nb-free steel were 710 °C and 874.9 °C, respectively. Based on the phase transformation temperatures, the heat treatment temperature was chosen. The base materials were put in the furnace at 1300 °C, 950 °C, 780 °C, and 500 °C, maintained for 15 minutes, and subsequently cooled in water at 30 °C to reproduce the targeted simulated microstructures of the CGH
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