Inclusion Characteristics and Acicular Ferrite Nucleation in Ti-Containing Weld Metals of X80 Pipeline Steel
- PDF / 7,733,264 Bytes
- 15 Pages / 593.972 x 792 pts Page_size
- 74 Downloads / 221 Views
RODUCTION
HIGH-STRENGTH low-alloy (HSLA) steels, such as the X80 pipeline steel, have been widely used in the constructions of long-distance oil and gas transportation systems, offshore structures, vessels, and other structures, due to their excellent combination of strength and toughness. However, the impact toughness of the weld metals tends to deteriorate after the welding thermal cycle because of the formation of coarse grain boundary allotriomorphic ferrite and well-aligned side plate ferrite (i.e., Widmansta¨tten) in the microstructures of the weld metals. A chaotic arrangement of laths and fine-grained interlocking microstructural characteristics of acicular ferrite (AF) can effectively retard crack propagation, noticeably improving the toughness.[1–3] Thus, AF is expected to develop fully in the microstructures of weld metals.
BINGXIN WANG is with College of Mechanical Engineering, Liaoning Shihua University, Fushun 113001, China. Contact e-mail: [email protected]. XIANGHUA LIU and GUODONG WANG are with the State Key Laboratory of Rolling & Automation, Northeastern University, Shenyang 110004, China. Manuscript submitted August 2, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The fine interlocking AF nucleates intragranularly in the form of separate laths on non-metallic inclusions, and competes with the grain boundary ferrite (i.e., Widmansta¨tten and lath bainite) during the austenite to ferrite transformation.[4] Hence, the factors controlling the AF formation mainly include prior austenite grain size, density and size of inclusions, and chemical compositions of the steels and/or weld metals.[4–8] However, the most essential condition is the ability of inclusions to nucleate the intragranular AF. Among some non-metallic inclusions, such as titanium nitride, vanadium nitride, and manganese sulfide, Ti-containing inclusions in particular have been known to strongly promote the formation of AF microstructure, and many studies have been performed to investigate the behavior and mechanisms of the Ti-containing inclusions inducing ferrite nucleation. Yamada et al.[9] and Takada et al.[10] pointed out that the TiO phase on the inclusions’ surface contributes to the heterogeneous nucleation of AF and that the AF nucleated on TiO shows the Baker–Nutting (B–N) orientation relationship with the TiO. This orientation relationship achieves good lattice coherency and decreases the interfacial energy between the AF and TiO, resulting in a decrease in activation energy for AF nucleation. Nako et al.[11] and Kang et al.[12] found that MnTi2O4 on the surfaces of the inclusions is responsible for the formation of AF. In
their studies, it was revealed that AF can have not only the B–N orientation relationship with MnTi2O4 but also the Kurdjumov–Sachs (K–S) orientation relationship with the austenite matrix. Formation of both B–N and K-S orientation relationships lowers the AF/MnTi2O4 and AF/austenite interfacial energies, giving rise to the AF nucleation. More recently, the Mn depletion phenomenon has attracted much att
Data Loading...
