Fig. 1 — Microstructure of the X80 base metal (250×). A B heated region. Some researchers pointed out a linear increase in MA with an increase in Nb content, but this effect has been found at rather high C content (Ref. 8). The corresponding hardness increase was attributed to precipitation of fine Nb(C, N) formed at cooling after the redissolution Fig. 2 — The proposed criteria for brittle fracture resistance. of Nb (Ref. 5). An investigation of the HAZ microstructures of two steels with 0.04% C and 0.07–0.10% Nb showed no difference in the prior austenitic grain size and, consequently, in the local hardenability. On the other hand, Nb reduced the size of the bainitic packet in the HAZ leading to an improvement in impact toughness (Ref. 9). Some researchers found that a small addition of Nb decreases toughness (Ref. 10), while others found either no significant effect of Nb addition in the case of low-C steels (Ref. 11) or increased toughness in very low C (~0.03%) steel (Ref. 12). The importance of very low C to ensure high HAZ toughness in two-pass submerged arc welds is emphasized in a few studies together with confirmation of the fact that without microalloying by Nb the strength of X80 cannot be achieved (Ref. 13). An investigation of coarse-grained HAZ of X80 grade steel with ~0.1% Nb using simulation of a single welding thermal cycle came to the conclusion that the heat input should be less than 30 kJ/cm to ensure good Charpy impact toughness (Ref. 14). As is well known, all properties including impact toughness are defined by the microstructure. Therefore, all discussions and differences of opinion about the role of Nb, which was often overshadowed or mixed with the dominating roles of C and Mn or Mo content, should be related to JANUARY 2014, VOL. 93 24-s WELDING RESEARCH Fig. 3 — The temperature dependence of the impact toughness in the HAZ of the following: A — X70; B — X80 grade steels depending on the cooling rate (shown on the curves), corresponding to a different heat input at welding. Table 1 — Chemical Composition of the Investigated Steels Grade Chemical Composition (%) C Si Mn S P Al Ti N2 Ca X-70 0.05 0.33 1.73 0.0005 0.006 0.033 0.013 0.0051 0.0002 Nb V Mo Cr Ni Cu B 0.056 0.001 0.002 0.17 0.012 0.014 0.0002 C Si Mn S P Al Ti N2 Ca X-80 0.06 0.30 1.56 0.002 0.014 0.037 0.014 0.004 0.0026 Nb V Mo Cr Ni Cu B 0.094 0.002 0.01 0.23 0.13 0.24 — Notes: H70 (HSLA) is the steel for offshore application in accordance with Standards Det Norske Veritas (DNV) Offshore Standard OS-F101, Submarine Pipeline Systems. X80 (HSLA) is the steel for the Cheyenne Plains Pipeline, U.S.A.
Welding Journal | January 2014
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