A B Fig. 8 — CCT diagrams of A — X70; B — X80 steel grades, built at cooling from 1300°C. lath size is 15 microns, prior austenite grain size is 40 microns. As shown for X70 grade (Fig. 4A), the studied composition shows a wide range of acceptable cooling rates for welding with large heat inputs, typical for factory-made longitudinal SAW, as well as with low heat inputs applied for field construction joints. Depending on the test temperature for a specific pipeline operation, the allowable range of postweld cooling rates may vary. For example, to guarantee a CVN value more than 120 J/cm2 at –20°C, the permissible range of cooling rates is from 2.7° to 70°C/s. It is worth noting that for field SAW at –7°C, all existing working instructions require preheating to 150°C, which means heat input should be 1.2 kJ/mm or higher to ensure a cooling rate no higher than 40°C/s. As can be seen for X80 grade (Fig. 4B), its high Nb content shows a wide range of acceptable cooling rates for welding with both high and low heat input. Depending on the testing temperature for a specific pipeline operation, the allowable range of postweld cooling rates may vary. For example, for a guaranteed level of toughness of more than 120 J/cm2 at –20°C, the permissible cooling rate range is from 2.7° to 40°C/s. Figure 6 presents the comparison of those permissible ranges of cooling rates for both investigated steels for impact toughness tested at –30°C. As shown, the X70 steel with 0.056% Nb can guarantee retaining 50% ShCVN (here 115 J/cm2 ) at cooling rates from 8° to 60°C/s, and the specified minimum value (here 70 J/cm2) at cooling rates from 3.8° to more than 100°C/s. Increase in Nb content results in slight changes of those values. The toughness of 115 J/cm2 at –30°C can be guaranteed at cooling rates from 7° to 20°C/s, whereas the level of 70 J/cm2 can be assured at cooling rates from 3° to 70°C/s. Microstructures obtained at various cooling rates are presented in Fig. 7. Figure 7A corresponds to the HAZ at very slow cooling and contains 50% bainite and 50% polygonal ferrite with the sizes of the bainite packet and ferrite grain of 30 and 35 microns, respectively. Figure 7B presents the microstructure of the SAW HAZ with a “hot pass” (preliminary temperature 100°C): 5% polygonal ferrite and 95% bainite, average bainite packet size is 15 microns, and prior austenite grain size (PAGS) is 70 microns. Microstructure of HAZ at SAW with a “cold pass” (20°C) is presented in Fig. 7C and contains 100% bainite of lath and globular morphology, and the bainite packet size is 10 microns and the PAGS is 60 microns. The microstructure that can ensure the highest low-temperature toughness is presented in Fig. 7D. It is 100% lath bainite with a packet size of 10 microns and PAGS of 45 microns. Effect of Nb on the Kinetics of Austenite Transformations The changes in impact toughness shown above reflect changes in microstructure resulting from the transformation of coarse-grained austenite in the HAZ for a specific thermal cycle. Investigations of phase transformations resulting in the building of continuous cooling transformation diagrams (CCT) were performed after the high-speed heating of dilatometer samples to a temperature of 1300°–1320°C. As shown in Fig. 8A, B, the kinetics of austenite transformation in both steels that were investigated is featured by bainite transformations in a wide range of cooling rates. The fact that Nb promotes the formation of lower temperature transformation banite-like products at a relatively high cooling rate is noted also at comparative investigation of effects of Nb and V (Ref. 15). Martensitic transformation is observed at high enough cooling rates, but they do occur in pipeline butt joints. Niobium slightly increases the stability of austenite, so that the formation of martensite in steel containing 0.094% Nb is observed at a cooling rate of 50°C/s, compared with 70°C/s at 0.056% Nb content. This effect is small and it is necessary to note that the actual cooling, which accompanies the root welding without preheating the weld, even with a cooling rate of 90°C/s, results in the volume fraction of martensite being not more than 25% and 10%, respectively, for the 0.094% and 0.056% Nb. As can be seen from the CCT, the formation of a significant amount of (low-carbon and therefore not very hard) martensite in these steels is impossible. Diffusion-controlled ferrite transformation is shifted, under the influence of niobium, to the slow cooling rates — up to 2.5°C/s at 0.094% Nb, and up to 4.2°/s at 0.056% Nb, i.e., toward significantly lower than the usual cooling rates during welding of thick-walled tubes under a layer of flux. Evaluation of Tendency to Cold Cracking During multipass welding, the HAZ cooling rate depends on the heat input and the temperature of the weld before the welding, beside the effect of the wall thickness. Processing of multipass butt-joint welding of pipelines varies depending on type of weld and heat input values as the following: • Root weld with heat input up to 0.55 kJ/mm; • Hot pass with heat input up to 1.2 kJ/mm; • Facing joint with GMA (CO2) welding with heat input up to 2.0 kJ/mm. The diagram of cooling rates vs. heat inputs for these types of butt-joint welding is shown in Fig. 9. Measurements of microhardness of dilatometric samples used at constructing CCT diagrams to characterize products of austenite transformations allow the evalu- JANUARY 2014, VOL. 93 28-s WELDING RESEARCH
Welding Journal | January 2014
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