Fig. 7 — Microstructure of HAZ depending on simulated thermal conditions (250×). • The temperature of minimum toughness required (here 70 J/cm2), which is usually defined by specifications for gas pipes (projection T3 in Fig. 2). Results and Discussion Investigation of HAZ Resistance to Brittle Fracture Evaluation of weldability of the steel containing 0.056% Nb based on T50 CVN (Fig. 3A) shows that during cooling of the external weld with a rate of 6°–8°C/s corresponding to the condition of complete precooling of the internal weld, the temperature of the average threshold ductilebrittle transition of the HAZ (here corresponding to CVN ~120 J/cm2) is –30°C (determined for 7°C/s). For welding over the “hot” joint (with its temperature of 100°C) and accordingly for the condition of a reduced cooling rate T50 ShCVN increases only up to –20°C (determined for 3.3°C /s). Evaluation of weldability of the steel containing 0.096% Nb by T50 CVN presented in Fig. 3B, under the same welding conditions, demonstrates that the ductilebrittle transition temperature (here, too, at CVN ~ 120 J/cm2) is also –30°C (determined for cooling rate 7°C/s, and for welding over the “hot” joint (again at 100°C) rises also to –20°C. Thus, the increase of Nb content does not negatively impact the brittle fracture resistance of the HAZ during welding with high heat input. The obtained experimental data were transformed to some diagrams depicted in Fig. 4. These diagrams present the CVN values vs. applied cooling rate at various temperatures of impact toughness measurements and thus allow us to define permissible ranges of cooling rates in the temperature region of phase transformations W8/5 (cooling rate from 800° to 500°C), which may ensure a prescribed level of brittle fracture resistance of the steel in the HAZ. As shown, these curves exhibit some extremes, pointing out a maximum possible impact toughness of the HAZ. At present, this possibility cannot be implemented due to lack of technical means to control postweld cooling. In particular, Fig. 5 presents the HAZ microstructure after simulation of multipass joint welding, the “hot pass” version. When very favorable microstructure with 100% lath bainite was obtained, bainite WELDING JOURNAL 27-s WELDING RESEARCH A B C D Table 4 — Calculated Values of Cooling Rates, Depending on the Preliminary Temperature of the Joint during Two-Pass, SAW Temperature of Pipe wall thickness, (mm) Internal Joint 16.4 25.4 before Welding, T, °C Heat input (E ), kJ/mm 3.4–4.0 4.8–5.4 Cooling rate (W800/500) (°C/s) 20°C 5–7 6–8 60°C 4–6 5–7 100°C 3–5 4–6
Welding Journal | January 2014
To see the actual publication please follow the link above