A Study on the Relationship between Microstructure and Impact Toughness with Different Heat Inputs and Post-Seam Annealing Temperatures in API X70 Electric Resistance Welded Steel Pipe

In this study, the influence of heat input and post-seam annealing (PSA) temperature on the microstructure and impact toughness of electric resistance welded (ERWed) API X70 steel pipe was investigated. The ERW seam welds of pipes were fabricated with low, moderate, and high heat inputs, and followed by the PSA process at 950 ℃. Regardless of heat input, the API X70 steel pipe exhibited good weldability, resulting in similar microstructure factors, i.e., metal flow angle (~70o), bondline width (~22 μm), and ferrite grain size (~4.4 μm). Although (Mn, Si)-rich oxide penetrators which are known to impair impact toughness were observed at the bondline, the area fraction was estimated to be as low as 0.011% in all the ERW seam welds. The impact toughness was not affected by these penetrators because of their minimal fraction, leading to ductile fracture at -20 ℃. However, different PSA temperatures (870, 950, and 1040 ℃) resulted in the different grain sizes of the seam welds. The ERW seam weld annealed at 1040 ℃ (PSA1040) exhibited the largest ferrite grain size near the bondline, whilst the smallest grain size was found in the ERW weld annealed at 870 ℃ (PSA870). In spite of having the smallest grain size, PSA870 exhibited the lowest impact toughness value, showing brittle fracture at –20 ℃. It was observed that the difference in residual stresses was not macroscopically detectable by the hole drilling method. The kernel average misorientation (KAM) maps indicated that the highest level of residual stress was observed near the bondline of PSA870, as evidenced by a high density of dislocations. This study provides instructive results, indicating that the impact toughness of ERW seam welds are significantly more affected by residual stresses than by the grain refinement effect.