Analysis of Results of Impact Bending Tests of Base Metal Samples and Welded Joints of Pipes From Low-Carbon Pipe Steels

Impact bending tests of rolled products, base metal and welded joints of pipes manufactured from low-carbon microalloyed steels are performed. The structure of different parts of welded joints is studied. Fractures of impact samples and compositions of nonmetallic inclusions on fracture surfaces are analyzed. The embrittling mechanisms and the causes of scattering of the impact toughness are considered. These may include coarse bainite within a large (deformed) austenite grain, a region near the fusion line with an unfavorable orientation of crystallographic cleavage planes {001} in a HFC welded joint, and large grains of grain-boundary ferrite in the weld and in the HAZ under arc welding. The critical grain size of the α-phase (the maximum fraction), at which the embrittling effect of the nonmetallic inclusions begins to be observed in the steels is 50 – 80 μm.