To obtain a higher strength in line pipes of diameter up to 1420 mm the controlled rolling of plates and coils or heat treatment of the product were applied. Metallurgical design based on purification, low carbon content, micro-alloying and controlled rolling has increased yield stress, ductility and low temperature behavior of the developed steels. Maximum homogeneous structure of the base metal and the weld joint, minimum residual stresses, which is beneficial for a better stress corrosion and fatigue resistance, were obtained through quench and tempering. Full scale tests have shown formed microstructure retards crack propagation from sharp defects notched in strip or plate end welds under both static and cyclic pipe loading. Combined laser–submerged are welding was tested for further improvement of properties of the end joints. Features of microstructure in weld, fusion and heat affected zone were studied. Advanced properties of the joints were found in laser welded steels but inherent high cooling rates must be reduced using different methods.

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