The ductile fracture arrest capability of gas pipelines is seen as one of the most important factors in the future acceptance of new high strength pipeline steels for high pressure applications. New North-European pipeline project is based on application of X70 pipes designed for high gas pressure of 9,8MPa. To study fracture propagation behavior in cold climate the full scale tests were carried out using longitudinally and spiral welded pipes with different microstructure of base metal. Results of the tests with crack propagation studies in predominately ferrite-pearlite (FP) and acicular ferrite (AF) steels possessing similar Charpy energy are presented. Crack propagation mechanisms in base metal and along spiral weld were studied and are presented with strain distribution along crack.

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.

Strengthening mechanisms applied for modern line pipe steel design were studied. Low carbon steels alloyed with Mn, Mo, V, Nb processed by the way of controlled rolling were developed for spiral welded X65-X80 line pipes up to 1420 mm diameter. Formation of the microstructure during steel processing was studied. The effects of typical microstructure for the steels on mechanical properties, strain hardening behavior and Bauschinger effect were studied. Main metallurgical factors affecting on strength measured in plates and pipes were revealed using physical and computer simulations.

A low-carbon low-alloy steel corresponding to Grade X42 according to API SPEC 5L and resistant to hydrogen sulfide environments has been developed. Heat treatment conditions have been optimized. The heat treatment recommended provides for the corrosion resistance and desired level of finished product mechanical properties. Welded joints of pipes made of the developed steel have successfully passed benchmark tests carried out at Astrakhan gas-condensate field.