Improving the Strength of Industrial Pipelines

Abstract

The main reason for pipeline weld failure is the formation of cold cracks in the heat-affected zone on account of the increased tendency to brittle failure. The cooling time after welding affects the tendency to cold cracking, and the critical cooling time is related to a parameter characterizing the cracking resistance for various steels. Basic principles are presented for welding low-alloy tube steels at low temperatures.The exploitation of new oil and gas deposits makes it necessary to construct industrial pipelines in northern regions of West Siberia, where building work can be done preferentially in the winter at low air temperatures (down to -50°C). A basic step is installation by welding, which largely determines the reliability, and in actual pipelines, cracking is most often due to welded joints.A survey of the reasons for welded joint failure in pipelines indicates that the main one is the formation of cold cracks in the heat-affected zone (HAZ) because of the elevated tendency to brittle failure.The cracking probability increases on welding at low air temperatures, which is due to the presence of quenched structures and extensive hydrogenation of the welded joint.Working reliability is largely determined by the proper choice of materials in combination with the best conditions for installation welding.There are no scientifically based criteria for choosing welding modes for negative temperatures, which hinders the definition of the best technology and often leads to unjustified complication and considerable expense on account of additional measures (heating before and after welding, thermal insulation of the installed components, and so on).Published data [1-5] and our studies show that one should use the parameter σ pmin characterizing the cracking resistance of steel in order to evaluate modes of welding and to define sound conditions for preventing coal cracks. For industrial pipelines of diameter 114-512 mm with wall thickness δ up to 16 mm, the critical value of σ pmin at low temperatures (down to -60°C) is 360-400 MPa [5,6].We have found that one should choose the conditions for heating installation joints in pipelines to eliminate cracking in the weld in the winter on the basis of the cooling time for the weld metal from 300 to 100°C, namely t 100 300 (Fig. 1), since as the cooling time for the metal in that temperature range increases, so does the release of hydrogen into the sur-