Abstract
Sulfide stress cracking (SSC) is one of the most dangerous types of pipelines destruction. Thermal impact of the welding process drives to heterogeneity of the microstructure and properties of the metal, which can lead to cracking of pipeline welded joints. Resistance to SSC of welded joints is determined by the thermal cycle of welding and cooling rate in the temperature range of austenite transformation. Due to performed studies based on simulation of welding heating the recommended range of cooling rates of 10–30 ° C/s was established, in which the resistance to SSC of welded joints is ensured. To calculate the cooling rates in coarse grained heat affected zone (CGHAZ) finite-element models of heat distribution were developed for longitudinal multi-electrode submerged arc welding (SAW) and multi-pass girth welding of pipes. Using the developed welding models, it was found that in order to achieve the cooling rate in CGHAZ it is necessary to reduce heat input up to 15-30% during multi-electrode SAW process of longitudinal welds of pipes . For multi-pass girth welding it is necessary to preheat the edges to be welded up to 100-300 °C depending on type of welding (GMAW or SMAW) and pipe wall thickness.
Highlights
Large diameter pipes (LDP) are manufactured from microalloyed ferrite-bainite steels
Despite the large amount of knowledge about the microstructure and properties formation in the base metal and welded joints of large diameter pipes, there is no data about the effect of the cooling rate in HAZ on the resistance of welded joints to stress cracking (SSC)
Studying the hardness change in coarse grained heat affected zone (CGHAZ) depending on the cooling rate (Fig. 2) and type of microstructure showed that in the area of ferrite-bainite and bainite transformations the hardness varies insignificantly from 205 to 220 HV at cooling rates of 0.1 – 20 °C/s
Summary
Large diameter pipes (LDP) are manufactured from microalloyed ferrite-bainite steels. Welded joints with pronounced structural, mechanical and electrochemical heterogeneity are the zone of predominant corrosion-mechanical destruction of pipes during transportation of hydrogen sulfidecontaining fluids and gases [4,5,6,7] It is connected with the rapid heating and cooling during the welding process, a significant changes of the microstructure and the mechanical and corrosion properties of steel in CGHAZ of welded joints. Despite the large amount of knowledge about the microstructure and properties formation in the base metal and welded joints of large diameter pipes, there is no data about the effect of the cooling rate in HAZ on the resistance of welded joints to SSC.
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