Studying On Microstructure,Properties And CCT Curves Of High Grade Pipeline Steel

Oil and natural gas are the main energy and important strategic resources for the development of national economy.The forming,laying,installation and maintenance processes of pipelines all rely heavily on pipeline steel welding techniques.Through welding,however,the rapid,uneven heating and cooling of complex thermal cycle process often result in the performance mismatch between the pipe material itself and the welding joint,especially within the welding heat affected zone?HAZ?.This may lead to a decrease in the operational safety and reliability of the pipeline.Therefore,how to obtain an excellent microstructure and excellent mechanical properties of the pipeline steel welding joint has long been a hot topic and a focus of concern in oil and gas transportation.Pipeline steel SHCCT curve is a diagram that relates the condition of the welding process and microstructure achieved by welding,which can effectively guide the design of process and execution of pipeline steel welding.This paper mainly concentrates on the microstructure evolution of X80,X90 and X100 high-grade pipeline steel under different welding conditions.First,fine samples,both solid and hollow,are made from the three kinds of pipeline steel and used to simulate the microstructure evolution of high grade pipeline steel in HAZ.In order to ensure the accuracy of simulation,all experiments were carried out on a high-precision quenching dilatometer?DIL805A/D?.Based on actual GMAW process parameters provided by Sichuan oil and Gas Contruction Engineering Co.Ltd.,three different thermal cycling processes are developed as follows.Process 1: heat to 950 ? at a heating rate of 10 ?/s,hold temperature for 10 min,then cool to room temperature with different t_8/5 values between 3 s and 600 sProcess 2: heat to 1100 ? at 150 ?/s and 200 ?/s,hold temperature for 10 s,then cool to room temperature with different t_8/5 values between 1.5 s and 600 sProcess 3: heat to 900 ??1100 ? or 1350 ??at 200 ?/s,hold temperature for 10 s,then cool to room temperature with different t_8/5 values between 1.5 s and 600 s.By using optical microscopy analysis technology,electron microscopy?SEM?and micro hardness testing method,the welding microstructure,mechanical properties and CCT?SHCCT?curves of high grade pipeline steel under different welding thermal cycles are determined.Subsequently,a comparative analysis between the three types of pipeline steel is conducted.The main conclusions as follows:?1?Under the condition of thermal cycling process 1: The microstructure of all three pipeline steels?X80,X90 and X100?exhibited a similar evolution process of P+PF?PF+QF?QF+GB? GB+BF.With the increase of cooling rate,the grain size decreases,and the microhardness increases.?2?Under the condition of the thermal cycle process 2: at different heating rates,the three kinds of pipeline steel exhibited different t_8/5 in bainitic ferrite?BF?formulation.The greater the heating rate,the greater the t_8/5 value for the bainite ferrite formulation in the X80 pipeline steel,and the smaller that in the X90 and X100 pipeline steel.The microstructure evolution of the three kinds of pipeline steel is similar.?3?Under the condition of the thermal cycle process 3: when the t_8/5 is between 300 s and 600 s,the microstructures of the critical zone and fine crystal zone of the three kinds of pipeline steel X80,X90,X100 at room temperature are different;when t_8/5 is between 1.5s and 120 s,the microstructure evolution of X80,X90,X100 pipeline steel in simulated welding critical zone and the fine grain area and the coarse grain zone is same,but the size and critical t_8/5 of the microstructure are both different.With the decrease of the t_8/5,microhardness of all three kinds of pipeline steel in the three thermal cycling processes increase.The CCT?SHCCT?curves of the three kinds of thermal cycling process of X80,X90,X100 pipeline steel not only provide reference for the design of heat treatment and welding process,but also provide effective experimental data for practical application of X80,X90 and X100 pipeline steel.