Study On Microstructure And Properties Of Inter-critically Reheated Coarse Grained Heat-affected Zone Of X80 Pipeline Steel

X80 pipeline steel has good strength,toughness and weldability,as well as good plasticity and ductile fracture resistance,and has been widely used in the world.In the connection of pipelines,welding is the most effective and common method,and for the connection of large thickness pipeline steel,double-sided welding and multi-pass welding are unavoidable.Study have shown that the inter-critically reheated coarse grained heat-affected zone（ICCGHAZ）that has undergone the second welding thermal cycle has become the zone with the weakest mechanical properties.In addition,both high p H environments and seawater environments can negatively impact pipeline steel properties,including corrosion resistance and hydrogen permeation properties.Therefore,in order to improve the safety and service life of pipelines,it is necessary to conduct in-depth research on the microstructure,corrosion resistance and hydrogen permeability of the ICCGHAZ of pipeline steel.In this research,the samples of coarse-grained heat-affected zone（CGHAZ）and ICCGHAZ of X80 pipeline steel with different peak temperatures and different cooling rates were obtained by welding thermal simulation technology,and the mechanism of microstructure evolution was studied and analyzed.The results show that the microstructure of the CGHAZs are mainly composed of granular bainite at different peak temperatures,and the acicular ferrite as the granular bainite complex microstructure can be better obtained at moderate cooling rates,which is the ideal microstructure for pipeline steel.The microstructure of ICCGHAZ is mainly granular bainite.As the secondary peak temperature gradually approaches Ac₃,the fraction of polygonal ferrite increases,and the prior austenite grain boundary changes from clear to blurred.And as the cooling rate gradually decreases,the fraction of lath bainite in the microstructure decreases,the fraction of polygonal ferrite increases,and the prior austenite grain boundary changes from clear to blurred.With the increase of the secondary peak temperature,the fraction and average size of M/A comstituents in the ICCGHAZs first increase and then decrease,changing from a"chain-like"distribution along the prior austenite grain boundary to a dispersed distribution.With the cooling rate decreases,the fraction and average size of M/A constituents continue to decrease,from a"chain-like"distribution along the prior austenite grain boundaries to a dispersed distribution.In addition,the first peak temperature will affect the critical temperature for severe coarsening of M/A comstituents in the ICCGHAZs.When the first peak temperature is 1350°C,the upper and lower critical peak temperatures for severe coarsening of M/A comstituents in the ICCGHAZs are higher than those at 1150°C and 1250°C.The differences in corrosion resistance and hydrogen permeation behavior between the CGHAZ and the ICCGHAZs of X80 pipeline steel were studied by electrochemical testing technology and electrochemical hydrogen permeation technology.The results show that in 0.5M Na₂CO₃-1M Na HCO₃ solution and 3.5 wt.%Na Cl solution,the order of the corrosion resistance and hydrogen permeation performance of the ICCGHAZs at different secondary peak temperatures and different cooling rates is consistent.It is found that the difference in corrosion resistance and hydrogen permeation behavior is mainly caused by the difference in the characteristics of M/A comstituents.With the fraction of M/A comstituents increases,the corrosion resistance and hydrogen permeation performance of the ICCGHAZ gradually decrease.With the average size of the M/A comstituents increases,the corrosion resistance and hydrogen permeation performance of the ICCGHAZ also decrease gradually.The functional relationship between corrosion resistance and hydrogen permeation behavior and the fraction and average size of M/A comstituents is obtained by the binary linear regression fitting.The reheated heat-affected zone（HAZ）sample was obtained by welding thermal simulation technology.Due to the difference in the peak temperatures of the two welding thermal cycles,the sample is divided into 5 regions（A～E）at an interval of 3 mm from the heating center to the base metal.There is a microstructure gradient between the regions,and the effect of the microstructure gradient on the corrosion resistance is explored.Through electrochemical and micro-electrochemical techniques in 0.5M Na₂CO₃-1M Na HCO₃ solution,it is found that the base metal exhibits the highest impedance and the most positive corrosion potential,the region（C）exhibits the lowest impedance and the most negative corrosion potential,the corrosion resistance of regions（A）and（B）is slightly better than that of region（C）.The difference in corrosion resistance of each region is attributed to the difference in microstructure type,M/A comstituents characteristics,grain and grain boundary angle characteristics.In addition,it is found that the impedance of regions（A）,（B）and（C）gradually tend to be consistent with the prolongation of immersion time,but they are still far worse than other regions,which is mainly attributed to the effect of corrosion products on corrosion resistance.