Effects Of Vanadium On Resistance To Hydrogen Corrosion And Mechanical Properties In X80 Pipeline Steel

At present,X80 grade pipeline steel has been widely applied to oil and gas transportation.With the rapid increase of energy demand in economic development,more and more newly discovered oil and gas fields have been exploited and utilized in remote areas.Pipelines often pass through extreme cold,frozen soil zones and mostly serve in wet soil.Moreover,transport medium contains acid substances such as hydrogen sulfide.Therefore,the failures of pipeline steel caused by hydrogen become increasingly prominent.Considering that fine nano-carbides can act as effective hydrogen traps to reduce the harm of hydrogen to pipeline steel,the effect and mechanism of different vanadium content on resistance to hydrogen corrosion and mechanical properties in X80 pipeline steel have been investigated systematically in this paper.Besides,the effects of different finish rolling deformation and tempering temperature after TMCP process on resistance to hydrogen induced ductility loss of the experimental steel has been analyzed,which could provide reference for the development and application of high vanadium X80 grade pipeline steel.The experimental results showed that the number of nano-scale carbides increased with increasing vanadium content in as-forged and tempered X80 grade pipeline steel.As vanadium content exceeded 0.12%,the average size of precipitates increased.The effective hydrogen diffusion coefficient decreased,the soluble hydrogen concentration increased and the HIC sensitivity decreased first and then increased with increasing vanadium content,which were mainly influenced by the amount and size distribution of nano-carbides in steel.The experimental steel with 0.12%vanadium content had the best performance on resistance to HIC failure.The effect of vanadium content on hydrogen-induced ductility loss of X80 grade pipeline steel has been investigated.The results showed that the precipitates with size less than 20 nm were mainly spherical or near spherical vanadium carbides,which had stronger binding energy with hydrogen atoms acted as hydrogen traps.Nonetheless,the ones with size larger than 20 nm were mostly ellipsoidal compound carbides containing niobium and vanadium.Increasing vanadium content led to the increase of the volume fraction of nano-scale precipitates,the number of dislocation density and hydrogen trapping efficiency.Meanwhile,it also caused the decrease of effective hydrogen diffusion coefficient,which improved the resistance to hydrogen-induced ductility loss.As a result,the experimental steel with 0.13%vanadium content had the lowest hydrogen embrittlement sensitivity index.Compared with high vanadium TMCP tempered X80 grade pipeline steel,the microstructure of non-tempered experimental steels were composed of lath bainite and a small amount of ferrite.The volume fraction of nano-scale precipitates in steel was lower,but the dislocation density was higher.High vanadium TMCP non-tempered experimental steel showed a worse performance on resistance to hydrogen-induced ductility loss.The experimental results showed that the proportion of ferrite decreased and the content of granular bainite increased with increasing FRD in high vanadium TMCP X80 grade pipeline steel.A large number of equiaxed grains appearred in the steel with 50%FRD,which indicated that the proportion of high angle grain boundaries increased,the deformation storage energy decreased and dynamic recrystallization occurred.The number of nano-scale precipitates increased first and then decreased with increasing FRD.The size of the precipitates in steel was mainly in the range of 10～15 nm and most precipitates in steel were vanadium-containing carbide particles(V4C3 or V8C7).R40 steel with 40%FRD had a largest number of nano-scale precipitates and the best resistance to hydrogen-induced ductility loss.The HIC sensitivity of four experimental steels with different FRD was not significantly different.The main HIC crack sources were the inclusion,such as Al2O3 and MnS,and large-scale C-rich phases.What’s more,the crack was more likely to extend within or between specific[100]oriented grains.Additionally,the researches on the effect of tempering temperature after TMCP process on the resistance to hydrogen-induced ductility loss of high vanadium X80 grade pipeline steel had been conducted.The results showed with the tempering temperature rising from 450℃ to 650℃,the number of nano-scale precipitates in steel increased gradually.However,as tempering temperature continued to rise to 700℃,the number of fine precipitates decreased and Ostwald ripening occurred.The microstructures of the steels tempered at 450～650℃ were composed of granular bainite,lath ferrite and a small amount of polygonal ferrite,while the experimental steel tempered at 700℃ was composed of coarse polygonal ferrite and its hardness decreased obviously.In addition,increasing tempering temperature caused the decreases of the content of lamellar cementite and dislocation density,which meant that dynamic recovery degree increased.Due to containing the most carbides,with size less than 30 nm,T650 steel had the lowest effective hydrogen diffusion coefficient,the highest soluble hydrogen concentration and hydrogen trap density.Therefore,the experimental steel tempered at 650℃ had the best resistance to hydrogen-induced ductility loss.At last,the effect of vanadium content on mechanical properties of X80 grade pipeline steel was studied.The results showed the experimental steel with the highest vanadium content(0.150 wt.%V)had the highest ductile-brittle transition temperature,-49.43℃.By contraries,the steel with the lowest vanadium content(0.036 wt.%V)had the lowest ductile-brittle transition temperature,-68.12℃,which had the best resistance to low temperature brittleness.The ductile-brittle transition temperatures of the other two experimental steels were between two parties,about-60℃.With the increase of vanadium content from 0.036 wt.%to 0.075 wt.%,the tensile strength and yield strength of the experimental steels were obviously improved.However,the further increase of vanadium content had a weak effect on the strength of experimental steels.Both precipitation strengthening and fine grain strengthening played important roles on the increase of steel strength.Comprehensive consideration of resistance to HIC and hydrogen-induced ductility loss,low temperature impact toughness and strength-plasticity properties,the X80 pipeline steel containing 0.11～0.13 wt.%vanadium,with 40%finish rolling deformation tempered at 650℃,possesses a good development value.