Research On Steels And Hot Induction Bending Processes For X80Hot Induction Bend

Hot induction bends are important accessories for long distance pipeline, moreover,the bands bear more complex stress than straight pipes during the operational process.This, in turn, necessitates the use of the heavy wall bends with higher strength. Withconstruction of the Second West-east pipeline, the needs of the heavy wall X80bends areoverwhelming, while, the experience in developing heavy wall X80bends lacks.Therefore, to carry out fundamental researchs for developing heavy wall X80bends isvery important, which have important significance for developing heavy wall X80bends,and construction for the Second West-east Pipeline.In this thesis, a new steel for heavy wall X80hot induction bend was designed basedon the results of effects of bending processes on microstructure and mechanical propertiesof the commercial X80steels by means of thermo simulation on a Gleeble-3500thermosimulator, optical metallography, scanning electron microscopy (SEM), electronback-scattered diffraction (EBSD), transmission electron microscopy (TEM). And then theeffects of bending processes on microstructure and mechanical properties of theself-designed bend steel, the weldability and changes of mechanical properties of heataffected zone (HAZ), the effects of bending processes on microstructure and mechanicalproperties of the weld matal were investigated. Furthermore, the relations of mechanicalproperties with microstructure of base metal, HAZ and weld mtal were discussed.The bending process simulation results of the commercial X80steels show that thesteels have higher low-temperature impact toughness, which can meet requirements of thetechnical standard for X80bend, while they have lower strength. C and alloy content havegreat effect on the strength of the bends. The strengh of the steels with low content of Cand alloy elements can only get the critical values of the technical standard at higherheating temperature and cooling rate. With the increasing the content of C and alloyelements in steel, the ferrite transformation is restrained, and phase transformationdecreasees, as a result, the strength increases. Moreove, addition of Nb and V in steels ishelp to keep higher strength of the bend after tempering because carbonitride precipitates.Therefore, the chemical compostions for heavy wall X80bends are designed. The bending process simulation results of the self-desgined X80bend steel show thatthe all strength and impact toughnes of the steel obtained in a wide range of processparamet are over the critical values of the standard for X80band. Meanwhile, with theincrease of heating temperature and cooling rate, the strength increases. However, for theimpact toughness, it seems to have optimal heating temperature and cooling rate. With theincrease of tempering temperature, the strength has little change, while the toughnessimproves. Therefore, the optical bending process parameters of the bend steel are: heatingtemperature at990℃, cooling rate at15℃/s and tempering temperature at600℃.The toughness of the coarse grain heat affected zone decrease markedly after weldingthermal cycle, while, which can be improved after bending heat treatment, and the impactenergies rise over100J, which is over the critical values. Comparing with the base matel,the strength of the HAZ is higher, and the tougness is lower. Moreover, the toughnessincreases and the strength decreases with the increase of the tempering temperature.Comparing with the base metal and HAZ, the weld metal appears lowlow-temperature impact toughness after bending heat treatment, which is one of importantworth considering issues. Moreover, the variation of mechanical properties of weld metalwith process parameters are difference. The strength increases with the increase of theheating temperature and cooling rate, while the toughness gets highest value when theheating temperatrue is at990℃, and the toughness inccreases with the increase of thecooling rate. In addition, with the increase of the tempering tempering temperatrure, thetoughness appeares decreasing trend. Therefor, the bending heat treatment for weld metalshould be heated at990℃, cooled at high cooling rate, and tempered at low temperature.The differences of the change rules of the mechanical properties with the processparameters attribute to the different initial mirostructres of base metal, HAZ and weldmetal, which affect the austenite states transformed during heating process, and then affectthe phase transformation and microstructures after cooling, consequently, affect themicrostructural variation during tempering process and final mechanical properties.The actual production process parameters, such as induction heating temperature,pressure of spray water, induction heating frequency and tempering temperature etc. havebeen optimized, and than the series of heavy wall X80bends for the Second West-east Pipeline have been developed.