Effect Of Production Process On Corrosion Resistance Of TP347HFG Steel Seamless Tube

Nowadays,various countries in the world attach great importance to the development of larger capacity power plant boilers with higher efficiency,energy saving and natural environment protection.Therefore,the improvement of steam temperature and pressure parameters is an inevitable problem to be solved.For many years,the research on ultra（super）critical technology in various countries has been continuous.At the same time,the superheater and reheater used in the boiler and other components need to bear high temperature and high pressure and corrosion resistance is the research concern of various countries.TP347HFG steel is one of the first materials in many parts used in thermal power units.TP347HFG austenitic heat resisting steel has excellent high temperature oxidation resistance and excellent high temperature lasting strength.But we know that the austenitic heat-resistant steel usually precipitates Cr₂₃C₆ along the grain boundary within the sensitization temperature range.The precipitation of high chromium carbide leads to intergranular corrosion caused by poor chromium in grain boundary.The service time of TP347HFG steel is seriously affected.This research is to optimize the forming process of TP347HFG seamless steel pipe,including high temperature softening,cold rolling and solid solution treatment process.The goal is to improve the corrosion resistance without compromising the mechanical properties.Therefore,the effects of high temperature softening,cold rolling and solution treatment on the microstructure and properties of TP347HFG seamless steel tubes were studied by means of metallographic observation,comprehensive mechanical properties test,EPR electrochemical test and other means.The comprehensive properties of the samples after high temperature softening,cold rolling deformation and solid solution treatment were compared and analyzed.Finally,the optimal production process parameters of high temperature softening,cold rolling deformation and solid solution treatment were determined.The optimum process conditions obtained from the test were applied to the production of TP347HFG seamless steel tube in the cooperation unit of the project.TP347HFG steel seamless tube is softened at 1250℃,1260℃,1280℃and 1300℃respectively.It was found that the grain coarsened with the increase of softening temperature.In addition,the intergranular corrosion sensitivity decreased from 8.4%to 1.18%with the increase of softening temperature.Considering the oxidation loss and grain coarsening in the production of stainless steel seamless tube,it is more difficult to follow up rolling.However,when the temperature is higher than 1280℃,the change trend of the intergranular corrosion sensitivity is small.Therefore,the optimal process parameters for the high temperature softening of TP347HFG steel seamless pipe were selected as 1280℃/10 min,water quenching.The optimum high temperature softening process was 1280℃/10 min.After 20%,40%,60%,70%four different degrees of rolling deformation.With the increase of rolling deformation,grain refinement occurs obviously.The grain breaking is more obvious at 60%and 70%and the grain breaking is relatively uniform at 70%.The seamless tubes with different cold-rolled deformations were treated by solid solution treatment.The solution treatment process of seamless tubes with different cold rolling deformations is the conventional process.The final study found that only 70%of the cold rolled deformation sample after solid solution to get uniform grain refinement.Therefore,70%cold rolling deformation is finally determined as the best cold rolling process parameter.After 70%rolling deformation,the optimal solution process parameters were sought.Based on the conventional process（heating temperature of1120℃and holding for 30 min）,the range of heating temperature was adjusted from 1090℃to 1180℃and the holding time was kept unchanged for 30min.The results show that when the solution temperature is in the range of 1120～1150℃,the grain size of the finished product is uniformly refined and meets the grain size requirements of 7～10.The sensitivity of the finished product to intergranular corrosion is 7.6%by using the optimized production process.Compared with the traditional process of 1250℃/10 min,60%rolling deformation and solution temperature in the range of 1090～1120℃,although the traditional process can achieve uniform grain refinement and meet the grain size requirements of 7～10,the intergranular corrosion sensitivity of the finished product is up to 12%.Therefore,compared with the traditional process,the intergranular corrosion resistance is significantly improved,and the optimal solution temperature is 1120～1150℃.On the basis of this solution process,the solution temperature was1150℃and the holding time was 5 min,15 min,30 min and 45 min.It was found that the intergranular corrosion sensitivity decreased with the extension of solution time.When the solution time was 45 min,the intergranular corrosion sensitivity decreased to 5.6%.Prolonged solution treatment time is beneficial to improve intergranular corrosion properties.So the optimal solution treatment process is 1120～1150℃for 45 min.Therefore,the softening temperature was increased from the conventional 1250℃to 1280℃to meet the requirements of microstructure and mechanical properties.The subsequent production process must be optimized as follows:the rolling deformation should be increased from 60%to more than 70%,the solution treatment temperature should be increased from 1090～1120℃to 1120～1150℃,and the solution time should be increased from 30 min to 45 min.The results of the trial production show that the TP347HFG seamless steel tube obtained by the optimized production process can significantly improve the corrosion resistance on the premise that the mechanical properties meet the requirements,and the intergranular corrosion sensitivity decreases from the original 12%to 5.6%.