Effect Of Phosphorus And Sulphur On High Temperature Ductility Of T/P92Ferritic Heat Resistant Steel

In recent years, heat resistant steels have been developed for high workingparameters in power plants. T/P92ferritic heat-resistant steel studied in this dissertationis a kind of new9Cr-2W-0.5Mo steel, which is developed based on T/P91steel. It iswidely used in the main steam pipes and overheaters of thermal power plant with a greatoxidation resistance and creep property.However, cracking often occurs in the materials during hot working andstraightening during the continuously cast process. It is usually caused by the impurityelements which lead to a ductility loss in the steel. Impurity elements P and S are alwaysremaining in the steel. In order to study the effect of the impurity elements on the hotductility of T/P92steel, hot tesile tests were carried out in the present work with a strainrate of10-3s-1on the T/P92steel samples. The samples were austenitized on1100oC,cooled to different temperatures from700oC to900oC and then held there for some timefollowed by tensile deformation. The high temperature ducility curves of the materialswere obtained by mesuring the reduction in area (RA) of the samples that were drawnfrom700oC to900oC. Then scanning electron microscopy (SEM) was used to analyzethe fracture morphology. After that, microstructures of the samples were studied byoptical microscopy. In addition field emission gun scanning transmission electronmicroscopy (FEG-STEM) was used to analyze the composition of grain boundaris. Atlast the change in the high temperature ductility curves of T/P92steel was studied aswell as the influence of impurity elements P and S based on all the experiments above.In the hot ductility curve, there is no obvious diffenence between theimpurity-doped and undoped samples from700oC to800oC, while the hot ductility ofthe samples doped with P and S is much lower than that of the undoped samples from800oC to900oC. SEM results show that all the specimens exhibit ductile fracture, andthe quantity and size of dimples on the fracture surface, which can also describe thechanges of hot ductility, are in good agreement with the hot ductility curves. Theimpurity elements have no obvious influence on the fracture morphology. TEM resultsdemostrate that alomost no second phase precipitates exist in the matrix, indicating thatthe ductility loss in the steels doped with impurity elements is not related to the secondphase precipitates. In addition, it is inferred from the stress-strain curves that there hasbeen no dynamic recrystallization occuring in the samples.The influence of impurities P and S on the T/P92steel from800oC to900oC isanalysed in detail. Both the impurity-doped and undoped steels attain the lowestductility point at about800oC, and they have no obvious difference with each other. Theductility loss on this point is mainly affected by the thin ferrite film networks alongprior austenite grain boundaries, and is not associated with impurity elements. The ductility of the P-doped steel has the maximum diffenence with the undoped steel at850oC. FEG-STEM microanalysis at this temperature indicates that there is obviousgrain boundary segregation of P. The concentration of P at the grain boundaries is quitea few times higher than that in the matrix, being maily resposible for the ductility losson this point, while there is no obvious grain boundary segregation in the S-doped steel.Ratained austenite is obseved in the S-doped steel sample after tesile testing above800oC, and there are many MnS inclusions, which should be mainly responsible for theductility loss of the S-doped steel obove800oC.