Effects Of Trace Tin And Cerium On Thehot Ductility Of Nuclear Pressure Vessel Steel SA508CL.3

SA508CL.3 is a typical nuclear pressure vessel steel. For the better use of continuous casting process to produce such a nuclear pressure vessel steel, this paper mainly studies the effect of the trace elements Ce and Sn on the hot ductility of this steel.Using the hot tension tests, he reduction of area(RA) and the true stress-stain curve of the specimens are obtained. Then scanning electron microscopy(SEM) is applied to analyze the fracture mechanism of the material; optical microscopy(OM) is used to analyze the effect of proeutectoid ferrite on the steel hot dutility and field emission scanning transmission electron microscopy(FEGSTEM) is adoped to analyse the effects of precipitation on the grain boudary and grain boundary segregation on the material hot ductility.The steel samples undoped, doped with Sn and doped with Sn+Ce are heated at 1300 oC for 3 minutes, and then cooled at a rate of 5 oC/s to different test temperatures followed by tensile deformation until fracture. The results indicate that hot ductility troughs occur for all the samples in the temperature range 650 oC to 1050 oC which apperas betwween 675 oC and 870 oC for the undoped samples, between 670 oC and 945 oC for the Sn-doped samples and between 690 oC and 875 oC, for the Sn+Ce-doped samples. The temperatures corresponding to the lowest for the three kinds of steels are 675 oC, 700 oC and 700 oC. Sn can make the trough wider and deeper and Ce can suppress detrimental effect of Sn.Both Sn and Ce have apparent grain boundary segregation. Sn makes the steel SA508 CL.3 tending to intergranular fracture, thereby deteriorating hot ductility. However, Ce may suppress, to a certain extent, Sn grain boundary segregation and meanwhile increase grain boundary cohesion, so that the hot ductili ty of the Sn+Ce-doped steel is improved.Besides the effect of Sn and Ce, the low hot duclity of the undoped steel at 675 ℃ and the Sn-doped steel between 675 oC and 700 oC results from the thin ferrite films precipitated at prior austenite grain boundaries, where the strain concentration in the ferrite films during deformation leads to the material failure.The low hot duclity of the undoped steel between 700 oC and 950 oC, the Sn-doped steel between 750 oC and 1000 oC, and the Sn+Ce-doped steel between 700 oC and 950 oC originates from austenite grain boundary sliding during deformation.The high hot duclity of the undoped steel between 950 oC and 1050 oC, the Sn-doped steel between 1000 oC and 1050 oC, and the Sn+Ce-doped steel between 950 oC and 1050 oC results from dynamic recrystallization during deformation.