Study On Microstructures And Properties Of Directionally Solidified T91 Steel

Accelerator Driven Systems?ADS?is a hot research topic in nuclear power research field all over the world.One of the key ADS system technologies is to use the liquid Pb-Bi alloy as a transmission medium to improve the thermal cycle efficiency of the nuclear power plant significantly.However,when the structural material carries Pb-Bi alloy,it will be subjected to the cycled changed temperature gradient,and the fatigue cracks along the flow direction will also initiate,which will easily cause the thermal fatigue failure to the structure.Therefore,it's necessary to improve the high temperature strength and thermal fatigue properties of the structural materials.It has been known that T91 steel is one of the important alternatives structural materials of the ADS system due to its good high temperature properties.In this paper,a method of directional solidification was utilized to tailor the grain orientation,mechanical properties and particular thermal fatigue property reasonably to meet the practical application needs of ADS system.Three samples with different solidification rates of 30?m/s,90?m/s and 180?m/s were prepared by the directional solidification method through liquid metal cooling?LMC?.There were columnar crystals which were mainly in the middle of the samples.The main phases of the initial solidification zone of T91 steel were martensite and retained austenite,the?-Fe and short rod like?-Fe were concentrated in the intermediate solidification zone,the range of columnar crystal width was50⁴00?m.In the final solidification zone,there were a number of lath martensite,delta ferrite,retained austenite and various carbonitride like Cr₂₃C₆,NbC,VN,etc.When the solidification rate was 90?m/s,the columnar crystal continuity was highest,and the directional microstructure was obvious.After the elastic strain test,it was found that the elastic modulus of the samples decreased after the directional solidification.The relationship of elastic modulus among three samples prepared by the directional solidification was E_90?m/s<E₁₈₀?m/s<E₃₀?m/s<E_{common casting}.When the solidification rate was 90?m/s,the elastic modulus of the sample was 39.23%lower than that of the common cast alloy,and decreased 50.66%at most.The elastic modulus in the middle of the sample would be lower.The elastic modulus increased significantly when the dendrites had more than two directions or there was martensite of which the phase boundary was uncertain.When the direction of columnar crystal and direction of drawing were closer,the elastic modulus was lower.The elastic modulus of dendrites was lower than that of columnar crystals.With the increasing of the number of thermal fatigue cycles,the hardness of both directional solidification samples and common casting samples decreased,and the reduction of common casting was higher than that of directional solidification.When the solidification rate was 90?m/s,the anti cyclic softening ability of samples was strongest.The ability of resisting thermal fatigue crack initiation for different samples could be expressed as follow:P_{common casting}<P₁₈₀?m/s<P₃₀?m/s<P₉₀?m/s.The ability of resisting thermal fatigue crack growth for different samples could be listed as follow:Qcommon casting<Q180?m/s<Q30?m/s<Q90?m/s.The tensile strengths of samples with different solidification rates of 30?m/s,90?m/s and 180?m/s increased by 69.23%,76.92%and 63.73%,respectively,which were higher than that of common casting alloy.The fracture of the common casting alloy was the mixture of cleavage and microporous aggregation,the fracture morphology of directionally solidified samples was mainly dimple.With increasing of solidification rate,the diameter and depth of dimples increased.There were small white particles Cr₂₃C₆ in the dimples,with the size of 2⁵?m.The hardness of T91steel increased gradually when the solidification speed increased.The microhardness of the samples with the solidification rate of 30?m/s,90?m/s and 180?m/s was14.4%,35%,44.4%,which were higher than that of the common casting alloy.