Effects of cooling rates on solidification microstructure evolution of 317L austenitic stainless steel
The increases of Mo element in 317L austenitic stainless steel make it have better chemical corrosion resis-tance.However,the higher Mo content will increase the precipitation tendency of the δ-ferrite phase during solidifica-tion,and promote the transformation of δ-ferrite into sigma phase.This type of Cr-Mo composite phase requires long-term homogenization treatment to eliminates it,but long-term high-temperature heat treatment will cause abnormal grain growth,which has the significant impact on subsequent corrosion performance.Therefore,control of the struc-ture during the solidification process is very significant.The cooling rate is one of the critical factors affecting the solidi-fication structure of stainless steel.The solidification microstructure evolution of 317L under three typical cooling rates(6,100,1 000℃/min)was observed by high-temperature confocal microscope(HT-CLSM).The solidification microstructure of the materials and the morphology,distribution and content of Cr-Mo composite phase was analyzed by metallographic microscope(OM),scanning electron microscope(SEM),energy dispersive spectrometer(EDS)and electron backscatter diffraction(EBSD).The results showed that the solidification type of 317L was FA.During the solidification process,the δ-ferrite phase first nucleated from the liquid phase,and the γ-austenite phase was formed by the peritectic reaction in the liquid phase.With the increased of cooling rates,the crystallization nucleation process of the material had a corresponding hysteresis,which lead to the rise of supercooling degree and the initial solidification temperature decrease slightly.At the same time,the diffusion behavior of Cr and Mo elements had been affected,while the content of the sigma phase increased first and then decreased with the increased of cooling rate,the content of ferrite phase decreased first and then increased.And the Cr,Mo and Ni elements in 317L had been segregated,in which the Cr and Mo segregated positively,the Ni segregated negatively,and the segregation of Mo was the most serious.
NETL
NSTL
万方数据
