317L奥氏体不锈钢中Mo元素的增加使其具有更加优异的抗化学腐蚀能力,然而较高的Mo元素含量会增加其凝固过程δ-铁素体相的析出倾向,同时促进δ-铁素体向sigma脆性相的转化,该类Cr-Mo复合相需要采用长时间的均质化处理进行消除,但长时间高温热处理会造成晶粒的异常生长,对后续腐蚀性能产生较大影响,因此在凝固过程中对其凝固组织的控制极为重要。冷却速率是影响不锈钢凝固组织的重要因素之一,利用高温共聚焦显微镜（HT-CLSM）观察了3种典型冷速（6、100、1 000 ℃/min）下317L的凝固组织演变过程,并借助金相显微镜（OM）、扫描电子显微镜及能谱仪（SEM-EDS）和电子背散射衍射技术（EBSD）等表征手段对其凝固组织以及Cr-Mo复合相形貌、分布和含量进行分析。结果表明,317L不锈钢为FA型凝固,在凝固过程中δ-铁素体相率先从液相中形核,并与液相发生包晶反应生成γ-奥氏体相;随着冷却速率的增加,材料的结晶形核过程存在相应的滞后性,导致其过冷度增加,初始凝固温度略有降低,同时Cr、Mo元素的扩散行为受到影响,致使sigma相含量随着冷却速率的增加先增加后降低,铁素体相含量先降低后增加;317L中的Cr、Mo、Ni元素均发生了偏析,其中Cr、Mo元素发生了正偏析,Ni元素发生了负偏析,且Mo元素的偏析最为严重。
Abstract
The increases of Mo element in 317L austenitic stainless steel make it have better chemical corrosion resistance. However,the higher Mo content will increase the precipitation tendency of the δ-ferrite phase during solidification,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 structure during the solidification process is very significant. The cooling rate is one of the critical factors affecting the solidification 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.
关键词
奥氏体不锈钢 /
冷却速率 /
凝固组织 /
HT-CLSM /
Cr-Mo复合相
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Key words
austenitic stainless steel /
cooling rate /
solidification structure /
HT-CLSM /
Cr-Mo composite phase
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脚注
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基金
山西省重点研发计划资助项目（202202050201019）; 国家自然科学基金资助项目（52271067）
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