9Cr18合金半固态触变压缩变形行为及组织演变

Deformation Behavior and Microstructure Evolution of 9Cr18 Alloy During Semi-Solid Compression

以9Cr18合金为研究对象,分别对9Cr18热轧态材料及半固态坯料进行触变压缩实验。通过OM和SEM研究了其在加热、半固态及变形冷却后的显微组织演变规律,分析了其压缩过程中的固液流动特性和应力-应变关系。研究表明,半固态坯料制备是保证材料发挥半固态变形特性的必备流程,坯料加热至半固态温度能够保证固液三维均匀分布,充分发挥液相流动特性。仅通过对轧态材料加热至半固态温度区间会导致液相沿原带状组织区域熔化析出,固液分布不均匀。热轧态材料带状熔化致使液相不能形成三维连通,液相流动只能在不同部位的若干区域进行,变形主要通过固相颗粒塑性变形完成,进入最后阶段变形抗力上升。半固态坯料变形过程中固液相分布均匀,当变形进行至触变阶段,液相由于受到向外侧压力梯度作用,在固相间隙中流动,固相颗粒予以协调,发生宏观固液分离,从而使变形抗力随之下降。9Cr18合金在半固态温度区间成形过程中表现出不同于传统热处理的组织演变规律。半固态温度范围内奥氏体溶解合金元素的能力较传统奥氏体化(1050℃)有极大提高,从而提高了奥氏体在快速冷却过程中的稳定性,在冷却后得到过饱和的亚稳奥氏体组织。这种半固态独特的组织演变过程为材料组织性能控制提供一种新的可能。

The compression behavior during semi-solid state is a fundamental basis for the following rheoforming or thixoforming. Coexist of solid/liquid phase leads to the unique deformation behavior. The chemical composition at each phase is different from conventional forming process. Deformation behav- ior and microstructure evolution are determined by various effects such as initial state, heating, cooling, etc. In this work, the semi-solid compression tests of 9Cr18 as hot-rolled material and semi-solid billet were conducted, respectively. Microstructure evolution during heating, semi-solid state, deformation and cooling was investigated by OM and SEM. Solid/liquid flow behavior and the relationship of stress-strainwere analyzed. The results showed the preparation of semi-solid billet is essential for the uniformity of sol- id particle and liquid phase, which would help to demonstrate the flow behavior. Only heating the as hot- rolled material to semi-solid led to the banded precipitation of liquid phase. The banded melting of as hot- rolled material made it hard for liquid phase to connect with each other. Liquid flow only happened in par- tial area and plastic deformation of solid particles was the main deformation behavior. The stress in- creased at the final stage. As for semi-solid billet, solid particles and liquid film coexisted uniformly. Macro separation of solid/liquid occurred as deformation came into thixotropic stage. Liquid flew towards outside and solid particles rotated, thus leading to the decrease of stress. Microstructure evolution at semi-solid state was different from conventional heat treatment. Solid austenite particles at semi-solid state could dissolve more alloying elements than normal austenization （1050 ~C）. This phenomenon would help to im- prove the stability of austenite and over-saturated meta-austenite was obtained after cooling. The special microstructure evolution during semi-solid state might provide a possible way to design a new heat treat- ment procedure.