冷却速率对奥氏体不锈钢凝固过程影响的原位观察

通过采用激光共聚焦扫描显微镜对AISI304奥氏体不锈钢的凝固过程进行了原位动态观察研究.发现当冷却速率为0.05℃·s-1时,奥氏体不锈钢以胞状晶方式凝固,其凝固模式为FA模式,即δ铁素体相先从液相中形核并长大,γ相在1 448.9℃时通过与液相发生包晶反应(L+δ→γ)在δ铁素体相界形成,当温度降到1 431.3℃时液相消失,δ铁素体相通过固态相变转变为γ相,富Cr贫Ni的残留铁素体位于胞状晶之间.当冷却速率为3.0℃·s-1时,奥氏体不锈钢以枝晶方式生长,冷却到1346.4℃时包晶反应在液相与δ铁素体相界之间进行,其残留铁素体位于枝晶干,与冷却速率为0.05℃·s-1时相比,其残留铁素体的数量增多,残留铁素体富Cr贫Ni的程度减轻. 

Effect of cooling rate on the solidification process of austenitic stainless steel by in-situ observation

Solidification processes of AISI304 austenitic stainless steel at cooling rates of 0.05℃·s-1 and 3.0℃·s-1 were observed in situ by confocal laser scanning microscopy. When the cooling rate is 0. 05℃·s-1 the solidification front is cellular morphology and the solidification mode belongs to FA mode. B ferrite appears firstly in liquid steel, and then γ phase precipitates prior at B-grain boundaries at 1 448.9℃ by means of the periteetic reaction (L+δ→γ). At 1 431.3℃ the liquid steel disappears. Finally the B ferrite transforms to γ phase by solid diffusion, and the retained ferrite which enriches in Cr and depletes in Ni locates between the cellular dendrites. When the cooling rate is 3.0℃·s-1, the solidification interface morphology is dendritic. The peritectic reaction takes place at interfaces between B-grains and the remained liquid at 1 346.4℃, and the retained ferrite locates in the dendrite arm. Compared to 0.05℃·s-1, the degree of enrichment in Cr and depletion in Ni in the residual ferrite decreases but the amount of them increases.