焊缝金属中针状铁素体晶粒长大行为

Study on grain growth behavior of acicular ferrite in weld deposited metal

以针状铁素体(AF)组织为基体组织的大热输入焊缝金属作为研究对象,采用金相显微镜(OM)、扫描电子显微镜电子背散射衍射装置(EBSD)、全自动静态相变仪等手段表征了焊缝金属内细长状的针状铁素体(AF)组织晶粒的取向特征,分析不同焊接热输入对焊缝金属内AF晶粒形核以及长大行为的影响规律.结果表明,在大热输入焊接条件下,当焊缝金属的冷却速度低于临界冷却速度时,AF晶粒以多边形铁素体形态从夹杂物边缘开始形核时,此刻与母相奥氏体(γ)间偏离K-S关系,当焊缝金属温度降低至相变开始温度后,AF晶粒以细长的针状开始长大,并且与母相奥氏体间满足K-S关系.但是随着焊接热输入降低,冷却速度逐步的加快,AF晶粒形核尺寸将越来越小,并且向着与母相奥氏体满足K-S关系的取向偏转速度加快,当焊缝金属的冷却速度超过临界冷却速度时,AF晶粒不用形核就可以迅速长大.

In this study,having acicular ferrite(AF)as the matrix the high heat input weld metal was taken as the research object and acicular ferrite(AF)grain orientation relationships(ORs)in the deposited metal was identified from Metallographic microscope(OM),electron bacK-Scatter diffraction device(EBSD),automatic static phase change instrument,etc,the influence of different heat input on the growth behavior of AF grain in the weld metal was analyzed.The results showed that,When the cooling rate of the weld metal is below the critical rate,AF in the weld metal nucleated from the inclusions with polygonal ferrite morphology,and deviated from K-S OR with respect to parent phaseγ.When the temperature of the deposited metal decreased to the transformation start temperature,AF grew up with slender needle morphology,and exhibited K-S OR with respect to parent phaseγ.However,with the decrease of the welding heat input,AF nuclear size reduced,and the approach to K-S OR with respect to parent phaseγwas accelerated,when the cooling rate of the weld metal exceeds the critical rate,AF grains can grow rapidly without nucleation.