Effect of secondary thermal cycle on intragranular ferrite

The influence of secondary thermal cycle on intragranular ferrite was investigated by physical simulation method.The rule of non-metallic inclusions distribution and the their sizes and density were mea sured with automatic image analyzer,and the chemical compositions and the figures of non-metallic inclusions that inducted the formation nucleation of intragranular ferrite had been analyzed with transmission electron microscopy and X-ray microanalysis.The results show that secondary thermal cycle has little effect on the chemical compositions,sizes and figures and density of non-metallic inclusions that inducted the formation nucleation of intragranular ferrite.The non-metallic inclusions beneficial to nucleation and growth of intragranular ferrite during secondary thermal cycle are oxidation products of Ti,Mn,Si,Al,such as TiO,MnO,SiO₂,Al₂O₃ and the sulphides of Mn and Cu,such as MnS,CuS and (Mn,Cu)S.As the cooling time from 800℃ to 500℃ is 10 seconds,the sympathetic nucleation of intragranular ferrite are found.First intragranular ferrite has large boundary energy and high density dislocation during the cooling process.These two factors are the main driving forces to the sympathetic nucleation and growth of intragranular ferrite.