Strength analysis of a ferrite plus bainite steel by AFM ultra-micro hardness tester

AFM ultra-micro hardness test, micro Vickers hardness test and TEM observat ion were conducted for a ferrite+bainite steel which consists of bainite an d polygonal ferrite. The results obtained are in the following. (1) AFM ult ra-micro hardness tests show that nanoscopic hardness is 155 near the cente r of bainitic ferrite grain and increases closer to its boundary. Nanoscopi c hardness is kept constant at 151 in the polygonal ferrite: grain. (2) Mic ro-Vickers hardness tests show that macroscopic hardness is 251 and 147 for the bainite and polygonal ferrite regions. respectively. (3) TEM observati ons show that many carbides precipitated along the grain boundaries of bain itic ferrite and the dislocation density was almost same in both grains of bainitic and polygonal ferrite. It is suggested from the above results that the strengthening by grain boun daries is important in the bainitic ferrite and this strengthening effect i s enhanced by carbides along grain boundaries.