Cementite Grain Boundary Walls Influence Yielding and Fracture of Steel and Iron

Atomic Force Microscopy (AFM) enabled viewing of the surface of metallographically prepared steel and iron, which surprisingly revealed grain boundary walls, The probability is very high that these walls are cementite. The upper yield point of hypoeutectoid ferritic steels is caused by dislocations that fracture these walls transversely. Here, the walls surround completely all of the grains. Then there is a large drop in stress to a minimum stress, the breakthrough stress. With continued deformation, the stress rises into the Lüders region and to the lower yield point. Where the walls are segmented in iron, dislocations can pass around the walls. This results in a gradual change from elastic to plastic deformation. These hypotheses have experimental support. Experiments show that the Cottrell atmosphere theory of yielding has a negligible effect.

AFM scans reveal cracks in the grain boundary walls. Above 77K, these cracks are primarily responsible for the brittle behavior of iron and steel at and below the ductile-brittle transition temperature. Scans, using the AFM scanning probe in contact mode, show that the top of the walls are flat like pearlite platelets. Using tapping mode scans, the walls fracture at their cracks. This leaves the walls with sharp peaks. For steels where the grain boundary walls completely surround all of the grains, cracks grow longitudinally within the walls from grain to grain. When these cracks are sufficiently long they fracture, according to the Griffith equation. For iron where the grain boundary walls do not surround completely all of the grains, cracks proceed longitudinally along the wall segments stopping at their ends. Then the stress must rise sufficiently for cracks to proceed into the ferrite grains that cause fracture. For cementite grain boundary walls, calculations correctly predict the stress and ductile brittle transition temperature that are required for fracture to occur.