Effects of grain boundary and triple line structures on carbide precipitation in type 304L stainless steel

grantor: University of TorontoType 304L stainless steel samples, solution treated and annealed at different sensitization temperatures, were used to investigate the correlation between carbide precipitation and grain boundary (GB) and triple line (TL) structure. The Orientation Imaging Microscopy (OIMTM) technique was employed in measuring the crystallographic parameters of the samples. Grain boundaries were characterized using (1) the coincident site lattice/displacement shift complete (CSL/DSC) model, and (2) the coincident axial direction/plane matching (CAD/PM) model. In addition, the Brandon criterion and the Palumbo-Aust criterion were used to obtain the allowable deviation from the exact CSL misorientations. The models used to characterize triple lines were (1) Bollmann's disclination model, (2) the Palumbo-Aust CAD model, (3) the CSL/GB model and (4) the CAD/GB model. Using the CSL/DSC model, it was found that more than 90% of special GBs ([Sigma] <= 29) and only about 20% of general GBs exhibited immunity to carbide precipitation. The upper limit of the low angle boundary for this material was found to be between 10° and 15°. GB carbide precipitation showed no correlation with the CAD/PM model. For Bollmann's disclination model, the percentage of special TLs (I-lines) immune to carbide precipitation, increased from 35% to 80%, when the heat treatment approached the ideal selective condition, while more than 80% of general TLs (U-Iines) exhibited susceptibility to carbide precipitation regardless of the sensitization conditions. Carbide precipitation was observed in all general TLs characterized by the CAD/GB model. No clear correlation between precipitation and triple line structure as per the CSL/GB and CAD models was found.M.A.Sc