PNNL

Abstract

Friction surfacing (FS) was investigated as a method for repairing cracks in 3-mm-thick 304L stainless steel (304L) plate. Simulated thru-cracks of 50 µm in width were created by tack welding two end milled surfaces. Friction surfacing was performed using two sizes of 304L consumable rod, with diameters of 9.52 mm and 12.7 mm, on two substrate conditions (clean and oxidized). Helium leak testing was used to evaluate the efficacy of the crack repair. Friction surfacing was able to both seal and repair cracks to a depth of 100 µm - 200 µm below the original surface, when using the 12.7-mm-diameter consumable rod for both substrate conditions. Helium leak rates of 10-10 atm-cc/sec were achieved, using tests designated as leak-tight by the ASTM N14.5 standard. Optical and scanning electron microscopy were used to investigate the coating microstructure and bond interfaces. In this study, the 12.7-mm-diameter rod performed better with disruption of the oxide layer on the oxidized substrate and providing a more homogenous coating. Mechanical properties of the coated samples were evaluated by performing micro-indentation, tensile, bending, and adhesion testing. Higher hardness in the coatings was observed due to the fine equiaxed grains resulting from dynamic recrystallization. Optimal process parameters using larger consumable rod (12.7 mm diameter) provided good mechanical properties of the coatings. Tensile and bending properties were comparable to those of uncoated specimen. No delamination of the FS coatings were observed during adhesion testing. The results demonstrate that friction surfacing is a viable option to repair cracks in stainless steels.