Microstructure Design of Powder-Pack Borided AISI 4140 Steel

This work attempts to design borided cases microstructure on AISI/SAE 4140 steel by changing the boron content in the pack boriding powder mixture. Three different amounts of B4C were used, yielding 1.21, 4.35 and 8.26 wt.% of boron in the pack boriding powder mixtures. The pack boriding process was carried out at 900 & DEG;C for 1, 2 and 4 h. Scanning electron microscopy and x-ray diffraction were performed to determine the microstructure, while instrumented indentation determined the mechanical behaviors. Scratch testing was carried out to evaluate the adhesion strength and failure modes of the borided layers. Ball-on-disc sliding wear tests were conducted against alumina balls to investigate the tribological properties. It was found that the lower amount of boron, i.e., 1.21 wt.%, resulted in borided cases with only Fe2B layer, while the high amount of boron, i.e., 8.26 wt.%, resulted in borided cases comprised of Fe2B/FeB phases. The borided layer with Fe2B/FeB presented high hardness; the highest wear rate was observed for the specimen with the highest boron content for 4 h.