MICROSTRUCTURAL CHARACTERIZATION AND IN-SITU TRANSMISSION ELECTRON-MICROSCOPY ANALYSIS OF LASER-PROCESSED AND THERMALLY TREATED FE-CR-W-C CLAD COATINGS

Rapidly solidified microstructures of Fe-Cr-W-C quaternary alloy deposited on low-carbon steel by laser cladding were investigated. The clad-coating alloy, a powder mixture of Fe, Cr, W, and C with a weight ratio of 10:5:1:1, was processed with a high-power continuous wave CO2 laser. The developed clad coatings possessed fine microstructures, uniform distributions of alloying elements, and high microhardness. Analytical electron microscopy and energy dispersive X-ray spectroscopy were used to characterize the crystal structures and microchemistries of the various phases in the clad coatings. The laser processed microstructure comprised fine primary dendrites of a face-centered cubic (fcc) austenitic gamma phase and interdendritic eutectic consisting of a network of pseudohexagonal M7C3 carbides rich in Cr in an fcc austenitic gamma phase. The interlamellar spacing in the eutectic matrix was about 20 nm. The relatively high microhardness, about 900 kg(f)/mm2, of such fine microstructures is attributed to the formation of complex ternary carbides uniformly distributed in the eutectic matrix. In situ transmission electron microscopy (TEM) of thermally treated clad coatings revealed that transformation of the primary gamma phase to body-centered cubic (bcc) ferrite (alpha phase) commenced after heating at 843 K for about 7 minutes. The transformation initiated at the interface of the primary dendrites and the eutectic and propagated gradually into the primary phase. Phase change of the interdendritic gamma austenite to a bcc alpha ferrite occurred after about 30 minutes of hold period at 843 K. Transformation of the M7C3 carbides did not occur even after heating at 843 K for about 3.2 hours. The growth of a thin M2O3 (M = Fe, Cr) oxide scale was detected after heating at 843 K for approximately 24 minutes. After cooling gradually to room temperature, the softened (723 kg(f)/mm2) microstructure consisted of primary dendrites with a bcc alpha ferrite crystal structure and interdendritic ternary eutectic of untransformed M7C3 carbides in alpha ferrite.