Microstructural characterization

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 al- loying 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 y phase and interdendritic eutectic consisting of a network of pseudohexagonal M₇C₃ carbides rich in Cr in an fcc austenitic γ 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 ter- nary carbides uniformly distributed in the eutectic matrix. In situ transmission electron micros- copy (TEM) of thermally treated clad coatings revealed that transformation of the primary γ phase to body-centered cubic (bcc) ferrite (α 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 γ austenite to a bcc α 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 kgy/mm²) micro- structure consisted of primary dendrites with a bcc α ferrite crystal structure and interdendritic ternary eutectic of untransformed M₇C₃ carbides in α ferrite.