Nonequilibrium microstructures and their evolution in a Fe-Cr-W-Ni-C laserclad coating

The laser-solidified microstructural and compositional characterization and phase evolution during tempering at 963 K were investigated using an analy tical transmission electron microscope with energy dispersive X-ray analysi s. The cladded alloy, a powder mixture of Fe, Cr, W, Ni, and C with a weigh t ratio of 10:5:1:1:1, was processed with a 3 kW continuous wave CO2 laser. The processing parameters were 16 mm/s beam scanning speed, 3 mm beam diam eter. 2 kW laser power, and 0.3 g/s feed rate. The coating was metallurgica lly bonded to the substrate, with a maximum thickness of 730 mu m, a microh ardness of about 860 Hv and a volumetric dilution ratio of about 6%. Microa nalyses revealed that the cladded coating possessed the hypoeutectic micros tructure comprising the primary dendritic gamma-austenite and interdendriti c eutectic consisted of gamma-austenite and M7C3 carbide. The gamma-austeni te was a non-equilibrium phase with extended solid solution of alloying ele ments and a great deal of defect structures, i.e. a high density of disloca tions, twins, and stacking faults existed in gamma phase. During high tempe rature aging, in situ carbide transformation occurred of M7C3 to M23C6 and M6C. The precipitation of M23C6, MC and M2C carbides from austenite was als o observed. (C) 1999 Elsevier Science S.A. All rights reserved.