Laser surface alloying of ferritic steel to enhance oxidation resistance

摘要

An attempt has been made to enhance the high temperature oxidation resistance (above 873 K) of 2.25Cr-1Mo ferritic steel by laser surface alloying (LSA) with co-deposited Cr using a continuous wave CO_2 laser. The main process variables chosen for optimizing the LSA routine were laser power scan speed of the sample-stage and powder feed rate. A detailed investigation indicates that the microstructure (studied by optical and scanning electron microscopy) and composition (determined by energy dispersive spectroscopic analysis) of the alloyed zone (AZ) are strong functions of the LSA parameters. Following LSA, microhardness of the AZ increases to 450-550 VHN as compared to 220 VHN of the underlying substrate. Isothermal oxidation studies in air by thermogravimetric analysis at 973 and 1073 K for up to 150 h reveal that LSA may significantly enhance the oxidation resistance of ferritic steel during exposure to 100/200 K above the current upper limit of use of the same steel for heat exchangers in thermal reactors. Post oxidation microstructural analysis suggests that an adherent and continuous Cr_2O_3 layer is responsible for the improvement in oxidation resistance. Finally, a detailed structure-property-LSA parameter correlation has been reported.