Laser in-situ combinatorial carbide coating on steel

by 1980- Singh, Anshul Kumar

The potential for synthesizing an in-situ grown ultra-fine carbide composite coating on the surface of steel during laser surface engineering was investigated. A 2.5 KW Nd:YAG laser was employed to modify the surface of a AISI 1010 steel deposited with a precursor powder mixture of Fe, Ti, Cr and C. With the help of laser surface engineering, carbide composite coating on the surface of plain C steel was achieved. It is envisioned that such a coating will offer superior tribological properties. Energy Dispersive Spectroscopy in supplement with X-ray Diffractometery indicated the evolution of TiC, Fe-Cr, and M7C3 as major phases in the coating. An oscillatory pattern for evolution of M7C3 was observed with respect to the laser power over the range of 900-2100 watts during processing. Although TiC was present in all the samples, the chromium carbides were absent in samples processed at certain laser powers. Corresponding to this behavior, variation in mechanical properties of the coating was observed. The hardness and wear properties of the samples without chromium carbides was inferior in comparison to samples with both TiC and chromium carbides. The roles of in-situ growth, refractory nature of the carbide particles, the nonequilibrium nature of the process and their contribution in successfully forming a composite coating have been described. Computational techniques were employed with the aim of studying possible reasons for phase evolution, stability of phases and solidification path and thus optimize parameters to tailor properties according to requirement. The temperature range of thermal transitions within the quaternary system iv (Fe, Ti, Cr and C) and the thermal stability of the evolved phases were studied with the help of differential scanning calorimetry (DSC). DSC studies indicated that the major exothermic reactions (formation of carbides) take place within 850-1150 oC. Temperature ranges for individual reactions were investigated. The evolved phases (TiC, M7C3, Fe-Cr and Fe3C) were characterized using X-ray diffraction (XRD). This multicomponent powder mixture, which was used as a precursor for synthesizing a composite coating on the surface of steel via laser surface engineering (LSE), was computationally investigated for thermal stability. The intended wear applications of the coating made thermal stability investigations imperative, as there is a localized heat buildup during wear, because of the contact between rubbing surfaces. Experimental evaluation (DSC) of thermal stability of the phases formed was done to supplement the computational investigations. The degradation of the coating due to prolonged stay at elevated temperatures (in oxidizing environments such as air) could lead to degradation in the properties of the coating. High temperature oxidation studies were done to investigate the oxidation kinetics of the composite coating. v