Fibre laser cladding of turbine blade leading edges: the effect of specific energy on clad dilution

摘要

Purpose - The present work aims at developing the laser cladding technology by means of an active fiber laser source applicable for hardfacing of martensitic steel turbine blades. It also aims to investigate two process parameter conditions to reproduce two different heat inputs, in order to highlight the effect of the thermal input on the thermal alteration and dilution of the substrate material and clad layer. Design/methodology/approach - The experimentation was performed initially at a sample level, reproducing the material and thickness of the blade leading edge, then on an industrial real component. Cladding process parameters were experimentally selected and two different process parameter conditions, at different specific energy, were determined. The microstructural and geometrical features of the clad samples were analyzed both by optical microscopy and scanning electron microscopy, in this latter case combining the information supplied by different probes, among which the EDX microanalysis to obtain chemical profiles. Hardness distribution was also evaluated by means of Vickers hardness tester. Findings - All the two investigated conditions were suitable for laser cladding of the blade leading edge, since a crack and pore free clad layer with a strong metallurgical bond to the substrate was obtained. The experimented two different heat inputs affected the extension of the HAZ as well the chemical and geometrical dilution. The clad integrity was preserved in both cases. The condition at higher specific energy was chosen to clad the turbine blade. The high specific energy condition was preferred because the iron dilution in the clad layer was inferior. Research limitations/implications - Further research is needed to correlate the chemical dilution and the thermal alteration introduced by the laser cladding process on such a kind of substrate at different process parameter conditions to the wear and corrosion resistance of the turbine blade. Practical implications - Laser cladding process with an innovative active fiber laser source of the leading edge of a steam turbine blade was developed. Progress achieved in laser cladding technology development is of practical value for manufacture of turbine blades, made of martensitic steels. Social implications - The paper investigates the effect of different energy input on the laser cladding of steam turbine blades, mainly used in coal, gas and nuclear plants to produce electricity by heating water to create steam. The laser cladding process is an effective technology to increase the steam blades toughness and resistance to creep, stress and corrosion. This increase in the turbine blade properties contributes to extend the life of such a critical components, decreasing cost and time of substitution and ensuring better service conditions. Originality/value - The most original aspect of the paper is related to the focus on the difference between the chemical and the geometrical dilution, being the first one mainly related to the corrosion and wear resistance of the clad layer, while the later mainly regards the clad layer adhesion to the substrate. More in general the paper presents one of the first experiments accomplished while making use of the active fiber laser source.