Авіаційно-космічна техніка та технологія (Apr 2018)
Experimental investigation of influence of thermal pulse deburring of gte turbine blades on the tendency to high-temperature gas corrosion
Abstract
One of the most important parts of gas turbine engines (GTE) are turbine blades, because from their operational characteristics depend the maximum gas temperature in a turbine, its reliability and service life, specific power and economy of an engine. Different mechanical damages, cracks, traces of general corrosion, changing their working section, are unacceptable on blades. Analysis of main causes of the parts destruction in GTE flow section shows that in most cases formation of shearing distortions and cracks occurs on blades edges in the surface layer of material. The main reason for appearance of these defects are sulphide-oxide and high-temperature gas corrosion. The basic development tendencies of blades reliability increasing show, that together with the development of new heat-resistant alloys another possible way to prevent the destruction of blade material is increasing of blade manufacturing quality. The final shape of turbine blades is often achieved by machining, which leads to formation of burrs on the edges. Thermal pulse deburring has a large number of technological advantages and is the most promising method for finishing treatment of the surfaces and edges of GTE blades. However, despite the numerous positive examples of the application of laser treatment of blade surfaces in order to increase the corrosion resistance, the mechanism of phase and structural transformations, occurring in the surface layers of heat-resistant steels and alloys of different compositions, is still not fully understood. To estimate the effect of thermal pulse deburring of gas turbine blades on their operational characteristics, accelerated tests of blade specimens on the tendency to high-temperature gas corrosion have been carried out. The tests consist of sequential chemical etching in electrolyte, electrochemical treatment and high-temperature treatment in aggressive gases. These tests allow to obtain the same corrosion layer on blades surfaces, like after real operation. Also influence of laser deburring on corrosion resistance was estimated in parallel. The experimental study was carried out on the example of treatment of GTE nozzle blades made from a heat-resistant alloy on a nickel basis ZhS26-VI. Specimens were obtained by cutting two new blades into small parts by hydroabrasive cutting method to ensure that there is no thermal impact on the material being processed and no burn-out of the alloying elements. Obtained specimens of GTE blades after the cutting have been undergone by additional machining to obtain burrs at the edges, which were removed by thermal pulse and laser deburring methods. Investigation of the surface layer state of specimens after accelerated tests for high-temperature gas corrosion has been carried out by means of microscopic analysis. For this purpose, microslices of specimens have been prepared. Using a comparative analysis of the corrosion layer thickness after the tests, it was shown that there is no influence of thermal pulse and laser deburring methods on the tendency to high-temperature gas corrosion.
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