APPLICATION OF THIN CERAMIC LAYERS TO PROTECT OF STEEL CONSTRUCTIONS AGAINST THE INFLUENCE OF HIGH- TEMPERATURE OXIDATION OF AIR ATMOSPHERE

Transcription

1 APPLICATION OF THIN CERAMIC LAYERS TO PROTECT OF STEEL CONSTRUCTIONS AGAINST THE INFLUENCE OF HIGH- TEMPERATURE OXIDATION OF AIR ATMOSPHERE Krzysztof Adamaszek a, Zbigniew Jurasz b a, b Automotive Research and Development Center BOSMAL, Sarni Stok 93, Bielsko-Biala, Poland, a b obrsm@bosmal.com.pl zbigniew.jurasz@bosmal.com.pl Abstract In this work will be presented the results of influence of heat-resisting insulated ceramic layers on increase of durability of high-heat loaded engine valves made of valve steel grade H10S2M according to Polish Standard. The inlet valves and stems of outlet valves heated up to 400 o C are produced from the heat-treated steels, e.g. H10S2M acc. to PN, containing inter alia c.a. 9.5% wt. Cr, 2.5%wt. Si and 0.8% Mo, whereas heads of outlet valve are heated sometimes up to 900 o C and made of austenitic steels contains c.a. 14% Cr, 14% Ni and also other elements c.a. 2.5% W and 0.3% Mo. The valves used as samples were covered by ceramic layers and annealed in electrical chamber furnace. The isothermal process of annealing was carried out at 1100, 1060, 1020, 980 and 940 C in the electrical chamber furnace at the gravitational flow of air. On the basis of conducted experiment it can be stated, that applied ceramic layers ensures good protection of steel surfaces against of the influence of high-temperature oxidation and decarburization of air atmosphere in range of temperature from 940 C up to 1000 C. 1. INTRODUCTION The valves belong to the most of high-heat and mechanical loaded parts of four-stroke engines. On durability of valves especially outlet valves except of heat-resistance properties influences also corrosion and cavitations resistance on interaction of flow flux of hot exhaust aggressive gases. The inlet valves and stems of outlet valves heated up to 400 o C are produced from the heat-treated steels, e.g. H10S2M acc. to PN, containing inter alia c.a. 9.5% Cr, 2.5%Si and 0.8% Mo, whereas heads of outlet valve are heated sometimes up to 900 o C and made of austenitic steels contains c.a. 14% Cr, 14% Ni and also other elements c.a. 2.5% W and 0.3% Mo [1]. During high-temperature annealing highly alloyed valve steels in gas environment contains dl O 2, CO 2 and water steam. The growth of scale on the surfaces of valve steels is reciprocal dt proportional to its thickness l [2]: dl k p =, (1) dt x and courses acc. to Tammann s law [3, 4], 2 = l 2 k p t + C (2) where; C - integral constant, is the measure of deviation from parabolic course of reaction in initial linear stage of oxidation. k - the rate of parabolic constant [µm/min -1/2 ] p 1

2 k p = l t, (3) where; l thickness of scale, t time of oxidation [5]. The rate of oxidation was determined from linear dependence of logarithm of parabolic constants lg k p (1), lg k p (2),..., lg k p (n) in function of reciprocal temperature 1/T 1, 1/T 2,..., 1/T n.. The aim of this work was to check the ability of applied thin ceramic layers on protection of inlet valve surfaces made of valve steel H10S2M oxidised in decarburization air atmosphere at temperatures in range from 900 up to 1100 o C. 2. SELF STUDY The research were carried out on the inlet valves (fig.1) made of industrial melt steel ferritic-perlitic contains 0.42%C, 0.46%Mn, 2.26%Si, 9.5%Cr, 0.38%Ni 0.86%Mo and samples made of the same grade of steel (i.e. H10S2M acc. to PN). Fig.1. The inlet valve after heat-treatment and chromium plating. The valve face paded by welding of stellite. Investigation were limited to the examination of inlet valves with (c.a. 1 mm thick - see fig. 2 a and 2 b) and without ceramic layers. Annealing oxidation process was carried out in chamber furnace at temperatures; 940, 980, 1020, 1060 and 1100 o C. Times of oxidation various respectively 32h50, 13h, 5h45, 4h45 and 3h. Fig. 2. The inlet valve before oxidation; a - without ceramic layer, b - coated by ceramic layer. The measurement of thickness of scales and ceramics layers was carried out by using metallographic microscope Neophot-3 at magnifications 50 and 400. The thickness of oxide layer for steel H10S2M initially growing rapidly from 11 till 36µm with rising of temperature from 940 up to 980 o C and shortening time of oxidation from 32h50 till 13h (fig.3a). 2

3 3. RESULTS AND DISCUSSION The thickness of oxide layer for steel H10S2M initially growing rapidly from 11 till 36 µm with rising of temperature from 940 up to 980 o C and shortening time of oxidation from 32h50 till 13h (fig.3a). Fig. 3a. The thin subsurface scale layer on the steel H10S2M oxidised at; a o C/13h. Not etched. Magn. 100 Whereas for higher temperatures than 1000 o C were noticed significant growth of layers, initially from 110 [µm] after annealing at temperature oxidation 1020 o C for 5h45, oxidation, up to 220 and 277 [µm] after oxidation at temperatures 1060 o C and 1100 o C respectively for 4h45 and 3h (fig.3b-c). b. c. Fig. 3b-c. The scales on the steel H10S2M oxidised in air at; b o C/5h45, c o C/4h45. Not etched. Magn. 400 Similar to the growing of scale the growth of decarburised layers have been observed. The decarburised layers growth courses initially slower up to 35 [µm] at temperature 980 o C for 13h (fig.4a), then up to 1350 [µm], after annealing at temperature 1100 o C for 3h (fig.4a, b). Fig. 4a-b. Ferrite of decarburised sublayer of steel H10S2M oxidised at, a o C/13h. Magn b o C/3h. Magn Etched with nital. 3

4 The later research reveals, that subsurface of ceramic layers at thickness c.a.1[mm] was without of any scales and decarburised layers after oxidation at all applied temperatures and times (fig.5). Fig. 5. The cross-section of valve through heat-resisting ceramic layer, a - external layer, b - compensate layer, c - steel H10S2M. Etched with nital. Magn Fig.6. a, b. The microstructure of valves coated by thin ceramic layer after oxidation in air: a o C/32h50, b o C/13h. Etched with nital. Magn c. d. Fig. 6.c,d. The microstructure of valves coated by thin ceramic layer after oxidation in air: c o C/5h45, d o C/4h45. Etched with nital. Magn In the subsurface areas were observed bainite with small precipitates of carbides. 4

5 Fig. 6e. The microstructure of valves coated by thin ceramic layer after oxidation in air: e o C/3h. Etched with nital. Magn CONCLUSION On the basis of obtained results it can be stated, that applied ceramic layer assurances good protection of external surfaces of tested valves from its influence on oxidation and decarburization of air atmosphere at temperatures in range from 940 up to 1100 o C. REFERENCES 1. BERNHARDT M.,DOBRZYŃSKI S., LOTH E.: Silniki samochodowe, Warszawa, WKiŁ, MROWEC S.: Kinetyka i mechanika utleniania stali. Katowice, Wyd. Śląsk, MROWEC S., Werber T.: Korozja gazowa metali. Katowice, Wyd. Śląsk, MROWEC S.: Zarys teorii utleniania metali. Katowice, Wyd. Śląsk, NEWKIRK J.B.: High - Temperature Materials Coatings and Surface Interactions, Tel-Aviv, Freund P.H.,