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Rola złożonego azotku Cr(V, Nb)N - fazy Z w wysokochromowych stalach martenzytycznych

Golański G., Kępa J.

Inżynieria Materiałowa

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2011

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Vol. 32, nr 6

917-922

PL

W pracy na podstawie danych literaturowych i badań własnych przedstawiono wpływ wydzielania złożonego azotku Cr(V, Nb)N - fazy Z na właściwości mechaniczne wysokochromowych, martenzytycznych stali zawierających 9÷12% Cr. Wydzielenia fazy Z powstają w czasie eksploatacji w wyniku przemiany lub/i rozpuszczenia się wydzieleń typu MX. Zanik drobnodyspersyjnych azotków, węglikoazotków bogatych w wanad i niob skutkuje pojawieniem się cząstek fazy Z. Objawia się to gwałtownym spadkiem wytrzymałości na pełzanie stali 12% Cr. Pokazano, że pierwiastkiem stymulującym wydzielanie fazy Z jest chrom, a ponadto niezbędnymi pierwiastkami do tworzenia tej fazy są niob lub tan- tal oraz azot. Optymalną temperaturą wydzielania fazy Z w stalach zawierających 9÷12% Cr jest temperatura 650°C. W żarowytrzymałych stalach o zawartości 9% Cr wpływ wydzieleń fazy Z jest niewielki i nie skutkuje tak drastycznym spadkiem wytrzymałości na pełzanie jak w przypadku stali zawierającej 12% Cr. Perspektywą wykorzystania fazy Z jest eksperymentalna stal zawierająca 12% Cr umacniana wydzieleniami tej fazy, co prawdopodobnie umożliwi opracowanie stali mogącej pracować w temperaturze powyżej 600°C.

EN

The paper presents the influence of precipitation of the complex nitride Cr(V, Nb)N - Z phase on the mechanical properties of high-chromium martensitic steels with 9÷12% Cr, based on the literature data and an independent study. Precipitations of the Z phase occur during use as a result of the transformation or/and dissolution of precipitates of the MX type. The disappearance of fine-dispersion nitrides, carbonitrides rich in vanadium and niobium results in the appearance of Z phase particles. It is revealed by a rapid decrease in the creep strength of 12% Cr steel. It has been shown that the element stimulating the precipitation of the Z phase is chromium, and the elements necessary for the formation of this phase are niobium or tantalum and nitrogen. The optimum temperature for Z phase precipitation in steels with 9÷12% Cr is the temperature of 650°C. In high-temperature creep resisting steels with 9% Cr the influence of Z phase precipitates is slight and does not cause such a drastic decrease in creep strength as in the case of steel containing 12% Cr. The prospect for using the Z phase lies in experimental steel with 12% Cr strengthened with the precipitates of this phase, which will probably make it possible to develop a steel that could operate at temperature above 600°C.

2

Gyrolite Formation in CaO–SiO2źnH2O–gamma-Al2O3–Na2O–H2O System under Hydrothermal Conditions

Baltakys K., Siauciunas R.

Polish Journal of Chemistry

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2007

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Vol. 81, nr 1

103-114

EN

The influence of gamma-Al2O3 and Na2O additives on the gyrolite formation process in the CaO–SiO2źnH2O–H2O mixture has been examined. It has been proved that gamma-Al2O3 is not recommended for the synthesis of gyrolite, because Al3+ ions stimulate the formation of C-S-H(I) and 1.13 nm tobermorite (where C – CaO, S – SiO2 and H – H2O). Na2O positively affects the formation of gyrolite: this compound is formed already after 8 hours of isothermal curing at 200 graduate C. On the contrary, in the pure mixtures gyrolite formed after only 32 h at 200 graduate C. In order to compare the cation exchange activity of synthesized gyrolite with (Al+Na)-substituted tobermorite, the ion exchange experiments were employed as a test reaction.

3

The influence of gamma-Al2O3 and Na2O on the formation of calcium silicate hydrates in the CaO-quartz-H2O system

Baltakys K., Siauciunas R.

Materials Science Poland

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2007

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Vol. 25, No. 1

185--198

EN

The influence of gamma-Al2O3 and Na2O additives on the formation and sequence of calcium silicate hydrates in the CaO-quartz-H2O system with CaO/SiO2 (C/S) molar ratio equal to 0.66 was determined. The synthesis was carried out in unstirred suspensions. The molar ratios of the primary mixtures were CaO/(SiO2 + Al2O3) = 0.66 and Al2O3/(SiO 2 + Al2O3) = 0 or 0.025. The amount of NaOH, corresponding to 5% of Na2O in the mass of dry material, was added in the form of solution. Due to the low solubility rate of quartz in the CaO-quartz-H2O system with C/S = 0.66 in the temperature range 150-200°C, low-base calcium silicate hydrates (Z-phase, gyrolite, pectolite, and others) do not form even after 72 hours of hydrothermal curing. It has been proved that gamma-Al2O3 and Na2O additives change the reaction mechanisms and composition of intermediate (final) products. In mixtures with Na2O, gyrolite starts to form already at 175°C. It should be noted that in the temperature range 175-200°C the final products are gyrolite and pectolite, and the intermediate compounds are C-S-H(l) and Z-phase. The formation of low-base calcium silicate hydrates in the CaO-quartz-Na2O-H2O system is slowed down when Al 2O3 is added because Al3+ ions stimulate the formation of tobermorite and prevent the formation of pectolite and Z-phas

4

Formation of gyrolite in the CaO-quartz-Na2O-H2O system

Baltakys K., Siauciunas R.

Materials Science Poland

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2007

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Vol. 25, No. 4

1089--1100

EN

Optimizing the duration and/or the temperature of hydrothermal synthesis of gyrolite has been investigated by adding NaOH solution into an initial mixture of CaO-quartz-H2O. The molar ratio of the primary mixture was C/S = 0.66 (C - CaO; S - SiO2). An amount of NaOH, corresponding to 5 % Na2O from the mass of dry materials, added in the form of solution and additional water was used so that the water/solid ratio of the suspension was equal to 10.0. Hydrothermal synthesis of the unstirred suspension was carried out in saturated steam at 150, 175, 200 oC. The duration of isothermal curing was 4, 8, 16, 24, 32, 48, 72 and 168 h. The temperature of 150 oC is too low for the synthesis of gyrolite; the stoichiometric ratio C/S = 0.66 is not reached even after 168 h of synthesis neither in pure mixtures nor in mixtures with addition of Na2O. Na+ ions significantly influence the formation of gyrolite from the CaO-quartz mixtures in the temperature range from 175 oC to 200 oC. Gyrolite is formed at 175 oC after 168 h and at 200 oC after 16 h of isothermal curing. On the contrary, in pure mixtures it does not form even after 72 h at 200 oC. Na+ ions also change the compositions of intermediate and final products of the synthesis. In mixtures with 5% Na2O, intermediate compounds are C-S-H(I) and Z-phase, and the final products are gyrolite and pectolite. Meanwhile, in mixtures without this additive, the main intermediate compounds alfa-C2S hydrate and C-S-H(II), and the main products are 1.13 nm tobermorite and xonotlite.