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Fundamentals physical metallurgical and corrosion characteristics of high alloyed austenitic stainless steel of type Cr21Ni33TiAl

Cihal V., Lasek S., Blahetová M., Turek L.

Zeszyty Naukowe. Mechanika / Politechnika Opolska

2010

z. 97

11-12

Stainless steel Cr21Ni33TiAl (Incoloy 800, NiCrTiAl33-21) can be used as heat or/and creep resistant for many applications. The study of stainless material Cr21Ni33TiAl was intent on a rate of workability, resistance to stress corrosion, susceptibility to intergranular corrosion and pitting, resistance in high temperature sulphate melt environments. Solutions of these partial tasks to define primary criterions for determination and utilization of basic characteristics of studied stainless steel Cr21Ni33TiAl. Convenient workability in the hot condition of this material gives a possibility of further alloying this base, e.g. by molybdenum for increasing of corrosion resistance in aggressive environment, if need to pitting corrosion. On the basis of stress corrosion testing in the chloride solutions seems the material Cr21Ni33TiAl after solution annealing, in the comparison of other examined materials, as very resistant to stress corrosion cracking, adequately to the content of nickel. Reducing of carbon content affects in the state after solution annealing only inexpressively its resistance to corrosion cracking. The sensitized material in the critical zone of temperature leads to increasing its sensibility to stress corrosion cracking which varies to intergranular course. This finding points out a requirement to study the steps on grain boundaries and the assurance of resistance to intergranular corrosion in the practice. The sensitizing of steel Cr21Ni33TiAl reduces its resistance to pitting corrosion in chloride environments. Resistance to intergranular corrosion of the material Cr21Ni33TiAl increases while lowering the content of carbon. Regarding the contents of titanium the virtue of stabilization seems to be at low contents of carbon as limited. Incubation time to intergranular corrosion (Rollason curve) is shorter in the comparison with the steel Cr18Ni10 at the same content of carbon. The long termed exposure at low temperatures in critical zone leads to considerable sensitivity to intergranular corrosion and pitting. There is eliminated at increased temperatures and quite shorter time. Electron microscopy testing confirms the priority precipitation of M23C6 carbides in grain boundary in all extent of critical temperatures. Precipitation of mild titanium carbides was proved in a little extent at the descended heat treatment. Removing of susceptibility to intergranular corrosion and by that also sensitivity to intergranular stress corrosion cracking is possible on the basis of reached results to aim by lowering the content of carbon under 0,02% and by right technological and heat treatment off the area of sensibility. While using the material Cr21Ni33TiAl also with the low carbon content by temperatures till 600°C there is the need to think about the danger of considerable degree of sensitizing, which at temperatures under 500°C can be removed by long-term persistence on the temperature. For increasing of the resistance of low carbon type of material Cr21Ni33TiAl to intergranular and pitting corrosion or intergranular stress corrosion cracking there is a need to take care for the problem of stabilization with using of workability in the cold condition in front on the final solution and stabilization annealing. High creep and heat resistance in air with presence of SO2 also in sulphate melt enables using of the material Cr21Ni33TiAl at the temperatures around 800°C. Regarding the examined quality of the low carbon type 02Cr21Ni33TiAl there is possible to recommend this one as a good basement for modification with molybdenum, copper and others for using in very aggressive conditions. The material Cr21Ni33TiAl is possible to use on the basis of examined qualities under the conditions by which there is a danger of occurrence of stress corrosion by the austenitic or austenite-ferritic stainless steels.

Use of image analysis in determining the thicknesses of coatings

Kalabisova E., Turek L.

Ochrona przed Korozją

2007

nr 2

66-69

Image analysis is an innovative solution in material characterization. One of the areas where image analysis can be put to use effectively is in establishing the thicknesses of coatings encountered in surface fi nishing applications. Galvanizing ranks among the most widely used corrosion protection techniques, and estimates of the useful service life of surface-finished substrate material hinge on establishing the correct thickness of the zinc layer applied in the galvanizing process. In applications where the fi lm thickness does not lend itself easily to measurements by common thickness gauges (such as on highly curved surfaces), a method which, in addition to fi lm removal by etching, is well-suited for measuring thicknesses is using the microscope. Prior to the measurement, polished metallographic sections are prepared from suitable specimens, and the measurement itself is conducted as per the standard EN ISO 1463 [1]. The thickness measurement proper is performed at regular intervals around the sample periphery, where the sample is allowed to travel within the optical system of the microscope and the readings are compared with a calibrated scale of the eyepiece. In case image analysis technique is used, the over-all, as-recorded image of the sample provides the input data for a software separation process by which the image of the Zn layer is isolated and, subsequently, evaluated using image analysis software. From comparisons of the results achieved it follows that the use of image analysis, which is also less time-consuming, yields more accurate results.

Metoda analizy obrazu to nowoczesne rozwiązanie stosowane w charakterystyce materiałów. Metoda ta znajduje praktyczne zastosowanie m.in. w ustalaniu grubości powłok w procesie obróbki powierzchniowej. Cynkowanie jest metodą najszerzej stosowaną w ochronie przed korozją i założona trwałość pokrytego powłoką podłoża po obróbce powierzchniowej wpływa na ustalenie odpowiedniej grubości powłoki cynku nałożonego w procesie cynkowania. W przypadkach gdy grubości nie można w łatwy sposób zmierzyć za pomocą standardowych urządzeń pomiarowych (np. gdy powierzchnie są mocno zakrzywione), oprócz techniki polegającej na usunięciu powłoki przez wytrawienie zastosowanie znajduje również metoda mikroskopowa. Przed wykonaniem pomiaru przygotowane zostają wypolerowane próbki metalografi czne, a sam pomiar jest wykonywany według normy EN ISO 1463 [1]. Grubość właściwa mierzona jest w równych odstępach w obrębie próbki, gdy próbka przesuwana jest w polu widzenia systemu optycznego mikroskopu, zaś odczyty porównywane są ze skalibrowaną skalą na okularze. W przypadku zastosowania metody obróbki obrazu, dane wejściowe uzyskane bezpośrednio z zarejestrowanego obrazu zostają przesłane do odpowiedniego programu, który następnie izoluje warstwę cynku aby później poddać ją obróbce. Na podstawie porównania wyników ustalono, że zastosowanie metody obróbki obrazu pozwala na uzyskanie bardziej dokładnych wyników w krótszym czasie.