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EN
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.
EN
The stainless steels and related alloys with sufficient resistance to a general corrosion can be susceptible to a localized corrosion in passive state (pitting, cracking, intergranular corrosion) in certain environment under specific conditions. The Drop Evaporation Test (DET) was developed for study of stainless materials resistance to stress corrosion cracking (SCC) at elevated temperatures 100 - 300 °C under constant external load using a chloride containing water solution. In the contribution the initiation and propagation of short cracks as well as pits were observed during the test by the travelling microscope method. The crack initiation and/or propagation can be influenced by the cyclic thermal stresses, when the diluted water solution drops cool down the hot sample. We attend to model stress changes in the testing specimens induced by cooling the heated sample by failing water solution drops. The modeling uses finite element analysis of formulated thermoelasticity problem.
8
Content available remote Stress Corrosion Cracking Evaluation of High Alloy Steel (X2CrNiMoN 22-5-3) by DET
EN
Stress corrosion cracking (SCC) can cause a dangerous failure of structural parts under specific combination of material, environment and loading. For testing of the resistance to SCC at higher temperatures (100-300°C) in water environments were proposed several methods. The Drop Evaporation Test (DET) is useful for comparison and evaluation of resistance of stainless steels and related alloys to SCC in chlorides containing water environments [1,2]. This test is valid in the cases, when evaporation of water environments can increase the content of anions (Cl-) and there is also possibility to form a salt deposition as a crevice on material surface. The advantage of DET is possibility of testing at higher temperature in the range 100~500°C using a lower Cl- water content (0-100 ppm) in convenient gas around the specimen. During DET there is a good possibility to observe a sample surface, initiation and propagation of pitting and corrosion cracking [3]. The process of thermal and corrosion fatigue is superposed on SCC mainly in the stage of initiation and propagation of short cracks especially in case of a higher difference of temperature between drops and heated sample. The DET simulates also the influence of heat transfer from material into environment on localized corrosion under evaporating conditions. The aim of this contribution is the study of short corrosion cracks growth on duplex steel of type X2CrNiMoN 22-5-3 (1.4462) under DET conditions by travelling microscope method.
EN
Segregation of phosphorus and silicon at the grain boundaries in austenitic stainless steels is known to make these steels susceptible to intergranular corrosion in strongly oxidizing environments. This behavior is more pronounced in steels containing phosphorus and normal contents of silicon, other conditions being equal. Considering the interrelationship between the changes in the relative free energy of the grain boundaries and the susceptibility to intergranular corrosion of high-purity austenitic stainless steels in HNO3+Cr6+ [1,2], associated with silicon, it would be of interest to evaluate the effect of phosphorus on the grain boundary energy in the steels of interest, booth with and without silicon, and to correlate this effect with the corrosion resistance of grain boundaries. The undesirable effect of phosphorus in these steels on the resistance of their grain boundaries in 27 % solution of HNO3+40 g.l-1 Cr6+ [3], can be eliminated by adding silicon in quantities ł2 % Si. The present paper shows the findings obtained during the research into the effect of the accompanying elements and silicon on the corrosion and electrochemical behavior of steel Cr20Ni20 with variable contents of phosphorus when exposed to nitric acid.
13
Content available remote Selection of stainless steels for the severe environments
EN
These last years, beside the austenitic-feritic (duplex) stainless steels grades, some high alloyed austenitic stainless steels with molybdenum contents and/or modified with nitrogen, wolfram and silicon contents have been developed. The nitrogen addition in these grades has been increased to 0,55% in order to obtain both high mechanical properties and high corrosion resistance. The steels with a very high nitrogen addition have a better microstructure stability in weld metal that those of austenitic steel with a moderated nitrogen addition. But in the heat affected zone can be the sensibility to the precipitation of fine nitrides. In the presence of both nitrogen and molybdenum and/or wolfram, significant improvements in the resistance of stainless steels to general and localized corrosion have been reported [1].
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