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| Longauerová M., Fedorová M., Longauer S., Kadlec J., Bořuta J., Fujda M., Janák G., Vojtko M. |
| HETEROGENEITIES OF CHEMICAL COMPOSITON IN CAST PRODUCTS |
| Keywords: Slabs|surface segregation|structure heterogeneities|centreline noncompactibilities|round blooms|central segregation| |
| No 1 (2007), p. 17-25 |
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| Dománková M., Marek P., Moravčík R. |
| EFFECT OF ANNEALING AT 650°C ON PRECIPITATION IN CHOSEN AUSTENITIC STAINLESS STEELS |
Abstract
Intergranular corrosion and stress corrosion cracking of austenitic stainless steels are the most important corrosion process that affects the service behaviour of these materials. Sensitisation behaviour of austenitic stainless steel is greatly influenced by several metallurgical factors such as chemical composition, degree of prior deformation, grain size, aging temperature-time. The exposition in the temperature range of 500-800°C leads to the grain boundary precipitation of chromium rich carbides (Cr,Fe)23C6 and to the formation of chromium depletion regions. If the chromium content near the grain boundaries drops under the passivity limit 12 wt.%, the steel becomes to be sensitised.
We deal with the influences of annealing on sensitisation behaviour in unstabilised AISI 316, AISI 304 and stabilised X6CrNiMoTi 17-12-2 austenitic stainless steels. The identification, morphology and size of grain boundary precipitates, which are responsible for sensitisation, were characterised for samples aged at 650°C for ageing duration corresponding to the onset of sensitisation. A systematic trend is observed in the experimentally determined sensitisation data of these typical stainless steels. This would eliminate the need for the independent generation of sensitisation data for stainless steel of specified composition, which is within the range investigated here. The database reported for these alloys will also help to recommend the limits of critical cooling rate to avoid sensitisation during fabrication.
For individual secondary phase identification transmission electron microscopy (TEM) of extraction carbon replicas was utilised. TEM observations were performed using JEOL 200 CX operated at 200 kV. Carbon extraction replicas were obtained from mechanically polished and etched surfaces. The replicas were stripped from the specimens in solution of CH3COOH : HClO4 = 4 :1 at 20°C and 20 V.
The precipitation in the experimental steel AISI 316 started after 5 hours aging at 650°C. M23C6 carbide was observed as first, later ?-phase and M6C carbide were identified. The phase ratio of M23C6 carbide decreased with increasing of aging time. It was in contrast to M6C carbide, which ratio increased with increasing of aging time. The similar tendency of the precipitation was observed in the experimental steel AISI 304 during aging at 650°C.
The precipitation in the experimental steel X6CrNiMoTi 17-12-2 started after 10 hours of aging at 650°C. We identified as first precipitates of MC carbide and ?-phase. M23C6 carbide and ?-phase were observed after 30 hours of aging. The proportion of ?-phase decreased with increasing of aging time. It was in contrast to ?-phase, which proportion increased with increasing of aging time.
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| Keywords: austenitic stainless steels|sensitisation|precipitation|secondary phases|intergranular corrosion| |
| No 1 (2007), p. 52-60 |
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