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| Podrábský T., Stránský K., Pospíšilová S., Rek A., Dobrovská J., Juliš M. |
| HETEROGENEITY ALLOY OF INCONEL 713 LC |
| Keywords: cast nickel alloy|chemical heterogeneity|mathematical-statistical analysis| |
| No 1 (2005), p. 68-73 |
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| Pecha J. |
| EFFECT OF HEAT TREATMENT ON MICRUSTRUCTURE AND PROPERTIES OF HIGH-CHROME STEEL USED IN POWER ENGINEERING |
Abstract
The presented paper pays attention to properties of heat-resisting, high-chrome steels used in extremely thermal loaded parts of power engineering plants. The steel feature is high brittleness at ambient temperature. The high brittleness causes a lot of problems at welding and cold forming. It is proved that high-chrome steel structure of a huge number of heats after delivery from mills is not pure ferritic as it might be expected.
Austenitic elements, though to a small extent, contribute to some austenite share in the basic ferritic matrix. Therefore it was studied with more details influence of individual elements on creation and stability of austenitic phase depending on temperature. It was studied influence of heat treatment parameters on features of heat resisting steel 17153.2 (25 Cr). Steel 17153 is most frequently applied in power engineering plants in places of high temperature of flue gas (above 800°C). For heat treatment of specimens there were used 3 temperatures of annealing and 5 methods of cooling down from annealing temperature. The programme of experiments in heat treatment was given with regard to the temperature interval of forming and annealing. The cooling methods respected the embrittlement types occurring at high-chrome steels. All the specimens were subject to analysis of optical and electronic microscopy. The experiments proved that only in the specimens annealed at 1200°C and cooled down in water it was found some amount of austenite in the basic ferritic matrix. The experiments resulted in the following conclusions. For steel plates manufacture there is an important condition to achieve a bigger last distortion at the lower limit of forming temperatures. This is the only procedure ensuring the carbidic phase to be dispersed uniformly and not to be accumulated at the grain boundary that would substantially influence the possibility to reach the optimal features including toughness. The condition is related particularly to the plate thickness bigger than 10 mm as there is a close relationship between the material thickness and grain size. It was proved presence of austenitic phase in the field of high temperatures due to certain nitrogen content. The presence of the austenitic phase mitigates growth of ferritic grain at high temperatures. Coarser ferrite grain is considered to be one of main reasons of low toughness. Another favourite effect of austenite consists in its higher carbon solubility at high temperatures resulting in mitigation of ferrite saturation with carbon. Due to these reasons it is necessary to check the content of austenitic elements in supplied heat resisting high-chrome steel plates. Based on the analysis results it can be taken a decision concerning annealing with fast quenching in water. In this way there are eliminated embrittlement consequences after cooling down from high temperature during plate manufacture
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| Keywords: heat resistance|spinels|brittle|ferit|austenit|structure|carbid|difraction|Cr equivalent|toughness| |
| No 1 (2005), p. 88-103 |
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