Abstract
	The present paper deals with the microstructure evaluation of T24 steel during creep exposition. Material of T24 pipe’s diameter 114, 3 x 13mm was made by Železiarne Podbrezová a.s. The given steel was subjected to the creep test under constant load 140 MPa and temperature 575 ?C with time to rupture 12 609 hour. The microstructure of as - received and as - crept material was studied in detail by means of light microscopy, scanning electron microscopy and transmission electron microscopy. Microstructure of the material after creep test was studied in three sections of the creep sample, fracture surface, area in distant 0,5 - 1 mm from the fracture surface and head. The aim was to study the influence of both testing temperature and local stresses on the micro-structural changes. In as - received material, carbide phases M23C6 were found to precipitates along the grain boundaries and M7C3 mainly along the sub-grain boundaries. The precipitation of the new carbide phases was observed in the material after the creep test. Carbides M6C precipitates along the grain boundaries while M2C were distributed inside of the grains. In the all states studied fine dispersion MX matrix carbides were observed. The particle sizes of M7C3, M6C, M2C carbides changed evidently and their chemical compositions changed slightly during the creep test. On the contrary carbides M23C6 are more dimensioned stable but their chemical composition has changed significantly during the creep test.
	New evaluation’s methods of secondary phase precipitation in steel T24 were designed.