INFLUENCE OF ELECTROLYTIC PLASMA CARBONITRIDING ON STRUCTURAL PHASE STATE OF FERRITIC-PEARLITIC STEELS

The change in phase composition and fine texture occurring in the  ferritic-pearlitic 0.18C – 1Cr – 3Ni – 1Mo – Fe, 0.3C – 1Cr – 1Mn – 1Si – Fe  and 0.34C – 1Cr – 1Ni – 1Mo – Fe steels under electrolytic plasma carbonitriding  was  investigated  by  transmission  electron  microscopy  (TEM) method conducted on thin foils. Carbonitriding was implemented by surface saturation with nitrogen and carbon in aqueous  solution under the temperature of 800  –  860  °C during 5  minutes. All  steels were investigated before and after carbonitriding. It was ascertained that in the original state steel is given as a mixture of grains  of pearlite and ferrite. Carbonitriding has led to creation of modified  layers: the bigger was the amount of pearlite before the beginning of  carbonitriding, the thicker was modified layer. Carbonitriding resulted  in significant qualitative changes in phase state and structure of steel.  It was revealed that in the surface area of modified layer along the  matrix, there were also particles of other phases: carbides, nitrides and  carbonitrides. In the course of removing from the surface of carbonitrided sample, their complete set and volume fractions decrease and at  the end of modified layer only one carbide phase is present in all steels,  i.e. cementite. It was found that matrix of all steels after carbonitriding is tempered packet (lath) and lamellar martensite. In the surface  area of carbonitrided layer the volume fractions of lath and lamellar  martensite depend on the original state of steel – the bigger was the  amount of pearlite in steel the less lath martensite and the more lamellar martensite was formed. Such a dependency cannot be observed in  the central area, and at the end of carbonitrided layer volume fractions  of martensite packets and plates are commensurate.

carbonitriding,  modified layer,  steel,  ferrite,  pearlite,  lath and  lamellar martensite,  phase composition,  carbides,  nitrides,  carbonitrides

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