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

Abstract

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.