Influence of electrolytic plasma nitriding mode on structural phase state of pearlitic steel

Abstract

The paper describes results of studies of phase transitions in structural phase state occurring in the type 0.34C-1Cr-1Ni-1Mo-Fe steel under electrolytic plasma nitriding in nitrogen-containing water solution. The nitriding voltages considered in the given study were 550 and 600 V. The research was conducted by means of X-ray diffraction electron microscopy. The specimens were studied in two states : 1) before modification (original state) and 2) after nitriding in the surface layer of the specimen. The study was conducted on thin foils. It was found that nitriding lead to significant changes in the structure of steel, namely in its phase composition and in the number of existing phases. In the original state the structure of steel was given as lamellar pearlite, ferritic carbide mix and fragmented ferrite. After 550 V nitriding it was lath martensite, plates of α-phase, with colonies of thin parallel plates of γ-phase and coarse grains of α-phase, containing γ-phase grains which were different in size and shape and were various-directional. Increase in nitriding voltage up to 600 V lead to change in the structure given as a lamellar non-fragmented pearlite and fragmented ferrite. The original state was marked by presence of particles of M3C cementite, after nitriding irrespective of the voltage it had the particles of M3C alloyed cementite, Fe3Mo3N nitride and Cr2C0.61N0.39 carbonitride. The sizes, volume fractions and locations of particles were dependent on nitriding voltage.