Microstructure evolution and properties of boronized low alloy steels and subsequent DLC/CrN coating

Abstract

The present study was focused on establishing an in-depth understanding of the pack-boronizing (a thermo-chemical heat treatment) behaviour of industrially employed low alloy steels (AISI EN41B and AISI 4140). Optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), electron probe microanalysis (EPMA), glow discharge optical emission spectroscopy (GDOES), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, nanoindentation, 2D surface-profilometer, and tribometer were used to investigate the specimens. Microstructure evolution in the boronized layer was studied in the present work. The critical role of alloying elements present in the steel was explored in terms of their migration behaviour during the growth of the boride front and, consequently, in understanding their role in the formation of boride, matrix, and transition region within the boronized layer. Boride morphology and the migration of alloying elements (during the development of a boronized layer) caused a variation in composition and fraction of phases from the surface to the core. Such variation affected the localised mechanical properties of the boronized layer. Maximum hardness of about 1800 HV0.1 was found in the near surface region of the boronized steel. Alloying elements such as Cr, Ni, Mo, and Mn revealed some solubility in the iron-boride. However, C, Al, and Si were entirely rejected by the boride's growing front. Al and Si were accumulated in the matrix and transition zone. The migration kinetics of Cr, Mn, Mo, and C was found to be almost equivalent to the rate of boride growth. However, Ni, Al, and Si were migrated at a slower rate. Boronizing treatment enhanced the low alloy steels' performance against wear and oxidation. Formation of oxide scale containing Fe2O3, Cr2O3, and B2O3 was revealed when the boride surface was exposed to atmospheric conditions. Contrary, Fe3BO6, Fe3BO5, Fe2BO4, and FeBO3 were the resulting compounds in the oxide scale during high-temperature oxidation. Among the investigated wear parameters, the maximum and minimum wear resistance of the boronized surface was ~46 and ~8 times the wear resistance of the non-boronized surface under the dry sliding conditions, respectively. Boronized layer helped in enhancing the load-bearing capacity of the steel; however, a very low coefficient of friction (CoF) was challenging under dry sliding. The observed CoF of the boronized surface was about 0.45-0.68. Synthesis of multilayer architecture by producing a DLC/CrN layer over the boronized surface effectively enhanced the tribological performance of the steel. Considerably low CoF (0.07-0.09) during dry sliding wear was achieved due to the deposition of DLC/CrN coating.