Original Article

Investigation of the microstructure, hardness and corrosion behavior of AISI 4340 steel subjected to pack boronizing process

Graduate School of Natural and Applied Sciences, Kirikkale University, 71451, Kirikkale, Turkey

^* e-mail: bunyaminyamanel@hotmail.com

Received: 21 July 2025
Accepted: 20 October 2025

Abstract

This study investigates in depth the improvement of the surface properties of AISI 4340 steel, widely used in the machinery industry, through a precisely controlled pack boronizing process. Steel samples were boronized using a novel boronizing agent containing two different activators at two different temperatures (950 °C and 1050 °C) for varying durations of 2,4, and 6 h. The microstructural evolution and properties of the boride layers formed after boronizing were comprehensively analyzed using optical microscopy and Scanning Electron Microscopy (SEM). X-ray diffraction (XRD) analysis precisely determined the phase composition of the boride layers. Surface hardness measurements revealed a remarkable 8-fold increase in hardness compared to untreated steel, highlighting the significant strengthening effect of the treatment. Furthermore, corrosion resistance tests conducted in strict accordance with ASTM G44-21 standards demonstrated a remarkable 14.11-fold improvement in the corrosion resistance of the boronized steel. The findings conclusively demonstrated a positive correlation between both treatment temperature and duration and the thickness of the formed boride layer. This study critically highlights the exceptional effectiveness of using a two-activator boriding agent in significantly enhancing the performance of AISI 4340 steel in demanding industrial applications.