Effect of Heat Treatments on the Mechanical and Electrochemical Corrosion Behavior of 38CrSi and AISI 4140 Steels

The aim of this investigation was to compare the microstructural, mechanical and electrochemical corrosion characteristics of 38CrSi steel with widely used AISI 4140 steel under various heat treatment processes to widen its field of applications. Experimental steels 38CrSi and AISI 4140 were subjected to austenitizing at 900 °C for 30 min followed by annealing, quenching in oil and quenching followed by tempering at 400 °C for 120 min. Microstructure of 38CrSi and AISI 4140 steels was characterized by light optical and scanning electron microscopy. Tensile, Charpy impact toughness and Rockwell hardness testing techniques were used to evaluate the mechanical properties. Electrochemical corrosion behavior was evaluated by a Tafel scan in tape water. Results show that microstructure of both steels comprised of ferrite and pearlite after annealing, packets and blocks of lath martensite with retained austenite after quenching and tempered lath martensite with retained austenite after quenching, and tempering with varying grain size distribution and volume fractions of different phases. Hardness and tensile properties of both steels varied in quite identical manner, while impact toughness of both steels varied in inverse manner. 38CrSi steel was found to be brittle, while AISI 4140 steel exhibited excellent properties under all heat treatment processes. In 38CrSi, the corrosion rate decreases as compared to the as-received sample from 0.515 to 0.486 mpy in quenching and tempering heat treatment, while in AISI 4140 steel the corrosion rate decreases during quenching from 0.620 to 0.472 mpy.