Effect of Heat Treatment on the Hardness and Microstructure of Locally Sourced Carbon Steel in Ogun State

This study investigates the effect of annealing, normalising, and quench-and-temper heat treatment cycles on the hardness and microstructure of medium carbon steel sourced from local steel merchants in Sagamu, Ogun State, South West Nigeria. Heat treatment is fundamental to engineering property optimisation in steel components, and understanding the response of locally available steel stock to standard treatment protocols is essential for practitioners operating in Nigeria's growing manufacturing sector. Steel bar specimens are cut, machined to standard dimensions, and subjected to annealing at 850 degrees Celsius furnace-cooled, normalising at 850 degrees Celsius air-cooled, quenching from 850 degrees Celsius in water and oil, and subsequent tempering at 200, 400, and 600 degrees Celsius. Microstructural examination is performed by optical microscopy after standard metallographic preparation and Nital etching. Brinell hardness is measured before and after each treatment. Carbon composition is verified by optical emission spectroscopy. Results confirm expected microstructural transformations: coarse pearlite after annealing, refined pearlite after normalising, and martensite after water quenching. Tempering at 400 degrees Celsius provides the optimal combination of hardness and toughness as indicated by impact energy measurements. Oil-quenched specimens show lower retained martensite hardness compared to water-quenched specimens due to the slower cooling rate. Keywords: heat treatment, carbon steel, microstructure, hardness, Ogun State.