Low temperature heat treatment of steel and the effect of quenching on the strength and oxidation behaviour

Mild steel is widely used in the engineering field because of its low carbon content that is often ductile and machinable. However, mild steel despite its hardness properties when exposed to corrosion mediums easily corrodes. This study investigates the effect of mild steel quenching after different heat treatment procedures and later exposed to corrosion environment (sodium chloride solution) to observe the corrosion behaviour of the metal. The mild steel samples were cut into dimensions of 75 mm x 25 mm x 4.5 mm and heat treated at temperatures of 100 ° C, 300 ° C, and 500 ° C, and quenched in both water and oil mediums. The temperature applied to the mild steel is expected to produce a phase structure in the pearlite region, and thereby improving its properties. The quenching medium influenced the microstructure of the mild steel, the hardness values as well as the corrosion resistant capabilities of the metal. The mechanical properties were tested using ultimate testing machine and Rockwell hardness Tester. Data obtained from this research, shows that the samples heat treated, water, and oil quenched all showed improved properties. It was observed that the percentage elongation for oil quenched at 100 °C, 300 °C, and 500 °C is 18.5 %, 14.1 %, and 12.4 % which is higher than water quenched which recorded 16.7 %, 12.3 %, and 9.8 % but the reverse was the results obtained for the other mechanical properties. The optical microscopy was used to observe surface morphology when the surface is exposed to the corrosion medium. It was observed that the oil quenched samples experience the lowest weight loss when compared to water quenched samples, and the as received sample had the highest weight loss. This shows that heat treatment and quenching provides some form of protection for the mild steel samples. (author)