Abstract
This research is focused on the effect of tempering on the microstructure and mechanical properties of 0.23%C austenitic dual phase steel locally manufactured in Nigeria. The as-received steel was normalized in order to annul the thermo-mechanical history of the steel. Thereafter, some of the normalized samples were austenetized at 850°C for 30 min and then slowly cooled in the furnace to the (α+ϒ) region and soaked for 30 min each at 790, 770, 750 and 730°C; and then quenched in hot water at 50°C. Again, some of the as-step quenched samples were tempered at 320°C for 1 h and air cooled. Mechanical testing and microstructures studies were conducted on all the heat treated samples. Optimum combination of properties was observed with samples intercritically step quenched at 770°C and tempered at 320°C for 1 h. Its hardness, impact strength, total elongation and ultimate tensile strength improved from 241.45 to 327.15 Hv, 0.26 to 1.66 J/mm2, 9.14 to 36.03% and 785.68 to 1569.28 N/mm2 respectively over the as-step quenched steel; representing 35.49, 538.46, 294.20 and 99.74% respectively. Microstructure photographs revealed duplex microstructure essentially comprised of ferrite and martensite with dispersion of carbide or retained austenite which is a typical characteristic of conventional dual phase steels.
Key words: Tempering, austenitic dual phase steel, mechanical properties, microstructures.
