Adefuye et al, 2020 17 www.etrj.com.ng © 2020 Faculty of Engineering, Lagos State University, Ojo. Nigeria. All rights reserved. (Print) ISSN 0794-2834 (Online) ISSN 2736-1969 Engineering & Technology Research Journal Volume 5(2) pp. 17-24 (July – September 2020) INFLUENCE OF QUENCHING AND TEMPERING HEAT TREATMENT ON TENSILE PROPERTIES AND TOUGHNESS OF COLD-DRAWN 0.12wt% C STEEL ADEFUYE O.A., RAJI N.A.*, J.O. OLALEYE, KUKU R.O. Mechanical Engineering Department, Lagos State University, Ojo. Nigeria * e-mail: nurudeen.raji@lasu.edu.ng Received: 18 th April 2020 Accepted: 5 th June 2020 Published: 20 th September 2020 https://doi.org/10.47545/etrj.2020.5.2.061 ABSTRACT Low carbon steel of 0.12 wt.% C steel cold drawn in 20, 25, 40, and 55% deformations of cold-drawn wires are characterised by brittle fracture when subjected to impact load because the process induces strain hardening. Experiments had been used extensively in industry to find the suitable heat treatment parameters for improved properties. The 0.12 wt.% C steel was heated to the region of austenite and hold for 30 minutes and 40 minutes for comparison, then rapidly cooled SAE 10W-40 engine oil followed by tempering at 400 deg. C. The yield strength of the drawn 25%, 40% and 55% steel reduce. The tensile strength reduces drastically for all the degree of cold-drawn steel. This was as a result of the dissolution of the steel carbon contents into the ferrite phase when heated above the AC1 temperature range and the tendency of the grain to grow due to prolong heating above recrystallisation temperature range. The impact toughness of the samples improves for the treated steel at 30 minutes duration of tempering reduces below the impact toughness of the non-treated steel for treatment at 40 minutes tempering duration for all the cold drawn steel. The toughness is also found to reduce with increasing cold drawn deformation and reduction rate tends to reduce with increasing cold-drawing. This procedure of heat treatment is extensively used for improving the toughness and hardness of the carbon steel. The study demonstrated the possibility of predicting the tensile and yield strength of 0.12 %wt. C steel. The correlation relationship established that the interdependence of the strength and the hardness is more reliable at low tempering duration of 30 minutes compared with the duration at 40 minutes. Keywords: Tensile strength, yield strength, quenching, SAE oil, tempering, hardness, toughness 1. INTRODUCTION The carbon metals harden during drawing process and causes reduction in the ductility of the metal but increases its strength [1]. The hardness is caused by generating dislocation and its movement within the structure of the carbon metal. This effect is known as work hardening and also known as strain hardening. Strain hardening is the increment in internal energy due to the increased dislocation density and density in vacancies and interstitials [2,3]. The changes in the mechanical properties of the metal as a result of the deformation often influence the performance of the resulting product of the process in service. Heat treatment for steel improves the mechanical properties of the steel. These properties include the yield strength, toughness, ductility, hardness and impact strength [4,5,6]. The various heat-treatment processes appropriate to plain- carbon steels are: normalizing, annealing, hardening, and tempering. Quenching involve heating the steel up to the region of the austenite and then rapidly cooled either in air, water or oil. The quenching procedure is used to obtain the martensite phase of the steel. Investigation of the impact of cold reduction size and annealing on the mechanical properties of High Strength Low Alloy steel (HSLA) was attempted [7]. Also, investigation into the effect of tempering on the internal friction of carbon steel established that when metals are plastically deformed the internal friction increases and the elastic modulus decreases [8]. The elastic modulus could be recovered by heat treatment of such deformed meta. The dislocation density and mobility usually depend upon the intensity of tempering. The increase of the tempering time reduces the martensite tetragonality due to the carbon segregation from the interstitial sites towards the dislocations.