The effect of ferrite volume fraction and morphology on mechanical properties and fracture toughness (K1C) of intercritically annealed and tempered structural steels

In this study, the effect of ferrite morphology and the volume fraction on the mechanical properties and fracture toughness values (K1C) have been investigated for 16MnCr5, 17CrNiMo6 and Ck22 steels with varying chemical compositions.Firstly, homogenization heat treatment have been applied to remove bands in as received steel bars. Then, two different starting microstructures were obtained by carrying out full annealing and quenching from austenite before intercritical temperature annealing in austenite region. Following this process, pre-heat treatment was performed on small specimen in different temperatures between AC1-AC3 Inter-critics temperatures. Dual phase steels that have two kinds of morphology as coarse and fine have been obtained and heat treatments regimes at % 40, % 20, % 10 ferrite volume fraction depending on starting microstructures. Similar microstructures were produced in mechanical test specimens by applying heat treatment regimes and tempering.Tensile and impact tests were employed for mechanical test specimens.Yield strength, tensile strength, fracture toughness, Elongation (%), strain hardening coefficient values and fracture surfaces were obtained from single axis tensile testing and various conclusions on impact toughness were also drawn from impact tests.The yield strength, fracture toughness, Elongation (%), impact toughness of steels showed lower values with decreasing amount of ferrite volume ratio while tensile strength and work hardening coefficient values have increased. In tempered specimens, yield strength, fracture toughness, Elongation and impact toughness values showed an increase while strain hardening coefficient value showed a decrease. Strength values of fine grained dual phase steels yielded better properties as opposed to coarse grained steels. The tempered specimens with % 10 ferrite also yielded improved fracture toughness values compared to other steels. Specimens showed brittle fracture while tempered ones showed ductile fracture.