Browsing by Author "Demirok, S"

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    Characterization of hot forged P285NH steel by metallurgical investigation and reliability analysisCharakterisierung von warmgeschmiedetem P285NH-Stahl durch metallurgische Untersuchungen und Zuverlassigkeitsanalyse
    Saklakoglu, N; Çivi, C; Hocalar, Ç; Demirok, S
    An extensive study was carried out to investigate the effect of cooling rate after hot forging process and normalization step on the hardness, strength and impact toughness and microstructure of P285NH steel. Understanding of the combined effect of cooling rate and normalization on the mechanical and microstructural properties of the steel would help to select conditions required to achieve optimum mechanical properties. The results indicated that the microstructures of all forging and cooling conditions were dominated by ferrite and pearlite phases with different morphologies and grain sizes according to various cooling rates. Conveyor cooling led to a formation of relatively fine acicular ferrite and pearlite grains in comparison to batch cooling which presented coarse polygonal ferrite with pearlite. Based on the data fluctuation of Charpy tests, the normal distribution provided a statistical analysis method for assessing the reliability. Through the statistical analysis of the distribution function, it can be concluded that normalization step is necessary for higher reliability. Both batch cooling and conveyor cooling did not give the required reliability level for safety components due to heterogeneities in the microstructure.
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    Experimental evaluation and FE simulation of phase transformations and tensile stresses in hot forging and controlled cooling
    Hocalar, C; Saklakoglu, N; Demirok, S
    This paper encompasses the development of a microstructure-based numerical model (FEM) of the conveyor cooling process after the hot forging of industrial steel with accu-rate predictions of the volume fraction of phases as yield and tensile strengths. An experi-mental procedure for validating the FEM was conducted using optical and scanning electron microscopy and tensile tests. Results showed very good agreement between the phase predic-tions of the 3D FEM model and those obtained from direct measurement of forged parts, with an average error of about 3.6 and 6.9 % for ferrite and pearlite phases, respectively. Tensile test results were evaluated at a 90 % reliability level, and very good agreements were obtained with an error of about 3 and 5 % for the yield and tensile strengths. The methodology could predict the phase transformations, and the mechanical properties during cooling after the hot forging of the steel were investigated.