Effect of Temperature and Strain Rate of The Hot Deformation of V Microalloyed Steel on Flow Stress

Effect of Temperature and Strain Rate of The Hot Deformation of V Microalloyed Steel on Flow Stress

Md Israr Equbal (Associate Professor, Department of Mechanical Engineering, JB Institute of Engineering & Technology, Moinabad, Hyderabad, India), Azhar Equbal (Assistant Professor, Department of Mechanical Engineering, RTC Institute of Technology, Anandi, Ormanjhi, Ranchi, India), Md. Asif Equbal (Assistant Professor, Department of Mechanical Engineering, Cambridge Institute of Technology, Tatisilwai, Ranchi, India) and R. K. Ohdar (Professor, Department of Forge Technology, National Institute of Foundry and Forge Technology, Hatia, Ranchi, India)
DOI: 10.4018/IJMFMP.2019010103

Abstract

Medium carbon micro-alloyed forging steels are employed in various automotive components. The impetus for the use of micro alloyed (MA) steels is cost reduction due to elimination of post-forging heat treatment. Compared to conventional quenched and tempered steels micro-alloyed steels can achieve similar or more superior properties simply by properly controlling the process parameters. Forging temperature, strain, strain rate and cooling rate are some of the important process parameters that influence the flow stress and final forging product quality. In the present study, hot compression test on a micro-alloyed steel grade 38MnSiVS5 were conducted on thermo-mechanical simulator (Gleeble-3500) to study the effect of temperature and strain rate on flow stress. The results indicate that the flow stress of 38MnSiVS5 steel is greatly affected by both deformation temperature and strain rate. Obtained true stress-true strain curves showed that the flow stress of the alloy increased by increasing the strain rate and decreasing the temperature, which can be represented in terms of an exponent type Zener-Hollomon equation. Finally, the constitutive equations for the flow behavior of 38MnSiVS5 microalloyed steel were determined.
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1. Introduction

Medium carbon micro-alloyed steels are considered for a wide array of engineering applications. Automobile components such as crank shafts, connecting rods, and wheel hubs are manufactured from these steels. The service performances and soundness of the components manufactured by these steels are largely depends on the applied thermo-mechanical processes and involved process parameters. A great amount of attention needs to be paid to the design of the thermo-mechanical processes by optimizing process variables to achieve a component that is defect free (Park, Yeom & Na, 2002). In metal forming processes such as rolling, forging, extrusion, wire drawing and sheet metal forming, the flow stress of the metal is a key factor for successful operation. Flow stress is defined as the stress required to sustain a certain plastic strain on the material. The forming pressure and the load are directly related to the flow stress, and therefore a lower flow stress is desirable in most of the forming operations. Factors such as strain, strain rate and temperature affect the flow stress of materials. The temperature plays an important role in forging of metals to attain optimum flow stress which in turn decreases the forging load and increases the die life. Alongside temperature strain rate also plays an essential role in the selection of forging process, as strain rate directly has an effect on the grain size and mechanical properties which ultimately affect the durability and fatigue life of the component. Because of the finite element method being extensively use, so as to characterize the true behaviour of work piece under various stages of metal forging, knowledge of the constitutive relationships relating process variables viz. strain rate and temperature to the flow stress of the deforming material is imperative, and hence it becomes important to evaluate the flow stress. Uni-axial hot compression test is the most common method employed for generating data and to study the flow behavior and workability of the materials (Goetz, Semiatin & Mater Eng, 2001). The generated data through uni-axial compression testing of cylindrical specimens are used to evaluate the constitutive flow behavior of materials (Lin & Liu, 2010) as well as to identify the optimum process parameter for thermo-mechanical processing (Lin, Chen & Zhong, 2008; Shi, McLaren, Selaars, Shahani & Bolingbroke, 1997; Takuda & Fujimoto, 1998). The present work involves uni-axial hot compression test of medium carbon vanadium micro-alloyed steel grade 38MnSiVS5 on Gleeble-Thermo Mechanical Simulator (TMS) to study the influence of forging temperature and strain rate on flow stress. The tests were carried out to analyze the effect of forging temperatures ranging from 900°C to 1100°C, with a 100°C interval. The strain rates were also varied in multiples of 10 from 0.2s-1 to 20 s-1 covering the appropriate processing range.

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