Correcting the Stress-Strain Curve in the Stroke-Rate Controlling Forging Process
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INTRODUCTION
THE forging process, carried out usually at temperatures higher than approximately two-thirds of the melting point of materials, is extensively used in manufacturing products, for improving the mechanical properties of materials, etc.[1–5] A fundamental method for investigating the working behavior of materials is by analyzing the true stress–true strain curves and by observing the microstructure of materials; this helps in understanding the intrinsic mechanical characteristics of materials, thereby optimizing the forging process. However, the friction and temperature rise of the specimen along with other factors have to be considered before further investigation.[6–9] In addition, the stroke-rate controlling process is generally carried out in industry. In this case, the strain rate will gradually increase with the strain level because of the decrease in the sample height. The effect of the strain rate on the stress-strain curve cannot be ignored, particularly in a large strain forging process. For a narrow range of the strain rate and temperature, it is experimentally observed that the flow stress during hot deformation may be related to the strain rate and temperature through an Arrheniustype rate equation:[10,11] Q r ¼ A_em exp ½1 RT where m denotes the strain-rate sensitivity; Q, the thermal activated energy; R, the gas constant; T, the experimental temperature; and A, the constant related to the characteristics of materials. In order to analyze the stress-strain
Y.P. LI, Researcher, H. MATSUMOTO, Assistant Professor, and A. CHIBA, Professor, are with Institute of Materials Research, Tohoku University, Sendai 980-8577, Japan. Contact e-mail: a.chiba@ imr.tohoku.ac.jp Manuscript submitted May 31, 2008. Article published online March 7, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
curves for further use in the stroke-rate controlling process, the correction into the strain-rate controlling process is also essential for most researchers and engineers. However, this correction is ignored in many studies. Therefore, in this research, the correction of the strokerate controlling process into the strain-rate controlling process will be interpreted in detail for the first time. In previous research, Li et al.[12] found that the instant friction coefficient in a hot forging process to large strain level is not a constant and follows the exponential growth equation ½2 ls ¼ l0 þ A expðe=e0 Þ where lS denotes the instant friction coefficient; l0, A, and e0 are determined by a fitting process. l0 + A is the initial friction coefficient, and it does not conform to the conventional viewpoint[13,14] that the friction coefficient as a function of the strain level is a constant because of the variations in conditions of the hot forging process. The interpretation of this theory has been described in detail, and the results have been shown to be in good agreement with the experimental results.[12] Processing maps, including both power dissipation and instability maps, are developed on the basis of the dynamic materia
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