Mohd Kaswandee Razali

An investigation of austenite grain growth (AGG) during the isothermal heat treatment of low-alloy steel is conducted. The goal is to uncover the effect of time, temperature, and initial grain size on SA508-III steel grain growth. Understanding this relationship enables the optimization of the time and temperature of the heat treatment to achieve the desired grain size in the studied steel. A modified Arrhenius model is used to model austenite grain size (AGS) growth distributions. With this model, it is possible to predict how grain size will change depending on heat treatment conditions. Then, the generalized reduced gradient (GRG) optimization method is employed under adiabatic conditions to characterize the model’s parameters, providing a more precise solution than traditional methods. With optimal model parameters, predicted AGS agree well with measured values. The model shows that AGS increases faster as temperature and time increase. Similarly, grain size grows directly in proportion to the initial grain size. The optimized parameters are then applied to a practical case study with a similar specimen size and material properties, demonstrating that our approach can efficiently and accurately predict AGS growth via GRG optimization.

Traditionally, simple flow stress laws including Hollomon and Swift models have been much employed. However, the simple models or laws could cover only limited part of materials including cold forgeable steels because they could be appropriately described by the models. Nowadays, special metals and new materials are being widely employed and new material models are needed for their accurate description of flow stress. In this paper, flow stress models of metallic materials at room temperature are reviewed and the tensile test is recovered with an emphasis on the post-necking strain hardening of material models. The necking point, consuming two degrees of freedom, is emphasized in characterizing the material constants because plastic deformation after it is very sensitive to itself. The well-known material models are first grouped into three categories such as Ludwik involving Hollomon, Voce, and Swift flow stress models. These three fundamental flow stress models are investigated in terms of tensile test to reveal their post-necking strain hardening behaviors using purposely the rigid-plastic finite element method, which predicts the necking point exactly in the engineering sense without any imperfection of cylindrical specimen.

In this study, plastic flow behaviors of aluminum alloys at elevated temperatures are studied to establish the engineering foundation of hot metal forming of aluminum alloys. Various material models of characterizing the flow behaviors for metal forming application are introduced and they were evaluated, comparing the experimental flow curves with the fitted flow curves (of Arrhenius model, improved Johnson-Cook model and Hensel-Spittel model and its variant) found from the related literature together with flow curves of the piecewise bilinear C-m model (power law model), the Ebrahimi et al. model and improved Ebrahimi et al. model (called PLF model). It has been shown that the accuracy of flow stress modelling is much dependent on the number of material constants and that the piecewise bilinear C-m model is the most attractive in terms of accuracy as well as practicability.

We develop a new flow model based on the Swift method, which is both versatile and accurate when used to describe flow stress in terms of strain hardening and damage softening. A practical issue associated with flow stress at room temperature is discussed in terms of tensile testing of a cylindrical specimen; we deal with both material identification and finite element predictions. The flow model has four major components, namely the stress before, at, and after the necking point and around fracture point. The Swift model has the drawback that not all major points of stress can be covered simultaneously. A term of strain to the third or fourth power (the “second strain hardening exponent”), multiplied and thus controlled by a second strain hardening parameter, can be neglected at small strains. Any effect of the second strain hardening exponent on the identification of the necking point is thus negligible. We use this term to enhance the flexibility and accuracy of our new flow model, which naturally couples flow stress with damage using the same hardening constant as a function of damage. The hardening constant becomes negative when damage exceeds a critical value that causes a drastic drop in flow stress.

SUS304 stainless steel is characterized by combined tensile and compression testing, with an emphasis on flow stress at higher strain and temperature. The plastic deformation behavior of SUS304 from room temperature to 400 °C is examined and a general approach is used to express flow stress as a closed-form function of strain, strain rate, and temperature; this is optimal when the strain is high, especially during automatic multi-stage cold forging. The fitted flow stress is subjected to elastothermoviscoplastic finite element analysis (FEA) of an automatic multi-stage cold forging process for an SUS304 ball-stud. The importance of the thermal effect during cold forging, in terms of high material strength and good strain-hardening, is revealed by comparing the forming load, die wear and die stress predictions of non-isothermal and isothermal FEAs. The experiments have shown that the predictions of isothermal FEA are not feasible because of the high predicted effective stress on the weakest part of the die.