Vitoon Uthaisangsuk

Investigation of Hot Deformation Behavior of Duplex Stainless Steel Grade 2507

Recently, duplex stainless steels (DSSs) are being increasingly employed in chemical, petro-chemical, nuclear, and energy industries due to the excellent combination of high strength and corrosion resistance. Better understanding of deformation behavior and microstructure evolution of the material under hot working process is significant for achieving desired mechanical properties. In this work, plastic flow curves and microstructure development of the DSS grade 2507 were investigated. Cylindrical specimens were subjected to hot compression tests for different elevated temperatures and strain rates by a deformation dilatometer. It was found that stress–strain responses of the examined steel strongly depended on the forming rate and temperature. The flow stresses increased with higher strain rates and lower temperatures. Subsequently, predictions of the obtained stress–strain curves were done according to the Zener–Hollomon equation. Determination of material parameters for the constitutive model was presented. It was shown that the calculated flow curves agreed well with the experimental results. Additionally, metallographic examinations of hot compressed samples were performed by optical microscope using color tint etching. Area based phase fractions of the existing phases were determined for each forming condition. Hardness of the specimens was measured and discussed with the resulted microstructures. The proposed flow stress model can be used to design and optimize manufacturing process at elevated temperatures for the DSS.

Effects of Isothermal Aging on Microstructure Evolution of Biomedical Co-Cr-Mo Alloy

Cobalt based alloys have been widely used in orthopedic implants. These alloys are an allotropic metal, which commonly exhibits two crystal structures, namely, FCC and HCP lattice. In this work, developed microstructure and hardness of a Co-Cr-Mo alloy after isothermal aging treatment were investigated. The applied aging procedure included soaking at the temperature of 850°C for five different holding times of 1, 3, 6, 9 and 12 h with subsequent water quenching. Microstructure examination, X-ray diffraction analysis and micro-hardness test were carried out for both as-received and heat-treated cobalt based alloys. The results showed that the FCC to HCP phase transformation occurred during the isothermal aging. It was observed that phase fraction of the identified HCP phase increased with longer aging time. Microstructure of the samples aged for 12 h showed very fine lamellae morphologies similar to a pearlitic structure with different orientations within each FCC grain. Apparently, these occurred lamellae structures could be well correlated with the formation of the HCP martensite. Additionally, it was found that in the Co-Cr-Mo alloy sigma phase precipitated early at the grain boundaries and further grew along these boundaries by increasing aging time. The hardness value of the examined alloy slightly increased with larger HCP phase fraction. The increased aging time certainly led to higher amount of the HCP martensite and consequently increased hardness and possible wear resistance properties.

Simplified identification of material parameters for Yoshida-Uemori kinematic hardening model

In sheet metal forming process of Advanced High Strength (AHS) steels, springback effect is one of the most critical problems for manufacturer. The springback of a formed part occurs due to residual stress released after deformation. FE simulations were often used to describe both forming and springback behavior of steel sheets. Recently, the Yoshida- Uemori (Y-U) kinematic hardening model has been successfully applied for the springback simulation. The model is capable of reproducing the transient Bauschinger effect, permanent softening and work hardening stagnation during a large deformation. In this work, method for determining materials parameter of the Y-U model was briefly presented. Initially, cyclic tests were performed under both tension and compression loads for the high strength steel grade JSC780Y and JSC980Y. FE simulations of 1-element model were carried in order to investigate predicted cyclic stress strain curves. Both Y-U model and a mixed isotropic-kinematic Barlat2000 model were used in the simulations. Stamping tests of hat shape sample were carried out for verifying the experimental and numerical results. It was found that the Y-U model provided more accurate springback results than the other model.

Investigation of Hot Deformation Characteristics of AISI 4340 Steel Using Processing Map

Uniaxial compression tests at various temperatures from 850°C to 1200°C and strain rates between 0.01 s-1 and 10 s-1 were carried out in order to determine hot working characteristic of the AISI 4340 steel. The plastic stress-strain responses at high temperatures of the steel were provided. Constitutive relationship between the flow stresses and the Zener–Hollomon parameters was primarily established by means of a hyperbolic sine function for the entire range of the investigated conditions. Afterwards, the power dissipation map and instability map were developed on the basis of the Dynamic Materials Model (DMM). The variation of efficiency of the power dissipation calculated as a function of strain rate sensitivity represented material behaviors according to the microstructure evolution. The peak efficiency indicated an optimum processing window for hot working. In this study, processing map was obtained by a superimposition of the power dissipation and the instability criterion. The domains of temperature and strain rate, in which material flow stability occurred, were determined. For the AISI 4340 steel, the processing maps exhibited a distinct domain with its peak efficiency at about 1050-1200°C and 0.01-0.1 s-1, in which the peak efficiencies of about 40-50% were shown for different strains. In combination with microstructure observations after hot deformation, dynamic recrystallization zone could be identified in the processing map at a certain strain.

Determination of Damage Criterion Using a Hybrid Analysis for Advanced High Strength Steel

Advanced High Strength (AHS) steels have been increasingly applied in the automotive industries due to their distinguished mechanical properties. Microstructures of these steels play an important role and are designed by constituent phases with distinct characteristics. AHS steels exhibit sophisticated damage mechanisms that complicate the prediction of material formability. In this work, Ductile Crack Initiation Locus (DCIL) was developed for describing failure behavior of dual phase steel sheet. A hybrid experimental and numerical analysis was used to determine the DCIL. Tensile tests of various sample geometries were experimentally carried out and crack initiation occurred during forming was identified by the Direct Current Potential Drop (DCPD) method. Then, FE simulations of the corresponding tests were performed to evaluate local stress triaxialities and equivalent plastic strains of the critical area. The damage curves for both crack initiation and localized necking were obtained. Additionally, the von Mises, Hill48 and Yld2000-2d yield criterion were defined in the calculations in order to examine effect of yield model on the resulted curves. To verify applicability of the damage curves, Nakazima test of uniaxial sample was taken into account.

Anisotropic Plastic Behavior of TRIP 780 Steel Sheet in Hole Expansion Test

Plastic behavior of advanced high strength steel sheet of grade TRIP780 (Transformation Induced Plasticity) was investigated using three different yield functions, namely, the von Mises’s isotropic, Hill’s anisotropic (Hill’48), and Barlat’s anisotropic (Yld2000-2d) criterion. Uniaxial tensile and balanced biaxial test were conducted for the examined steel in order to characterize flow behavior and plastic anisotropy in different stress states. Additionally, disk compression test was performed for obtaining the balanced r-value. According to the different yield criteria, yield stresses and r-values were calculated for different directions and then compared with experimental data. To verify the modeling accuracy, a hole expansion test was carried out experimentally and numerically by FE simulation. Stress-strain curve from the biaxial test was described using voce and swift hardening models. Punch load and stroke, final hole radius, and strain distribution on specimen surface along the hole circumference and the specimen diameter in rolling and transverse directions were determined and compared with the experimental results. It was found that the simulations applying Yld2000-2d yield function provided an acceptable agreement. Consequently, it is noted that the anisotropic yield potential significantly affects the accuracy of the predicted deformation behavior of sheet metal subjected to hole expanding load.

Finite Element Modeling for Hot Forging Process of AISI 4340 Steel

Hot forging is a common, but important manufacturing process in metal forming industries. Forming at high temperature, flow stress of material decreases and thus lower load is required. Furthermore, mechanical properties of material regarding its microstructure characteristics can be improved according to heating and cooling conditions. Hot forging process for a spacer wheel made of steel AISI 4340 was investigated by means of FE simulation. Initially, hot compression test was performed using a thermo-mechanical simulator for determining flow curves. Different temperatures and strain rates within the process range were considered. A Zener-Hollomon parameter was applied to describe the experimentally obtained flow curves. To verify accuracy of the FE simulations, multi-step hot compression test was carried out at different forming temperatures. Subsequently, force development and final shape calculated for the investigated part were analyzed. Reliability of the FE results was significantly influenced by the defined flow curve or hot deformation behavior of material.

Tension-Compression Tests for Springback Prediction of AHS Steel Using the Yoshida-Uemori Model

In sheet metal formingprocess of automotive parts, springback effect is crucial, in particular, foradvanced high strength (AHS) steels. Most structural components of new vehiclesshow very complex shapes that require multi–step forming procedures.Therefore, finite element (FE)simulation has been often used to describe plasticdeformation behavior and springback occurrence of formed metal sheets.Recently, the kinematic hardening Yoshida–Uemorimodel has showed great capability for predicting elastic recovery of material. In this work, the AHSsteel grade JSC780Y wasinvestigated, in which tension–compressiontests were carried out. From resulted cyclic stress–strainresponses, material parameters were identified using different fitting methods.Determined model parameters were firstly verified by using simulations of 1–elementmodel. The most appropriate parameter set was thenobtained. Finally, a Hat-Shape forming test was performed and springback waspredicted and compared with experimental results.

Determination of Stress-Strain Curve of Dual Phase Steel by Nanoindentation Technique

The advanced high strength (AHS) steels, for example, dual phase (DP) steels, transformation induced plasticity (TRIP) steels and complex (CP) steels principally exhibit multiphase microstructure features. Thus, mechanical behavior of the constituent phases significantly affects the resulting overall properties of such AHS steels. Novel material characterization techniques on micro- and nano-scale have become greatly more important. In this work, stress-strain response of the DP steel grade 1000 was determined by using the Nanoindentation testing. The DP steel showed the microstructure containing finely distributed martensite islands of about 50% phase fraction in the ferritic matrix. The nano-hardness measurements were firstly performed on each individual phase of the examined steel. In parallel, finite element (FE) simulations of the corresponding nano-indentation tests were carried out. Flow curves of the single ferritic and martensitic phases were defined according to a dislocation based theory. Afterwards, the load and penetration depth curves resulted from the experiments and simulations were compared. By this manner, the proper stress-strain responses of both phases were identified and verified. Finally, the effective stress-strain curve of the investigated DP steel could be determined by using 2D representative volume element (RVE) model.

Fe-Sn Intermetallics Synthesized via Mechanical Alloying-Sintering and Mechanical Alloying-Thermal Spraying

Iron (Fe)-tin (Sn) intermetallics were synthesized by using two different routes. These two routes had a common synthesis step, in which Fe powder (19 wt. %) was mechanically alloyed with Sn powder (81 wt. %) for 25 h under argon atmosphere. The mechanically alloyed powders were then treated with different heating routes. In the first route, the compacts of the mechanically alloyed powders were sintered at different temperatures for different times. It was found that the FeSn2 content increased with increasing temperature and time. There were small traces of Fe, Sn and FeSn found in the sintered materials. In the second route, the mechanically alloyed powders were plasma-sprayed using different currents of 300, 400 and 500 A. It was found that the porous coatings produced by plasma sprayng consisted of mixed Fe, Sn, FeSn2, SnO, FeO and Fe3O4 for all employed currents.