Chih-Chun Hsieh

Overview of Intermetallic Sigma () Phase Precipitation in Stainless Steels

The 𝜎 phase which exists in various series of stainless steels is a significant subject in steels science and engineering. The precipitation of the 𝜎 phase is also a widely discussed aspect of the science and technology of stainless steels. The microstructural variation, precipitation mechanism, prediction method, and effects of properties of 𝜎 phase are also of importance in academic discussions. In the first section, a brief introduction to the development and the precipitation characteristics (including morphologies and precipitation sites) of 𝜎 phase in stainless steels is presented. In the second section, the properties effect, prediction method, processing effect, elemental addition, retardation method and Thermo-Calc simulation of the 𝜎 phase in stainless steels are highlighted.

Microstructural characterization of Al/Mg alloy interdiffusion mechanism during accumulative roll bonding

In this study, the Accumulative Roll Bonding (ARB) process was used with a snap-stack procedure to reduplicate an Al (1100)/Mg (AZ31) alloy. Samples underwent four rolling and stacking cycles four times, which produced a 24-layer structure. The ARB process creates a multilayer compound between Al/Mg layers with excellent bonding characteristics. The excellent bonding characteristics were due to atomic diffusion. Diffusion couples between Al and Mg were investigated to study the composition of the formation in the Al−Mg system. Layers of intermetallic compound (IMC) Al3Mg2 and Al12Mg17 were observed. The composition-depth curves of the diffusion zone were determined by electron microprobe analyses of the IMCs.

Growth of intermetallic phases in Al/Cu composites at various annealing temperatures during the ARB process

The purpose of this study is to discuss the effect of annealing temperatures on growth of intermetallic phases in Al/Cu composites during the accumulative roll bonding (ARB) process. Pure Al (AA1100) and pure Cu (C11000) were stacked into layered structures at 8 cycles as annealed at 300 °C and 400 °C using the ARB technique. Microstructural results indicate that the necking of layered structures occur after 300 °C annealing. Intermetallic phases grow and form a smashed morphology of Al and Cu when annealed at 400 °C. From the XRD and EDS analysis results, the intermetallic phases of Al2Cu (θ) and Al4Cu9 (γ2) formed over 6 cycles and the AlCu (η2) precipitated at 8 cycles after 300 °C annealing. Three phases (Al2Cu (θ), Al4Cu9 (γ2), and AlCu (η2)) were formed over 2 cycles after 400 °C annealing.

Effect of vibration on microstructures and mechanical properties of 304 stainless steel GTA welds

This study investigates the microstructures and mechanical properties of 304 stainless steel at various vibration frequencies during simultaneous vibration welding. The experimental results demonstrated that simultaneous vibration welding could accelerate the nucleation and grain refinement of the microstructures. The effect of the grain refinement was more evident at the resonant frequency (375 Hz) and a minimum content of residual δ-ferrite (4.0%). The γ phase grew in the preferential orientation of the (111) direction with and without vibration. The full width at half maximum of the diffraction peak widened after the vibration, which was attributed to the grain refinement. The residual stress could be efficiently removed through simultaneous vibration welding when the amplitude of the vibration was increased. Furthermore, the lowest residual stress (139 MPa) was found when the vibration frequency was 375 Hz. The hardness and Young’s modulus exhibited slight increases with low and medium frequencies. The hardness values were increased by 7.6% and Young’s modulus was increased by 15% when the vibration frequency was resonant (375 Hz).

Dispersion strengthening behavior of σ phase in 304 modified stainless steels during 1073 K hot rolling

The dispersion strengthening behavior of the σ phase in 304 modified stainless steel as hot-rolled at 1073 K has been investigated in this study. The morphology, quantity and chemical composition of the δ phase were analyzed using optical microscopy (OM), X-ray diffractometry (XRD), ferritscope (FS), and image analysis (IA). The amounts of σ phase in the stainless steels increased gradually at 1073 K as the reduction ratio increased from 0% to 75%. The XRD analyses showed that a higher reduction ratio enhanced the conversion of δ-ferrite (110) to σ phase (542). The σ phase was precipitated homogeneously at the recrystallized ferrite grains when the reduction ratio was increased from 0% to 75%.

Evolution of microstructure and residual stress under various vibration modes in 304 stainless steel welds.

Simultaneous vibration welding of 304 stainless steel was carried out with an eccentric circulating vibrator and a magnetic telescopic vibrator at subresonant (362 Hz and 59.3 Hz) and resonant (376 Hz and 60.9 Hz) frequencies. The experimental results indicate that the temperature gradient can be increased, accelerating nucleation and causing grain refinement during this process. During simultaneous vibration welding primary δ-ferrite can be refined and the morphologies of retained δ-ferrite become discontinuous so that δ-ferrite contents decrease. The smallest content of δ-ferrite (5.5%) occurred using the eccentric circulating vibrator. The diffraction intensities decreased and the FWHM widened with both vibration and no vibration. A residual stress can obviously be increased, producing an excellent effect on stress relief at a resonant frequency. The stress relief effect with an eccentric circulating vibrator was better than that obtained using a magnetic telescopic vibrator.

Effects of vanadium content on the microstructure and dry sand abrasive wear of a eutectic Cr-Fe-C hardfacing alloy

In this study, the effects of vanadium on the morphology and wear behavior of a eutectic Cr-Fe-C hardfacing alloy were discussed. The alloys tested contained different amounts of vanadium, ranging from 0 to 2.39 wt%. A fibrous V4C3 was found when the alloy contained 0.93 wt% vanadium. The addition of vanadium was found to decrease the fraction of eutectic M23C6 and increase the width of the interspaces between the eutectic cells. The DTA results revealed that V4C3 formed just before the eutectic α+M23C6 during solidification. The surface hardness was shown to increase with increasing vanadium content, which also caused the hardness deviation and wear loss to decrease; however, the addition of vanadium was not shown to affect the hardness of eutectic α+M23C6. The V4C3 could be scratched off during the wear test due to the increase in the width of the interspaces between the eutectic cells; therefore, the alloys that contained 0.93 and 2.39 wt% vanadium exhibited similar wear loss results.

Microstructural development of brass alloys with various Bi and Pb additions

In the study, using the gravity casting method, adding 1.52%Pb, 0.5%Bi, 1%Bi and 1.5%Bi into the brass (Cu-40%Zn) alloy. The microstructural changes from the Widmanstätten into the networked structures when Pb was added to 1.5%. The microstructure was an acicular Widmanstätten when Bi contents were 0.5% and 1% and it was a plate Widmanstätten when Bi contents were 1.5%. There were four kinds of precipitation morphologies of Bi particles. The precipitation morphologies of Bi particles can be divided into a globular (<1 μm), a disc (=1 μm), discontinuous massive (>1 μm), and continuous block structures (about 20∼30 μm). The Pb particles were embedded in the networked α phase and the Bi particles precipitated at the α/α and the α/β′ grain boundaries. The XRD analysis showed the high proportion of β′ phase with 0.5% Bi-brass and 1% Bi-brass and indicated a lower one with Pb-brass and 1.5% Bi-brass.

Microstructure and abrasive wear properties of Fe-Cr-C hardfacing alloy cladding manufactured by Gas Tungsten Arc Welding (GTAW)

A series of Fe-Cr-C hardfacing alloys is deposited by gas tungsten arc welding and subjected to abrasive wear testing. Pure Fe with various amounts of CrC (Cr:C=4:1) powders are mixed as the fillers and used to deposit hardfacing alloys on low carbon steel. Depending on the various CrC additions to the alloy fillers, the claddings mainly contain hypoeutectic, near eutectic, or hypereutectic microstructures of austenite γ-Fe phase and (Cr,Fe)7C3 carbides on hardfacing alloys, respectively. When 30% CrC is added to the filler, the finest microstructure is achieved, which corresponds to the γ-Fe+(Cr,Fe)7C3 eutectic structure. With the addition of 35% and 40% CrC to the fillers, the results show that the cladding consists of the massive primary (Cr,Fe)7C3 as the reinforcing phase and interdendritic γ-Fe+(Cr,Fe)7C3 eutectics as the matrix. The (Cr,Fe)7C3 carbide-reinforced claddings have high hardness and excellent wear resistance under abrasive wear test conditions. Concerning the abrasive wear feature observable on the worn surface, the formation and fraction of massive primary (Cr,Fe)7C3 carbides predominates the wear resistance of hardfacing alloys. Abrasive particles result in continuous plastic grooves when the cladding has primary γ-Fe phase in a hypoeutectic structure.

The Effect of Oscillating Traverse Welding on Performance of Cr-Fe-C Hardfacing Alloys

In this study, a series of experiments involving Cr-Fe-C hardfacing alloys is conducted to evaluate the effect of oscillating traverse welding on microstructure and performance of clad alloys. The alloys are designed to exhibit hypoeutectic, eutectic, and hypereutectic morphology. The morphology of the heat-affected zone (HAZ) of the unmelted metal, the solidified remelted metal, and the fusion boundary exhibited distinct characteristics. In the hypoeutectic and the eutectic alloys, the same lamellar eutectic structure can be observed as the solidified structure, and they also showed the same evolution in the HAZ. In the hypereutectic alloy, the incomplete weld pool blending results in a eutectic morphology instead of a fully hypereutectic morphology. The hardness result reveals that, for the hypereutectic alloy, the eutectic region, instead of the HAZ, is the weak point. The wear test shows that the hypoeutectic alloy exhibits the same wear behaviors in both the remelted metal and the HAZ, and so is the hypereutectic alloy; the eutectic alloy remelted metal and the HAZ have different wear morphologies.