Phase Transformation of δ→σ in Multipass Heat-Affected and Fusion Zones of Dissimilar Stainless Steels

Chih-Chun Hsi,Weite Wu 대한금속·재료학회 2011 METALS AND MATERIALS International Vol.17 No.3

The purpose of this report is to discuss the phase transformation from δ-ferrite to σ phase in various welding regions of dissimilar stainless steel welds. Experimental results indicated the grain refinement was very obvious in the second and the third pass fusion zones (304-1 and 304-2) when the welding pass increased from 1 to 3. This refining phenomenon of δ-ferrite was attributed to the recrystallization. The order of morphological development of δ-ferrite in the multipass fusion zones was as follows: massive→columnar→equiaxed. The σphase precipitated in the third pass fusion zone (304-2) and its solidification order is L→δ+L→δ→δ+γ→δ+σ+γwith a composition of σ phase (60 wt.%Fe-35.7 wt.%Cr-4.2 wt.%Ni) of the third pass fusion zone. The precipitation mechanism of the σ phase was as a eutectoid decomposition of δ→σ+γ2.

Microstructural Characterization of Al/Mg Alloy Interdiffusion Mechanism during Accumulative Roll Bonding

( Ming-Che Chen ),( Chih-Chun Hsieh ),( Weite Wu ) 대한금속재료학회 ( 구 대한금속학회 ) 2007 METALS AND MATERIALS International Vol.13 No.3

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.

Effect of Vibration on Microstructures and Mechanical Properties of 304 Stainless Steel GTA Welds

Chih-Chun Hsieh,Chien-Hong Lai,Weite Wu 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.4

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).

Growth of Intermetallic Phases in Al/Cu Composites at Various Annealing Temperatures During the ARB Process

Chih-Chun Hsieh,Ming-Shou Shi,Weite Wu 대한금속·재료학회 2012 METALS AND MATERIALS International Vol.18 No.1

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.

Effects of Vanadium Content on the Microstructure and Dry Sand Abrasive Wear of a Eutectic Cr-Fe-C Hardfacing Alloy

Hsuan-Han Lai,Chih-Chun Hsieh,Chi-Ming Lin,Weite Wu 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.1

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.

Dendrite Evolution of Delta (δ) Ferrite and Precipitation Behavior of Sigma (σ) Phase during Multipass Dissimilar Stainless Steels Welding

Chih-Chun Hsieh,Xiaofei Guo,Chia-Ming Chang,Weite Wu 대한금속·재료학회 2010 METALS AND MATERIALS International Vol.16 No.3

The purpose of this study is to discuss the development of dendrite δ-ferrite and the precipitation behavior of σ phase in the overlapping heat-affected zones, or the fusion zones, during the multipass dissimilar stainless steels welding process. The color metallographic and color separation techniques were used to clearly identify the δ, σ, and γ phases. The experimental results indicated that the grain refinement was very obvious in the second and third pass fusion zones (304-1 and 304-2) when the welding pass was increased from 1 to 3. This refining phenomenon was attributed to the higher cooling rate and the degree of supercooling. The δ phase precipitated in the secondary and the tertiary dendrite arms of the δ-ferrite and it was able to grow on the dendrite arms along the [111] crystallographic in a 45o angle. The σ phase was also able to form stably after the phase decomposition of δ-ferrite into globular and lacy morphologies.

Microstructural Development of Brass Alloys with Various Bi and Pb Additions

Chih-Chun Hsieh,Jia-Siang Wang,Paxon Ti-Yuan Wu,Weite Wu 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.6

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.

Lattice Relations and Solidification of the Complex Regular Eutectic (Cr,Fe)-(Cr,Fe)23C6

Hsuan-Han Lai,Chih-Chun Hsieh,Chi-Ming Lin,Weite Wu 대한금속·재료학회 2017 METALS AND MATERIALS International Vol.23 No.3

The eutectic (Cr,Fe)-(Cr,Fe)23C6 showed a triaxial fishbone structure and could be categorized as a “complexregular structure”. In this study, the lattice relations of the fishbone (Cr,Fe)23C6 were examined and the solidificationprocess was observed using a transmission electron microscope and a confocal laser scanning microscope. Forone of the three fish bones in a eutectic cell, parallel (Cr,Fe)23C6 lamellas at one side of the spine had the samelattice direction, as did those in the (Cr,Fe) phase. The lattices of neighboring (Cr,Fe)23C6 and (Cr,Fe) phases werenot coherent. Lamellar (Cr,Fe)23C6 on opposite sides of a spine had different lattice directions, and their latticeboundary was in the spine. By using the confocal laser scanning microscope, the solidification of lamellar eutecticstructure could be observed. At the low cooling rate of 5oC·min-1, parallel lamellas would grow thick blocksinstead of thin plates. To obtain a thin lamellar eutectic structure, the cooling rate should be higher, likethe rate in welding.

Microstructural Evolution with Various Ti Contents in Fe-Based Hardfacing Alloys Using a GTAW Technique

Chih-Chun Hsieh,Yi-Chia Liu,Jia-Siang Wang,Weite Wu 대한금속·재료학회 2014 METALS AND MATERIALS International Vol.20 No.4

The aim of this study is to discuss the effect of microstructural development with different Ti contents inFe-based hardfacing alloys. A series of Fe-Cr-C-Si-Mn-xTi alloy fillers was deposited on SS400 low carbonsteel substrate using oscillating gas tungsten arc welding. The microstructure in the Fe-based hardfacing alloywithout Ti content addition included: the primary γ, eutectic γ+(Fe,Cr)3C, eutectic γ+(Fe,Cr)2C and martensite. With increasing Ti contents, the microstructures showed the primary TiC carbide, γ phase and eutectic+(Fe,Cr,Ti)3C. The amount and size of TiC carbide in the hardfacing layers increased as the Ti contentincreased. However, the eutectic γ+(Fe,Cr,Ti)3C content decreased as the Ti content increased. Accordingto the results of the hardness test, the lowest hardness value (HRC 54.93) was found with 0% wt% Ti andthe highest hardness (HRC 60.29) was observed with 4.87 wt% Ti.

Microstructure and Abrasive Wear Properties of Fe-Cr-C Hardfacing Alloy Cladding Manufactured by Gas Tungsten Arc Welding (GTAW)

Jie-Hao Chen,Chih-Chun Hsieh,Pei-Shing Hua,Chia-Ming Chang,Chi-Ming Lin,Paxon Ti-Yuan Wu,Weite Wu 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.1

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 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.