• Proceedings of the KWS Conference
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• 2002.10a
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• pp.408-411
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• 2002

Effect of in-situ vibration on the properties of A-grade steel SMA weldment has been investigated. Welding was performed on the steel fixed at the experimental jig under the mechanical vibration of a given frequency. The applied frequency varied from 39 to 43.5 Hz (harmonic frequency). For weldments formed under the vibration with a sub-harmonic frequency, both the columnar width of the weld metal and the prior austenite grain size of the HAZ near the fusion line clearly decreased. This indicates that the vibration increase the cooling rate after welding. Vibration effect was also found at the weld metal formed at the center region of the weldment. The weld metal showed liner microstructure both in columnar zone and in equiaxed zone with thinner grain boundary ferrite. However mechanical properties of the weld metal did not exactly follow the microstructural changes developed under the vibration. The weld metal formed under the vibration revealed higher yield and tensile strength but lower ductility and impact toughness, compared with the conventional weld metal.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.266-272
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• 2002

This study reviews the factors controlling the weld metal cracking and shows the difference from those of HAZ cracking. It further reviews the recent progresses made in consumable design for improving the crack resistance in the high strength weld metal. Previously the controlling factors for weld metal cracking were regarded as weld metal strength, diffusible hydrogen and weld metal height. However an overall review presented in this article shows that the cold crack resistance can be improve significantly through the microstructural control and that an increase in tensile strength is not necessarily related to a decrease in the resistance to cold cracking.

• Journal of Welding and Joining
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• v.34 no.5
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• pp.47-53
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• 2016

The aim of this study is to consider effect of weld elastic modulus on simulations of stress concentration and fatigue life for pressure vessel. The investigations include analysis with ADINA and WINLIFE softwares for whole body model about using condition of the boiler vessel. Values of weld elastic modulus were divided by 5 steps in butt weld area of the boiler vessel body. The stress concentration of the butt weld more was increased in case of higher elastic modulus of weld area because of higher difference of material properties between matrix and weld. It was concluded that the fatigue lives were decreased along increasing stress concentration due to high elastic modulus of weld. The matrix microstructure was estimated as pearlitic structure of ${\alpha}$ ferrite and pearlite. And the microstructures of welds along 5 steps of elastic modulus were estimated as bainitic fine pearlite and martensite as increasing elastic modulus.

• Proceedings of the KWS Conference
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• 2010.05a
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• pp.70-70
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• 2010

In the present study, to estimate the mechanical properties of 800 MPa grade weld metal, welding was carried out using 800 and 600 MPa grade flux cored arc welding (FCAW) consumable and characteristics of the weld metals were investigated. The chemical composition of weld metals was investigated by an optical emission spectroscopy (OES) method. The microstructure of weld metals was analyzed by optical microscopy (OM) and secondary electron microscopy (SEM). The compositions and sizes of inclusions which are the dominant factors for the nuclei of acicular ferrite were analyzed by an transmission electron microscopy (TEM). In addition, mechanical properties of the weld metals were evaluated through tensile tests and charpy impact tests. Mostly the acicular ferrite phase which has high strength and toughness was observed. The 600 MPa grade weld metal was consisted of 75% acicular ferrite and 25% ferrite which was formed at high temperature (grain boundary ferrite, widmanstatten ferrite, polygonal ferrite). However, the 800 MPa grade weld metal was composed of about 73% acicular ferrite and 27% low temperature phase (bainite, martensite). Toughness was considerably decreased due to the increase of tensile strength (from 600 MPa to 800 MPa). The sizes of inclusions which were observed in both weld metal were $0.4{\sim}0.8\;{\mu}m$, it is effective size to form acicular ferrite.

• Proceedings of the KWS Conference
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• v.43
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• pp.123-124
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• 2004

This study was investigated on the impact toughness and microstructure of welded metal and heat affected zone for Hi Nitrogen TiN Steel. With welding procedures, welding heat input applied were 30, 79 and 264 kJ/cm. TiN steel has shown very small prior austenite grain size for all the welding heat input applied, which was considered to result from the effect of TiN particles. In case of single SAW and EGW welding, the dilution rate of base metal into the weld was not high, resulting that there were no significant effects of base metal chemical composition on the mechanical properties of welds. However, TSAW with double Ypreparation carried very high dilution rate so that TiN steel has impaired the toughness of weld metal because N content in the weld was increased through the dilution of base metal.

• Journal of Power System Engineering
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• v.15 no.1
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• pp.45-50
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• 2011

The effect of Mn and Ti contents in filler wire on the microstructure and mechanical property of weld metal has been investigated after hybrid welding with ultra fine grained (UFG) steel. The microstructure and distribution of alloy compositions at the top region of weld zone were quite different with those at the bottom region after hybrid welding. The bottom region of weld zone contained higher Mn and Ti contents, and consequently the hardness of bottom region was higher than that of top region. With the increase of Mn and Ti contents in filler wire, the volume percent of acicular ferrite in weld metal decreased, and the weld zone showed higher hardness and better impact property.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.59-64
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• 2002

Effects of the aging treatments on the microstructure and strength of heat affected zone(HAZ) in the welds of a age-hardened Al-Mg-Si alloy, 5N01-T5, were investigated. The base metal aging treatments before MIG welding were conducted at 423K to 473K for 28.8ks Post weld heat treatment(PWHT) to recover the HAZ strength was performed at 448K for 28.8ks. Microstructure observations, hardness measurements and tensile tests were conducted to study properties of the MIG weld joints. The position of the softest region in HAZ where the hardness insufficiently recovered after natural aging and PWHT was at a distance of approximately 15mm from the center of the fusion zone. Hardness of the softest regions after natural aging and PWHT decreased with increase in the base metal aging temperature. TEM observation clarified that strengthening ${\beta}$"(Mg$_2$Si) precipitates and coarse ${\beta}$′ precipitates affected the hardnes of HAZ. Incomplete recover of hardness in HAZ after PWHT was caused by the precipitating of non-hardening ${\beta}$′ phase during the weld thermal cycle. In order to examine the effects of weldheat input and welding speed, the laser weld joints were also investigated and compared with the MIG weld ones. Laser welding had the narrower width of the softened regions in HAZ compared with MIG welding. The hardness of the softest regions of the laser welds after PWHT was higher than that of the MIG welds. Quantitative relations between hardness of the softest region and base metal aging temperature were obtained for both welding processes. Accordingly, the equations to estimate the strength of the weld joints after PWHT with varying base metal temperatures were proposed for MIG welding and laser welding.

• Journal of Welding and Joining
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• v.2 no.1
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• pp.4-17
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• 1984

Overlaid weld metals of austenitic stainless steel in a pressure vessel of power reactor are usually post-weld heated for a long period of time after welding. The PWHT is considered as a kind of sensitizing and it is important to check the soundness of the weld metal after PWHT, especially about the precipitation of carbides. The purpose of this report is to obtain information on the relation between the change of microstructure and Post-Weld Heat Treatment in the overlaid weld metals. Metallurgical aspects of the problem on austenitic stainless steel heated at $625^{\circ}C$, $670^{\circ}C$, $720^{\circ}C$ and $760^{\circ}C$ for 3, 10, 30, 100 and 300 hours have been investigated by means of optical-micrography, micro-hardness measurement, scanning electron microscope and electron-probe micro analysis. From the results obtained, the following conclusions are drawn; 1) The PWHT above $625^{\circ}C$ for a long time causes a diffusion of carbon atoms from low alloy steel into stainless steel, and consequently carbon is highly concentrated at the boundary layer of stainless steel. 2) C in ferritic steel migrated to austenitic steel and carbides precipitated in austenitic steel along fusion line. At higher temperatures, the ferrite grains coarsened in the decarburized zone. 3) In the change of microstructure of stainless steel overlaid weld metal, the width of carbides precipitated zone and decarburized zone increased with increase of PWHT temperature and time. 4) At about $625^{\circ}C$ to $760^{\circ}C$, chromium carbides, mainly $M_{23} C_6$, precipitate very closely in the carburized layer with remarkable hardening. 5) Precipitation of delta ferrite from molten weld metal depends on solidification phenomenon. There was a small of ferrite near the bond in which the local solidification time was short, comparing with after parts of weld metal. Shape and amount of ferrite were not changed by Post-Weld Heat Treatment after solidification.

• Journal of Welding and Joining
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• v.14 no.6
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• pp.93-100
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• 1996

Effect of a flux composition on weld metal toughness in submerged arc welding with 60kgf/$\textrm{mm}^2$ grade C-Mo type wires was investigated and interpreted in terms of weld metal microstructure and hardenability. Flux workability was also studied by characterizing a weld bead profile. Compared to other weld metals, .weld metal used alumina basic flux with nickel showed lowest oxygen content, highest hardenability and the most acicular ferrite. The highest impact toughness of that weld metal, however, was attributed to the tough matrix due to the nickel rather than to the larger amount of acicular ferrite. Manganese silicate flux had better workability than alumina basic flux, showing broader welding conditions resulting in a depth-to-width ratio of 0.5. The composition of oxides in the weld metal was dependent on the flux composition, showing MnO-SiO$_2$-TiO in manganese silicate flux and MnO-SiO$_2$-Al$_2$O$_3$-TiO in alumina basic flux. MnO-SiO$_2$composition in both oxides was similar to a tephroite.

• International Journal of Advanced Culture Technology
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• v.3 no.1
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• pp.169-178
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• 2015

This research aims to study the effects of various welding parameters in gas metal arc welding (GMAW) process on welding penetration, microstructure and hardness of AS3578-A350 high strength steel with the thickness of 10 mm. The welding process parameters were a welding current of 100-200A, an arc voltage of 20-30V, a welding speed of 20-60 cm/min and a gas shielding type of Ar and $Ar+CO_2$. The summarized experimental results are as follows. An increase of the welding current and voltage affected to increase the penetration depth of the joint. However, when the welding speed was decreased, it increased the penetration depth of the joint. Using the Ar gas for shielding the weld area, produced the higher penetration depth and the less narrow weld bead than the joint that was shielded by the mix gas of $Ar+CO_2$. The variation of the welding process parameters affected to produce the various microstructures of weld metal and heat affected zone and also showed the various kind of hardness along the weld joint.