• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.632-638
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• 2002

Electron beam (EB) and Gas tungsten arc (TIG) welds were performed on 12.7 mm thick Ti-6Al-4V plate (ASTM Titanium Grade 5). Charpy-V toughness and hardness, as well as, microstructure of the welds and penetration from the macrostructure were studied. It appears that by EB welding rather smaller $\beta$-grains than with TIG welding can be obtained. Next to the fusion line the $\beta$-grain size in the HAZ was 50 ${\mu}{\textrm}{m}$l while in the weld metal it was 150 ${\mu}{\textrm}{m}$. Charpy-V toughness of the EB weld metal was equal or even better to that of base metal, which shows that the $\alpha$-martensite per se is not particularly brittle if only the grain size is fine enough. This is similar to behavior of low carbon martensite in steel. The grain size was studied with light optical and scanning electron (SEM) microscopes. Thus for products, for products which can be manufactured automatically with very narrow fit, the EB welding of Ti-6Al-4V appears to yield satisfactory toughness without any complex post weld heat treatment. ill this study as in earlier studies the TIG welds gave lower toughness than that of the base material due to the higher heat input and slower cooling as compared to EB welding.

• Journal of Welding and Joining
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• 제32권6호
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• pp.29-34
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• 2014

This paper has an purpose to study the effect of C on the toughness of YS 460 MPa FCAW weld metal. These effects were evaluated by charpy impact and CTOD test about 4 FCAW weld metal containing various C and Si content in relation to microstructure. Increase of C content was helpful to increase AF volume fraction and reduce PF(G) and FS volume fraction by increasing super cooling rate for ferrite transformation. Also, Increase of C content up to 0.045wt% made the strength and impact toughness higher by increasing AF volume fraction. The weld metal containing higher C content indicated higher CTOD value. It is because the volume fraction of PF(G) and FS, can play a role as crack initiation site, was reduced. Effect of C on the strength and elongation of weld metal was higher with an increase of Si contents.

• Journal of Welding and Joining
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• 제13권4호
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• pp.55-64
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• 1995

A study was made to examine the effects of postweld heat treatment(PWHT) on the toughness and microstructures in the weld heat affected zone(HAZ) of Cu-bearing HSLA-100 steel. The Gleeble thermal/mechanical simulator was used to simulate the weld HAZ. The details between toughness and PWHT of HAZ were studied by impact test, optical microscopy(O.M.), scanning electron microscopy (SEM), transmission electron microscopy(TEM) and differential scanning calorimetry(DSC). The decrease of HAZ toughness in single thermal cycle comparing to base plate is ascribed to the coarsed-grain formed by heating to 1350.deg.C. The increase of HAZ toughness in double thermal cycle comparine to single thermal cycle is due to the fine ferrite(.alpha.) grain transformed from austenite(.gamma.)formed by heating to .alpha./.gamma. two phase region. Cu precipitated during aging for increasing the strength of base metal is dissolved during single thermal cycle to 1350.deg.C and is precipitated little on cooling and heating during subsequent weld thermal cycle. It precipitates by introducing PWHT. Thus, the decrease of toughness in triple thermal cycle of $T_{p1}$ = 1350.deg.C, $T_{p2}$ = 800.deg.C and $T_{p3}$ = 500.deg.C does not occur owing to the precipitation of Cu. The behaviors of Cu=precipitates in HAZ is similar to that in base plate. PWHT at 550.deg.C shows highest hardness and lowest toughness, whereas PWHT at 650.deg.C shows reasonable toughness, which improves the toughness of as-welded state.state.

• 대한기계학회논문집
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• 제11권2호
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• pp.314-321
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• 1987

본 논문에서는 용접열영향부(heat affected zone: HAZ)의 인성에 영향을 미치 는 인자중에서 잔유응력의 영향은 중대한 문제로 주목되고 있으며, 이로 인한 용접재 의 기계적 성질의 저하를 경함하기 위한 방법으로 용접후열처리(post weld heat trea- tment: PWHT)에 의한 제거방법을 이용하고 있다.그러나 이 열처리는 보톤 600.deg. C 이 상의 고온에서 실현되기 때문에 열처리과정에서 템퍼링(tempering)효과로 인한 HAZ의 열화가 때때로 발생하고 있어서 용접 시공상 문제로 되고 있다. 또한 PWHT시에는 가 열중이나 냉각중에도 열처리효과가 진행되고 있다고 볼 수 있으므로, PWHT가열속도는 기계적 성질의 개선이라는 이유외에 경제적 측면에서도 중요시 된다. 특히 용접조직 의 포화는 파괴의 양식에 직접반영되어, 파면형태의 차로 나타나므로, 본 조직의 포화 와 관련시켜 검토하기 위하여 열처리된 HAZ를 대상으로 작용된 응력의 크기와 가열속 도가 HAZ의 파괴인성에 어떠한 영향을 미치는지를 COD파괴인성시험과 미소경도측정, 그리고 주사전자현미경관찰을 통하여 고찰하였다.

• Journal of Welding and Joining
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• 제13권4호
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• pp.46-54
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• 1995

The cold cracking susceptibility of a variety of weld metals deposited by GMAW with several kinds of commercial solid wires for high strength-toughness steel was investigated. G-BOP test and LB-TRC test were carried out to study the effects of preheat, chemical composition and hydrogen level on the weld metal cold cracking. The results obtained are as follows. 1) 10% CPT obtained by G-BOP test was the most valuable criteria for evaluating the cold cracking susceptibility of weld metals compared with percentage of cracking at room temperature and crack free temperature, and it had good correlation with the results of LB-TRC test. 2) Cold cracking susceptibility of weld metals was high in the row of MG100A, MG100C, MG100D and MG100B. Welds deposited with MG130 and MG80 showed similar icidents of cracking with MG100C and MG100B respectively, even though their strength levels were different. 3) Diffusible hydrogen level in weld metals which has good relation with hydrogen content in wire itself was the most critical factor for controlling the cold cracking susceptibility of weld metal.

• 대한기계학회논문집
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• 제8권2호
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• pp.97-103
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• 1984

본 연구에서는 이에 주목하고 잔존하는 잔유응력을 제거한 상태에서 PWHT가 용접 HAZ조직의 파괴인성에 어떠한 영향을 미치고 있느가와 잔유응력이 잔존된 상태에 서의 PWHT 유대시간과 가열속도가 파괴인성에 어떠한 영향을 미치는가를 알아보기 위 하여 공시재에 무응력과 일정응력을 가한 상태에서 유대시간과 가열속도를 변화시켜가 며 PWHT를 실족한 후 소성굽힘에 의한 COD파괴인성 시험 등을 통하여 PWHT의 영향을 검토하였다.

• Journal of Welding and Joining
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• 제13권3호
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• pp.65-76
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• 1995

Weldability of DS100 and HY type high strength-toughness steel plates, tentatively produced as domestic production, was investigated. DS100 and DS130A had nearly same hardenability in HAZ in spite of its difference in Ceq. Based upon the y-groove test results, cold cracking susceptibility of DS130 was superior to that of DS100 because of its lower hydrogen level in weld metal. Solidification cracking tested by the Trans-Varestraint test was occured in all of the weld metals, and its susceptibility was high in the row of DS100, DS130A and DS130B. However, no liquation cracking and ductility-dip cracking tested by the Longi-Varestraint test with 6.0% augmented strain were detected in base metal and reheated weld metal. Toughness in the GMA welding joint was satisfied with the relative Mill Spec, even though welded joint of DS100 had relatively low impact energy especially at the weld metal.

• 한국해양공학회지
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• 제26권6호
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• pp.27-32
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• 2012

The effects of reheating during welding on the microstructure and impact toughness of weld metal in 25% Cr super duplex stainless steels were investigated. Using different heat inputs, weld metals with different reheated regions were obtained. This showed that, depending on the reheating temperature, the microstructure in the reheated region was quite different from that of the as-deposited microstructure. When reheated into the ${\gamma}+{\alpha}$ temperature range, fine intragranular austenite was formed in the as-deposited columnar structure. However, when reheated above the ${\alpha}$ solvus temperature range, most of the columnar structure disappeared and fine equiaxed austenite and ferrite were formed. Because of the larger amount of fine austenite in the reheated region, a higher impact toughness was obtained in the weld metal with a higher amount of reheated region.

• 열처리공학회지
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• 제8권1호
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• pp.53-64
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• 1995

A study was made to characterize the microstructures and mechanical properties of the base metal and the heat-affected zone(HAZ) in Cu-bearing HSLA-100 steel. The Gleeble thermal/mechanical simulator was used to simulated the weld HAZ. The relationship between microstructure and toughness of HAZ was studied by impact test, O. M, SEM, TEM, and DSC. The toughness requirement of military specification value was met in all test temperatures for the base metal. The decrease of HAZ toughness comparing to base plate is ascribed to the coarsed-grain and the formation of bainite. Obliquely sectioned Charpy specimens show that secondary crack propagate easily along bainite lath. Improved toughness(240J) at HAZ of $Tp_2=950^{\circ}C$ is due to the fine grain, and reasonable toughness(160~00J) in the intercritical reheated HZA is achieved by the addition of small amount of carbon which affects the formation of "M-A". Cu precipitated during ageing for increasing the strength of base metal is dissolved during single thermal cycle to $1,350^{\circ}C$ and is precipitated little on cooling and heating during subsequent weld thermal cycle. Thus, the decrease of toughness does not occur owing to the precipitation of Cu.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.229-234
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• 2002

The variation of HAZ toughness with nitrogen content and weld cooling rate was investigated and interpreted in terms of both microstructure and the amount of free nitrogen. The presence of free nitrogen in HAZ was investigated by internal friction measurement and its amount was measured by hydrogen hot extraction analysis. Both nitrogen content and weld cooling rate influenced HAZ microstructure and high toughness was obtained at a mixed microstructure of acicular ferrite, feffite sideplate and polygonal ferrite. If nitrogen content is too low or cooling rate is too fast, bainitic microstructure is obtained and toughness is low. On the other hand, if nitrogen content is too high or cooling rate is too slow, coarse polygonal ferritic microstructure is obtained and toughness is deteriorated again. ill addition to the microstructural change, high nitrogen content also resulted in a large amount of free nitrogen. Therefore, nitrogen content should be kept as low as possible even if the mixed micostructure is obtained. In this experimental condition, the maximum toughness was obtained at 0.006% nitrogen content when weld cooling time ($\Delta$t$_{8}$5/)) is 60s.TEX>5/)) is 60s.