Kook-Soo Bang

Evaluation of weld metal hot cracking susceptibility in superaustenitic stainless steel

Solidification cracking susceptibilities of two types of superaustenitic stainless steel, 254SMO and SR50A, were evaluated by transverse Varestraint tests. The susceptibilities were compared with those of conventional austenitic stainless steel 316L, and factors influencing the difference of susceptibility were discussed. The comparison showed that 254SMO and SR50A are more sensitive to solidification cracking than 316L. In the transverse Varestraint tests, both total and maximum crack lengths are longer in the superaustenitic stainless steel. Because of the longer maximum crack length, the superaustenitic stainless steel also has a wider brittleness temperature range of cracking than 316L: about 178 °C for the superaustenitic stainless steel and 43 °C for 316L. It is believed that straight subgrain boundaries owing to the cellular dendritic solidification and segregations of sulfur and phosphorus in the subgrain boundaries of superaustenitic stainless steel make it more sensitive to solidification cracking. In addition to the solidification cracking, reheat cracking is also observed within the previous weld bead in the superaustenitic stainless steel because of fully austenitic solidification with significant segregations. This suggests that caution should be given to the occurrence of reheat cracking when superaustenitic stainless steel is multi pass welded.

Quantitative Analysis on the Effects of Welding Parameters on Diffusible Hydrogen Contents in Weld Metal Produced by FCAW Process

The effects of welding parameters such as contact tip-to-work distance (CTWD), voltage, and current on the weld metal diffusible hydrogen contents (HD) were investigated and rationalized by the calculation of heating time and amount of heat generated in the extension length of flux cored wire. As CTWD increased from 15 to 25mm, HD decreased from 8.46 to 5.45mL/100g deposited metal. Calculations showed that, with an increase of CTWD, the amount of heat generated increased from 46 to 92J in addition to an increase of heating time. Increase of current from 250 to 320A, however, gave little variation of HD. It showed that no significant change in the amount of heat generated was found, and heating time was decreased with an increase of current. It also showed that CTWD is more influential than voltage in relatively lower heat input ranges, while voltage is more in higher input ranges

Electrical properties of piezoelectric PZT thick film by aerosol deposition method

Lead zirconate titanate (PZT) thick films with thickness of were fabricated on silicon substrate by aerosol deposition method. As-deposited films on silicon were annealed at the temperatures of . The electrical properties of films deposited by PZT powders were characterized using impedance analyzer and Sawyer-Tower circuit. The PZT powder was prepared by both conventional solid reaction process and sol-gel process. The remanent polarization, coercive field, and dielectric constant of the thick film with solid reaction process were , 30 kV/cm and 1320, respectively. On the other hand, the PZT films by sol-gel process showed a poor dielectric constant of 635. The reason was probably due to the presence of pores produced from organic residue during annealing.

Microstructure and Impact Toughness of Weld Metal in Multipass Welded Super Duplex Stainless Steel

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 temperature range, fine intragranular austenite was formed in the as-deposited columnar structure. However, when reheated above the 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.

Variation of Toughness and Porosity Formation in Weld Metal with Al Content in Self-Shielded Arc Welding Wire

Abstract Three different welding wires were used to study the effects of Al content on weld metal toughness and porosity formation in self-shielded arc welding. Weld metal microstructure showed that while wire with 1.3% Al content contains coarse δ-ferrite, wires with less than 0.5% Al content showed no such phase. In addition to the microstructural differences, cleanliness in weld metal was also different among wires. It showed that weld metal toughness was influenced by the δ-ferrite formation, cleanliness and Ni addition. Even though wires with less than 0.5% Al content showed higher weld metal toughness, they showed relatively poor workability, forming porosities in weld bead in lower arc voltages.Key Words : Self-shield arc welding wire, Al content, Weld metal microstructure, Toughness, porosity 1. 서 론 셀프실드아크용접 (SSAW)은 외부로부터 특별한 차폐가스를 사용하지 않는 용접법으로 용접 중 흄 발생이 많고 용접금속 기계적성질이 열악하여 가스용기 설치가 어려운 고층건물이나 한냉지에서 송유관 용접 같은 옥외용접에 주로 사용되어 왔다. 하지만 용접금속의 기계적성질, 특히 충격인성의 지속적인 향상으로 최근에는 해양구조물 등에도 그 적용이 확대되고 있다

Low Heat Input Welding to Improve Impact Touglness of Multipass FCAW-S Weld Metal

Multipass self-shielded flux cored arc welding with different heat inputs (1.3-2.0 kJ/㎜) was conducted to determine the effects of the heat input on the proportion of the reheated region, impact toughness, and diffusible hydrogen content in the weld metal. The reheated region showed twice the impact toughness of the as-deposited region because of its fine grained ferritic-pearlitic microstructure. With decreasing heat input, the proportion of the reheated region in the weld metal became higher, even if the depth of the region became shallower. Accordingly, the greatest impact toughness, 69 J at -40℃, was obtained for the lowest heat input welding, 1.3 kJ/㎜. Irrespective of the heat input, little difference was observed in the hardness and diffusible hydrogen content in the weld metal. This result implies that low heat input welding with 1.3 kJ/㎜ can be performed to obtain a higher proportion of reheated region and thus greater impact toughness for the weld metal without the concern of hydrogen cracking.

Effects of chemical composition on the microstructure and mechanical properties of FCAW-S weld metal containing 2% Ni

In this study the effects of alloying elements on the microstructure and mechanical properties of 600MPa grade FCAW-S weld metals containing 2% Ni were examined. Carbon, manganese and aluminum contents were varied in the ranges of 0.075%–0.101%, 1.19%–1.69%, and 0.66%–1.49% respectively. Regardless of the Al content, all of the weld metals showed a bainite dominant microstructure with no δ-ferrite. This indicates that when a weld metal contains 2% Ni, the Al content can be increased up to around 1.5% without concern about the deterioration of impact toughness due to the presence of δ-ferrite. The tensile strength of the weld metals varied from 595 MPa to 702 MPa dependent upon the chemical composition. Multiple regression analysis showed that while C and Mn have strong influences on the tensile strength, Al has little influence. Therefore, the Pcm index of weld metals could be used as an indication of their tensile strength. Impact toughness of the weld metals was influenced most by tensile strength and showed that the 50J transition temperature increased by 36 °C when the tensile strength was increased by around 100 MPa. Therefore, an excessive increase of the tensile strength should be avoided to attain higher impact toughness. Even when inclusion mean diameters were increased from 0.588 μm to 0.708 μm with an increase of the Al content from 0.66% to 1.49%, the size difference showed little influence on the impact toughness of the weld metals in this experiment.

Effects of Welding Parameters on Diffusible Hydrogen Contents in FCAW-S Weld Metal

The effects of the welding parameters, contact tip-to-workpiece distance (CTWD), current, and voltage on the diffusible hydrogen content in weld metal deposited by self-shielded flux cored arc welding were investigated and rationalized by comparing the amount of heat generated in the extension length of the wire. This showed that as CTWD increased from 15mm to 25mm, the amount of heat generated was increased from 71.1J to 174.8J, and the hydrogen content was decreased from 11.3mL/100g to 5.9mL/100 g. Even if little difference was observed in the amount of heat generated, the hydrogen content was increased with an increase in voltage because of the longer arc length. A regression analysis showed that the regression coefficient of voltage in self-shielded flux cored arc welding is greater than that in arc welding. This implies that voltage control is more important in self-shielded flux cored arc welding than in arc welding.

Effects of Diffusible Hydrogen Content and Hardness on Cold Cracking in High Strength Weld Metal

*Division of Advanced Materials Science and Engineering, PKNU, Busan, KoreaKEY WORDS: Weld metal 용접금속, Cold cracking 저온균열, Diffusible hydrogen content 확산성수소량, Hardness 경도ABSTRACT: The effects of the diffusible hydrogen content and hardness on the cold cracking in high strength weld metal were investigated . The diffusible hydrogen contents were influenced by welding paramet ers such as the voltage and contact tip-to-work distance (CTWD) . The diffusible hydrogen content increased with an increase in voltage. However , it was decreased with an increase in CTWD. CTWD also influenced the weld metal hardness,especially when the wire used had a higher stren gth than the base metal. This showed that weld metal hardness h ad a more powerful effect on weld metal cold cracking than the diffusible hydrogen content in this experiment.교신저자 방국수: 부산광역시 남구 용당동 산 100, 051-629-6379, ksbang@pknu.ac.kr

Development of Clean Welding Consumables

를 위해서는 접합부 균열발생 및 성능저하의 원인이 되는 수소함량을 극소화하기 위한 접합소재 제조기술이 요구되며 이에 대응한 접합부 수소거동에 대한 해석기술이 필요하며, 접합소재의 과학적 플럭스 설계기술 도입과 DB화를 통한 고유원천소재 제조기반 확보가 요구된다. 접합소재의 저수소화를 위하여 미국의 링컨사는 접합소재에 불소(F)를 첨가하여 수소를 낮추는 기술을 개발하였으나, 불소 역시 불화물을 형성하여 환경에 유해하다. 접합소재의 실용화시 다량의 fume 발생으로 환경을 저해하는 요소로 작용하여 Cs 을 첨가한 flux를 설계함으로서 fume 발생량을 기존의 절반으로 줄이는 기술 등이 개발되고 있다. 또한, 탄소배출권 등의 제약으로 CO