ATOS 80 고장력강의 용접조건에 따른 제 특성에 관한 연구

Abstract

A Study on the Characterzation of Properties in the Welding Conditions of ATOS80 High-strength Steels

Baek, Jung-Hwan Advisor : Prof. Choi, Byung-Ky, Ph.D. Department of Mechanical Engineering Graduate School of Chosun University

In this study, mechanical properties were investigated and compared such as tensile strength, yield strength, elongation, hardness, and brittleness in ATOS80 specimens which were welded using a flux cored arc welding. Welding failures were revealed through macroscopic examination of the welded part, microscopic weld examination, and radiograph testing. The results are presented as follows:

1. For a tensile strength test, performance of a specimen which was welded with 4 passes under the same conditions was better than that of a specimen welded with 5 passes. There was no significant difference found when gas used was 15ℓ/min. or 20ℓ/min. and it was suggested that the elongation for pure carbon dioxide shielding gas was 2～6% higher than that for mixed gas because the pure carbon dioxide had more deoxidation effect. The ratio of CO2 and argon of 20:80 produced better results than other ratios of mixed gases and there was no significant difference for preheating.

2. In a hardness test of welded specimens, the hardness of the specimen which was welded with 5 passes was higher than that of a specimen welded with 4 passes under the same welding conditions because of micro fissures as heat input into the weld is large when the specimen is welded with 5 passes. In comparing data on the mix of protective gases and constraints, when the bevel angle was 50° rather than 40° or 60°, the hardness was more stable. The 5mm-thick specimens welded had more stable hardness than the 3mm-thick welded specimens, and at a mix ratio 30:70 for CO2 and argon with more carbon dioxide, hardness was lower. The reason that the hardness value was stable was due to the higher deoxidation effect. 3. In an impact test of the weld specimens, it was discovered that the specimen welded with 4 passes had a higher impact value than that of the specimen welded with 5 passes. A shield gas flow rate of 20ℓ/min. had the highest impact value, and it was believed that when 25ℓ/min. of gas was used, the weld could not be properly shielded because of turbulence. Also when the CO2 was 100% and the gap welding of the unconstrained weld was 50° the absorption energy was the highest because internal stress was contained in the weld. The impact test for weld specimens which were not preheated had a higher absorption energy than the preheated weld specimen it was probably because ductility increased due to coarse fissures.

4. In a macroscopic test of the welds, the specimens were welded with 4 passes, shield gas flow rate of 20ℓ/min. and a gap of 50° had less failures and a normal dendritic structure was found in the entire weld, but the rest of the specimens had weaker micro-structures and dendritic structures developed in terms of root gaps or the mixed gas ratio. In the case where the weld specimen was 5mm thick and the gas ratio was 20:80 for CO2 and argon there were no welding failures, but the other specimens had welding failures.

5. In a microscopic tests of the weld, under the same welding conditions for 4 passes, shield gas flow rate of 20ℓ/min. and gap welding of 40°and 50°, the ground of the specimen was ferrite with a micro martensite structure, and the boundary was ferrite ground with a pearlite structure; but the other welds had a ferrite ground with some Bainite or Pearlite structures on the micro martensite structure. In terms of the root gap or mixed gas ratio, all of the specimens had stable micro structures.

6. In a radiograph test, it was discovered that the specimens were welded under the same welding conditions of 4 passes, shield gas flow rate of 20ℓ/min, and gap welding of 50°, and steel thickness of 3mm and 5mm, the mixed gas ratio of 20:80 for CO2 and argon had no or the least welding failures, but the other specimens had a lot of welding failures.

[ Desired Effects ] This study was recommended that through tests of v-shaped flux cored arc welding of 12mm thick ATOS80 high tensile steel, the following conditions are the most appropriate: 4 pass welding, unconstrained, 100% CO2 shielding gas, 5[mm] of root gap, 20ℓ/min of shielding gas, 50°of gap welding, 200[A] of welding current, 24[V] of arc voltage, preheating to 160[℃] and flux arc welding ceramic for backing.