Effect of Welding Processes on Microstructure and Mechanical Properties of Ultrahigh－Strength D6AC Alloy Steel Weldments

• Main Authors: Huang, Yi-Zhang, 黃羿彰
• Other Authors: Li, Yi-Kang
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/45035340789913214414

碩士 === 大葉大學 === 醫療器材設計與材料碩士學位學程 === 104 === D6AC is an ultra-strength alloy steel that is refined through vacuum arc remelting (VAR). It is low in sulfur, phosphorus, and impurities and is more strengthen and toughness that AISI 4140 and 4340. Currently, D6AC is only used in the aerospace industry and the military. When a large aircraft nozzle and engine metal steel shell manufacturing, the straight seam joint can be taken by GTAW, coupled with quenching achieve hardening effect. Circumferential joint can be taken by EBW. Therefore, this study collected D6AC for GTAW multi-level welding and EBW welding of weldments analyzed. Test results: GTAW weld organization is columnar structure; base metal is tempered martensite. HAZ impact value as the operating temperature rises gradually rise, and when the maximum value at 600 ℃, lifting about 276%; impact value bead shall have the same trend, an increase of approximately 236%. The tensile strength decreases with temperature is slightly lower, by about 8 percent, but elongation has 211 percent rate increased. An impact test on an EBW weldments indicated that average room-temperature impact value was 92.5 J/cm2. When the operating temperature reached 600 ℃, the average impact value increased to 151% at 140 J/cm2. The fractured-surface microstructures observed using SEM were quasi-cleavage fracture planes. These microstructures gradually transformed into spiral-shaped structures as the operating temperature rose. An energy-dispersive X-ray spectroscopy (EDS) and TEM analysis indicated blue brittleness in the weldments at 400 ℃and red brittleness at 600 ℃, Produce MnO2, Fe3O4 etc. precipitate phase.