합금 조성 및 공정 조건 변화에 따른 비조질강 마찰용접부의 미세조직 및 기계적 특성 변화에 대한 연구 = Effects of chemical composition and welding conditions on the microstructure and mechanical properties of friction-welded joints of non-quenched and tempered steel

다국어 초록 (Multilingual Abstract)

The increasing demand for construction and industrial machinery has intensified the need for structural materials tthat provide both high mechanical performance and manufacturing efficiency. Non-quenched and tempered (NQT) steels have recently attracted considerable attention as promising alternatives to conventional quenched–tempered steels, as they can achieve competitive properties without heat treatment, offering advantages in energy consumption and production costs. Friction welding is also widely utilized for structural components because of its high productivity and solid-state joining characteristics, which effectively minimize defects within the heat-affected region. In this study, the effects of alloy composition and post-weld heat treatment (PWHT) on the microstructure evolution and mechanical properties of friction welded joints between NQT steels and S45C steel were investigated. Four types of NQT steels (Si-free, Nb-free, Nb-0.01, and Si-0.58) were prepared and friction-welded with S45C, and PWHT was additionally applied to the Si-0.58 steel. The microstructures of welded joints were characterized by optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), and transmission electron microscopy (TEM). For evaluating the mechanical properties of welded joints, Vickers hardness, Charpy U-notch impact, and tensile testing were performed. In the friction welded-joint, the microstructural regions appear progressively from the bond line. As the distance from the bond line between the NQT steels and the S45C steel increases, a thermomechanically affected zone (TMAZ) and a heat-affected zone (HAZ) are identified. The TMAZ and HAZ of the NQT steels consist of bainite and grain boundary ferrite, whereas those of the S45C steel consist of grain boundary ferrite and pearlite. The addition of Nb enhances the mechanical properties of the friction welded-joint by promoting the precipitation of fine MC carbides and refining the austenite grains. In the Si-containing alloy, retained austenite is generated in the friction welded-joint because cementite precipitation is suppressed. In the Gleeble-simulated HAZ tests, an increase in peak temperature and heat input caused coarsening of the prior austenite grains, which led to a reduction in the resulting mechanical properties. Although the FGHAZ condition (1000°C) possessed the smallest PAG size, it exhibited the lowest mechanical properties owing to the significantly increased ferrite fraction. Post-weld heat treatment (PWHT) decomposed the retained austenite into ferrite and carbides, thereby lowering the mechanical performance compared with the as-welded Si-0.58 steel. Through these observations, the relationships between microstructural evolution and mechanical behavior in friction welded-joints of NQT steels were examined with respect to variations in alloy composition and processing conditions.

목차 (Table of Contents)

• 목차 = 1
• Abstract = 3
• List of tables and figures = 6
• 제1 장. 서 론 = 13
• 제2 장. 이론적 배경 = 15
• 제1 절. 탄소강의 개요 = 15
• 1) 탄소강 = 15
• 2) 비조질강 = 17
• 제2 절. 마찰용접의 개요 = 20
• 1) 마찰용접 = 20
• 2) 마찰용접 시 미세조직 변화 = 21
• 제3 장. 실험 방법 = 22
• 제1 절. 시편 준비 = 22
• 1) 마찰용접 = 22
• 2) 열영향부 재현 = 28
• 제2 절. 미세조직 분석 = 30
• 제3 절. 기계적 특성 평가 = 31
• 제4 장. 결과 및 고찰 = 32
• 제1 절. 화학성분 별 초기 미세조직 = 32
• 1) 비조질강의 초기 미세조직 = 32
• 제2 절. 화학성분 별 기계적 특성 = 38
• 1) 비조질강의 충격특성 = 38
• 2) 비조질강의 상온 인장특성 = 41
• 제3 절. 화학성분 별 마찰용접부 미세조직 = 43
• 1) 마찰용접부 미세조직 = 43
• 제4 절. 화학성분 별 마찰용접부 기계적 특성 = 50
• 1) 비커스 경도 시험 = 50
• 2) 마찰용접부의 충격특성 = 52
• 3) 마찰용접부의 상온 인장특성 = 57
• 제5 절. 비조질강 용접 열영향부 재현 = 59
• 1) 재현 열영향부 미세조직 = 59
• 2) 재현 열영향부 경도 및 충격특성 = 64
• 제6 절. 공정 조건 변화에 따른 마찰용접부 특성 = 68
• 1) 마찰용접 후 PWHT 전후 미세조직 = 68
• 2) 마찰용접 후 PWHT 전후 기계적 특성 = 76
• 제5 장. 결론 = 82
• References = 85