Correlation of the microstructure and fracture toughness of the heat-affected zones of an SA 508 steel
- PDF / 4,382,046 Bytes
- 13 Pages / 612 x 792 pts (letter) Page_size
- 82 Downloads / 184 Views
I. INTRODUCTION
IN heat-affected zones (HAZ) of an ASME SA 508 steel, a low-alloy steel used for structural components of atomic power plants, microstructural changes such as grain coarsening, carbide precipitation, and martensite formation generally occur and cause a deterioration in toughness and an increase in sensitivity to brittle fracture.[1] Microstructures of the HAZ after large-heat-input welding are affected by factors such as chemical composition, welding condition, and peak reheating temperature. According to the distance from a fusion zone, the HAZ subjected to welding thermal cycles is composed of a coarse-grained region reheated above 1100 8C, a fine-grained region reheated between 900 8C and 1100 8C, a partially transformed region reheated between 700 8C and 900 8C, and a tempered region reheated below 700 8C. Among these HAZ regions, the most vulnerable to embrittlement is the coarse-grained region located near the bond, which is generally referred to as a local brittle zone (LBZ).[2–6] However, in multipass welding, this LBZ can be changed because of a repeated heat effect. Particularly, the multipass welded HAZ shows a very complex microstructure resulting from complicated thermal history, which makes the evaluation of fracture toughness on specific HAZ regions very difficult, and thus thermal-cycle-simulation testing is typically used. In this case, quantitative analyses are also difficult because of the microstructural complexity.[7] The degree of embrittlement in the LBZ varies SANGHO KIM and SUK YOUNG KANG, Research Assistants, and SUNGHAK LEE, Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790784 Korea. SEI J. OH, Postdoctoral Research Associate, and SOON-JU KWON, Professor, are with the Department of Materials Science, Pohang University of Science and Technology, Pohang, 790-784 Korea. JOO HAG KIM, Senior Researcher, and JUN HWA HONG, Director, are with the Nuclear Materials Technology Development Team, Korea Atomic Energy Research Institute, Dajeon, 305-600 Korea. Manuscript submitted December 14, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
with material chemistry and welding conditions. Under the conditions for the easy LBZ formation, the LBZ fatally affects toughness of the welded region. Therefore, assuring fracture toughness in this region and quantitatively evaluating it are very critical to the safety of structures. This is particularly so when SA 508 steels are applied to structures for atomic power plants. Many studies have been made on microstructural factors associated with the embrittlement phenomenon,[8–11] but very few can sufficiently explain its mechanism. Since the HAZ microstructure changes with thermal cycles in a highly complicated manner, quantitative and systematic analyses are extremely difficult.[7] Optical microscopy, electron microscopy, and X-ray diffraction have been used for the microstructural analysis of the HAZ, but its quantitative analysis is hardly possible because of the mixed ph
Data Loading...
