Novel Concept of Creep Strengthening Mechanism using Grain Boundary Fe 2 Nb Laves Phase in Austenitic Heat Resistant Ste
- PDF / 362,420 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 74 Downloads / 169 Views
Novel Concept of Creep Strengthening Mechanism using Grain Boundary Fe2Nb Laves Phase in Austenitic Heat Resistant Steel Imanuel Tarigan1, Keiichi Kurata1, Naoki Takata1,2, Takashi Matsuo1,2, Masao Takeyama1,2 1 Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, S-8-8, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8552, Japan. 2 Consortium of the Japan Research and Development Center for Metals (JRCM), Tokyo, Japan. ABSTRACT The creep behavior of a new type of austenitic heat-resistant steel Fe-20Cr-30Ni-2Nb (at.%), strengthened by intermetallic Fe2Nb Laves phase, has been examined. Particular attention has been given to the role of grain boundary Laves phase in the strengthening mechanism during long-term creep. The creep resistance increases with increasing area fraction () of grain . . . boundary Laves phase according to equation /0 = (1-), where 0 is the creep rate at = 0. In addition, the creep rupture life is also extended with increasing without ductility loss, which can yield up to 77% of elongation even at = 89%. Microstructure analysis revealed local deformation and well-developed subgrains formation near the grain boundary free from precipitates, while dislocation pile-ups were observed near the grain boundary Laves phase. Thus, the grain boundary Laves phase is effective in suppressing the local deformation by preventing dislocation motion, and thereby increases the long-term creep rupture strength. This novel creep strengthening mechanism was proposed as “grain boundary precipitation strengthening mechanism” (GBPS). INTRODUCTION A major challenge to realize the advanced ultra-super critical (A-USC) power plants is to develop materials with 105 h creep rupture strength more than 100 MPa at 973K. Under this condition, the ferritic steels cannot be used and only Ni-base alloys can meet the requirement, although they are expensive. On the other hand, even though they are fcc based, creep rupture strengths of the commercial austenitic steels are far below the required condition, due to the low thermal stability of metallic carbides used in these steels. Thus, it is a big breakthrough if we can develop austenitic heat-resistant steels to meet the condition. We have proposed a new type of austenitic heat-resistant steel Fe-20Cr-30Ni-2Nb (at.%) strengthened by the thermodynamically stable intermetallic phases of Fe2Nb TCP (Topologically Close-Packed) phase, together with Ni3Nb GCP (Geometrically Close-Packed) phase [1,4], and demonstrated that the creep rupture strength of this model steel at 973 K is superior to that of conventional austenitic heat resistant steels strengthened by metallic carbides [2,3,5,6]. The creep rate and microstructure analysis under a relatively low stress (973 K/120 MPa) revealed that the creep rate continuously decreases by five orders of magnitude during the transient creep stage for several hundreds hours due to the precipitation of metastable Ni3Nb-" (D022) phase within grain interiors, and then the creep rate increases due to the transformation of
Data Loading...
