The Role of Thermomechanical Processing in Creep Deformation Behavior of Modified 9Cr-1Mo Steel
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DIFIED 9Cr-1Mo steel (ASME Grade 91) is extensively used for the fabrication of high-temperature components for fossil fuel fired and nuclear power plants, due to its inherent advantages such as high thermal conductivity, low coefficient of thermal expansion, and high resistance to oxidation and stress corrosion cracking.[1–10] Conventionally, the steel has been used in the normalized and tempered (NT) condition. The steel derives its strength mainly from grain and subgrain boundaries decorated by M23C6 (M = Cr) carbides and intralath regions having MX (M = V, Nb; X = C, N) precipitates rich in V and Nb and transformation-induced dislocation density. The contribution of dislocation structure to creep strength has been reported by researchers.[11] On creep exposure, the steel weakens because degrading mechanisms, such as precipitate coarsening and recovery of the dislocation structure, become operative. The coarsening rate of
P. SHRUTI and G.V.S. NAGESWARA RAO are with the National Institute of Technology, Warangal, 506004, India. T. SAKTHIVEL and K. LAHA are with the Indira Gandhi Centre for Atomic Research, Kalpakkam, 603102, India. Contact email: [email protected] T. SRINIVASA RAO is with the National Institute of Technology, Thiruchirapally, 620015, India. Manuscript submitted June 7, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
M23C6 precipitates is much faster than that of the MX precipitates. As a result, inter barrier spacing increases and dislocation motion become less restricted. To hinder the migration of boundaries and to impede the movement of dislocations, the density of obstacles, i.e., number of precipitates, has to be increased. In this study, thermomechanical treatment (TMT) is adopted to refine the microstructure and, hence, improve the properties of the steel. The kinetics of the processing are designed so as to cause an enhanced precipitation of MX. This is with a view that increased population of the relatively stable MX precipitates ensures microstructural stability even at elevated temperatures. Klueh[3] reported an enhancement in tensile and creep properties after following a TMT process. Tan et al.[6,7] reported that strengthening of TMT processed steel was attributed to refined prior austenite grains, increased density of martensite packets and laths, and especially an increase in the number of fine precipitates. Hollner et al.[12] studied the effect of TMT on high-temperature mechanical properties of modified 9Cr-1Mo steel. They concluded that TMT processing reinforces the pinning of the dislocations by the MX particles, which in turn improves the mechanical properties. The changes in substructure of the steel by dislocation multiplication, its interaction with barriers, and rearrangement of dislocations through the diffusion-controlled dislocation climb process occurs in various stages of creep regimes. The process of dislocation rearrangements by the climb
process follows first-order reaction kinetics. Analyses of different regimes of creep in pure metals and precipitation-hardened al
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