Effect of Laves Phase on Ductile-Brittle Transition of 12 Pct Cr Steel
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I.
INTRODUCTION
A new generation of 9 to 12 pctCr steels is widely used for turbine components such as steam pipelines, rotors, blades, disks, and valves of fossil power plants, which operate under ultra-super critical conditions with steam pressures of 24 to 35 MPa and temperatures of 580 C to 610 C.[1–5] Heat treatment of these steels consists of normalizing at a temperature of ~ 1050 C and tempering at 720 C to 780 C. Under normalizing, the austenite transforms to lath martensite with a body-centered cubic (bcc) structure. Martensite has a four-level hierarchy in its morphology, i.e., prior austenite grains (PAGs) are divided into packets, blocks, and laths with a high dislocation density in the lath interiors.[6] Tempering leads to the precipitation of M23C6 carbides and M(C,N)-type carbonitride particles. M23C6 carbides with sizes of 60 to 100 nm are precipitated mainly on the boundaries of PAGs, packets, blocks, and laths and M(C,N)-type carbonitride particles are distributed uniformly throughout the matrix. The microstructure after tempering is usually called a tempered martensite lath structure (TMLS) because the characteristic morphology of martensite is
J. BORISOVA, V. DUDKO, R. MISHNEV, and R. KAIBYSHEV are with the Belgorod State University, Pobeda 85, Belgorod, Russia, 308015. Contact e-mail: [email protected] Manuscript submitted November 5, 2018. Article published online May 20, 2019 3528—VOLUME 50A, AUGUST 2019
retained after tempering owing to the precipitation of M23C6 carbides and M(C,N)-type carbonitrides that exert high Zener drag forces on the PAG boundaries, packets, blocks, and lath boundaries and serve as effective barriers to dislocation motions. The superior creep resistance of these steels is attributed to three factors. First, the TMLS contributes to an increase in strength owing to internal stresses inherent in martensite.[1,2,6–11] Second, dispersion strengthening is provided by nanoscale boundary M23C6 carbides and fine M(C,N)-type carbonitrides homogeneously distributed in the lath interiors.[1,7,9,12–14] Third, solution strengthening is attributed to Cr, Co, Mo, and W solutes in a ferritic matrix.[1,13,15,16] Martensitic steels with Cr ranging from 9 to 10.5 wt pct are tough in tempered conditions.[17–26] For instance, the impact toughness of P91 and P92 steels at room temperature is 180 and 250 J, respectively.[22,25] However, additional alloying by substitutional solids, and lowering N and increasing B, may decrease the fracture toughness.[24] About 50 years ago, 12 Cr steels were introduced in service for high-temperature rotors and blades of steam turbines operating up to 565 C.[1] At present, a new generation of these steels with improved creep strength providing service at temperatures up to 650 C has been developed.[1] The fracture toughness of these steels is a critical property for their application as structural materials for rotating turbine parts and components.[1] The fracture toughness required for a turbine application must be at a level of 70 MPa 9 m0
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