Laves-phase precipitates in a low-carbon 9% Cr martensitic steel during aging and creep at 923 K

Effect of Tungsten on Creep Behavior of 9%Cr–3%Co Martensitic Steels

“…The 1 wt %W additive provides the precipitation of the W-rich М6С carbides and Laves phase even under tempering. No formation of thermodynamically stable W-rich Laves phases was reported in the conventional 9%Cr martensitic steels after tempering [1,2,[7][8][9][10][11][12], and the appearance of the less stable W-rich M6C The lath thickness was approximately 0.4 µm for both steels. The high dislocation density of approximately 2 × 10 14 m −2 was observed within the lath and subgrain interiors.…”

“…This depletion does not occur in the 9%Cr steel containing no or low amount of W [7]. Depletion of solid solution by these elements highly deteriorates the creep resistance [7,10,11]. It is worth noting that, at present, the most of experimental data on the effect of W on creep behavior were obtained for cobalt-free high chromium martensitic steels.…”

“…Stability of TMLS is provided by M 23 C 6 -type carbides and MX (where M is V and/or Nb, and X is C and/or N) carbonitrides precipitated during tempering at boundaries and within ferritic matrix, respectively, and boundary Laves phase particles precipitated during creep [1,2,[4][5][6][7][8][9][10][11][12]. MX carbonitrides, which are highly effective in pinning of lattice dislocations, play a vital role in superior long-term creep resistance of the high chromium martensitic steels, whereas boundary M 23 C 6 carbides and Laves phase particles exerting a high Zener drag force stabilize the TMLS [1,2,5,[7][8][9][10][11][12][13][14]. This dispersion of secondary phase particles withstands short-term creep [4,9,15].…”

“…This dispersion of secondary phase particles withstands short-term creep [4,9,15]. The Laves phase provides effective stabilization of TMLS and therefore promotes creep resistance, although their effect on the rearrangement of lattice dislocation is negligibly small [9,11,15]. However, the particles of Laves phase grow with a high rate under creep condition and, at present, these boundary precipitations are considered to be responsible for the creep ductility of the P92-type steels during long-term aging [16].…”

“…These elements provide an effective solid solution strengthening [1]. It was shown [21] that addition of 1% W gives +35 MPa increase in the creep rupture strength at 600 • C for 1000 h. However, in contrast with cobalt, the tungsten and molybdenum have limited solubility within ferrite, and their excessive content leads to precipitation of such W-and Mo-rich particles as Laves phase Fe 2 (W,Mo) or M 6 C carbides [1,7,[9][10][11][12]22,23]. This depletion does not occur in the 9%Cr steel containing no or low amount of W [7].…”