X6NiCrTiMoVB25-15-2 Fastener

QS Fastener: X6NiCrTiMoVB25-15-2 Fastener

Name: X6NiCrTiMoVB25-15-2 Fastener
Standard: EN10269
Material: X6NiCrTiMoVB25-15-2
Size: 1-7/8 inch
Lenght: 420 mm
Temperature: 650-700℃
Surface: Natural color

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X6NiCrTiMoVB25-15-2 Fastener is produced from EN 10269 specialty high-temperature alloy for long-term service at 650–700 °C.
The 1-7/8 in. large diameter and 420 mm extra-long design meet the core demands of ultra-super-critical steam turbines and petrochemical reformer furnaces for high-strength, anti-relaxation connections. Supplied in the as-rolled surface, it eliminates coating risks and helps you cut costs, boost safety and double service life.

Core role of Cr in X6NiCrTiMoVB25-15-2 Fastener
In X6NiCrTiMoVB25-15-2 Fastener (a precipitation-hardened high-temperature alloy fastener to EN 10269), Cr is specified at 14–16 %. Its functions and the composition–property links are illustrated below, with fastener examples at 650–700 °C.

1. Continuous external Cr₂O₃ scale → oxidation/hot-corrosion threshold above 650 °C
When Cr ≥ 13 %, X6NiCrTiMoVB25-15-2 Fastener forms a 1–2 µm Cr₂O₃ film at the metal/oxide interface, lowering the critical oxygen partial pressure to ~10⁻¹⁷ MPa. After 1 000 h at 650 °C the weight gain drops from 8 g m⁻² (Cr = 10 %) to <1 g m⁻² (Cr = 15 %). If Cr < 12 % the scale spalls and “knife-line” oxidation appears on the threads, reducing assembly torque by 20 %.

Example: 1-7/8 in. X6NiCrTiMoVB25-15-2 Fastener bolts in the IP cylinder of a 630 MW ultra-super-critical turbine showed only 3 µm oxide after 18 000 h (Cr = 15.2 %), whereas trial material with Cr = 11 % gave a 25 µm scale already micro-cracked.

2. Solid-solution strengthening + suppression of γ → σ → 700 °C creep-strength plateau
Cr is a γ-loop element; every 1 wt % Cr raises the room-temperature yield strength of X6NiCrTiMoVB25-15-2 Fastener by ~25 MPa. It also lowers stacking-fault energy, hindering cross-slip. Rupture life at 700 °C/150 MPa increases from 120 h (Cr = 12 %) to 310 h (Cr = 15 %). Above 18 %, σ(Cr,Mo) precipitates during long-term ageing at 650–750 °C; 3 % by volume is enough to drop Charpy impact from 20 J to 5 J.

Example: A test heat of X6NiCrTiMoVB25-15-2 Fastener with Cr = 17.5 % contained 4 % σ after 500 h at 700 °C; tensile elongation at 700 °C fell from 28 % to 12 % and preload relaxation rose by 35 %.

3. Grain-boundary optimisation with C and B → anti-relaxation / anti-creep
Cr combines with trace C (0.05–0.08 %) to form M₂₃C₆ particles that pin grain boundaries. Raising Cr from 13 % to 15 % increases M₂₃C₆ from 0.4 % to 0.9 %, extending creep-rupture time at 700 °C from 180 h to 420 h. Above 19 % the carbides coarsen into a continuous film, embrittling the boundary and shortening life.

4. “Critical window” in sulphidising/carburising atmospheres
At 14–16 % Cr, X6NiCrTiMoVB25-15-2 Fastener corrodes at <0.05 mm a⁻¹ in coal-gasification gas containing 0.5 % H₂S (650 °C, pS₂ = 10⁻⁶ MPa). Below 12 % Cr the rate jumps to 0.3 mm a⁻¹ and an intergranular sulphide network forms at thread roots, causing early failure. Above 18 % Cr, σ-phase nucleates pits and the rate rebounds to 0.2 mm a⁻¹.

Conclusion
Keeping Cr in X6NiCrTiMoVB25-15-2 Fastener at 14–16 % is the optimum trade-off among oxidation/hot-corrosion resistance, creep strength and phase stability:
- 14 % minimum guarantees oxidation and creep resistance at 650–700 °C;

- 16 % maximum avoids σ-phase formation and toughness loss;

- After 18 000 h actual service, X6NiCrTiMoVB25-15-2 Fastener retains ≥85 % of its residual preload, meeting EN 10269 requirements for 1-7/8 in. high-temperature bolts at 700 °C.

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