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21CrMoV5-7 Creep-Rupture (EN 10269 / 1.7709)
The creep-rupture performance of 21CrMoV5-7 (Werkstoff 1.7709) is defined in EN 10269 Annex A as the 100,000-hour stress-rupture envelope. The grade holds approximately 260 MPa at 540 deg C and 180 MPa at 550 deg C at the 100,000-hour rupture point per the Larson-Miller correlation. Above 550 deg C the V4C3-only chemistry runs out of margin and design must step up to Durehete 1055.
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100,000-Hour Stress-Rupture Envelope
| Temperature | Stress at 10^5 h rupture |
|---|---|
| 450 deg C | ~470 MPa |
| 475 deg C | ~400 MPa |
| 500 deg C | ~340 MPa |
| 525 deg C | ~290 MPa |
| 540 deg C | ~260 MPa |
| 550 deg C | ~180 MPa |
Larson-Miller Correlation
The Larson-Miller parameter (LMP = T(20 + log t) where T is in Kelvin and t is rupture time in hours) collapses the time-temperature creep envelope onto a single master curve. For 21CrMoV5-7 the LMP envelope passes through LMP=20.5 at 290 MPa and LMP=21.0 at 250 MPa, giving the engineer a one-equation route from any design temperature and design life to the allowable stress.
Creep Rate
The 1 percent creep strain at 100,000 hours is approximately 80 percent of the stress-rupture value at the same temperature. For 540 deg C: 1 percent creep at 200 MPa applied stress at 100,000 hours. The 0.2 percent creep strain at the same conditions is approximately 65 percent of the rupture stress, giving 170 MPa.
Microstructure Driver
The creep performance is driven by the V4C3 carbide dispersion formed during the 680 to 740 deg C temper. The fine V4C3 precipitates pin dislocation motion during long-time elevated-temperature service. As the carbides coarsen with extended exposure, the creep envelope drops; the rate of coarsening is approximately Arrhenius with an activation energy of 320 kJ/mol.
Frequently Asked Questions
Q. What is the 100,000-hour rupture stress for 21CrMoV5-7 at 540 deg C?
Approximately 260 MPa to EN 10269 Annex A. Use the Larson-Miller correlation to interpolate to any design temperature and design life within the qualification envelope.
Q. Does 21CrMoV5-7 have a 200,000-hour rupture envelope?
Not in EN 10269 base qualification. The 200,000-hour projection is supplied as part of the VdTUV Wb 350 extended qualification scope for German pressure-vessel work. The projection extrapolates the 100,000-hour Larson-Miller envelope with a -20 to -30 MPa offset at 540 deg C.
Q. What is the Larson-Miller parameter?
LMP = T (20 + log t) where T is absolute temperature (Kelvin) and t is rupture time (hours). The parameter collapses the time-temperature creep envelope onto a single master curve. For 21CrMoV5-7 the LMP envelope passes through LMP=20.5 at 290 MPa.
Q. Why does 21CrMoV5-7 fall off above 550 deg C?
The V4C3 carbide dispersion coarsens faster at higher temperatures. Without titanium-and-boron grain-boundary pinning (which Durehete 1055 has) the grain-boundary creep cavitation dominates above 550 deg C and the long-term rupture stress drops sharply.
Related Properties Pages
Drill into 21CrMoV5-7 properties: Chemical Composition · Mechanical Properties · Heat Treatment · Elevated-Temperature Properties · Hardness · Welding Procedure · Machinability. Standards: EN 10269 · vs ASTM A193 B16. Back to the 21CrMoV5-7 Alloy Hub.
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