20 Ductile fracture initiated at inclusions and second-phase particles Transgranular fracture due to nucleation of voids and void coalescence are postponedGrain boundary sliding , wedge crack s and voids growth -> intergranular fractureAt high temp, matl necks down to rupture( absence of nucleation and coalescence of voids)Creep cavitation : nucleation and growth of grain-boundary voids (third stage)Extremely high stresses normal to a grain boundary are required to nucleate a cavity.Cavity growth will occur if
21 Mechanisms of intergranular nucleation Two types of intergranular cracks :Wedge-shaped cracks (w-type cracking)Round or elliptical cavities (r-type crack).(From W.D. Nix and J. C. Gibeling, in Flow and Fracture at ElevatedTemperatures, ed, R. Raj (Metals Park, Ohio: ASM, 1985).)
22 Critical Factors in Creep The rate of damage or strain rateThreshold temperaturesLow creep ductility is:More severe for higher tensile strength materials and welds.More likely in a coarse-grained material than a fine-grained material.Promoted by certain carbide types in some Cr-Mo steels.
23 Creep and Stress Rupture Affected Units or Equipment Heater tubes, tube supports, hangers and other furnace internals.Piping and equipment, such as hot-wall catalytic reforming reactors and furnace tubes, hydrogen reforming furnace tubesWeld and HAZ of Catalytic Reformer Tubes and ReactorsWelds of dissimilar materials (ferritic to austenitic welds)All boiler and fired heater tubes are susceptible.
24 Appearance or Morphology of Creep Damage Change in dimensionsCreep voidingChange in microstructure (SEM, OM)Change in hardnessDimensionalReplication and hardness
25 Prevention / Mitigation of Creep To prevent creep damage - minimize the metal temperatureTo avoid stress concentratorsLow creep ductility can be minimized by the careful selection of chemistry for low alloy materials (e.g. PWHT of Cr-Mo Steel)Creep damage is not reversible - repair or replace the damaged component.Alloys with improved creep resistance may be required for longer life (e.g. Nb, V, Mo, Ti)Visual inspection followed by thickness measurements may be required to assess remaining life of heater tubes
26 Typical fractograph of creep crack growth IntergranularMixed Mode : Intergranular and Transgranular
31 Monkman and Grant relationship Creep strength could be estimated from rupture strength with sufficient accuracy for design purposesMonkman and Grant relationshiplogtr + Clogєs = K, where K and C are constant and tr is rupture life and єs is the minimum creep rateCreep properties of copper tested at 688K. Make an estimate of (a) the rupture time at 27.6 MPa and (b) the stress required to give a rupture time of sStress , MPaMinimum creep rate,% s-1Time to fracture, s48.334.527.63.0 x10-47.5 x10-53.6 x10-556402640-
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