BMFB 4283NDT & FAILURE ANALYSIS

Lectures for Week 11

Prof. Qumrul Ahsan, PhD Department of Engineering MaterialsFaculty of Manufacturing Engineering

11.0 Creep and Stress Rupture

11.1 Introduction11.2 Creep Tests10.3 Mechanism of Creep Deformation and 10.4 Fracture at Elevated Temperature 10.5 Creep Data

Issues to address

/G > 10-2

10-4< /G <10-2

/G < 10-4

• Ductile fracture initiated at inclusions and second-phase particles

• Transgranular fracture due to nucleation of voids and void coalescence are postponed

• Grain boundary sliding , wedge crack s and voids growth -> intergranular fracture

• At 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

.(From W.D. Nix and J. C. Gibeling, in Flow and Fracture at ElevatedTemperatures, ed, R. Raj (Metals Park, Ohio: ASM, 1985).)

Mechanisms of intergranular nucleation

• Two types of intergranular cracks :• Wedge-shaped cracks (w-type cracking)• Round or elliptical cavities (r-type

crack)

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Critical Factors in Creep• The rate of damage or strain rate

• Threshold temperatures

• Low 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.

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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 tubes

• Weld and HAZ of Catalytic Reformer Tubes and Reactors

• Welds of dissimilar materials (ferritic to austenitic welds)

• All boiler and fired heater tubes are susceptible.

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Appearance or Morphology of Creep Damage

DimensionalReplication and hardness

• Change in dimensions• Creep voiding• Change in microstructure (SEM, OM)• Change in hardness

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Prevention / Mitigation of Creep

• To prevent creep damage - minimize the metal temperature

• To avoid stress concentrators

• Low 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

Typical fractograph of creep crack growth

Mixed Mode : Intergranular and Transgranular

Intergranular

• Creep strength could be estimated from rupture strength with sufficient accuracy for design purposes

• Monkman and Grant relationship logtr + Clogєs = K, where K and C are constant and tr is rupture life and єs is the minimum creep rate

Creep 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 12000 s

Stress , MPa Minimum creep rate,% s-1

Time to fracture, s

48.334.527.6

3.0 x10-4

7.5 x10-5

3.6 x10-5

56402640

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