1. BMFB 4283 NDT & FAILURE ANALYSIS
Lectures for Week 11
Prof. Qumrul Ahsan, PhD
Department of Engineering Materials
Faculty of Manufacturing Engineering

2. Issues to address 11.0 Creep and Stress Rupture 11.1 Introduction
11.2 Creep Tests
10.3 Mechanism of Creep Deformation and Fracture at Elevated Temperature
Creep Data

13. /G > 10-2
10-4< /G <10-2
/G < 10-4

20. 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

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 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.

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 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.

24. Appearance or Morphology of Creep Damage
Change in dimensions
Creep voiding
Change in microstructure (SEM, OM)
Change in hardness
Dimensional
Replication and hardness

25. 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

26. Typical fractograph of creep crack growth
Intergranular
Mixed Mode : Intergranular and Transgranular

31. Monkman and Grant relationship
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 s
Stress , MPa
Minimum creep rate,
% s-1
Time to fracture, s
48.3
34.5
27.6
3.0 x10-4
7.5 x10-5
3.6 x10-5
5640
2640 -