BMFB 4283 NDT FAILURE ANALYSIS Lectures for Week

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

Issues to address 11. 0 Creep and Stress Rupture 11. 1 Introduction 11. 2 Creep Tests 10. 3 Mechanism of Creep Deformation and 10. 4 Fracture at Elevated Temperature 10. 5 Creep Data

/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

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 Elevated. Temperatures, ed, R. Raj (Metals Park, Ohio: ASM, 1985). )

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 coarsegrained material than a finegrained material. – Promoted by certain carbide types in some Cr-Mo steels. 22

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. 23

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

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 25

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

• 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 688 K. 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 48. 3 34. 5 27. 6 Minimum creep rate, Time to fracture, s % s-1 3. 0 x 10 -4 7. 5 x 10 -5 3. 6 x 10 -5 5640 2640 -