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Hydrogen Embrittlement of Pipeline Steels: Causes and Remediation

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Hydrogen Embrittlement of Pipeline Steels: Causes and Remediation 1 HYDROGEN EMBRITTLEMENT OF PIPELINE STEELS: CAUSES AND REMEDIATION P. Sofronis, I. Robertson, D. Johnson University of Illinois at Urbana-Champaign Hydrogen Pipeline R&D Project Review Meeting Oak Ridge National Laboratory, Oak Ridge TN January 5-6, 2005 January 2005 2 Hydrogen Embrittlement: Long History M.L. Cailletet (1868) in Comptes Rendus, 68, 847-850 W. H. Johnson (1875) On some remarkable changes produced in iron and steels by the action of hydrogen acids. Proc. R. Soc. 23, 168-175. D. E. Hughes (1880) Note on some effects produced by the immersion of steel and iron wires in acidulated water, Scientific American Supplement, Vol. X, No 237, pp. 3778- 3779. Literature is voluminous January 2005 3 Hydrogen Embrittlement: Long History Proc. R. Soc. 23, 168-175, 1875 January 2005 4 Hydrogen Embrittlement: Long History Proc. R. Soc. 23, 168-175, 1875 January 2005 5 Hydrogen Embrittlement: Definition Material degradation caused by the presence of hydrogen under load. It is manifested in z Strain hardening rate z Tensile strength z Reduction in area z Fracture toughness z Elongation to failure z Crack propagation rate Degraded material often fail prematurely and sometimes catastrophically after many years of service Degradation is influenced by z Microstructure and operating conditions January 2005 6 Hydrogen-Induced Crack Propagation in IN903 Static crack in vacuum. Hydrogen gas introduced abc main main crack crack 0 s thinning 17s thinning 21s de f Crack 29s Micro-crack 32 s 39s formed linkage January 2005 7 Crack Propagation in IN 903 Due to Hydrogen Direction of crack hole advance Deformation band January 2005 8 Hydrogen Embrittlement Mechanisms Several candidate mechanisms have evolved each of which is supported by a set of experimental observations and strong personal views Viable mechanisms of embrittlement z Stress induced hydride formation and cleavage ¾ Metals with stable hydrides (Group Vb metals, Ti, Mg, Zr and their alloys) ¾ Supported by experimental observations z Hydrogen enhanced localized plasticity (HELP) ¾ Increased dislocation mobility, failure by plastic deformation mechanisms ¾ Supported by experimental observations z Hydrogen induced decohesion ¾ Direct evidence is lacking ¾ Supported by First Principles Calculations (DFT) Degradation is often due to the synergistic action of mechanisms January 2005 9 Hydrogen Enhanced Localized Plasticity (HELP) Failure is by localized shear processes occurring along slip planes: shear localization Transgranular fracture surfaces are highly deformed despite the fact macroscopic ductility is reduced (localized shear processes occurring along slip planes) Intergranular fracture occurs by localized ductility in the region adjacent to the grain boundaries Applicable to all systems z Non-hydride forming systems (Fe, Ni, Al, 304, 310, 316 stainless steel, Ti3Al, Ni3Al) z Hydride forming systems α-Ti, β-Ti z Alloy and high purity systems z Bcc,fcc, hcp Underlying principle is the hydrogen-induced shielding of the Interactions between microstructural defects January 2005 10 Features on Intergranular Surfaces of a 0.28pct C Steel Fractured in Hydrogen Decrease in the density and size of ductile features (tear ridges) as a function of crack length. Observations such as these led Beachem to conclude that hydrogen impacts plastic processes. C. D. Beachem, Met. Trans. 3,437, (1972) January 2005 11 Fracture Surface of a Beta 21S Alloy in Absence of Macroscopic Ductility (Brittle Failure) 30 20 10 Plastic elongation (%) 0 0.00 0.10 0.20 0.30 Hydrogen concentration (H/M) January 2005 12 Slip Lines on Brittle Intergranular Facets 310s stainless steel, 5.3 at% H 310s stainless steel, 5.3 at% H Ulmer and Altstetter, in Hydrogen Effects on Materials Behavior Moody and Thompson, p. 421 January 2005 13 Instrumentation: Controlled Environment Transmission Electron Microscope CELL Aperture cones Specimen JEOL 4000 Controlled Pumping axis Environment Transmission port JEOL Electron4000 Environmental Microscope cell TEM Objective Lens Pole-piece D. K. Dewald, T. C. Lee, J. A. Eades, I. M. Robertson and H. K. Birnbaum Review of Scientific Instruments, 62, 1438, 1991. January 2005 14 Instrumentation: Stages and Samples Single-tilt straining stage Double-tilt straining stage Single-tilt, high T straining Single-tilt, low T straining Double-tilt stage Double-tilt, high T stage January 2005 15 Influence of Hydrogen on Dislocation Mobility (Fe under constant Load and Increasing H Pressure) January 2005 16 Dislocation Motion in Fe due to Introduction of Hydrogen Gas Increasing hydrogen gas pressure 14:60 14:77 18:50 18:90 20:60 20:71 30:40 30:64 36:16 36:41 38:41 41:21 January 2005 17 Hydrogen Effect on Dislocation Mobility in Ti January 2005 18 Hydrogen-Deformation Interactions Solute hydrogen atoms interact with an applied stress field z Hydrogen-induced local volume dilatation (2cm3/mole in Fe) z Hydrogen-induced local elastic moduli changes (measurements in Nb) Solute hydrogen diffuses through normal interstitial lattice sites (NILS) toward regions of lower chemical potential, i.e. z Tensile hydrostatic stress z Softened elastic moduli January 2005 19 Iso-concentration Contours of Normalized Hydrogen Concentration around two Edge Dislocations in Niobium as the Separation Distance Between the Dislocations Decreases January 2005 20 Effect of Hydrogen Atmospheres around the Two Dislocations: Shear Stress on Dislocation 2 vs Separation along the Slip Plane Hydrogen reduces the interaction between dislocations January 2005 21 Reversibility of Hydrogen Effect on Adding and Removing Hydrogen Pressure increase from 15 to 75 torr Pressure decrease from 75 to 9 torr Material: high-purity Al black - initial position white - final position January 2005 Pressure increase from 15 to 75 torr 22 Influence of Hydrogen on Dislocation Separation in a Pile-up (310S Stainless Steel) January 2005 23 Hydrogen-Carbon Interaction Iso-concentration countours of normalized hydrogen concentration around an edge dislocation and a carbon atom with a tetragonal axis [100] Carbon atom is modeled as a stress center with a tetragonal distortion January 2005 24 Shielding of Interaction Between Edge Dislocation and Carbon Atom January 2005 25 Hydrogen Effect on Dislocation Cross Slip in Aluminum An Alternative Explanation of Increasing Slip Localization Change in line direction in (c) indicates cross-slip process has initiated. Cross-slip process halted due to hydrogen, (c) and (d). Comparison images Cross slip process Increasing hydrogen pressure constant hydrogen pressure decreasing hydrogen pressure. resumes as hydrogen pressure reduced (f). JanuaryCross-slip 2005 in progress. Cross-slip process halted. Cross slip process resumes 26 HELP: Conclusions Hydrogen enhances the mobility of dislocations and decreases the separation distance between dislocations in a pile-up Hydrogen restricts cross-slip by stabilizing edge character dislocations. Hydrogen enhances crack propagation rates Hydrogen reduces the stacking-fault energy of 310S stainless steel by 20% January 2005 27 HELP and Hydrogen Embrittlement Intermediate temperatures Low temperatures High temperatures or high strain rates or low strain rates Atmosphere moves with •No atmposphere dislocation •No hydrogen effect Atmosphere lags behind •Shielding Embrittlement KT> WB −6 Ni 200<T << 300K;ε 10 s−1 CC/exp(/)0 = WB KT Fe 77<T < 400K •Both Ni and pure Fe hardened by hydrogen at Ni is not embrittled −−51 ε>10 s T >473K T > 473K •Ni is hardened by hydrogen at •At higher strain rates atmosphere T < 200K moves but lags behind hardening •Increasing the temperature •Pure Fe is hardened at gives serrated yielding January 2005T <100K 28 Critical Issues for the HELP Mechanism How does the hydrogen effect on the microscale affect the mechanical behavior on the macroscale? How does hydrogen enhance slip localization? What are the synergistic effects of other solutes on HELP type fracture, particularly at grain boundaries? What is the actual mechanism by which the enhanced plasticity leads to fracture? z Localized microvoid coalescence (seen in situ TEM)? z Stroh crack due to compressed pile-ups? January 2005 29 MODELING OF HELP Connecting the microscopic to the macroscopic January 2005 30 Modeling of Hydrogen-Induced Instabilities in Plane Strain Tension Multiplicity of solutions Initially Initially homogeneous homogeneous deformation deformation localizes into localizes into a band of a narrow neck intense shear Important note Shear band bifurcation or shear localization is a precursor to material failure. How does hydrogen affect these instabilities? January 2005 31 Hydrogen in Equilibrium with Local Stress and Plastic Strain CCLT+ Dilatational strain Flow stress reduction Hydrogen trapped Problem Elastoplasticity at dislocations is p CC,ε TL fully coupled Stress-Plastic strain Plastic response assumptions: • Material rate independent Lattice hydrogen • von Mises yielding • Hardens isotropically C CCσ , 0 L kk 0 • H-induced softening • H-induced lattice dilatation Equilibrium theory Solution incremental in time (load) January 2005 32 Condition for Shear Banding Bifurcation 2 2 1 ∂σ ∂σ ∂c hcr 11++ννβµ+ h = Y + Y equals =−−+βµ N 3 ∂ε p ∂c ∂ε p G ( ) 23II 91( −ν ) 2 2 43−+NII 1ν 2 +−+( ) βµsin2 θ σeeOσ ( )()0 GG 24 3( 1−ν ) 0.002 c0=0.1 c =0.3 hh, 0 cr h c0=0.5 0.001 bifurcation points Dependence of the critical h cr and tangent modulus h on the 0 macroscopic strain for initial ε22 σ I =σ 22 h cr hydrogen concentrations of H/M = 0.1, 0.3, 0.5
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