Hydrogen Damage

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Hydrogen, in small quantities, damages metallic materials and reduces their serviceability. Several phenomena associated with hydrogen are shown here.

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  • Hydrogen Damage

    1. 1. HYDROGEN DAMAGE OF METALLIC MATERIALS T.K. G. NAMBOODHIRI PROFESSOR OF METALLURGICAL ENGINEERING (Retired) BANARAS HINDU UNIVERSITY VARANASI
    2. 2. Introduction <ul><li>Hydrogen-most ubiquitous element </li></ul><ul><li>Minute amounts can damage metals </li></ul><ul><li>Can be easily picked up by metals during melting, casting, working, fabrication and use </li></ul><ul><li>Many forms of hydrogen damage </li></ul>
    3. 3. Classification of hydrogen damage HYDROGEN ENVIRONMENT EMBRITTLEMENT CRACKING DEGRADATION OF OTHER MECHANICAL PROPERTIES LOSS IN TENSILE DUCTILITY SLOW STRAIN RATE EMBRITTLEMENT HYDROGEN EMBRITTLEMENT HYDRIDE EMBRITTLEMENT HYDROGEN DAMAGE SOLID SOLUTION HARDENING HIGH STRAIN RATE EMBRITTLEMENT CREATION OF INTERNAL DEFECTS POROSITY SHATTER CRACKS , FLAKES, FISH EYES BLISTERING HYDROGEN ATTACK HYDROGEN STRESS
    4. 4. Solid solution hardening. <ul><li>Hydrogen hardens metals like Nb,Ta and Ti at concentrations within solid solubility limits. These metals may be Embrittled at high strain rates. </li></ul>
    5. 5. HYDROGEN EMBRITTLEMENT HYDROGEN EMBRITTLEMENT HYDROGEN ENVIRONMENT EMBRITTLEMENT HYDROGEN STRESS CRACKING LOSS IN TENSILE DUCTILITY & OTHER MECHANICAL PROPERTIES
    6. 6. Hydride embrittlement <ul><li>Seen in hydride forming metals like Ti, V, Zr </li></ul><ul><li>Low strain rate embrittlement : At low H concentrations. Similar to H.E.. Caused by stress-nucleated hydrides </li></ul><ul><li>High strain rate embrittlement : At high H concentrations. Due to hydride precipitates. Enhanced by high strain rates. </li></ul>
    7. 7. Creation of internal defects <ul><li>Blistering : Formation and collection of molecular hydrogen at internal defects like grain boundaries, inclusions and laminations, build-up of high pressures leading to formation and growth of blisters </li></ul><ul><li>Shatter cracks and flakes : Internal fissures seen in large forgings due to segregated H </li></ul><ul><li>Fish-eyes : bright patches of embrittled regions visible on fracture surfaces of weldments. </li></ul>
    8. 8. Creation of internal defects <ul><li>Micro-perforations: Exposure to high hydrogen pressures at R.T. may develop small fissures or micro-perforations </li></ul><ul><li>Porosity: Liquid metals containing large amounts of H liberate it during solidification causing porosity in Fe, Al and Mg. H content, cooling rate, external pressure and H partial pressure control porosity </li></ul><ul><li>Hydrogen attack : Exposure of steels to high pressure H at high temperatures causes H to react with carbon of steel to produce methane. Leads to decarburization, dissolution of carbides and weakening of the steel. </li></ul>
    9. 9. CHARACTERISTICS OF IMPORTANT H. D. PROCESSES <ul><li>HYDROGEN EMBRITTLEMENT </li></ul><ul><li>HYDROGEN INDUCED BLISTER CRACKING </li></ul><ul><li>HYDROGEN ATTACK </li></ul>
    10. 10. Characteristics of Hydrogen Embrittlement <ul><li>Strain rate and temperature sensitivity </li></ul><ul><li>Delayed failure </li></ul><ul><li>Sub critical cracking and threshold stress intensity </li></ul><ul><li>Effects on tensile properties </li></ul><ul><li>Fracture mechanisms </li></ul>
    11. 11. Strain rate and temperature sensitivity of H.E. <ul><li>Most pronounced at slow strain rates and ambient temperatures </li></ul><ul><li>Decreases monotonically with increasing strain rate </li></ul><ul><li>Minimum at around room temperature </li></ul>
    12. 12. Delayed failure in H.E. <ul><li>Incubation period independent of stress </li></ul><ul><li>Time to failure increases with decreasing stress </li></ul><ul><li>Discontinuous crack propagation </li></ul><ul><li>Lower critical stress below which no failure </li></ul><ul><li>H causes delayed failure in many materials </li></ul><ul><li>Parameters strongly dependent on H content </li></ul>
    13. 13. Effects of H on tensile properties <ul><li>Increased/decreased/unaffected Y.S. </li></ul><ul><li>Strain localization/Luders band/serrated yielding </li></ul><ul><li>Increased/decreased flow stress and work hardening rate </li></ul><ul><li>Increased screw dislocation velocity and dislocation multiplication </li></ul><ul><li>Loss in tensile ductility </li></ul>
    14. 14. Fracture mechanisms in H.E. <ul><li>H promotes all fracture mechanisms </li></ul><ul><li>MVC in low strength steels </li></ul><ul><li>Ductile, quasi-cleavage or I.G. fractures in high strength steels </li></ul><ul><li>I.G.failure in Ni </li></ul><ul><li>I.G. or Transgranular cleavage(fluted morphology) in Ti alloys. </li></ul><ul><li>I.G., cleavage or M.V.C. in Al alloys. </li></ul>
    15. 15. H induced fracture morphology
    16. 16. Hydrogen Induced Blister Cracking (HIBC) <ul><li>Metallurgical parameters: Non-metallic inclusions, Sulfur content, alloy segregation and microstructure. Very low S levels and inclusion shape control to eliminate HIBC nucleation. Reduction in hardness of segregated zones to prevent HIBC propagation. </li></ul><ul><li>Environmental parameters: Hydrogen fugacity. A threshold fugacity below which HIBC does not occur. </li></ul>
    17. 17. HYDROGEN INDUCED BLISTERS <ul><li>Top picture shows hydrogen induced blisters in a pipe line carrying hydrogen sulfide containing fluids </li></ul><ul><li>Bottom picture shows the cross section of a blister </li></ul>
    18. 18. Hydrogen Induced Blister Cracking (HIBC) <ul><li>Hydrogen blisters nucleate on inclusion stringers and cause cracking when their internal gas pressure exceeds the fracture stress- HSLA/Line pipe steels. Straight or step-wise cracking </li></ul>
    19. 19. HYDROGEN ATTACK <ul><li>Equipment in contact with hydrogen at temperatures above 260 C and hydrogen partial pressures above 690 KPa may suffer from high temperature hydrogen attack. </li></ul><ul><li>Atomic hydrogen diffuses readily in steels and reacts with carbon present to form methane which collects at grain boundaries and in minute voids. </li></ul><ul><li>Methane pressure build-up leads to cracking </li></ul>
    20. 20. Hydrogen attack : Nelson curves for steels <ul><li>Nelson curves show safe operating limits of temperature and hydrogen partial pressure for steels. Empirical curves drawn from actual experience </li></ul>
    21. 21. Prevention of Hydrogen damage <ul><li>Metallurgical control: Alloy chemistry, crystal structure, microstructure and substructure. </li></ul><ul><li>Environmental control: Inhibition by organic compounds like nitriles, sulfoxides, azoles and amines. </li></ul>
    22. 22. CONCLUSIONS <ul><li>SMALL QUANTITIES OF HYDROGEN CAN SERIOUSLY AFFECT THE PROPERTIES OF MANY STRUCTURAL MATERIALS </li></ul><ul><li>HYDROGEN DAMAGE HAS MANY FORMS </li></ul><ul><li>CAREFUL SELECTION OF MATERIALS, PROCESSING STEPS AND ENVIRONMENTS CAN PREVENT HYDROGEN DAMAGE </li></ul>

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