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Lecture 9 (26/Sept/05) : Codes & Specifications, Weldability (partially done in 2005)

Lecture 9 (26/Sept/05) : Codes & Specifications, Weldability (partially done in 2005). Prof. T.W. Eagar Fall 2005 MIT. Rules for Testing & Design. By order of importance: 1. Code 2. Specifications Not a code No force of Law 3. Recommended practice 4. Guide

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Lecture 9 (26/Sept/05) : Codes & Specifications, Weldability (partially done in 2005)

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  1. Lecture 9 (26/Sept/05):Codes & Specifications, Weldability (partially done in 2005) Prof. T.W. Eagar Fall 2005 MIT

  2. Rules for Testing & Design By order of importance: • 1. Code • 2. Specifications • Not a code • No force of Law • 3. Recommended practice • 4. Guide • Rules for Designing and Building Pressure Vessels • AWS Codes [buildings and bridges (considers fatigue more)]

  3. Design History

  4. Weldability of Steels • See Phase diagram • Easiest to weld is low carbon • High carbon steels are a pain to weld • Cast irons are terrible to weld (critical flaw size down)

  5. Iron-Carbon Phase Diagram

  6. Welding Steels • Austenitic (Face Centered Cubic) • Easy to weld • Low corrosion resistance • Ferritic (Body Centered Cubic) • Magnetic • H - cracking due to high Cr content • Martensitic (Body Centered Tetragonal) • Quenched and tempered • Incredibly tough, great grain size (gives both strength and toughness) • Can get 300ksi • Very difficult to weld • 4. Duplex (Austenite + Ferrite) • Very good to weld

  7. History of Welding (not done in 2005) • 1850’s: Bessemer figured out how to get high enough temperatures to melt steel by preheating air • 1880’s: Carnegie started US Steel and became richest man in world • 1807: Sir Humprey Davies discovered electric arc • Edison and Westinghouse provided enough electricity to do arc welding in 1880’s

  8. History of Welding (not done in 2005) • at first bare steel electrodes were used but that makes porous weld metal because of nitrogen escape from the solidifying metal • Goteborg, Sweden: muddy electrodes were found to create nonporous weld metal • The beginning of ESAP Corp • start of mineral flux covered electrode • AWS Electrode Designation: E7018 (70 ksi tension strength, 8 is about the flux) • can have low hydrogen ... but have to bake out water • In US at the same time, a paper-wrapped electrode was found to make better welds ... so cellulose is also used as flux ... the decomposition of the cellulose makes shielding atmosphere with lots of hydrogen • eg. E6011 • This was all called stick welding, Metal Manual Arc Welding or Shielded Metal Arc Welding • Flux formulations still somewhat of a black art, one uses old tobacco stalks (concentration of rubidium)

  9. History of Welding (not done in 2005) • 1920’s Comfort Avery Adams of Harvard founded American Welding Society • 1930’s Big Inch Pipeline from LA to NY was built with welding, the first critical strucuture • 1940’s welding took off with WWII ships • 1930’s Submerged Arc Welding ... powdered flux on work covers the arc from the bare electrode ... much faster welding than SMAW • 1940’s Gas Tungsten Arc Welding (GTAW or TIG (tungsten inert gas welding)) ... uses nonconsumable tungsten electrode in argon or helium shielding gas • great for welding Al • 1950’s Gas Metal Arc Welding (GMAW) or MIG (metal inert gas) bare metal wire in argon or helium shielding gas • 1960’s Lincoln Electric made self-shielded MIG ... basically the flux is inside the hollow tube electrode instead of on the outside like in SMAW ... can also had metals to react with nitrogen to remove

  10. Hydrogen Cracking • Venn diagram: need all three of the following items to have a problem • Hydrogen • Stress • Susceptible microstructure • Typically concerned with hardness >Rc30 approx 120ksi

  11. Hydrogen Cracking • Nitrogen problem has been solved but today the problem (for welding steels) is hydrogen cracking • like with WWII Liberty ships • hydrogen comes from flux or cellulose • Hydrogen dissolves more in liquid steel than solid steel • Can go to inclusions or other flaws, promotes other mechanisms • Sometimes called delayed cracking, often hours after the welding, up to days, shipyards wait a week then do NDT • but not months ... hydrogen cracking occurs within a few hours of welding unless trapped in porosity .. then it could take a bit longer • Have to bake out hydrogen in oven, will diffuse out but takes time and can’t let it diffuse through • SMAW requires less than tenth of a percent of moisture to avoid hydrogen cracking • on a warm day, there’s enough humidity in the air to exceed hydrogen cracking limits with non-gas-shielded welding

  12. Susceptibility to Hydrogen Cracking • Source: BA Granville, Cold Cracking in Welds in HSLA Steels, Welding of HSLA Steels, Proc. Int. Conf., ASM, NOV 1976

  13. Explanation of Hydrogen Cracking Plot • Used in Welding of Steels • Zone II, traditional steels • HY130 is near middle, HY100, HY80 below • Carbon equivalent is a measure of the hardenability of the steel, • Preheat required is a function of the carbon equivalent (to avoid hydrogen cracking) • Boiler and pressure vessel code: “shall not weld” below 50degF without preheating. • Things start to get many monolayers of water on surface

  14. Controlling Hydrogen Cracking(not done in 2005) • Preheat • Postheat • Heating can be done many ways: • Induction heating • Resistance heating • Local flame heating • Large flame furnace • Shot Peen • Stress Relieve • Wait 48 hrs to non-destructively test • Hydrogencracking usually shows itself within 1 week, but in low temp conditions can take much longer

  15. Controlling Hydrogen Cracking(not done in 2005): Preheat Baseplates • Reduces hydrogen • One of largest single costs in a military shipyard • Typical 200degF • HY100 on SeaWolf 400degF “blue jelly” suits • Workers have to wear water cooled suits to weld submarines • Typically use electric resistance heaters, large % of the electricity usage • Susceptibility to H2 cracking maximizes at about room temp, but you should not weld below 32 deg F

  16. Controlling Hydrogen Cracking(not done in 2005): Postheat • reduce hydrogen • keep the temperature • diffuses out (analogy CO2 bubbles from ginger ale) • time depending on thickness and temperature • SeaWolf doing postheating as well • Diagram of weld prep, ¾” thick, then do a postheat, if went to full • 4” then wouldn’t get the hydrogen out for weeks • Can help temper the weld metal

  17. Peening (not done in 2005) • Very empirical • Use Almen gauge, about 1”x 4” of the material, • Hit it, see how much it distorts • Measure height of the bow • Not much science to peening • Other choices have better control • Was the only way to weld heavy armor plate • Not used much anymore

  18. Other Problems in Welding Steels (besides Hydrogen Cracking) • Useful Book: Weldability of Steels, good appendix, 80pages with welding parameters • Solidification Cracking • Laminar Tearing • Reheat Cracking • Welding through primers (can get phosphor) • Cavities and Porosity • Slag Inclusions • Lack of fusion/penetration • Imperfections in shape • but most problems come from hydrogen cracking

  19. Welding of AluminumAluminum Alloys • Non-heat treated • 1000 series, 99+% Al, 1100 is pure • 3000 series, Mn to increase strength • highest volume production alloy (for cans) • 4000 series Si • used for welding • 5000 series, Mg solid solution strengthening • Solid solution strengthening, 4-20ksi annealed yield strength, 18-40 ksi • Work hardened yield strength • Heat treated (precipitate hardened) • 2000 series, Cu • eg 2024 can’t be welded ... it cracks, but 2219 can be welded (used in space shuttle) • typically need to diffusion weld these alloys • 6000 series, Mg + Si • eg 6061-T6 (very common alloy): 6061 refers to composition, T6 refers to tempering and aging • 7000 series Zn eg 7075 (aircraft) • 8000 series other • hard to weld and maintain the same strength • 7-14 ksi annealed, 25-72 ksi heat treated, high specific strength

  20. Aluminum is Canned Energy • half the cost of aluminum is due to energy cost to make it • if you have cheap energy, make aluminum and ship it • Aluminum is made in Quebec, Norway, Venezuela and Russia, places with cheap energy (mainly hydroelectric) • Major producers are Alcan and Alcoa

  21. Aluminum Can Be Strengthened by Precipitates • learned this in 1920’s • quench aluminum, heat it up to allow precipitates to grow and strengthen Al • doubles yield strength • these alloys are hard to weld, because welding messes up precipitates • if this alloy is welded and then stressed, it will fail at half the yield strength that it could have

  22. Joining of Aluminum • Most aluminum casting alloys are heavily alloyed, lower melting point, easier to cast • High strength solid solution aluminum alloy typical 30ksi yield strength, about the same or less than garden variety steel • Welding can almost match basemetal strength • Heat treated alloys up to 70ksi strength, for welding have difficulty meeting the base metal strength, say 50% • Significant weakness in the heat-affected zone • Must heat treat high performance Al alloys to get strength

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