Module 8

Module 8 Welding Metallurgy for the Welding Inspector Cairo Inspection Company Eng. Ibrahim Eldesoky ASNT –NDT-LEVEL III ASME AUTHORIZED INSPECTOR

Metallurgy “ The science dealing with the internal structure of metals and metal properties ”

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Welding Metallurgy “ Concerned with the changes occurring in metals during welding ”

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Discussion Topics  Structures  Changes on heating  Changes on cooling  Heat treatments  Alloying  Phases

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Atomic Structure  Matter made up of atoms  Atoms are very small  Electrons orbit atomic nuclei  Have a home location in solids  Oscillate around home position

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Atomic Structure Simple Cubic

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Home Location

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Solids  Minimum internal energy  Atoms fixed in place  Atoms have ‘home’ position  Atoms vibrating  Orbiting electrons  Crystalline structure

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Liquids  Higher internal energy  Atoms free to move  Atoms vibrating more  Orbiting electrons  No fixed structure

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Solid vs Liquid

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Thermal expansion on Heating Thermal contraction on Cooling

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Welding Stresses (Tensile)  Remain after welding is completed (residual stress)  Can cause distortion  (welding stress exceeds yield strength of material)  Can cause cracking  (welding stress exceeds tensile strength of material)  Residual stress can be as high as yield strength  Relieved by three methods Cairo Inspection Company (CIC)

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Stress Relieving  Thermally  Vibratory  Peening

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Crystal Structures  BCC - Body Centered Cubic  FCC - Face Centered Cubic  HCP - Hexagonal Close Packed  BCT - Body Centered Tetragonal

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Common Crystal Structures

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Metal Solidification

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Grain Size Effects  Fine grained materials Have good tensile strength Have good ductility Have good low temperature properties  Coarse grained materials Have slightly lower strength Are slightly less ductile Have good high temperature properties Cairo Inspection Company (CIC)

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Alloying “ Adding elements to change mechanical or physical properties ”

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Interstitial Alloying

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Interstitial Alloying Results in Lattice Distortion

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Substitutio nal Alloying

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Substitutional Alloying Results in Lattice Distortion

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Microstructures  Metals composed of grains (crystals)  Very small  Separated by grain boundaries  Different phases within grains

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Microstructure of Iron

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Iron-Carbon Phase Diagram

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Phases in Steel  Ferrite  Cementite  Austenite  Bainite  Pearlite  Martensite

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Cooling Rates  Slow  Moderate  Fast

- Pearlite - Bainite - Martensite

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- softest - hardest

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Lamellar Appearance of Pearlite

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Martensite

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Tempering “ Restores ductility and toughness and reduces strength and hardness ”

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Effects of Tempering Temperatures Alloy is AISI 4142

Tempering temperature is in degrees F

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Effects of Tempering Temperatures Alloy contains 12% Cr

Note reduction in impact strength at 885 F (475 C)

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Continuous Cooling Diagram & Jominy Hardenability Diagram for 8630 steel. Phase formed on cooling varies with cooling rate.

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Weld Area & Fe - Fe3C Diagram

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Heat Input

Joules/inch = current x voltage x 60

travel speed (in./min)

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Preheat  Slows cooling rate  Produces less martensite  Reduces risk of cracking  Reduces distortion  Reduces hydrogen

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Carbon Equivalent ( CE ) CE = % C + % Mn + % Ni + % Cr + % Cu + % Mo 6 15 5 13 4

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Examples of Preheating vs C.E. C.E. < 0.45 0.45 - 0.60 >0.60

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Preheat Temperature

Optional 2000 - 4000 F 4000 - 7000 F

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Heat Treatments for Steels  Super-critical - above transformation temp. Annealing Normalizing Quenching (to harden)  Sub-critical - below transformation temp. Tempering Stress Relieving Cairo Inspection Company (CIC)

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Diffusion “ Movement of atoms within a solution, be it a solid, liquid or gas”

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Diffusion of Lead and Gold Atoms

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Solubility Dissolving Salt into Water  Solid into liquid  Diffusion mechanism  Solubility limit  Temperature effects

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Solid Solubility  Unique to metals  Solid into solid  Solubility limit  Temperature effects

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Examples of Solid Solubility  Gold and lead bars  Carburizing  Nitriding

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Stainless Steels  Ferritic  Martensitic  Austenitic  Precipitation Hardening (PH)  Duplex

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Sensitization of Austenitic Alloys “ Formation of chromium carbides between 800 - 1600 degrees F ” (Sensitization reduces the corrosion resistance in many environments)

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Corrosion Attack of Austenitic Stainless Steel

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Avoiding Sensitization  Heat treatment ( SAWQ )  Stabilized grades  Low carbon grades

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Other Common Alloys  Aluminum Pure aluminum Aluminum-copper Aluminum-magnesium  Copper Pure copper Brasses Bronzes Cairo Inspection Company (CIC)

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Main Content Page Layout•

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The End …… any questions?

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