WELDING METALLURGY

WELDING METALLURGY

1. HEAT FLOW

   • Amps x Volts = Watts
   • Heat & Light = Watts
   • Efficiencies; SAW 90-99%, GTAW 25-50%, GMAW, FCAW, SMAW in between

2. ENERGY INPUT

   • (Volts x Amps)/Speed
   • Higher = Larger the weld, wider the HAZ, slower cooling rate

3. COOLING RATES

   • Heat input to the weld
   • Plate thickness
   • Initial temperature of plate

4. ZONES IN A WELD

   • Regions >723C will partially transform to Austenite ferrite
   • >910C complete Austenite
   • Low energy input on thick plate; cool rate may be as fast as quench and hard martensite
   • Higher energy on thin plate/preheat; slow cool rate and softer microstructure
   • Coarse grain = low toughness
   • hard martensite = cracking susceptibility

5. HYDROGEN CRACKING

   • Weld metal or HAZ to H embrittlement related to Chem Comp. and microstructure
   • Hydrogen content
   • Stress / Strain level of crack initiation
   • Temperature; cracking can occur after weld cooled to <100C

6. HYDROGEN

   • H can cause severe embrittlement = reduction in ductility / drop of fracture stress of notched steel
   • Diminishing effect w/ increasing Temp, 150-200C H has no effect
   • Effect is Max. at slow or steady rate of impact loading

7. STRESSES

   • Restraint
     • Higher in Thicker Plates and Groove welds
   • Stress Concentrations
   • Avoid Cracking, Control;
     • Stress
     • Microstructure
       • Hardenability (Alloy Content)
         • Higher = Cooling rate decreases
       • Weld cooling rate
     • Hydrogen content
     • Temperature

8. HAZ STRUCTURE

   • Zone I; <0.10% Carbon
   • Zone II; C & C maganese steels; cooling rate controlled
   • Zone III; >0.10% Carbon; Hydrogen controlled

9. HYDROGEN CONTROL

   • Higher carbon alloy; preheat alone may be insufficient = PWHT may be required
   • If HAZ is excessive hard; softening by tempering at temperature may be needed.

10. CRACK TESTS

    • Controlled Thermal Sensitivity (CTS)
      • Fillet weld 2 blocks bolted together; thermal severity or cooling rate changed by using diff thickness plates
    • Tekken Test
      • Y Groove prep promotes HAZ cracks, straight part encourages weld metal crack.
      • Higher restraint test with restraint increase with thickness
    • Gapped Bead-On-Plate (G-BOP)
      • Welds are heated with torch to a dull red in vicinity of gap
      • discolouration of fracture surface indicates crack present before heating

11. LAMELLAR TEARING

    • Non-metallic inclusions that flatten out during rolling results in ductility THROUGH the thickness being lower than other directions
    • Occur in large welds where strains are normal to plate surface; T-Butt and corner are most susceptible
    • Main factors
      • Type of Joint
      • Through-thickness ductility of steel
      • Restraint of the joint
      • Other WPS factors; i.e. H content

12. WELD METAL

    • Solidified weld metal will have a microstructure differing from that of parent
    • More like a rapid cool casting
    • Meeting equal strength but may have much lower carbon content

13. DILUTION

    • Composition depends on
      • Composition of Electrode
      • Composition of parent metal
      • Chemical reactions w/ weld metal, flux/shield gas
    • Weight of Parent Metal Melted / Weight of Total melted Metal

14. CHEMICAL REACTIONS

    • Deoxidizing elements; Silicon, Manganese, Aluminum react with O to form non-metallic inclusions

15. SOLIDIFICATION

    • Weld speed high = elongated weld pool and Xtls grow from side of pool towards each other
    • Weld speed low = growth of Xtl will originate from back of weld pool

16. SOLIDIFICATION CRACKING

    • Liquid trapped when Xtl grow on cooling.
    • Cracks can run along centre of weld, and even entire length
    • Rule-of-thumb; Root run Current should not exceed 10x groove angle

17. EFFECTS OF IMPURITIES

    • Sulphur to Iron lowers melting temp. to as low as 988C
      • Sulphur offset by adding Manganese
    • Copper contact tip may arc to the workpiece

18. WELD METAL MICROSTRUCTURE

    • Weld metal not worked like a plate and will have solidification structure
    • Austenite grains of hot weld metal are usually large and elongated
    • Weld metal composition is different and it may contain many small oxide inclusions
    • Weld metal cooling rate is usually much more rapid than that of a plate after rolling
    • When Austenite is transformed, Ferrite formation at grain boundaries is typical
    • If reheated >910C microstructure transforms back to small austenite and on cool produces fine ferrite
      • Heat from one pass refines the grains of previous passes

19. STRESS RELIEF

    • Heating welded fab to 625C to reduce stress.
    • Below transformation temp. So no major microstructure changes
    • Hard areas will be softened by tempering
    • Properties; Tensile strength reduceed

20. WELD METAL AND HAZ TOUGHNESS

    • Coarse grain in HAZ of weld metal can lead to decrease in fracture toughness
      • Heat input restrictions so that austenite grain growth in HAZ is restricted and weld cools sufficiently quickly