WELDING METALLURGY

HEAT FLOW
1. Amps x Volts = Watts
2. Heat & Light = Watts
3. Efficiencies; SAW 90-99%, GTAW 25-50%, GMAW, FCAW, SMAW in between
ENERGY INPUT
1. (Volts x Amps)/Speed
2. Higher = Larger the weld, wider the HAZ, slower cooling rate
COOLING RATES
1. Heat input to the weld
2. Plate thickness
3. Initial temperature of plate
ZONES IN A WELD
1. Regions >723C will partially transform to Austenite ferrite
2. >910C complete Austenite
3. Low energy input on thick plate; cool rate may be as fast as quench and hard martensite
4. Higher energy on thin plate/preheat; slow cool rate and softer microstructure
5. Coarse grain = low toughness
6. hard martensite = cracking susceptibility
HYDROGEN CRACKING
1. Weld metal or HAZ to H embrittlement related to Chem Comp. and microstructure
2. Hydrogen content
3. Stress / Strain level of crack initiation
4. Temperature; cracking can occur after weld cooled to <100C
HYDROGEN
1. H can cause severe embrittlement = reduction in ductility / drop of fracture stress of notched steel
2. Diminishing effect w/ increasing Temp, 150-200C H has no effect
3. Effect is Max. at slow or steady rate of impact loading
STRESSES
1. Restraint
  1. Higher in Thicker Plates and Groove welds
2. Stress Concentrations
3. Avoid Cracking, Control;
  1. Stress
  2. Microstructure
    1. Hardenability (Alloy Content)
      1. Higher = Cooling rate decreases
    2. Weld cooling rate
  3. Hydrogen content
  4. Temperature
HAZ STRUCTURE
1. Zone I; <0.10% Carbon
2. Zone II; C & C maganese steels; cooling rate controlled
3. Zone III; >0.10% Carbon; Hydrogen controlled
HYDROGEN CONTROL
1. Higher carbon alloy; preheat alone may be insufficient = PWHT may be required
2. If HAZ is excessive hard; softening by tempering at temperature may be needed.
CRACK TESTS
1. Controlled Thermal Sensitivity (CTS)
  1. Fillet weld 2 blocks bolted together; thermal severity or cooling rate changed by using diff thickness plates
2. Tekken Test
  1. Y Groove prep promotes HAZ cracks, straight part encourages weld metal crack.
  2. Higher restraint test with restraint increase with thickness
3. Gapped Bead-On-Plate (G-BOP)
  1. Welds are heated with torch to a dull red in vicinity of gap
  2. discolouration of fracture surface indicates crack present before heating
LAMELLAR TEARING
1. Non-metallic inclusions that flatten out during rolling results in ductility THROUGH the thickness being lower than other directions
2. Occur in large welds where strains are normal to plate surface; T-Butt and corner are most susceptible
3. Main factors
  1. Type of Joint
  2. Through-thickness ductility of steel
  3. Restraint of the joint
  4. Other WPS factors; i.e. H content
WELD METAL
1. Solidified weld metal will have a microstructure differing from that of parent
2. More like a rapid cool casting
3. Meeting equal strength but may have much lower carbon content
DILUTION
1. Composition depends on
  1. Composition of Electrode
  2. Composition of parent metal
  3. Chemical reactions w/ weld metal, flux/shield gas
2. Weight of Parent Metal Melted / Weight of Total melted Metal
CHEMICAL REACTIONS
1. Deoxidizing elements; Silicon, Manganese, Aluminum react with O to form non-metallic inclusions
SOLIDIFICATION
1. Weld speed high = elongated weld pool and Xtls grow from side of pool towards each other
2. Weld speed low = growth of Xtl will originate from back of weld pool
SOLIDIFICATION CRACKING
1. Liquid trapped when Xtl grow on cooling.
2. Cracks can run along centre of weld, and even entire length
3. Rule-of-thumb; Root run Current should not exceed 10x groove angle
EFFECTS OF IMPURITIES
1. Sulphur to Iron lowers melting temp. to as low as 988C
  1. Sulphur offset by adding Manganese
2. Copper contact tip may arc to the workpiece
WELD METAL MICROSTRUCTURE
1. Weld metal not worked like a plate and will have solidification structure
2. Austenite grains of hot weld metal are usually large and elongated
3. Weld metal composition is different and it may contain many small oxide inclusions
4. Weld metal cooling rate is usually much more rapid than that of a plate after rolling
5. When Austenite is transformed, Ferrite formation at grain boundaries is typical
6. If reheated >910C microstructure transforms back to small austenite and on cool produces fine ferrite
  1. Heat from one pass refines the grains of previous passes
STRESS RELIEF
1. Heating welded fab to 625C to reduce stress.
2. Below transformation temp. So no major microstructure changes
3. Hard areas will be softened by tempering
4. Properties; Tensile strength reduceed
WELD METAL AND HAZ TOUGHNESS
1. Coarse grain in HAZ of weld metal can lead to decrease in fracture toughness
  1. Heat input restrictions so that austenite grain growth in HAZ is restricted and weld cools sufficiently quickly

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WELDING METALLURGY

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	Created by Grant Aden over 10 years ago