Weldability and metallurgy

Weldability and Metallurgy of Welding
Heat
Sources: Arc, Flames, Resistance, Friction, explosion, impact, chemical, radiation, energy beams (Most common is Arc)
Effects of Heat:
Residual Stresses and Distortion
Reduction of ductility or a degree of hardening in HAZ leading to cracking
Deterioration of toughness properties (primarly in HAZ)
Loss of strength in HAZ of certain workhardened, quenched, and tampered material
Calculation of Heat Input:
Weldability and Metallurgy of Welding
Weld Thermal Cycles - Time Temperatue History
Weldability and Metallurgy of Welding
Weld Thermal Cycles – Temperature Distribution Around moving Arc
Weldability and Metallurgy of Welding
Welding Metallurgy
Welding metallurgy deals with
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Interaction of different metals
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Interaction of metals with Gases and chemical
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Changes in Physical Properties
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Solubility of gases in metals
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Effect of impurities
For welding processes, reaction times are in order of minutes, seconds and fraction of seconds
Weldability and Metallurgy of Welding
Structure of Metals
Liquid State – Free movement of atoms
Solid State - Space Lattices (imaginary lines connecting atoms of symmetrical arrangements)

Pure Metals
1. Common Space Lattices
2. (over a dozen types are possible)
3. Allotropic Forms and Allotropic Change
Alloys
4. Solid Solution
5. (Interstitial & Substitutional)
6. Intermetallics (Chemical Compounds
with complicated crystal structures)
4. Phases in an alloy
Weldability and Metallurgy of Welding
Solidification
Does not occur simultaneously throughout the melt
Formation of multiple nuclei at different points
Growth – Adding on of additional crystals as temperature decreases
Growth Directions - Six directions normal to the main axis of crystal – Cube
Dendritic Growth – Growth in a tree like pattern with branches and sub Branches
Grains – Nuclei occur randomly and dendrites from different nuclei are at odd angles with one another.
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They cannot merge as they meet.
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They grow untill they meet dendrites from adjacent crystals.
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This gives rise to irregularly shaped areas called grains
Weldability and Metallurgy of Welding
Phase Transformations
1. With Temperature-Allotropic Transformation (Iron,
Titanium, Zirconium, Cobalt etc)
2. Recrystalization (melts or Resolidifies)
Weldability and Metallurgy of Welding

Equilibrium Diagram, Constitution Diagram or Phase Diagram
Weldability and Metallurgy of Welding
Equilibrium Diagram and Welding Processes
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Phase diagrams are based on equilibrium conditions
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For Welding – temperature changes are rapid and equilibrium conditions rarely occur
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Still, Equilibrium diagrams are the best available tools to determine phases
Hardenability
Heat Treatment process to increase hardness is similar to the process occurring in welding
Hardenability depends on several factors, such as
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Grain Size and Microstructure
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Deformation in crystals
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Temperature to which steel has been heated
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Rate of Cooling (CCT and TTT diagrams)
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Alloying elements
Weldability and Metallurgy of Welding
Welds
Factors during welding
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Changes in temperature
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Changes in dimensions
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Growth of crystals and Grains
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Phase Transformations
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For Arc Welding – Thermal Cycles are most important (depend on process, procedure, metal and mass)
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Rate of change decrease as the distance from weld increases
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Many different cooling rates will result – thus many different microstructures will result
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The system is more complex when filler metal is added across the arc
Weldability and Metallurgy of Welding
Welds – Thermal Cycles
Weldability and Metallurgy of Welding
Equilibrium Diagram and Welding Processes
Weldability and Metallurgy of Welding
Welds – Different Microstructures
Weldability and Metallurgy of Welding
Welds – Changes in Composition
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When the composition of filler is different from base metal there is a variation in composition across the weld (dilution also occurs)
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This combined with changes in cooling rates will produce a range of microstructures across the weld
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Each microstructure will have its own properties
Weldability and Metallurgy of Welding
Welds – Changes in Composition
Weldability and Metallurgy of Welding
Stresses and Distortion
Weldability and Metallurgy of Welding

Stresses and Distortion