Weldability and metallurgy

Weldability and metallurgy

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:

Weld Thermal Cycles - Time Temperatue History

Weld Thermal Cycles – Temperature Distribution Around moving Arc

Welding Metallurgy

Welding metallurgy deals with

• Interaction of different metals

• Interaction of metals with Gases and chemical

• Changes in Physical Properties

• Solubility of gases in metals

• Effect of impurities

For welding processes, reaction times are in order of minutes, seconds and fraction of seconds

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

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

They cannot merge as they meet

They grow untill they meet dendrites from adjacent crystals

This gives rise to irregularly shaped areas called grains

Phase Transformations

1. With Temperature-Allotropic Transformation (Iron,

Titanium, Zirconium, Cobalt etc)

2. Recrystalization (melts or Resolidifies)

Equilibrium Diagram, Constitution Diagram or Phase Diagram

Equilibrium Diagram and Welding Processes

Phase diagrams are based on equilibrium conditions

For Welding – temperature changes are rapid and equilibrium conditions rarely occur

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

Grain Size and Microstructure

Deformation in crystals

Temperature to which steel has been heated

Rate of Cooling (CCT and TTT diagrams)

Alloying elements

Factors during welding

Changes in temperature

Changes in dimensions

Growth of crystals and Grains

Phase Transformations

For Arc Welding – Thermal Cycles are most important (depend on process, procedure, metal and mass)

 Rate of change decrease as the distance from weld increases

 Many different cooling rates will result – thus many different microstructures will result

 The system is more complex when filler metal is added across the arc

Welds – Thermal Cycles

Equilibrium Diagram and Welding Processes

Welds – Different Microstructures

Welds – Changes in Composition

When the composition of filler is different from base metal there is a variation in composition across the weld (dilution also occurs)

This combined with changes in cooling rates will produce a range of microstructures across the weld

Each microstructure will have its own properties

Welds – Changes in Composition

Stresses and Distortion

Stresses and Distortion