How does the fusion zone of a weld solidify?

Freezing starts at unmelted parent grains and columnar dendrites grow toward the weld centre

Equiaxed grains form at the weld centre first and grow outward

It stays amorphous because cooling is slow

Grains are recrystallised without melting

What is the HAZ, and which region is hard and brittle in hardenable steels?

Unmelted metal heated enough to change structure; the coarse-grained region next to the fusion line may contain martensite

Melted metal; the fine-grain zone is brittle

Unaffected parent metal; the whole zone is brittle

The slag layer; the weld centre is brittle

Hydrogen-induced cold cracks in the HAZ of high carbon equivalent steel are reduced by

Preheating, low-hydrogen electrodes and controlled heat input

Increasing heat input and cooling quickly

How does a higher power density heat source (laser or electron beam) change the weld compared with an oxy-fuel flame?

Deeper, narrower weld with smaller HAZ, less distortion and higher speed

Shallower weld with more distortion

Same weld but cheaper equipment

Practice · 6 marks

Weld metallurgy, HAZ and heat sources

Short answer

Fusion weld has fusion zone with cast columnar grains, fusion boundary, heat-affected zone with coarse then fine grains, and unaffected parent metal. Common defects are porosity, slag inclusions, lack of fusion, incomplete penetration, undercut, cracks and distortion. Energy sources range from oxy-fuel flame with wide HAZ to laser/electron beam with high power density, narrow deep weld and small HAZ.

Describe the metallurgy of a fusion weld, showing the zones of the weld and the heat-affected zone (HAZ) in steel. List the common weld defects, and compare the characteristics of the energy sources used in welding.

Metallurgy of a fusion weld

During welding, the parent metal and the filler melt in the weld pool and then solidify. Solidification starts from the unmelted grains of the parent metal at the fusion boundary (epitaxial growth), and columnar dendrites grow toward the weld centre, in the direction of heat flow. The weld metal therefore has a cast (dendritic) structure with some segregation of solute.

 Weld bead
  _______________________
 | fusion zone (weld metal)   cast, columnar grains
 |------ fusion boundary ----
 | HAZ: coarse grain (near the boundary)
 |      fine grain (normalised)
 |      partly transformed
 |------------------------------
 | unaffected parent metal

Zones (from the weld centre outward):

Fusion zone - melted and resolidified metal, a mixture of filler and parent metal.
Fusion (partially melted) boundary.
Heat-affected zone (HAZ) - not melted, but heated enough to change its structure: a coarse-grained region next to the fusion line (hard and brittle, with martensite in hardenable steels), then a fine-grained normalised region, then a partially transformed region.
Parent metal - unaffected.

The properties of the HAZ depend on the peak temperature, the time at temperature and the cooling rate. Steels with a high carbon equivalent form martensite in the HAZ, which leads to hydrogen-induced cold cracking. This is avoided by preheating, using low-hydrogen electrodes and controlling the heat input.

Common defects

Porosity (trapped gas) and slag inclusions
Lack of fusion and incomplete penetration
Undercut and overlap
Cracks - hot cracks (during solidification) and cold cracks (hydrogen-induced)
Distortion and residual stress

Energy sources compared

Source	Typical power density	Features
Oxy-fuel flame	about $10^3$ W/cm $^2$	Wide heating, slow, large HAZ, low cost
Electric arc	$10^4$ - $10^5$ W/cm $^2$	Most common; portable; moderate HAZ
Plasma arc	about $10^5$ - $10^6$ W/cm $^2$	Narrow arc, deep penetration
Laser beam, electron beam	$10^6$ - $10^8$ W/cm $^2$	Keyhole welding; narrow, deep weld; small HAZ; expensive

A higher power density gives deeper and narrower welds, a smaller HAZ, less distortion and a higher welding speed.

✓ Answer checked by Claude Opus 5.5 (high effort)

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