---
title: "Weld metallurgy, HAZ and heat sources"
subject: "Manufacturing and Production Processes"
chapter: 12
key: "semester_4/manufacturing_and_production_processes:weld-metallurgy-haz-defects-energy-sources"
source: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/weld-metallurgy-haz-defects-energy-sources
site: BCT Engineering
---

# Weld metallurgy, HAZ and heat sources

Manufacturing and Production Processes · Chapter 12: Material Joining Processes

- Asked in: year not given, 6 marks
- Video: [Introduction to Welding Metallurgy](https://www.youtube.com/watch?v=8WlAb8MlTI4) — Bueno's Handyworks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/weld-metallurgy-haz-defects-energy-sources
- Whole chapter: https://www.bctengineer.com/md/semester_4/manufacturing_and_production_processes/chapter-12

## Question

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.

## 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.

## Answer

#### 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.

```text
 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):
1. **Fusion zone** - melted and resolidified metal, a mixture of filler and parent metal.
2. **Fusion (partially melted) boundary.**
3. **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.
4. **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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does the fusion zone of a weld solidify?
   A. Equiaxed grains form at the weld centre first and grow outward
   B. Freezing starts at unmelted parent grains and columnar dendrites grow toward the weld centre
   C. It stays amorphous because cooling is slow
   D. Grains are recrystallised without melting
   Answer: B. Epitaxial growth from the parent grains gives columnar dendrites growing toward the centre along the heat flow, a cast structure with segregation. (Key point: Fusion zone solidification)

2. What is the HAZ, and which region is hard and brittle in hardenable steels?
   A. Melted metal; the fine-grain zone is brittle
   B. Unmelted metal heated enough to change structure; the coarse-grained region next to the fusion line may contain martensite
   C. Unaffected parent metal; the whole zone is brittle
   D. The slag layer; the weld centre is brittle
   Answer: B. The HAZ is not melted but its structure changes; next to the fusion line the coarse-grained region can form hard, brittle martensite. (Key point: HAZ structure)

3. Hydrogen-induced cold cracks in the HAZ of high carbon equivalent steel are reduced by
   A. Increasing heat input and cooling quickly
   B. Preheating, low-hydrogen electrodes and controlled heat input
   C. Using a damp electrode coating
   D. Welding without any shielding
   Answer: B. Preheating slows cooling and low-hydrogen electrodes limit hydrogen, avoiding martensite and cold cracking. (Key point: Avoiding cold cracking)

4. How does a higher power density heat source (laser or electron beam) change the weld compared with an oxy-fuel flame?
   A. Wider weld and larger HAZ
   B. Deeper, narrower weld with smaller HAZ, less distortion and higher speed
   C. Shallower weld with more distortion
   D. Same weld but cheaper equipment
   Answer: B. Higher heat intensity gives keyhole-type narrow, deep welds, small HAZ and less distortion, though the equipment is expensive. (Key point: Energy source comparison)

## Related in Manufacturing and Production Processes

- [Gas flames, arc welding and heat input](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aoxyacetylene-flames-smaw-heat-input-numerical) — Manufacturing and Production Processes, chapter 12

## Related in other subjects

- [Resistance spot welding heat and efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Aresistance-spot-welding-heat-numerical) — Workshop Technology, chapter 10
- [Notes on powder metallurgy and heat treatment](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Apowder-metallurgy-heat-treatment-notes) — Machine Design II, chapter 3
- [Energy from D-T fusion and U-235 fission](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Afusion-fission-energy-mass-defect) — Energy Resources, chapter 7
- [Common acoustic defects](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aacoustic-defects) — Building Technology, chapter 1