A joint can contain changed parent material outside the melted section.
Locate the material regions
TWI defines the heat-affected zone as unmelted parent metal altered by the thermal cycle. It adjoins the fusion region but is a different material region. Primary reference: TWI technical explanation.
A section drawing should retain the original parent material, fusion boundary and surrounding altered region separately. Surface colour is another observation; a visible tint line cannot automatically establish the complete boundary of a microstructural change.
Read a hypothetical section
Imagine a fictional polished section showing a melted central region two units wide. A separately characterized altered band extends one additional unit on each side. Under this deliberately simple geometry, the combined affected span is four units, while the melted span remains two.
Describing all four units as melted would change the interpretation. Describing only two units as affected would omit the surrounding region. These invented coordinate units establish no process capability, heat-affected width or acceptance criterion.
Keep the evidence attached
Build a section record with distinct labels for fusion, parent material and the method used to identify an altered region. Retain magnification and orientation if images support the labels.
Ask which material property matters at each location and what observation supports that conclusion. This makes a fabrication discussion specific enough to request the relevant section or material evidence rather than judging the whole joint from a surface photograph.
Customer Questions
Is the heat-affected zone necessarily melted?
It is the thermally changed, unmelted parent region.
What is the fictional melted span?
Two units; the wider combined span is four.
Does a tint boundary define the complete region?
Keep its observation separate from the material-characterization evidence.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
Discuss Your Machine Requirement
Share your product, container, required output and the evidence needed for your technical review.
Request a Technical Review