Welding Metallurgy

Welding Metallurgy

‘Welding is the joining of two or more pieces of metal by applying heat or pressure, or both, with or without the addition of filler metal, to produce a localized union through fusion or recrystallization across the interface’.

1. Weld Metal All metal melted during the making of a weld and retained in the weld.

2. Weld Zone The zone containing the weld metal and the HAZ.

3. Parent Metal Metal to be joined or surfaced by welding.

4. Fusion Penetration The depth to which the parent metal has been fused.

Welding Metallurgy

5. Weld Junction The boundary between the fusion zone and the HAZ.

6. Root (of Weld) The zone on the side of the first run farthest from the welder.

7. Fusion Zone The part of the parent metal that is melted into the weld.

8. Heat Affected Zone (HAZ) The HAZ is the subject of continuing interest since it involves a wide range of temperatures from the welding operation that can significantly alter the base metal's metallurgy and associated mechanical and physical properties. 

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Welding Metallurgy

Weld Metal

Heat Affected Zone HAZ

Unaffected Base MetalUnaffected Base Metal

Grain Coarsened HAZ

Grain Refinement HAZSubcritical HAZ Intercritical HAZ

Heat Affected Zone

Weld Metal

Unaffected Base MetalUnaffected Base Metal

Grain Coarsened HAZ

Grain Coarsened HAZThe peak temperatures reached in the Grain Coarsened HAZ region range from 1090°C to 1480oC, depending on the carbon content.

Another way to describe this temperature range in metallurgical terms, is that it extends from much above the upper critical transformation temperature to just below the solidifying temperature.

Weld Metal

Unaffected Base MetalUnaffected Base Metal

Grain Refinement HAZ

Grain Refinement HAZThis region comprises temperature from just above the lower critical transformation temperature and up to 816oC. These temperatures are within the normalizing heat treatment range and are very conducive to austenitic grain refinement

Weld Metal

Unaffected Base MetalUnaffected Base Metal

Intercritical HAZ

Intercritical HAZThe temperatures in this region includes the intercritical ranges, between the lower and upper critical temperatures.

Weld Metal

Unaffected Base MetalUnaffected Base Metal

Subcritical HAZ

Subcritical HAZ

The subcritical-HAZ includes the tempered area of the Fe-Fe3C phase diagram. Should the base metal be in the tempered condition (i.e. quenched and tempered) the heat of welding may be sufficient for further tempering, thereby reducing the tensile strength and hardness in this region. There are no phase transformations which take place in the tempered area since the lower critical transformation temperature is not exceeded.

Single-Run V Butt Weld

Single-Run V Butt Weld

(1) Weld Metal, columnar crystal formation(2) Parent metal, overheated zone, grain growth.(3) Parent metal, refining zone, complete recrystalization.(4) Parent metal, refining zone, partial recrystalization. – Equiaxed structure(5) Parent metal, unaffected structure

Single-Run T Fillet Weld

Single-Run T Fillet Weld (1) Weld Metal, columnar crystal formation(2) Parent metal, overheated zone, grain growth.(3) Parent metal, refining zone, complete recrystalization.(4) Parent metal, refining zone, partial recrystalization.

– Equiaxed structure(5) Parent metal, unaffected structure

Multi-Run V Butt Weld

Grain Growth

1. Cold worked fibrous crystals2. New crystals begin to form3. Recrystallisation in progress4. Recrystallisation nearly complete5. Equiaxed crystal grains6. Grain growth

By using a microscope, this is the effect heat has on a peice of Low Carbon Steel grain structure.

The Effect On Materials When Subjected To A Heat Source

Heat Affected Zone

NOTE - flamein static position