Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 119
the grain boundaries there will be common to both. The region of the component
adjacent to the weld is known as the heat-affected zone (HAZ).
The heat-affected zone
The temperature–distance profile across the weld is a result of the balance
between the rate of heating and the rate at which heat is conducted away (the
base material forming a large heat sink). A more intense heat source will give
a steeper profile and the HAZ will be confined to a narrower region. The
changes in microstructure that take place in the HAZ will depend on the
material being welded and upon its thermal and mechanical history. For
example, if the material is in the cold-worked condition, it will recrystallize
and, if it is age-hardened, the precipitate distribution will be changed. Different
changes will occur in different temperature regimes of the HAZ and Fig.
3.32 is a schematic diagram illustrating the possible regimes in the HAZ
adjacent to a weld in a low-carbon steel. The adjacent iron–carbon phase
diagram indicates the temperatures concerned in the case of a 0.15 wt% C
steel and the various regimes are self-explanatory.
Residual stresses and cracking
When a weld is deposited, the volume of material being heated and cooled
Solidified weld
1600
Liquid
Peak temperature (T p )
Solid-liquid transition zone
Grain growth zone
Recrystallized zone
Temperature
(°C)
1400
1200
1000
Partially transformed zone 800
Tempered zone
600
Unaffected base
material
400
Liquid + γ
γ
γ + Fe 3 C
α + Fe 3 C
200
0.15 1.0
Heat affected zone
Fe
Wt% C
3.32 Schematic diagram of the heat-affected zone of a 0.15% C steel
indicated on the Fe-Fe 3 C phase diagram.