What is Heat Affected Zone or HAZ in Welding?
Most people think that a weld is only the portion of metal fused during welding. This is a very limited way of looking at a weld. A more accurate way is to consider a weld as the area affected by the heat input during welding. According to this definition, a weld is composed of three main parts illustrated in below Figure :
1. fused zone (weld metal)
2. bonding zone (fusion line)
3. base metal heat-affected zone (HAZ)
So what is HAZ?
The area of the base metal next to the weld that does not melt but is changed by the heat from the welding process. In a way, this area is heat treated by the welding process, that is, its mechanical properties have been altered. In theory, the HAZ refers to all areas of the base metal heated to above ambient temperature during welding. In practice, the term HAZ is used to describe the areas altered by welding heat input.
Width of HAZ?
The width of the HAZ depends primarily on heat input and thermal conductivity (heat dissipation in base metal). If heat input is decreased or thermal conductivity increased, the HAZ size will decrease. This means that a weld made with SMAW process will normally produce a narrower HAZ than one made with FCAW (using a large diameter electrode). Similarly, stainless steels will have a larger HAZ than carbon steels, since the thermal (heat) conductivity is lower than steel.
Properties and transformation during welding in HAZ?
The iron-carbon phase diagram shows that a phase transformation starts when the temperature reaches 723°C. At that temperature BCC transforms into FCC. Since weld cooling rates from temperatures above 723°C may be rapid, hardening of the weld area is common. The heat affected zone (HAZ) is a very important area because weld faults may occur in this zone.
As can be seen in above Figure, the transformations that take place in the HAZ depend on the highest temperature attained at each point of the zone during welding.
This figure illustrates what happens to a steel that has been cold worked before welding. This is the case for most rolled plates that did not receive a heat treatment after rolling. Where the temperature is minimal, grains (deformed by rolling) will use the heat provided by the welding process to recrystallize. Long grains (deformed by rolling) will transform into several smaller grains. Areas where the temperature rises above 723°C will show the effects of phase transformations (BCC – FCC – BCC). Near the weld fusion line, where temperatures are just below the melting point, very large grains form. This is generally the weakest part of a weld.
Click here to calculate the Transformation temperature in Steel using its alloying elements.
When it is important to limit grain growth in the HAZ, the welder should be following strict welding procedures and limit heat input by using small (stringer) weld beads when possible. Weaving is commonly used, but should be limited to plain low-carbon steels where heat treatments have lesser effects.