Skip to content

Heat-affected Zone: All Aspects of HAZ That We Should Know

Due to its outstanding strength and durability, TMT (Thermo-Mechanically Treated) bar is a commonly utilized material in the construction industry. The Heat Affected Zone (HAZ) forms around the surface of the bar exposed to high temperatures during the butt jointing procedure when two bars are linked together to produce a continuous length. The integrity of the TMT bar in butt joints we examine the HAZ phenomena and talk about how to mitigate its consequences.

Understanding Heat Affected Zone (HAZ)
  •  When performing a Butt Joint, a substantial amount of heat is generated due to the high electric current used in the process. The intense heating and rapid cooling can cause changes in the microstructure of the TMT bar, leading to the formation of the Heat Affected Zone (HAZ). The size of the HAZ can vary depending on several factors, including cooling rate, temperature, material composition, and heat exposure duration. Its typical range of HAZ is 5-10mm from the joint interface.
Effect of the HEAT ZONE
  • Heat-Affected Zone is a region of the TMT bar that has been altered by the heat generated during the butt joint. The HAZ can significantly impact the properties of the joint, particularly the Strength and Ductility.
  • The high electric current used in the butt joint induces significant heat. That excess heat will change in microstructure and form undesirable properties.
When a Cold forged TMT bar is used in Butt joints, the forged butt joint will have a higher strength and ductility compared to a non-forged butt joint. This is because the cold forging process results in a denser and stronger TMT bar with a more uniform microstructure, which allows the butt joint to be stronger and more reliable.
  1. Non-forged Butt Joint:
    Non-forged TMT bars can  also be used in the butt joint to create a joint between two bars. Non-forged butt joint has quite good                  strength as per IS 9417, which is widely used in construction. The non-forged butt joint fulfills the recommendation of IS 9417:2018          which is the tensile strength of the butt joint not less than 90% of the actual tensile strength of the TMT bar. However, as compared to      Cold Forged Joints, the Non-forged Joint has lower strength. 

Note: No doubt HAZ is an important parameter in the Butt Joint however the butt joint strength of the TMT bar has reached up to 90% tensile strength of the steel bar that is accepted by IS 9417:2018. But if we want to increase joint strength above 90%, we have to Cold Forge the TMT bar before execution of the Butt joint.

1. Cold Forged Butt Joint:
The Cold forging process involves shaping the TMT bar after applying compressive pressure that can change the internal structure of the bar. Cold Forging of the bar enhances the strength and durability of the butt joint.
Cold Forging will improve several properties:
·       Strength
·       Durability
·       Ductility
Strength: Cold forging of the TMT bar increase the strength of the bar, making them more Resistant to Breaking and Bending.
Durability: Cold forging of the TMT bar also improves the durability of the bar. it makes them better able to withstand harsh environmental conditions such as exposure to temperature fluctuation, moisture, etc.
Ductility: Cold forging of TMT bar improve the ductility of a bar, making them more flexible and resistant to cracking under stress. 
Visual parameters of Forged Butt Joint:
  The Breaking Point of Joint:
After testing a forged butt joint in a universal testing machine, the breakpoint of the TMT bar will not occur at the joint because of the cold forging. During butt joint execution due to high-temperature HAZ will induce, that heat zone will be overcome by cold forging of TMT Bar so that self-bar will break.  
Bar Break Forged Butt Joint
Bend Section of Butt Joint:
As per IS 9417:2018, the Butt Joint should be bent at 60 degrees angle without any deformation.
Joint Uniformity:
The internal surface of the joint can be analysed by cutting the side portion of the forged butt joint.
Joint Cross-Section:
The enlarged cross-section of rebar by Cold Forging will improve and densify the grains structure of metal so that after joint execution cross-section of the joint will achieve more uniformity.

When butt-joining TMT bars, the Heat Affected Zone (HAZ) is an important factor to take into account. The quality and durability of the joint may be protected, maintaining the structural integrity of building projects, with the right knowledge and management of the HAZ. We can successfully counteract the impacts of HAZ and enhance the Butt Joint strength by the use of the Cold Forge Technique.