Weld Heat Input, also known as welding heat input, is the amount of heat input to a unit length of weld seam during the welding process. It is a key parameter for measuring the heat applied to the workpiece during the welding process, and has a crucial impact on the quality and performance of the welded joint.

Simply put, it quantifies the intensity of "heating" during welding.

Concept

1. Definition: The welding line energy (E or Q) is usually calculated using the following formula:
    `E = (η* I * U) / v`
    `E ': Welding line energy (unit: joules/millimeter or kilojoules/millimeter)
    `η `: Welding thermal efficiency (dimensionless, usually between 0.6-0.9, depending on the welding method)
    `I: Welding current (ampere)
    `U: Welding voltage (volts)
    `V `: Welding speed (mm/s) - This is the key denominator

2. Physical significance:

The molecule 'η * I * U' represents the actual output of the welding power source and the power effectively used for melting metal and heating workpieces (unit: watt=joule/second).
The denominator 'v' represents the welding speed, which is the distance traveled by the welding gun/torch per unit time.
Therefore, 'E=power/speed' represents the energy consumed per unit length of weld seam. The slower the speed, the longer it takes for a unit length of weld seam to receive heat, and the greater the linear energy; The larger the current and voltage, the more heat is generated, and the greater the line energy.

Why is welding line energy very important?

The linear energy directly affects the heating and cooling process of the welding pool and its surrounding area (heat affected zone HAZ), which in turn affects:

1. Depth and width of fusion: An increase in line energy usually leads to an increase in depth and width of fusion.

2. Size and organization of heat affected zone:
Excessive linear energy: can lead to a wide heat affected zone, significant grain growth, a sharp decrease in material toughness (especially impact toughness), and a decrease in hardness (for certain steel grades). For easily quenched and hardened steel, excessive linear energy may actually reduce the tendency for cold cracking due to slower cooling, but the loss of toughness caused by microstructure coarsening is usually the main problem.
Insufficient linear energy: may lead to defects such as insufficient melting depth, lack of fusion, and slag inclusion. For easily quenched and hardened steel, a small linear energy will significantly accelerate the cooling rate, greatly increasing the risk of producing hard and brittle martensite and cold cracks.

3. Welding residual stress and deformation: The greater the linear energy, the more total heat input, and the wider the range of uneven heating, which usually leads to greater welding residual stress and deformation.

4. Sensitivity to welding defects, such as cold cracks, hot cracks, reheat cracks, and laminar tearing, is closely related to the cooling rate controlled by linear energy.

5. The mechanical properties of weld metal and heat affected zone, including strength, toughness, hardness, plasticity, etc., are dominated by microstructural changes under the influence of linear energy.

Application and Control of Welding Line Energy

1. The core of process parameter formulation: When welding engineers formulate welding process specifications, line energy is one of the core parameters that need to be strictly regulated and controlled. Different materials, thicknesses, joint forms, and performance requirements all require different ranges of linear energy.

2. Material Applicability:
Low carbon steel and low-alloy high-strength steel: usually have strict maximum linear energy limits to prevent excessive deterioration of toughness in the heat affected zone. For quenched and tempered steel, the restrictions are particularly strict.
Easy to quench hard steel: Minimum and sometimes maximum linear energy requirements need to be specified to ensure that the cooling rate is not too fast and does not produce too much martensite, leading to cold cracking.
Austenitic stainless steel: To avoid intergranular corrosion, lower line energy and faster cooling rate are usually required.
Titanium alloy: To avoid overheating and grain coarsening, lower linear energy is usually used.

3. The influence of welding methods: The thermal efficiency (η) of different welding methods varies:
* Submerged arc welding: highest thermal efficiency (η≈ 0.85-0.99)
* Molten electrode gas shielded welding: relatively high (η≈ 0.75-0.90)
* Welding rod arc welding: medium (η≈ 0.70-0.85)
* Tungsten inert gas welding: minimum (η≈ 0.45-0.75)
* Laser welding/electron beam welding: The heat source is concentrated, and the total heat input may be low, but the power density is extremely high. The concept of linear energy still applies, but the calculation and influence have their own particularities.

4. Control method: Control the line energy by adjusting the welding current (I), welding voltage (U), and most importantly, welding speed (v). Improving welding speed is the most effective way to reduce line energy while ensuring good fusion.

Summary

The welding line energy is the heat received per unit length of the weld seam, and is a key bridge connecting welding process parameters with the final quality and performance of the welded joint.

Understanding and precise control of line energy is crucial for:

* Obtain good weld formation
* Avoid welding defects
* Ensure that the weld metal and heat affected zone have the required mechanical properties (especially toughness)
* Control welding stress and deformation
Crucial. It is an essential core parameter in welding process qualification, welding procedure formulation, and welding production quality control.