What is Heat Input in Welding?

The concept of heat input in welding processes is very important because it greatly affects the cooling rates of welded joints. As the heat input increases, the cooling rate of the welded joint reduces, giving room for increased grain size as well as the Heat Affected Zones (HAZ), causing changes in properties of the material such as reduced toughness.

Conversely, a reduced cooling rate prevents the formation of a hard and brittle phase called martensite. But in cases where the welding materials are susceptible to hardening and hydrogen embrittlement, then the heat input should just be above a minimum threshold. This implies that heat input during welding can be kept either below or above the minimum threshold depending on the nature of the material to be welded. When there is low heat input in welding for certain materials, it also leads to welding defects.

There is this misconception that heat input is easily controlled during welding, but this is not entirely the case. It is in fact a difficult parameter to control during welding.

Heat Input - Definition

In simple terms, heat input is the proportion of heat provided to the work-piece which leads to the formation of a weld. It is usually calculated in units of energy per unit length. In most cases the unit length are either in millimetres or inches (KJ/mm or KJ/in). Those working in imperial units (USA) use inches while those working with S.I. units (majority of the world) utilize millimetres.

Welding Heat Input Formulae

How to calculate heat input in welding:

Now let us look critically at the above equation and further understand what it portrays. It can be derived from the equation that we can have a steady heat input even if there is an increase in the voltage or current, as long as there is a proportionate increase in travel speed of the weld. This suggests that there will be no change in the mechanical properties and microstructure of the material as long as heat input is kept constant.

Theoretically, it is achievable. However in practical terms this is not always the case because the different welding processes have diverse thermal efficiencies, also called ‘arc efficiencies’. Arc efficiency generally highlights the amount of energy used to heat up the work-piece and also melt the filler metal. An arc efficiency of 1 will depict 100% of the energy was used to form the weld, while an arc efficiency of 0.55 shows that 55% of the energy was used while 45% was lost unproductively lost to the surrounding environment.

Common arc efficiencies are; Submerged Arc Welding (SAW): 0.83; Gas Metal Arc Welding (GMAW): 0.83; Gas Tungsten Arc Welding (GTAW); 0.65; Plasma Arc Welding (PAW); 0.46.

Effect of Heat Input in Welding

Weld heat input plays a major role in the properties of a weld. Mechanical properties and toughness of a weld are hugely determined by the microstructure of the weld. As illustrated in the diagram below, heat input determines the cooling rate of a weld pool which directly affects the final microstructure and mechanical properties of the weld. The cross sectional area of a weld is proportionate to the heat input. This implies that the major effect of high heat input in welding is that more of the filler material melts per unit length giving rise to a bigger weld bead.

Image credit: R.S. Funderburk

The most significant effect of heat input in welding is that it determines the cooling rate of a weld which in turn determines the microstructure of the weld metal. Similarly, a change in the microstructure of a weld directly affects its mechanical properties. Therefore, the need to always control heat input during welding can never be overemphasized.

How to Reduce Heat Affected Zone in Welding (HAZ)

The formation of Heat Affected Zone (HAZ) during welding largely depends on the weld heat input, although other factors such as weld geometry and environmental conditions also play a part. In simplest terms, when a high amount of thermal energy is channelled on a metal workpiece, the Heat Affected Zone becomes bigger.
When materials of high strength and toughness are being welded, precaution is of great importance. A lot of defects may arise when weld heat input is either too high or too low. If the travel speed is too slow, excessive grain growth occurs in the weld and HAZ. Therefore to minimize the size of HAZ, travel speed has to be increased.
Therefore, to reduce weld defects and achieve slim HAZ, heat input must be controlled with high precision. This will further help to minimize the cost of post-weld treatments.

How to Control Heat Input in Welding

Heat input in welding is a difficult parameter to control during operations. This is because, during welding, the parameters that can be monitored are the voltage, current and speed of the weld flow, but it is after the welding operation that we can truly ascertain whether the recommended limits of heat input were achieved.
It always leads to unfavourable circumstances when we discover the weld bead was not produced within the allowable heat input threshold. A lot of questions arise, such as, should the enter weld bead be grinded? Should the weld bead be treated instead? There is no definite answer to these questions. It will all depend on the circumstances and severity of the weld defects.
Therefore it can be seen that to control heat input in welding is a major concern of most welding professionals. To be able to produce weld beads that possess the quality to pass various quality assurance tests, one must always be conscious of weld heat input and other related parameters.

Why is the Control of Heat Input Important in Welding?

The most crucial reasons why heat input must be controlled during welding are explained below:

1. To minimise the possibility of hydrogen embrittlement, also called hydrogen assisted cracking (HACC). This commonly happens when some steels are welded.


2. To ensure ferritic materials retain their high toughness after the welding operation.


3. To minimise the formation of oxides on the weld surface. This is popular with titanium, stainless steel, and nickel-based alloys.


4. To ensure the microstructure of stainless steel is not altered due to the growth of other constituents which negatively affect their ability to resist corrosion.

Conclusion

As we have discovered, heat input during welding is a very important parameter which welders always pay attention to. When not controlled properly, it can lead to wastage of money in order to treat or re-weld the joint. It can also lead to catastrophic failure of the welded joint during operation. The effect of low heat input in welding can also be as catastrophic as the effect of high heat input depending on the welding material.