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Physics Formulas

Heat Input Formula

Under the arc welding process, heat input and arc energy are the two important measures of energy. This energy is supplied to the piece of object to form a weld. Both are measured in the units of energy per unit length. It refers to the characters like weld length, a bead, and the diameter of a weld spot or cross-section. The amount of heat input is into a weld and is a function of time. Let us study the heat input formula here.

What is Heat Input?

The vast majority of fabricators are not bothering about the heat input. For many purposes this is okay. But when we are welding on materials whose microstructure is sensitive, then it is important to know about the heat input. This is the reason why heat input is critical in some specific applications.

It is because of a huge bearing on the cooling rate. Typically weldment measures the faster cooling rates because they cause embrittlement in the heat-affected zone. For example, while dealing with materials susceptible to hydrogen-induced cracking in which adequate heat input is critical.

Heat Input Formula


Heat input is a critical parameter for the arc welding processes. The Heat input will ensure to control the sound weld quality. Definition of Heat input says that the amount of electrical energy that is supplied to a weld during the welding process is the heat input.

The Formula for Heat Input

The heat input formula is:

\(Heat\;input = \frac{Voltage \times current \times time}{distance\;travelled \times 1000}\)

The accurate measurement of the arc voltage is the difficulty here. But mostly measure the load voltage at the output terminals of the current source.

Heat input affects the cooling rates during the welding process. It affects the microstructure of the weld metal and hence heat-affected zone (HAZ). A change in the microstructure directly affects the mechanical properties of the weld metal and heat-affected zone (HAZ). Thus, it is very important to control the heat input to achieve a sound microstructure and a good quality weld.

Thus, the heat input is the arc energy under the European system. So, the following formula holds true with regard to the European system; Heat input = Thermal efficiency × Arc energy

Solved Examples for Heat Input Formula

Q.1: Compute the heat input of the engine which operates in 2000 V with a current of 5500 calories. The distance is 10 meters with time 40-sec duration.

Solution: Given data parameters are as follows,

  • Voltage = 2000 V
  • Current = 5500 A
  • Distance travelled = 10 m
  • Time = 40 sec

Heat-input = \(\frac{Voltage \times current \times time}{distance\;travelled \times 1000}\)

Substitute all the values in the given formula.

Heat-input = \(\frac {2000 \times 5500 \times 40}{10 \times 1000}\)

Therefore, Heat input is 44000 Joules per meter.

Q. 2: A heat engine operates at a voltage of 1500 V. Flowing current is 3400 A. Find out the heat input if the distance is 24 meters with time 80 sec.

Solution: Given parameters are,

  • Voltage = 1500 V
  • Current = 3400 A
  • Distance = 24 m
  • Time = 80 sec

Heat-input = \(\frac{Voltage \times current \times time}{distance\;travelled \times 1000}\)

Substitute all the values in the given formula we get,

\(= \frac {1500 \times 3400 \times 80} {24 \times 1000}\)

= 17000

Therefore, Heat input will be 17000 joules per meter.

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