# How to calculate heat input

Heat input is a critical parameter for arc welding processes. The Heat input shall be controlled to ensure sound weld quality.

Heat input may be defined as **“The amount of electrical energy that is supplied to a weld during the welding process”.**

**How to calculate Heat input?**

There are several ways of calculating the heat input. We shall discuss the two most common methods of calculating the heat input.

##### 1. American system (given in ASME BPVC Section IX – QW 409.1 (a) and various AWS standards):

### Heat input:

Where,

- Heat input is expressed in J/In (Joule/Inch) or J/mm (Joule/mm)
- Voltage is expressed in Volts
- Travel speed is expressed in In/min or mm/min

The unit for heat input obtained by this formula shall be either in J/In or J/mm. To get the results in KJ/in or KJ/mm, divide the result by 1000.

**Example 1:** If a welder takes 2 minutes to complete a 18 inches long weld. He keeps the voltage at 24 volts and current 120 amps. What is your heat input?

**Answer:**

Given data parameters are;

**Travel Speed** = Length of Weld/Time to weld = 18 inches/2 minutes = 9 in/min

**Voltage** = 24 volts

**Current** = 120 amps

**Heat Input** = (24 X 120 X 60)/ 9 = 19200 J/in

= 19.2 KJ/in (Divided by 1000 to obtain the result in KJ/in.)

##### 2. European system (given in EN ISO 1011-1 and PD ISO/TR 18491)

An additional parameter of Thermal efficiency (process efficiency or arc efficiency) is used while calculating heat input as per European standards.

### Heat input:

Value of thermal efficiency is different for different arc welding processes, see table 1:

**Table – 1**

**Why Heat input is so important?**

Heat input affects the cooling rates in welds and thereby it affects the microstructure of the weld metal and that of the heat affected zone (HAZ). A change in microstructure directly affects the mechanical properties of weld metal and heat affected zone (HAZ). Therefore, it’s very important to control the heat input to achieve a sound microstructure and a good quality weld.

**Note: The heat input (as calculated by ASME standards) is referred to as “arc energy” under the European system. Hence the following formula holds true with regard to European system;**

**Heat input = Thermal efficiency X Arc energy**

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