What is heat input in welding, its formula, online calculator, and unit


Heat Input in Welding, and how to calculate it?

The heat input in welding influences the weld cross-sectional area. High heat input welds have a large weld bead and commonly for submerged arc welding, achieve high dilution; the associated slow cooling results in austenite grain growth in the HAZ, giving low toughness. If a high heat input arc weld has a deep narrow shape, this can contribute to a risk of solidification cracking. Low heat input welds, such as multi-pass welds, have smaller weld beads. The fast cooling can give hard brittle zones in the  HAZ  and a  risk of hydrogen cracking, which means that the heat input must be controlled or preheat used.

Heat can be technically transferred into the matter by three different ways depending on the aggregate state of the material as:

  1. Conduction
  2. Convection &
  3. Radiation

In welding joints, most of the heat transfer takes place by conduction. Not all of the 100% heat is transferred to the weld pool due to heat losses take place during heat transfer.

Importance of heat input

For most of the carbon steels, low alloy steels, Cr-Mo steels as well as stainless steels, abrupt cooling from the heat of the welding to be avoided. Because of the risk of the hardening, cracking, change in the weld metal and HAZ microstructure etc. Depending upon the type of material, its thickness, and heat input, the maintenance of a lower preheat temperature and upper inter-pass is required. For such materials, heat input shall be chosen to be matched to the welding process.

The heat input during welding can be viewed as a main influencing factor on the properties of ferritic and ferritic-austenitic stainless-steel welds in particular.  This influences the time/temperature cycle occurring during welding.

Heat input formula & calculation in ASME Section IX & AWS D1.1

Heat input is considered as a Supplementary Essential variable in the BPVC ASME Section IX. So, for a WPS requiring CVN testing/ toughness, the increase in heat input shall be considered as the Essential Variable. Clause QW 409.1 is applicable to SMAW, GTAW, GMAW (MIG-MAG)SAW, FCAW, PAW & EGW processes and shall be determined by:

Where travel speed is in inch/ minute or mm/min. Using a dividend factor of 1000, the heat input value obtained will be in kJ/inch or kJ/mm as shown below:

AWS D1.1 2020 Edition, clause 6.8.5 uses same equation as in ASME Section IX given above. For waveform-controlled welding processes, heat input shall be determined by:

Example:
1. Calculate the heat input for a procedure qualification test coupon welded with a current range of 140A to 190 Amp, 16-18 Volts and travel speed of 80 mm/min to 110 mm/min.

Solution:

Considering the practical situation here in this example, the heat input shall be having two values as:

1.     Minimum heat input

2.     Maximum heat input

For minimum heat input we will take the current and voltage on the lower side as it’s a multiplication factor, and travel speed on higher side as travel speed is dividend factor. So,

Minimum Heat input (J/min) = (140 X 16 X 60)/110   = 1221.8 J/min or 1.22 kJ/mm

Whereas, the

Maximum heat input (J/min) = (190 X 18 X 60)/80      = 2565 J/min or 2.57 kJ/mm

Here, the unit of travel speed if is inches, the heat input will be in Joules/inch or kJ/inch.

Heat input Calculation according to ISO/TR 18491:2015 or EN 1011-1

When a WPS/ PQR qualification is carried out according to ISO/ BS/ EN or DIN standards such as DIN EN ISO 15614-1, the heat input shall be determined by the following formula:

Where

k is the thermal efficiency for the welding process given in Table 1;

U is the arc voltage measures as near as possible to the arc, in Volts;

I is the welding current, in Ampere;

V is the travel speed in mm/s.

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