Welding Heat Input

Arc energy is the amount of heat generated in the welding arc per unit length of weld, and is usually expressed in kilojoules per millimeter length of weld (kJ/mm) Heat input (HI) for arc welding is calculated from the following formula:

Arc energy (kJ/mm) = Volts x Amps / Travel Speed (mm/sec.) x 1000

Heat input is the energy supplied by the welding arc to the work piece and is expressed in terms of arc energy x thermal efficiency factor.

The thermal efficiency factor is the ratio of heat energy introduced into the welding arc to the electrical energy consumed by the arc.

Heat input values into the weld for various processes can be calculated from the arc energy by multiplying by the following thermal efficiency factors;

SAW (wire electrode)                                1.0

MMA (covered electrode)                          0.8

MIG/MAG                                                0.8

FCAW (with or without gas shield)            0.8

TIG                                                          0.6

Plasma                                                      0.6

Example

A weld is made using the MAG welding process and the following conditions were recorded;

Volts: 24

Amps: 240

Travel speed: 300mm per minute

Arc energy (kJ/mm) = Volts x Amps / Travel Speed (mm/sec.) x 1000

                                  = 24 x 240 x 60/ 300 x 1000

                                  = 345600/ 300,000

Arc Energy               = 1.152 or 1.2 KJ/ mm

Heat Input (KJ/mm) = Volts x Amps x 60 x K/ Travel Speed (mm/sec.) x 1000

                                  = 24 x 240 x 60 x 0.8/ 300 x 1000

                                  = 276, 480/ 300,000

                                  = 9.91216 KJ/mm

Heat input is mainly influenced by the travel speed.

Welding position and the process have a major influence on the travel speed that can be used.

For manual and semi-automatic welding the following are general principles:

Vertical-up progression tends to give the highest heat input because there is a need to weave to get suitable profile and the forward travel speed is relatively slow.

Vertical-down welding tends to give the lowest heat input because of the fast travel speed that can be used.

Horizontal-vertical welding is a relatively low heat input welding position because the welder cannot weave in this position.

Overhead welding tends to give low heat input because of the need to use low current and relatively fast travel speed.

Welding in the flat position (down hand) can be a low or high heat input position because the welder has more flexibility about the travel speed that can be used.

Of the arc welding processes, SAW has potential to give the highest heat input and the highest deposition rates and TIG and MIG/MAG can produce very low heat input.

Typical heat input values for controlled heat input welding will tend to be in the range ~1.0 to ~3.5kJ/mm.