Heat Input on a WPS Is Written in Joules per Millimetre
A welding procedure specification almost always states a heat-input limit in kilojoules per millimetre — 0.5 to 2.5 kJ/mm covers most structural work. The formula that produces the number, however, does not give kilojoules. Multiply volts by amps by 60 and divide by travel speed in millimetres per minute and the result is joules per millimetre, so every calculation ends one factor of a thousand away from the unit the procedure is written in. That single step is where welding engineers, inspectors and procedure qualification records meet the kilojoule-to-joule conversion, and where a misplaced decimal quietly puts a run outside its qualified range.
What the Number Means Before It Gets Converted
Arc Energy Comes Before Heat Input
The Efficiency Factor Follows the Process
Travel Speed Is the Lever in the Welder's Hand
Interpass Temperature Rides Alongside
Checking a Run Against the Procedure Before the Arc Strikes
Work in joules per millimetre while the arithmetic is happening, then present the answer in the unit the procedure uses. Recording both columns is what makes a welding data sheet reviewable months later.
Assemble the arc energy in J/mm
Multiply the arc voltage by the welding current, multiply by 60 to turn per-second into per-minute, and divide by travel speed in mm/min. At 28 V, 250 A and 400 mm/min that is (28 × 250 × 60) ÷ 400 = 1 050 J/mm.
Apply the process efficiency factor
That MIG run at 0.8 gives 1 050 × 0.8 = 840 J/mm of heat input. Enter 840 in the joule field and the kilojoule side reads 0.84 kJ/mm, comfortably inside a 0.5–2.5 kJ/mm band.
Reverse the pair to read a limit downwards
When the procedure hands you a ceiling instead of a measurement, the swap arrows put kilojoules on the input side: type 1.5 and read 1 500 J/mm, the number the travel-speed rearrangement actually needs.
Put the digits on the welding data sheet
The copy control above each field gives back the bare figure with no unit and no thousands spacing, so a heat-input value transfers cleanly into a qualification record or an inspection form. Ctrl+C inside a field returns the same thing.
Welding Parameter Sets and the Energy They Put Into the Joint
Each row runs one set of arc parameters through the formula and then through its process efficiency factor. Arc energy is shown in both units so the thousandfold step is visible, and the last column is the number a procedure would actually be checked against.
| Process and run | V · A · mm/min | Arc energy (J/mm) | Arc energy (kJ/mm) | Heat input (kJ/mm) |
|---|---|---|---|---|
| GTAW root pass, thin wall | 12 · 110 · 60 | 1 320 | 1.32 | 0.792 (k 0.6) |
| SMAW 3.2 mm low-hydrogen fill | 24 · 130 · 150 | 1 248 | 1.248 | 0.9984 (k 0.8) |
| GMAW short-circuit on 3 mm sheet | 19 · 120 · 500 | 273.6 | 0.2736 | 0.21888 (k 0.8) |
| GMAW spray transfer on 10 mm plate | 28 · 250 · 400 | 1 050 | 1.05 | 0.84 (k 0.8) |
| FCAW gas-shielded fill pass | 26 · 220 · 300 | 1 144 | 1.144 | 0.9152 (k 0.8) |
| Same GMAW run, travel dropped to 150 | 28 · 250 · 150 | 2 800 | 2.8 | 2.24 (k 0.8) |
| SAW single wire, 12 mm butt | 32 · 500 · 500 | 1 920 | 1.92 | 1.92 (k 1.0) |
| SAW heavy deposit, slow travel | 34 · 650 · 400 | 3 315 | 3.315 | 3.315 (k 1.0) |
Rows four and six are the same machine settings; only the travel speed changed, and the heat input rose from 0.84 to 2.24 kJ/mm — from mid-range to the top of a typical band, without touching a knob. The last row is over 2.5 kJ/mm and would need either a faster run or a procedure written to allow it, while the short-circuit row at 0.21888 kJ/mm sits below the usual floor and belongs on thin sheet rather than on a thick restrained joint.
What This Field Pair Gives a Procedure Check
Both J/mm and kJ/mm Visible at Once
The formula lands in joules and the procedure is written in kilojoules; keeping both boxes populated means the comparison against a limit never depends on a shift of three decimal places done in the head.
Heat-Input Values Ready for the WPS Form
Copied output carries no unit label and no spaced thousands, so a value drops straight into a welding data sheet or a qualification record without cleanup.
Read a kJ/mm Ceiling Back Down to J/mm
Rearranging for travel speed needs the limit in joules; the arrows put the kilojoule figure from the procedure on the input side and hand back the value the rearrangement expects.
A Comma-Written 1,25 kJ Reads Correctly
European procedures print the decimal comma, and a value typed that way is accepted as it stands, with spaces ignored, so a figure copied off a paper WPS needs no reformatting.
Heat Input Questions From the Welding Procedure
Where do the volts, amps and the 60 come from in the J/mm formula?
Volts times amps is watts, which is joules per second — the rate the arc delivers energy. Travel speed is conventionally recorded in millimetres per minute, so the 60 converts seconds into minutes and leaves joules per millimetre of weld. At 24 V and 130 A the arc is putting out 3 120 W; run it at 150 mm/min, which is 2.5 mm/s, and each millimetre receives 3 120 ÷ 2.5 = 1 248 J. The 60 in the standard form just saves that intermediate step.
Arc energy or heat input — which one does the procedure limit?
It depends on the code, which is exactly why the record has to say. Heat input is arc energy multiplied by the thermal efficiency of the process: 1.0 for submerged arc, 0.8 for stick, flux-cored and MIG, 0.6 for TIG, following ISO/TR 18491 and long-standing AWS practice. A TIG root at 1 320 J/mm of arc energy is only 792 J/mm of heat input — 1.32 against 0.792 kJ/mm. Reporting the larger figure where the smaller is required makes a compliant run look non-compliant, and the reverse mistake is worse.
What goes wrong in the heat-affected zone above and below the range?
Too much energy holds the metal hot for longer, coarsening grain in the heat-affected zone and dropping toughness, which shows up as poor impact results and can be severe in the coarse-grained region next to the fusion line. Too little energy cools the joint too fast, encouraging hard martensitic microstructure that, together with any residual hydrogen and restraint, invites cold cracking hours after the weld looks finished. The band on a procedure — often 0.5 to 2.5 kJ/mm, or 500 to 2 500 J/mm — is the window between those two failure modes for that steel and thickness.
My WPS caps heat input at 1.5 kJ/mm — how fast must I travel?
Turn the limit into joules and rearrange for speed. A cap of 1.5 kJ/mm is 1 500 J/mm; for a MIG process at 0.8 efficiency the arc energy may reach 1 500 ÷ 0.8 = 1 875 J/mm. With 28 V and 250 A the minimum travel speed is (28 × 250 × 60) ÷ 1 875 = 224 mm/min. Anything slower breaches the limit. That is a number a welder can actually work to, unlike the kilojoule figure it came from, and it is worth marking on the run sheet next to the amperage.
Why does a procedure qualification record heat input at all?
Because the qualification test only proves the properties that were produced under the conditions used. Mechanical and impact results belong to the cooling rate of the test coupon, and heat input is the single parameter that summarises volts, amps and travel speed into that cooling rate. Recording it fixes the range that production welding may use, so a joint welded far outside the qualified band is no longer covered by the test evidence — even if the visual and the radiograph both pass.
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