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Weld Arc Heat Input Calculator

Calculate true thermal energy transferred into the base metal (Joules per inch) using amperage, voltage, and travel speed. Critical for ASME IX and AWS D1.1 WPS compliance.

Arc Energy

Welding Process

Travel Speed (IPM)

Welding Amperage150 A

Welding Voltage22 V

True Heat Input

13.2 kJ/in

Joules per Inch13,200 J/in

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The Apprentice Corner 📚

What is Heat Input? Heat input is the amount of electrical energy (Joules) dumped into one inch of the weld over time. Amps and Volts determine the raw power, but how fast you move your hands (Travel Speed) determines how much of that power concentrates in a single spot.

If you move your hands too slowly, you pump massive amounts of Joules into the metal, which can warp it, change its metallurgical grain structure, or blow a hole right through it.

The Journeyman's Note ⚡

"ASME BPVC Section IX dictates that if you are welding Impact-Tested materials (like pressure pipelines), Heat Input is an Essential Variable. You CANNOT exceed the qualified Heat Input limit from the PQR without physically re-testing the metal to prove you haven't destroyed its Charpy V-Notch toughness properties."

For estimation purposes only. Always consult a licensed professional before beginning work. Full Trade Safety Notice →

What is Arc Heat Input: The WPS Parameter That Controls Weld Metallurgy?

Heat input is the thermal energy deposited per unit length of weld. It is the single most important variable controlling cooling rate, HAZ width, grain size, and mechanical properties. ASME Section IX and AWS D1.1 both require heat input to be recorded on every WPS/PQR because it directly determines whether the weld zone will be tough and ductile or brittle and susceptible to cracking.

Mathematical Foundation

Heat Input (AWS/ASME Standard)

HI = \frac{V \times A \times 60}{S}

HI

= Heat Input in Joules per inch (J/in)

V

= Arc Voltage (Volts)

A

= Welding Current (Amps)

S

= Travel Speed (inches per minute)

60

= Conversion factor from seconds to minutes

Heat Input (kJ/mm — Metric)

HI = \frac{V \times A}{S \times 1000}

S

= Travel speed in mm/s for the metric version

Laws & Principles

ASME Section IX Essential Variable: Heat input is an essential variable for P-Number 1 through P-Number 11 materials. Exceeding the qualified heat input range by more than 10% invalidates the WPS and requires requalification.
AWS D1.1 Table 4.5: Heat input limits for prequalified WPS are defined by material group and thickness. Exceeding the limit produces unacceptable grain coarsening in the HAZ.
Maximum Heat Input: Too much heat input causes grain coarsening, reduced toughness, and excessive distortion. Quenched and tempered steels (A514, HY-80) are especially sensitive — high heat input re-austenitizes the HAZ and destroys the tempered microstructure.
Minimum Heat Input: Too little heat input causes lack of fusion, incomplete penetration, and excessively fast cooling rates that produce brittle martensite in the HAZ.
Travel Speed vs. Heat Input: Doubling travel speed halves heat input. This is the easiest variable for a welder to control in production.

Step-by-Step Example Walkthrough

" A welder is running GMAW (MIG) on A572 Gr 50 structural steel at 28V, 250A, traveling at 12 inches per minute. The WPS specifies a maximum heat input of 55,000 J/in. "

1. Apply formula: HI = (V x A x 60) / S
2. HI = (28 x 250 x 60) / 12
3. HI = 420,000 / 12 = 35,000 J/in
4. Convert to kJ/in: 35,000 / 1,000 = 35 kJ/in
5. Compare to WPS maximum: 35,000 < 55,000 J/in. Within limits.

Final Result: Heat input is 35,000 J/in (35 kJ/in). Well within the WPS maximum of 55 kJ/in. The welder has room to slow down slightly for better penetration without exceeding limits.

Quick Answer: What Is Weld Heat Input and Why Does It Matter?

Heat input is the thermal energy deposited per inch of weld, measured in Joules per inch (J/in). The formula is HI = (Volts × Amps × 60) / Travel Speed (in/min). It matters because it controls the cooling rate of the weld zone. Too much heat input causes grain coarsening and reduced toughness. Too little causes lack of fusion and brittle martensite. ASME Section IX and AWS D1.1 both require heat input to be recorded on every Welding Procedure Specification (WPS) because exceeding the qualified range by more than 10% invalidates the procedure.

Heat Input Formula

Heat Input (J/in) = (Voltage × Amperage × 60) / Travel Speed (in/min)

Heat Input (kJ/mm) = (Voltage × Amperage) / (Travel Speed mm/s × 1000)

The 60 in the US formula converts seconds to minutes. Some codes also apply a thermal efficiency factor (0.8 for GMAW, 0.6 for GTAW, 1.0 for SAW) but AWS D1.1 uses the raw calculation without efficiency correction.

Heat Input Failures

The Q&T Steel Disaster

A fabrication shop welds A514 quenched-and-tempered steel (100 ksi yield) using SAW at 32V, 550A, 10 in/min. Heat input: (32 x 550 x 60) / 10 = 105,600 J/in. The WPS maximum for A514 is 45,000 J/in. The high heat input re-austenitized the HAZ (heated it above the A3 transformation temperature) and destroyed the tempered martensite microstructure. Post-weld tensile tests show the HAZ yield strength dropped to 62 ksi, below the 100 ksi minimum. The entire weld run (40 feet of high-value connection) must be removed and re-welded with proper parameters. Cost: $22,000 in labor and rejected material.

The Travel Speed Adjustment

A CWI reviews production data and notices the welder running a multi-pass groove weld at 25V, 200A, 8 in/min. Heat input: (25 x 200 x 60) / 8 = 37,500 J/in. The WPS range is 25,000-45,000 J/in. The welder is within limits but the CVN (Charpy) toughness samples from this heat input range have been marginal. The CWI recommends increasing travel speed to 10 in/min: (25 x 200 x 60) / 10 = 30,000 J/in. This 20% reduction in heat input narrows the HAZ by roughly 15% and improves Charpy toughness by 12 ft-lbs. The adjustment costs nothing and takes 5 seconds to implement.

Typical Heat Input Ranges by Process

Process	Typical Range (kJ/in)	Typical Range (kJ/mm)	Notes
GTAW (TIG)	8-25	0.3-1.0	Lowest heat input; best for thin materials
GMAW (MIG) Short Circuit	15-35	0.6-1.4	Low-moderate; good for thin-to-medium
GMAW (MIG) Spray	30-60	1.2-2.4	Moderate-high; thick plate flat/horizontal
FCAW	25-55	1.0-2.2	Similar to MIG spray; good all-position
SMAW (Stick)	20-45	0.8-1.8	Moderate; varies with electrode size
SAW (Submerged Arc)	50-150+	2.0-6.0+	Highest; not suitable for Q&T steels

Note: These are typical production ranges. Actual WPS limits depend on material, thickness, and code requirements. Always verify against your qualified WPS.

Pro Tips for Heat Input Control

Do This

✓Record actual volts, amps, and travel speed for every production weld. ASME IX and AWS D1.1 require the as-welded parameters to match the qualified WPS range. A CWI or third-party inspector can reject the entire weld if the recorded heat input exceeds the qualified maximum by more than 10%.
✓Use a stopwatch to measure travel speed. Time the weld pass over a known distance. Welders consistently overestimate their travel speed by 15-25%. A welder who thinks they are traveling at 12 in/min is often moving at 9-10 in/min, which increases heat input by 20-33% and may push the weld outside the WPS range.

Avoid This

✗Don't use SAW on quenched-and-tempered steels without specific WPS validation. Submerged arc welding deposits the highest heat input of any common process (50-150+ kJ/in). On Q&T steels like A514, this re-austenitizes the HAZ and destroys the tempered microstructure, reducing yield strength below the minimum specification. Use GMAW or FCAW with controlled parameters instead.
✗Don't use the machine dial reading as your actual voltage. The voltage at the arc is lower than the machine setting due to cable resistance (voltage drop). On a 100-foot lead cable, the drop can be 2-4 volts. Use the actual arc voltage from the machine readout or a separate meter at the weld head. A 3-volt error at 250A changes heat input by roughly 10%.

Frequently Asked Questions

What happens if heat input is too high?

Excessive heat input causes: (1) Grain coarsening in the HAZ, which reduces Charpy V-notch (CVN) toughness dramatically. (2) A wider HAZ, creating a larger zone of degraded material. (3) Excessive distortion and residual stress. (4) On quenched-and-tempered steels, loss of the tempered microstructure and a drop in yield strength. On stainless steels, excessive heat input causes chromium carbide precipitation (sensitization), making the weld zone susceptible to intergranular corrosion.

What happens if heat input is too low?

Insufficient heat input causes: (1) Lack of fusion between the weld bead and the base metal or between passes. (2) Incomplete penetration at the root. (3) Excessively fast cooling rates that produce brittle martensite in the HAZ of hardenable steels. (4) Cold lapping (bead sitting on top of the base metal without fusing). Low heat input is the most common cause of lack-of-fusion defects found during radiographic or ultrasonic inspection.

How does heat input relate to preheat requirements?

Heat input and preheat both slow the cooling rate, but they work differently. Heat input is the energy deposited BY the arc. Preheat is the temperature of the base metal BEFORE the arc strikes. Higher preheat reduces the temperature gradient, slowing cooling. Higher heat input increases the total thermal mass of the weld zone. AWS D1.1 Annex I calculates minimum preheat based on both carbon equivalent (CE) AND heat input. A low-heat-input process (TIG at 10 kJ/in) may require higher preheat than a high-heat-input process (SAW at 80 kJ/in) on the same material because the TIG weld cools faster.

What is the difference between heat input in J/in and kJ/mm?

They are the same measurement in different units. To convert: 1 kJ/mm = 25.4 kJ/in = 25,400 J/in. So 35,000 J/in = 35 kJ/in = 1.38 kJ/mm. US codes (AWS D1.1, ASME Section IX) typically use J/in or kJ/in. European and ISO codes (EN ISO 15614) use kJ/mm. Always confirm which unit your WPS uses before recording parameters.