4.3 Heat Input, Arc Energy, Polarity & Parameters

4.3 Heat Input, Arc Energy, Polarity & Parameters

Quick Answer: Heat input in kJ/mm is typically (V × I × 60) / (v × 1000) with travel speed in mm/min. Combined with preheat/interpass and correct polarity, it controls cooling rate and bead shape. Inspectors measure parameters and keep them inside WPS ranges—qualification is voided if essential variables drift outside those windows.

Process and parameter control is a major IWI-S theme (exam-meta weight for processes and parameters is high). You must calculate heat input, explain polarity effects, and know how to verify parameters on site without guessing.

Why Parameters Matter Metallurgically

Welding is localised heat treatment plus liquid metal addition. Cooling rate through critical temperature ranges influences:

• HAZ hardness and toughness
• Risk of hydrogen cold cracking (with hydrogen and restraint)
• Solidification structure and hot-cracking susceptibility
• Distortion and residual stress patterns

Cooling rate is driven by heat input, preheat/interpass, joint thickness, and heat sinks (fixtures, backing bars). Two welds with identical “look” can have different metallurgical outcomes if heat input differs.

Heat Input Formula (kJ/mm)

The standard teaching formula for arc energy / heat input is:

Heat input (kJ/mm) = (V × I × 60) / (v × 1000)

Where:

• V = arc voltage (volts)
• I = welding current (amperes)
• v = travel speed (mm/min)
• 60 / 1000 converts to kJ per mm of weld length

Worked example

Suppose V = 24 V, I = 200 A, travel speed v = 300 mm/min:

Heat input = (24 × 200 × 60) / (300 × 1000) = 288000 / 300000 = 0.96 kJ/mm

If travel speed drops to 200 mm/min with the same V and I:

Heat input = (24 × 200 × 60) / (200 × 1000) = 1.44 kJ/mm

Slower travel raises heat input; faster travel lowers it. Voltage and current increase also raise heat input if speed is fixed.

Units discipline

• If speed is given in cm/min or in/min, convert carefully before using the formula.
• Some documents report heat input in kJ/cm (numerically 10× kJ/mm for the same weld)—read the unit on the WPS.
• Do not mix wire-feed speed with travel speed; they are different variables.

Arc Energy vs Heat Input and Efficiency Factors

Many training texts distinguish:

• Arc energy — electrical energy calculated from V, I, and travel speed (the formula above without efficiency).
• Heat input — energy estimated to enter the workpiece, sometimes arc energy × efficiency factor (η).

Approximate efficiency factors often taught for order-of-magnitude comparison (values vary by source and process setup):

Use the definition required by the governing WPS, code, or qualification standard. Some procedures qualify on arc energy without η; others specify heat input with efficiency. On the exam, apply the formula as stated in the question. On site, follow the fabricator’s qualified method and do not invent η values.

Travel Speed Measurement

Travel speed is the variable most often misestimated by eye. Practical methods:

1. Timed length — mark a known weld length (e.g. 100 mm), time the arc travel with a stopwatch, compute v = length / time (convert to mm/min).
2. Mechanised readout — tractor or robot display/logger (verify calibration status if QC requires it).
3. Video or pendant records in automated cells.

For multipass welds, measure per pass if the WPS sets heat-input limits per pass. Average speeds across long stops can hide noncompliant peaks.

Preheat and Interpass Interaction

Heat input alone does not define cooling rate. Preheat raises the starting temperature so the weld cools more slowly; interpass temperature is the temperature of the joint before the next pass. Inspection implications:

• High heat input plus high preheat can produce soft, coarse HAZ and may reduce strength/toughness if limits are exceeded.
• Low heat input plus low preheat on hardenable steels raises HAZ hardness and hydrogen-cracking risk.
• WPS often lists minimum preheat, maximum interpass, and heat-input window together—verify the set, not one number in isolation.
• Measure temperature with approved methods (contact thermocouple, calibrated crayons/pyrometers as allowed) at the locations the procedure specifies.

Polarity Effects

Polarity sets which terminal is positive and where more electron-related heating occurs.

Common conventions (arc welding teaching model)

• DCEP (Direct Current Electrode Positive) — electrode positive / workpiece negative. Often puts more heat into the electrode/wire, supporting higher melt-off rates in many consumable-electrode processes; deep penetration behaviour depends on process (classic teaching also links DCEP to good penetration in SMAW for many electrodes).
• DCEN (Direct Current Electrode Negative) — electrode negative / workpiece positive. Often puts more heat into the work, useful when you want less electrode heating (e.g. some TIG setups with DCEN for tungsten longevity and work heating).
• AC — alternating polarity; used for aluminium TIG cleaning action and some SAW/MMA electrodes designed for AC.

Exact penetration and bead-shape outcomes depend on process + consumable + parameters. The inspector rule is absolute: polarity on the machine and cable hookup must match the WPS. Wrong polarity is a frequent cause of unstable arc, tungsten damage (TIG), poor penetration, or excessive spatter.

Also verify cable polarity labels after maintenance—swapped leads are a classic site error.

Other Parameters the Inspector Watches

Beyond V, I, and v:

• Wire-feed speed (MIG/MAG/FCAW/SAW) — primary current driver on CV systems.
• Stick-out / electrode extension — affects resistance heating of wire and actual current.
• Gas type and flow — shielding effectiveness; drafts and blocked nozzles matter.
• Weaving width and technique — can change effective heat input and sidewall fusion.
• Pulse settings (where used) — average vs peak current must still satisfy procedure intent.

Why WPS Ranges Matter

A Welding Procedure Specification records parameters that produced acceptable mechanical and NDT results during procedure qualification (supported by WPQR). Ranges exist because:

1. Essential variables have qualified windows—outside them, properties are not demonstrated.
2. Production needs tolerance for real equipment, but not unlimited freedom.
3. Codes define which variables are essential, supplementary essential, or nonessential—changing essential variables beyond limits requires requalification.

Inspector actions:

• Confirm the correct WPS revision is at the workstation.
• Verify amperage, voltage, travel speed, polarity, gas, consumable against that WPS.
• Stop or raise NCR when values are outside range (per project authority matrix).
• Do not “approve by eye” a beautiful bead welded at unrecorded or out-of-range settings.

Measuring Voltage and Current

Practical verification:

• Calibrated clamp meters / in-line meters for current; measure during steady welding, not only at strike.
• Voltage should be arc voltage (near the process), not confused with open-circuit voltage.
• For long leads, voltage drop can make panel meters disagree with true arc voltage—know where the WPS expects measurement.
• Prefer simultaneous V, I, and speed samples for heat-input calculation.
• Automated data acquisition is preferred evidence when the QC plan requires it; spot checks still validate sensors.

Document readings with time, joint ID, welder ID, and WPS number so traceability survives audit.

Exam Calculation Discipline

On WTE/WIE-style questions:

1. Write the formula.
2. Convert units.
3. Compute carefully.
4. Compare to a stated limit (e.g. max 1.5 kJ/mm).
5. Link the result to a metallurgical consequence if asked (HAZ hardness, cracking risk, toughness).

Exam Traps

• Using wire-feed speed as travel speed in the heat-input formula.
• Reporting heat input without checking kJ/mm vs kJ/cm.
• Ignoring preheat when discussing cooling rate.
• Assuming any polarity is fine if the arc “looks stable.”
• Believing a WPS range is a suggestion rather than the qualified window.

Parameter control is how inspectors connect electrical settings to metallurgical outcomes. Master the formula, the measurements, and the WPS discipline.