MIG Welding Temperature: How Hot It Gets and How to Control It

You just finished a bead on 1/8-inch mild steel, and it looks tall, lumpy, and sits on top of the plate like a worm on a sidewalk. You crank the voltage up, and now you’re blowing holes through the joint. Somewhere between those two attempts is the temperature you actually wanted, but your machine doesn’t have a temperature dial, only volts and wire speed. That gap, between what welders say and what the machine controls, is where most MIG problems are born.

MIG Welding Temperature: The Short Answer Before the Deep Dive

That tall, lumpy bead sitting on top of 1/8-inch plate is telling you something useful: the arc melted wire, but it never delivered enough energy to melt the steel underneath. Crank the voltage and you swing to the opposite failure, blowing through the plate. The sweet spot exists, but your machine will never label it “temperature.” It gives you volts and wire feed speed, and expects you to turn those into a sound weld.

So here is the short answer. MIG welding temperature is not a single setting. It is the result of four variables working together: voltage, which sets arc length; wire feed speed, which sets amperage and how much filler enters the puddle; travel speed, which decides how long the arc stays in one place; and shielding gas, which affects heat transfer and arc stability. Change any one and the heat changes. Ask whether you mean arc plasma, molten puddle, base metal preheat, or interpass temperature, and the answer changes again.

The arc plasma at the core of a GMAW arc typically runs about 6,000 to 10,000 degrees F (3,300 to 5,500 C). That is hotter than the melting point of any common structural metal. The molten puddle is much cooler, usually around 2,700 to 2,800 degrees F, because steel melts and gets pushed aside before it can absorb much more. Mild steel itself melts at roughly 2,600 to 2,800 degrees F (1,430 to 1,540 C). Aluminum melts near 1,221 degrees F (660 C), about half that, which is a big reason MIG welding aluminum feels like a completely different process.

That gap between arc and puddle explains an important distinction: hot enough to fuse versus hot enough to ruin the metal. Fusion requires the puddle and the adjacent base metal to reach melting at the joint root. Ruin happens when excess heat spreads into the surrounding steel, widening the heat-affected zone, increasing distortion, softening quenched-and-tempered grades, or burning through thin sheet. A weld can be hot enough to fuse and still be overheated.

In practice, a welder controls or measures three temperatures. Heat input is the calculated energy delivered per inch or millimeter of weld, usually expressed in kJ/in or kJ/mm. It is not shown on the machine display; you calculate it from voltage, amperage, and travel speed (TWI explains the underlying formula). Preheat is the bulk temperature of the base metal before the arc starts. Interpass temperature is the maximum allowed base metal temperature between passes. Preheat and interpass protect the steel’s cooling behavior, not just the puddle.

Those numbers are not universal. A thick restrained joint may need preheat to slow cooling and prevent cracking; 1/8-inch mild steel usually needs none. A quenched-and-tempered steel may have a strict heat input cap and a low interpass ceiling, while austenitic stainless often tops out far below carbon steel limits. The material, thickness, joint restraint, and filler decide the window.

This guide will walk through the physics behind the arc and puddle, the voltage–wire speed–travel speed triangle, the heat input math with worked examples, preheat and interpass limits, distortion and cracking, material-specific windows, troubleshooting by symptom, measurement tools, and the safety side. On code work, preheat and interpass requirements are governed by specifications such as AWS D1.1 and ASME Section IX. Use the principles here to understand the why, then follow the applicable welding procedure specification for the numbers you actually apply.

What ‘Temperature’ Really Means in MIG Welding: Four Different Numbers

Ask five welders what temperature they run on 1/8-inch steel and you will get answers in volts, amps, or wire speed. None of those is a temperature. In MIG welding, the word describes at least four separate things, and confusing them is how a sensible setting turns into a cold lap or a burn-through.

1. Arc Plasma Temperature: Hotter Than the Metal Can Stay

The arc is not a flame. It is ionized gas carrying current between the wire and the work, and its core can reach roughly 6,000–10,000°F (about 3,300–5,500°C), as described in Miller’s welding resource library. That heat exists for milliseconds and is concentrated in a small column. Voltage sets arc length: a long arc spreads heat over a wider area, overheats the surrounding plate, and invites undercut and spatter. A short arc concentrates energy into a tighter spot but can stub, spatter, or leave a narrow, ropey bead.

2. Puddle Temperature: Where Fusion Actually Happens

The puddle is the molten mixture of filler wire and base metal. For carbon steel it sits around 2,700–2,800°F regardless of whether the machine reads 18 V or 24 V. To fuse, it must exceed the liquidus of both materials. Extra energy superheats the pool, making it more fluid and easier to wash out, but a wet-looking puddle is not proof of penetration. A puddle can look lively while riding on unmelted base metal underneath.

3. Preheat Temperature: The Starting Condition

Preheat is the bulk temperature of the base metal before the arc starts, measured a short distance from the joint rather than in the puddle or the arc. It is generally a minimum requirement. Its jobs are to slow cooling, drive off surface moisture, and reduce cracking risk in thick or highly restrained steel. Thin mild steel often needs none; thick sections and higher-carbon-equivalent steels are another story.

4. Interpass Temperature: The Ceiling You Must Respect

Interpass temperature is the maximum allowed base-metal temperature between passes. Unlike preheat, it is a ceiling, not a target. Let it climb and you can lose toughness in the heat-affected zone, over-temper quenched-and-tempered steel, widen the HAZ, and add distortion. The usual shop fix is air cooling, sometimes with compressed air. Water is not a shortcut; it can introduce cracking and hardness problems.

5. Ambient Temperature and Humidity: The Practical Extra

The fifth practical number is shop temperature and humidity. A cold plate pulls heat out of the weld, changing the cooling rate. Wind strips shielding gas. Damp air and moist plate increase the chance of porosity and hydrogen pickup. These conditions rarely appear in a code table, but they absolutely change how the arc behaves.

Temperature	What it is	Typical value or limit	How it is controlled
Arc plasma	Ionized gas between wire and work	About 6,000–10,000°F at the core	Voltage, current, arc length
Puddle	Molten filler plus base metal	About 2,700–2,800°F for carbon steel	Amperage, travel speed, transfer mode
Preheat	Bulk base-metal temperature before welding	A minimum set by the WPS or code; often none on thin mild steel	Torch, blankets, induction, oven
Interpass	Base-metal temperature between passes	A maximum set by the WPS or code; material-specific	Air cooling, time, travel sequence
Ambient	Shop air and starting plate condition	No single number; avoid wind, frost, and condensation	Wind breaks, heating, ventilation

Cooling Rate Is the Number Behind the Numbers

Codes focus on preheat and interpass because both control the hidden variable: cooling rate. Cooling rate, not peak temperature alone, largely determines hardness and toughness in the heat-affected zone, a relationship TWI’s technical FAQs explain in detail. Two welds run at the same amperage can carry very different heat input if travel speed differs. Change from 10 to 20 inches per minute and the energy per inch can halve. One bead may be quenched by a cold fixture, the other may cool slowly on a preheated plate. The amperage on the display looks identical, but the steel ends up with a different microstructure. Aluminum makes the point vividly, since it conducts heat away so fast that the surrounding plate acts like a heat sink; our aluminum MIG welding guide covers that behavior in practice.

Keep those numbers separate and the settings start to make sense. Arc temperature is a consequence of voltage and current. Puddle temperature is what actually fuses metal. Preheat is the starting condition. Interpass is the limit you protect. Ambient conditions sit underneath all of it, quietly changing the result.

Voltage, Wire Feed Speed, and Travel Speed: The Real Temperature Controls

Your MIG machine has three knobs that actually matter, and none of them says “temperature.” Voltage, wire feed speed, and travel speed work together like the legs of a three-legged stool. Move one, and the other two have to adjust or the whole thing tips over. Master how these three interact and you stop guessing at heat. You start predicting it.

Here is the plain-English version before we get into numbers: voltage shapes the arc, wire feed speed sets how much energy is available, and travel speed decides how much of that energy lands on each inch of joint. Everything else in this section is detail hung on that frame.

Voltage: The Arc Length Knob

Voltage controls arc length. Crank it up, and the arc stretches out longer, spreading heat over a wider area. The bead flattens, wets out further at the toes, and the puddle gets more fluid. Push it too far and you get undercut along the edges, spatter peppering the plate, and a bead that looks wide and washed out.

Drop the voltage and the arc shortens and concentrates. The bead turns narrow, high-crowned, and ropey. In extreme cases the wire stubs into the plate and the puddle freezes before it can tie into the sidewalls, which is textbook cold lap.

Think of voltage as the shape of the heat, not the amount of it. A long arc at the same amperage covers more surface area but delivers less intensity per square inch. This is why raising voltage alone rarely fixes a lack-of-fusion problem. It can make the weld look prettier while the root stays cold. In practice, voltage is the fine-tuning knob you touch in half-volt increments once wire feed speed is close.

Wire Feed Speed: The Amperage Dial

On a constant-voltage MIG machine, wire feed speed is your amperage control. Feed wire faster and the machine pushes more current to melt it. More current means more melt-off, more deposition, and a hotter puddle. Feed it slower and everything cools down.

There is a ceiling. Push wire faster than the arc can consume it and the wire drives into the plate, stubs, and either burns back with a pop or birds-nests at the drive rolls. That upper limit is the machine telling you the arc voltage can no longer sustain that burn-off rate.

Because these two controls are locked together, many machines publish a “synergic” curve that pairs each wire speed with a voltage. Learning to set wire speed first, then voltage, keeps you from chasing your tail between two knobs that fight each other when adjusted out of order.

Travel Speed: The Multiplier Nobody Watches

Travel speed is the most underrated of the three, and it changes heat input inversely. Slow down and you deposit more energy per inch of weld. Speed up and you spread the same energy over more distance.

But there’s a trap. Slowing down past a certain point stops adding penetration and starts subtracting it, because the arc begins riding on top of a puddle that has piled up ahead of it. The arc no longer reaches the root. You get a wide, tall, cold weld that looks substantial and fuses to nothing. Faster travel, within reason, drives the arc into the base metal and produces better penetration than a crawl.

Watch the puddle, not the clock. A healthy bead rides just behind the arc with a leading edge that bites into the plate. When the puddle runs out ahead of the arc, you are moving too slowly for the current you set.

The Amp-Per-Thousandth Rule of Thumb

For mild steel, a widely used shop starting point is roughly 1 amp per 0.001 in (0.025 mm) of material thickness. So 1/8 in plate lands near 125 A, 1/4 in near 250 A, and 16-gauge sheet near 60 A.

Treat this as a launch pad, not a law. Joint configuration, position, fit-up, wire diameter, gas mix, and transfer mode all shift the real number. A tight square-groove butt needs different current than a loose fillet on the same thickness. Use the rule to get in the neighborhood, then let the puddle tell you the rest.

Starting Points by Thickness and Transfer Mode

The table below assumes 0.030–0.035 in solid wire, 75/25 Ar/CO₂ for short circuit, and an argon-rich mix (85–90% Ar) for spray. These are common starting values, not a WPS.

Thickness	Transfer	Voltage	Amperage	Travel Speed	Gas Flow
18 ga (0.048 in)	Short circuit	15–17 V	50–70 A	Fast, stitch if needed	15–20 CFH
16 ga (0.060 in)	Short circuit	16–18 V	60–80 A	Moderate	15–20 CFH
1/8 in (0.125 in)	Short circuit	18–20 V	110–140 A	10–14 in/min	20–25 CFH
3/16 in (0.188 in)	Short circuit	19–22 V	140–170 A	10–14 in/min	20–25 CFH
1/4 in (0.250 in)	Short circuit	21–24 V	170–200 A	10–14 in/min	20–25 CFH
1/4 in (0.250 in)	Spray	26–29 V	220–260 A	12–18 in/min	25–35 CFH
3/8 in (0.375 in)	Spray	28–31 V	250–300 A	12–18 in/min	25–35 CFH

Shielding gas flow for mild steel GMAW typically sits around 15–25 cubic feet per hour. That range holds for indoor work with decent wind protection. Outdoors or near a fan, you either build a screen or watch your porosity climb — and no voltage adjustment will save you.

Transfer Modes and Their Temperature Character

The transfer mode changes the character of the heat more than any single setting. It’s worth understanding each one, because the wrong mode guarantees problems regardless of how carefully you tune the knobs.

• Short circuit: The wire touches the puddle, shorts out, and detaches dozens to hundreds of times per second. Very low heat input, excellent for thin sheet and out-of-position work. On thick sections it is prone to lack of fusion because the arc never gets hot enough to dig in before the next short.
• Globular: Large droplets transfer irregularly across a wide arc. Unstable, spattery, and hard to control. Most operators treat globular as a region to pass through on the way to spray, not a mode to weld in.
• Spray: Fine droplets blast axially across the arc at high current density. Deep penetration, high deposition, and a smooth bead — but it needs current above the transition level, commonly well over 200 A for 0.035 in wire in argon-rich mixes, plus a gas blend with enough argon to support it. Not viable on thin sheet.
• Pulsed spray: Alternates a high peak current with a low background current, so metal transfers in spray mode at average currents low enough for thin material and vertical work. This is the mode that gives you spray-like fusion with controllable heat input.

Material behavior changes these windows substantially. Aluminum, for instance, demands different gas, different wire feed, and a very different relationship between voltage and arc stability — the aluminum MIG welding practical guide covers those shifts in detail, and prep matters even more than settings when you’re chasing clean fusion, as shown in this guide to aluminum cleaner for welding.

Turning These Knobs Into an Actual Setting

Start from thickness, not from the display. Pick your transfer mode first, because short circuit and spray live in entirely different voltage and amperage neighborhoods. Then set wire feed speed to land near the amp-per-thousandth target, set voltage to match the arc length you want, and only then dial travel speed while watching the puddle.

Adjust one variable at a time and run a test bead on scrap of the same thickness. If the bead sits high and crowned with poor tie-in, add wire feed speed before you touch voltage. If it’s flat, wide, and undercutting at the toes, back the voltage down. If the puddle outruns the arc and the bead looks poured rather than fused, increase travel speed.

Those three decisions — shape, energy, energy per inch — account for most of what a welder means when they say “temperature.” Once you can read them off the puddle instead of the dial, the rest of this article’s math and material windows will make a lot more sense. For the underlying arc behavior and manufacturer guidance on transfer modes, the Miller welding resource library and TWI’s technical FAQs are worth bookmarking.

Heat Input Math: Turning Volts and Amps Into a Number You Can Compare

Voltage and wire feed speed get most of the attention, but neither one tells you how much energy actually goes into the plate. Travel speed decides that. Put the three together and you get heat input, a single number you can compare from one weld to the next, one procedure to another, and one machine to a completely different machine.

The formula in plain English

For inch-pound work, heat input is:

HI (kJ/in) = (60 × volts × amps) ÷ travel speed in inches per minute

Volts times amps gives watts, or joules per second. Multiplying by 60 converts the minutes in your travel-speed figure into seconds. Dividing by inches per minute leaves joules per inch, and dividing by 1,000 turns that into kilojoules per inch.

For metric procedures:

HI (kJ/mm) = (volts × amps × 60) ÷ (travel speed in mm/min × 1,000)

To convert between the two, remember that 1 kJ/in is approximately 0.0394 kJ/mm. That single conversion factor is enough to compare a shop chart written in kJ/in with an ISO or ASME procedure written in kJ/mm.

Three welds, three very different numbers

Voltage	Amperage	Travel speed	Heat input (kJ/in)	Heat input (kJ/mm)
19 V	130 A	12 in/min	12.35	0.487
24 V	180 A	15 in/min	17.28	0.681
26 V	220 A	25 in/min	13.73	0.541

Run the arithmetic and the pattern becomes obvious. The first weld is mild: 60 × 19 × 130 ÷ 12 equals 12,350 joules per inch, or 12.35 kJ/in. The second is the hottest per inch at 17.28 kJ/in, even though its voltage and amperage are lower than the third weld. The third looks like the aggressive one, with 26 volts and 220 amps, but a fast 25 in/min travel spreads that energy over more inches and brings it down to 13.73 kJ/in.

That is the counterintuitive lesson. A fast, high-current weld can have a fierce arc and still deposit less energy per inch than a slower, milder weld. Peak temperature and total energy are not the same thing.

What about thermal efficiency?

The formula above gives arc energy at the torch. Not all of that energy reaches the workpiece; some is lost to radiation, spatter, and the surrounding atmosphere. For GMAW, a thermal efficiency factor of roughly 0.8 is commonly used when an estimate of delivered heat is needed, a convention covered in resources such as TWI’s technical guidance. Applying 0.8 to the first example gives about 9.9 kJ/in.

Do not apply the factor automatically. Some procedures define heat input as calculated arc energy, while others require efficiency to be included. Read the definition in the applicable specification before you compare your number with a limit.

Maximum and minimum limits both matter

Many quenched-and-tempered structural steels specify both a maximum heat input and a maximum interpass temperature. The heat input cap protects the heat-affected zone from softening and toughness loss; the interpass cap prevents the whole joint from soaking at temperature. Austenitic stainless steels also have practical ceilings, since excess heat worsens distortion and can leave the steel sensitized.

Minimum heat input matters too. On hardenable steels, an extremely fast cooling rate can produce hard, crack-prone microstructures, especially when hydrogen is present. The answer is not always “more heat”; it is the range that the procedure was qualified to use. Aluminum behaves differently again because its conductivity pulls heat away so quickly, which is why the complete guide to aluminum MIG welding treats heat input and travel speed as a paired decision rather than separate settings.

Procedure qualification records exist precisely to bracket these variables. A WPS and its supporting PQR list the qualified heat input range, and production welding should stay inside it. For code work, the requirements come from documents such as AWS D1.1 and ASME Section IX, not from a generic shop chart.

Write the heat input number on your parameter card next to the volts, amps, and travel speed. It takes ten seconds, and it turns a vague argument about “running hot” into a comparison you can actually repeat.

Preheat and Interpass Temperature: The Limits That Protect the Steel

Preheat and interpass temperature are the two numbers a MIG welder can control before and between passes, and they often matter more than a small tweak to voltage. One prepares the steel for the arc. The other stops the steel from getting too hot after the arc has moved on.

Get either wrong on thick plate, high-carbon steel, or a restrained joint, and the weld may pass a visual inspection yet fail later through cracking or lost toughness. That is why procedure specifications treat these temperatures as limits, not suggestions.

Preheat Is a Cooling-Rate Tool, Not a Comfort Setting

Preheat is the bulk temperature of the base metal before welding begins. Its main jobs are to slow the cooling rate, drive off surface moisture, and give hydrogen trapped in the weld time to escape. Slow cooling also helps avoid hard, brittle microstructures in the heat-affected zone.

Hydrogen-assisted cracking becomes more likely as carbon equivalent, section thickness, joint restraint, and cooling rate increase. A fast-cooling weld on heavily restrained steel is the classic bad combination. TWI’s technical FAQs explain the underlying hydrogen-cracking mechanism in more detail.

Preheat also matters when the shop is cold. A plate sitting at 40 degrees F pulls heat out of a small puddle very quickly. Without preheat, the weld may lack fusion or crack before anyone notices a problem.

How Much Preheat? Start With Five Inputs

There is no universal preheat number for every steel. The amount depends on:

• Carbon equivalent: higher CE means greater hardenability and more cracking risk.
• Thickness: thick sections conduct heat away faster and restrain shrinkage more.
• Joint restraint: stiff T-joints and repairs on loaded members are less forgiving than free edges.
• Hydrogen level: moisture, mill scale, paint, and filler condition all affect hydrogen pickup.
• Code or WPS: the applicable procedure specification sets the enforceable minimum.

For alloy grades, filler choice and heat treatment belong in the same conversation. Our guide to alloy steel welding electrodes covers selection and safe use. On code work, AWS D1.1 and related specifications govern preheat by material group and thickness, so always work from the project’s current WPS.

Heating and Checking It

Common heating methods include oxy-fuel torches, resistance heating blankets, induction coils, and shop ovens for smaller parts. Torches are flexible but easy to overheat locally. Blankets and induction give more even heat, which is why they are preferred on thicker or code-critical joints.

Verify preheat with temperature-indicating crayons, contact thermocouples, or an infrared thermometer. Infrared readings can be misleading on shiny, mill-scaled, or recently ground steel because emissivity changes. A temp stick or contact probe avoids that problem.

Interpass Temperature: The Ceiling You Should Not Chase

Interpass temperature is the maximum base-metal temperature allowed between passes. Exceeding it can reduce toughness, over-temper quenched-and-tempered steel, widen the heat-affected zone, and increase distortion. The weld may look fine while the metallurgy quietly suffers.

AWS D1.1 commonly caps interpass temperature at 650 degrees F for carbon steel unless the engineer or WPS states otherwise. Austenitic stainless is often limited to about 350 degrees F to protect corrosion resistance and limit sensitization. Some quenched-and-tempered grades allow only 400 to 500 degrees F.

Material	Typical interpass ceiling	Main concern
Carbon steel to AWS D1.1	650 degrees F, unless WPS says otherwise	Toughness and distortion
Austenitic stainless	About 350 degrees F	Sensitization and corrosion resistance
Quenched-and-tempered steel	Often 400 to 500 degrees F	Over-tempering the HAZ

These are starting points, not universal law. Values differ by material group, thickness, and code edition. If the WPS is stricter, follow the WPS.

When a part runs too hot, let it cool in still air or use compressed air. Never water-quench a weld to bring interpass down. Rapid quenching can create hard zones and cracking, especially on alloy steel.

A Field Checklist Before the Arc Starts

1. Confirm the steel grade and thickness against the drawing or WPS.
2. Check the specified preheat minimum and interpass maximum.
3. Clean moisture, oil, paint, and heavy scale from the joint.
4. Heat evenly, using blankets or induction where practical.
5. Measure near the joint and at depth, not only at the surface.
6. Record the reading before tacking or welding.
7. Recheck interpass before every additional pass.

Preheat buys time for hydrogen to escape and cooling to stay controlled. Interpass keeps that benefit from turning into damage. Respect both limits and the steel will do its part.

How Temperature Creates Distortion, Residual Stress, and Cracking

Every weld is a small contest between expansion and restraint. The arc heats a narrow strip of metal until it wants to grow in every direction, but the colder plate around it refuses to move. The hot metal yields instead, squashing plastically against that restraint. When the arc leaves, the weld and heat-affected zone cool, contract, and pull. The surrounding metal holds them back, leaving the weld zone locked in tension. Residual tensile stress is a normal byproduct of restrained contraction, not a defect by itself.

What You See and What You Don’t

The visible results are familiar: bowing along a beam, camber in a long panel, angular distortion that lifts the flange of a T-joint, fillet legs that shrink as they cool, and oil-canning across thin sheet. Localized heating with restraint is the root cause of angular distortion in fillet welds. The top of the joint receives far more heat than the root, so the flange curls toward the weld as the top cools and shortens. Oil-canning is especially common on thin material, including aluminum sheet, where the same mechanics show up with even less margin. See our guide to aluminum MIG welding for the material-specific prep and settings.

Distortion	Cause	Countermeasure
Bow along a beam	Weld shrinks on one side of the neutral axis	Balance welds, preset, backstep
Angular T-joint	More heat at the flange than the root	Alternate sides, limit interpass
Camber in a panel	Long weld along one edge	Skip weld, clamp, preset
Oil-canning	Thin sheet buckles from local shrinkage	Lower voltage, faster travel, backing
Fillet leg shrink	Weld volume contracts on cooling	Correct size, fewer passes
Cracking	Restraint plus a hard microstructure	Preheat per WPS, control interpass

The invisible consequences matter just as much: tensile residual stress, a wider heat-affected zone, higher cracking risk in hardenable steel, and reduced fatigue life. A part can look straight and still be primed to crack. TWI’s technical guidance treats distortion and residual stress as one family of cooling-related problems, which is the right way to think about them.

Countermeasures That Actually Work

Think in two directions: less heat per inch, and heat placed more evenly. Balanced welding about the neutral axis, backstep welding (which reverses normal progression to spread heat and reduce peaking), skip welding, and alternating passes all even out the temperature profile. Pre-setting the joint or offsetting fixtures gives the shrinkage somewhere to go. On thin sheet, keep interpass low and use faster travel with lower voltage rather than more amps.

Excessive interpass temperature increases shrinkage and can worsen distortion on multi-pass work, which is why AWS D1.1 and project procedures cap it. Use the least heat input consistent with fusion. Heat straightening can rescue a bowed part, but only within controlled temperature limits and with slow, even cooling; overheating turns a straightening job into a metallurgical one. Peening can relieve stress in narrow, controlled circumstances, but heavy peening can damage the weld toe and mask cracks instead of preventing them.

“Just weld it hotter” is often the worst advice for sheet metal. More heat means more contraction, more oil-canning, and a greater chance of burn-through. Thin-gauge work rewards higher travel speed and lower voltage because the goal is to fuse quickly and leave before the surrounding plate absorbs the energy.

Consider a 1/4-inch plate T-joint that bows after three passes. The welder did nothing obviously wrong with voltage or wire speed, but let interpass climb to 700°F. Each pass started on already-expanded, softer steel, and the final contraction pulled the flange like a drawstring. Checking interpass with a temp stick and letting the part air-cool between passes would have cost ten minutes and saved the straightening job.

Control the contraction and you control the shape. Temperature is the lever; sequence and restraint decide how hard it pulls.

Temperature Windows by Material: Steel, Stainless, Aluminum, and Galvanized

A single voltage and wire speed combination can run beautifully on mild steel and fail miserably on stainless or aluminum. The machine has not changed. The material’s melting point, thermal conductivity, and tolerance for heat have. Every metal has a temperature window where fusion happens cleanly without wrecking its properties, and learning those windows is what separates a weld that passes inspection from one that cracks next month.

Mild Steel: Forgiving, but Not Immune to Cold

Mild steel melts somewhere around 2,500 to 2,800°F (1,370 to 1,540°C). It is the most forgiving common material because it conducts heat moderately and tolerates a wide range of cooling rates. For 1/8-inch and thinner sections, preheat is generally unnecessary. In fact, adding heat to thin sheet usually makes distortion worse.

Cold plate is a different story. A 40°F plate pulled in from a winter truck can chill the puddle fast enough to cause lack of fusion, especially on the first pass. The arc has to overcome the plate’s heat sink before it can wet into the joint. Letting the steel reach shop temperature, or warming it gently, solves more cold-lap problems than turning up the voltage. Follow the WPS or applicable code for thicker sections, where preheat becomes a requirement rather than a courtesy.

HSLA and Quenched-and-Tempered Steel: Respect the Ceiling

High-strength low-alloy and quenched-and-tempered steels are strengthened by careful heat treatment, and welding is an uncontrolled heat treatment. Too much heat input softens the heat-affected zone and sacrifices strength. Too little, combined with a fast cool, can produce a hard, crack-prone microstructure. Their procedures usually specify a heat input cap in kJ/in, a minimum preheat, and a low-hydrogen approach with clean, dry consumables.

Interpass temperature matters just as much. Many Q&T procedures hold the maximum around 400 to 500°F, because letting the part soak hotter can over-temper the parent metal. Never assume a number from a forum applies to your grade. The steel mill’s data and the qualified WPS govern.

Austenitic Stainless: Heat That Stays Put

Austenitic stainless, including 304 and 316, has much lower thermal conductivity than carbon steel. Heat concentrates near the arc instead of spreading away, so the same amperage that feels mild on steel can produce severe distortion and a wide, sugared HAZ on stainless. Use lower amperage, faster travel, and a tight arc. Keep interpass temperature around 350°F, and never water-quench between passes.

The bigger danger is sensitization. When austenitic stainless is held roughly in the 800 to 1,500°F range, chromium carbides can precipitate at the grain boundaries and leave the steel vulnerable to intergranular corrosion. Multiple passes on standard 304 make that exposure worse. Low-carbon 304L or stabilized grades such as 321 and 347 reduce the risk, but controlling heat remains the first defense.

Aluminum: The Heat Sink Problem

Aluminum melts at about 1,221°F (660°C), yet it often needs more amperage than steel of the same thickness. Its high thermal conductivity pulls heat away from the puddle almost as fast as the arc adds it. Thin sections may need no preheat at all, while thick sections sometimes benefit from a controlled preheat to help the arc establish a puddle. Too much preheat, however, can make the metal mushy and hard to control.

For 6061-T6, the heat-affected zone will be softer than the parent metal unless the part is re-heat-treated after welding. That is a design consideration, not a setting error. Cleanliness is critical with aluminum, so a proper aluminum cleaning routine matters as much as the temperature. For feed, gas, and technique details, see this aluminum MIG welding guide.

Galvanized Steel: Temperature Meets Chemistry

The zinc coating on galvanized steel melts at about 787°F (419°C) and boils near 1,660°F (907°C), both well below the steel’s melting point. The arc turns that coating into vapor, which causes porosity, spatter, and a dirty puddle. Grinding the zinc back from the joint and using strong fume extraction matter far more than any voltage tweak. Zinc oxide fume can cause metal fume fever, so follow OSHA welding guidance and wear proper respiratory protection.

Material Temperature Comparison

Material	Melting Point / Range	Typical Preheat	Typical Max Interpass
Mild steel	2,500–2,800°F	None on thin sections; per WPS on thick	Often 650°F unless specified
HSLA / Q&T steel	Varies by grade	Per WPS, often 100–400°F	Often 400–500°F
Austenitic stainless	About 2,550–2,650°F	Usually none	About 350°F
Aluminum	About 1,221°F	None thin; sometimes 150–250°F thick	Keep work cool; avoid prolonged soak
Galvanized steel	Steel core melts near 2,500°F+	Usually none	Same as base steel, but coating limits heat

The common thread is that temperature windows are about protecting properties, not just achieving fusion. A weld that looks fine can still have a softened HAZ, a sensitized grain boundary, or a zinc-contaminated root. Match the heat to the metal, verify the limits, and let the procedure, not habit, set the ceiling.

Common Temperature Mistakes and How to Troubleshoot Them

Almost every MIG problem that gets blamed on “bad temperature” is actually a specific, diagnosable fault. Stop adjusting randomly and start reading the symptom. A cold, tall bead and a blown-through hole are not mysteries — they are the machine and your hand telling you exactly which variable is out of line. Work symptom first, cause second, fix last, and you will stop chasing settings.

The Master Symptom–Cause–Fix Table

Symptom	Most Likely Cause	Fix
Cold lap / lack of fusion	Amperage too low, voltage too low, travel too fast, wrong polarity, mill scale left on	Raise wire feed speed first, verify polarity, grind the joint clean, then slow travel slightly and watch the puddle tie into the toe
Burn-through on thin sheet	Too much heat for the gauge: excessive voltage, slow travel, oversized wire	Drop voltage, speed up, step to 0.023 in wire, use stitch or pulse, add copper backing
Excessive spatter	Voltage outside the sweet spot, inductance set wrong, low gas flow, worn contact tip, long stick-out	Fine-tune voltage in 0.5 V steps, adjust inductance, reset flow to roughly 15–25 CFH, replace tip, shorten stick-out
Porosity	Gas stripped by drafts, low flow, plugged nozzle, moisture, galvanized coating, oily surface	Build a wind break, check flow at the nozzle, clean or replace the nozzle, dry the plate, grind zinc back
Undercut	Voltage too high, travel angle too steep, travel speed too fast	Reduce voltage, flatten gun angle to about 10–15 degrees of push, slow slightly
Ropy, high-crowned bead	Voltage too low, travel too fast, puddle freezing before it wets out	Raise voltage to flatten the bead, slow travel, confirm the arc is leading the puddle
Birdnesting	Wire feed problem, not temperature: kinked liner, wrong drive roll tension, spool bind, tip clog	Trim wire, check liner and drive rolls, replace contact tip, verify the wire spool spins freely
Burn-back into the tip	Wire stops feeding while the arc keeps running: slip in the drive rolls, wrong feed tension	Increase drive roll tension, check for a crushed liner, confirm the tip bore matches wire diameter
Erratic arc and inconsistent penetration	Worn or oversized contact tip letting the wire wander	Replace the tip with the correct bore size and re-test on scrap

The Big Two: Cold Lap and Burn-Through

These two failures sit at opposite ends of the heat spectrum and account for most rework. Cold lap happens when the filler sits on top of the base metal without melting into it — the classic worm-on-a-sidewalk bead. The puddle froze before it could wet the sidewall.

Fix it in this order: raise wire feed speed to add current, confirm correct polarity for your process, grind off mill scale and rust, then slow travel just enough that the puddle visibly bites into the toe. Raising amperage before voltage is the reliable sequence, because voltage alone widens the arc without adding much penetration.

Burn-through is the mirror image. On 20-gauge sheet, the heat that fuses a 1/4-inch plate will open a window in seconds. Drop voltage first, increase travel speed, and switch to thinner wire. Stitch welding — short beads with cooling pauses — plus a copper backing bar handles the rest. Pulsed spray transfer is the professional answer for thin material because it delivers heat in controlled packets; if you are working aluminium sheet, our guide to aluminium MIG welding covers the pulse settings in detail.

Operator Variables That Change Your Real Heat

Two welders can run identical panel settings and produce completely different welds. The difference is in the hands.

• Stick-out length. Long wire stick-out increases resistive heating of the wire and reduces actual welding current. Push the gun closer and your amperage climbs without touching the dial — which is why a welder who suddenly extends for reach gets a cold, spattery bead.
• Gun angle. A steep angle concentrates heat and narrows the bead; too steep and you dig a trench. Keep it near perpendicular with a slight lead.
• Drag versus push. Dragging the gun (backhand) generally increases penetration and heat concentration compared with pushing. Push gives a flatter, wider, cleaner-looking bead with less penetration — good for thin sheet, risky on thick plate.
• Joint fit-up. A gapped joint demands less heat than a tight one, because the arc has less metal to sink into. Opening a gap without reducing settings is a fast route to burn-through.

Two mechanical details also masquerade as temperature faults. A worn or oversized contact tip causes erratic arc starting and inconsistent penetration, because the wire no longer exits on centre. And any drafty condition can strip shielding gas and create porosity even when flow settings look correct — the gauge says 20 CFH, but the wind carries it away before it reaches the puddle. Weld indoors, build a screen, or increase flow modestly while shielding the nozzle.

For fume, fume extraction, and ventilation requirements around these fixes, the OSHA welding and cutting overview is the practical starting point. And if the metal in question is aluminium rather than steel, contaminated or poorly prepped surfaces cause porosity that no voltage tweak will cure — start with proper aluminium cleaner for welding before you touch the machine.

Measuring and Controlling Temperature in the Shop

All the theory in the world does not help if you cannot tell whether a plate is actually at 250°F or just feels warm. Shop temperature control comes down to three habits: measuring with a tool that tells the truth, setting the machine so it repeats, and building a routine that catches problems before the arc starts.

Tools That Give You a Real Reading

• Temperature-indicating crayons (temp sticks). Mark the plate near the joint. The mark melts at its rated temperature, giving a clear yes-or-no check for preheat and interpass. They are cheap, fast, and unaffected by surface shine.
• Contact and magnetic thermocouples. A probe held against the steel, or a magnetic pad on a flat surface, gives a direct reading and works well for thicker sections that need to soak.
• Infrared thermometers. Convenient, but they read low on shiny, scaled, or polished metal because emissivity is low and inconsistent. A reading of 180°F on bright plate might really be 300°F. Temp sticks and contact probes avoid that error.
• Thermal cameras. Useful for surveying a whole part and spotting hot spots, though the same emissivity caveat applies. Use them for comparison, not as the final word on code work.
• Weld coupons. A test piece of the same thickness, cut and bent or inspected, confirms that your settings produced fusion, not just a pretty surface.

Measure the Metal, Not the Air

Preheat means the base metal is hot through the thickness, not just on the surface. Heat within a few inches of the joint on both sides where possible, let the reading stabilize, and brush or grind the measurement spot clean. A torch can put 400°F on the surface while the center of a thick plate is still cold. Give the part time to soak, then verify again before welding. The same discipline applies to interpass: measure the actual joint area before the next pass, not the far end of the fabrication.

Machine-Side Control

Make a parameter card from the WPS or a reliable settings chart and tape it to the machine. Record voltage, wire feed speed, gas flow, and travel speed. Synergic programs are helpful because one knob adjusts a matched set of parameters, but verify the result on scrap rather than assuming the display matches the arc.

Duty cycle matters too. A 60 percent duty cycle at rated amperage allows six minutes of welding in any ten-minute period, leaving four minutes for cooling. Ignore that and the machine drifts as it heats, changing output without changing the dial. If the arc starts acting differently after twenty minutes, stop and let the machine cool.

Control the Environment

Wind scatters shielding gas, so build a wind break before blaming the wire. Cold shop air pulls heat out of thin plate and can cause lack of fusion on a winter morning. Humidity raises hydrogen pickup risk, and moisture in the gas line or on the plate makes porosity more likely. Preheating frozen plate is not optional; it removes ice and moisture and gives the weld a fighting chance. For aluminum, reflectivity and fast heat conduction make contact measurement even more important, and our Aluminum MIG Welding guide covers the material-specific details.

A Repeatable Shop Routine

1. Verify the material grade and thickness.
2. Look up the WPS or a proven settings chart.
3. Clean the joint and check fit-up.
4. Preheat if required, then prove it with a temp stick or contact probe.
5. Tack and run a test bead on scrap of the same thickness.
6. Weld with a consistent travel speed and stick-out.
7. Check interpass temperature before every additional pass.
8. Record the settings, preheat, and interpass readings.

Finally, treat temperature control as part of hot-work safety. OSHA 29 CFR 1910.252 sets requirements for welding protection, ventilation, and fire prevention in general industry. NFPA 51B provides the hot-work permit and fire-watch framework, including monitoring the area after welding ends. A hot plate is not just a metallurgical concern; it is a fire risk that deserves the same attention as your settings.

Putting It All Together: A Temperature Workflow That Prevents Rework

If you take one mental model away from this guide, make it this: voltage shapes the arc, wire feed speed sets the energy available, travel speed converts that energy into heat per inch, and preheat and interpass decide how the steel responds. Four separate decisions, not one “temperature” knob. A welder who holds that model can walk up to a strange machine and get close on the first bead, because they know which variable to move and which to leave alone.

The Tape-to-the-Machine Checklist

Print this, laminate it, and stick it to the side of your machine. Each step exists because skipping it is how a weld goes cold or blows through.

1. Identify the metal and thickness. Not “steel” — what steel, measured, not guessed. Mill scale and paint hide thickness.
2. Confirm process and gas. Solid wire on 75/25 argon-CO2 is not the same animal as flux core on CO2.
3. Set a starting point from a settings chart or, on code work, the WPS. Never from memory of a different job.
4. Verify polarity and stick-out. Solid wire runs electrode positive; stick-out is roughly 3/8 to 1/2 inch for short circuit.
5. Clean the joint. Grind mill scale, rust, oil, and zinc back to bright metal at least an inch either side.
6. Preheat if required, and prove it with a temp stick a few inches from the joint, not with your hand near the plate.
7. Run a test bead on scrap of the same thickness. Thirty seconds of scrap beats twenty minutes of grinding.
8. Adjust one variable at a time: voltage, then wire speed, then travel speed.
9. Monitor interpass before every pass after the first. Let it cool in air; never quench.
10. Inspect and record the settings, the joint, the thickness, and what worked.

When to Stop and Call Engineering

Humility is a technical skill. Sometimes the correct move is to put the gun down and ask, because the failure mode is not cosmetic.

• Unknown steel grade. A mystery plate could be A36 or a quenched-and-tempered grade that loses half its toughness in the heat-affected zone.
• Thick, restrained joints. High restraint plus fast cooling is the classic recipe for hydrogen-assisted cracking.
• Any code work. Field adjustments outside a qualified WPS range invalidate qualification — the weld is no longer proven to perform.
• Repair welds on loaded structures. Someone calculated those loads; guessing at heat input adds risk you cannot see.
• Anything quenched and tempered. Heat input caps and interpass ceilings on these grades are narrow and unforgiving.

For shop-floor boundaries on hot work, permits, and fire watches, the OSHA 1910.252 welding standard is the baseline, and the TWI technical FAQ library is a solid plain-language reference when you need to justify a decision.

Building the Skill, Not Just the Setting

Numbers get you in the neighborhood; puddle reading gets you home. Deliberate practice means running coupons of known thickness — say 1/8, 3/16, and 1/4 inch — and varying one parameter per bead, so “too cold” becomes a shape rather than a number. Ten minutes of that teaches more than an hour of production welding.

Keep a notebook. Recording settings by thickness and joint type builds a reference that beats generic wall charts, because it accounts for your machine, your cable length, and your travel speed. And here is the humbling truth: most defects blamed on “bad temperature” trace back to travel speed, stick-out, or joint preparation. The arc was fine. The hand was fast, the stick-out was an inch long, or the fit-up had a gap that swallowed the puddle.

Moving to aluminium, that discipline pays off even more, because heat runs away from the puddle instead of staying in it. Our complete practical guide to aluminum MIG welding covers the settings side of that shift in detail.

The Closing Idea

Temperature mastery is heat-input mastery plus patience. The welder who understands why a weld is cold — low amps, fast travel, or a plate at 40 degrees F — will always beat the one who only knows which knob to turn, because they can fix a problem they have never seen before.

To keep going, work backwards through the fundamentals: settings logic, then gas selection, then machine setup. Get those three right and temperature stops being a mystery and becomes a number you control.

Frequently Asked Questions

What temperature does MIG welding reach?

The arc plasma in GMAW commonly reaches roughly 6,000-10,000 degrees F (3,300-5,500 C) at its core. The molten puddle itself is much cooler, near 2,700-2,800 F for mild steel, because metal melts and flows away before it absorbs more energy. The base metal nearby rarely exceeds a few hundred degrees except in thin sections.

Is MIG welding hotter than stick welding?

The arc temperatures are broadly similar, but stick welding usually delivers higher heat input because it runs higher currents and slower travel speeds on thicker material. MIG can be either hotter or colder depending on transfer mode: short-circuit MIG is a low-heat process, while spray transfer MIG concentrates a lot of energy into a small area.

What is a normal interpass temperature for MIG welding?

For carbon steel, many procedures cap interpass temperature at 650 degrees F, though the exact limit comes from the applicable code or welding procedure specification. Austenitic stainless is often limited to around 350 degrees F, and quenched-and-tempered steels may be capped lower still to protect toughness.

Do I need to preheat mild steel for MIG welding?

Usually not for thin mild steel in a warm shop, because the process already adds enough heat. Preheat becomes important on thick sections, cold plate, high-carbon-equivalent steel, and highly restrained joints, mainly to slow cooling and prevent cracking. Follow the WPS or code when the work is structural.

Why does my MIG weld look cold and sit on top of the metal?

That appearance usually means insufficient heat input at the joint, often from low wire feed speed, low voltage, or travel speed that is too fast for the thickness. Dirty mill scale, long stick-out, or reversed polarity can produce the same look. Raise amperage first, confirm polarity and gas, clean the joint, then adjust travel speed.

Can I weld galvanized steel with MIG, and what temperature matters?

You can, but the zinc coating melts at about 787 degrees F and vaporizes near 1,660 degrees F, producing toxic fume, porosity, and spatter. Grind the coating back from the weld zone, use good fume extraction and respiratory protection, and expect the arc to behave differently where coating remains.

How do I calculate heat input for a MIG weld?

Use heat input in kJ/in = (60 x volts x amps) / travel speed in inches per minute. For metric, divide by travel speed in mm/min and by 1,000 to get kJ/mm, then optionally multiply by an efficiency factor near 0.8 for GMAW. The result lets you compare procedures and stay inside WPS limits.

Does higher voltage mean more penetration in MIG welding?

Not directly. Voltage mainly changes arc length and bead shape; too much voltage widens and flattens the bead and can cause undercut, while too little causes a narrow, ropey bead with poor fusion. Penetration is driven more by amperage, transfer mode, and travel speed, with voltage playing a supporting role in how the arc transfers heat.

Why does my MIG welder weld differently after an hour of work?

Thermal drift in the machine, cable and connection resistance, contact tip wear, and a changing base metal temperature all affect the arc. Check duty cycle, let the machine cool if it is derated, inspect the gun and liner, and re-verify settings on scrap before continuing critical work.