MIG Welding Temperature: How Hot It Gets and Why It Matters

You flip the switch on your new MIG welder, look for the temperature dial, and find nothing but volts and wire speed. So you crank both up, and the next thing you know you have burned a hole through 18-gauge sheet while the ¼-inch plate beside it still shows a cold, lumpy bead that never fused. The truth is that MIG welding does have a temperature — several of them, in fact — and understanding which one you are actually controlling is the fastest way to stop guessing and start welding consistently.

MIG Welding Temperature: What the Question Really Means

There is no temperature dial on a MIG welder, and that absence trips up almost everyone the first time they use one. You get voltage and wire-feed speed, plus maybe a chart inside the door. The useful answer is that MIG welding does reach extraordinary temperatures, but the machine is not set in degrees. It is set in electrical and mechanical conditions that produce a particular amount of heat in a particular place.

At the arc, temperatures climb into the thousands. TWI describes the electric arc used in arc welding as an intense heat of around 6,500°F (about 3,600°C) that melts metal at the joint (TWI). The molten pool is cooler than the plasma but still hot enough to turn steel into liquid. The surrounding plate stays warm long after the arc switches off. Those are three different temperatures, and each answers a different question.

The Three Temperatures a Welder Actually Manages

Arc and plasma temperature is the heat of the ionized gas carrying current from the wire to the work. Published measurements for gas tungsten and gas metal arcs report values from roughly 5,000 K to 30,000 K, depending on current density, arc length, and shielding gas (arc temperature research). That is not the temperature of your part. It is the temperature of the bridge doing the melting.

Base metal and weld pool temperature is what determines fusion. The pool must be hot enough to melt the parent metal, not just the filler. Metal droplets in short-circuiting transfer with mild steel wire have been measured at roughly 2,200 K to 2,700 K, far below plasma temperature but more than enough to fuse steel (droplet temperature measurements). The heat-affected zone stays hot after the arc moves on, which is why cooling rate matters as much as peak temperature.

Preheat and interpass temperature is the bulk temperature of the plate before welding and between passes. It is measured away from the arc, not inside the puddle. Preheat slows cooling; interpass limits stop the part from getting too hot. Both are process controls, not welding temperatures in the arc sense.

Temperature	Rough range	What it answers
Arc plasma	5,000–30,000 K	How the arc melts and transfers metal
Weld pool and droplets	Droplets around 2,200–2,700 K	Whether fusion actually occurs
Preheat and interpass	Often 50–500°F, depending on code and alloy	Cooling rate, cracking risk, distortion

Why Confusing Them Creates Expensive Mistakes

Most bad decisions start with one temperature being used as shorthand for another. Turn up voltage because the bead looks cold, and you may get a wider arc, more spatter, and burn-through on thin sheet without fixing the real problem: dirty metal, poor fit-up, or too little wire. Add preheat to cure lack of fusion caused by the wrong polarity, and you simply add distortion. Keep welding pass after pass without checking interpass, and stainless can lose corrosion resistance while quenched and tempered steel can lose strength.

Material makes the confusion worse. Aluminum conducts heat away so quickly that the arc feels cold, yet its low melting point makes burn-through easy. Steel tolerates more heat because it melts at a much higher temperature. That is why temperature advice cannot be separated from thickness and alloy. If aluminum is part of your work, the practical settings and cleaning steps in this aluminum MIG welding guide are a useful companion to the temperature theory here.

What MIG and GMAW Mean Here

GMAW, or gas metal arc welding, is the formal name for what most people call MIG. It uses a continuously fed consumable wire electrode and a shielding gas. Strictly speaking, MIG uses an inert gas such as argon, while MAG uses an active gas such as CO2 or a mixed gas. In shop talk, “MIG” usually covers both, and this article follows that convention.

What This Article Covers

You will see how hot the arc really gets, how melting points compare across steel, stainless, aluminum, and brazing wire, and why volts and wire speed replace a temperature dial. We will work through heat input math, preheat tables, interpass limits, material-specific practice, defect diagnosis, and shop measurement. By the end, “MIG welding temperature” should stop being one confusing number and become a set of controllable conditions.

How Hot Is a MIG Arc, Really? The Science in Plain Numbers

Stand close to a MIG arc and the heat hits your face before your eyes adjust. The light is intense, the sound is a fast crackle, and the metal goes from grey to orange in seconds. It is tempting to ask for one number: how hot is that arc? The honest answer is that a MIG arc is not one temperature. It is a steep temperature gradient, and the number you get depends on where the thermometer is pointed.

A useful reference point comes from optical emission spectroscopy of GMAW with 100% CO₂ shielding. At 300 A and 33.5 V, researchers measured plasma temperatures above 8,000 K at the center of the arc. That is roughly 7,700°C or 13,900°F. Move away from the core and the numbers fall quickly. The same study found the maximum temperature at the wire droplet’s arc root reached around 2,500 K, about 2,230°C or 4,040°F.

Measurement point	Approximate temperature	What it tells you
Plasma core	Above 8,000 K (7,700°C / 13,900°F)	The hottest, most ionized part of the arc
Droplet at the arc root	About 2,500 K (2,230°C / 4,040°F)	Where the wire is being melted and detached
Droplet surface	About 3,100 K (2,830°C / 5,120°F)	Superheated surface; close to iron’s boiling point
Weld pool	Near the alloy’s melting range, plus superheat	Much cooler than the plasma, but still molten

Why the Metal Never Reaches Arc Temperature

The plasma is hot, but it is also thin and fast-moving. Most of its energy is dumped into a small area at the arc root, and the base metal conducts that heat away almost as quickly as it arrives. That is why an 8,000 K arc can melt a narrow strip of steel while the plate 50 mm away is still cold enough to touch.

The molten pool is much cooler than the plasma. It sits near the melting range of the alloy, with some superheat, and the heat-affected zone below it is cooler still. The arc is a heat source, not a furnace that brings the whole part to arc temperature. This distinction matters because a weld can have a violently hot arc and still lack fusion if the heat is spread too thin or the travel speed is too high.

Shielding Gas, Current Density, and Arc Length

The shielding gas changes the arc’s personality, not just its temperature. Argon-rich mixes produce a stable, constricted arc that supports spray transfer and gives a narrow, deep profile. CO₂ runs broader and more oxidizing, with a harsher arc and more spatter, but it is inexpensive and effective on carbon steel. Helium additions raise the arc voltage and put more heat into the workpiece, which is why helium mixes show up on thick aluminum and stainless work. The fundamentals of arc welding explain how the gas column carries current and shapes the heat.

Current density has just as much influence. A small-diameter wire at high amperage concentrates current into a tiny spot, raising the local temperature and melting rate. A larger wire at the same amperage spreads the current and feels softer. Arc length, controlled mainly by voltage, changes the picture too. A short arc keeps energy focused; a long arc widens, loses heat to the surrounding air, and produces a flatter, wider bead with less penetration.

Arc Temperature Is Not the Same as Heat Input

Think of a blowtorch and an oven. The blowtorch flame is hot enough to blister paint in an instant, but it cannot heat a roast all the way through. An oven at a lower temperature can. Temperature describes intensity at a point; heat input describes the total energy delivered per unit length of weld. A MIG arc can be extremely hot and still deliver modest heat input if it travels fast. That is the daily reality of thin-sheet welding.

This is also why a “hotter arc” does not automatically mean “more penetration.” Penetration depends on where the energy lands, how long it stays there, the transfer mode, and the metal’s thermal conductivity. Aluminum is the classic example: its surface melts at a relatively low temperature, but it conducts heat away so fast that the arc can look fierce while fusion stays shallow. Our aluminum MIG welding guide covers how travel speed, cleaning, and machine setup interact on that material.

Droplet temperature adds another layer. Research on GMAW electrode droplets reports surface temperatures near 3,100 K, close to or above the boiling point of pure iron depending on the alloy. That same work notes surface temperature varies by more than 100 K between spray and globular transfer, which helps explain why fume composition changes when you switch transfer modes. You can read the study through its DOI record.

So the next time someone asks how hot MIG welding gets, give them the range: a plasma core in the thousands of kelvin, a droplet around 2,500 to 3,100 K, and a weld pool that is cooler still. Then ask which temperature they actually need to control.

Melting Points vs. MIG Temperatures: Steel, Aluminum, Stainless and More

Start with the number that reframes this whole conversation. In steel arc welding, the molten pool is generally limited to roughly 2,300 K to 2,800 K — about 3,680°F to 4,580°F — because heat escapes as metal evaporates off the surface. The plasma above it runs far hotter, but the puddle itself plateaus. That ceiling still sits above the melting range of every common base metal, which is exactly why an arc can fuse steel in a fraction of a second and just as easily blow a hole through 18-gauge sheet. The AWS Welding Handbook covers the heat-flow side of this in detail; for the arc itself, TWI’s primer on what arc welding is is a good plain-language starting point.

Most alloys don’t melt at one temperature, either. They soften and melt across a range, and that range is what decides how much forgiveness your process gives you.

Metal / filler	Approx. melting range	Typical MIG note
Mild steel (A36, 1018)	1,425–1,535°C (2,600–2,800°F)	Wide plastic band; forgiving heat window
Steel filler wire / rod	~1,650°C (2,800°F)	Melts near the base metal; fusion is easy to achieve
Austenitic stainless (304, 316)	~1,400–1,450°C (2,550–2,650°F)	Low conductivity, heat stays put, distortion climbs
Aluminum alloys (6061, 5052)	~580–660°C (1,080–1,221°F)	Melts low, sinks heat away fast, burns through suddenly
Silicon bronze filler (ERCuSi-A)	~1,000–1,060°C (1,830–1,940°F)	Brazing wire; wets steel without melting it
Zinc coating (galvanized)	Melts ~420°C (740°F); vaporizes ~910°C (1,670°F)	Coating is gone long before the steel is

Why Aluminum Feels Cold but Burns Through

Aluminum melts at roughly 660°C (1,221°F), less than half the melting point of steel, yet it is notorious for feeling “cold” under the arc. The reason is thermal conductivity: aluminum pulls heat away from the joint so aggressively that the puddle stays small and sluggish, so you push more amps — and then the section suddenly collapses because there was very little temperature margin left. Steel behaves the opposite way. Its higher melting point gives you a wider window between “fused” and “ruined.” If aluminum is your material, work through the settings and technique in our complete aluminum MIG welding guide before blaming the machine, and remember that surface prep matters as much as heat — see prepping aluminum right.

Melting the Filler vs. Melting the Base Metal

Here is where a lot of welds quietly fail. The filler wire melts first because it is thin and sits in the hottest part of the arc. That molten droplet can sit on top of cold base metal, freeze, and look like a bead — a classic cold lap. Real fusion requires the base metal edges to reach their melting range too, even briefly. A weld that looks neat, sits slightly proud, and shows a rounded toe is often a weld that never bonded. Test it by cross-sectioning a coupon or simply running a chisel along the toe; if the bead peels, temperature never reached the base metal.

Galvanized, Brazing and the Low-Temperature Trap

Galvanized steel is a temperature problem disguised as a welding problem. The zinc coating melts around 420°C (740°F) and vaporizes near 910°C (1,670°F), so it is boiling off while the steel underneath is still solid. That produces fume, porosity, and spatter — not a lack of heat. Per Teandersen technical guidance, brazing wire melts near 1,060°C (1,940°F) versus roughly 1,650°C (2,800°F) for typical steel filler, which is why silicon bronze lets you join galvanized or heat-treated sheet without melting the base metal or destroying its temper. If you are working with higher-strength grades, the same logic explains why filler choice and heat ceiling go together — see alloy steel welding electrodes for the selection side.

Why Your MIG Machine Has Volts and Wire Speed Instead of a Temperature Dial

A MIG welder is not a thermostat. It is a power supply with a wire feeder attached, and the two controls you see reflect that. You are not choosing a temperature. You are choosing how long the arc is and how fast metal enters it. Puddle temperature is the result of those choices, plus how quickly you move.

Constant Voltage: One Knob for Arc Length, One for Current

GMAW runs on a constant-voltage, or CV, characteristic. The power source holds a nearly fixed voltage across the arc while wire feed speed determines how much current flows. Feed wire faster and the contact-tip-to-work distance shortens, the arc length changes, current rises, and the wire burns off faster until the system settles at a new balance. That self-regulating loop is the reason arc welding processes can be semi-automatic at all.

In practice, each control has a clear job:

• Voltage sets arc length and bead shape. Higher volts give a wider, flatter bead; lower volts give a narrower, more crowned bead.
• Wire feed speed sets amperage and deposition rate. More inches per minute means more current and more metal.
• Travel speed decides how much of that energy lands in any given inch of plate.

AWS publishes typical GMAW and FCAW procedure data for carbon steel, stainless and aluminum in its Everyday Pocket Handbook. Those charts are built on voltage, wire feed speed and travel speed, not degrees.

What “More Heat” Actually Means

When a welder says “it needs more heat,” they usually mean amperage. Heat delivered to the joint is volts × amps ÷ travel speed, so turning up voltage alone mostly makes a longer, wider arc that spreads similar energy over more area. Turning up wire speed adds amps and filler metal at the same time. Slowing down adds heat without changing the arc at all. That is why three different adjustments can each “fix” a cold-looking bead in three different ways.

Starting Points for Mild Steel (Short-Circuit Transfer)

These are reasonable starting settings for 0.030 in. solid wire and 75/25 argon-CO₂ on clean, fitted mild steel. Treat them as launch points, not gospel.

Material thickness	Voltage (approx.)	Wire feed speed	Notes
18 ga (1.2 mm)	15–16 V	150–180 ipm	Watch for burn-through; tack generously
16 ga (1.5 mm)	16–17 V	180–220 ipm	Stitch or backstep on gaps
14 ga (2 mm)	17–18 V	220–260 ipm	Comfortable short-circuit range
1/8 in (3 mm)	18–19 V	260–320 ipm	Bevel above 1/4 in if possible
3/16 in (5 mm)	19–20 V	320–380 ipm	Consider spray for flat work
1/4 in (6 mm)	20–22 V	380–450 ipm	Preheat likely per code; multipass

Tuning by Sound and Bead Profile

You do not need a thermometer to dial in short-circuit. You need your ears and a look at the toe line. A healthy sizzle sounds like frying bacon: steady and fast. A harsh crackle with spatter spraying out means the wire is stubbing into the puddle, usually because wire speed is too high for the voltage or stick-out is too short. A hissing, spitting arc with a tall crowned bead means the arc is too long, so the puddle sits on top of the plate instead of biting in.

Then read the bead. The toes should wet in and blend, not sit like a rope on a table. A high, narrow bead with a visible seam at the edges is cold lap, which we tackle later in the troubleshooting section. A wide, flat, grey bead with undercut along the edges calls for less voltage or more travel speed.

Inductance: The Third Setting That Changes Puddle Feel

On short-circuit transfer, inductance controls how fast current can rise when the wire contacts the puddle. More inductance gives a slower rise, a softer arc and a wetter, more fluid puddle, which suits thicker material and smoother beads. Less inductance gives a sharper, faster-freezing puddle and a crisper sound, which helps on thin sheet and out-of-position work. Synergic machines usually hide this as an “arc control” or “sharpness” slider. Once you find the right setting for your gas and thickness, you rarely touch it again.

Transfer Modes Change the Heat Equation

Short-circuit transfer works at low current and is the standard choice for thin sheet and vertical work, but it delivers heat in bursts rather than a steady stream. Globular transfer is the awkward middle ground: large droplets, a rough arc, spatter and limited positional ability. Spray transfer needs higher current and an argon-rich shielding gas. The droplets become a fine axial mist, deposition jumps, and the arc is really only practical flat and horizontal. Pulsed spray uses a synergic waveform to alternate peak and background current, giving spray-like transfer at lower average heat input. That is why it is popular for stainless and for aluminum MIG welding, where thin sections would otherwise run away from you.

On a synergic machine you pick wire type and diameter, set thickness or amperage, and the box derives voltage and feed speed. That is convenient, but it hides the relationship. Learn what each variable does on a manual machine and the synergic display stops being a black box.

Once you accept that volts, wire speed and travel speed are the real controls, the “temperature” question becomes an energy question. That has a formula, which is where heat input comes in.

Heat Input: The Number That Actually Predicts What Temperature Does to Your Weld

Two welders can set the same 25 volts and 200 amps and still put wildly different amounts of energy into the plate. One travels at 4 mm/s, the other at 8 mm/s. The first leaves a broad bead and a wide heat-affected zone. The second moves so fast the arc barely fuses the toes. Voltage and amperage tell you the arc’s power at any instant. Heat input tells you how much of that power each millimetre of joint actually receives, and that is what shapes cooling rate, hardness, distortion, and toughness.

The Formula, in Two Conventions

Heat input is energy per unit length, usually expressed in kilojoules per millimetre. In the ASME and AWS convention it is arc voltage multiplied by arc current, divided by travel speed, with no efficiency factor applied:

Heat input (kJ/mm) = (volts × amps) ÷ (travel speed × 1,000)

That denominator assumes travel speed in mm/s. If your speed is in mm/min, multiply the top by 60 first. The result is often called arc energy, though many procedures and inspectors label it heat input anyway. The AWS D1.1 structural welding code and ASME construction codes use this convention when they set procedure limits.

The European approach adds a process efficiency factor, k, because not all arc energy reaches the workpiece. The multiplier is roughly 0.8 for MIG/MAG, 0.8 for stick and flux-cored, 1.0 for submerged arc, and 0.6 for TIG:

Heat input (kJ/mm) = (k × volts × amps) ÷ (travel speed × 1,000)

Neither version is wrong. They answer slightly different questions. Arc energy describes what the arc generates; the efficiency-adjusted value estimates what the metal absorbs. Problems begin when a procedure written in one convention is compared directly with a procedure written in the other.

Three Worked Examples You Can Copy

Take a MAG weld at 25 V, 200 A, and 4 mm/s. Arc energy is (25 × 200) ÷ (4 × 1,000) = 1.25 kJ/mm. Applying 0.8 efficiency gives 0.8 × 25 × 200 ÷ 4 ÷ 1,000 = 1.0 kJ/mm.

Now a stainless job at 22 V, 140 A, and 3.5 mm/s. Arc energy is 22 × 140 ÷ 3,500 = 0.88 kJ/mm. With 0.8 efficiency, actual heat input is about 0.70 kJ/mm.

Finally, a heavy mild-steel pass at 30 V, 280 A, and 3 mm/s. Arc energy is 30 × 280 ÷ 3,000 = 2.8 kJ/mm. Efficiency-adjusted heat input is 2.24 kJ/mm.

Settings	Travel speed	Arc energy	Heat input (k = 0.8)
25 V, 200 A	4 mm/s	1.25 kJ/mm	1.0 kJ/mm
22 V, 140 A	3.5 mm/s	0.88 kJ/mm	0.70 kJ/mm
30 V, 280 A	3 mm/s	2.80 kJ/mm	2.24 kJ/mm

Travel speed sits in the denominator, so doubling it halves heat input even when the voltage and amperage displays never move. Two beads that look similar can therefore have completely different cooling histories. It is also why “turn it up” is incomplete advice. Raising amps without adjusting travel speed may barely change energy per millimetre, while slowing down almost always does.

What Counts as High or Low Heat Input?

Controlled heat input welding commonly falls between about 1.0 and 3.5 kJ/mm. Low-alloy steel is often run near 2.5 kJ/mm to slow cooling, give hydrogen time to escape, and avoid a hard, crack-prone heat-affected zone. In the referenced technical guidance, cold cracking is mainly a concern when heat input drops below roughly 3 kJ/mm, especially in thicker sections and higher-carbon-equivalent steels.

Austenitic 300-series stainless is usually kept below about 1.5 kJ/mm. The concern flips: too much heat and slow cooling can promote sensitization, carbide precipitation, greater distortion, and loss of corrosion resistance. The arc welding fundamentals published by TWI are a useful starting point for how arc energy interacts with the material.

Aluminum behaves differently again. Its high thermal conductivity pulls heat away from the joint quickly, so the same nominal heat input produces a much narrower hot zone than it would in steel. A process tuned for steel may feel cold on aluminum. For a material-specific starting point, see this complete practical guide to aluminum MIG welding.

Position Changes the Number More Than You Think

Vertical-up welding usually needs a slower travel speed and a slight weave to control the puddle, and both raise heat input. Vertical-down is faster and deposits less energy per millimetre, but it can sacrifice fusion on thicker material. Flat and horizontal sit somewhere between. That is why a procedure that passes flat may need revalidation when moved to vertical-up.

Heat input drives cooling rate, and cooling rate drives microstructure. Fast cooling can harden the heat-affected zone and raise cracking risk. Slow cooling can soften it, but may reduce toughness and increase distortion. More heat input does not automatically mean a better weld; it means a different thermal history with different consequences.

Arc Energy vs. Heat Input: Why the Terms Blur

Strictly speaking, arc energy is the energy generated by the arc per unit length. Heat input is the energy absorbed by the workpiece, after losses to radiation, spatter, and the surrounding atmosphere. The efficiency factor accounts for that gap. In North American codes the unadjusted calculation is frequently called heat input anyway. The practical lesson is simple: state your convention, especially when comparing procedure sheets or writing a WPS.

Preheat Temperature for MIG Welding: When Cold Steel Becomes the Enemy

Preheat is the temperature the base metal is sitting at before the arc ever strikes. It sounds like housekeeping, but on thick sections, higher-carbon steels, and cold winter mornings it is often the difference between a weld that passes inspection and one that cracks overnight. Preheat rarely announces itself. The bead can look perfect, the spatter clean, and the failure still shows up hours later.

So let’s separate opinion from code: what preheat does, when you need it, and how to read the governing table without a code book on your bench.

What Preheat Actually Does

Heating the joint before welding does three jobs at once:

• It slows the cooling rate. Hot steel next to a hot weld pulls heat away less aggressively, so the weld and heat-affected zone cool gradually instead of being quenched by a cold plate.
• It gives hydrogen time to escape. Moisture, rust, primer, and residual lubricant introduce hydrogen. Slow cooling buys diffusion time so hydrogen leaves the metal instead of collecting at a crack tip.
• It reduces cracking risk and residual stress. A smaller thermal gradient means less contraction stress locking into a restrained joint.

Preheat is optional on thin mild steel that is clean, dry, and welded in a comfortable shop with low restraint. A 1/8-inch bracket at 65°F needs nothing. It becomes necessary on thick sections, high carbon equivalent, restrained joints, cold ambient conditions, and whenever the procedure says so.

Reading AWS D1.1 Table 5.11 Without a Code Book

For structural steel work in the US, the reference is AWS D1.1 Structural Welding Code – Steel. In the 2025 edition, Table 5.11 replaced what older hands call Table 3.2. It sorts base metals into seven categories, A through G, across four thickness bands. Categories group steels by strength and metallurgical family, not trade name, so check the specification first and the category second.

GMAW on ordinary carbon steel usually lands in Category B, the everyday bucket for mild steel such as A36. Category A is largely limited to very low-carbon steels; higher letters carry stricter requirements for alloy and quenched-and-tempered material.

Category B steel, GMAW — thickness	Minimum preheat
Up to 3/4 in	32°F (0°C)
Over 3/4 through 1-1/2 in	50°F (10°C)
Over 1-1/2 through 2-1/2 in	150°F (66°C)
Over 2-1/2 in	225°F (107°C)

Two details catch people out. The same value serves as both minimum preheat and minimum interpass temperature, one number with two obligations. And the footnote: when base metal is below 32°F, D1.1 requires preheat to at least 70°F and holding that temperature during welding. Frozen steel behaves like a heat sink, and the first inch of weld absorbs the punishment.

Carbon Equivalent and When the Table Steps Aside

Thickness alone is a crude proxy for hardenability. Carbon equivalent (CE) is better because manganese, chromium, molybdenum, vanadium, and similar elements each raise hardening and cracking risk beyond what carbon alone suggests. A 1-inch plate of A36 and a 1-inch micro-alloyed plate can share a thickness band and behave nothing alike.

When CE climbs, or the steel is quenched and tempered, restraint is severe, or service is fatigue or fracture critical, the table stops governing. A qualified WPS takes over. AWS notes that alternative methods in Annex B can produce significantly lower preheat values than the conservative table, but they must be qualified, not assumed. If you run alloy consumables, this guide to alloy steel welding electrodes explains how filler choice interacts with preheat and cracking risk.

Heating Methods and Where to Measure

Common methods include oxy-fuel torches with a heating head, propane or natural-gas weed burners (watch for moisture in the flame), and electric resistance blankets, the most controllable option for code work. An air-acetylene torch works for small parts, as this air acetylene welding guide covers, but lacks output for thick structural sections.

Measure on the face you are heating, and on both faces when the section is thick, because a plate can read 200°F on top and 80°F at mid-thickness. Take readings a few inches back from the joint and again ahead of the arc. Temperature-indicating crayons and contact thermocouples are reliable field tools. Infrared guns depend on emissivity and mislead on shiny or scaled surfaces. AWS guidance on preheat and interpass practice treats measurement location as part of the requirement.

Finally, a 95°F summer afternoon is not preheat. Air temperature is not metal temperature. A 2-inch plate that sat in the shade all night is still a heat sink at noon. Check the steel, not the weather app.

Interpass Temperature: The Upper Limit Most Home Welders Ignore

You finish a root pass on a stainless flange, the metal is still ticking and popping as it cools, and you drop the next bead straight on top because the puddle looks happy. Nothing cracks. Nothing smokes. Three weeks later the part is back on your bench with rust-colored tea stains along the weld toe, and nobody can explain why a weld that looked perfect is now corroding in a food plant. That is the interpass temperature ceiling — the limit almost nobody checks, because a weld that is too cold usually looks ugly, while a weld that is too hot looks great right up until it fails.

What interpass temperature actually means

Interpass temperature is the temperature of the base metal immediately before you lay the next pass. It has two boundaries. The minimum is the preheat you calculated. The maximum is a ceiling set to protect the metallurgy you paid for. Both are measured on the metal, not the arc, and both belong in the procedure.

The measurement location is where most home welders go wrong. Codes require the interpass temperature to be maintained for a distance at least equal to the thickness of the thickest part, and never less than 75 mm (3 in), in all directions from the weld, as laid out in AWS D1.1. In practice, for a maximum, most inspectors read about 1 in (25 mm) from the weld toe on the heavier side. Reading the weld cap itself tells you almost nothing useful — it is always hotter than the surrounding plate.

The limits that matter in a real shop

These are the numbers worth memorizing, and the reasoning behind each one is different.

Material	Typical max interpass	Why the ceiling exists
300-series austenitic stainless, nickel alloys	~150°C (300°F)	Preserve corrosion resistance; limit sensitization
Quenched and tempered steels	~150°C (300°F)	Protect strength and toughness from tempering
Aluminum 5XXX	~65°C (150°F)	Reduce hot cracking risk
Aluminum 6XXX / castings	~120°C (250°F) typical	Alloy-dependent; check the WPS
Copper alloys	Varies widely	Depends on alloy family and temper

For austenitic 300-series stainless and nickel-base alloys, keeping the maximum around 150°C (300°F) is the standard way to hold corrosion resistance. Push past it and chromium carbides can precipitate at the grain boundaries in the sensitization window, stripping chromium from the matrix and leaving the weld toe hungry for rust. Quenched and tempered steels carry the same 150°C ceiling for a different reason: extra heat tempers the steel behind the weld, and you quietly lose the yield strength the steel was specified for. Aluminum 5XXX alloys are usually capped near 65°C (150°F) because hot, slow-cooling metal plus a lot of restraint is a recipe for hot cracking.

The AWS guidance on preheat and interpass makes the same point from the other direction: a maximum that exists in a procedure is not a suggestion, and no qualified procedure should be run on memory.

How to hold the ceiling without losing your day

You do not need a water bath. You need patience and a plan.

• Stagger the sequence. Jump from one side of the joint to the other instead of stacking beads in one hot spot.
• Use dwell time deliberately. Let the part cool while you set up the next joint, not while you stand over it.
• Cool with air. A fan moving ambient air is fine. Never quench a hot weld — water on stainless invites cracking, and on Q&T steel it can harden the HAZ into something brittle.
• Clamp on a heat sink. A copper or aluminum backing bar pulls heat out of thin sections and does more for interpass control than any dial.
• Carry two crayons. One at the minimum, one at the maximum. Two temperature-indicating crayons are the accepted field method for checking interpass temperature, and they work on dull, dirty, real-world surfaces where an IR gun is guessing.

Thin aluminum is where this bites hardest. A 3 mm sheet can go from comfortable to too hot in two passes, and the alloy will not warn you before it cracks. If you are working through aluminum MIG welding, treat the interpass ceiling as part of the setup, not an afterthought.

The takeaway

Preheat gets all the attention because cold steel cracks loudly and immediately. Interpass temperature fails quietly, months later, in the form of corrosion, lost strength, or a crack that starts at the toe. Set the ceiling before you strike an arc, measure it where the code says to measure it, and let the part cool on its own schedule instead of yours.

Temperature by Material: Aluminum, Stainless, Thin Sheet and Galvanized Steel

Every material you feed through a MIG gun responds to heat differently. The same voltage and wire speed that flatters ¼-inch mild steel can destroy 18-gauge sheet or leave aluminum sitting in a puddle of oxide. Once you understand what each metal wants, temperature stops being a mystery and becomes a set of repeatable choices.

Aluminum: Preheat to Flatten the Gradient, Not to Add Heat

Aluminum melts near 1,221°F, roughly half the melting range of mild steel. That sounds like it should be easy to fuse, but aluminum conducts heat away from the arc about five times faster than steel. The result is a narrow, frustrating window: the metal under the gun is still cold while the surrounding plate wicks energy away, so you push more amps, and suddenly the section collapses.

That is why preheat for aluminum is commonly cited in the 120°F to 350°F range for thicker sections, with 350°F treated as an upper limit. The goal is not to supply fusion energy. The arc does that. Preheat narrows the thermal gradient so the puddle forms predictably and the plate does not act like a heat sink. On thin sheet, skip it entirely or keep it at the low end, because preheat plus a hot arc is a fast route to burn-through.

Cleaning matters more than temperature. Aluminum oxide melts far above the base metal, traps hydrogen, and causes porosity and erratic wetting. Degrease, then wire brush with a stainless brush reserved for aluminum only. Our guide to prepping aluminum for welding covers the sequence, and the complete aluminum MIG welding guide walks through machine setup and push technique. Remember that a bright, fluid-looking aluminum puddle represents far less energy than a steel puddle of the same size.

Stainless Steel: Low Heat Input, Tight Interpass, Clean Backside

Austenitic stainless is not difficult to melt. It is difficult to keep metallurgically healthy. Published filler-metal guidance typically caps heat input around 1.5 kJ/mm for 300-series stainless, with interpass temperature held near 150°C (300°F), as summarized in AWS guidance on preheat and interpass. Stay in that envelope and you limit sensitization, the chromium carbide precipitation that steals corrosion resistance from the grain boundaries.

Duplex stainless follows a different recipe, roughly 0.5 to 2.5 kJ/mm with interpass near 150°C, because it needs a balanced ferrite-to-austenite ratio rather than a fast quench. For both families, back purging protects the root from oxidation; a sugared, black backside is a corrosion crack waiting to happen. Use skip welding and balance your passes around the neutral axis to control distortion.

Thin Sheet: The Burn-Through Triangle

On material under about 1/8 inch, burn-through is controlled by three variables: amperage, travel speed, and fit-up gap. Reduce any one and the hole gets less likely. In practice, travel faster before you turn down the voltage, because slowing down to fix a cold bead usually makes the next problem worse. A copper or aluminum backing bar pulls heat out of the root, stitch welding limits accumulation, and pulsed MIG lets you use a higher peak current with a lower average heat input.

Galvanized Steel: Zinc Changes Everything

Zinc melts at about 420°C (740°F) and vaporizes at about 910°C (1,670°F), so any arc welding destroys the coating locally no matter how carefully you set the machine. The vapor expands violently, causing porosity, spatter, and a plume of metal fume. Grind the coating back roughly an inch from the joint, weld, then recoat. Ventilation is not optional here; review the hazards on OSHA’s welding and cutting page.

For thin galvanized sheet, silicon bronze brazing wire melts around 1,060°C (1,940°F) versus roughly 1,650°C (2,800°F) for typical steel filler, per Teandersen technical guidance. Lower temperature means less zinc vaporization and less risk of embrittling high-strength steel.

Material	Typical preheat	Interpass ceiling	Main risk
Aluminum (thick sections)	120°F to 350°F	65°C to 120°C by alloy	Burn-through, porosity
300-series stainless	Usually none	About 150°C (300°F)	Sensitization, distortion
Duplex stainless	Usually none	About 150°C	Wrong phase balance
Thin mild steel	None	Per WPS	Burn-through, warping
Galvanized steel	None	Per WPS	Zinc fume, porosity

Material choice sets the boundaries, but the number that tells you whether you stayed inside them is heat input. That is where the next section picks up.

Troubleshooting by Temperature Symptom: Cold Lap, Burn-Through, Cracking and Spatter

A MIG weld rarely fails for mysterious reasons. It fails because too little heat, too much heat, or the wrong kind of heat met the joint at the wrong moment. The bead tells you which. Read the defect, work backwards to heat input, and stop guessing at the dial.

Map the Defect to the Cause First

Before touching a dial, match what you see to a thermal signature. This table is the fast map.

Symptom	Likely thermal cause	Quick check	Fix
Cold lap / lack of fusion	Too little energy for the thickness, travel too fast, arc on the bead	Settings vs. thickness; arc placement	Small voltage bump, slower travel, aim at the root
Burn-through / undercut	Too much energy, or travel too slow on thin metal	Thickness vs. settings; gaps	Cut voltage and wire speed, speed up, add backing
Porosity	Contamination or gas loss disturbing the pool	Gas flow, nozzle, cleanliness	Clean the joint, fix gas delivery, shorten stickout
Delayed cracking	Hydrogen trapped in a fast-cooling, hard weld	Preheat and interpass records	Specified preheat, slow cooling, low-hydrogen practice
Excessive spatter	Voltage and wire speed out of balance	Transfer mode; does the arc pop or hiss?	Match volts to wire feed, shorten stickout, check polarity
Distortion	Unbalanced heat concentrated in one area	Pass sequence, travel direction	Balance side to side, backstep, heat sinks

Cold Lap and Lack of Fusion: The Energy Deficit

Cold lap is what happens when the filler melts but the base metal under it does not. The bead can look tidy while sitting on the steel like a rope on a table. The cause is thermal: the joint never reached the fusion boundary because current was too low for the thickness, travel was too fast for the arc to wet in, or the arc pointed at the puddle instead of the root. Heat input matters because it drives cooling, and cooling governs distortion, shrinkage, and unwanted phases in stainless steel (TWI).

The fix is usually a small voltage increase, slower travel, and a hard look at where the arc lands. Tilt into the joint, hold a consistent stickout, confirm polarity.

Burn-Through and Undercut: The Energy Surplus

Burn-through cuts a hole. Undercut melts a toe and leaves a trench with no filler to fill it. On thin sheet a couple of volts swings the outcome, so the better move is a slightly higher wire feed, a shorter arc, and faster travel, not turning voltage down until the puddle freezes. Backing bars, stitch welds, and pulse buy margin.

Cracking: The Defects You Find Later

Hydrogen-assisted cold cracks in steel can show up many hours after the arc goes out, so a weld that looks sound at the end of the shift proves nothing. That delay is the whole argument for preheat and controlled cooling. On higher-strength steels, AWS D1.1 sets a maximum interpass of 400°F (200°C) up to 1-1/2 in and 450°F (230°C) above that for A709 HPS 70W and A852 Grade 70 (AWS D1.1). There is a ceiling too: preheat much above roughly 500°F (260°C) can undo the toughness benefit a moderate interpass gives some steels (AWS Welding Digest). Log both numbers and inspect after a delay.

Spatter, Porosity, and Distortion

Spatter that spits and pops signals an unstable arc, usually a voltage and wire-speed mismatch. A hissing, spraying arc usually means voltage is high; stubbing means voltage is low or stickout is too short. Porosity is mostly contamination and shielding, not heat: zinc coating, oil, moisture, and gas loss all disturb the molten pool. Distortion comes from concentrated, unbalanced heat, so alternate sides, stagger passes, and lean on travel speed, your main handle on energy per inch. Aluminum needs its own approach to cleaning and gradient control, covered in this guide to aluminum MIG welding and these tips on prepping aluminum right.

A Troubleshooting Sequence That Works

1. Verify material and thickness. The metal picks the settings, not your habit.
2. Check polarity and gas. Wrong polarity or a nearly empty cylinder mimics a heat problem.
3. Confirm contact-tip-to-work distance and gas flow. Long stickout robs you of current and shielding.
4. Adjust voltage and travel speed in small steps, then re-weld a test piece. Vertical-up runs hottest and vertical-down coldest, so position changes the answer.

Work that order and you will spend less time wondering why a weld that looked fine came back to bite you.

Measuring Temperature in the Shop, Staying Safe, and Putting It All Together

Everything above only pays off if you can actually tell how hot your metal is, and that’s where most home shops fall apart. You nod along at preheat minimums and interpass ceilings, then head back to the bench and guess. Guessing works right up until a cracked fillet shows up two days later, stainless loses its corrosion resistance, or you lean a forearm on a plate that stopped glowing five minutes ago and is still sitting at 400°F. Here’s how to close the loop: measure it properly, stay safe around it, and turn it into a workflow you repeat every time.

The Temperature Measurement Toolkit

You have three realistic options in a small shop, and they are not interchangeable.

• Temperature-indicating crayons — wax-like sticks rated to one specific temperature. The mark melts within a couple of seconds once the surface reaches that set point. Cheap, fast, accepted for code work. Buy the numbers you need: 150°F, 300°F, 400°F.
• Contact thermocouples — a probe against the surface feeding a digital meter. Ideal for interpass tracking and logging a cooling curve, but slower, and it needs a clean surface and firm contact.
• Infrared (IR) guns — instant, point-and-shoot, and the most misused tool in the shop. More on that below.

Placement matters as much as the instrument. ISO 13916:2017 guidance widely cited in industry material calls for measuring at four times the material thickness from the groove edge, capped at 50 mm, for plate up to 50 mm thick. It also suggests waiting roughly two minutes per 25 mm (1 in) of thickness before reading a heated surface so the through-thickness temperature can equalize. Reading a 1-inch plate ten seconds after the torch leaves tells you about the surface, not the section.

Why IR Guns Struggle (and Why Crayons Win on Code Work)

Infrared thermometers infer temperature from emitted radiation, so the reading depends on emissivity — how efficiently the surface radiates. Shiny aluminum is the classic problem child: low, inconsistent emissivity means the gun can be hundreds of degrees off, usually low. Polished stainless is nearly as bad, and fresh mill scale versus a wire-brushed surface will read differently at the same real temperature.

That’s why a melted crayon mark is the field standard for code work. It’s a direct physical event at a known temperature that doesn’t care about surface finish. If the mark melts, you’re at temp. No calibration menu, no emissivity setting, no argument. If you do lean on IR, restrict it to rough, dark, oxidized surfaces, hold the gun perpendicular and close, and treat the number as a trend rather than a certification. And remember the hard limit: no handheld gun measures arc temperature. Anyone quoting one figure for “MIG temperature” is oversimplifying something that spans thousands of degrees across a few millimeters.

Staying Safe Around Heat You Can’t See

Arc radiation and UV. The arc throws intense ultraviolet and infrared radiation. UV causes “arc eye,” a painful corneal inflammation that shows up hours later, and it burns skin like sunburn only faster. Correct shade, sleeves, a buttoned collar, and no gaps matter — and nearby helpers need protection too, because UV reflects off bright surfaces and travels.

Hidden hot metal. Steel stops glowing red around 1,000°F and looks completely ordinary at 500°F, which is still hot enough for a serious contact burn. Assume every weldment is hot until measured. Chalk a “HOT” mark or verify with a crayon before you grab it.

Fume, and how temperature changes what you breathe. This is the connection most welders miss. GMAW fume composition is dominated by evaporation from the electrode rather than the base plate, and higher droplet surface temperature increases metal vapor and shifts that composition. Cranking parameters to “get more heat” changes the chemistry coming off the wire. Manganese is a known concern in steel welding; hexavalent chromium is the concern with stainless. Weld hotter, ventilate better.

OSHA 29 CFR 1910.252 sets the general requirements for welding, cutting and brazing, covering personnel protection and fire prevention, and the agency’s welding and cutting safety resources spell out ventilation expectations plainly. Local exhaust at the arc, open doors, a fan pulling air out of your breathing zone — none of it is glamorous, and all of it is cheaper than the alternative.

The Workflow That Ties It Together

1. Identify the metal. Mild steel, stainless, aluminum, galvanized. This alone decides half your settings. For aluminum, cleaning isn’t optional — see our guide to prepping aluminum right.
2. Find the governing thickness. Not the average — the thickest part at the joint. That drives preheat and code bands.
3. Set the machine by thickness and transfer mode. Short-circuit for thin sheet, spray or pulsed spray for thicker plate.
4. Calculate heat input. Volts × amps ÷ travel speed. Keep stainless under roughly 1.5 kJ/mm and low-alloy steel near 2.5 kJ/mm unless the WPS says otherwise.
5. Apply preheat if required. Check the code table, check carbon equivalent, then verify with a crayon. A warm day doesn’t count.
6. Cap interpass. Measure between passes at the specified location and let it cool if you’re over.
7. Inspect. Cold lap, undercut, spatter patterns, heat tint — all of them are temperature stories written on the surface.

Temperature control in MIG welding is a system, not a dial. Volts, wire speed, travel speed, preheat, interpass and cooling rate all pull on the same outcome, and the instrument you check it with matters as much as the numbers you punch in. Build the measurement habit, respect the heat you can’t see, and the welds stop surprising you.

Frequently Asked Questions

What temperature does MIG welding reach?

The arc plasma in GMAW is typically measured in the thousands of Kelvin — published studies report values from roughly 5,000 K up to 8,000 K and higher at the arc center — while the molten steel weld pool and wire droplet sit closer to 2,200–3,000 K (about 3,500–4,900°F). The base metal away from the pool stays much cooler, which is why the heat-affected zone matters as much as the arc itself.

Is there a temperature setting on a MIG welder?

No. MIG machines are set by voltage and wire feed speed (or a single synergic thickness/amperage number), not by degrees. Voltage mainly controls arc length and wire feed speed controls current, so ‘more heat’ is really a combination of those two plus travel speed.

Do I need to preheat steel for MIG welding?

Often not for thin mild steel at room temperature, but preheat becomes important as thickness, carbon equivalent and restraint increase. Under AWS D1.1, A36 welded with GMAW requires no preheat above freezing up to 3/4 in, but rises to 50°F, 150°F and 225°F in the thicker bands, and any steel below 32°F must be brought to at least 70°F.

What is a safe maximum interpass temperature for MIG welding?

It depends on the material. Austenitic stainless and quenched and tempered steels are commonly capped around 150°C (300°F), aluminum alloys are often limited to 65–120°C (150–248°F), and AWS D1.1 caps certain high-performance steels at 400–450°F. Always follow the project WPS over any generic number.

Why does my MIG weld look fine but fail a bend test?

That is the classic signature of a cold weld: the bead sits on top of the metal without real fusion, usually from low amperage, fast travel, wrong polarity or a poor ground. Raising voltage slightly, slowing travel and cleaning the joint typically fixes it, but destructive testing is the only way to be sure.

Can I use an infrared thermometer to check preheat?

You can, but accuracy suffers on shiny or low-emissivity surfaces such as aluminum and stainless, and readings vary with distance and angle. Temperature-indicating crayons and contact thermocouples are the accepted methods where code compliance matters.

How hot does MIG welding get on aluminum?

The arc is just as hot, but aluminum melts near 1,221°F and conducts heat away quickly, so the puddle forms suddenly and burns through easily. Preheat of roughly 120–350°F on thicker sections reduces the thermal gradient and helps fusion without pushing the metal to its limit.

Does higher voltage mean higher temperature in MIG welding?

Higher voltage lengthens the arc and usually widens the bead, but it does not automatically mean more heat into the part. Total energy delivered depends on voltage times amperage divided by travel speed, so a wide, long arc moving fast can put in less heat than a tight arc moving slowly.

Is MIG welding hot enough to warp 1/8-inch steel?

Yes. Thin steel warps from localized heating and shrinkage even though the arc is brief. Lower heat input, shorter stitches, back-stepping, clamping and allowing parts to cool between passes control distortion more effectively than simply turning the machine down.