What Produces the Heat During a Shielded Metal Arc Weld? The Real Physics of Stick Welding

A first-year apprentice once told me he thought the stick electrode was basically a match that the machine lit for him — until he dragged a cold 6010 across plate, heard that sharp crackle, and watched the steel turn liquid under a glowing blue-white cone. Nothing was burning, no gas torch was involved, yet in seconds the metal was a puddle. So where does all that heat actually come from in shielded metal arc welding?

The Question Every New Welder Asks: What Actually Makes the Heat in SMAW?

A first-year apprentice once told me the stick electrode was basically a match the machine lit for him. Then he dragged a cold 6010 across a plate of mild steel, heard that sharp bacon crackle, and watched the metal turn liquid under a glowing blue-white cone. No gas torch was anywhere near the joint, yet in seconds the quarter-inch plate was a puddle and two pieces had become one.

So where does all that heat actually come from in shielded metal arc welding? It is one of the first questions every welding student asks. If you have ever stood under a stick arc wondering whether something in that flux coating is secretly combusting, you are in good company. The answer is more interesting than most people expect.

What Shielded Metal Arc Welding Actually Is

Shielded metal arc welding, abbreviated SMAW and known colloquially as stick welding, is defined in the standard sense as a process that produces coalescence of metals by heating them with an arc between a covered metal electrode and the base metal work piece, with shielding obtained from decomposition of the electrode covering. That definition, used by NIOSH and the CDC, holds the story in miniature: the arc does the heating, the coating does the shielding.

That combination makes stick welding remarkably durable as a process. Because the shielding comes from the rod itself, there is no gas bottle, no regulator, no torch hose to drag around: just a machine, a ground clamp, a holder, and a box of electrodes. That portability is a major reason electric arc welding remains the most widely used type of fusion welding, from shipyards and pipelines to the local repair shop.

The Heat Is Electrical, Not Chemical

Here is the point that separates welders who understand their craft from machine operators. The electrode coating does not burn, the rod does not ignite, and no chemical reaction is doing the heavy lifting. As safety training literature from SAIF explains, the movement of electrons through the arc provides the major source of heat required in welding. You are forcing a large current through a narrow gap of gas that does not want to conduct. Making it conduct anyway is what releases the heat.

NIOSH notes in its welding, brazing, and thermal cutting guidance that the extreme heat needed to melt metal, exceeding 5,000 °C in some cases, is produced by an electric arc between the work and an electrode. Mild steel melts at roughly 2,750 °F (about 1,510 °C), so the arc pours far more energy at the joint than the metal needs. Your technique is what keeps the excess from becoming a defect.

What This Article Covers

That abundance of energy is why understanding heat is the biggest lever you have for improving your welding. Over the next several sections, we break the physics down into shop-floor language:

• Arc physics — plasma formation and the three voltage-drop zones that generate heat.
• Amperage, voltage, and resistance — how your machine settings become thermal energy in the puddle.
• Polarity — why DCEN, DCEP, and AC shift the heat balance between rod and workpiece.
• The flux coating — what it really does, and why the belief that the rod “burns” is the most common misconception in the trade.
• Real temperatures — arc core figures, heat input math, and how much energy reaches the joint.
• Safety — UV, infrared, fumes, and OSHA filter-shade requirements tied to arc heat.
• Practical control — arc length, travel speed, weave width, and preheat, so you direct the heat instead of fighting it.

We will also compare the process with its neighbors where that helps. If you have background in setting up a MIG welder or the electrode classification system, much of this will connect.

One thing worth saying up front: this is not arcane theory. Every burn-through on thin sheet, every cold lap on a corner joint, every sad bead that failed to stack traces back to a heat problem. Learn where the heat comes from and how it distributes itself, and stick welding gets noticeably easier. If you are just starting out, our beginner’s guide to stick welding difficulty pairs well with the physics here.

Let us start with the arc itself.

The Short Answer: The Electric Arc Is the Heat Source

Strip away the flux, the smoke, and the crackling noise, and every stick weld runs on one single idea: an electric arc. The SAIF defines an arc as “a sustained, luminous discharge across a gap in an electric circuit”, and adds the point that settles our apprentice’s match theory once and for all: the movement of electrons through the arc provides the major source of heat. Nothing is burning. Nothing is combusting. The heat is electrical energy being converted into thermal energy, in a gap you can hold a business card across.

That is the short answer. The longer answer, the one that actually makes you a better welder, is about where in that gap the heat is made and why the machine’s dials matter so much. If you are still building confidence with the fundamentals, our honest beginner’s guide to stick welding is the right companion piece.

Current Does Not Jump. It Is Carried.

Here is the part that trips up almost everyone. When you strike an arc, it feels like electricity is leaping across the air gap like a tiny lightning bolt. Physically, that is not what happens. The ISBE shielded metal arc welding curriculum explains it plainly: current “does not jump the gap; it is conducted by a mass of ionized gas.” That mass of gas is plasma.

When the electrode tip gets close enough to the workpiece, the voltage stresses the air between them until atoms are stripped of electrons. Flux vapors and shielding gases present in the gap get ripped into positive ions and free electrons, and suddenly that “air” is a conductive fluid. Now the circuit is complete. Electrons stream from the cathode toward the anode, ions drift the other way, and this plasma column glows white-blue because it is absurdly hot.

The reason this matters for heat control is simple: the plasma is the heat generator, and its stability depends on what is vaporizing into it. Flux ingredients, base metal vapors, and atmospheric gases all ionize and feed the arc column. That is why an arc struck with a bare rod sounds harsh and wanders, while a flux-coated rod like a 7018 holds a smooth, quiet puddle. To understand what those coatings are actually made of, see our full breakdown of welding electrodes: types, numbers, polarity, and selection.

The Three Voltage-Drop Zones

An arc is not one uniform tube of heat. It behaves like three electrical regions in series, and each one drops voltage and turns that drop into heat. Total arc power is the product of arc voltage and current, and arc voltage is essentially the sum of the cathode drop, the plasma drop, and the anode drop, as detailed in the NPTEL lecture on the physics of the welding arc. Expressed as the familiar power formula:

Q = V × I

• Cathode fall zone — a very thin layer, fractions of a millimeter, right at the cathode surface where voltage drops steeply. Electrons accelerating out of this zone gain energy and slam into nearby material, dumping heat into whichever surface is acting as the cathode.
• Plasma column — the visible arc stream itself. Current flowing through a resistive gas generates heat along its entire length. This is why arc length changes heat distribution so dramatically: a longer arc means a longer resistive column, and more voltage dropped usefully nowhere.
• Anode fall zone — the matching thin layer at the anode. Electrons arriving here give up their kinetic energy and their condensation energy directly into the anode surface.

Because roughly half the arc’s heat is generated at each electrode interface, the split between them is governed by current direction. That is the whole story behind polarity, which the next section unpacks.

A Very Short History of a Very Hot Idea

The physics did not change over time; our ability to control it did. The same ISBE curriculum traces the timeline: the electric arc was discovered in 1801, used commercially with carbon electrodes in 1881, and in 1887 a bare metal rod was found to melt off and act as filler metal. Then came the breakthrough that made modern stick welding possible: in 1910, a flux coating was added to the electrode.

That single addition turned an unstable, smoky curiosity into the most portable fusion process on earth. As TWI notes in its arc welding overview, arc welding processes remain the most widely used fusion welding processes in industry, and SMAW is still the fallback whenever the work leaves the shop. Compare that with a purely chemical heat source like aluminothermic rail welding, and the contrast is stark: thermite gets its heat from a reduction reaction, while SMAW gets every degree of it from electrons in motion.

Note: The same electron flow that makes the arc is also what heats your cable runs, your holder, and the stick-out of rod protruding from the clamp. Poor welding connectors do not waste a little efficiency; they steal heat that belongs in the puddle.

So when you strike your next arc, picture it correctly: not a match, not a flame, but a sustained electrical discharge conducted by ionized gas, converting voltage and amperage into concentrated, controllable heat. Everything after this, from machine settings to travel speed, is simply about steering that heat.

From Wall Power to Weld Pool: How Amperage and Voltage Become Heat

The electricity coming out of your shop wall is the same electricity melting the steel in front of you. Nothing about it changes except its pressure and volume. To understand what produces the heat during a shielded metal arc weld, follow that power on its journey from the service panel to the puddle.

The Supply Chain: 460 Volts In, 22 Volts Out

Shop power arrives at your machine at 220, 380, or 460 volts depending on your country and facility. That voltage is far too high to hold an arc; strike one directly across line voltage and you get a violent, uncontrolled short circuit rather than a weld. Inside the welder, a transformer or an inverter’s high-frequency switching circuit steps the input down dramatically. As the Illinois State Board of Education’s welding curriculum puts it, a typical welder transforms 220-volt input to usually between 15 and 25 volts at the electrode. The amperage dial on the front of the machine controls how many amps flow through the circuit once the arc is struck.

In other words, the machine trades voltage for amperage: high pressure becomes high flow. That low-voltage, high-current output makes the arc both powerful and survivable, low enough in voltage that you can safely hold the stinger, high enough in current to melt steel. If you have compared settings between processes, our guide to how to set up a MIG welder shows the same principle playing out with wire feed.

Three Electrical Quantities, Three Roles in Heat

Most beginner confusion about heat comes from mixing up three different electrical ideas. In practical shop terms:

• Amperage (current) is the measure of electrical current flowing through a circuit and an indication of the heat being produced. This is why the amp dial is really a heat dial. The ISBE welding curriculum states it plainly: amperage is the primary indicator of heat in the arc.
• Voltage is the electrical pressure that pushes current across the arc gap. You don’t set it directly; it responds to arc length. Stretch the arc and voltage climbs, shorten it and voltage drops. Voltage sustains the arc, but amperage does the heavy melting.
• Resistance is the opposition to the flow of current in a circuit and, again per the ISBE curriculum, it is what causes electric energy to be transformed into heat.

That last point answers a second question hiding inside the first. Heat in SMAW doesn’t come only from the arc gap. It is produced everywhere current meets resistance: the welding cables, the electrode holder, and, most importantly, the bare length of electrode sticking out from the holder, called the stick-out. Any place current flows through something imperfect gets hot.

Doing the Math: How Many Watts Is That?

Electrical power in watts is simply voltage times amperage. Set your machine at 80 amps and strike an arc at roughly 22 volts, and you have:

P = V × I = 22 V × 80 A ≈ 1,760 watts

In household terms, that is like running two large space heaters, concentrated into a gap you could hide behind a pencil. Now consider that a 1/4-inch 7018 might run at 160–180 amps. The same math gives roughly 3,500–4,000 watts concentrated into a few millimeters of glowing plasma. That is why steel doesn’t stand a chance.

Resistance Heating: The Heat Inside the Rod Itself

Here is where it gets interesting. Researchers modeling electrode behavior describe melting rate with a two-part formula:

Melting rate = a × I + b × L × I²

The first term is arc heat, proportional to current. The second term is Joule heating, or I²R resistance heating, of the electrode extension: that bare stick-out between the holder and the arc. As documented in NPTEL’s welding technology course, this resistive term grows with the square of the current and with stick-out length. Doubling the amps quadruples the resistance heating, and holding a longer stick-out warms the rod core before it even reaches the arc.

Experienced welders exploit this intuitively. Overhead or root passes often call for a slightly longer stick-out to preheat the rod, soften it, and keep the puddle manageable. When you need crisp penetration on cold, thick plate, you shorten up. Resistance heat becomes a tuning tool, not a side effect.

So when you turn that dial and hear the crackle, hold the full picture together: wall power transformed into low-voltage, high-current output, converted into thermal energy in three places at once, the plasma of the arc, the work and electrode surfaces, and the resistive body of the rod itself. Amperage tells you how much heat is being made. Voltage tells you the arc is staying lit. Resistance quietly turns a chunk of that electricity into heat along the way. Master those three, and the puddle becomes predictable.

Three Places Heat Is Born in an SMAW Circuit

When a welder says “the rod made the heat,” they’re only half right. The heat in a shielded metal arc weld is not born in one single spot. It’s generated in three distinct places inside the welding circuit, and understanding where each one sits is the difference between running a controlled puddle and fighting a wandering, spattery arc.

1. The Arc Column: The Hottest Zone

The plasma column between the electrode tip and the workpiece is the primary furnace. Current flows through a channel of ionized gas that can reach temperatures far beyond what the steel itself can survive, which is exactly why the metal under the arc goes liquid so quickly. Almost all of the light, noise, and violent energy you see and hear comes from this zone. As TWI’s technical overview of arc welding describes, the arc is a sustained electrical discharge, and the column converts electrical energy into thermal energy continuously while the arc is lit.

The column is also where the flux vapors and shielding gases do their quiet work. Those gases ionize, feed conductive particles into the stream, and keep the plasma stable. A stable column transfers heat smoothly; an unstable one flickers, and flickering heat means an inconsistent penetration profile.

2. The Electrode Tip and Work Surface: The Interface Heat

The second zone is split across two thin boundaries: the anode fall and the cathode fall regions. Electrons crossing these gaps slam into the metal surfaces, and that kinetic energy becomes heat right at the interface. The practical result is that roughly half of the arc’s heat develops at the electrode side and roughly half at the work side, with the exact split depending on polarity.

Heat generation on the workpiece side depends on polarity for DC welding, and on AC it is shared back and forth between the workpiece and the electrode, as explained in NPTEL’s Physics of Welding Arc lecture. This is the mechanism behind everything in the next section about DCEN versus DCEP: you are not changing how much heat exists, you are changing which side of the joint receives it.

3. Resistance Heating: The Humble Contributor

The third source is plain resistive heating, following the same I²R law that makes a toaster element glow. It shows up in the cable runs, the electrode holder, and especially the length of bare electrode extending from the holder to the arc, commonly called the stick-out. Increase that stick-out and you add resistance, which adds heat in the rod before the arc ever contributes. A well-maintained welding connector matters here for more than just current flow; corroded or loose connections waste energy as heat in the wrong place.

Where the Heat Goes Next

Once generated, the energy splits three ways:

• Conduction pulls heat away from the fusion zone into the surrounding base metal, creating the heat-affected zone.
• Convection carries heat away through the rising plume of hot gas above the puddle.
• Radiation throws heat outward as light and infrared energy, which is also why your skin and eyes need protection.

Only part of the arc heat is actually used for melting; the rest is lost through conduction into the base metal, convection, and radiation to the surroundings, according to the same NPTEL arc physics material. That is why arc length matters so much: a long arc opens the column up to radiation and convective loss before the energy ever reaches the plate.

Why Consumable Processes Keep More Heat

Here is the part most beginners never hear. SMAW, MIG, and submerged arc are all consumable-electrode processes, meaning they use heat at both the cathode and the anode to melt filler and base metal simultaneously. Non-consumable GTAW only develops useful melting on one side of that exchange, so much more energy wanders off. The efficiency gap is dramatic:

Figures from NPTEL’s welding arc lectures. The submerged arc wins because the flux blanket traps virtually all of the radiation and convective loss, and SMAW borrows a milder version of the same trick: the covering melts into a slag blanket that insulates the puddle and pushes heat back into the joint instead of letting it escape.

Note: Useful heat is the energy that ends up in the fusion zone. Everything else — cable loss, radiation, convective plume, and over-conducted base metal — is overhead. The skill in stick welding is mostly the skill of keeping overhead low while the puddle stays fed.

That framing is exactly why welders who learn to read their puddle, rather than chasing amperage numbers alone, produce sounder welds. If you want the full picture of how this heat behaves once it reaches the plate, our companion piece on weld burns on arms shows how quickly stray radiative and conductive heat injures unprotected skin.

Polarity and Heat Balance: DCEN, DCEP, and AC Explained

Two welders can run the exact same rod at the exact same amperage and get completely different results: one gets a shallow, wide bead with fast rod consumption, the other a deep, narrow penetration profile. The setting that explains the difference isn’t amperage at all. It’s polarity, which is simply the direction current flows through the welding circuit.

Since the arc is the heat source, and the arc is made of moving electrons, which way those electrons travel decides where most of the heat lands. That’s the whole idea behind this section, and once it clicks, you’ll understand heat control at a level most hobby welders never reach.

How Electrons Actually Move in the Arc

Conventional “current” and electron flow run opposite each other, so it’s easiest to think in terms of electrons, which are what actually carry energy into the metal:

• DCEN (Direct Current Electrode Negative), also called straight polarity: electrons flow from the electrode into the workpiece.
• DCEP (Direct Current Electrode Positive), also called reverse polarity: electrons flow from the workpiece into the electrode tip.
• AC (Alternating Current): the direction reverses continuously, roughly 100 to 120 times per second, because the current crosses zero on every half-cycle of the 50 or 60 Hz supply.

That last point matters more than it sounds. As the Illinois State Board of Education welding curriculum notes, the arc is technically extinguished every time current passes through zero, then re-ignited as voltage builds back up. On 60 Hz AC that happens about 120 times per second. The arc looks and sounds continuous to your eye, but electrically it’s a rapid strobe of mini arcs.

Where the Heat Goes: DCEN vs. DCEP

Electrons arriving at a surface dump kinetic energy into that surface as they decelerate and strike it, so whichever element receives the electron stream absorbs more of the arc’s thermal energy. According to the Australian Welding Institute’s SMAW overview, DCEN builds more heat on the electrode side, which increases the electrode melting rate but decreases weld penetration. DCEP does the opposite: it drives more heat into the electrode tip and the immediate arc zone, producing deeper penetration into the base metal.

Because DCEP pushes heat deeper, it’s the workhorse polarity for structural stick welding, which is why most common electrodes, including the ubiquitous E7018, are specified for DCEP or AC. If you’re setting up your machine for these rods, our 3/32 E7018 amperage, polarity and settings guide walks through the exact dial numbers.

The AC Advantage: Balance and Oxide Cleaning

AC splits its heat evenly between the electrode and the work, giving a genuine middle ground between melting rate and penetration. But its second trick is just as valuable: each polarity reversal acts like a tiny scrubbing pulse at the workpiece surface, helping break up the tenacious oxide film on aluminum and the mill scale, rust, and oil contamination found on real-world steel. That cleaning action is why electrodes like E6011 and E6013 perform so well on dirty, painted, or rusty plate when run on AC. AC also minimizes arc blow, the magnetic deflection you’ll notice when welding large fillets or out-of-position joints on DC, because the magnetic field reverses with the current and never pushes the arc sideways for long enough to matter.

Why Polarity Matters More Than Most Beginners Realize

Think about what polarity really is from a heat standpoint: it’s free heat management. Before you touch the amperage dial, you can shift where the arc’s energy lands simply by changing current direction. Rod burning too fast and blowing through on thin steel? Trying DCEN may give you a shallower, cooler-feeling puddle at the same amperage. Can’t get enough fusion into a heavy joint? DCEP at the same setting will dig deeper.

This is also why polarity appears in nearly every welding procedure specification. Our pipe welding procedures guide covers how WPS documents lock polarity in alongside amperage, travel speed, and preheat, because changing it changes the joint’s heat input and therefore its metallurgy. For a deeper look at how rod classification numbers encode these recommendations, see what electrodes are in welding: types, numbers, polarity and how to choose.

One last practical point: changing polarity doesn’t change the arc’s radiation output, so your welding PPE requirements stay exactly the same. The arc is just as harsh on your eyes at DCEN as it is on DCEP.

The Flux Coating: It Doesn’t Create the Heat, but It Shapes and Protects It

Walk into any shop and you’ll hear it repeated like gospel: “The flux is what burns the rod.” It’s an easy mistake. When you strike an arc, the coating sizzles, smokes, and glows, and the rod seems to vanish into the puddle almost eagerly. But the flux coating is not the fuel. Nothing in that covering releases significant heat on its own. Every degree of heat in a shielded metal arc weld comes from the electric arc described earlier. The flux is a manager of that heat — a shaper, a shield, and a cleaner — not its source. Once you accept that, several stick-welding puzzles resolve: why a bare rod won’t run properly, why different coverings change arc behavior so dramatically, and why slag does far more than sit on your bead looking ugly.

What the Covering Actually Does: Four Jobs, One Arc

The U.S. Environmental Protection Agency’s welding emissions documentation states it plainly: “The electrode covering provides stability to the arc and protects the molten metal by creating shielding gases by vaporization of the cover” (EPA AP-42, Chapter 12.19). In other words, the coating doesn’t burn — it vaporizes, and it does so only because the arc’s heat decomposes it. Educational welding reference material from ISBE’s technical curriculum groups the flux’s responsibilities into four functions, and they’re worth memorizing:

• Protect the molten metal from the atmosphere. Liquid steel at 2,800°F will soak up oxygen and nitrogen from the air in a heartbeat, leaving you with porosity and a brittle, weakened deposit.
• Create a gas blanket. As the covering vaporizes, it releases a cloud of shielding gases — carbon dioxide, carbon monoxide, water vapor, and hydrogen depending on chemistry — that physically displaces air around the arc and the puddle.
• Float impurities into slag. The molten flux reacts with oxides in the puddle, gathers them, and lifts them to the surface where they freeze into a protective glassy layer.
• Stabilize the arc. Ionizable constituents such as rutile and potassium compounds keep the plasma column conducting smoothly, which is why coated rods run so much steadier than bare wire.

That’s a remarkable amount of work for a coating that contributes essentially zero fuel to the process.

The Slag Blanket: An Underrated Heat-Management Tool

The fourth phase of the flux — the slag — is where the coating directly changes how heat behaves in your joint. As the ISBE material puts it, the slag “covers the bead to protect it from the air and to slow the rate of solidification and cooling.” Read that slowly: it slows cooling. A bare molten puddle radiates heat away furiously in every direction and solidifies almost instantly. Cover it with molten slag and you’ve essentially put a thermal blanket over the weld zone.

This matters for two practical reasons. Slower cooling gives dissolved gases more time to escape before the metal freezes, which reduces porosity. It also lets the heat already in the joint do more useful metallurgical work — better fusion at the toes, a smoother bead profile — instead of being wasted to the atmosphere as radiant energy. In that small way, the flux genuinely helps a higher percentage of the arc’s heat stay where you want it.

Flux Chemistry Changes Arc Voltage — Which Changes Your Heat

Here’s where the coating quietly circles back to heat. Different covering chemistries produce different arc voltages, and since heat output is the product of amps times volts, voltage shifts translate directly into energy shifts at the arc. A cellulose-rich covering like E6010 runs a longer, more forceful, deeper-penetrating arc at a given setting, while a low-hydrogen basic covering like E7018 produces a shorter, quieter, more controllable arc. Same amperage dial setting, noticeably different thermal behavior. Electrode selection is really a heat-control decision: our guide to what electrodes in welding actually are and how to choose them breaks down the AWS classification system in depth, but the short version for heat is that the last digit tells you the covering type, and the covering type tells you how the arc will deliver that heat.

E6010 vs. E7018: Two Coverings, Two Very Different Experiences

E6010 uses a cellulose-based covering heavily loaded with organic material. When the arc heat hits it, the cellulose decomposes into a vigorous gas stream that blasts into the joint, giving deep penetration and that characteristic aggressive crackle. It’s an excellent choice for root passes and dirty or rusty plate, because the forceful arc action physically cleans the puddle.

E7018 is the opposite temperament: a basic low-hydrogen covering built on calcium carbonate and calcium fluoride, with iron powder for deposition efficiency. The arc is smooth and buttery, the slag peels cleanly, and the weld metal is tough and crack-resistant. If you’re setting one up on the machine for the first time, the settings in 3/32 E7018 amperage and polarity give you a reliable starting point before you fine-tune by puddle behavior.

The trade-off is fume. Because these coatings vaporize so much material, SMAW is one of the fume-heaviest processes around: NIOSH’s welding health literature puts SMAW fume generation rates at roughly 300 to 800 mg per minute, compared to just 3 to 7 mg/min for GTAW (NIOSH, Respiratory Hazards of Welding). That’s up to a hundredfold difference created almost entirely by the coating doing its job. Good ventilation and proper respiratory protection aren’t optional here — see our complete guide to welding PPE for how to build a fume-protection setup that actually works.

The takeaway is the one a beginner should tattoo on the inside of their helmet: the arc makes the heat, the flux decides how well you keep it, direct it, and protect the metal with it. Understand that division of labor, and you’re already ahead of most people holding a stinger.

How Hot Is the Arc, and How Much of That Heat Reaches the Metal?

Ask ten welders how hot a stick arc gets and you will get ten answers, each rounded to whatever number sounds most impressive. The real figures come from published technical sources, and they are more dramatic than most beginners expect. But arc temperature is only half the story. The other half is how much of that heat actually lands in the joint instead of drifting into the shop air.

The Temperature Numbers People Search For

According to TWI’s technical FAQ on arc welding, an electric arc produces intense heat of roughly 6,500°F (about 3,600°C) — enough to melt steel at the joint instantly. That is the figure you will see quoted most often, and it is a fair working number for a stable welding arc.

It is also a conservative one. Welding education curriculum from the Illinois State Board of Education puts arc heat at the moment of strike between 6,000°F and 9,000°F, reflecting the spike that happens when the gap first breaks down. NIOSH measurement data cited by CDC/NIOSH reports arc temperatures recorded above 5,000°C — roughly 9,000°F — and notes that welding spatter has been observed spraying up to 35 feet from the work area. That single detail tells you how much energy is being released: the metal is not just melting, it is being ejected at speed.

Put those side by side and the point is obvious: the arc runs more than twice the temperature needed to melt steel. That excess is why a weld pool forms so fast, why thin stock blows through so easily, and why heat control matters far more than raw power.

Why Arc Temperature and Heat Input Are Not the Same Thing

Procedure writers do not spec arc temperature. They spec heat input — the amount of energy actually deposited into the work, usually expressed in joules per millimetre or kilojoules per inch. The standard formula used in welding procedure specifications is:

Heat Input = (Voltage × Amperage × 60 × Efficiency) ÷ Travel Speed

Efficiency is the part beginners skip. Not every watt at the arc reaches the plate. Some radiates away, some convects into the shielding gas and atmosphere, some is lost to spatter, and some conducts backward into the electrode, holder and cables. Processes differ here: arc efficiency for SMAW is commonly assumed in the 75–90% range depending on the source, electrode type and arc length, while gas tungsten arc welding sits noticeably lower because more energy radiates from an uncovered arc.

That heat does not sit on the surface. It conducts downward and sideways through the plate, creating the heat-affected zone: the band of base metal next to the weld that never melted but had its microstructure rewritten by the temperature cycle. Too much heat input grows the HAZ, softens or embrittles the material, and can trigger cracking in hardenable steels. Too little produces cold lap and lack of fusion. WPS documents for critical work — see Pipe Welding Procedures: The Complete Guide to WPS, Positions, Passes & Inspection — lock these ranges down precisely because the mechanical properties depend on them.

Where the Missing Heat Goes

Three things steal heat from the joint:

• A long arc. Every extra millimetre of arc length adds voltage, which adds total power — but a larger share of it leaves as radiation and convection instead of entering the puddle. It also widens the arc cone, scattering energy over a bigger area.
• Spatter and open pool exposure. Molten metal thrown from the puddle carries heat with it. Slag-covered consumable processes retain heat better than bare-arc processes for exactly this reason.
• Fast travel and cold base metal. Heat arriving faster than the plate can absorb it piles up locally, driving burn-through and undercut instead of deep, clean fusion. Our Stick Welding Thin Metal: Amp Settings, Techniques & Burn-Through Fixes guide walks through dialling this back on light gauge.

So the direct answer to the question: an electric arc of roughly 6,500°F, produced by electrical discharge rather than combustion, is what produces the heat during a shielded metal arc weld — and how much of that heat becomes a sound weld depends on amperage, voltage, polarity, arc length and travel speed working together.

Common Myths and Mistakes About Heat in Stick Welding

Every shop has its own folklore about heat, and some of it survives only because nobody ever tested it. Bad heat assumptions don’t just cost you a nice-looking bead; they cause undercut, lack of fusion, porosity, brittle heat-affected zones, and far more fume than the job should produce. Here are the myths that cause the most damage on real jobs.

Myth 1: The Rod and Flux Are “Burning” Like a Torch

Nothing in an SMAW arc is fuel the way propane is fuel. The electrode core and coating are consumed by heat the arc produces, not the other way around. The flux coating decomposes in the arc plasma and releases shielding gases and slag, but that decomposition is driven by arc energy from the power source. Think of the rod as filler that gets melted, not a match that keeps itself lit, and you stop blaming the consumable for heat problems that are really electrical settings problems. For a refresher on what the numbering actually means, see What Are Electrodes in Welding? Types, Numbers, Polarity & How to Choose.

Myth 2: A Long Arc Still Delivers the Same Heat

A longer arc does produce a higher voltage, so the numbers on the display go up. But a large share of that extra energy leaves as radiation and convected hot gas instead of going into the puddle, and the extra voltage widens the arc cone, throwing spatter and washing the toes. Arc length also governs strike behavior: if the electrode is not instantly pulled away after striking, it fuses and sticks; pulled too far, the arc extinguishes. The habit that fixes most of this is keeping arc length roughly equal to the rod diameter.

Myth 3: Cranking Amperage Compensates for Fast Travel

Turning the dial up because you’re moving fast usually gives you the worst of both worlds: deep, narrow penetration with a thin, fast-freezing puddle that never wets out at the toes. You get undercut and a cold-looking joint profile instead of fusion. As EPA AP-42 notes, actual emissions and heat behavior depend on base metal, voltage, current, arc length, travel speed, and electrode angle together, so heat is a system setting, not a single knob. Slow down first, then trim the amperage. Setting charts like 3/32 E7018 Amperage: Settings, Polarity & How to Run It give a defensible starting point per diameter.

Myth 4: Polarity Barely Matters for Heat

It matters a great deal. Swapping DCEN and DCEP shifts roughly half of the arc heat between the workpiece and the electrode tip, changing penetration shape, burn-off rate, and how fast the rod is consumed at the same amperage. Running an electrode on the wrong polarity often shows up as shallow welds or an unstable arc that the welder “fixes” with more amps. AC flips polarity about 100–120 times per second, which balances heat and helps break oxide films on dirty steel, one reason 6011 and 6013 stay popular for field work.

Myth 5: Stick Always Penetrates Deeper Than MIG

Penetration is a function of heat density, arc force, travel speed, and joint geometry, not process labels. A short-circuit MIG set properly on 1/8 in plate can out-penetrate a stick rod run cold and fast. The honest comparison is the one you make on your own coupon, with settings documented. If you want the wider picture, Is Stick Welding Hard? An Honest Beginner’s Guide walks through where each process is genuinely easier.

Myth 6: Stick-Out Resistance Heat Is Negligible

That bare length of electrode between the holder and the arc is a resistor, and it follows I²R just like your cables and holder do. It adds a little heat to the rod core, but it also wastes energy and shortens the effective arc. It’s small next to arc heat, so never use a long stick-out as a substitute for proper settings, and keep your connections tight as covered in Welding Connectors Explained.

Myth 7: Preheat Is Optional on Heavy Sections

Thick, constrained, or high-carbon work can crack if the surrounding metal pulls heat away too fast from a cold joint. In preheating operations, metal may be preheated to as much as 600°F (SAIF), controlled with a temperature stick or infrared thermometer rather than guesswork. For code work, interpass and preheat requirements belong in the written procedure, as in Pipe Welding Procedures: The Complete Guide to WPS, Positions, Passes & Inspection.

Quick Troubleshooting: Symptom to Heat Cause

Undercut, cold lap, and related defects have documented acceptance limits, so check Welding Defects Undercut: Causes, Acceptance Limits, and How to Fix It before grinding your way out of trouble. And because burn-through on thin stock is really a heat-control failure, Stick Welding Thin Metal: Amp Settings, Techniques & Burn-Through Fixes is worth a read alongside Stick Welding Sheet Metal.

Safety: Managing Arc Heat, Radiation, Fumes, and Burn Hazards

Every joule of electrical energy that turns into heat in that arc is trying to reach your eyes, your skin, and your lungs. A stick arc is essentially an unfiltered sun sitting 14 inches from your face. Understanding where the heat is born is only half the job — the other half is knowing what that heat produces besides a weld, and what it takes to contain it.

What the Arc Throws at You: UV, IR, and Visible Light

The plasma column described earlier is a powerful radiator. It emits visible light, infrared (IR) radiation you feel as radiant heat on your face and hands, and ultraviolet (UV) radiation intense enough to produce two distinct injuries:

• Photokeratitis and arc flash — UV burns the cornea and conjunctiva. It is essentially a sunburn of the eye: painful, slow to heal, and entirely preventable. Symptoms often appear hours after exposure, which is why beginners routinely underestimate it.
• Erythema (skin burns) — exposed forearms and necks blister exactly as if sunburnt. Arc burns are one of the most common beginner injuries in stick welding; our guide on weld burns on arms: causes, first aid, and prevention covers treatment and the gear that stops it happening twice.

UV also degrades clothing, cracks rubber, and yellows plastics. The flux blanket absorbs and scatters part of the radiation, which is exactly why SMAW is less radiatively aggressive than gas-shielded processes — more on that in a moment.

OSHA’s Minimum Filter Shade Table for SMAW

Under 29 CFR 1910.133, employers must provide filter lenses or plates of a shade number that gives at least the minimum protection required for the work. For shielded metal arc welding, the OSHA table reads:

In plain terms: below 60 A you need a minimum of shade 7, 60–160 A calls for shade 8, 160–250 A for shade 10, and 250–550 A for shade 11. Going darker than the minimum for comfort is fine and common; going lighter is not, and sunglasses under your helmet do not count. Auto-darkening helmets are acceptable provided they darken within the time limits of the standard and the light state is no lighter than shade 3.

Helmets, Screens, and the 35-Foot Rule

The regulation that surprises most new welders is 29 CFR 1910.252(a)(2), which requires employees to be protected by helmets or hand shields during all arc welding or arc cutting operations. The only carve-out is submerged arc welding, where the arc hides under granular flux. There is no such thing as “just tacking real quick” without a shield.

The same standard covers bystanders. Welding screens and booths must have a finish of low reflectivity so bystanders are not receiving bounced UV, and anyone within the arc’s line of sight needs the same eye protection as the welder. Fire prevention is equally codified: combustible materials should, where practicable, be relocated at least 35 feet (10.7 m) from the work, or otherwise protected with fire-resistant covers, and a fire watch with an extinguisher is mandatory when sparks can travel to hidden combustibles.

Fumes, Gases, Ventilation, and Heat Stress

That same flux decomposition that shields the puddle also releases fumes, and the hazard depends entirely on the electrode: a mild-steel E7018 is comparatively tame, while galvanized, stainless, nickel-alloy, and especially cadmium-coated or lead-painted base metals can produce highly toxic fume. General mechanical ventilation must keep airborne contaminant exposure below accepted limits — a common benchmark is 35–50 ft³/min per square foot of contaminated floor area, and never less than 2,000 ft³/min at the booth. Confined spaces add oxygen-deficiency and ignition risks on top of fume, and require atmosphere testing, forced ventilation, and a permit system before anyone strikes an arc. The OSHA welding, cutting, and brazing fact sheet is the fastest place to check the full requirement set.

Do not overlook plain heat stress. Standing over a puddle producing 1,700+ watts in a hot shop, wearing leather and a helmet, is physiologically demanding. Dehydration and heat exhaustion degrade judgment long before they feel like a medical problem — and a welder with impaired judgment is the one who skips the shield and burns an eye.

The Full PPE Baseline

The reference document behind all of this is AWS/ANSI Z49.1, Safety in Welding, Cutting, and Allied Processes. The practical baseline it establishes is straightforward: flame-resistant jacket and leathers, gauntlet gloves, safety-toe boots, and a welding cap to stop sparks lodging in hair or running down your collar. Clothing must be free of oil and grease, cuffs closed, and no cuffs or pockets where sparks can collect. For the complete kit breakdown and what each item actually protects against, see our welding PPE: the complete guide to protective gear.

The Physics-to-Protection Checklist

1. An arc at 6,000°F+ radiates as well as heats — treat every strike as a light source you must filter.
2. Match your filter shade to amperage using the OSHA table above; increase shade for out-of-position or overhead work.
3. Helmet or hand shield on for every arc strike, including tack welds, per 1910.252.
4. Move or cover combustibles 35 feet away, or post a fire watch.
5. Ventilate deliberately, and treat any confined space as permit-required until proven otherwise.
6. Respect heat stress: schedule breaks, hydrate, and lighten up before your decision-making slips.

The arc does not care who it burns. Everything in this section exists because the same physics that makes SMAW a capable fusion process — a plasma column hotter than the surface of the sun’s photosphere, concentrated into a gap you control by hand — has no natural regard for eyes, skin, or lungs. Protection is not an accessory to the process. It is part of the circuit.

Practical Tips: Controlling SMAW Heat Like a Professional

Knowing where the heat comes from is half the battle. The other half is putting it exactly where you want it. Once you accept that amperage sets the size of the fire and your technique decides where that fire sits, heat control stops being luck and becomes a dial you can turn.

Start With Amperage, Not the Arc

Your machine dial is the single biggest heat control you own. A practical starting point used across the trade is roughly 1 ampere for every 0.001 in of electrode diameter: a 3/32 in rod near 90 A, a 1/8 in rod near 125 A, a 5/32 in rod near 160 A. From there, trim by 5–10 A based on position, plate thickness, and how the puddle is actually behaving.

Manufacturer data always wins over rules of thumb. AWS A5.1 classification sheets give the qualified range for each electrode, and our detailed settings guides walk through the numbers for common rods, including 3/32 E7018 amperage, polarity and how to run it and 5/32 E7018 amperage, polarity and technique.

Hold the Arc Length to the Diameter of the Rod

Every extra millimetre of arc length is voltage that heats the air instead of the joint. The standard teaching from the Illinois State Board of Education welding curriculum is simple: keep the arc length roughly equal to the diameter of the electrode’s bare end. For a 1/8 in rod, that means a 1/8 in arc, heard as a steady frying-bacon crackle rather than a hiss.

That same source sets the geometry around the puddle:

• Bead width: about 1.5 to 2 times the diameter of the bare end of the electrode.
• Work angle: 90° to the plate on a flat butt joint.
• Travel angle: 5–15° in the direction of travel.

Push or drag changes how much arc force digs into the plate, which is why the reasoning is covered separately in push or pull stick welding: which technique is right and why.

Match Polarity, Then Read the Puddle

Polarity decides how the arc’s heat divides between the rod and the work. Set it wrong and you will chase the problem with amperage, making it worse. Confirm the electrode’s required connection — many low-hydrogen rods run DC+ while fast-freeze rods have their own preference — using what are electrodes in welding: types, numbers, polarity and how to choose. If your machine cannot deliver the required polarity, that is a rod-selection problem, not a heat problem.

Travel Speed and Weave: The Fine Adjustment

Amperage sets the heat input rate; travel speed decides how much of it each inch of joint receives. Watch the leading edge of the puddle. A fluid, shiny toe that wets out cleanly means the balance is right. If the puddle piles up and lags behind the arc, you are moving too slow or running too hot. If the rod skips and leaves a narrow, high crown, you are outrunning the heat. Keep weave widths to 2 to 2.5 times the rod diameter — wider than that and the trailing solidified metal never re-melts.

Thin Metal: Spread the Heat Out

On sheet and thin tube, the goal is to stop the joint from sitting at melting point continuously. Techniques that work:

• Stitch or skip welding — short tacks with cooling pauses between them.
• Back-stepping — weld against the direction of travel in short segments so each pass preheats the previous one.
• Heat sinks and backing bars — copper or aluminium blocks pull heat away from the edge.
• Lower amperage and a smaller rod — a 1/16 or 3/32 rod stays within its range at amps that will not blow through.

Our stick welding thin metal: amp settings, techniques and burn-through fixes guide goes deeper, and stick welding sheet metal covers rod choices specifically.

Thick and Hard Steel: Control the Cooling Rate

Heat control works in both directions. On heavy sections and carbon steels, preheat and interpass temperature keep the cooling rate slow enough to avoid cracking and a brittle heat-affected zone. Procedures for critical work specify preheat, interpass limits, and bead sequencing — the kind of detail documented in pipe welding procedures: the complete guide to WPS, positions, passes and inspection. Position work tightens everything further, since overhead puddles want to fall; the overhead stick welding guide covers the 4F and 4G settings.

Choosing Between Stick, MIG and TIG

Heat tolerance often decides the process. SMAW is described by NIOSH as the simplest, least expensive and most widely used arc welding process, capable of welding all ferrous metals in all positions (CDC/NIOSH) — ideal outdoors, on dirty plate, or in the field. MIG is faster and easier to start but brings its own setup and safety considerations, as outlined in how to set up a MIG welder. TIG gives the finest heat control on thin material, see the complete guide to TIG welding sheet metal. Still deciding whether stick is for you? Is stick welding hard? is an honest read.

Conclusion: The Direct Answer, and a Working Checklist

So, what produces the heat during a shielded metal arc weld? An electric arc — a sustained electrical discharge between the flux-coated electrode and the workpiece, converting electrical energy into thermal energy at roughly 6,500°F. The flux coating shields and shapes that heat; it does not create it. Everything else in your hands is about directing it.

• Set amperage from electrode diameter, then adjust by puddle behaviour.
• Hold arc length equal to the rod’s bare-end diameter.
• Keep a 90° work angle and a 5–15° travel angle.
• Match polarity to the electrode’s data sheet.
• Limit weaves to 2–2.5 rod diameters.
• Use stitch, back-step and heat sinks on thin material.
• Preheat and control interpass temperature on thick or hard steel.
• Protect yourself: correct lens shade, screens, ventilation and full welding PPE are non-negotiable.

Frequently Asked Questions

What produces the heat during a shielded metal arc weld?

The heat comes from the electric arc — a sustained electrical discharge between the flux-coated electrode and the base metal. As electrons and ions cross the arc gap, their electrical energy converts into intense thermal energy that melts both the rod tip and the plate.

How hot does a stick welding arc get?

Arc temperatures are typically around 6,500°F (about 3,600°C), with strike temperatures reported anywhere from 6,000°F to 9,000°F and NIOSH citing measured arc temperatures above 5,000°C. That far exceeds steel’s melting point of roughly 2,750°F (1,510°C).

Does the flux coating burn to create heat?

No. The flux coating is not the fuel — the arc is. The coating decomposes in the arc’s heat to release shielding gases and slag that protect the molten pool, and it also stabilizes the arc and can influence arc voltage.

What does amperage control in SMAW?

Amperage is the measure of current flow and the main indicator of how much heat the arc produces. Higher amperage increases heat, penetration, and electrode melt-off rate; lower amperage is used for thinner material to avoid burn-through.

How does polarity change the heat balance?

With DCEN more heat is driven toward the workpiece, while DCEP pushes more heat into the electrode and generally increases penetration. AC reverses polarity over 100 times per second, distributing heat evenly between the electrode and the work.

Does all the arc heat go into the weld?

No. Only a fraction is used for melting; the rest is lost through conduction into the surrounding base metal, convection, and radiation. Arc efficiency for SMAW is commonly cited in the 66–85% range, with the slag blanket helping retain heat better than open arcs.

Why does arc length affect heat?

A longer arc increases voltage and spreads the arc column, losing more energy to radiation and spatter instead of concentrating it in the puddle. Keeping arc length roughly equal to the electrode diameter focuses heat and produces a cleaner, deeper weld.

What safety gear is needed for arc heat and radiation?

OSHA requires helmets or hand shields for all arc welding, with filter shades matched to current — typically shade 10 for 160–250 A SMAW. Add flame-resistant clothing, dry insulated leather gloves, and screens to protect bystanders from UV and spatter.