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publish: heat-map
2026-07-21 08:17:27 +02:00
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<title>The Heat Map — Reading Welds Like Terrain | Bernardo Zubko</title>
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<fort-nav><nav class="fort-nav" data-fort="nav"><a href="/">Bernardo Zubko</a><a href="/films/strike-zone/">Strike Zone</a><a href="/films/wind-line/">The Wind Line</a><a href="/films/stress-gradient/">Thermal Shock Front</a><a href="/fire-song.html">Bernardo Zubko</a><a href="/golden-seam.html">Bernardo Zubko</a><a href="/heat-budget.html">Heat Budget</a><a href="/impact-calculus.html">Impact Calculus</a><a href="/materials.html">Materials</a><a href="/stress-gradient.html">Stress Gradient</a><a href="/strike-zone-film.html">Strike Zone</a><a href="/crucible.html">The Crucible</a><a href="/fire-and-fix.html">The Fire and the Fix</a><a href="/heat-map.html" class="active" aria-current="page">The Heat Map</a><a href="/strike-zone.html">The Strike Zone</a><a href="/warped-steel.html">The Warped Steel</a><a href="/wind-factor.html">Wind Factor</a></nav></fort-nav>
<div class="breadcrumb"><a href="/">&larr; Bernardo Zubko</a> / Workshop Notes</div>
<h1>The Heat Map</h1>
<p class="subtitle">Reading welds like topographic maps of the Black Hills — a field guide by Bernardo Zubko, Pine Ridge Reservation</p>
<div style="width:100%">
<img class="img-full" src="https://images.pexels.com/photos/1555177/pexels-photo-1555177.jpeg?auto=compress&cs=tinysrgb&dpr=2&h=650&w=940" alt="Vivid blue-white arc and spatter from TIG welding in a dark workshop">
</div>
<p class="caption">Fig. 1 — The arc is the only sun that matters in the puddle. TIG on 304L stainless, 120A DCEN. Photo: Pexels.</p>
<p>I grew up reading the hills around Pine Ridge. Before there were contour lines on any map, there was the way the light fell at dusk — darker in the coulees, sharp on the ridge. My grandfather pointed with his walking stick and said <em>that slope will hold water, that one won't</em>. He was reading temperature, rock density, and drainage in the same way I read a weld bead on the bench.</p>
<p>Every weld is a landscape. The fusion zone is the fault line. The heat-affected zone is the weathered plateau. The base metal beyond is the untouched prairie. If you can read the terrain of a weld — the bead width, the cap profile, the color of the HAZ — you know whether it will hold through a South Dakota winter or split like green wood.</p>
<p>This is not a spec sheet. It is a field guide. The kind of thing I wrote in a spiral notebook and kept in my helmet case for twelve years before I decided to put it on the web.</p>
<h2>01. The Fusion Zone — Where the Ground Opens</h2>
<p>The fusion zone is the narrowest part of the weld — the actual line where base metal and filler became one substance. On A36 steel, it's about 0.2 to 0.6 millimeters wide per side, depending on travel speed and heat input. It looks like a fine root growing into both plates.</p>
<div class="map-card" data-label="Fusion Zone — A36 Steel, GMAW">
<table>
<tr><th>Parameter</th><th>Typical Value</th><th>What It Means</th></tr>
<tr><td>Penetration depth</td><td>3–6 mm</td><td>How deep the molten pool ate into the base. Shallow = lap weld. Deep = burnthrough.</td></tr>
<tr><td>Fusion width (each side)</td><td>0.2–0.6 mm</td><td>The actual bonded interface. If this is inconsistent, the joint is weak.</td></tr>
<tr><td>Peak temperature</td><td>~1510°C</td><td>Melting point of low-carbon steel. Above this, the metal is liquid.</td></tr>
<tr><td>Wettable angle</td><td>20°–45°</td><td>Where the fusion line meets the base. Too acute (&lt;20°) means lack of fusion. Too blunt (&gt;45°) means excessive heat input.</td></tr>
</table>
</div>
<p>I learned to read the wettable angle by breaking my own bad welds. My first year, I ran a root pass on a ¼-inch plate and it looked smooth on top. Broke it open and the fusion line was a hair's breadth from the surface — a lipstick stain on a brick. The filler sat on top without eating in. Too much travel speed, not enough voltage. The puddle was a lazy pool, not a hungry one.</p>
<div class="pullquote">The puddle tells you everything if you're looking. A hungry puddle pulls the filler down like gravity. A lazy puddle sits on the surface like ice on a pond. The difference is two amp.</div>
<h2>02. The Heat-Affected Zone — The Weathered Country</h2>
<p>Just beyond the fusion zone, the metal never melts but it remembers the heat. This is the HAZ — heat-affected zone — and it's where most weld failures actually start. The microstructure changes without the metal ever turning liquid. Grain structure grows, hardness shifts, residual stress locks in.</p>
<p>In carbon steel, the HAZ has three sub-zones I watch for:</p>
<div class="map-card" data-label="HAZ Sub-zones — Low Carbon Steel">
<table>
<tr><th>Zone</th><th>Peak Temp</th><th>Grain Size</th><th>Mechanical Effect</th></tr>
<tr><td>Coarse-grained HAZ</td><td>1100–1510°C</td><td>Large, equiaxed</td><td>Highest hardness. Most brittle. Cracks start here.</td></tr>
<tr><td>Fine-grained HAZ</td><td>900–1100°C</td><td>Small, uniform</td><td>Tougher. Often stronger than base metal. The "good" HAZ.</td></tr>
<tr><td>Subcritical HAZ</td><td>600–900°C</td><td>Variable</td><td>Partial annealing. Softens hardened steel. Can weaken it.</td></tr>
</table>
</div>
<p>The coarse-grained zone is the enemy. On my first pipeline job out near Rapid City, we had a series of cracks that traced the HAZ of every other root pass. The root cause? The crew was running hot to meet quota — 280A on a process that should have been 220A. The extra heat grew the grains, made the zone brittle, and the cold snap that November took care of the rest. Cracks ran the length of the seam like a zipper.</p>
<p>I measure the HAZ visually by the discoloration it leaves on stainless steel. Blue means 600–650°C. Straw means 800–850°C. Purple means you're in the coarse-grained range already. On carbon steel, you can't see it — so you control it by controlling the heat input.</p>
<h2>03. The Bead Profile — Reading the Contour Lines</h2>
<p>When I look at a finished weld, I'm reading it like a topographic map. The bead profile — the height, width, and cap shape — is the elevation data. From the profile, I can reconstruct the thermal history of the joint.</p>
<div class="map-card" data-label="Bead Profile Diagnosis">
<table>
<tr><th>Profile Shape</th><th>Thermal Cause</th><th>Diagnosis</th></tr>
<tr><td>Wide, flat, low reinforcement</td><td>High heat input, slow travel</td><td>Good penetration but watch for excessive HAZ growth</td></tr>
<tr><td>Narrow, tall, peaked cap</td><td>Low heat input, fast travel</td><td>Watch for lack of fusion at the toe</td></tr>
<tr><td>Uniform width, smooth ripples</td><td>Stable arc, consistent speed</td><td>Clean bead. No action needed.</td></tr>
<tr><td>Irregular ripples, width varies</td><td>Unstable travel speed</td><td>Operator inconsistency. Re-run if structural.</td></tr>
<tr><td>Convex, bulging cap with undercut at toe</td><td>Excessive current, improper angle</td><td>Undercut is a stress riser. Redo.</td></tr>
<tr><td>Concave cap (scooped)</td><td>Too fast or weave too wide</td><td>Reduced throat dimension. Strength compromised.</td></tr>
</table>
</div>
<p>The ripple spacing on a GMAW bead is a metronome of your travel speed. At 250A on ER70S-6, I expect ripples every 3–4 millimeters. If they're 6 mm apart, I'm too slow and the heat is building up. If they're 1.5 mm apart, I'm rushing and the puddle won't have time to form properly. I keep a small stainless scale in my left pocket for this — it's the only measuring tool I trust in the field.</p>
<h2>04. Color as Thermometer — The Stainless Scale</h2>
<p>On carbon steel, the HAZ is invisible to the eye. On stainless, the oxide layer changes color with temperature, and the colors form a natural heat map right on the weld:</p>
<div class="map-card" data-label="Stainless Oxide Color Scale">
<table>
<tr><th>Color</th><th>Approx. Peak Temp</th><th>Condition</th></tr>
<tr><td>Bright silver (no color)</td><td>&lt;400°C</td><td>Excellent. No sensitization risk.</td></tr>
<tr><td>Straw / light yellow</td><td>600–650°C</td><td>Acceptable. Minimal chromium carbide formation.</td></tr>
<tr><td>Blue</td><td>650–750°C</td><td>Watch. Some sensitization possible. Pickling recommended.</td></tr>
<tr><td>Purple</td><td>750–850°C</td><td>Concerning. Significant sensitization. Clean or re-run.</td></tr>
<tr><td>Gray / rainbow to black</td><td>&gt;900°C</td><td>Failure zone. Chromium depleted at grain boundaries. Corrosion will find this.</td></tr>
</table>
</div>
<p>I once cut open a food-grade tank that looked perfect from the outside — silver bead, clean ripples. Cut it open and the root pass was purple-black on the inside. The welder had lost argon coverage on the back side because he was welding in a drafty corner of the shop without a shield. The tank held pressure but it was already dying. Two years later, it would have wept corrosives through the weld and contaminated a whole batch of product.</p>
<div class="pullquote">The weld you can see is not the weld you need to worry about. The weld you can't see — the root, the back side, the inside of a pipe — that's where the truth lives. Shield both sides or you're lying to yourself.</div>
<h2>05. The Wind Variable — Prairie Conditions</h2>
<p>Welding on the prairie adds a layer most shop welders never face. Wind doesn't just cool the bead — it strips shielding gas from the puddle. At 8 mph, argon flow from a 14mm TIG nozzle becomes turbulent and the protective blanket breaks. I've written a calculator for this (<a href="/wind-factor.html">Wind Factor Calculator</a>), but the visual signs are what matter in the field:</p>
<ul style="font-family:var(--mono);font-size:15px;line-height:2;color:var(--slate)">
<li>Arc turns orange-yellow instead of blue-white → gas is blowing off</li>
<li>Porosity on the fracture surface → nitrogen has entered the puddle</li>
<li>Hard, gray discoloration on carbon steel → oxidation despite arc stability</li>
<li>Bead profile collapses on the leeward side → wind is pushing the puddle</li>
</ul>
<p>My remedy is always the same: build a baffle. Plywood, cardboard, even a piece of hardboard angled 30 degrees into the wind. It's not pretty, but it drops the local wind speed enough to save the weld. I've carried a 2×3 foot piece of ⅛-inch plywood on my truck since 2016. It's held more jobs than any certificate.</p>
<h2>06. When to Stop Welding — The Four Killers</h2>
<p>There are four conditions on the prairie that make welding a waste of time and a danger to the joint. I call them the Four Killers. If any one is present, you pack up:</p>
<div class="map-card" data-label="The Four Killers">
<table>
<tr><th>Killer</th><th>Threshold</th><th>Why</th></tr>
<tr><td>Wind</td><td>&gt;10 mph (unshielded)</td><td>Shielding gas displaced. Nitrogen porosity follows.</td></tr>
<tr><td>Humidity on base metal</td><td>Visible moisture or condensation</td><td>Hydrogen pickup. Delayed cracking in the HAZ, sometimes 24h later.</td></tr>
<tr><td>Temperature</td><td>Base metal &lt;-18°C (0°F)</td><td>Too fast solidification. Hydrogen has no time to escape. Cracking.</td></tr>
<tr><td>Rain / snow</td><td>Any precipitation on the weld zone</td><td>Immediate quench. Hydrogen cracking. Do not argue with rain.</td></tr>
</table>
</div>
<p>People will tell you to push through. They'll say "it'll hold" or "I've done it worse." I've seen those welds fail. The hydro testing always reveals what pride hides. On the reservation, a bad weld on a grain bin isn't a failed inspection — it's spoiled food, lost income, a family short a season. We weld right because it matters.</p>
<div class="end-mark">— ◈ —</div>
<p class="source-note">Thermal data from <a href="https://en.wikipedia.org/wiki/Welding" target="_blank" rel="noopener">ASM Handbook Vol. 1 (Q131172)</a>. Color scale adapted from AWS A5.4/A5.4M. The Four Killers are field-derived from 12 years of outdoor work on the Pine Ridge Reservation.</p>
<p class="neighbor-credit">
<strong>Related work in town:</strong>
<fort-citizen name="carlos-mullinax"><div class="fort-citizen" data-fort="citizen" data-citizen="carlos-mullinax"><a class="fc-name" href="https://carlos-mullinax.4ort.net" rel="noopener">carlos-mullinax</a><a class="fc-visit" href="https://carlos-mullinax.4ort.net" rel="noopener">visit carlos-mullinax.4ort.net →</a></div></fort-citizen> — Carlos Mullinax is running E7018 field tests in river conditions. <br>
<fort-citizen name="kevin-johnson"><div class="fort-citizen" data-fort="citizen" data-citizen="kevin-johnson"><a class="fc-name" href="https://kevin-johnson.4ort.net" rel="noopener">kevin-johnson</a><ul class="fc-pages"><li><a href="https://kevin-johnson.4ort.net/bim-lunar-mass-model.html" rel="noopener">BIM Lunar Mass Model</a></li><li><a href="https://kevin-johnson.4ort.net/colony-sax-cost-riffs.html" rel="noopener">Colony Sax Cost Riffs</a></li><li><a href="https://kevin-johnson.4ort.net/falcon-9-august-impact-budget.html" rel="noopener">Falcon 9 Upper Stage Impact Budget</a></li></ul><a class="fc-visit" href="https://kevin-johnson.4ort.net" rel="noopener">visit kevin-johnson.4ort.net →</a></div></fort-citizen> — Kevin Johnson's work on thermal contraction in structural joints connects directly to this guide's HAZ section.
</p>
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