238 lines
12 KiB
HTML
238 lines
12 KiB
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<body>
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<header>
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<nav><a href="index.html">← Home</a></nav>
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<fort-nav><nav class="fort-nav" data-fort="nav"><a href="/">Augusto Torres</a><a href="/films/precision-in-fabrication/">Index</a><a href="/blueprint-reading.html">Blueprint Reading for Fabricators</a><a href="/mole-negro.html">Mole Negro</a><a href="/welding-distortion.html" class="active" aria-current="page">Thermal Distortion in Welding</a></nav></fort-nav>
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<h1>Thermal Distortion in Welding</h1>
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<div class="subtitle">Shop Notes — Why Metal Warps and How to Stop It</div>
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</header>
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<main class="container">
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<section>
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<img class="hero" src="https://images.pexels.com/photos/4588610/pexels-photo-4588610.jpeg?auto=compress&cs=tinysrgb&dpr=2&h=650&w=940" alt="Grinding sparks flying from metal work">
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<p>Every fabricator hits this wall eventually: you lay down a perfect bead, let it cool, and the piece is twisted. The distortion wasn't in your hands — it was in the physics. Heat goes in one side, metal expands, heat leaves, metal contracts... and nothing lines up anymore.</p>
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<p>After two decades on the floor, here's what I've learned about keeping metal flat when the arc won't let you.</p>
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<div class="toc">
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<strong>Contents</strong>
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<ol>
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<li><a href="#why">Why Metal Warps</a></li>
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<li><a href="#types">Types of Distortion</a></li>
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<li><a href="#control">Control Techniques</a></li>
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<li><a href="#sequence">Weld Sequence Matters</a></li>
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<li><a href="#fixtures">Fixturing & Restraint</a></li>
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<li><a href="#post">Post-Weld Correction</a></li>
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</ol>
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</div>
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</section>
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<section id="why">
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<h2>Why Metal Warps</h2>
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<p>Metal expands when heated. That's not theory — that's what you feel under your fingers when a hot plate cools and pulls. The coefficient of thermal expansion for mild steel is roughly <strong>11.7 × 10⁻⁶ /°C</strong>. That small number compounds over the length of a weld.</p>
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<p>When you weld, the heat-affected zone (HAZ) gets to 2,500°F+ locally. The rest of the plate is still room temperature. That gradient is the enemy. As the weld pool solidifies and cools, it pulls inward — creating residual stress that distorts the part.</p>
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<div class="warn">A rule from the floor: if you can feel the plate get hot to the touch during welding, you're putting too much heat in. Slow the travel, drop the amperage, or add more breaks.</div>
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</section>
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<section id="types">
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<h2>Types of Distortion</h2>
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<div class="spec-block">
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<table>
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<tr><td>Angular</td><td>One side draws more than the other; the joint rotates. Most common in fillet and groove welds on thin plate.</td></tr>
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<tr><td>Longitudinal</td><td>The entire part shortens along the weld axis. Critical on long seams and plate assemblies.</td></tr>
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<tr><td>Lateral</td><td>The plate buckles perpendicular to the weld. Shows up as a banana curve.</td></tr>
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<tr><td>Wrinkling</td><td>Compression buckling in thin sheet metal — looks like a crumpled paper bag. Almost always a fixturing failure.</td></tr>
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</table>
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</div>
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<p>I've seen every one of these on the line. Angular distortion is the one that catches people most often — you think the joint is square, but measure it after cooling and you're 3-5 degrees off. That stacks up fast on a multi-pass assembly.</p>
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</section>
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<section id="control">
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<h2>Control Techniques</h2>
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<h3>Preheat</h3>
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<p>Bringing the whole assembly to a uniform temperature before welding reduces the gradient. For mild steel, 150-250°F preheat is standard. For thicker sections or higher-carbon steels, go higher. Preheat doesn't eliminate distortion — it makes the thermal profile more manageable.</p>
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<h3>Heat Input Management</h3>
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<p>Lower amperage, faster travel speed, and shorter beads all reduce total heat input. On TIG, this means running lean — 120A isn't always better than 80A with better technique. I've trained guys who thought more amperage meant faster work, and ended up scrapping twice as much.</p>
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<h3>Intermittent Welding</h3>
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<p>For long seams, don't run the full length. Stitch weld — 2 inches on, skip 4, go back and fill. This gives each segment time to cool and settle before the next pass. It looks messy but it holds flat.</p>
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</section>
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<section id="sequence">
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<h2>Weld Sequence Matters</h2>
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<p>This is where experience matters most. The order in which you lay welds determines how the distortion distributes.</p>
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<h3>Rule 1: Weld from the center outward</h3>
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<p>Starting in the middle and working both ways lets distortion cancel out symmetrically. Start at one end and walk the length, and you'll drag the whole part behind you.</p>
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<h3>Rule 2: Alternate sides on a joint</h3>
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<p>If you're welding both sides of a butt joint, do half on side A, flip, do half on side B. Each pass on one side pulls the part back toward center. Don't do all of side A then all of side B — you'll twist it like a wet rag.</p>
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<h3>Rule 3: Big to small</h3>
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<p>Weld the largest, highest-heat passes first. The smaller finishing passes add minimal distortion by comparison. Put the heavy work in while the part is still restrained and fresh.</p>
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</section>
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<section id="fixtures">
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<h2>Fixturing & Restraint</h3>
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<p>Clamps, jigs, and backing bars are your best defense. A properly fixtured part will resist distortion forces that would twist an unclamped one into junk.</p>
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<p>On the line, I use toggle clamps with soft jaws to avoid marring, backed by angle iron. For thin sheet (below 16 ga), I vacuum-bag onto a flat platen. Heavy stuff (1/2" and up) goes in a weld positioner where I can rotate and distribute heat evenly.</p>
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<div class="warn">Never weld without restraint on a precision part. If the jig takes longer to build than the weld, it's still faster than reworking a twisted part.</div>
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</section>
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<section id="post">
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<h2>Post-Weld Correction</h2>
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<p>Sometimes you need to correct what's already done. Two approaches:</p>
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<h3>Heat Straightening</h3>
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<p>Apply localized heat with an oxy-acetylene torch to the high side, let it cool. The contraction pulls the part back toward flat. Works best on mild steel. You need feel for this — too much heat and you're making it worse.</p>
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<h3>Mechanical Straightening</h3>
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<p>Press, hammer, or bend the part back. This works on small deflections but introduces new residual stress. It's a band-aid, not a cure. I'd rather do it right the first pass than spend an hour hammering a thing back into shape.</p>
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</section>
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<section>
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<h2>Source & References</h2>
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<p>Metal fabrication is a <a href="https://www.wikidata.org/wiki/Q4115400" target="_blank">recognized occupation (Wikidata Q4115400)</a> involving cutting, bending, and assembling processes. The thermal behavior of metals during welding is well-documented in metallurgical engineering literature.</p>
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<p class="backlink"><a href="index.html">← Back to home</a></p>
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<p>Augusto Torres · Shop notes from two decades on the floor · <a href="index.html">Home</a></p>
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