# Welding **Welding** is a fabrication process that joins two pieces of material, usually metals or [[Thermoplastic|thermoplastics]], by causing them to coalesce into one piece: the joint is heated until it melts, or pressed until the surfaces bond in the solid state, often with a filler added, so that the result is one continuous body rather than two parts held by a fastener or an adhesive. It differs from brazing and [[Solder|soldering]], where only a lower-melting filler melts. Every welding process is a way of delivering concentrated energy to a line on a workpiece, from a gas flame, an electric arc, a resistance-heated contact, an electron or [[Laser|laser]] beam, or friction, and every one leaves behind a fusion zone and a [[Heat-affected_zone|heat-affected zone]] whose properties differ from the metal around them. In the microsim below the reader drives a torch across a thick [[Steel|steel]] plate and changes its travel speed *v* (or its absorbed power *q*). The temperature field around the source is Rosenthal's moving point source, `T − T₀ = (q/(2·π·k·r))·exp(−v·(x + r)/(2·α))`, drawn live in the torch's frame; the 1,500 °C isotherm outlines the melt pool and the 723 °C isotherm the outer edge of the heat-affected zone, both shrinking in width and stretching into teardrops as the torch speeds up, with the width of the heat-affected band read out. On the [[Materials_science|Materials science]] flagship this article is the *Welding* section of Part IV, Fundamentals › Processing; the [[Gas_tungsten_arc_welding|gas tungsten arc welding]] page reuses the same field with its own power preset. ## Etymology The verb *weld* descends from the older verb *well* in the sense of heating a metal until it flows; the smith who "welled" two bars together at white heat was welding them before the modern spelling settled.[^oed] The forge weld it named is older than the word by two millennia, documented from the [[Iron_Age|Iron Age]] onward; the fusion processes below are inventions of the last century and a half.[^cary-helzer] ## History Until the nineteenth century the only weld was the blacksmith's: two pieces of iron heated to a sweating white heat, fluxed with sand, and hammered together on the anvil. The electric arc changed that. Nikolay Benardos and Stanisław Olszewski patented carbon-arc welding in the mid-1880s, Nikolay Slavyanov substituted a consumable metal electrode a few years later, and the coated electrode that made manual arc welding practical was patented by Oscar Kjellberg in the first decade of the twentieth century; in the same period Elihu Thomson developed resistance welding, Hans Goldschmidt's aluminothermic reaction gave the railways [[Exothermic_welding|thermite welding]], and the oxyacetylene torch appeared in France in 1903.[^cary-helzer][^lincoln] Submerged arc welding was introduced in the 1930s, gas tungsten arc welding in the early 1940s and gas metal arc welding in the late 1940s, each in effect a new way of shielding the molten pool from the air.[^lincoln] Electron-beam welding came out of the French atomic-energy programme in the late 1950s, the laser followed in the 1960s, and friction stir welding was invented at The Welding Institute in 1991.[^cary-helzer][^mishra-ma] Rosenthal's 1941 analysis of the moving heat source, the microsim's equation, dates from the middle of that sequence.[^rosenthal1941] ## Processes Welding processes are usually sorted by their energy source and by whether the parent metal melts. Fusion processes, gas, arc, resistance and beam welding, melt a pool that solidifies into the joint; solid-state processes bond without melting. In all of them the power absorbed, the speed of travel and the thermal properties of the metal fix how large the pool is and how fast the joint cools. ### Gas welding Oxy-fuel welding burns a fuel gas, usually acetylene, in [[Oxygen|oxygen]] to produce a flame hot enough to melt steel, with a filler rod fed by hand. The gas ratio sets the flame between carburizing, neutral and oxidizing, and the neutral flame neither adds carbon to the steel nor burns it out.[^cary-helzer] The process is slow and its heat diffuse, so it has largely given way to arc processes for fabrication, but the same torch cuts steel by burning it in a jet of oxygen and remains the portable process of the repair shop. ### Arc welding [[Arc_welding|Arc welding]] uses an electric arc between an electrode and the work as its heat source. In shielded metal arc welding the electrode is a consumable rod whose coating decomposes into a protective gas and a slag; in gas metal arc welding a bare wire is fed continuously through a torch under a [[Shielding_gas|shielding gas]] such as [[Argon|argon]] or carbon dioxide; in gas tungsten arc welding a non-consumable tungsten electrode carries the arc and filler is added separately; in submerged arc welding the arc runs beneath a blanket of granular flux. The arc's power is voltage times current, and the fraction absorbed by the work, the arc efficiency, has been measured calorimetrically at about 0.67 for gas tungsten arc, 0.84 for gas metal arc and 0.91 for submerged arc welding.[^dupont-marder] A gas metal arc at 250 A and 24 V therefore puts about 5 kW into the plate, the figure the microsim uses as its default *q*. [[Plasma_(physics)|Plasma]] arc welding constricts the arc through a nozzle. ### Resistance welding Resistance welding passes a large current through the parts to be joined and lets [[Joule_heating|Joule heating]] at the contact melt a nugget between them, while the electrodes squeeze the joint. Spot welding of sheet, the process that assembles car bodies, delivers thousands of amperes for a fraction of a second between copper electrodes; seam welding rolls the electrodes along the joint, and projection welding concentrates the current at embossed points.[^cary-helzer] No filler or shielding gas is needed, and the heat is generated exactly where the joint is. ### Energy beam welding Electron-beam and laser-beam welding focus energy into a spot small enough to vaporize a channel through the metal. The beam advances behind a keyhole of vapour, producing a deep, narrow weld with a heat-affected zone far smaller than an arc leaves.[^cary-helzer] Electron beams need a vacuum, which limits part size but gives a perfectly clean atmosphere; laser beams work in air or under shielding gas and are readily carried by fibre to a robot. The microsim's stretched, narrow isotherms at high *v* are the shape of a beam weld. ### Solid-state welding Solid-state processes bond without a molten pool. The forge weld is the oldest; friction welding spins one part against the other until the interface is hot and plastic, then forges them together; friction stir welding plunges a rotating pin into the seam of two plates and stirs the softened metal across the joint without melting it, which suits [[Aluminium|aluminium]] alloys that crack or lose strength when fusion welded.[^mishra-ma] Ultrasonic welding scrubs thin sheets or wires together at high frequency, and diffusion bonding holds clean surfaces under heat and pressure until atoms [[Diffusion|diffuse]] across the interface. Because nothing melts, these joints keep the wrought [[Microstructure|microstructure]] of the parent metal. ## Geometry Welds are described by the joint they close and the shape of the deposit. The five basic joints are butt, lap, corner, tee and edge, and thick sections are prepared with a square, V, U or J groove so the arc can reach the root. The weld itself is a groove weld, which fills a prepared joint, or a fillet weld, which lies in the corner of a lap or tee joint with a roughly triangular cross-section; the fillet's strength is set by its throat, the shortest distance from the root to the face. A welding symbol on a drawing encodes all of this on a reference line with an arrow to the joint. The geometry also sets the heat flow: a thick plate conducts heat away in three dimensions, which is the case Rosenthal's point-source solution and the microsim describe, while a thin sheet conducts in two and cools more slowly for the same heat input. ## Quality A weld is judged by the same properties as the parent metal, strength, toughness, [[Fatigue_(material)|fatigue]] life and [[Corrosion|corrosion]] resistance, and by the discontinuities that degrade them: porosity, slag inclusions, lack of fusion, undercut at the toe, and cracks. Most cracks trace back to the thermal cycle the microsim shows: the pool shrinks as it solidifies, the plate around it resists, and the joint is left with residual stress and, in hardenable steels, a brittle microstructure that the stress can crack. ### Heat-affected zone Around the fusion zone lies the heat-affected zone, the band of parent metal that never melted but was heated enough to change. Its extent is set by the temperature field, and for a point source moving at speed *v* across a thick plate Rosenthal solved the heat equation in closed form:[^rosenthal1941][^rosenthal1946] `T − T₀ = (q/(2·π·k·r))·exp(−v·(x + r)/(2·α))` Here *T*₀ is the plate temperature, *q* the absorbed power, *k* the [[Thermal_conductivity_and_resistivity|thermal conductivity]], α = k/(ρc) the [[Thermal_diffusivity|thermal diffusivity]], *x* the coordinate along the travel direction from the source and *r* the distance from it. The microsim's steel preset takes room-temperature handbook values for iron, k = 80 W/m·K,[^openstax-up2-1-6] c = 452 J/kg·K[^openstax-up2-1-4] and ρ = 7,800 kg/m³,[^openstax-up1-14-1] which give α = 2.3 × 10⁻⁵ m²/s (derived); the pool boundary is drawn at 1,500 °C, just below the 1,538 °C melting point of pure iron because carbon lowers it, and the outer edge of the heat-affected zone at 723 °C, the eutectoid temperature below which [[Pearlite|pearlite]] is stable.[^callister-fec] A carbon steel's conductivity is below iron's and changes with temperature, so the preset is ILLUSTRATIVE and the sim prints its constants on screen. Two consequences are worth reading off the screen. Directly behind the source, where `x = −r`, the exponential is 1 and the temperature falls as `q/(2·π·k·r)` whatever the speed: the length of pool trailing the torch, `q/(2·π·k·(T_m − T₀))`, does not depend on *v* at all. With q = 5 kW that is 6.7 mm for the 1,500 °C pool and 14.3 mm for the 723 °C edge (derived). The width does depend on *v*, because the exponential cuts the field off to the sides and ahead: solving for the widest point of each isotherm gives a pool half-width of 4.5 mm and a heat-affected edge at 7.6 mm at 5 mm/s, 3.7 and 5.9 mm at 10 mm/s, and 2.9 and 4.5 mm at 20 mm/s, so the heat-affected band on each side narrows from 3.1 to 2.3 to 1.6 mm as the speed quadruples (derived). Faster travel at the same power buys a narrower zone and a longer, teardrop-shaped pool. ### Lifetime extension with after treatment methods A welded joint's fatigue life is often shorter than the parent metal's, because the weld toe is a stress concentration standing in a field of tensile residual stress. Post-weld treatments attack both: grinding or re-melting the toe with a tungsten arc smooths the notch, and hammer, needle or ultrasonic impact peening deforms the toe to leave a compressive residual stress, the physics that shot peening applies to a [[Work_hardening|work-hardened]] spring. The International Institute of Welding publishes recommendations for these methods and the fatigue credit each may be given in design.[^iiw-haagensen] Post-weld heat treatment relieves residual stress and tempers hard microstructure in the heat-affected zone. ## Metallurgy The fusion zone is a small casting: crystals grow into the pool from the unmelted grains at its boundary and follow the solidification front as columnar grains that meet at the centreline, where segregation and porosity collect.[^cary-helzer] The heat-affected zone of a [[Carbon_steel|carbon steel]] is a sequence of [[Heat_treating|heat treatments]] laid side by side: next to the fusion line the metal was heated far into the [[Austenite|austenite]] field and its grains coarsened, farther out it was just austenitized and its grains refined, and beyond the 723 °C isotherm it was merely tempered or, if it had been cold-worked, [[Recrystallization_(metallurgy)|recrystallized]] and softened. How fast each band cools decides what it becomes, and Rosenthal's equation gives that too. Differentiating the centreline field with respect to time, using `dx/dt = −v`, gives the cooling rate at temperature *T* as `2·π·k·(v/q)·(T − T₀)²` (derived). At 550 °C, near the nose of the pearlite transformation, the 5 kW steel preset cools at about 140 K/s at 5 mm/s and 280 K/s at 10 mm/s (derived): doubling the speed doubles the cooling rate, and doubling the power halves it. A hardenable steel cooled that fast forms [[Martensite|martensite]] in its heat-affected zone, hard, brittle and, if [[Hydrogen|hydrogen]] from moist flux has dissolved in the pool, prone to cracking hours after the weld is finished. The countermeasures follow from the equation: preheating raises *T*₀ and lowers the cooling rate at every temperature, a higher heat input per unit length `q/v` does the same, and dry, low-hydrogen consumables starve the crack. Austenitic [[Stainless_steel|stainless steels]] do not harden but can sensitize to intergranular corrosion, and aluminium alloys lose their [[Precipitation_hardening|precipitation-hardened]] temper wherever the weld heat over-ages them. ## Unusual conditions Welding is done in water, in vacuum and in the field. Underwater welding is either wet, with waterproofed electrodes, or dry, inside a hyperbaric chamber sealed around the joint; the quenching water raises cooling rates far above those on the microsim's plate, and the American Welding Society's underwater welding code classifies the resulting welds by the service they may be trusted with.[^aws-d36] Electron-beam welding already runs in vacuum, which makes it the natural process for orbit. On pipelines in the open, wind strips shielding gas from the arc and cold steel raises cooling rates, so field procedures specify wind shields and preheat. ## Safety issues The arc is an intense source of ultraviolet light: seconds of unprotected exposure inflame the cornea, the "arc eye" that a filtered helmet prevents. Welding fume, the fine oxide particulate condensed from metal vapour, is the other principal hazard. Manganese from steel consumables and hexavalent [[Chromium|chromium]] from stainless steel are the constituents of most concern, and in 2017 the International Agency for Research on Cancer classified welding fumes as carcinogenic to humans.[^iarc118] Fume extraction at the source is the standard control.[^niosh] Compressed gases and flashback in oxy-fuel work, [[Ozone|ozone]] from the arc's ultraviolet light, electric shock and fire from smouldering spatter complete the list. ## Costs and trends The cost of a weld is dominated by labour, and the arithmetic that welding engineers use divides it by the deposition rate of the process and the operator factor, the fraction of the shift during which the arc is actually burning.[^lincoln] That arithmetic explains the trade's direction: wire-fed processes displaced the stick electrode because the arc need not stop to change rods, and robotic welding raises the operator factor toward unity. Higher travel speed cuts cost per metre directly, and the microsim shows the price: a narrower heat-affected zone but a faster cooling rate and the metallurgical risks that come with it. ## Plastic welding Thermoplastics weld because they soften reversibly above their [[Glass_transition|glass transition]] or melting range: heated surfaces pressed together interdiffuse their [[Polymer|polymer]] chains and, on cooling, are one piece. Hot-gas welding with a filler rod mirrors gas welding of metal, hot-plate welding melts two faces against a heated tool before pressing them together, and ultrasonic and laser welding deliver the heat through the part to the interface. Thermosets, whose cross-linked networks char rather than flow, cannot be welded. ### Solvent welding Solvent welding replaces heat with a solvent that swells and softens the polymer at the joint. Pressed together while soft, the surfaces interdiffuse, the solvent evaporates, and the joint hardens into a continuous polymer bridge. Solvent cements for PVC pipe, formulated with dissolved polymer to fill the gap, are specified by standard and are the reason plastic plumbing is assembled without a flame.[^astm-d2564] ## See also - [[Heat-affected_zone]] - [[Gas_tungsten_arc_welding]] - [[Exothermic_welding]] - [[Arc_welding]] - [[Thermal_conduction]] - [[Heat_treating]] - [[Work_hardening]] - [[Solder]] ## References [^rosenthal1941]: Rosenthal, D. (1941). "Mathematical theory of heat distribution during welding and cutting." *Welding Journal* 20 (5): 220s–234s. (No DOI; not a Portal Book; page to pin against the journal record.) [^rosenthal1946]: Rosenthal, D. (1946). "The theory of moving sources of heat and its application to metal treatments." *Transactions of the ASME* 68: 849–866. (No DOI; page to pin against the journal record.) [^openstax-up2-1-6]: Ling, S. J.; Sanny, J.; Moebs, W. (2016). *University Physics Volume 2*. OpenStax (Portal Book 078). Chapter 1, Temperature and Heat, §1.6 Mechanisms of Heat Transfer, Table 1.5 (thermal conductivities), chapter pp. 17–74 (table page to pin). https://openstax.org/books/university-physics-volume-2/pages/1-6-mechanisms-of-heat-transfer [^openstax-up2-1-4]: Ling, S. J.; Sanny, J.; Moebs, W. (2016). *University Physics Volume 2*. OpenStax (Portal Book 078). Chapter 1, §1.4 Heat Transfer, Specific Heat, and Calorimetry, Table 1.3 (specific heats), chapter pp. 17–74 (table page to pin). https://openstax.org/books/university-physics-volume-2/pages/1-4-heat-transfer-specific-heat-and-calorimetry [^openstax-up1-14-1]: Ling, S. J.; Sanny, J.; Moebs, W. (2016). *University Physics Volume 1*. OpenStax (Portal Book 077). Chapter 14, Fluid Mechanics, §14.1 Fluids, Density, and Pressure, Table 14.1 (densities), chapter pp. 665–720 (table page to pin). https://openstax.org/books/university-physics-volume-1/pages/14-1-fluids-density-and-pressure [^callister-fec]: Callister, W. D.; Rethwisch, D. G. (2010). *Materials Science and Engineering: An Introduction*, 8th ed. Hoboken: Wiley. Chapter 9, Phase Diagrams, the iron–iron carbide phase diagram (eutectoid temperature and the melting point of iron). Not a Portal Book; page to pin. [^dupont-marder]: DuPont, J. N.; Marder, A. R. (1995). "Thermal efficiency of arc welding processes." *Welding Journal* 74 (12): 406s–416s. (No DOI; page to pin against the journal record.) [^mishra-ma]: Mishra, R. S.; Ma, Z. Y. (2005). "Friction stir welding and processing." *Materials Science and Engineering: R: Reports* 50 (1–2): 1–78. https://doi.org/10.1016/j.mser.2005.07.001 [^cary-helzer]: Cary, H. B.; Helzer, S. C. (2005). *Modern Welding Technology*, 6th ed. Upper Saddle River, New Jersey: Pearson Prentice Hall. Chapter 1 (history and survey of processes) and the process chapters. Not a Portal Book; page to pin. [^lincoln]: The Lincoln Electric Company (2000). *The Procedure Handbook of Arc Welding*, 14th ed. Cleveland: Lincoln Electric. Section 1 (historical development of fusion joining) and the cost-estimating section. Not a Portal Book; page to pin. [^iiw-haagensen]: Haagensen, P. J.; Maddox, S. J. (2013). *IIW Recommendations on Methods for Improving the Fatigue Strength of Welded Joints* (IIW-2142-110). Cambridge: Woodhead Publishing for the International Institute of Welding. Not a Portal Book; page to pin. [^aws-d36]: American Welding Society. *AWS D3.6M, Underwater Welding Code*. Miami: American Welding Society. (Edition year and clause to pin.) [^iarc118]: IARC Working Group on the Evaluation of Carcinogenic Risks to Humans (2018). *Welding, Molybdenum Trioxide, and Indium Tin Oxide*. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Volume 118. Lyon: International Agency for Research on Cancer. (Page to pin.) [^niosh]: National Institute for Occupational Safety and Health (1988). *Criteria for a Recommended Standard: Welding, Brazing, and Thermal Cutting*. DHHS (NIOSH) Publication No. 88-110. Cincinnati: NIOSH. (Page to pin.) [^astm-d2564]: ASTM International. *ASTM D2564, Standard Specification for Solvent Cements for Poly(Vinyl Chloride) (PVC) Plastic Piping Systems*. West Conshohocken, Pennsylvania: ASTM International. (Edition year to pin.) [^oed]: *Oxford English Dictionary*, s.v. "weld, v." and "well, v." Oxford University Press. (Entry date to pin.) ### Sources - Ling, S. J.; Sanny, J.; Moebs, W. (2016). *University Physics Volume 2*. OpenStax. Portal Book 078. https://open.umn.edu/opentextbooks/textbooks/university-physics-volume-2 - Ling, S. J.; Sanny, J.; Moebs, W. (2016). *University Physics Volume 1*. OpenStax. Portal Book 077. https://open.umn.edu/opentextbooks/textbooks/university-physics-volume-1 - Rosenthal, D. (1941, 1946). The moving-heat-source papers cited above. Not Portal Books. - Cary, H. B.; Helzer, S. C. (2005). *Modern Welding Technology*, 6th ed. Pearson Prentice Hall. Not a Portal Book. - The Lincoln Electric Company (2000). *The Procedure Handbook of Arc Welding*, 14th ed. Not a Portal Book. - Callister, W. D.; Rethwisch, D. G. (2010). *Materials Science and Engineering: An Introduction*, 8th ed. Wiley. Not a Portal Book. <!-- MATTERSIM:BEGIN g24 — Matter & Energy Cluster microsim (framework build, specs/sims/Welding.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework), pending deploy:** *Welding* will play here once `https://wikitube-3d-microsims.netlify.app/matter/Welding.html` is live. <!-- pending: <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/matter/Welding.html" data-title="Welding"></div> --> <!-- MATTERSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Welding) : [Wikitube](https://en.wikitube.io/wiki/Welding) · pinned revision [1370204962](https://en.wikipedia.org/w/index.php?oldid=1370204962) · 2026-09-11 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Materials_science]]. --- *Matter & Energy Cluster child articles, wave 1 · 2026-09-11 · drafted · Materials_science row M21 · sim pending (matter/Welding).*