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tldr: The scope of this question is potentially larger than it appears. Disadvantages apply to personal, hobby, research-level soldering such as prototype design by engineers. Disadvantage also apply to industrial production, all industries involving electronics.Disadvantages of lead-free solders:The flux core in the wire and lead-free fluxes contain harsh reducing agents that are highly irritating to wet membranes like the sinuses and the eyesThe flux used in lead-free soldering has a shelf life because exposure to oxygen reduces the effectiveness of the core in the solder. For this reason, many solder companies have introduced expiration dates for their flux-core solders instead of just printing the DOM (date of manufacture).lead-free solder is mostly, tin, silver, copper :: Sn, Ag, Cu, or other metals. The temperature required to get a lead-free solder alloy into its molten, “eutectic”* state which allows for proper wetting is greater than that required for soldering with Sn60Pb40 or Sn63Pb37 Lead-tin solder; significantly hotter in centigrade or Fahrenheit (C or F).Table source linkGood old Sn60Pb40 (or Sn63Pb37) undergoes a phase change from solid to liquid (molten) at a [specific] eutectic point. 183 degrees C (highlighted in yellow)SAC (SnAgCu305) is likely the most common lead-free solder. SAC305 is a lead-free alloy that contains 96.5% tin, 3% silver, and 0.5% copper. The phase change from solid to liquid occurs over a range (217–220)The phase change from solid to liquid is not at a single [eutectic] point or temperature value for this lead-free alloy, SAC 305, and so is technically a non-eutectic solder.Wetting is a property of liquids. Wetting is crucial to the formation of acceptable or superior solder joints. Wetting occurs as a phase change from solid to liquid (flux is crucial for this wetting as well), then heating it to the eutectic temperature.Flux allows for heated solid solder to wet and be wet in open air. This is similar to how surfactants such as soaps help water (liquid) become “wetter.”Flux helps solder wet properly, as the solder moves to the two surfaces being heating temporarily with the soldering iron tip.The less-than-desirable “eutectic range” for the alloy itself can account forless than ideal wettinga different appearance from other metallic alloysLead-free solders also differ in their greater surface tension, specifically, and which can be seen exaggerated in specialty solder alloys such as Sn96Ag04, tin solder or Silvergleem. This is a solder alloy used for jewelry, for example. The surface tension of lead-free solders results in playfulness on the tip, flux is essential (but caustic or corrosive)technique must be modified, if accustomed to normal electronicslead-free solder will require specialized, (upgraded) tips, especially if the tip temperature is determined by the model number of the tip (e.g. Metcal brand), and not a variable temperature dial, tips which could be expensive to replace lead-solder tipsthere is a good amount of reason to consider that the higher temperatures required for making good solder joints with lead-free solder could be responsible for the premature failure of active semiconductor components as a trend, some of which have low temperature exposure tolerances.When soldering is done with leaded-solder, there is somewhat of a correlation between the qualitative appearance of the joint (shiny, smooth) and the electrical conductivity and mechanical strength, the two most important qualities provided by Mr. solder joint.When lead-free solder is used, appearance and qualities like “shininess” are not really an indication of a good solder joint.If you are totally new to soldering and you have a lot of it to do (for example, to complete a large engineering project that will determine your final grade)you may not learn how to solder sufficiently well if you use lead-free when learning to solder. (i.e., don’t waste your time!!)This situation would be made worse if one is also attempting to learn using lead-free solder that is old, expired, oxidized or using lead-free flux that is expired.It would be worse or least ideal to the also use old, oxidized soldering tip(s) and/or a soldering iron lacking power in Wattage available, or temperature prevision.Learning what good soldering looks like, through first-hand experience is easiest achieved by using the old leaded solder, it is the starting point for soldering (even with mandated lead-free ROHS in industry) and how to adjust your technique to the idiosyncratic demands of lead-free alloysIn addition, the inability to make good solder joints leads to other issues:hidden resistances in solder joint(s) and which become worse over timesolder joints so poor that they begin to act as thermal intermittentscold, dry joints that are not conductive as they should be and lack mechanical strengthtrying to troubleshoot the source of an issue when the problem was that one or two solder joints on an IC chip simply do not have a good electrical connection (in terms of conductivity) to the circuit because of a lack of experience in spotting defective solder joints when using lead-free. This could lead to hours pr days of frustration and running in circles when circuit analysis won’t help you figure it out because it could be one crappy solder joint.The alternative:Use leaded solder for your projectsWash your hands every time you touch solder, just as you do after say, doing woodwork or working on a car.Do not eat, drink, smoke, apply cosmetics, till you wash your hands thoroughly and you are done soldering, just like doing any DIY project.Keep your area clean and out-of-reach to children or pets. Wipe up your work area and static-dissipative mat to clean up solder balls and bits.Use a solder (lead-waste) container, or substitute for one, e.g., a medium sized coffee can where contaminated items can be dumped (when they are cool).Contain leaded solder waste such as solder balls and bits in that container such as solder wick, brass wool for conditioning your solder tips, bad components, etc.Dispose of this waste at electronic-recycling collection events or contact your waste management company.Unless you are a business that requires so much soldering that you would be classified as a small, medium, or large waste-generator:you don’t have to use lead-free. Doing so doesn’t mean you are more conscientious or a “better person.” Simply manage the handling and disposal of the waste, just like you do when you change the oil in your car.Alloys containing lead versus lead saltsSolder as an alloy is composed of tin and lead and which has a super-strong outer surface layer of oxidation.Solid solder, as in the case of solder wire, has an oxidative coating on the surface. This coating results from exposure to ambient air which contains oxygen that bonds to surface atoms. The oxidative coating on metals may be held strongly. For example, the oxidative coating on aluminum is an invisible barrier that makes it corrosion proof.Years ago, certain products had lead incorporated into them, notably in indoor house paint. The paint contained lead, but not as metal but as lead salts, or molecular compounds containing lead. If ingested, as paint chips that eroded off walls and eaten by infants, the lead would be easily digested and easily absorbed by the body as toxic, heavy metals.Is it possible to handle pure lead (Pb) metal slabs and get some on the hands? Then not wash one’s then eat a bag of popcorn? Pure lead is “soft” and can rub off on hands but solder is mostly tin, not pure lead metal.Leaded solder is best to learn. In fact, if you paid money to learn from a training organization that offers certifications, you would be trained using leaded solder.They don’t have the time and patience for you to not show some good results from training.Leaded solder is how people have learned soldering and continue to learn to solder correctlyedit, in case this question was meant for industry, I touch upon here, a case where the rubber meets the road regarding lead-free versus leaded solder allows used in automated production:An ideal solder joint with have two (2) key characteristics:good electrical conductivity that results from the proper reflow and wetting within a non-oxidized and contaminant free solder jointmechanical strength of the solder joint that connects two different surfaces togetherIn electronics, through-hole components and through-hole board designs were dominant. The processes for soldering through-hole components into plated through-hole barrels (PTH) that transect the board, were highly reliable yet they were based on lead-containing alloys.Wave machines solder pots, utilizing molten solder pools, to solder all the components to a board well, reliably, and verifiable quickly by visual inspection, were the standard.That said, currently it is not feasible or even possible to maintain pools of molten lead-free solder, using the lead-free alloys currently available.to be clear: It is possible to create a molten pool that is restricted in area during the process of selective soldering, for brief periods, while under or inside a non-oxidative environment. The process is done in a nitrogen gas saturated system and not done in ambient air. I could be done under argon Ar(l) a completely unreactive or noble, gas.here is a wonderful video the demonstrates selective soldering: link I know the video is 10 yrs old but selective soldering for lead-free processes still looks similar to that today in 2020.it’s optional, premium technologyMost electronics today are fabricated using surface mount technology or SMT technology. The components are attached only to the surface of a board, and not through it.SMT technology utilizes solder paste, distributed in precise volumes onto the board surface using stencils and a squeegee. Components (SMD or surface mount devices) are tacked onto a board using a pick and place machine (thousands of them and teeny-tiny). The solder paste provides enough tackiness (stickiness) to hold the components in place and during convection reflow. This process for surface mount components works using lead-free solder pastes works generally well.The exception is for components that are so important that they must be soldered into a board designed for through-hole technologies using plated through-holes and this regards many I/O connectors that provide, for example, many functions or many signal channels and many small pins within many holes.Because molten solder pools are not great with lead-free alloys, such as with wave-soldering machines, many manufacturers must now rely on a process called “intrusive soldering.”Intrusive soldering is the use of surface mount convection reflow ovens to solder through-hole components, while still using SMT-purposed solder paste. Intrusive soldering is also called Pin-in-Paste (PIP) or Pin-in-hole-paste (PIHP) soldering. This is done when waves [of molten solder] were used previously for those. Intrusive soldering is a new approach to soldering based on, and also resulted from, RoHS (lead-free) prescribed protocol.Source, images: http://www.ami.ac.uk/courses/topics/0226_pip/index.htmlNotice the concave shapes (versus convex) or bumpy appearance to the solder. Lead-free solders tend to have greater surface tension. Often will also see darker solder at the surface, usually as partially reflowed solder paste when everything out of a wave machine looks as shiny metal that clearly reached a state of liquidus, displaying properties of wetting and of solder that reached its eutectic temperature—the point or narrow temperature range for phase change; solid becomes liquid.Intrusive soldering however, is mostly experimental since each process and oven temperature profile must be tweaked for every particular assembly. It is an ad hoc process, mostly trial-and-error—tweaking or adjusting oven temperature profiles to produce desirable outcomes. Often the first production run of a single circuit board with these connectors will be defective and scrapped (something now expected), and the oven temperatures tweaked on the next run.Due to the experimental nature of intrusive soldering, agreement by convention will be difficult to standardize or attain.Not everyone agrees that Pin-in-Paste (PIP) or Pin-in-hole-paste (PIHP), aka intrusive soldering, can allow for standardized improvements or even produce acceptable solder joints (results similar to those allowed for previously with leaded solders) with the currently available alloys.For this reason, through-hole connectors must be soldered by hand for lead-free alloys and cannot depend on convection reflow methods whenever a connector is used in a very important circuit board, especially in the case of mission-critical class-3 circuit boards, such as those placed in satellites which are very difficult to service once they’re in orbit. For this reason, intrusive soldering has not yet been accepted by IPC as a reliable method that can be standardized. Instead, IPC urges direct oversight by an astute soldering technician/process engineer and the onus is on them to get the process right by tweaking it for every individual assembly, but for class-2 boards (consumer level) at best.There are some perceived advantages to cost, to using this unconventional means of installing through-hole components with convection reflow of solder pasted just as a reduction in the number of process steps, taken with a grain of salt because the solder joints created with intrusive soldering simply cannot reach target criteria, but only acceptable conditions at best.A solder joint can be Target (nearly perfect), or Acceptable (not too shabby), or non-conforming or Defective.Only hand-soldering of through hole components with lead-free solder wire and not paste, can allow for Target solder joints.Intrusive soldering of through-hole components using solder paste in a convection reflow process:results in “Acceptable” solder joints, at bestmore easily allows for non-conforming characteristics (solder joints with insufficient solder, solder joints with voids or pockets of air, cold (non-reflowed) solder jointscomes with a known trade-off of having solder joints that do not have the same conductivity and mechanical strength of through-hole components installed with leaded solders (but may suffice for say, cheap consumer electronics).Soldering through-hole components with lead free solder paste and using SMT convection reflow is often problematic compared to the older process which was highly consistent, highly reliable in producing target solder joints.There was a huge advantage to established processes involving lead-containing solders and technologies, but these are no longer in use whenever lead-free alloys are required, and also because of the different chemistries involved with leaded-and lead free solders.The closest thing today for lead-free solders to the past wave technology with leaded solder, is called selective soldering. Selective soldering requires a specialized machine with cameras and PC, but also cooling with liquid nitrogen. So it add cost to the process where arguably the electronics don’t have to be 100% perfect in aesthetics as long as they’re 100% functional.The disadvantage was that lead-waste in electronics is not and was not being processed as waste, properly, only an estimated 15% of old electronics is recycled appropriately.This is an example of a challenge or disadvantage presented by the transition from leaded to lead-free solders.edit: One other big disadvantage to lead-free solder alloys that should be mentioned involves the formation of microscopic metallurgical “whiskers,” more specifically “tin whiskers” and also “zinc whiskers” that have been increasingly associated with lead-free solder alloys.Image:Image, same source:The formation of these whiskers is as yet not well-understood. What is clear from just the images above is that these develop over time and essential create tiny short-circuit or bridging conditions, instead of discrete, isolated components.For this other reason, highly and critically important soldering electronics does not use lead-free solders. The best examples of highly critical soldering involves Class 3 electronics [mentioned above] (mission-critical electronics with absolutely the lowest possible fail rates, such as in jet planes, defense applications, rockets and satellites, other examples follow), which still utilize lead-based alloys still, because of the tin-whisker problem correlated with lead-free solder alloys in electronics.[Effects of]:Whiskers [Wikipedia] can cause short circuits and arcing in electrical equipment. The phenomenon was discovered by telephone companies in the late 1940s and it was later found that the addition of lead to tin solder provided mitigation.[4]The European Restriction of Hazardous Substances Directive (RoHS), that took effect on July 1, 2006, restricted the use of lead in various types of electronic and electrical equipment. This has driven the use of lead-free alloys with a focus on preventing whisker formation, see § Mitigation and elimination. Others have focused on the development of oxygen-barrier coatings to prevent whisker formation.[5]Airborne zinc whiskers have been responsible for increased system failure rates in computer server rooms.[6]Zinc whiskers grow from galvanized (electroplated) metal surfaces at a rate of up to a millimeter per year with a diameter of a few micrometres. Whiskers can form on the underside of zinc electroplated floor tiles on raised floors due to stresses applied when walking over them; and these whiskers can then become airborne within the floor plenum when the tiles are disturbed, usually during maintenance. Whiskers can be small enough to pass through air filters and can settle inside equipment, resulting in short circuits and system failure.Tin whiskers don't have to be airborne to damage equipment, as they are typically already growing in an environment where they can produce short circuits. At frequencies above 6 GHz or in fast digital circuits, tin whiskers can act like miniature antennas, affecting the circuit impedance and causing reflections. In computer disk drives they can break off and cause head crashes or bearing failures. Tin whiskers often cause failures in relays, and have been found upon examination of failed relays in nuclear power facilities.[7] Pacemakers have been recalled due to tin whiskers.[8]Research has also identified a particular failure mode for tin whiskers in vacuum (such as in space), where in high-power components a short-circuiting tin whisker is ionized into a plasma that is capable of conducting hundreds of amperes of current, massively increasing the damaging effect of the short circuit.[9]The possible increase in the use of pure tin in electronics due to the RoHS directive drove JEDEC and IPC to release a tin whisker acceptance testing standard and mitigation practices guideline intended to help manufacturers reduce the risk of tin whiskers in lead-free products.[10]Silver whiskers often appear in conjunction with a layer of silver sulfide which forms on the surface of silver electrical contacts operating in an atmosphere rich in hydrogen sulfide and high humidity. Such atmospheres can exist in sewage treatment and paper mills.(Another such atmosphere that is high is hydrogen sulfide gases and involve humidity is at the deep sea vents at the bottom of the ocean. So very small deep-sea submarines or so submersibles such as the ALVIN may also be susceptible.)Whiskers over 20 µm in length were observed on gold-plated surfaces and noted in a 2003 NASA internal memorandum.[11]The effects of metal whiskering were chronicled on History Channel's program Engineering Disasters 19.image from the link below:Doctoral student unravels 'tin whisker' mysteryYong Sun, a mechanical engineering doctoral student at the University of South Carolina's College of Engineering and Computing, has solved part of the puzzle.…Sun's findings were published in the Scripta Materialia, a materials science journal. This fall he won the prestigious Acta Student Award, one of only six to receive the honor. A team of editors from Acta Materials, Scripta Materialia and Acta Biomaterials evaluated the applications and Sun beat out students from the world's top universities, including MIT."This shows that our research in material science is reaching an international audience," Sun said. "It is nice to be recognized for our work."The importance of that work goes well beyond extending the operating life of consumer electronics. NASA has verified multiple commercial satellite failures it attributes to tin whiskers. Missile systems, nuclear power stations and heart pacemakers also have fallen victim to tin whiskers over the past several decades and they are also considered a suspect in reported brake failures in Toyota vehicles.While manufactures had been able to control some whiskers by mixing small amounts of lead into tin solder, the 2006 European Union ban on lead in most electronic equipment had ignited a debate among scientists about whether whiskers would remain a perpetual problem. Some observers even predict that it's only a matter of time before miniature devices built after the ban start failing en masse.Xiaodong Li, a professor in USC's Department of Mechanical Engineering who served as an adviser on the research, said Yong's work likely will prompt manufacturer to design lead-free products that diffuse stress."This (research) is a very big deal. As we move toward nano-scale devices, this is a problem that needs to be solved," Li said.Read more at: https://phys.org/news/2012-12-doctoral-student-unravels-tin-whisker.html#jCpFinally, it should be noted that IPC, the industry standard setting organization for electronic solder joints, has received and documented examples of 63Sn/37Pb solder with tin whiskers, provided by documented cases from NASA and Goddard Space Flight.So even the usual solder used for aerospace application has been demonstrated to present a potential for tin whiskers if the unit was ultimately retrievable.My first experiences with soldering did not include any mention of safety or the implications of heavy metals, but played with my elder brother’s soldering equipment when he took a course on electronics. I played with solder casually, didn’t even wash my hands, so I look back and wondered if I had been exposed to more-than-healthful levels of lead.There are many threats that are toxic to biological life forms, many of which are never publicized or addressed. The article I recently added mentions that there have been few or singular solutions to mitigate the problem of tin whiskers, besides adding small amounts of lead to mitigate tin whiskers and device failure in the field.Tin whiskers have also been documented by NASA for aerospace electronics on Sn63Pb37 alloy, the standard military/aerospace solder alloy, in very-long-service satellites.It’s “two steps forward, one step back,” (adding lead minimizes the problem better than any alternative)while the good intentions may be misplaced in the first place and possibly a hindrance to engineering pursuits. In the case of cardiac pacemakers, I would think that lead has a sufficiently good reason to be used, perhaps with special insulation, polymeric, or conformal coatings, the RoHS directive in medical devices may be overly rigid when exceptions should be made, such as with pacemakers.Thank you for reading

Well the first making of steel was recorded in India in 400 BCBut the most important process of steel making is bessemers process made by Henry Bessemers an english inventor in the 19th century.Read these if you are interested or skipHenry Bessemer - WikipediaThe Entire History of SteelThe story of steel begins long before bridges, I-beams, and skyscrapers. It begins in the stars.Billions of years before humans walked the Earth—before the Earth even existed—blazing stars fused atoms into iron and carbon. Over countless cosmic explosions and rebirths, these materials found their way into asteroids and other planetary bodies, which slammed into one another as the cosmic pot stirred. Eventually, some of that rock and metal formed the Earth, where it would shape the destiny of one particular species of walking ape.On a day lost to history, some fortuitous humans found a glistening meteorite, mostly iron and nickel, that had barreled through the atmosphere and crashed into the ground. Thus began an obsession that gripped the species. Over the millennia, our ancestors would work the material, discovering better ways to draw iron from the Earth itself and eventually to smelt it into steel. We’d fight over it, create and destroy nations with it, grow global economies by it, and use it to build some of the greatest inventions and structures the world has ever known.Metal From HeavenKing Tut had a dagger made of iron—a treasured object in the ancient world worthy of few more than a pharaoh. When British archaeologist Howard Carter found Tutankhamun’s tomb nearly a century ago and laid eyes on this object, it was clear the dagger was special. What archaeologists didn’t know at the time was that the blade came from space.Tutankhamun’s meteoric iron dagger.POLYTECHNIC UNIVERSITY OF MILANIron that comes from meteorites has a higher nickel content than iron dug up from the ground and smelted by humans. In the years since Carter’s big discovery, researchers have found that not only King Tut’s dagger but also virtually all iron goods dating to the Bronze Age were made from iron that fell from the sky.To our ancestors, this exotic alloy must have seemed like it was sent by entities beyond our understanding. The ancient Egyptians called it biz-n-pt. In Sumer, it was known as an-bar. Both translate to “metal from heaven.” The iron-nickel alloy was supple and easily hammered into shape without breaking. But there was an extremely limited supply, brought to Earth only by the occasional extraterrestrial delivery, making this metal of the gods more valuable than gems or gold.It took thousands of years before humans started looking beneath their feet. Around 2,500 BC, tribesmen in the Near East discovered another source of dark metallic material hidden underground. It looked just like the metal from heaven—and it was, but something was different. The iron was mixed with stones and minerals, lumped together as ore. Extracting iron ore wasn’t like picking up a stray piece of gold or silver. To remove iron from the subterranean realms was to tempt the spirit world, so the first miners conducted rituals to placate the higher powers before digging out the ore, according to the 1956 book The Forge and the Crucible.But pulling iron ore from the Earth was only half the battle. It took the ancient world another 700 years to figure out how to separate the precious metal from its ore. Only then would the Bronze Age truly end and the Iron Age begin.The Long Road to the First SteelTo know steel, we must first understand iron, for the metals are nearly one and the same. Steel contains an iron concentration of 98 to 99 percent or more. The remainder is carbon—a small additive that makes a major difference in the metal’s properties. In the centuries and millennia before the breakthroughs that built skyscrapers, civilizations tweaked and tinkered with smelting techniques to make iron, creeping ever closer to steel.Around 1,800 BC, a people along the Black Sea called the Chalybes wanted to fabricate a metal stronger than bronze—something that could be used to make unrivaled weapons. They put iron ores into hearths, hammered them, and fired them for softening. After repeating the process several times, the Chalybes pulled sturdy iron weapons from the forge.MICHAEL STILLWELLWhat the Chalybes made is called wrought iron, one of a couple major precursors to modern steel. They soon joined the warlike Hittites, creating one of the most powerful armies in ancient history. No nation’s weaponry matched a Hittite sword or chariot.Steel’s other younger sibling, so to speak, is cast iron, which was first made in ancient China. Beginning around 500 BC, Chinese metalworkers built seven-foot-tall furnaces to burn larger quantities of iron and wood. The material was smelted into a liquid and poured into carved molds, taking the shape of cooking tools and statues.Neither wrought nor cast iron was quite the perfect mixture, though. The Chalybes’ wrought iron contained only 0.8 percent carbon, so it did not have the tensile strength of steel. Chinese cast iron, with 2 to 4 percent carbon, was more brittle than steel. The smiths of the Black Sea eventually began to insert iron bars into piles of white-hot charcoal, which created steel-coated wrought iron. But a society in South Asia had a better idea. India would produce the first true steel.Around 400 BC, Indian metalworkers invented a smelting method that happened to bond the perfect amount of carbon to iron. The key was a clay receptacle for the molten metal: a crucible. The workers put small wrought iron bars and charcoal bits into the crucibles, then sealed the containers and inserted them into a furnace. When they raised the furnace temperature via air blasts from bellows, the wrought iron melted and absorbed the carbon in the charcoal. When the crucibles cooled, ingots of pure steel lay inside.An example of an early clay crucible discovered in Germany.SSPL/GETTY IMAGESIndia’s ironmasters shipped their "wootz steel" across the world. In Damascus, Syrian smiths used the metal to forge famous, almost mythological “Damascus steel” swords, said to be sharp enough to cut feathers in midair (and inspiring fictional supermaterials like the Valyrian steel of Game of Thrones). Indian steel made it all the way to Toledo, Spain, where smiths hammered out swords for the Roman army.In shipments to Rome itself, Abyssinian traders from the Ethiopian Empire served as deceitful middlemen, deliberately misinforming the Romans that the steel was from Seres, the Latin word for China, so Rome would think that the steel came from a place too distant to conquer. The Romans called their purchase Seric steel and used it for basic tools and construction equipment in addition to weaponry.Iron’s days as a precious metal were long over. The fiercest warriors in the world would now carry steel.Holy Swords and Samurai SteelAccording to legend, the great sword Excalibur was imposing and beautiful. The word means "cut-steel." But it wasn’t steel. From the age of King Arthur through Medieval times, Europe lagged behind in iron and steel production.A Medieval Broad Sword with Viking Blade bearing the arms of the De Bohun family. Photo by Chris Radburn/PA Images via Getty Images)CHRIS RADBURN/PA IMAGESGETTY IMAGESAs the Roman Empire fell (officially in 476), Europe spun into chaos. India still made its sensational steel, but it couldn’t reliably ship the metal to Europe, where the roads were unkempt, merchants were ambushed, and people feared plague and illnesses. In Catalonia, Spain, ironworkers developed furnaces similar to those in India; the “Catalan furnace” produced wrought iron, and lots of it—enough metal to make horseshoes, wheels for carriages, door hinges, and even steel-coated armor.Knights brandished specially crafted swords. They were forged by twisting rods of iron, a process that left unique herringbone and braided patterns in the blades. The Vikings interpreted the designs as dragon coils, and swords like King Arthur’s Excalibur and El Cid’s Tizona became mythological.The best swords in the world, however, were made on the other side of the planet. Japanese smiths forging blades for the samurai developed a masterful technique to create light, deadly sharp blades. The weapons became heirlooms, passed down through generations, and few gifts in Japan were greater. The forging of a katana was an intricate and ritualized affair.Japanese smiths washed themselves before making a sword. If they were not pure, then evil spirits could enter the blade. The metal forging began with wrought iron. A chunk of the material was heated with charcoal until it became soft enough to fold. After it cooled, the iron was heated and folded about 20 more times, giving the blade its arcing shape, and all through the forging and folding, the wrought iron’s continued exposure to carbonaceous charcoal turned the metal into steel.Katana signed by Masamune, considered Japan’s greatest swordsmith from the Kamakura period, 14th century.TOKYO NATIONAL MUSEUM AT UENOA swordsmith used clay, charcoal, or iron powder for the next step, brushing the material along the blade to shape the final design. Patterns emerged in the steel that were similar to wood grain with swirling knots and ripples. The details were even finer than the dragon scales of European blades, and Japanese katanas were given names like “Drifting Sand,” “Crescent Moon,” and "Slayer of Shuten-dōji," a mythological beast in Japanese lore. Five blades that remain today, the Tenka-Goken, or “Five Swords Under Heaven,” are kept as national treasures and holy relics in Japan.Of Iron and CoalThe first blast furnace looked like an hourglass.Along the Rhine Valley in present-day Germany, metalworkers developed a contraption that stood about 10 feet high, with two bellows placed at the bottom, to accommodate larger quantities of iron ore and charcoal. The blast furnace got blazing hot, the iron absorbed more carbon than ever, and the mixture turned into cast iron that could be easily poured into a mold.It was the ironmaking process the Chinese had practiced for 1,700 years—but with a bigger pot. Workers dug trenches on the foundry floor that branched out from a long central channel, making space for the liquid iron to flow. The trenches resembled a litter of suckling piglets, and thus a nickname was born: pig iron.MICHAEL STILLWELLIron innovation came just in time for a Western world at war. The invention of cannons in the 13th century and firearms in the 14th century generated a hunger for metal. Pig iron could be poured right into cannon and gun barrel molds, and Europe started pumping out weapons like never before.But the iron boom created a problem. As European powers began to stretch their power across the globe, they used up tremendous amounts of timber, both to build ships and to make charcoal for smelting. A single English furnace required about 240 acres of trees per year, according to the book Steel: From Mine to Mill, the Metal That Made America by Brooke C. Stoddard. The British Empire turned to the untapped resources of the New World for a solution and began shipping metal smelted in the American colonies back across the Atlantic. But smelting iron in the colonies destroyed business for the ironworks in England.The answer to Britain’s fuel woes came from a cast iron pot maker. Abraham Darby spent much of his childhood working in malt mills, and in the early 1700s, he remembered a technique from his days of grinding barley: roasting coal, a combustible rock. Others had tried smelting iron with coal, but Darby was the first to roast the coal before smelting. Roasted coal maintained its heat far longer than charcoal and allowed smiths to create a thinner pig iron—perfect for pouring into gun molds. Today, Darby’s large blast furnace can be seen at the Coalbrookdale Museum of Iron.England had discovered the power of smelting with coal. But it still wasn’t making steel.The Clockmaker and the CrucibleBenjamin Huntsman was frustrated with iron. The alloys available to the clockmaker from Sheffield varied too much for his work, particularly fabricating the delicate springs.An untrained eye doctor and surgeon in his spare time, Huntsman experimented with iron ore and tested different ways of smelting it. Eventually he came up with a process quite similar to the ancient Indian method of using a clay crucible. However, Huntsman’s technique had two key differences: He used roasted coal rather than charcoal, and instead of placing the fuel inside the crucible, he heated iron and carbon mixtures over a bed of coals.The ingots that emerged from the smelter were more uniform, stronger, and less brittle–the best steel that Europe, and perhaps the world, had ever seen. By the 1770s, Sheffield became the national fulcrum of steel manufacturing. Seven decades later, the whole country knew the process, and the steelworks of England burned bright.In 1851, one of the first world's fairs was held in London, the Great Exhibition of the Works of Industry of All Nations. The Crystal Palace was built with cast iron and glass for the event, and almost everything inside was made of iron and steel. Locomotives and steam engines, water fountains and lampposts, anything and everything that could be cast from molten metal was on display. The world had never seen anything like it.The Bessemer BreakthroughHenry Bessemer was a British engineer and inventor known for a number of unrelated inventions, including a gold brass-based paint, a keyboard for typesetting machines, and a sugarcane crusher. When the Crimean War broke out in Eastern Europe in the 1850s, he built a new elongated artillery shell. He offered it to the French military, but the traditional cast iron cannons of the time were too brittle to fire the shell. Only steel could handle the controlled explosion.The crucible steelmaking process was much too expensive to produce items as large as cannons, so Bessemer set out to find a way to produce steel in larger quantities. One day in 1856, he decided to pour pig iron into a container rather than let it ooze into a trench. Once inside the container, Bessemer blasted air through perforations on the bottom. According to Steel: From Mine to Mill, everything remained calm for about 10 minutes, and then suddenly sparks, flames, and molten pig iron came bursting from the container. When the chaos ended, the material left in the container was carbon-free, pure iron.Oil painting by E.F. Skinner showing steel being produced by the Bessemer Process at Penistone Steel Works, South Yorkshire. Circa 1916.SSPLGETTY IMAGESThe impact of this explosive smelting incident is hard to overstate. When Bessemer used the bellows directly on the molten pig iron, the carbon bonded with the oxygen from the air blasts, leaving behind pure iron that—through the addition of carbon-bearing materials such as spiegeleisen, an alloy of iron and manganese—could easily be turned into high-quality steel.Bessemer built a machine to carry out the procedure: the “Bessemer Converter.” It was shaped like an egg with an interior clay lining and an exterior of solid steel. At the top, a small opening spewed flames 30 feet high when the air blasted into the furnace.Almost immediately, though, a problem arose in Britain's ironworks. It turned out that Bessemer had used an iron ore containing very little phosphorus, while most iron ore deposits are rich in phosphorus. The old methods of iron smelting reliably removed the phosphorus, but the Bessemer Converter did not, producing brittle steel.MICHAEL STILLWELLThe issue vexed metallurgists for two decades, until a 25-year-old British police clerk and amateur chemist, Sidney Gilchrist Thomas, found a solution to the phosphorus problem. Thomas discovered that the device’s clay lining was not reactive with phosphorus, so he replaced the clay with a lime-based lining. It worked like a charm. The new method, which churned out five tons of steel in 20 minutes, could now be used across England’s ironworks. The old Huntsman crucible process, which produced a paltry 60 pounds of steel in two weeks, was obsolete. The Bessemer Converter was the new king of steel.American SteelOn the other side of the Atlantic, massive iron ore deposits remained untapped in the American wilderness. In 1850, the United States was producing only a fifth as much iron as Britain. But after the Civil War, industrialists began turning their attention to the Bessemer process, sparking a steel industry that would generate vastly more wealth than the 1849 California Gold Rush. There were roads to build between cities, bridges to construct over rivers, and railroad tracks to lay into the heart of the Wild West.Andrew Carnegie wanted to build it all.No one accomplished the American dream quite like Carnegie. The Scottish immigrant arrived in the country at age 12, settling in a poor neighborhood of Pittsburgh. Carnegie began his ascent as a teenage messenger boy in a telegraph office. One day, a high-ranking official at the Pennsylvania Railroad Company, impressed by the hardworking teen, hired Carnegie to be his personal secretary.Andrew Carnegie.LIBRARY OF CONGRESSThe “Star-Spangled Scotsman” developed a business acumen and worked his way up the ladder in the railroad industry, making some savvy investments along the way. He owned stakes in a bridge-building company, a rail factory, a locomotive works, and an iron mill. When the Confederacy surrendered in 1865, the 30-year-old Carnegie turned his attention to building bridges. Thanks to his mill, he had the mass production of cast iron at his disposal.But Carnegie knew he could do better than cast iron. A durable bridge needed steel. About a decade before Sidney Thomas refined the Bessemer Converter with a lime-based lining, Carnegie brought the Bessemer process to America and acquired phosphorus-free iron to produce steel. He established a steel mill in Homestead, Pennsylvania, to manufacture the alloy for a new type of building that architects called “skyscrapers.” In 1889, all of Carnegie’s holdings were consolidated under one name: the Carnegie Steel Company.By this point, Carnegie was single-handedly producing about half as much steel as all of Britain. Additional steel companies started sprouting up around the country, creating new towns and cities, including an iron mining town in Connecticut named "Chalybes" after the ironmakers of antiquity.America was suddenly steamrolling its way to the top of the steel industry. But things were about to get rocky at Carnegie’s Homestead Steel Works, right across the Monongahela River from Pittsburgh.To keep manufacturing costs down, wages were low. The salary for the 84-hour workweek was less than $10 in 1890 (about $250 today)—and that for backbreaking labor in the hot steel mills. Accidents were common, and in Pittsburgh, the air was so heavily polluted that a writer for The Atlantic Monthly called Steel City, “hell with the lid taken off.”Pittsburgh’s Strip District neighborhood looking northwest from the roof of Union Station.NASAIn July 1892, tensions boiled over between the Carnegie Steel Company and the union representing workers at the Homestead mill. The company chair, Henry Clay Frick, took a hard stance, threatening to cut wages. The workers hanged an effigy of Frick, and he responded by surrounding the mill with three miles of barbed-wire fence, expecting hostilities. The workers voted to strike and were subsequently fired, leading to a nickname for the fenced mill: “Fort Frick.”About 3,000 strikers took control of Homestead, forcing out local law enforcement. Frick hired 300 agents from the Pinkerton Detective Agency to guard the mill, and on the morning of July 6, 1892, a civil battle ensued. Men gathered at the riverbank, throwing rocks and firing guns at the Pinkerton agents trying to get ashore in boats. The strikers used whatever they could find as weapons, rolling out an old cannon, igniting dynamite, and even pushing a burning train car into the boats.Order was restored when a National Guard battalion of 8,500 entered the town and placed Homestead under martial law. Ten people were killed in the clash. Frick was later shot and stabbed in his office by an anarchist who heard of the strike, but survived. He left the company shortly after, and in 1897, Carnegie hired an engineer named Charles M. Schwab (not to be confused with the founder of the Charles Schwab Corporation) to serve as the new president. In 1901, Schwab convinced Carnegie to sell his steel company for $480 million. Schwab’s new company merged with additional mills to form the United States Steel Corporation.The Pennsylvania state militia arrives to quell the hostilities, art in Harpers Weekly by Thure de Thulstrup.LIBRARY OF CONGRESSThe American steel industry continued to explode into the 20th century. In 1873, the United States produced 220,000 tons of steel. By 1900, America accounted for 11.4 million tons of steel, more than the British and successful German industries combined. The new United States Steel Corporation was the largest company in the world, manufacturing two-thirds of the nation’s steel.It was a rate of production never before seen across the globe, but the steel foundries were just getting warmed up.Metal of War and PeaceDisagreements at U.S. Steel led Charles Schwab to find a new job presiding over a different, rapidly growing company: Bethlehem Steel. In 1914, two months into the Great War, Schwab received a secret message from the British War Office. Hours later, he bought a ticket to cross the Atlantic under a false name. In Europe, he met with England’s Secretary of State for War who wished to place a large order—with a catch. The British wanted Bethlehem to build $40 million in weaponry for England, and do no business with the Crown’s enemies. Schwab accepted and went to his next meeting, this one with the First Lord of the Admiralty, Winston Churchill. Churchill placed an order of his own: submarines for the Royal Navy to combat German U-boats, and he needed them immediately.HMS E34, a British E-class submarine in a floating dock. She was commissioned in March 1917, sank the U-Boat UB-16 off Harwich in the North Sea on 10 May 1918, and was mined near the the Frisian islands on 20 July 1918. The sub was lost with all the crew.UNITED KINGDOM GOVERNMENTBut Schwab had a problem. Neutrality laws in the U.S. prevented companies from selling weapons to WWI combatants on either side of the trenches. Undeterred, Bethlehem Steel sent submarine parts to an assembling plant in Montreal ostensibly for humanitarian rebuilding efforts—and American steel started leaking into the Allied war effort.The need to skirt neutrality laws disappeared when the United States officially entered World War I in April 1917. In 1914, when the war was just getting started, the United States produced 23.5 million tons of steel—more than twice its production 14 years earlier. At war’s end in 1918, production had doubled again. American steel gave the Allies a decisive advantage in the fight against the Central Powers.Empire State Building under construction with the Chrysler Building in the background, 1930.IRVING BROWNING/THE NEW YORK HISTORICAL SOCIETYGETTY IMAGESWhen the war ended, U.S. steelmaking emerged stronger than ever. Art Deco towers began to sprout up among the New York and Chicago skylines, with the vast majority of the steel coming from two companies: U.S. Steel and Bethlehem Steel. Iconic structures such as Rockefeller Center, the Waldorf-Astoria Hotel, the George Washington Bridge, and the Golden Gate Bridge were built with Bethlehem steel. In 1930, the company’s steel went into the then-tallest skyscraper in the world: the Chrysler Building. Less than a year later, the Empire State Building, with 60,000 tons of steel supplied by U.S. Steel, would reach higher than Chrysler to become the enduring symbol of Manhattan.But skyscrapers weren’t the only innovation sparked by the explosion in steel production. The material went into a bonanza of cars, home appliances, and food cans. (Two up-and-coming companies, Dole and Campbell’s, were becoming all the rage thanks to the long shelf life of their canned goods.) Bethlehem Steel and U.S. Steel’s assets were valued higher than those of the Ford and General Motors Companies.It was truly the age of steel—but trouble was not far off.Following the stock market crash of 1929, steel production slowed as the economy tumbled into the Great Depression. American steelworkers were laid off, but the mills never went completely dark. Railroad tracks still spread across the country, canned food remained popular, and as Prohibition drew to a close, a new steel product emerged: the steel beer can, introduced in the 1930s by Pabst for its Blue Ribbon brew.Steel Pabst Blue Ribbon can from the early 1940s.STEEL CANVASFollowing the Depression, the metal-hungry engines of war again ignited the foundries of the world. Germany moved to occupy land in Denmark, Norway, and France, gaining control of new iron mines and mills. Suddenly, the Nazis were capable of producing as much steel as the United States. In the East, Japan took control of iron and coal mines in Manchuria.When the attack on Pearl Harbor brought America into World War II, the U.S. government banned production of most steel consumer goods. The industrialized nations of the world, hurtling headfirst into world war, began rationing steel for a select few purposes: ships, tanks, guns, and planes.The American mills melted metal 24 hours a day, often with primarily female workforces. The economy began to boom again, and soon American steel production was more than three times larger than that of any other country. During World War II, the U.S. manufactured 25 times more steel than it did during World War I. And once again, the steelworks of the New World played a decisive role in the Allies’ victory.When the war was over at last, the U.S. lifted its ban on steel consumer goods. With more than half the world’s steel now American-made, the markets for cars, home appliances, toys, and reinforcing bars (rebar) for construction were as lucrative as ever. Steel from leftover ships and tanks was melted down in enormous furnaces to be reused in bridges and beer cans.But overseas, a dire need to rebuild, and the introduction of new steelmaking technology, was about to help foreign steel companies flourish.The Road to Modern SteelEven with mills churning non-stop during wartime, manufacturers had not yet perfected the art of smelting steel. It would take an idea dreamed up 100 years before the end of WWII to revolutionize the process once more—and ultimately, to dethrone the U.S. as the world’s steel king.German scientist and glassmaker William Siemens, living in England to take advantage of what he believed to be favorable patent laws, realized in 1847 that he could lengthen the amount of time a furnace held its peak temperature by recycling the emitted heat. Siemens built a new glass furnace with a small network of firebrick tubes. Hot gases from the melting chamber exited through the tubes, mixed with external air, and were recycled back inside the chamber.It took nearly 20 years for Siemens’ glassmaking furnace to find its way into metallurgy. In the 1860s, a French engineer named Pierre-Emile Martin learned of the design and built a Siemens furnace to smelt iron. The recycled heat kept the metal liquefied for longer than the Bessemer process, giving workers more time to add the precise amounts of carbon-bearing iron alloys that turned the material to steel. And because of the additional heat, even scrap steel could be melted down. By the turn of the century, the Siemens-Martin process, also known as the open hearth process, had caught on all over the world.MICHAEL STILLWELLJump forward to the 20th century, when a Swiss engineer named Robert Durrer found an even better way. Durrer was teaching metallurgy in Nazi Germany. After World War II wound down, he moved back to Switzerland and experimented with the Bessemer process. He blasted pure oxygen into the furnace (rather than air, which is only 20 percent oxygen), and found that it removed carbon from the molten iron more effectively.Durrer also discovered that by blowing oxygen into the furnace from above, rather than below as on a Bessemer Converter, he could melt cold scrap steel into pig iron and recycle it back into the steelmaking process. This “basic oxygen process” separated all traces of phosphorus from the iron, too. The method combined the advantages of both the Bessemer and Siemens-Martin furnaces. Thanks to Durrer's innovations, producing vast quantities of steel became cheaper yet again.While nations in Europe and Asia immediately adopted the basic oxygen process, American mills, still at the top of the industry, soldiered on using the Siemens-Martin process in confident contentment—unwittingly opening the door for foreign competition.Stainless Steel and the Decline of the American MillIn 1912, a British metallurgist named Harry Brearly was looking for a way to preserve the life of gun barrels. Experimenting with chromium and steel alloys, he found that steel with a layer of chromium was particularly resistant to acid and weathering.Brearly started selling the steel-chromium alloy to a friend working in cutlery, calling it “rustless steel”—a literal moniker befitting an engineer. His friend, Ernest Stuart, who needed to sell the knives to the public, came up with a catchier name: stainless steel.A company called Victoria was forging steel knives for the Swiss Army when it caught wind of the new anticorrosive metal from Great Britain. The company promptly changed the metal in its knives to inox, which is another word for the alloy that’s derived from the French word for stainless, “inoxydable." Victoria rebranded itself as Victorinox. Today, there is a good chance you could find one of their red pocketknives in your desk drawer.Suddenly stainless steel was all over the world. The anticorrosive, glimmering metal became a critical material for surgical tools and home goods. The hubcaps at the top of the Chrysler Building are made of stainless steel, which helps them retain their silver sheen in the sunlight. In 1959, workers broke ground in St. Louis to build the stainless steel Gateway Arch, which remains the tallest man-made monument in the Western Hemisphere.The Gateway Arch in St. Louis standing 630 feet tall.DANIEL SCHWEN/WIKIMEDIA ​But just as St. Louis was building the Gateway to the West, the rest of the world was catching up with American steel production. Low wages overseas and the use of the basic oxygen process made foreign steel cheaper than American steel by the 1950s, just as the steel industry took a hit from a cheaper alloy for home goods: aluminum.In 1970, U.S. Steel’s run as the world’s largest steel company ended after seven decades, supplanted by Japan’s Nippon Steel. China became the world’s top steelmaker in the 1990s, and Bethlehem Steel closed its plant in Bethlehem in 1995. It wasn’t until the late 20th century that most American steel mills finally adopted the basic oxygen process. As of 2016, the United States ranked fourth in steel production according to the World Steel Association.The Sustainable Future of SteelMuch of the world’s stainless steel is made in mini mills. These metalworks do not make steel from scratch, but rather melt down scrap steel for reuse. The most common furnace in a mini mill—the electric arc furnace, also invented by William Siemens—uses carbon electrodes to create an electric charge to melt down metal.The spread of mini mills in the last half-century was a critical step toward recycling old steel, but there is a long way to go to achieve fully sustainable smelting. Forging steel is a well-known emitter of greenhouse gases. The basic oxygen process, still used widely today, was developed almost a century ago, when the ramifications of climate change were only just entering circles of scientific research. The basic oxygen process still burns coal, emitting about four times more carbon dioxide than electric furnaces. But phasing out the oxygen blasts entirely for the electric arc is not a sustainable solution—only so much scrap steel is available for recycling.Today, metallurgists are in the early stages of developing eco-friendly steel production methods. At MIT, researchers are testing new electricity-based technologies for smelting metals. These electric smelting techniques have the potential to significantly reduce greenhouse gas emissions if they can be improved to work on metals with higher melting points, such as iron and steel.A chart of electrolysis of a molten semiconductor.MIT/MICHAEL STILLWELLAdditional ideas that have been used to limit car emissions are being tested as well. Last February, an Austrian manufacturer called Voestalpine began constructing a mill designed to replace coal with hydrogen fuel—technology that is likely at least two decades away. As a stopgap, the Chinese government even enforced limits on its country’s steel output last year.The stakes have changed in the 21st century. But the question remains the same as it ever was, the same as it was for those manning the crucibles of India, the blast furnaces of Germany, and the foundries of America. How do we get better at making steel?