Metal Identification – How to Tell Metals Apart

Besides proficiency as a welder, new hires in a metal shop are expected to know a little something about the products they weld on. This section of Welders Universe describes the products and classifications used in construction and fabrication. At a minimum, an entry-level welder should have the ability to:

  • Distinguish between the common metals
  • Recognize common shop materials (channel, tubing, elbows, flanges, etc.) and structural members (I-beams, trusses, girders, etc.)
  • Understand the basic metalworking processes (especially heat treatment) and vocabulary

Welding school graduates should also be able to:

  • Identify unmarked metals using a spark test, magnet or other means
  • Interpret the meaning of stock classification numbers on labels and supply bins

Visually Identifying Common Metals

Carbon Steel: This is an inexpensive metal used for commercial buildings, bridges , pipelines, and other types of construction. After iron is extracted from the ore, its carbon content is manipulated to produce different grades of steel. Steel with higher amounts or carbon is harder, which is why cutting tools have a lot of it. On the downside, harder can mean brittle. Most structural steel contains lower amounts or carbon in order to remain ductile. The flexibility allows the metal to respond to a variety of loads, impacts, temperatures and seismic disturbances without cracking. Low-carbon steel can also take a lot of heat in welding without any serious consequences. High carbon steels are more sensitive to heat. Usually, they require a pre or post heat treatment to relieve any stresses or changes in grain size and lattice structure that may develop whenever the metal is welded.

Steel is a dense, relatively heavy material that easily rusts, so the surface must be painted, galvanized, cleaned often, encased in concrete, or protected in some other way. Freshly grinded carbon steel looks shiny and metallic; otherwise it has a dull, dark (but still metallic) color. On a grinder, steel produces lots of sparks. As a rule, the greater the spark bursts, the higher the carbon content of the steel. There are four basic grades of steel, and not easy to tell apart by appearance alone:

  • Mild Steel – used in pipe and steel framing 0.05% to 0.3% carbon
  • Medium Carbon Steel – forging and vehicle parts 0.3% to 0.6 % carbon
  • High-Carbon Steel – springs and wire 0.6% to 1% carbon
  • Tool Steel – cutting and drilling tools 1% – 2% carbon

If the iron content is higher than 2%, the metal is classified as wrought or cast iron. At this point it becomes much harder to weld with any success.

Stainless Steel: This is carbon steel with ten percent or more chromium added to prevent rust and other forms of corrusion. For this reason, stainless steel is the preferred metal for containers that hold food, liquids and chemicals. Restaurant kitchens are packed with stainless steel counters, pots, coffee urns and, of course, silverware. You can recognize it by its grainy look and silvery gleen. Other alloying elements commonly included are nickel and molybdenum.

There are three categories of stainless steel based on the lattice structure of the metal when it’s manufactured: martensitic, ferritic and austenitic. These classifications are impossible to distinguish visually. However, there are still ways to tell the apart. the austenitic type is non-magnetic.

  • Austenitic – Most common and extensive range of stainless steels and non-magnetic. The type most frequently used is referred to as 18-8 (18% Chromium and 8% Nickel). It’s highly corrosion resistant and thus used for food and chemical equipment, architectural and structural applications
  • Ferritic – Usually made from a straight chromium alloy with no additions. It’s magnetic and not heat treatable. Mostly used for automotive trim and cooking utensils
  • Martensitic – Used to make fasteners, pump shafts and turbine blades. Magnetic.

Here are two other less common stainless steel classifications:

  • Precipitation Hardening – A chromium-nickel blend that’s made stronger by a special heat treatment. The stainless steels may be referred to by their chromium-nickel percentages, such as 13-8, 15-5 or 17-7. Used to make valves, gears and petro-chemical equipment.
  • Duplex – A blend of three alloys: chromium, nickel and molybdenum. It’s more resistant to stress corrosion cracking than austenitic, yet tougher than ferritic alloys. Used for pipelines.

Aluminum: This metal is extracted from bauxite ore, then subjected to lengthy processing, part of which involves electrolysis. Because of its complicated recipe, aluminum only came into vogue during modern times. Besides soda cans and aluminum foil, it’s used to construct airplanes, automotive components and other things that require the toughness of steel, but not the weight. It’s not ferrous, so it doesn’t rust, and always remains shiny. Aluminum doesn’t produce any sparks on a grinder.

Chromoly: A well-known commodity around custom auto shops, this metal is a class of alloyed steels. Chromoly stands for chromium-molybdenum, but nickel and other elements may be included into the recipe. The metal is used for gears, piston pins, connectors, frames and crankshafts. It can be subjected to a heat treatment known as case-hardening to make it more durable. Chromoly produces lots of sparks on a grinder.

Titanium: This lightweight material is wildly expensive, so you’re not likely to find it casually lying around a shop. Yet it pays for itself with excellent mechanical properties and a strength-to-weight ratio that’s highest among the metals. Its high corrosion resistance incudes salt water, chlorine and even acids. Welded titanium pipe and equipment (like pressure vessels and tanks) are used by the petrochemical industry. The metal has long been incorporated in aerospace applications, and more recently in high-end motorcycles, bicycle frames, tennis rackets and golf clubs. Because they’re non-toxic, titanium plates, hips and screws are also planted nowadays inside human beings with good results.

Cast Iron: Best known as the metal of pancake griddles, skillets and soup pots, this metal is chalk full of carbon. It cannot be formed because high carbon content means brittleness. Instead, the metal has to be poured into molds, which is why it’s called “cast” iron. You see it everywhere in metal shops, since cast iron forms the base of many stationary shop tools. The metal is heavy-duty and resists deformation, making it the perfect choice for long-lasting, level surfaces for work tables and saws.

Wrought Iron: In ancient times, iron ores were smelted in charcoal fires. Since charcoal is pure carbon, the process adds lots of carbon, producing what’s called wrought iron. In a blacksmith’s forge, air or oxygen is used to stoke a fire that may reduce the amount of carbon from 4 or 5 percent to about 2 percent. Now the iron is ductile enough to form (or forge) with hammers, punches and other tools. Wrought iron is used for ornamental ironwork, door latches and hardware, barnyard tools and other implements that are part of blacksmithing.

Copper: Onee the go-to metal for plumbing, the role of copper in the world has diminished somewhat since the advent of PVC and copper ore’s rising price tag. Even pennies are made of zinc now, with a copper coating. However, this noble metal is still used in electric wire and transformers, since it conducts electric current better than anything else and resists corrosion.

Bronze: This is a combination of copper and a second metal, usually tin, which produces a strong, tough substance that resists corrosion better than iron. In antiquity, bronze first gained its widespread reputation in cutting knives and swords. (Ancients initially combined copper with arsenic to make the bronze, not tin, and that meant an early trip to the grave for many bronzesmiths.) Before the advent of stainless steel, bronze was the primary metal for boat and ship fittings, and it’s still used to construct ship propellers. TIG welders use silicon-bronze brazing rod on sheet metal panels, since it generates less distorion and warping.

Brass: More than decorative because of its shiny, gold-like look, brass is used plumbing and electrical connectors because it resists static electricity and sparks. You may also be familar with brass on musical instruments (like tubas and horns), due to exceptional acoustic properties. The metal is softer and lighter than bronze and combines copper with zinc (rather than tin). In a welding shop, most of the fittings on compreseed gas cylinders and torches — such as the flashback arrestors in the photo above — are made of brass.

Galvanized Steel: This is carbon steel that has been submerged in molten zinc. The zinc provides a protective coating that resists rust and corrosion. As in the familiar Cyclone fencing, steel that’s galvanized appears a solid, bright gray color. Zinc is considered a toxic substance, so you you shouldn’t weld on the metal until the zinc coating has been ground away around the weld joint. And you shouldn’t grind zinc without wearing a respirator or sturdy N-95 dust mask.

Lead: Contrary to popular belief, pencils are not made out of lead but graphite. Lead is an extremely dense metal used in medical settings as a shield from X-rays and other forms of radiation. The metal is gray and has a low melting point, which makes it ideal for soldering. However, it’s no longer used as plumbing pipe or to solder canned foods, because it’s toxic enough to cause nerve disorders.

Next: Metal Classifications

 

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