Tag Archives: anneal

Episode 107 — Steel

steelThis episode of The Engineering Commons finds Adam, Brian and Jeff talking about steel, the most common of engineering metals.

  • Jeff and Brian consider whether comic book hero Superman should have been called the “Man of Osmium,” or the “Man of Molybdenum,” rather than the “Man of Steel.”
  • Steel is not an element one can find in the periodic chart; it is rather an iron alloy.
  • One of the earliest sources of iron was iron meteorites.
  • A key component in steel is carbon, which is alloyed with iron in percentages (by weight) from about 0.5 to around 4 percent.
  • Iron ore is rock that can be mined from the ground and processed to obtain fairly pure iron.
  • Once treated as a waste product, Taconite is now mined for its iron content, which is about 20 to 30 percent by weight.
  • The largest deposit of iron ore in the United States is the Mesabi Iron Range, located in the state of Minnesota.
  • Hematite is a type of iron ore that is reddish in color, and has a high iron content (between 55 and 70%).
  • According to at least one website, the “ite” suffix used in many geological and metallurgical names comes from the Greek word “lithos,” meaning rock or stone.
  • Brian notes that, due to its once high economic value, an aluminum cap sits atop the Washington Monument in Washington, DC.
  • Named for Henry Bessemer, the Bessemer process made large-scale steel production economically feasible.
  • Coke is the primary fuel used in heating iron ore to a temperature where the iron melts and flows out of the blast furnace.
  • There are fourteen different metallic lattice structures found in nature, but the two most common are the Body-Centered Cubic (BCC) and the Face-Centered Cubic (FCC) arrangements.
  • Allotropy describes the capacity of certain elements to exist in two or more different phases while in a common physical state.
  • An alloy is the mixture of two or more metals, or the mixture of a metal with one or more additional elements.
  • At room temperature, iron exists in a BCC crystalline structure known as ferrite, or alpha iron.
  • When heated to a temperature between 912 and 1,394 °C, iron assumes a FCC crystal structure known as austenite.
  • Wrought iron is iron with a very low carbon content; this makes it easy to bend, form, and weld.
  • Low carbon steel is iron that has been alloyed with between 0.05 and 0.25% carbon (by weight). This is also known as “mild steel,” and is easily machined, formed, and welded.
  • Medium-carbon steel has a carbon content of between 0.3 and 0.6% by weight. The increased carbon content gives these steels greater hardness and wear resistance.
  • High-carbon steels have a carbon percentage of between 0.65 to 1%, giving such steels a very high surface hardness that makes them useful for cutting tools.
  • Cast iron has a carbon percentage of around 4%; this type of iron alloy is easily cast, but is rather brittle.
  • Rapid cooling of a heated material is known as quenching. The process of quenching a low-carbon steel heated to the austenite phase produces a very hard and skewed BCC crystal structure known as martensite.
  • Pearlite is a microstructure composed of alternating layers of ferrite and cementite.
  • The final phase structure for steel can be determined from Time-Temperature Transformation and Continuous Cooling Transformation diagrams.
  • Annealing is a heat treatment that reduces the number of crystalline dislocations, and thus increases a metal’s workability.
  • In contrast to annealing, tempering is a heat treatment used a remove some, but not all, of the hardness that has been intentionally introduced into a metal.
  • Adam notes that quenched steel can be very brittle.
  • Welding can introduce unwanted hardening or softening of joined materials.
  • Steel can be brought to shape using either hot-forming or cold-forming processes.
  • Steel specifications are available from numerous engineering bodies, including the ASTM and SAE.
  • One well-known corrosion-resistant family of steels is Hastelloy, produced by Haynes International.

Thanks to Dennis Hill for use of the photo titled “fpx090412-06.” Opening music by John Trimble, and concluding theme by Paul Stevenson.