Steel-making 101 (or) Steeling Christians

Steel is a common metal alloy that is used in many applications: buildings, bridges, automobiles, reinforced concrete (concrete with steel reinforcing bars, or rebar), and fasteners (nails, bolts, and screws). How would you feel about a comparison between steel production and our Christian walk? Let’s stretch…

Steel is an alloy, or mixture, of iron (Fe) and carbon (C). The amount of carbon that can be added to iron and the associated phase (solid, liquid, or a mixture of the two) that is obtained depends on the temperature. This information is arranged in an equilibrium phase diagram, where the vertical axis is temperature, the horizontal axis is percent carbon, and the individual areas indicate different phases. We identify steel as iron that has between 0.002% and 2.1% carbon added to it.

Iron-carbon phase diagram

Iron-carbon phase diagram

The carbon atoms are much smaller than the iron atoms so they fit in the spaces within the atomic lattice (structure) of the iron. This is called an interstitial alloy. These interstitial carbon atoms serve to impede the motion of dislocations (imperfections) in the iron’s atomic structure when a force is applied. This causes the mixture of iron and carbon (steel) to be stronger than iron alone.

Interstitial alloy of carbon in iron

Interstitial alloy of carbon in iron

To obtain steel’s high strength (resistance to deformation) and ductility (the ability to deform without fracture), a sequence of events must be followed.

  • The steel is first annealed. This means that its temperature is raised to a high level where it is soft (this is the austenite phase in the top, left of the phase diagram). At this high temperature, the internal crystal structure is modified to obtain recovery, recrystallization, and grain growth.
  • The austenite is then plunged into water (sometimes saltwater or oil) to rapidly cool the microstructure. This “freezes” the interstitial carbon in place and creates a non-equilibrium phase called martensite. Martensite is very hard, but it is also brittle. It is like a glass hammer. It is hard, but not yet very useful. The carbon trapped in the iron microstructure distorts it, which causes the brittle behavior. Martensite can only be obtained from the rapid quench — it is not present in the equilibrium phase diagram.
  • To retain hardness, but restore ductility and make the steel useful, the martensite is tempered. This is a precise process of heating and cooling over time to obtain the desired material properties.

Let’s now draw some parallels to our Christian walk.

  • God begins drawing us toward Him and away from our obsession with self. This annealing process is required to soften our hard hearts and awaken the desire for our Creator and Father. This annealing may be an internal “temperature rise” or it may be external “heat”. God’s desire is to win our hearts, though, so He lovingly leads us to a microstructural change of recovery, recrystallization, and grain growth.
  • When we accept the need for Jesus as our Savior and are baptized, we are quenched. We are plunged into the water where a new microstructure (alignment of our atoms) is formed that can only be obtained through this very special celebration of our new life. It is not available or seen until we are obediently quenched by our acceptance of Jesus.
  • God is not finished with this new birth, however. Now He begins a lifelong process of tempering. Sometimes it is heating, sometimes it is cooling, but it is a controlled, precision process designed to improve our strength and ductility. We are increasingly able to resist the forces of the world against us (strength) even as we increase our compassion for those around us (ductility).

As a Christian and engineer, it is so comforting to see the character of our loving God reflected through both the universe and our lives. Wherever you are in God’s steel-making process, understand that it is a journey and that you are not alone. The Master Craftsman is at work and He loves you very much!


About Tony Schmitz

Tony Schmitz received his BS in Mechanical Engineering from Temple University in 1993, his MS in Mechanical Engineering from the University of Florida in 1996, and his PhD in Mechanical Engineering from the University of Florida in 1999. He is a mechanical engineering professor at the University of North Carolina at Charlotte.
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