Fig. 1.  Pure iron is a weak engineered material

This is the second in a series of articles on the influence of alloying elements in heat treatment and the broader topic of their role in physical metallurgy (i.e. the science of making useful products out of metals).  
Iron has a number of properties that make it attractive to use in everyday products: it is plentiful, inexpensive and easy to extract from its ores (i.e. raw-material state). Pure iron also has other attractive properties such as good magnetic and electrical characteristics. However, pure iron is weak and, as such, is a poor choice as an engineering material. In fact, you might be surprised to know that iron is not as strong as most plastics (Fig. 1). However, iron can be alloyed to make it harder and produce steel that dramatically improves its usefulness as an engineered material.   

Iron has two highly interesting characteristics. First, it has “asymmetric rotation” of its electrons, that is the electrons can spin in the same direction. This characteristic means that iron can easily combine with other elements that will change the properties of iron by alloying. Secondly, iron is “polymorphic” (Fig. 2), that is the crystal structure can “flip” (or, as metallurgists say, transform). These two unique characteristics, especially when combined with heat treatment, result in changes to its physical, mechanical and metallurgical properties.

Fig. 2.  Polymorphic transformation of iron from body-centered cubic (BCC) to face-centered cubic (FCC) at 910°C (1670°F)

Effects of Alloying Elements on Polymorphic Transformations
We can add a variety of alloying elements to iron to create iron-based alloys. For example, adding carbon strengthens the material on heat treatment and controls where the “flip” (i.e. transformation) occurs.

Polymorphic transformation temperatures are as follows:
  • A3 temperature → 910°C: BCC α FCC γ
  • A4 temperature → 1400°C: FCC γ BCC δ
Notes:
1.) A3 is the temperature at which transformation of ferrite to austenite is completed during heating.
2.) A4 is the temperature at which delta ferrite transforms to austenite during cooling.

Some elements (Ni, Mn, Co, Cu) raise the A4 temperature, and lower A3, i.e. they stabilize austenite further and increase the range of temperature over which austenite can exist as a stable phase. Other elements (Cr, W, V, Mo, Al, Si) raise the A3 temperature and lower A4, that is they restrict field over which austenite may exist.

More to follow…