This is the third in a series of articles on the influence of alloying elements in heat treatment and the broader topic of their role in physical metallurgy (that is, the science of making useful products out of metals).

Popular alloying additions to steel include: aluminum (Al), chromium (Cr), cobalt (Co), niobium/columbium (Nb, Cb), copper (over 0.6%), manganese (over 1.6%), molybdenum (Mo), nickel (Ni), silicon (over 0.6%), titanium (Ti), tungsten (W), vanadium (V) and zirconium (Zr).

As a general rule, to obtain greater strength add carbon (C), manganese (Mn) or nickel (Ni); to obtain better corrosion resistance add chromium (Cr) or copper (Cu); for better machinability add lead (Pb), sulfur (S) or selenium (Se); for better properties at high temperature add tungsten (W) or molybdenum (Mo). Let’s explore some of these elements in more detail (Table 1).

Since carbon is such an important element, we should list the general effects of adding carbon to steel, namely:
  • Raising the ultimate strength
  • Enhancing the abrasive (wear) resistance
  • Improving the uniformity of hardness
  • Increasing the depth of hardening
  • Intensifying the fineness of fracture
  • Lowering the hardening and quenching temperature
Carbon is also responsible for:
  • Reducing the shock resistance
  • Increasing the resistance to machinability
  • Lessening the heat conductivity
  • Lowering the ductility and toughness
  • Diminishing the electrical or magnetic conductivity
To negate some of the negative effects of carbon, we add other elements such as:
  • Chromium, which is a carbide-former, increases both the hardness and wear resistance of the case and strengthens the core
  • Nickel to increase strength and toughness in the core
  • Vanadium to improve the strength and ductility, especially elasticity and shock resistance
  • Molybdenum to raise the ultimate strength, hardness and toughness and to promote the retention of high hardness at elevated service temperature
Each alloying element addition, acting alone or in combination with one another, produces different effects or intensity of the effects of another addition.  

More to follow...