This is the first in a series of blogs 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).  

Metal parts can be made in a variety of ways, depending on the shape, properties and cost of the finished product. The desired properties may be electrical, mechanical, magnetic or chemical in nature. All of them can be enhanced by alloying and heat treatment. The cost of a finished part is often determined more by its ease of manufacture than by the cost of the material. This has led to a wide variety of ways to form metals and competing technologies.[1]  

The decision to use alloying elements in steel is an important one since they are expensive materials. However, the extra cost may be partly offset by the greater ease with which most alloy steels can be heat treated.  

The Alloying Elements of Steel [2]
Plain-carbon steels contain traces of certain elements (Si, Mn, S, P) that unavoidably entered the steel during the iron and steelmaking processes. These elements are not generally considered alloying elements unless their content exceeds the amount corresponding to the steelmaking process.  

Steel is considered alloyed when, in addition to the elemental constituents naturally present (Fe and C), other alloying elements are added intentionally during the steelmaking process to ensure specific properties can be achieved (that cannot be produced without alloying).  

The most important and most frequently applied alloying elements of steel are manganese (Mn), nickel (Ni), chromium (Cr), tungsten (W), vanadium (V), molybdenum (Mo), titanium (Ti), niobium (Nb) and boron (B).Note: Other blogs have discussed the influence of some if not all of these elements. 

The reason we add these alloying elements to steels is to:
  • Improve mechanical properties (e.g., strength, ductility and toughness)
  • Increase resistance to corrosion (i.e. chemical resistance)
  • Improve certain physical properties (e.g., magnetic and electrical properties)
  • Improve manufacturability (e.g., formability, weldability and machinability)
These objectives can be achieved in various ways, depending on the relationship of the alloying elements to iron (Fe) and to the fundamental alloying element in all steels, namely carbon (C).  

Alloying-Element Characteristics
Alloying elements influence the mechanical properties of steels through both solid-solution and metallic-compound formation as well as through carbide formation. These effects are explained by the equilibrium phase diagrams as well as the non-equilibrium phase transformation diagrams (i.e. TTT & CCT diagrams).  

The alloying elements can form solid solutions (substitutional or interstitial) and metallic compounds with iron. Only alloying elements (e.g., Cr, Mn, Ni, Co, V) with atomic diameters similar to that of iron can form substitutional solid solutions with iron. Interstitial solid solutions are formed in steel by elements with small atomic diameters (e.g., C, N, B). Many other elements form metallic compounds with iron (e.g., nitrogen, silicon and phosphorous. The greater the differences between the atomic radii of elements forming the metallic compounds, the more stable the metallic compounds. These differences are greatest in the case of carbon and nitrogen, and carbides and nitrides are usually more stable in steels as opposed to other metallic compounds.  

Alloying elements that form carbides in steels are called carbide-forming elements. One important item for heat treaters to remember is that carbides require time to dissolve into solution on heating and in service maintain their hardness up to a much higher temperature than martensite.  

Future blogs will offer more. Stay tuned.