Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 5M

Year: Dec 2014

Alloying elements are added to achieve certain properties in the material. As a guideline, alloying elements are added in lower percentages (less than 5%) to increase strength or hardenability, or in larger percentages (over 5%) to achieve special properties, such as corrosion resistance or extreme temperature stability

1. Elements which tend to stabilize austenite.

   • Al, Cr, Si, Mo, W, P, are ferrite stabilizers, they tend to form solid solution with alpha iron
   • They have greater solubility in ferrite – BCC
   • Generally have similar BCC structure
   • They decrease the amount of C present in γ-Fe
   • Favors formation of free carbides in steel
   • The ferrite form is Delta ferrite since it can exists from melting point to room temp

2. Elements which tend to stabilize ferrite.

   • Ni, Mn, Co are austenite stabilizers, they tend to form solid solution with gamma iron
   • They have greater solubility in austenite
   • They have FCC crystal structure
   • They do not combine with carbon present in γ to form simple or complex carbide, therefore carbon remains in the solid solution in the γ
   • 13% Mn steels are austenitic at room temperature called as Hadfield Steel.
   • C and N are also austenite stabilizers(interstitial solutes in FCC)

3. Elements which tend to form carbides.

   • Some alloying elements, notably manganese, chromium, molybdenum, and vanadium, dissolve in theferrite phase and also form carbides. Such elements as nickel, copper, and silicon do not form carbides in steel. When present in the amounts usually found in alloy steels, they dissolve in and strengthen the ferrite.

4. Elements which tend to graphitize the carbide.

   • Si, Co, Al and Ni. Only a small proportion of these elements can be added to the steel before graphite forms during processing, with attendant ruin of the properties of the steel. Their present makes the carbides unstable.