Effect of Individual Alloying Elements
Sulphur
Sulphur is not a desirable element in steel because in interferes along hot rolling and forging resulting in hot-shortness or hot embrittlement. Sulfur however, is helpful in developing free cutting nature. Therefore sulfur up to 0.33% is added in free cutting steel. Or else, sulfur is restricted to 0.05% in open hearth or BOF steel & to 0.025% in electric furnace steel.
Phosphorous
It produces cold shortness which decreases impact strength at low temperature. So its percentage is normally restricted to level of sulfur. It is helpful in free cutting steels & is added up to 0.12%. It also improves resistance to corrosion.
Silicon
Silicon is present in all steels but is added up to 5% in steels utilized as laminates in transformers, motors & generators. For providing toughness it is significant constituent in steel utilized for spring, chisels and punches. It has a good effect in steel that it combines with free O2 & form SiO2 and enhances strength and soundness of steel casting (upto 0.5%).
Manganese
1.2 to 1.4% of Mn produces extremely tough, wear resistant and non-magnetic steel called Hadfield steel. It is important ingredient of free cutting steel upto 1.6%. Mn combines with S, forming MnS. For this reason Mn should be 3 to 8 times the S. Mn is effective in increasing hardness and hardenability.
Nickel
It is good in enhancing strength, hardness and toughness whereas maintaining ductility. 0.5% of Ni is good for parts subjected to impact loads at room and very low temperatures. Higher amounts of Ni help improve the corrosion resistance in presence of Cr as in stainless steel. In steel Nickel results in good mechanical properties after annealing & normalising and hence large forgings, castings and structural parts are made in Ni-steel.
Chromium
Chromium is common alloying element in tool steels, stainless steel, corrosion resistant steel (4% Cr). It forms carbide and generally improves hardness, wear and oxidation resistant at elevated temperature. It improves hardeanbility of thicker sections.
Molybdenum
Molybdenum is commonly present in high speed tool steel, carburising steel and heat resisting steel. It forms carbide along high wear resistance & retaining strength at high temperatures. Mo usually enhances hardeability and helps improve the effects of other alloying elements as Ni, Mn and Cr.
Tungsten
It is important ingredient of tool steel and heat resisting steel and generally has same effects as Mo but 2 to 3% W has same effect as 1% Mo.
Vanadium
Like Mo, V has inhibiting influence on grain growth at high temperature. V carbide possesses highest hardness and water resistance. It improves fatigue resistance. It is important constituent of tool steel and may be added to carburising steel. Hardeability is markedly increased because of V.
Titanium
In stainless steel, Addition of Ti does not permit precipitation of Cr carbide since Ti is stronger carbide former and fixes are carbon.
Cobalt
It imparts magnetic property to high C steel. In the presence of Cr, Co does not permit scale formation at high temperature by increasing corrosion resistance.
Copper
Atmospheric corrosion resistance of steel is increased by addition of 0.1 to 0.6% copper.
Aluminium
Aluminium in percentage of 1 to 3 in nitriding steels is added to improve the hardness by way of forming Al nitride. 0.01 to 0.06% Al added during solidification produces fine grained steel castings.
Boron
Very small percentage (like 0.001 to 0.005) of B is effective in enhancing hardness, particularly in surface hardening boriding treatment.
Lead
Less than 0.35% Pb improves machinability.
The effects of alloying element in respect of various desired effects are summarized below :
a. Hardenability – Si, Mn, Cr, Ni, W, Mo, B
b. Toughness – Si, Ni
c. High temperature strength – Cr, Mo, W
d. Corrosion resistance – Cr, Mo, W
e. Wear resistance – Cr, Mo, W, V
f. Low temperature impact strength – Ni
g. Atmospheric corrosion resistance – Cu
h. Machinability – S, P, Pb
i. Fatigue strength – V
j. Surface hardening – Al