Alloy Steels
Carbon steels in their commercial forms always contain certain amounts of other elements. Many of these elements enter the steel from the ores and it is difficult to remove them during the process of steel making. All commercial steels contain varying amounts of Mn, Si, S and P and frequently varying amounts of such elements in Cr, Ni, Mo and V. If alloying elements other than carbon are present only in small amounts (e.g. Mn upto 0.8%, Si upto 0.3%, etc.) then the steel is usually called low alloy steel or plain carbon steel. Sulphur and phosphorus when more than 0.05% of either is present, tend to make steel brittle, so that during steel making these elements are reduced to at least this value. Si has little effect on strength and ductility if less than 0.2% is present. As the content is raised to 0.4% the strength is raised without effecting ductility, but above 0.4% Si, the ductility is impaired. Si is added as deoxidiser and that part which does not make silicon dioxide remains in steel as impurity.
Mn is another alloying element which is present in most steels. If it exists in solid solutin in the ferrite it has a strengthening effect. It may also exist in forms of Mn3C which forms part of the pearlite of MnS. Upto 1% of Mn has strengthening effects on steel and its presence in excess of 1.5% induces brittleness in steel. Excess Mn is added to melt during steel making to bring its level to desired value. It also acts as a deoxidiser.
Intentional addition of many other elements modifies the structure of steel and hence improves its properties. Steels to which such intentional additions have been made (including those steel which contain Mn in excess of 1% or Si in excess of 0.3%) are known as alloy steels. One particular effect of alloying is that it enables martensite to be produced with low rates of cooling and permits larger sections to be hardened than is possible with plain carbon steel.
The important elements that are used to alloy with steel in varying quantities are Ni, Cr, Mo, W, Mn and Si. The bcc metals like Cr, W and Mo when alloyed with steel tend to form carbides which reduce the proportion of Fe3C in the structure. On the other hand the fcc elements like Ni, Al, Cu and Zr do not form carbides. Mn which has three allotropic complex structures also forms carbide.
Various advantages in terms of improved mechanical properties & corrosion resistance are obtained by adding one or several alloying elements.
The various advantages of alloy steel are following :
1. Higher hardness, strength and toughness on surface and over bigger cross-section.
2. Better hardenability & retention of hardness at higher temperature (good for creep and cutting tools).
3. Higher resistance against corrosion & oxidation.
The alloying elements influence the properties of plain C steel in four ways:
1. By strengthening ferrite whereas forming a solid solution. The strengthening effects of several alloying elements are in this order: Cr, W, V, Mo, Ni, Mn and Si.
2. By forming carbides that are harder & stronger. Carbides of Cr and V are hardest and strongest against wear specifically throughout tempering. High alloy tool steel use this effect.
3. Ni and Mn lower the austenite formation temperature whereas other alloying elements raise this temperature. Most of elements shift eutectoid composition to lower C percentage.
4. Most of elements shift the isothermal transformation curve (TTT) to lower temperature, therefore lowering the critical cooling rate. Ni, Mn, Cr and Mo are prominently effective in this respect.