The iron-iron Carbide Equilibrium Diagram
The temperature at which the allotropic changes take place in iron is influenced by alloying elements, the most important of which is carbon. The portion of the iron-carbon alloy system which is of interest is shown in Fig -17-. This is the part between pure iron and an interstitial compound, iron carbide, Fe3C, containing 6.67 % carbon. The portion of the diagram in the upper left-hand corner is known as the delta region because of the (S) solid solution.
In this portion the pretectic reaction takes place. The equation of the pretectic reaction may be written as:
In this part of the diagram the solid solution formed when carbon dissolves in BCC iron is called (s) solution.
Alloy Steel
Alloy steel can be defined as steel containing by weight at least:
(0.1 % MO or V), or (0.3 % W or CO), or (0.5 % Cr or Ni), or
(2 % Mn).
These elements are asses to a plain carbon steel in order to improve its properties or, sometimes, introduce new properties. Some of the most important are:
1. Increase hardenability.
2. Improve strength at ordinary temperature.
3. Improve mechanical properties at either high or low temp.
4. Improve toughness at any minimum hardness or strength.
5. Increase wear resistance.
6. Increase corrosion resistance.
7. Improve magnetic properties.
Thus, small amounts of Ni and Cr may be added to produce a general improvement in mechanical properties, whilst larger amounts of these metals will introduce such phenomena as the stabilization of the austenite phase at room temperature. The alloying elements added may either simply dissolve in the ferrite or they may either combine with same of the carbon forming carbides.
The principal effects which these alloying elements have on the microstructure and properties of a steel can be classified as follows:
1. The Effect on the Allotropic Transformation Temperature
The allotropic transformation temperatures referred to have are those at 910 ˚C, where, on heating, the body-centered cubic () [BCC] of pure iron changes to one which is FCC ( ), and at 1400 ˚C, where FCC structure, changes back to BCC (S) iron. These change points are designated A3 and A4 respectively.
The A1 temperature is at 723 ˚C, where the
change take place in plain carbon steel, whilst the (A2) temperature is at 769 ˚C, above which pure iron looses its magnetic properties (This point has no structural significance).
* Some elements like Ni, Mn, Co and Cu raise the (A4) temperature and lower the (A3) temperature, as shown in Fig. 18 In this way these elements, when added to a carbon steel, tend to stabilize austenite () and increase the range of temperature over which austenite can exit as a stable phase.
Fig -18- Relative effect of an alloying element on the allotropic transformation temp. A3 and A4
Other elements, the most important of which include Cr, W, V, Mo, Al and Si, have the reverse effect, in that they tend to stabilize ferrite (X) by raising the A3temperature and lowering the A4 , as indicated in Fid . 19.
Fig -19- Relative effects of the addition of an alloying element on allotropic. Transformation. Tempe – raptures at A3 and A4 tending to stabilize.
Such elements restrict the field over which () may exist, and these form what is often called a () loop.
The element of the – stabilizing group generally have a Fcc lattice. Since this is similar to that of austenite, these, elements therefore retard the transformation of () of (). At the same time these elements retard the precipitation of carbides and again this has the effect of stabilizing ().
The – stabilizing elements are usually those with a BCC lattice. These will dissolve more readily in – iron than in – iron and at the same time diminish the solubility of carbon in austenite – In this way stabilize ferrite.
2- The Effect on the Stability of the Carbides:
Some of the allaying elements form very stable carbides when added to a plain carbon steel this generally has a harden my effect on the steel, Its influences the hardening temperature and soaking time. Particularly when the carbides formed are harder than iron carbide it self. Such elements include Cr, W, V, Mo,Ti and Mn.
Fig -20- show the effect these alloying element on probable hardening
Other element has a graphitizing effect on the iron carbide, that is they tend to make it unstable so that it breaks up, releasing free graphitic carbon. Element which tend to cause graphitization include co, cu & Al. forming graphitic carbon causes reduction in the value of strength , ductility , and impact strength , in order to reduce this effect we anther alloying element have reversible effect Which is the element that form carbides.
3- The Effect on Grain Growth
The rate of crystal growth is accelerated at high temperature, by the presence of some elements, like chromium (Cr). Grain growth is retarded by other element like (Ni) Nickel and V (vanadium), whose presence thus produce a steel which is less sensitive to the temperature conditions of heat treatment .
4- The displacement of the Eutectoid point
The addition of an alloying element to carbon steel displaces the eutectoid point towards the left of the equilibrium diagram. That is, a steel can be completely prearlitic even though it contains less than 0.83% carbon as in Fig. 21
Fig -21- The effect of Mn and Ti on the displacement of the eutectoid point in steel.
5. The Improvement in Corrosion Resistance:
The corrosion resistance of steel is improved by the addition of element from thin but dense and adherent oxide films which protect the surface of the steel from further attack.
6. Effect on the Mechanical Properties:
One of the main reasons for alloying is to effect improvement in the mechanical properties of a steel. These improvements are generally the result of physical changes. For example, hardness is increased by stabilizing the carbides; strength is increased when alloying elements due to refinement of grain in Fig 22 shows the minor effect of (Cr) in annealed steels compared with steel cooled in air on Tensile strength.