When a steel is annealed this means that the steel (EN 8, 0.4 % Carbon) is heated to 900 0C. The steel is then cooled slowly which causes only a few nuclei to form. This means the metallic crystals can grow unimpeded to a large size, which causes the metal to be more ductile and mallabel than normalised steel. This is because the carbon atoms, which are present, are distributed along the crystal edges and since only a few crystals are present the impurity will be highly concentrated along these grain boundaries.
When a steel is normalised this means that the steel (EN 8, 0.4 % Carbon) is heated to 900 0C. The steel is then cooled rapidly which leads to the formation of a sudden shower of nuclei. Since there are many crystals they are small in size, which is generally tougher and stronger but more brittle. This is because the carbon in the steel can’t form in the crystal structure they are forced to the grain boundaries, because there are many crystals there will be many grain boundaries when the crystal meet so the carbon will be relatively more evenly distributed throughout the steel. Rather than spread out in the metal crystal lattice as in annealed steel.
The interstitial solid solution of carbon in FCC iron is known as Austenite. It can contain up to 2.0% Carbon and is a stable phase between 723 and 14930C.
The interstitial solid solution of carbon in BCC iron is known as ferrite. This is stable below 9100C and can contain a maximum of only 0.02% carbon.
Iron carbide Fe3 is generally known as Cementite
Perlite is a mixture of two phase’s ferrite and cementite.
Metals have different hardness because the atoms in the lattice structure of different metals elements have different bond energies. The higher the bond energy the harder it will be to break the bond. Since metals are made up elements each will have a different hardness.
Method
Four steel specimens are provided that have been heated in a furnace for 30 minutes at 900 0C, The first specimen was then quenched in water, the second quenched in oil, the third was allowed to cool in the furnace annealing it the fourth specimen has been normalised.
Each specimen is prepared for microscopic examination by grinding and polishing. A mirror like surface free from any visually observed scratches is needed for this experiment. This is to ensure when looking at the surface with an optical microscope, that the pyramid shape is easily recognisable. The hardness is then tested using the Vickers Hardness Machine.
The Vickers pyramid hardness test uses a square based diamond pyramid as the indentor. In this test, the diagonal length of the square impression is measured by means of a microscope which has a variable slit built into the eyepiece. The width of the slit is adjusted so that its edges coincide with the corners of the impression, and the relative diagonal length of the impression is then obtained from a small instrument geared to the movement of the slit, and working on the principle of a revolving counter. The ocular reading thus obtained is converted to Vickers Pyramid Hardness Number (VPN) by reference to tables.
Results
The steel which has been quenched in water is the hardest of the four specimens with annealed been the weakest. This can be seen in the table of results above.
Discussion
Steel is essentially an alloy of iron and carbon containing up to roughly 2.0% carbon. The relatively small carbon atoms dissolve interstitially in iron. The maximum solubility of carbon in the FCC form of iron is 2.0% and this sets the limit of the carbon content of steel. As the FCC form changes to BCC any carbon in solution in excess of 0.02% will be precipitated from solid solution assuming that it is cooling slowly enough to permit this. Under the slowest rate of cooling possible carbon does not precipitate as graphite but as iron carbide, Fe3C. Like most metallic carbides this is a hard substance. Consequently, as the carbon content of steel increases so does its hardness.
Once an alloy has solidified atoms move with greater difficulty into positions of equilibrium and often require considerable thermal activation to effect this state. This also applies to the polymorphic transformation which takes place in iron and consequently in carbon steels. For a 0.4% carbon steel the transformation, austenite→ferrite begins at approximately 820°C.
In general terms, hardness can be defined as the ability of a material to resist surface abrasion. The phase transformation required to obtain hardness in the steel varies between each specimen. Each specimen was cooled at a different rate; this affects the phase transformation between specimens.
The first specimen was cooled in water. Like oil quenching, in this process the steel is rapidly cooled in water. This causes considerable distortion of the structure. The relationship between the temperature at which transformation of austenite occurs and the structure and properties produced can be studied in a time-temperature-transformation (TTT) graph. A similar effect occurs in water quenching as in oil quenching. The austenite is rapidly cooled allowing the FCC transformation to BCC to occur before carbon is able to form carbide crystals. The resultant metallurgy of this specimen is that it is considerably harder than the other specimens that were cooled at a slower rate. This is shown in the results of the experiment.
The second specimen was cooled in oil. Phase transformations in alloys are time-dependent, therefore when austenite is cooled at a high rate the transformation from FCC to BCC is so great that it occurs before carbon is able to form carbide crystals. Therefore, the faster the rate of cooling the harder the steel. Thus, this specimen had a high hardness value.
The third specimen was annealed. This process requires the steel not be removed from the furnace to cool, but left in the furnace air. This process of heating then slowly cooling causes the steel to be softer and therefore more machinable. Whilst the tensile strength is little affected by this treatment, ductility is increased. This specimen took the longest to cool because there was less heat loss from the steel into the surrounding environment, and the warm air in the furnace prolonged the heat insulation of the steel. Its metallurgy should then follow that it was the least hard. This is shown in the results of the experiment.
The fourth specimen was normalised. This is done to refine the grain of steel, which improves its ductility and toughness. After a short period of ‘soaking’, the steel is removed from the furnace and cooled in still air when the fine-grained austenite will transform to fine-grained ferrite and pearlite. This cooling process takes a long time because although the initial heat loss from the steel specimen is high on removal from the furnace, the subsequent heat loss decreases gradually over time; thus the cooling rate is slow.
Conclusion
In summary, the effects that different rates of cooling have on steel are shown to increase the hardness of steel by rapid cooling in water (1st specimen) or decrease the hardness by softening it through annealing (3rd specimen).
Refernces
Higgins, R.A. (1994) Properties of Engineering Materials 2nd edition. London: Arnold
