OJBECTIVE
The aim of this experiment is to study the
cooling rate effects on the microstructure as well as hardness properties of 0.45% Carbon Steel. RESULTS
Microstructures of carbon steels vary with
cooling rates. The different microstructures for four different medium carbon steel specimens having composition of 0.45% carbon with different cooling rates can be observed below.
Specimen D1, Furnace Cooled
Specimen D2, Air Cooled
Ferrite Pearli te
Specimen D3, Fan Cooled
Specimen D4, Water Quenched
1 Ferrite Pearli te Ferrit Pearli te Martens ite
Rockwell Hardness and Converted Vickers’ Hardness Number (HV) of Specimens
Spe cim en Hardnes s Reading 1 Hardnes s Reading 2 Hardnes s Reading 3 Hardnes s Reading 4 Av era ge Vic ker s' Har dne Ro ck wel l Vic ker s' Har Ro ck wel l Vic ker s' Har Ro ck wel l Vic ker s' Har Ro ck wel l Vic ker s' Har 2
Har dne ss dne ss, HV 1 Har dne ss dne ss, HV 2 Har dne ss dne ss, HV 3 Har dne ss dne ss, HV 4 ss, HV D1, Fur nac e Co ole d HR B 88. 5 18 5.8 3 186 .99 7 D2, Air Co ole d HR B 94. 5 21 6.8 75 21 3.8 75 D3, HR 22 219 3
Fan Co ole d B 95. 5 2.9 41 .95 4 D4, Wa ter Qu enc hed HR C 64. 5 81 4.2 86 769 .57 2
Cooling Rates of Specimens
Cooling rate is given by ΔT/t, where T = temperature in °C and t = time taken for cooling to occur in seconds.
ΔT = 1000°C - 25°C = 975°C
Speci men ΔT (°C) Tim e (s) Cooling rate (°C/s) D1, Furnace Cooled D2, Air Cooled D3, Fan Cooled D4, Water Quenched
Graph of Average Vickers’ Hardness (HVavg)
Vs Cooling Rate ( Δ T/t)
DISCUSSION 1.
Cooling Rate – Hardness Relation
As observed in the above graph of Vickers’ Hardness Vs Cooling Rate, a higher cooling rate results in a greater hardness value of the specimen.
2.Cooling Rate – Microstructure Relation
The different cooling rates of the specimen cause different formation of microstructures. As the hypereutectoid steel cools from
1000°C in the austenite γ phase as shown in Diagram 1 above, the temperature of the steel crosses the upper critical temperature into the “γ + α” region. The transformation of austenite to pearlite begins by formation of cementite nuclei at austenite grain
boundaries. Carbon diffuses from the surrounding austenite to the cementite, depleting the austenite and transforms to ferrite. The rejection of carbon from the ferrite region causes the formation of
additional nuclei of cementite and the
process continues, resulting in the formation of alternating cementite and ferrite. Here, proeutectoid α forms until it reaches the eutectoid isotherm at 723°C of which the remaining austenite ɣ transforms into
Diagram 2
In diagram 2, the Continuous Cooling
Transformation Diagram, Curve X represents a rate of cooling during a normalizing (air cooling) process. Transformation of unstable austenite beings at K and ends at N, with fine pearlite produced. A faster cooling rate will expose the steel longer between the 2 curves and more unstable austenite will be
transformed to pearlite. In addition, faster cooling rate produces finer pearlite grains. However, if the cooling rate is very rapid as represented by Curve Y above, the unstable austenite will persist until it reaches the
critical temperature Ms at O where it directly
transforms to martensite.
It should be noted that although the above diagram is mainly for Eutectoid steels,
similar concepts can be applied for the
hypoeutectoid steel used in this experiment. The differences being the position of the curves as well as the critical cooling rates required for the formation of martensite.
As observed in the experiment, Specimen D1 went through the slowest cooling rate, resulting in lesser and coarser pearlite grains as compared to D2 and D3. Specimen D4 was cooled at a very rapid rate, resulting in the formation of martensite instead.
3.Microstructure – Hardness Relation
From the results of the experiment, it can be seen that the furnace cooled specimen has
the lowest hardness value, followed by air cooled, fan cooled and water quenched. As ferrite grains are pure iron with BCC structure, its orderly metallic bonding is not distorted by compounds such as cementite found in pearlite and the structure is not distorted as compared to body-centered tetragonal lattice found in martensite. The absence of distortion causes dislocation to occur easily, which means that a lesser
amount of stress is required for deformation to occur. This means that ferrite is the
softest as compared to pearlite and
whereby it has the most ferrite grains and least pearlite grains as compared to the other specimens.
Due to the presence of cementite in pearlite, it distorts the orderly crystal lattice and this inhibits the occurrence of dislocation. As finer grains of pearlite have more phase
boundaries as compared to coarse grains, this further inhibits dislocation motion, making fine grained microstructures stronger and harder. This is apparent in the comparison between D2 and D3, where D3 has more and finer pearlite grains.
In specimen D4, it consist of Martensite, which is a single-phase, supersaturated solution of carbon in ferrite with carbon atoms located interstitially in a
body-centered tetragonal lattice. The excessive supersaturation distorts the normal BCC structure to body-centered tetragonal, resulting in changes to its mechanical
properties such as an increase in strength, hardness and decrease in ductility.
Furthermore, the BCT structure has
relatively few slip systems, reducing the occurrence of dislocation.
From the above comparison, it can be
observed that ferrite is the softest, followed by coarse pearlite, fine pearlite and lastly martensite.
CONCLUSION
This experiment well illustrates the effect of cooling rates on the microstructures as well as hardness of a specimen. By changing the cooling rates of carbon steel, we can control and alter the microstructure of the material. This in turn gives us the desired mechanical properties for the material’s specific purpose.
