Cooling Rate Effect

LAB REPORT (Informal)

ME2151-1

Cooling Rate Effect Semester 5

1. Objective This experiment was conducted to investigate the effect of cooling rate on the microstructure and hardness of commercial steel. Steel specimens having composition of wt0.45% of carbon with different cooling rates, particularly furnace cooled, air cooled, fan cooled and water quenched, were investigated. 2. Background Steel containing wt0.45% carbon is heat to the austenite phase and then cooled down. When cooled sufficiently slow (equilibrium condition), the composition of the steel can be determined by referring to the iron-carbon phase diagram. The ferrite (proeutectoid ferrite) phase forms as the steel cools, and it can only contain a maximum of wt0.02% carbon. The excessive carbon atoms diffuses out and form Fe3C (cementite) with Fe atoms. This Fe3C serves as a nucleus and layers of ferrite and Fe3C are formed subsequently. Thus a pearlite phase is shaped and the grains grow as the carbon continues to diffuse. When cooled down to room temperature, proeutectoid ferrite (white) and pearlite (dark) can be observed under the microscope. Steel containing wt 0.8% will go through similar transition when heat treated; however, as the phase diagram shows, no proeutectoid ferrite will be formed when it is cooled. Thus, it is very difficult to observe the micro structural differences under a low magnification level, since all could be seen are dark pearlite phases only. That’s way wt0.8% carbon cannot be used for this experiment.

I.

3. Procedure Four steel specimens with different heat treatment processes (furnace cooled, air cooled, fan cooled and water quenched) were provided. They were labeled as D 1, D2, D3, and D4

II.

respectively. Observe the specimens under the optical microscope with a magnification of ×200.

III.

Distinguish and label the phases in the microstructure of the specimens. Determine the hardness of the four specimens using Rockwell Hardness Tester. Scale B

IV. V.

was used for D1 – D3, while scale C was used for D4. Converted the hardness value to Vickers Hardness through the convention table. Analyze the results and establish the relationship amongst cooling rate, microstructure

and hardness of specimens. 4. Results and Discussions

The microstructures of the specimens are shown in the appendix, and the phases are properly labeled. The hardness of the specimens is shown in the following table.

D1 D2 D3 D4

(RB) (RB) (RB) (RC)

Rockwell Hardness 2 3 4 83.0 85.0 84.5 94.0 96.5 95.0 95.5 95.0 95.0 58.0 59.0 59.0

1 85.0 96.0 97.0 57.5

Vickers hardness Avg. 84.4 95.4 95.6 58.4

186.1 222.2 223.7 662.0

Relationship between Hardness and Heat Treatment Process 12 10 8

Vickers Hardness

6 4 2 0

Sample procedure of converting Rockwell Hardness to Vickers Hardness: The table on the right is extracted from the conversion table. By interpolation, we get: Vickers Hardness = 660+

58.4−58.3 × ( 670−660 )=662.0 58.5−58.5

Rockwell Hardness (Scale B) 58.8 58.3

Vickers Hardnes s 670 660

Observing the microstructures, we find that different specimens have different amount of proeutectoid ferrite grains. The amount is decreasing with the increasing speed of cooling, and there are not proeutectoid ferrite grains present in the water quenched specimen. The difference can be explained as follow: when steel cools faster, the carbon atoms have less time to diffuse out of the solid solution (austenite), thus less proeutectoid ferrite is forms; there is also less time for the formation of Fe3C nucleus and the growth of pearlite grains, thus the pearlite grains are

finer. When steel cool very rapidly, there is insufficient time for the carbon atoms to diffuse, they form interstitial impurities and distort the Fe lattice so that body-centered tetragonal structure is formed in stead of the normal b.c.c. structure. This phase is called martensite. It is meta stable and hard. The hardness test shows that hardness increases as the steel is cooled faster. Faster the cooling process is, finer grains are formed and the larger amount of grain boundaries more significantly hinter the propagation of dislocation. As a result, hardness of steel is increased. It is also noted that water quenched specimen is very hard. Apart from the very fine grains, it also has supersaturated carbon atoms, which are interstitial impurities and have an effect of blocking dislocation propagation. These two factors contribute to the extreme high hardness of the steel. Water quenching is widely used to produce hard metals. Long ago, blacksmith used this method to produce high quality swords; and in the modern days, chemicals solutions are sometimes used to further improve the mechanical properties of quenched metals. There is probably significant error in the measurement of the hardness of air-cooled specimen. Though the trend is properly reflected, the exact value may not be accurate. The value ought to be considerably higher than that of the air cooled specimen. The reason may lies in the fact that the specimen used in the experiment has gone through a number of tests and there is limited space room left for new tests. 5. Conclusion Through this experiment, the effect of cooling rate on the microstructure and hardness of steel containing wt0.45% carbon specimens. Proeutectoid ferrite phase is formed in the furnace cooled, air cooled and fan cooled specimens, and the amount decreases as the cooling process becomes faster. A special phase, martensite is formed when steel is water quenched. Along with the difference in the microstructure, there are also differences in the hardness among the specimens. The faster the cooling process is, the harder the steel is; and the water quenched steel exhibits exceptional hardness.