Me 2151-1 Cooling Rate Effect

ME 2151-1 COOLING RATE EFFECT SESSION: 2009/2010

1. Objectives

To study the effect of cooling rates on the microstructure & hardness of 0.45% carbon steel

2. RESULTS (2a)

Results

Of

Observations

On The

Microstructures

of

Specimens Below are the results obtained for the observations on the microstructures of the various specimens used during the experiment, and all the important phases are labeled.

Specimen D1 Furnace Cooled. 400 ×

Specimen D2 , Air Cooled. 400 ×

Specimen D3 Fan Cooled. 400 ×

Specimen D4 Water Quenched. 400 ×

(2b)

Results of Rockwell Hardness Test On Specimens

Rea

Vickers

Rea

Vicker

Rea

Vickers

Rea

Vickers

Average

d

Numbe

d

s

d

Numbe

d

Numbe

Vickers

rate(oC/s

mg

r

mg2

Numb

mg3

r

mg

r

Number

)

1 Furnace

83.

Cooling

0

Air

94.

Cooling

0

Fan

96.

Cooling

0

er

deviation

Cooling

4

163.9

83.0

163.9

81.0

157.7

79.0

151.0

159.1

8.1

0.01

213.8

94.5

216.9

94.5

216.9

93.5

210.6

214.5

3.9

0.3

225.9

95.5

222.9

94.5

216.9

94.0

213.8

219.9

8.0

15

Water

59.

Cooling

0

675.0

59.5

686.0

59.0

675.0

58.0

654.0

672.5

16.7

3. DISCUSSIONS AND ANALYSIS OF RESULTS (3a)

Comments On The Observed Microstructures And Effects Of

Cooling Rate On Microstructures The physical properties (color, crystalline form if solid, density, etc.) may differ widely, but identical chemical compounds can be formed from the various allotropes of the one element. In this very experiment, we are considering 0.45% carbon steel, and therefore only need to look at the Fe-C phase diagram up to around 0.45 % C.

The effect of rate of cooling is studied using samples of different cooling rates but with the same carbon content. We will use

480

samples D1(furnace cooled), D2(air cooled), D3(fan cooled) and D4(water cooled), which have 0.45% C to study this effect.

The microstructure of D1 (furnace cooling) consist largest of white space and dark space. Which means material stands under temperature between 500-700 for a very long time. So the ɑ phase forming process have been doing for a long time. The result is vary big amount of ɑ phase in the finally product. Compare to specimen D2, the faster cooling rate the less ɑ phase has time to form. But more pearlite phase appears. In specimen D3, the cooling rate comes to 15 oC/s .The ɑ phase just does not have enough time to form and almost all space is filled with pearlite phase. However, the D4 shows another phase caused by very fast cooling rate. Both ɑ phase and pearlite phase have no time to form. A supersaturated solid solution of carbon in b.c.t iron is formed.

(3b)

Effects Of Cooling Rate On Microstructures And Hardness

Now, we shall see how the different in the cooling rate affects the hardness and strength of steel. In the hardness test done in the experiment, we observed that the hardness increases as the rate of cooling increases. Under water cooling (D4), the material's cooling rate is too fast. Martensite phase is formed. This specimen is the strongest

among

the

4

specimen. It is probably because martensite phase has a body-centered tetrahedral, which is much stronger than the

pearlite

phase

(

a

comparison with both ferrite

and orthorhombic) structure. Comparing D1, D2 and D3: D1 is weakest because it has the finest ɑ phase of all 3. The ɑ phase is the softest structure appears on the diagram.

D2 and D3 have very similar results in hardness test. Which probably caused by following reasons. First, in D3 there is a large degree of adherence between the two phase. Which would probably weak the pearlite phase structure. However, the strong and rigid cementite phases severely restricts the deformation of the softer ferrite phases in the regions adjacent to the boundary thus reinforcing the ferrite. Second, in D2 there is lots of ferrite phase exist with pearlite phase. In this way, lots of phase boundaries are formed. They serve as barriers to dislocation motion in the same as do grain boundaries. So for D2, there are more boundaries that the dislocations have to pass during plastic deformation. This greater restriction to dislocation motion gives D2 a greater hardness. The two diagrams below summarised the different in hardness for the various type of microstructures formed under different cooling rates.

Conclusion: From this experiment, I learn that by changing the cooling rate, we can control the microstructure which in turn will give us the desired mechanical properties.