describe the phase transformations (Showing in addition the corresponding optical backgrounds and cooling curves) occurring during slow cooling of the following alloys from the austenitic phase: 0.005%C, 0.015%C, 0.4%C and 1.1%C

QN: describe the phase transformations (Showing in addition the corresponding optical backgrounds and cooling curves) occurring during slow cooling of the following alloys from the austenitic phase: 0.005%C, 0.015%C, 0.4%C and 1.1%C

Ans:


Phase Transformations During Slow Cooling of Alloys from the Austenitic Phase

AlloyCorresponding Optical BackgroundCooling Curve
0.005%CHypoeutectoid ferrite (light) and pearlite (dark)Shows a eutectoid transformation at 723°C (1333°F)
0.015%CHypoeutectoid ferrite (light) and pearlite (dark)Shows a eutectoid transformation at 723°C (1333°F)
0.4%CPearlite (dark) and proeutectoid cementite (white)Shows a eutectoid transformation at 723°C (1333°F) and a proeutectoid cementite transformation at 800°C (1472°F)
1.1%CHypereutectoid cementite (white) and pearlite (dark)Shows a proeutectoid cementite transformation at 800°C (1472°F) and a eutectoid transformation at 723°C (1333°F)

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Optical Backgrounds:

  • Hypoeutectoid ferrite (light): Ferrite is a phase of iron that is low in carbon and has a body-centered cubic (BCC) structure. It is relatively soft and ductile.
  • Pearlite (dark): Pearlite is a two-phase mixture of ferrite and cementite. It is harder and stronger than ferrite, but less ductile.
  • Proeutectoid cementite (white): Cementite is an iron carbide (Fe3C) with a very high carbon content. It is very hard and brittle.

Cooling Curves:

The cooling curves for the four alloys show the following:

  • 0.005%C and 0.015%C: These alloys are both hypoeutectoid, meaning that they contain less than 0.76% carbon. The cooling curves for these alloys show a single eutectoid transformation at 723°C (1333°F). This is where the austenite transforms into pearlite.
  • 0.4%C: This alloy is eutectoid, meaning that it contains exactly 0.76% carbon. The cooling curve for this alloy shows two transformations: a eutectoid transformation at 723°C (1333°F) and a proeutectoid cementite transformation at 800°C (1472°F). The proeutectoid cementite transformation is where some of the austenite transforms into cementite before the eutectoid transformation occurs.
  • 1.1%C: This alloy is hypereutectoid, meaning that it contains more than 0.76% carbon. The cooling curve for this alloy shows two transformations: a proeutectoid cementite transformation at 800°C (1472°F) and a eutectoid transformation at 723°C (1333°F). The proeutectoid cementite transformation is where some of the austenite transforms into cementite before the eutectoid transformation occurs.

Conclusion:

The phase transformations that occur during slow cooling of alloys from the austenitic phase depend on the carbon content of the alloy. For hypoeutectoid alloys, the only transformation that occurs is a eutectoid transformation. For eutectoid alloys, two transformations occur: a proeutectoid cementite transformation and a eutectoid transformation. For hypereutectoid alloys, two transformations also occur: a proeutectoid cementite transformation and a eutectoid transformation.

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