Phase Diagram

Phase Diagrams Fe-Mn, Fe-Co, Fe-Mo

 

In pure iron, the A4 (1394 °C) and A3 (912 °C) transformations take place at constant temperatures. If an element enters into solid solution in iron — forming in that way a binary alloy — each of these transformations are required by the Phase Rule to occur over a range of temperature. Some elements, such as manganese, raise the A4 and lower the A3 transformation temperatures below increasing, in effect, the extent of the gamma field in the iron-carbon phase diagram.

Fe-Mn phase diagram shows which phases are to be expected at equilibrium for different combinations of manganese content and temperature.  The melting point of iron and manganese at the pressure of 101325 Pa is 1538 °C and 1246 °C, respectively.

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Bainite

 

Bainite is an acicular microstructure (not a phase) that forms in steels at temperatures from approximately 250-550°C (depending on alloy content). First described by E. S. Davenport and Edgar Bain, it is one of the decomposition products that may form when austenite (the face centered cubic crystal structure of iron) is cooled past a critical temperature of 727 °C (about 1340 °F). Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite.

Bainite Microstructure

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Time-Temperature-Transformation (TTT ) Diagram

T (Time) T(Temperature) T(Transformation) diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy. When austenite is cooled slowly to a temperature below LCT (Lower Critical Temperature), the structure that is formed is Pearlite. As the cooling rate increases, the pearlite transformation temperature gets lower.

Figure 1a. TTT Diagram

In Figure 1a the area on the left of the transformation curve represents the austenite region. Austenite is stable at temperatures above LCT but unstable below LCT. Left curve indicates the start of a transformation and right curve represents the finish of a transformation. The area between the two curves indicates the transformation of austenite to different types of crystal structures. (Austenite to pearlite, austenite to martensite, austenite to bainite transformation.)

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Fe-Fe3C T-T-T Diagram

 

Fe-Fe3C T-T-T Diagram, Adapted from Callister pg. 295, Fig. 10.6

The time-temperature transformation curves correspond to the start and finish of transformations which extend into the range of temperatures where austenite transforms to pearlite. Above 550 C, austenite transforms completely to pearlite. Below 550 C, both pearlite and bainite are formed and below 450 C, only bainite is formed.

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