Chp.9 Phase Transformations

Question 1

What is the Transformation rate?

Answer 1

The reciprocal of the time necessary for a reaction to proceed halfway to its completion.

Question 2

What is a Phase Transformation?

Answer 2

A change in the number and/or character of the phases that constitute the microstructure of an alloy.

Question 3

What is nucleation?

Answer 3

The initial stage in a phase transformation. It is evidenced by the formation of small particles (nuclei) of the new phase that are capable of growing.

Question 4

What is free energy?

Answer 4

A thermodynamic quantity that is a function of both the internal energy and entropy (or randomness) of a system. At equilibrium, the free energy is at a minimum.

Question 5

What is superheating and supercooling?

Answer 5

supercooling: Cooling to below a phase transition temperature without the occurrence of the transformation.

superheating: Heating to above a phase transition temperature without the occurrence of the transformation.

Question 6

What is a Isothermal transformation diagram (TTT)?

Answer 6

A plot of temperature versus the logarithm of time for a steel alloy of definite composition. Used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy.

Question 7

What’s the difference between coarse and fine pearlite?

Answer 7

coarse pearlite: Pearlite for which the alternating ferrite and cementite layers are relatively thick. Occurs in higher temps.

fine pearlite: Pearlite in which the alternating ferrite and cementite layers are relatively thin. Occurs in lower temps.

Question 8

What is Bainite?

Answer 8

An austenitic transformation product found in some steels and cast irons. It forms at temperatures between those at which pearlite and martensite transformations occur. The microstructure consists of alpha-ferrite and a fine dispersion of cementite.

Question 9

What is Spheroidite?

Answer 9

Microstructure found in steel alloys consisting of spherelike cementite particles within an alpha-ferrite matrix. It is produced by an appropriate elevated-temperature heat treatment of pearlite, bainite, or martensite, and is relatively soft.

Question 10

What is Martensite?

Answer 10

A metastable iron phase supersaturated in carbon that is the product of a diffusionless (athermal) transformation from austenite.

Question 11

Explain what an athermal transformation is.

Answer 11

A reaction that is not thermally activated, and usually diffusionless, as with the martensitic transformation. Normally, the transformation takes place with great speed (i.e., is independent of time), and the extent of reaction depends on temperature.

Question 12

What is alloy steel?

Answer 12

alloy steel is a ferrous (or iron-based) alloy that contains appreciable concentrations of alloying elements (other than C and residual amounts of Mn, Si, S, and P). These alloying elements are usually added to improve mechanical and corrosion-resistance properties.

Question 13

What is a continuous cooling transformation diagram (CCT)?

Answer 13

A plot of temperature versus the logarithm of time for a steel alloy of definite composition. Used to indicate when transformations occur as the initially austenitized material is continuously cooled at a specified rate; in addition, the final microstructure and mechanical characteristics may be predicted.

Question 14

What is Tempered Martensite?

Answer 14

The microstructural product resulting from a tempering heat treatment of a martensitic steel. The microstructure consists of extremely small and uniformly dispersed cementite particles embedded within a continuous alpha-ferrite matrix. Toughness and ductility are enhanced significantly by tempering.