ASSIGNMENT

Assignment

Assignment

Part a

Austenitization occurs when the temperature of the material exceeds its characteristic austenitization temperature. The temperature at which austenite starts to form in equilibrium conditions is the Ae1 temperature. In higher temperatures the volume fraction of austenite increases at the expense of other phases, until the material is fully austenitic. The temperature at which the transformation to austenite is complete is the Ae3 temperature. If heating conditions apply, the transformation temperatures are raised. Then designations Ac1 and Ac3 are used to represent the critical temperatures . Ac1 temperature increases slightly with increasing heating rate. This movement is quite insensitive to carbide distribution and carbon content. In contrast to the elevation of the Ac1 temperature, the Ac3 temperature is structure sensitive and varies considerably with the heating rate .(Lenel 2004 201-205)

Austenitization of a microstructure consisting of spherical carbides in a matrix of ferrite, a, begins by nucleation of austenite, ?, about carbides in a/a grain boundaries . Austenite film grows until it completely envelopes the carbide. It has been assumed that if the austenitization temperature of the surrounding ferrite is not exceeded, further growth of austenite occurs only by carbon diffusing through the austenite envelope from the dissolving carbide to the advancing a/? boundary. Carbon diffusion through the austenite nodule is the rate controlling process  and .

At sufficiently high temperatures ferrite transforms into austenite without carbon diffusion. In such a case, the properties of the formed austenite depend mainly on the extent of carbide dissolution during austenitization. It has been presented that in the case of alloy carbides, the dissolution rate is determined by the diffusivity of the metallic alloy elements  and . Therefore, the austenitization temperature has a pronounced effect on dissolution due to the temperature dependence of diffusion of carbide forming alloying elements . Alloy carbides are thermodynamically more stable in high temperatures and may be unaffected by the thermal cycle , or complete dissolving may require long austenitization times and high temperatures. (Judd 2008 206-242)

Since diffusion is the rate controlling process in carbide dissolution, the extent of change depends on the number of diffusive jumps which take place during the thermal cycle . The number of atoms diffusing in unit time across unit area through a unit concentration gradient is known as the diffusivity or diffusion coefficient. It depends on the temperature and concentration of the alloy . The variation of diffusion coefficient D (m2 s-1) with temperature is given by

(1)

where D0 is the frequency factor (m2 s-1), Q the activation energy (J mol-1) and R is the gas constant (J mol-1 K-1). Under isothermal conditions the characteristic diffusion distance d (m), assumed to be predominantly controlled by volume diffusion in austenite, is given by

(2)

d2=2Dt.

For a thermal cycle T(t) the characteristic diffusion distance is given by

(3)

Carbon may also diffuse before the a/? transformation , but in such low temperature the diffusivity is sluggish and the effect caused by diffusion in the a-phase is insignificant.

Dissolution rate of carbides is relevant since the carbides act as sources of carbon to the ...