Continuous-cooling transformation diagrams

Fig. 18. Continuous-cooling transformation diagram for a Type 4340 alloy steel, with superimposed cooling curves illustrating the manner in which transformation behavior during continuous cooling governs final microstmcture (1). Ae is critical temperature at equiHbrium. Ae is lower critical...

Atkins M Atlas of continuous cooling transformation diagrams for engineering steels . British Steel Corporation, Sheffield, 1977. 

Given the isothermal transformation (or continuous-cooling transformation) diagram for some iron-carbon alloy, design a heat treatment that will produce a specified microstructure. 

It is important to note that the treatments relating to the kinetics of phase transformations in Section 10.3 are constrained to the condition of constant temperature. By way of contrast, the discussion of this section pertains to phase transformations that occm with changing temperatirre.This same distinction exists between Sections 10.5 (Isothermal Transformatiorr Diagrams) and 10.6 (Continuous-Cooling Transformation Diagrams). 

Superimposition of isothermal and continuous-cooling transformation diagrams for a eutectoid iron-carbon alloy. 

With regard to the representation of the martensitic transformation, the M(start), M(50%), and M(90%) lines occur at identical temperatures for both isothermal and continuous-cooling transformation diagrams. This may be verified for an iron-carbon alloy of eutectoid composition by comparison of Figures 10.22 and 10.25. 

Figure 10.27 Continuous-cooling transformation diagram for a eutectoid iron-carbon alloy and superimposed cooUng curves, demonstrating the dependence of the final microstructure on the transformations that occur during coohng.

In summary, isothermal and continuous-cooling transformation diagrams are, in a sense, phase diagrams in which the parameter of time is introduced. Each is experimentally determined for an alloy of specified composition, the variables being temperature and time. These diagrams allow prediction of the microstructure after some time period for constant-temperature and continuous-cooling heat treatments, respectively. 

Isothermal and continuous-cooling transformation diagrams make possible the prediction of microstructural products for specified heat treatments. This feature was demonstrated for alloys of iron and carbon. 

The addition of some alloying elements (other than carbon) shifts pearlite and bainite noses on a continuous-cooling transformation diagram to longer times, making the transformation to martensite more favorable (and an alloy more heat-treatable). 

Figure 10.40 shows the continuous-cooling transformation diagram for a 0.35 wt% C iron-carbon alloy. Make a copy of this fignre, and then sketch and label continnous-cooling cnrves to yield the following microstrnctnres ... 

Briefly explain why there is no bainite transformation region on the continuous-cooling transformation diagram for an iron-carbon alloy of entectoid composition. 

Figure 10.40 Continuous-cooling transformation diagram for a 0.35 wt% C iron-carbon alloy.

Continuous-Cooling Transformation Diagrams Mechanical Behavior of Iron-Carbon Alloys... 

Figure 13.2 Continuous-cooling transformation diagram for the crystallization of a lunar glass (35.5 wt% Si02,14.3 wt% TiOj, 3.7 wt% AljOj,...

The role of continuous-cooling transformation diagrams in the heat treatment and control of microstructure was presented (for iron-carbon alloys) in Chapter 10. We discussed in this chapter how this type of diagram is employed in designing heat treatments to crystallize glass-ceramics. The following concept map represents this relationship for processing these materials. 

Figure 9.10 shows the so-called continuous cooling transformation diagram , i.e., the relationship between cooling condition and crystallization kinetics, of a polymer under quiescent condition. In the diagram, the cooling curves are plotted... 

Figure 9.10. Continuous cooling transformation diagram under quiescent condition. A higher number on the cooling curve indicates a higher cooling rate.

---