DUCTILE-TO-BRITTLE TRANSITION During World War II when a number of welded transport

ships, away from combat, suddenly split in half. The vessels

were constructed of a steel alloy that possessed adequate

ductility according to room-temperature tensile tests….but the

brittle fractures occurred at relatively low ambient

temperatures, at about 40C.

One of the primary functions of Toughness tests is to

determine whether or not a material experiences a ductile-to-brittle transition with decreasing temperature and, if so,

the range of temperatures over which it occurs.

The ductile-to-brittle transition is related to the temperature

dependence of the measured impact energy absorption. This

transition is represented for steel by curve A in Figure 9.20.

Frequently, the percent shear fracture is plotted as a function

of temperature—curves B in Figure 9.20.

Page 1 of 10

At higher temperatures the CVN energy is relatively large,

with a ductile mode of fracture.

As the temperature is lowered, the impact energy drops

suddenly over a relatively narrow temperature range, below

which the energy has a constant but small value; that is, the

mode of fracture is brittle.

Page 2 of 10

Appearance of the failure surface is indicative of the nature

of fracture, and may be used in transition temperature

determinations.

For ductile fracture this surface appears fibrous or dull (or of

shear character); conversely, totally brittle surfaces have a

granular (shiny) texture (or cleavage character).

Over the ductile-to-brittle transition, features of both types will

exist (Figure 9.21).

For many alloys there is a range of temperatures over which the ductile-to-brittle transition occurs (Figure 9.20);

This presents some difficulty in specifying a single ductile-to-brittle transition temperature.

Page 3 of 10

T1 = FTP = Fracture transition plastic (temperature at which

the fracture changes from totally plastic to substantially

brittle).

T5 = NDT = Nil ductility temperature ( temperature at which

fracture initiates with essentially no prior plastic deformation)

Page 4 of 10

Not all metal alloys display a ductile-to-brittle transition.

Those having FCC crystal structures (including aluminum-

and copper-based alloys) remain ductile even at extremely

low temperatures.

However, BCC and HCP alloys experience this transition.

For these materials the transition temperature is sensitive to

both alloy composition and microstructure.

For example,

1) Decreasing the average grain size of steels results in a

lowering of the transition temperature. Hence, refining the

grain size both strengthens and toughens steels.

2) In contrast, increasing the carbon content, while increasing

the strength of steels, also raises the CVN transition of steels,

as indicated in Figure 9.22.

Page 5 of 10

Page 6 of 10

Page 7 of 10

Page 8 of 10

Page 9 of 10

Page 10 of 10