Material selection for low-temperature condition

Why do materials that are sound at room temperature fail before design life when exposed to temperature below room temperature, even though its tensile properties are good enough to withstand the applied load?. What happens to material properties when they are exposed to a temperature much below room temperature?. How materials must be selected in this case?.

These questions were boggling my mind when I reviewed the case study of titanic and liberty ship failures.

Finally found an interesting fact or criteria that causes these failures. Below article will briefly tell about that.

A tensile test is not sufficient to predict material fracture behavior under some circumstances when materials are exposed to temperature below room temperature. In this case, impact testing is used to understand characteristics of materials under the following conditions low temperature, high strain rate, and high triaxial state of stress.

Ductile to brittle transition temperature:

One of the primary purposes of the impact test is used to determine the ductile to brittle transition effect with decreasing temperature.

At lower temperatures, the materials that are ductile at room temperature will become brittle. This is known as ductile to the brittle transition temperature. At that time impact energy of material gets drastically reduced.

General Ductile to brittle transition temperature graph

Charpy test:

The most preferred method for impact testing is the Charpy test. During impact testing, energy observed by the material at a particular temperature is tested.

There are no explicit criteria for ductile to brittle transition temperature. A particular energy value is considered for all the testing temperatures. The energy value at which the components fail is considered to be 50% brittle and 50% ductile.

Suppose you are given a case where you need to select a material for low-temperature application without any comprise in tensile properties. How would you do that?. What criteria would you take to make the design safe?.

The below example will provide a deeper understanding of that

The particular impact energy of 27J is considered for testing the material at different temperatures. Q235 actually stands for low carbon steel with 235Mpa yield strength. For Q235 steel irrespective of quality grade, all the tensile properties remain the same. Q235 steels are used in making tanks as it has good weldability and formability criteria. I will select the quality grade based on the application temperature required as tensile properties are constant. For each temperature application, the chemical composition of steel alone varies.

The below image will give an idea of how different metals behave incased of Ductile to brittle transition temperature.

Medium and low carbon FCC metals as well as most HCP metals have high notch toughness and brittle fracture is not a problem unless there is some special chemical environment.

Some examples are Aluminium and copper alloys.