LOW TEMPERATURE CARBON STEEL –
Before explaining the LTCS we must know about the term DBTT.
What is called DBTT?
In case of any metal as the temperature increase the strength decreases. Reverse also should be true. As temperature decreases the strength should increases. As the temperature of material decreases, the ductile natures of material change to brittle.
For example if you take a rubber tube, dip it in liquid nitrogen and then hammer the same, the rubber tube breaks like glass. Thus each material has temperature at which properties of material changes from ductile to brittle. This temperature is called ductile to brittle transition temperature.
What is low temperature carbon steel?
Carbon steel can be converted from ductile to become brittle around zero degree C. The properties of material can be modified during the manufacturing stage so that DBTT can be further lowered. Such steel are called low temperature carbon steel (LTCS).
These steel are subjected to impact test loading at the low temperature to satisfy the low temperature design requirement. Killed carbon steel can be used up to -29 degree C. Below this temperature usually austenitic stainless steel can be used.
LTCS is normally used in cryogenic service or fluids that can cause a reduction in temperature in case of a leak. For example in case of LPG is a gas at ambient condition.
But it is transported or stored as liquid under pressurize condition. In case of minor leaks, the escaping liquid gets converted into gas very rapidly by collecting the heat from surrounding. This can result in a low temperature zone and if DBTT is reached, then pipe or equipment material can fail in a catastrophic manner.
To avoid this LTCS is used. These steel undergo special manufacturing process and the steel should have minimum impact load test value.