Vessels and pipelines designed to operate at very low temperatures(cryogenic) must be fabricated from materials and with techniques suitable to the service conditions.
The problems connected with the behavior of normal steels at low temperatures were first brought to attention at the end of World War II, when eight ships of the Liberty class where involved in spectacular disasters by their sudden brittle fracture in two parts under conditions of cold weather and high seas.
The investigations that followed were instrumental in discovering a new property, unheard of up to that time, called Fracture Toughness. Furthermore it was found that, in certain materials, ductility is strongly dependent upon temperature.
A special Drop-Weight Nil-Ductility-Test was introduced to help rating materials and processes and to determine the temperature above which a dynamic crack is arrested.
In normal steels perfectly ductile at room temperature, a gradual passage to brittle fracture susceptibility upon impact was found, strongly dependent on decreasing temperatures. The temperature range where this phenomenon occurs has been called the Ductile to Brittle Transition Temperature (DBTT). Structures made for service at low temperatures must therefore be made of materials that maintain adequate ductility at sub freezing conditions. Cryogenic alloys include austenitic stainless steels, manganese stainless steels, 9 Nickel steels, maraging steels, titanium, aluminum and nickel alloys.
A low cost way to measure the resistance of a weldment to impact fracture is performed by means of an Impact Toughness Testing on a specimen of specified geometry like the Charpy V-notch (CVN) test (ASTM E23). The amount of energy absorbed at any specified temperature during fracture is measured and recorded.
This test does not measure an inherent material property but results in a relative measure of impact toughness. The results permit to rate different materials and procedures in a database to be used for design and for inspection.
The specimen is oriented to have the notch and the expected plane of fracture run longitudinally through the weld metal. The test is conducted on welded specimens at the specified low temperature. The CVN absorbed energy result should be at least as specified by the requirements.
Cryogenic Tanks are used for Liquefied Natural Gas (LNG), Cryogenic Ammonia and other products at cryogenic temperatures for storage and/or transportation.
by:PWL
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