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Crack arrest checks for new LPG storage tank steels

TWI's large scale test to determine structurally representative crack arrest properties
TWI's large scale test to determine structurally representative crack arrest properties

Connect, September/October 1998

A TWI Member company has been studying the possibility of using alternative materials for new liquefied propane gas (LPG) storage tanks with an operating temperature of -50◦C.

Previous TWI group sponsored projects have shown that the crack arrest properties of 1.5% nickel thermomechanically controlled processed (TMCP) steel and their weldments are suitable for LPG applications. However, recent trends in steel making and the high cost of nickel have resulted in development of micro-alloyed TMCP steels with nickel contents of approximately 0.5%. These steels are reported to possess very similar crack arrest properties.

TWI was called in to provide independent data, and to ascertain that the low temperature properties of these steels will not be affected by welding. A work programme was carried out consisting of ten large-scale double-tension crack arrest tests on parent steels, weld metal and HAZ regions to study structurally representative crack arrest behaviour. Charpy, 'Pellini' drop-weight and fracture toughness initiation (CTOD) complemented the test programme.

Results show that these two steels generally have excellent initiation toughness properties at temperatures down to -50◦C. Also, the programme illustrates that there are 0.5% nickel TMCP steel grades and associated higher nickel weldments available which will arrest fast running brittle cracks for LPG storage tank operating conditions. However, crack arrest is borderline for other 0.5% nickel TMCP steels and their weldments. The use of 0.5% nickel steel for LPG storage tank fabrication is hence a possibility, but independent evaluation of properties under structurally realistic conditions is needed before any decision is made.

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