Crack arrest checks for new LPG storage tank steels

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

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

Previous TWI joint industry projects had shown that the crack arrest properties of 1.5% nickel thermomechanically controlled processed (TMCP) steel and their weldments were suitable for LPG applications. However, the development of micro-alloyed TMCP steels with nickel contents of approximately 0.5% reported to possess very similar crack arrest properties, offered at a lower cost alternative.

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 structurally representative large-scale double-tension crack arrest tests on parent steels, weld metal and HAZ regions to study crack arrest behaviour. Charpy, 'Pellini' drop-weight and fracture toughness initiation (CTOD) complemented the test programme.

Results showed that the 0.5% Ni steels generally had excellent initiation toughness properties at temperatures down to -50°C. Furthermore, the programme demonstrated that there were 0.5% Ni TMCP steel grades and associated higher nickel weldments available which could arrest fast running brittle cracks under LPG storage tank operating conditions. For other 0.5% Ni TMCP steels and their weldments, however, crack arrest was borderline. The use of 0.5% Ni steel for LPG storage tank fabrication was hence a possibility, but independent evaluation of properties under structurally realistic conditions was required for safe material selection.