TS-99
A Study on Usage of Steel Plate having thickness more than 40mm without Post Weld Heat Treatment for Pressurized LPG Cargo Tanks
Koichi NISHIFUJI* and Hiromi SHIIHARA*
ABSTRACT
There are many demands of large size pressurized LPG cargo tank having diameter over 10m and made of steel plate thickness over 40mm. ClassNK's Rule requires that the pressure vessel made of steel plate having thickness over 40mm with welding should be conducted with Post Weld Heat Treatment (PWHT). However, there is few appropriate facility for PWHT m the world ClassNK, LPG tank manufacturers and steel metal industries have been proceeding a study with analysis by fracture mechanism and experiments in order to product LPG tanks made of welded steel plate having thickness more than 40mm without WHT. The result shows that there is not deferent strength and toughness between welded steel plate having 40, 50 and 60mm with PWHT and without PWHT at -10℃. So it is recognized from this result that the steel plate having thickness of 50mm can be used for pressurized LPG tank without PWHT.
Key Words: PWHT, Pressurized LPG cargo tank, Brittle fracture, Critical stress intensity factor, Critical COD
1. INTRODUCTION
In case of manufacturing a pressurized LPG tanks for installation on ship. Post Weld Heat Treatment (PWHT) is required by NIPPON KAIJI KYOKAI (hereinafter, to be referred as ClassNK) rules. The following effects are expected by PWHT carried out at the aim of removal or restriction of residual stress generated on the welded parts and cold formed parts;
1) Prevention of embrittlement due to strain hardening
2) Increase of ductility of welded parts
3) Improvement of stability of the shape
A dominant effect expected by PWHT is to prevent brittle fractures of the tanks used at low temperature such as lower than -10℃. ClassNK's Rules have exemption of PWHT to LPG tank made of the steel plate having trickiness not more than 40mm for usage at ambient temperature subject to having the enough impact test value of the base metal and welded parts.
On the other hand, there is strong demand of large size pressurized LPG tanks over 10m of diameter and over 40mm of steel plate thickness since 1990. However, there is few appropriate facility in the world to carry out PWHT to such large size tanks.
High strength quenched and tempered rolled steel plate, which have tensile strength over 700N/mm2 (HT70) is generally used as shell plate for pressurized LPG tank in order to decrease thickness of steel plate for pressurized LPG tanks. This material essentially includes alloy components such as Mo, Cr, V in order to achieve high tensile strength. These components make steel material sensitive against stress relief cracking. And welded pans for high tensile steel become embrittlement due to precipitation hardening by PWHT. For the reasons of above mentioned, HT70 is generally used for pressurized LPG tanks without PWHT.
ClassNK, LPG tank manufacturers and steel material industries have been proceeding a study based on fracture mechanism with experiments to usage of steel plate having more than 40mm without PWHT[1], The experiment of the notched and angular distorted wide cross joint tensile test was carried out in addition to tensile test, bending test impact test and so on using large steel plate of thickness over 40mm.
2. ITEM OF EXPERIMENTS
The aim of the experiments is confirmation of strength and fracture toughness for welding parts without PWHT compared to it carried out.
The following tests were carried out in addition to the notched and angular distorted wide cross joint tensile test.
1) Base metal test
・Chemical compositions analysis
