Takahiro Kamo

Influence of martensitic islands on fracture behaviour of high heat input weld HAZ

A study was made on the relation between CTOD value and M–A constituent for the single-heat-cycled weld heat-affected zone (HAZ) of YS 320–360 N/mm2 class thermomechanical control process steels with increased heat input. It was found that the M–A constituent disappeared and lost its deterioration effect on the HAZ CTOD toughness when the heat input exceeded about 15 kJ/mm, although this boundary heat input depended on the steel chemistry. On the other hand, the austenite grain size increased monotonically with increasing the heat input. But the austenite grain size could not be the controlling factor of the HAZ toughness, and its effect deferred between base metals. However, the HAZ toughness was related to the fracture facet size for the large heat input conditions. This fracture facet size, which represents the fracture toughness, is considered to be a measure of the uniformity of the transformed microstructure from austenite in the HAZ.

Development of 7%Ni-TMCP Steel Plate for LNG Storage Tanks

Demand of natural gas continues to increase in the recent years due to the rise of environmental issue and the drastic increase of crude oil price. These events led to the increase of constructions of Liquefied Natural Gas (LNG) storage tanks worldwide. The inner tank material for above ground LNG storage tanks have mostly been made of a 9% nickel steel plate over the last 50 years as it has excellent mechanical properties under the cryogenic temperature of −160deg-C. During this period, the LNG storage tanks made of 9%Ni steel plate have been operated safely at the many LNG export and import terminals in the world. Meanwhile, technologies of steel making, refinement, design, analysis, welding and inspection have been improved significantly and enabled enlarging volumetric capacity of the tank 2–3 times. There was a tendency for nickel price to increase in recent years. In such a circumstance lowering Ni content has focused attention on the 9%Ni steel as nickel is an expensive and valuable rare metal and a 7%Ni steel plate was eventually researched and developed by optimizing the chemical compositions and applying Thermo-Mechanical Controlled Process (TMCP). As a result, it was demonstrated that 7%Ni-TMCP steel plate had excellent physical and mechanical properties equivalent to those of 9%Ni steel plate. In order to evaluate fitness of the 7%Ni-TMCP steel plate and its weld for LNG storage tanks a series of testing was conducted. Several different plate thicknesses, i.e. 6,10,25,40 and 50 mm, were chosen to run large scale fracture toughness tests including duplex ESSO tests, cruciform wide plate tests as well as small scale tests. It was concluded that the 7%Ni-TMCP steel plate warrants serious consideration for use in LNG storage tanks. This paper reports details of the research and development of the 7%Ni-TMCP steel plate.Copyright

Application of 7%Ni-TMCP Steel Plate to Large Scale LNG Storage Tank

Increase of natural gas demand has led the increase of constructions of Liquefied Natural Gas (LNG) storage tanks worldwide. The inner tank material for above ground LNG storage tanks has mostly been made of 9% nickel steel plate over the last 50 years as it has excellent mechanical properties under the cryogenic temperature of −160deg.C. During this period, the LNG storage tanks made of 9%Ni steel plate have safely been operated. Meanwhile, technologies of steel making, welding, design, construction and inspection of 9%Ni steel plate have been improved significantly and enable us to achieve enlarging volumetric capacity of the tank to 2–3 times.It is known that nickel is an expensive and valuable rare metal and recent tendency of increase in nickel price has influenced the price of 9%Ni steel plate. In such a circumstance lowering the Ni content would save cost and this has led to an extensive research and development resulting in the 7%Ni-TMCP steel plate. This 7%Ni-TMCP steel plate developed is characterized by adjustment of chemical compositions and application of suitable Thermo-Mechanical Controlled Process (TMCP).In order to evaluate fitness of the 7%Ni-TMCP steel plate and its weld for LNG storage tanks, a series of testing was conducted. Several different kinds of plate thicknesses from 6 to 50 mm were chosen to run large scale fracture toughness tests such as duplex ESSO tests, cruciform wide plate tests as well as small scale tests. As a result it was demonstrated that the 7%Ni-TMCP steel plate had a quite excellent resistance to brittle fracture of the inner tank steel and its welds exposed to the cryogenic temperature of LNG.Furthermore, other various issues such as extent of applicable heat input, repair weld, difference of physical constants and so on were investigated and found acceptable.In Japan Ministry of Economy, Trade and Industry (METI) approved to apply this steel to new LNG storage tanks of Osaka Gas. After that Osaka Gas decided to construct a new full containment LNG storage tank applying 7%Ni-TMCP steel in Senboku terminal 1. The capacity is 230,000m3, which is larger than present largest scale of 180,000m3 in Japan. The construction is planned to start in September 2012 and be completed by November 2015.Copyright

Development of 550 MPa Yield Strength Steel Plate for Offshore Structures

Offshore structure steel with high strength of YS550MPa has been investigated. As for offshore structure steel, high toughness in welded joints is required in addition to that in base metal. TMCP type steel of up to YS420MPa grade is used widely, and up to YS500MPa grade is reported in some papers. However, steel of higher strength grade with good toughness and weldability will be beneficial to structures in strict conditions. To reach the YS550MPa requirement, hardening effect by Cu precipitation was utilized. Steel plates were designed with micro-alloyed low C-Mn-Cu-Ni-Cr-Mo system. The combination of the copper precipitation and TMCP technology can increase strength without deteriorating toughness and weldability. Heat treatment for Cu precipitation was carried out to optimize the balance of strength and toughness of the base metal. The developed steel also shows good HAZ CTOD toughness up to 76.2mm thickness in several welding conditions including after PWHT. The newly developed steel has the possibility to increase the flexibility to design large-sized structures.© 2004 ASME