1. Introduction.
The welding process are one of the most common assembly techniques
used for assembly, fabrication and welding storage tank, piping, vessels and
marine carrier of LNG, however, the welding procedure for steel alloy
requires high level of skills on welding that is strictly controlled welding
parameter for balancing avoidance of surface integrity imperfection so the
only way to enhanced the surface integrity and weld quality attributes of
welded joint by controlling the parameters of welding process.
1.1. Materials design for LNG.
Demand for natural gas continues to increase due to the environmental issues and the rise of
crude oil price Moreover, the large volume of shale gas that developed over the last decades
leads to an increased number of constructions of LNG storage systems worldwide.
In order to design large-scale structures like LNG tanks with the purpose of
failure prevention, it is necessary and important to clarify mechanical
properties of materials under actual circumstances. ( Johnson W.1972) .
Application of metal in cryogenic operating conditions (-163 °C and
atmospheric pressure) such as liquified natural gas (LNG) plant need careful
consideration on fitness of materials to withstand sudden brittle fracture
damages during operation because of cryogenic temperatures.
In particular, an understanding of structural mechanical behavior at cryogenic temperatures is
vital for designing LNG carriers, vessels and pipelines, etc., because the inner walls of these
structures are in direct contact with LNG at 110 K (-163 C), therefore prediction the mechanical
behavior at low temperatures with high reliability provide precise understanding of the
mechanical behavior and fracture characteristics of the main materials used in the structure and
which needed first for design. Doo-Hwan Park (2015), the safety of LNG storage system during
operating can be guaranteed by proper material selections as well as optimum microstructural
and match fit chemical composition between parent metal and weldment.
Investigate the mechanical and microstructural characteristics of the
materials used in liquefied natural gas (LNG) is essential factor to ensure
their ability to withstand such cryogenic condition because totally change in
mechanical properties and metallurgy occur when materials undergo to
cryogenic temperature. Therefore, mechanical performance of both base material (BM)
and weldment is dramatically influenced by the decreasing temperature down to cryogenic
temperature which defined temperature below -150°f (-101°c), Moreover, the low-temperature
environment also results into a reduction in impact toughness for the BM and WJ, which is
pointed out by some researchers, Xin J, Fang C(2017) .
Brittle fractures in the aerospace, oil and gas, chemical industry, nuclear and fossil-fuel power
generation, and other industries are not common, but when they occur, they may result in a
sudden, unexpected, and catastrophic failure, followed by the potential release of chemicals,
toxics, or even a fire or explosion. therefore, awareness of cryogenic temperature brittle fracture
is essential for anyone working in such industries, the goal to reduce the risk of brittle failure by
learning how to properly identify and manage potential cold temperature embrittlement and
brittle fracture risks, Daniel J. Benac(2016) .
Stainless steel, nickel steel and invar alloys have mainly been used as cryogenic materials in
ships and offshore structures, these materials show excellent mechanical properties at low
temperatures and superior corrosion resistance however, cryogenic materials are expensive
because nickel, which is commonly used in such materials, has become increasingly expensive
cost over the years. Therefore, various studies have attempted to reduce the cost of cryogenic
materials by using carbon steel nickel alloy, manganese steel alloy and aluminum alloy, Doo-
Hwan Park (2015).
One of the most common materials for the LNG storage systems has been 9% nickel steel over
the last 50 years as it has excellent mechanical properties under cryogenic temperature,
Recently, there have been efforts for lowering the nickel contents due to the increased nickel
price as well as the high price of nickel-based welding consumables. In particular, the weld
material cost for 9% nickel steel is known to amount about 25% of the entire construction cost of
each LNG cargo tank . Takahiro, K., Kazushige, A., Tomoya, K., Kazushi, O (2011) In this
respect, 7% nickel steels are recently developed for reducing associated costs mainly for
cryogenic applications. The newly developed 7% nickel steels are known to have improved
toughness comparable to that of 9% nickel steels by TMCP and microalloying technology.
Nickel steel has many benefits, such as good strength, outstanding corrosion resistance, and
applicability in a wide range of temperatures. As shown in Table 1(ICG Code; IGC: London,
UK, 2014) .
Various temperature application of nickel steels (ICG Code; IGC , UK, 2014) .
Many applications operate at sufficiently low temperature conditions where most structural steels
become very brittle and, therefore, unsuitable for use in safety-critical structures. So, the
materials used in the vessels or storage tanks which keep the natural gas at liquefaction
temperatures need to remain ductile and crack resistant with a high level of safety.
