The materials commonly used in low-temperature LNG tanks are 9%Ni steel, 5%Ni steel, and some Austenitic stainless steel grade. 9%Ni steel, specified in ASTM A353and A553, which is characterized by high strength, excellent low-temperature toughness, easy processing, and excellent welding performance, has been widely used in LNG storage tanks and shipping. Its minimum working temperature can reach to-196℃, is the best toughness steel plate material used in a cryogenic environment.

In order to maximize its low-temperature toughness, a 9Ni steel plate is usually treated by quenching and tempering or twice normalizing and heating. However, the weld metal cannot be heat treated, so how to ensure the low-temperature toughness of the welded joint, prevent welding cracks and arc magnetic bias blowing are the problems welders must face, which depends on the type of welding material, welding line energy and welding process. In welding practice, Nickel-based welding materials are generally used for welding 9Ni LNG steel tanks, which mainly include:

1, Ferritic stainless steel W (Ni) =11%. The weld metal has poor low temperature toughness when welding 9Ni steel with the same welding material.

2, Austenitic stainless steel Ni-Cr alloy (Ni13%~Cr16%). The strength is slightly higher than Ferrite steel while poor the low temperature toughness and the linear expansion coefficient are significantly different from 9Ni steel.

3, Ni alloy (Ni-Cr-Fe alloys containing about 40% Ni and Ni-Cr-Mo alloy containing about 60% or more Ni). The nickel alloy welding materials offer good low temperature toughness and good cold crack resistance such as welding wires EnNiCrMo-6 and EnNiCrMo-3, which are the most commonly used welding materials for 9Ni steel plate, their linear expansion coefficient is close to 9Ni steel, do not need pre-welding and post-welding heat treatment, especially suitable for outdoor construction.

ENiCrMo-3 or ENiCrMo-6 welding wires and 9Ni steel are types of low carbon steel and their carbon content and their linear expansion coefficient are basically similar at room temperature and high temperature, therefore to avoid thermal stress caused by thermal expansion and cold contraction. Compared with other low alloy steels of the same strength, 9Ni steel has better crack resistance and almost no cold crack under low hydrogen conditions. But when using low nickel and high manganese Austenite electrodes, due to the dilution of the base metal in the fusion zone will appear high hardness martensite belt, sensitive to hydrogen embrittlement. However, Ni-base alloy material can basically avoid the occurrence of high hardness martensite zone in the fusion zone, and avoid the tendency of cold and hot crack in 9Ni steel weld.

The welding methods of 9Ni steel in the LNG storage tank are mainly SMAW and SAW. The circular seam is covered by submerged arc welding and the vertical seam is covered by arc welding. In order to avoid the problems such as cold crack, hot crack, low temperature toughness reduction, arc magnetic bias blowing, and so on, the following control measures can be taken to achieve the ideal effect during welding of 9Ni steel:

  • Necessary preparation before welding

Clean the surface of welding groove by using the organic solution or grinding before welding; When the temperature is lower than 5℃, the base metal must be preheated before welding; The welding wire shall be kept in strict accordance with the requirements and shall not be exposed to the air for a long time. Try not to preheat before welding, and the temperature between layers should not exceed 100℃. The preheating temperature and interlayer temperature directly affect the cooling rate after welding. The slower the cooling rate and the more conducive to grain growth, thus reducing the low temperature toughness

  • Strictly control line energy

Welding with small heat input is generally controlled within 0.7 ~ 3.0kJ/mm. The larger the heat input, the longer the residence time of the welding heat cycle and the coarser the microstructure grain of the joint, which increases the thermal crack sensitivity and leads to the decrease of low temperature toughness. Small heat input will increase weld passes so that the subsequent weld passes will play a tempering on the previous weld pass and improve the low temperature toughness.

  • Reduced magnetic deflection

9Ni steel is a strong magnetic material that may be magnetized in the process of processing and transportation. DC welding machine may further accelerate the magnetization, resulting in arc magnetic bias blowing, affecting the quality of welded joints, especially the more dense magnetic field lines at the LNG storage tank doorway leading more magnetic deflection blowing. We can control it by the following methods:

(1) Measure the magnetism of 9Ni steel and degaussing when necessary, control the magnetic induction intensity below 50GS, and select welding materials that can prevent arc magnetic deflection blowing like EnNiCrMo-6 and EnNiCrMo-3 wires.

(2) Use an AC welding machine as far as possible during welding.

(3) Use a grinding wheel. As carbon arc gouging adopts a DC welding machine of which air gouging current is usually above 500A, so the external DC strengthening magnetic field is formed between the air gouging, DC welding machine and tank wall. The strong residual magnetism is easily generated in the tank wall when the carbon gouging is over, which leads to magnetic deflection of the welding arc.