Steels Welding for Cryogenic Applications, Natural gas is more efficiently stored and transported in liquid phase, which involves taking the material to temperatures below -163 º C. This requires economic and materials that work well at low temperatures, such as steel to 9% Nickel, which has been developed to have good toughness and impact resistance at low temperatures to prevent the spread of cracks, good ductility and strength traction. This article will review the metallurgy and weldability of these steels, general welding and the precautions to be taken into account to make a weld with this steel.

Cryogenic Materials Metallurgy

The most critical property of steels for cryogenic applications is their tenacity. Ferritic materials show a change in their mechanical behavior when exposed to low temperatures, and this is manifested by a reduction in the toughness of steel, characterized by a change from ductile to brittle behavior as the temperature decreases below the critical temperature of transition. This temperature can not be assessed on all materials depending on their crystal structure, in the case of steels, this temperature is presented in ferritic steels while not shown in the austenitic.

WELDING OF STEEL TO 9% nickel.

Usually these steels are welded in the condition after heat treatment. The preparation of the board must be done carefully, should be avoided sharp edges to try to induce no magnetization in the plates. Surfaces must be carefully cleaned with acetone or some organic solvent to remove contaminants that
can cause defects in the weld.

Aspects in the manufacture of welding
• Evaluation of the welding processes to use
• Material supply to use
• Welding Procedure

Evaluation of welding processes to employ:
Welding processes such as submerged arc welding (SAW), shielded metal arc welding gas (GMAW), arc welding electrode tungsten (GTAW) and coated electrode welding (SMAW) can be employed, however the SMAW process turns out to be a viable and flexible to weld in any position or material and field. For this process are commonly used basic electrodes.

Filler material used:
Input materials to vary from those of ferritic alloys high percentage of Ni (80Ni/20Cr/0.26C), they are generally used in high temperature applications. Ferritic alloys around 12% Ni are cheap, but they are not accepted for the sizes of storage tanks today. The primary objective in selecting the filler metal is to get a counterpart to the base metal that is tenacious, ductile to reduce residual stresses in the HAZ (heat affected zone) (elong.> 35%), and with a coefficient of thermal expansion low and similar to the base metal to prevent thermal fatigue in the union. Because the liquid natural gas tanks are subject to continual expansion and contraction, the constant of thermal expansion filler materials should be similar to the base material. Figure 7 shows that the filler material ENiCrMo-6 is closer to thermal expansion of the base material at different temperatures.

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