Steelmaking plays an important role in securing the properties need­ed for cryogenic services. It produces steels with features such as (a) fully deoxide steel (fully killed steel), (b) fine grain, (c) heat treatment, quench and tempered (d) alloy with nickel, (e) minimal use of carbon, and (f) reduction in the sulfur and phosphorus content to a lower level than will normally be found in other alloys used at room temperature. Steel that contains nickel, with controlled grain size, heat treatment, and other features mentioned above, can improve fracture toughness.

Steel Nickel Content Lower Service Temperature

The increasingly strong demand for cryogenic service has focused on the need for strength, toughness, and assurance against brittle fail­ure when services temperatures decrease toward absolute zero or lower. Construction welds for cryogenic services has become a spe­cialized science.

When making welded pipe joints on the three category of alloys, attention must be given to the preparation, which is similar to that for material or alloys discussed for high temperature and pressure in serv­ice. The root bead must be uniform all around. Both pipes to be weld­ed should be properly aligned, with the internal alignment consistent in terms of the internal walls. During the welding, all efforts must be made to avoid undercut, poor restart, and deep crevices along the fusion line closest to the bevel, In addition, the cover pass should not have a high crown with a sharp transition into the base metal, but instead a gradual transition. This will depend on the size of the electrode being used and the technique. The root pass should be made with the GTAW process along with pulsed current control. There should be no internal defects. Any sign of undercut on the surface of the filler should be ground away.

Nickel Steel of the 9% Grade

This grade of steel was initially developed to provide strength and extreme toughness at very low temperatures. It would provide an alloy with the required toughness and strength at very low temperature, 262°F, needed for tank that store liquified natural gas. These tanks are heat treated and supplied as quench and tempered steel, with a maximum thickness of two inches. The welds are not subjected to any heat treat­ment. Preheating is very seldom applied unless the temperature has dropped to temperatures lower than 70°F.

The electrodes used for welding this type of material are high nickel - chrome iron electrodes specified as AWS-E-NIC, Fe-2 and E-NiCrFe-3. The SMAW, combined with the electrode specified above, requires less heat and the application of stringer beads. The flow of the puddle is not as fluid as those from either low alloys or high alloys such as stainless steel. In addition, the puddle will not wash up to the sides to any appre­ciable degree. Therefore, the welder will have to deliver the weld metal in the area where it is needed by manipulation of the electrode. If the electrode size is large, it may compromise the quality of the weld. Supervisors should be cautious finding the appropriate size electrode to use; a large size can end up requiring a lot of time grinding.