Welding materials of steel for low temperature pressure vessels
(1) Basic requirements
Low temperature pressure vessel pressure elements or pressure elements and non-pressure elements welding electrode arc welding electrode should be selected NB/ T47018.1~47018.7 "Welding materials for pressure equipment ordering technical conditions" low hydrogen alkaline electrode. The submerged arc welding flux should be alkaline or neutral.
Welding joints of non-pressure accessories welded with low-temperature pressure elements, when the bearing capacity is large at low temperatures, the same requirements should be applied to the welding joints of pressure elements; When the load is small, the corresponding welding material can be selected according to the weldability requirements.
The selection of steel welding materials for low-temperature pressure vessels must ensure that the welding joint contains the least harmful impurities sulfur, phosphorus, oxygen and nitrogen, especially nickel-containing steel should strictly limit the impurity content, because the increase of impurity content will significantly reduce the toughness of the welded joint.

(2) ferritic steel
Welding between ferritic steels should generally use ferritic welding materials (except 9% Ni steel). The low temperature impact test temperature of the welded joint and the low temperature impact work requirements of the weld metal and the heat affected zone are the same as that of the base material, and shall not be lower than the performance of the base material.
Welding materials for dissimilar steel welding between ferritic steels are generally selected according to the base material on the side with higher toughness requirements. The impact test temperature of the welded joint metal shall not be higher than the lower of the base material on both sides. The welding process evaluation of dissimilar steel and the heat treatment state of the product welding test plate should be the same as the final use state of the container, and the performance inspection should meet the following requirements.

① The tensile and bending test requirements of welded joints are lower in the base material on both sides.
② The impact energy requirements of the heat affected zone on the lower strength side of the welded joint metal are based on the requirements of the base material on the lower strength side, and the impact energy requirements of the heat affected zone on the higher strength side are based on the requirements of the base material on the higher strength side.

(3) Austenitic steel
The selection of welding materials between austenitic steels shall meet the following requirements.
① The carbon content of weld metal is not more than 0.10 %.
② The chemical composition of the weld metal should conform to E0-19-10, E00-19-10, E00-23-13 in GB/ T 983 "Stainless Steel electrode" and GB 4233 "Stainless steel rod and wire for inert gas shielded welding" and GB 4242 "Stainless steel wire for welding" H0Cr21Ni10 , H00Cr21Ni10, H0Cr26Ni21 requirements.
③ When the design temperature is lower than -100 ℃, the low-temperature Charpy (V-notch) impact test in the welded joint area should be carried out according to NB/ T 47014 "Welding Process Assessment of Pressure Equipment", and meet the requirements of Table 13-4.
Note: The average value of the impact work of 3 samples at the test temperature shall not be lower than the specified value in the table, and the impact work value of 1 sample is allowed to be lower than the specified value in the table, but not lower than 70% of the specified value.
(4) Welding between ferritic steel and austenitic steel
For dissimilar steel welding between ferritic steel and austenitic steel, Cr23Ni13 or Cr26Ni21 high chromium nickel or nickel-based welding materials should generally be selected, and stress relief heat treatment is no longer carried out after welding. This kind of dissimilar steel welding process evaluation and product welding test plate shall meet the following requirements.
① The tensile strength of the welded joint is not lower than the minimum tensile strength of the base material on both sides.
② The impact energy of the heat affected zone on the side of the ferritic steel should be based on the tensile strength of the ferritic steel, and meet the requirements in Table 13-4. The impact energy of weld metal should also meet the requirements in Table 13-4.

③ The joint should be subjected to side bending test, and the test method should be carried out according to NB/T 47016 "Mechanical properties of welding specimens of Pressure equipment products". After bending test, in the weld and heat affected zone on the tensile surface, there shall be no single opening defect with a length greater than 3mm in any direction. The angular opening defect of the sample shall be excluded, but the angular opening defect length caused by non-fusion, slag inclusion or other internal defects shall be included.

Commonly used steel welding materials for low-temperature pressure vessels