Welding material selection rules for same  and different steel


 Welding of low carbon steel and low alloy steel

Low-carbon steel and low-alloy steel are common ferrite steel, and the welding between them and low-alloy steel of different materials belong to the same kind of steel welding. This kind of steel welding between the low-grade material, refers to the strength level or alloy element content of low material selection of welding materials, to ensure that the weld metal properties meet the low-grade material can. The choice of low-grade materials than high-grade materials welding performance is better, the price is cheaper, conducive to reducing manufacturing costs.

Welding of low-alloy and medium-alloy heat-resistant steels

Because of the discontinuity of the chemical composition of the weld seam of the same kind of dissimilar steel, the discontinuity of the performance will occur. If this discontinuity has a greater impact on the use of performance, the principle can not be low-grade selection of welding materials. For example, if SA213-T91 is welded with SA213-T22, the T91 base metal near the fusion line on the T91 side will be seriously carbonized if the welding material of 2.25 CR-1MO is selected according to the general low-grade principle, at the same time, the weld near the fusion line of T91 side decarburized seriously, resulting in decarburization layer. This is because the chromium content of T91 is about 9% and the carbon content of 2.25 CR-1MO welding wire is about 2.25% . After annealing treatment after welding, the chromium content in the heat-affected zone of T91 side is much larger than that of the weld side, a large amount of carbon will shift to the base metal to produce carburizing layer, resulting in increased hardness, microstructure more hardened, and weld side of serious decarburization, low hardness, microstructure softening, so that the joint performance deterioration. If 9Cr-1Mo welding material is used, the weld metal will be carburized and the base metal decarburized on the side of T22 fusion line.

Choice of welding material for dissimilar steel welding

When welding carbon steel, low alloy steel and Austenitic stainless steel dissimilar steel, the welding material should be selected according to the working temperature and the working condition of the joint. When the working temperature of this kind of dissimilar steel joint is below 315, the Austenitic stainless steel welding material with high content of CR and Ni alloy can be chosen. Select suitable CR and Ni Austenitic stainless steel based on the chemical composition of carbon steel (alloy steel) and austenitic steel and the welding fusion ratio, according to certain Ni-equivalent and Cr-equivalent microstructures, to avoid the weld to produce a large number of martensite structure. Of course, there will be narrow martensite band near the fusion line of carbon steel or low alloy steel. By reducing the carbon content of the welding material, the martensite structure will become low carbon martensite with better plasticity, also can guarantee the joint to have the good performance.

When the bearing pressure of dissimilar steel joints, at the temperature of 315or above, the use of nickel-based welding materials. For example, ECRNIFE-2, Ercrnife-3, etc. , the main reasons are as follows. If ordinary Austenitic stainless steel welding materials are used, the following problems arise:

a. thermal stress and thermal fatigue failure at high temperatures due to the large difference between ferrite and austenite Coefficient of thermal expansion.

b. because of the great difference of alloy element content, the welding joint will produce serious decarburization layer and carburization layer at high temperature, which will worsen the high temperature performance.

c. the local microstructure of the weld is hardened by the martensite band near the fusion line.

This can be avoided by using a nickel-based welding material for the following reasons:

a. the Coefficient of thermal expansion of the nickel-based material is between ferrite and austenite.

b. the nickel base material will not produce decarburization and carburization in the welded joint.

c. no martensite structure will be produced by ni-based material welding.

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Post time: Jan-12-2023