Why stainless steel can not be welded with carbon steel electrode is because it will cause intergranular corrosion of stainless steel.

What is intergranular corrosion?

Intergranular corrosion is a type of localized corrosion that spreads inward along the boundaries between metal grains. Is due to the difference between the grain surface and internal chemical composition and grain boundary impurities or the existence of internal stress, will destroy the combination between grains, greatly reduce the mechanical strength of the metal, mechanical properties deterioration, can not withstand the knock, is a very dangerous corrosion.

Carbon is the main element that causes intergranular corrosion. Usually, the carbon content of carbon steel is higher than that of stainless steel. This is the reason why stainless steel is easy to cause intergranular corrosion by welding with carbon steel electrode. Therefore, low-carbon or even ultra-low-carbon stainless steel electrode should be selected for welding stainless steel, such as Austrian 102, Austrian 132, Austrian 002, etc.

In addition, for the impact toughness and crack resistance and other high requirements of the important structure, should not choose acid electrode. The so-called acidic electrode refers to the electrode containing a large number of acidic oxides (such as silicon dioxide, titanium dioxide, etc.) in the drug skin.

This is because: ① the oxide in acidic electrode makes it more oxidizing, and the weld metal contains more oxygen, which makes the alloy elements burn out more; ② the coating of acidic electrode contains more silica, and some silicon exists in the form of silica inclusions in the weld, resulting in poor mechanical properties of the weld, especially the plasticity and impact toughness are lower than those of alkaline electrode; ③ the coating of acidic electrode does not contain fluorite (CaF2), the hydrogen removal capacity is low; the hydrogen content of the weld metal is high, the cold crack resistance is poor, and the manganese content of the desulfurization element in the weld is low, and the desulfurization effect is poor.
In summary, the important structure of welding should not choose acid electrode, but should choose the weld metal oxygen content is low, alloy transition coefficient, sulfur and hydrogen content is also low alkaline electrode.