The alloying elements in steel are the main factor in the formation of a dual-phase structure. Adjusting the chemical composition of the steel and adding ferrite-forming elements to the weld will promote the weld to form a dual-phase structure of austenite plus ferrite.
The advantage of forming a dual-phase structure is that the austenite in the two-way structure has a larger carbon concentration and the carbon has a tendency to diffuse into ferrite; and the larger the concentration of chromium in ferrite, the chromium has to austenite The trend of proliferation.
Both carbon in austenite and chromium in ferrite diffuse to the junction of the two phases. Because the diffusion rate of carbon is very high, it is possible to cross the boundary from austenite to form chromium carbide with chromium; and because chromium is in the ferrite The diffusion rate in the the body is much faster than in the austenite, so once a chromium-depleted layer appears at the grain boundary, chromium can be replenished from the ferrite quickly, and the chromium-depleted layer disappears and the austenite crystal The amount of chromium carbide precipitated on the boundary is reduced, the distribution is discontinuous, and the directionality of the single austenite columnar crystal is disrupted, so as to prevent the chromium-depleted layer from penetrating between the crystal grains to form a concentrated channel for the corrosive medium, and reduce the intergranular corrosion. Corrosion tendency.
Although the formation of a dual-phase structure is beneficial to prevent intergranular corrosion, attention should be paid to the control of ferrite content. When the ferrite content exceeds 5%, not only the impact toughness of the weld will drop sharply, but also the tendency of intergranular corrosion will increase. The practice has proved that about 5% of ferrite can obtain satisfactory resistance to intergranular corrosion.
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