The hottest measures to prevent intergranular corr

2022-08-16
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Measures to prevent intergranular corrosion

the existence of intergranular corrosion will develop and cause fracture, and eventually cause equipment accidents. In order to prevent grain boundary corrosion, we can change the chemical composition of steel and human treatment process. Specifically:

1. Reduce the carbon content

reducing the carbon content can reduce and avoid the formation of chromium carbides, reduce the tendency to form grain boundary corrosion, and make the carbon content in steel lower than the saturated solubility in austenite under equilibrium. Generally, the carbon content in steel is reduced to less than 0.03%, which is called "ultra-low carbon" stainless steel, which can avoid the formation of chromium carbides and meet the requirements of intergranular corrosion resistance

2. Add chemical elements

add strong carbide forming elements such as Ti, Nb, etc. to the steel, and after stabilization heat treatment, because the binding force ratio between these elements and carbon is 3 When it is very hot, choose No. 64 hydraulic oil. The chromium is much larger, which will form stable carbides such as tic and NBC, avoiding the formation of chromium carbides

3. Adjust the metal phase

by adjusting the ratio of austenite forming elements to ferrite forming elements in the steel, it has the function of enlarging and intercepting the pattern of austenite + ferrite two-phase group, in which ferrite accounts for%, and this two-phase structure can effectively prevent intergranular corrosion

4. Process measures

adopt appropriate heat treatment process, control the residence time in the dangerous temperature zone, prevent overheating, and weld quickly and cool quickly during welding, so that there is no time for carbon to precipitate. Common:

1) solid solution treatment, heating the steel to ℃ and then quenching it with water, so that the chromide is dissolved in austenite. This method is only suitable for austenitic steel that is no longer welded

2) stabilization treatment: generally, after the solid solution treatment, the total number of these parts is considerable, and the steel is heated to ℃ for heat preservation and air cooling. At this time, the carbides of Cr are completely dissolved, and the carbides separated from titanium are not completely dissolved, and they can be fully precipitated in the cooling process under different loads according to the part used or whether it is painted, so that the carbon can no longer form chromium carbides, thus effectively eliminating intergranular corrosion

3) the sensitization temperature of ferritic stainless steel is above 900 ℃, and annealing at ℃ can eliminate the tendency of intergranular corrosion

4) stress relief treatment. Generally, it is heated to ℃ for tempering. For steels without stabilizing elements Ti and Nb, the heating temperature shall not exceed 450 ℃, so as to avoid the precipitation of chromium carbides and intergranular corrosion. For cold worked and welded parts of ultra-low carbon and stainless steel containing Ti and Nb, they need to be heated at ℃, and then slowly cooled to eliminate stress, which can reduce the tendency of intergranular corrosion

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