Stainless steel pipes are mostly made of austenitic stainless steel containing more than 8% nickel, and the ferrite content in austenitic stainless steel generally does not exceed 8%.
At high temperatures, carbon steel, alloy steel and stainless steel are all face-centered cubic austenite lattices, and there are 3 carbon atoms in a cube. In the case of carbon steel and low-alloy steel, cooling after austenitization reduces the distance between iron atoms and shrinks the austenite, transforming into a ferrite lattice with only one atom of carbon, and it is easy to precipitate carbides from the austenite.
Because austenitic stainless steel contains more than 8% nickel, the atomic weight of nickel is 58.71, which is larger than the atomic weight of iron 55.847. After the nickel atoms replace the iron atoms in the austenite lattice, the austenite lattice is expanded, and when the solution is heated When the water is sprayed for rapid cooling, the crystal lattice shrinks quickly, and carbon is too late to precipitate. The carbon atoms are fixed in the austenite lattice, which is the solution treatment.
If the austenitic stainless steel is heated and quickly cooled down, the carbon atoms will be solid-dissolved in the austenite lattice, and carbides will not precipitate at the grain boundaries, avoiding the formation of chromium carbide, avoiding the depletion of chromium at the grain boundaries, and avoiding the combined oxidation of oxygen and iron, thereby avoiding Produce intergranular corrosion.
Similarly, austenitic stainless steel cools rapidly, carbon atoms are dissolved in the austenite crystal lattice, there are no carbides on the grain boundaries, and there is no obstacle when the metal deforms, so the stainless steel becomes very soft, has a large elongation, and has good toughness; and After the ferrite-based carbon steel is quenched, carbides are precipitated during rapid cooling, and the toughness decreases; the faster the cooling, the coarser the precipitated carbides and the lower the toughness. Therefore, the solid solution of austenitic stainless steel must be heated and quickly cooled to avoid carbide precipitation.
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