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Home » FAQs » Carburizing process for maintaining corrosion resistance of austenitic stainless steel
Carburizing process for maintaining corrosion resistance of austenitic stainless steel
2017-05-20 14:04:43

 Austenitic stainless steel has good corrosion resistance, and has good toughness, workability, weldability and heat resistance, but the disadvantages are hardness, wear resistance and fatigue resistance. Carburizing treatment of austenitic stainless steel is an effective surface strengthening method. Unfortunately, conventional carburizing process has a great damage to the corrosion resistance of austenitic stainless steel. Therefore, the development of a carburizing process does not damage the corrosion resistance of austenitic stainless steels, austenitic stainless steel has to resolve corrosion resistance and good mechanical properties at the same time the dilemma, can greatly improve the scope of application.

The corrosion resistance of stainless steel is mainly due to the addition of a certain amount of Cr in steel. On the one hand, Cr reaches a certain content in iron matrix can make iron electrode potential to obtain a leaping rise; on the other hand, the Cr element formed on the stainless steel surface with a layer of dense oxide film, the shielding effect of the corrosive environment. Why conventional carburizing will damage the corrosion resistance of austenitic stainless steel? This is because in high-temperature conditions, Cr atoms in austenitic stainless steel are easy to combine with C atoms to produce chromium carbide, which first precipitates on the austenite grain boundary of carburized layer and forms a network distribution. Because of the large radius of the Cr atom, it is difficult for the internal Cr to diffuse to the poor Cr layer of the surface layer, which results in the partial chromium depletion on the surface, and the dense Cr2O3 oxide film on the stainless steel layer is also destroyed.
Therefore, in the condition of no damage to the corrosion resistance of austenitic stainless steel, the precondition of carburizing is to ensure that the carbide is not precipitated. Because the carbides of chromium are formed in a certain temperature range in high temperature range, carburizing must be carried out in the proper low temperature range to avoid the formation and precipitation of carbides. At this temperature, due to the smaller atomic radius of C, with the gap diffusion mechanism, after carburizing C atoms can diffuse into the lattice of austenitic stainless steel, forming a solid solution; and Fe, Cr atomic radius is larger, only to exchange mechanism in the absence of sufficient diffusion, diffusion activation energy under the condition of Fe and Cr atoms cannot mobile.
This ensures that Cr's carbides do not form. Chromium carbide generated at 550 degrees, so low temperature carburizing austenitic stainless steel will be below 550 DEG C conditions, so it can not damage the original performance in stainless steel corrosion resistance under the condition of improving the surface strength.
LTCSS (Lowtemperaturecolossalsupersaturation) technology developed by American Swagelok company has been put into use at present. The main technological steps are as follows: pretreatment of austenitic stainless steel surface before treatment is called alloy surface activation. The activation process adopts pure HCl and N2 gas mixture, holding 250 hours for 2 hours. HCl can effectively remove the chromium oxide passivation film on surface of austenitic stainless steel, and the addition of N2 to under atmospheric conditions, creating a non oxidizing environment, so as to ensure the matrix of chromium atoms are not oxidized again, avoid passivation film rebuild. Then, a hardening layer of about 70 microns can be obtained by carburizing for 20 hours at 470 c.. After LTCSS processing of 316 stainless steel compared with untreated material, have significantly improved performance, the surface hardness is improved from the original 400HV to 1000HV; the fatigue limit increased from 200MPa to 325MPa; the corrosion resistance in NaCl solution, 0.6mol/L, anodic pitting potential increased from +140mV to +990mV.

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