| Austenitic stainless steels, with well corrosion resistance, heat resistance and welding performance, etc, were widely used in petrochemical & energy, electricity &transportation and aviation & navigation, and other fields. However, its use was severely restricted in harsh environments because of its low hardness & poor wear resistance characteristics. Therefore, its surface hardening was the most important method to expand the scope of application. Chemical treatment was an effective way to improve the strength of austenitic stainless steel surface. The surface strength could be improved by nitriding, carburizing or carbonnitriding under the conventional conditions. However,the high temperature alloyed tissue was generated under the conventional conditions,resulting in corrosion resistance of austenitic stainless steels degrading greately.With the development of national economy, wear & corrosion resistant materials(i.e. austenitic stainless steels treated to improve surface strength without damaging its corrosion performance) were demanded, and the hardening way of austenitic stainless steels had been on research. Low-temperature chemical heat treatment technology,without prejudice to the corrosion resistance of austenitic stainless steel premised, was the way to improve surface hardness, which had been concerned for the technical workers over the past decades. By using the self-developed low-temperature gas carburizing furnace, the technology of removing the austenitic stainless steel surface passivation film was exploited, the best parameters combination were optimized through screening and analysis during the low-temperature gas carburizing process, and the austenitic stainless steel low-temperature gas carburizing layer structure, composition distribution & strengthing mechanism and analysis of the corrosion resistance and friction properties & the matching relationship between corrosion potential and total depth of carburizing layer were investigated to reveal corrosion and strengthening mechanisms under the low temperature resulting in establishing the enhanced corrosion curve. At the same time, the austenitic stainless steel rare earth catalytic infiltration under low-temperature gas carburizing conditions was developed to improve the surface hardening speed orderly, Rare earth catalytic mechanism of austenitic stainless infiltration at low-temperature gas carburizing conditions was studied, and a good foundation was laid to quicken low-temperature gas carburizing technology of austenitic stainless steels.(1) Gas carburizing technology of the austenitic stainless steel was studied under low temperature conditions, so that the technology of surface passivation film removed of austenitic stainless steels was developed, the relationship of the carburizing gas composition, carburizing temperature, carburizing time and other parameters were grasped, and the ideal surface corrosion resistant reinforcing layer was obstained. The results showed that: in the carburizing process conditions of the carburizing temperature450 °C ~ 500 °C and carburizing time of 32 h ~ 72 h, the total thickness of low-temperature gas carburizing layer and the highest hardness value reached 78 μm and1250 HV, respectively. The value of surface hardness was three times above than that of austenitic stainless steel substrate and the strengthen effect was obvious. Without prejudice to austenitic stainless steel corrosion resistance, the best process parameters of low-temperature gas carburizing surface hardening technology were carburizing temperature 470 °C, carburizing time 48 h. Two pre-treatmen of carburizing process was carried out and the parameters were temperature 250 °C and time 2 h, respectively.(2) The carbon atoms distribution & diffusion of low-temperature gas carburizing layer, its effect on crystal structure of the substrate and carbide precipitatin were studied,and its strengthening mechanism was explored. XRD analysis showed that, the lattice constant of austenitic stainless steels was changed in varying degrees as a result of the infiltration of carbon atoms; EDS and ICP analysis showed that, the carbon content of austenitic stainless steels low-temperature gas carburizing layer was gradually reduced from the surface to the substrate, which the highest carbon atoms content was about 5%higher than that of the austenitic stainless steel substrate concentration of carbon balance,showing a supersaturated characteristic form(S phase); The hardness gradient of austenitic stainless steels low-temperature gas carburized layer was relationship with changes in the organization and the carbon concentration gradient of carburizing layer. S phase was a major factor to the strengthening surface of austenitic stainless steels. The strengthening way before carbide precipitation was solid solution strengthening way, and solid solution strengthening and dispersion strengthening jointed action after the carbon precipitation.(3) The corrosion and wear properties was investigated. Results showed that,austenitic stainless steels could be completely solid solution strengthening and the loss of basic chromium elements(maintaining corrosion resistance of the substrate) of austenitic stainless steels was prevented by the strict low-temperature gas carburizing technology; At the same time, the strength of austenitic stainless steels and tribologicalproperties were improved greatly due to the carbon solid solution strengthening strengthening,achieving enhanced corrosion effect.(4) Considering austenitic stainless steel low-temperature gas carburizing technology industrial development, rare earth catalytic infiltration was developed under the low-temperature gas carburizing conditions and the problem of production efficiency during the low-temperature gas carburizing process was solved. Meanwhile, the mechanism of rare earth catalytic infiltration was discussed and the effects of rare earth catalytic effect on the corrosion reinforcing layer infiltration organization and performance were investigated. The results showed that: the proper rare earth catalyst to improve the low-temperature gas carburizing penetration rate had a significant effect, the total thickness & hardness of carburizing layer were increased by 23.4 % & 16.3 %,respectively. With the development of this technology, the austenitic stainless steel low-temperature gas carburizing technology for industrial application process was further promoted. |
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