The function of some groups of stainless steel can be classified to 5 groups: Martensite stainless steel, Ferrite stainless steel, Austenite stainless steel, Austenite-Ferrite stainless steel, Austenite - Martensite stainless steel
This stainless steel group contains from 12% to 17% Cr. If the Cr content is at the lower limit (12 ~ 13%) then we have to control the carbon content C is not exceed 0.4% to avoid creating too much chromium carbide. Because Chromium Carbide depleted Cr content in the alloy background and reduce corrosion resitance. If the Cr content is up to 17%, the amount of C can be as high as 0.9 ~ 1.1% to increase the mechanical properties (stiffness) and still remain corrosion resistance.
The suitable temperature for heat treatment of this steels is: 950 ~ 1100 ° C. The coolling environment may be oil or air (due to of high content of % Cr so it should be easy to fabricate). The Ram temperatures depends on the specific requirements but must pay attention to avoid brittle ram type II in the temperature of 350 ~ 575 ° C (by quenching in oil. If stainless steel was cooled slowly will forms Cr23C6 steel brittle and reduce the corrosion resistance).
Martensite stainless steel has a high corrosion resistance in freshwater environments, because of the passive effect of chromium so that it is not corrode in acidic HNO3 (but corrosive in other acid). This stainless steels with low carbon content (eg: grade 403 and 410 of U.S) is used for stainless steel jewelry, stainless fasteners, heating equipments (
Ferrite stainless steel has a higher elastic limit than Austenite stainless steel but the plastic deformation is lower, so, they are suitable for the normal fabrication (rolling, drawing, typing , stamping, etc.). Corrosion resistance of them depend on chromium content. In order to limit the local corrosion (point corrosion), we have to increase the chromium content above 20% and better increase more 2% of Mo alloy to be used in the marine enviroment, seawater and acid.
The most important characteristics of this type of stainless steel is very good mechanical properties and almost has no phenomenon of brittle ferrite of stainless steel, high elastic limit steel is over three times austenite. In addition, this group also has great corrosion resitance, especially in condition of high pressure (stress corrosion), or concentrated corrosion (point corrosion) and in the environment strong erosion (exhaust valve hole, pipe chemical vapor ...)
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