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  • E-mali: sxthsteel@sxth-group.com
  • Tel: +86-29-89636557
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  • What is the intergranular corrosion of austenitic stainless steel? ( 3 )
    Aug 03, 2018

    ( following from previous pages )

    There are 10% ~ 50% volume of delta ferrite in the steel, which can improve the intergranular corrosion tendency of austenitic stainless steel. Due to the precipitation of delta ferrite at 500 to 800 C, the precipitation of (Cr, Fe) 23C6 in the delta / gamma phase delta phase shows a point shape, excluding the precipitation in the austenite grain boundary (Cr, Fe) 23C6, and the diffusion coefficient of chromium in the delta phase is 103 times higher than that in the gamma phase, and the chromium poor region is not produced. It must be pointed out that not only the precipitation of chromium carbide will cause chromium depletion at grain boundaries, but also the precipitation of chromium nitride and_phasewill cause chromium depletion at grain boundaries.

    In short, in order to prevent the tendency of intergranular corrosion of austenitic steels, the following measures can be taken in the composition design and heat treatment process of steels:

    In the composition design of steel, the carbon content in the steel is reduced on the one hand; on the other hand, the stable carbide forming elements (Ti, Nb) are added to the steel to precipitate special carbides and eliminate the intergranular chromium poor zone. For this reason, the content of titanium or niobium in steel is 0.8% or more than Ti equal to 5 (C%-0.02%), and 1% or more Nb is equal to 10 (C%-0.02%). In addition, the contents of impurity elements such as nitrogen, phosphorus, silicon and boron must be strictly restricted in steel.In heat treatment process, austenitic stainless steel is usually quenched at 1050 ~1100 ~C (solution treatment) to ensure the content of carbon and chromium in the solid solution. For the non stable steel, the austenite composition is homogenized and the chromium poor area is eliminated. For the stable steel, the chromium carbide is transformed into a special carbide of titanium and niobium, which ensures the chromium content required for corrosion resistance.