Duplex Steels. Part II: Carbides and Nitrides

Abstract

As mentioned in Part I, duplex stainless steels have a very complex precipitation behavior due to the high amount of alloying elements they contain. Thermodynamically, this leads to the formation of many phases (Fig. 1). Diffusion rates for interstitial elements in ferrite are approximately 100 times higher and have solubilities that are 100 times lower than those in austenite. This leads to the result that formations of carbides, nitrides, and intermetallic phases occur in the ferritic phase only. The precipitated phases can be distinguished regarding their kinetic formation, controlled due to element solubility or diffusion. The growth of phases, which are dependent on element solubilities, cannot be avoided due to technically practicable cooling rates, in general (austenite, carbides, nitrides, and e-phase). In contrast, diffusion-dependent phase formation is associated with prolonged treatment time at critical temperatures (v-, r-phase, and 475 C embrittlement) and can be suppressed by rapidly quenching. The precipitation behavior of duplex stainless steels shown in Fig. 1 can be divided into two temperature ranges. In the upper temperature range between 600 and 1,000 C, the nitrides of type CrN, Cr2N, carbides of type M23C6, and vand r-phases can develop. In the lower temperature range of 300–550 C, the formations of p-, Zand a0-phases take place (475 C embrittlement). In the same temperature range, e-phase can occur in coppercontaining duplex steels. The solubilities of carbon and nitrogen in duplex steels are limited and become lower with decreasing temperature. Both elements prefer to concentrate in the austenite, due to their higher solubility for interstitial elements and to a greater number of tetrahedral sites. The ferrite can dissolve very small amounts of carbon and nitrogen in octahedral sites and, for this reason, the formations of carbides and nitrides occur exclusively in the ferritic phase and at the ferrite–austenite phase boundaries. An overview of carbides and nitrides, which have been identified in duplex steel by various studies, is shown in Table 1. Precipitation of carbides and nitrides greatly reduces the corrosion resistance and toughness of the duplex steels and thus should be avoided. Figures 2, 3, and 4 show examples of the negative effects of carbides and nitrides on corrosion behavior.