Sensitization ferritic stainless steels

Straight chromium ferritic stainless steels are less sensitive to stress corrosion cracking than austenitic steels (18 Cr-8 Ni) but are noted for poor resistance to acidic condensates. 

In ferritic stainless steels, Cr diffusion is faster leading to more rapid sensitization over a lower temperature range as shown in Fig. 20. This is because of rapid carbide and nitride formation [64]. At higher temperatures, Cr diffusion occurring in body-centered cubic ferrous-based materials is considerably more rapid than in face center cubic (f.c.c.) materials. This tends to level concentration profiles and mitigate ICC susceptibility even for diffusion-controlled precipitation. However, sensitization can occur even in rapidly cooled ferritic material due to more rapid carbide formation kinetics. There are other... 

Sensitization diagrams (Figs. 19 and 20) for austenitic and ferritic stainless steels of varying carbon contents roughly follow the TTT behavior of carbide precipitation [65]. Increasing carbon content facilitates more extensive carbide precipitation at shorter... 

Fig. 20 Time-temperature-sensitization behavior diagrams for austenitic and ferritic stainless steels illustrating different time-temperature regimes for sensitization. (Ref. [61], from Corrosion of Stainless Steels A. John Sedriks, copyright John Wiley Sons.

The clearest approach to avoiding sensitization in ferritic stainless steels is to minimize the interstitial C and N contents. However, the exact levels allowed depend on exact alloy composition. The greater the Cr and Mo contents are in the alloy, the more resistant the alloy will remain for higher interstitial levels [70]. For 18Cr-2Mo steels, the C -b N level must be as low as 60 to 80 ppm. Ti and/or Nb can also be added to ferritic steels. For 26Cr-lMo steels, the minimum stabilizer content required is given by [108] ... 

Ferritic stainless steel has the reputation of being less sensitive to intergranular corrosion than austenitic stainless steel. This type of corrosion can nevertheless take place under certain conditions of thermal treatment [20]. The diffusion coefficients of both carbon and chromium in ferrite are larger than in austenite. Grain boundary precipitation of carbides and nitrides of chromium can therefore occur at temperatures of 540-600 °C already. The behavior differs from that of austenitic stainless steel, which becomes sensitized at higher temperatures only. Because of the larger diffusion... 

Austenitic stainless steels are sensitive to stress corrosion cracking in chloride-containing environments (Chapter 11). On the other hand, they are generally more resistant to hydrogen embrittlement than ferritic stainless steels. 

Alloy contents of stainless steels, particularly nickel, determine the sensitivity of the metal to SCC. Ferritic stainless steels, which are nickel-free, and the high-nickel alloys are not subject to SCC. An alloy with a nickel content of greater than 30% is immime to SCC. The most common grades of stainless steel (304, 304L, 316, 316L, 321, 347, 303, 302, and 301) have nickel contents in the range of 7-10% and are the most susceptible to SCC. 

Sensitization in ferritic stainless steels is introduced by high temperature heat treatment (above 925°C) and relieved by heating for a short time between 650 and 815°C, which is opposite to the observations on austenitic steels. Annealing sensitized ferritic steel at 788°C for several minutes will eliminate intergranular attack. The susceptibility is reduced by the addition of titanium or niobium. The presence of carbon or nitrogen is necessary to cause sensitization. 

Ferritic steels. Types 430 and 434, are resistant to SCC in MgCh at 140°C. High purity ferritic stainless steels are subjected to SCC in boiling 30% sodium hydroxide in tests exceeding 1000 h and in 42% MgCl2 in a sensitized condition. Types 430 and 446 stainless steels are subject to chloride SCC in the welded conditions. In the presence of high residual levels of copper (0.37%) and nickel (1.5%), the alloys become susceptible to SCC in 42% MgCU solution. 

Conventional ferritic stainless steels containing the same amount of carbon and chromium as austenitic steels undergo sensitization more rapidly than the austenitic steels. The ferritic steels generally become sensitized when quenched from 927-1149 °C. Sensitization is eliminated or minimized by annealing between 732 and 843°C for a sufficient length of time. If the steel is held for a sufficiently long period of time, chromium diffuses back to the grain boimdaries and sensitization is eliminated. 

Annealing is the only way to correct a sensitized stainless steel. Because different stainless steels require different temperatures, times, and quenching procedures, the user should contact the material supplier for such information. A number of tests can detect sensitization resulting from carbide precipitation in anstenitic and ferritic stainless steels. The most widely used tests are described in ASTM Standards A 262 and A 763. More detailed information on sensitization of stainless steels can be fonnd in varions books on metallurgically influenced corrosion. 

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