Austenite summary

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

A schematic summary of the alloying metals that affect the anodic polarization curve of stainless steel is shown in Fig. 4.16. The addition of 8% nickel to an alloy containing 18% chromium forms austenitic structure SS Type 304. The addition of Mn and N increases the stability of austenitic steel. The chromium content of stainless steel affects the anodic polarization curves as shown in Fig. 4.16. Nickel promotes repassivation in a corrosive environment, but concentrations higher than 30% reduces the passivation current, the critical current density, and increases the critical pitting potential. Nitrogen... 

S] Gardand, P. O., Steinsmo, U., Dmgli, J. M., and Solheim, P., High, Alloyed Stainless Steels for Chlorinated Seawater Applications A Summary of Test Results for Eleven Austenitic and Duplex Materials, Paper 646 NACE CORROSION/93, NACE Nashville, TN, 1993. 

Table 8 provides a summary of the redox potentials of various tests for austenitic and ferritic stainless steels. Because both carbides and molybdenum-rich sigma-phase may result in intergranular attack in highly oxidizing solutions, the nitric acid test should be specified for materials to be used in nitric acid or other highly oxidizing environments. 

Figure 4.12 Summary of oxide scale structure and composition formed on austenitic Fe-Cr-Ni alloys after exposure in 25 MPa SCW at 550°C for 500 h in a static autoclave [45].

Fig. 8.15 [32] contains a summary of the swelling results of the CEA irradiation CASTOR program obtained on several different austenitic nuances irradiated at 450°C and 71 dpa in a Phenix capsule. The prime objective of this program was to improve the material specification by identifying the effect of additive elements on swelling and creep of 316 and 15/15-type steels. 

A ranking system that is at least semiquaiMitative has been devdoped (Ref 26). In this syston, the value of a normalized erosion resistance, defined as the maximum rate of volume loss of a reference matoial divided by the maximum rate of volume loss for the material being evaluated, is computed. This allows comparison of materials that have been tested under different sets of conditions, provided that the refoence material has been tested under each the diffoent sets of conditions. Figure 8 is a summary of ntxmal-ized erosion resistance for a wide variety of alloys tested at different conditions, using 18Cr-8Ni austenitic stainless steel with a hardness of 170 HV as the... 

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