Level ferritic steels

Fig. 5. Radioactivity after shutdown per watt of thermal power for A, a Hquid-metal fast breeder reactor, and for a D—T fusion reactor made of various stmctural materials B, HT-9 ferritic steel C, V-15Cr-5Ti vanadium—chromium—titanium alloy and D, siUcon carbide, SiC, showing the million-fold advantage of SiC over steel a day after shutdown. The radioactivity level after shutdown is also given for E, a SiC fusion reactor using the neutron reduced...

In the ferritic steels the effects of nitrogen and carbon are indistinguishable one from the other and the normal incidental level is sufficient to cause weld decay susceptibility. Thus in the super ferritic group both carbon and nitrogen are controlled to a low practicable level and sufficient titanium is added to stabilise both elements. 

Connections by welding are used if the materials applied can be welded safely, and only up to the pressure level of 700 bar. The usual range of materials comprises low-alloyed- and higher-alloyed ferritic steels as well as austenitic types. 

Work on one particular ferritic steel, Fecralloy, for fabrication of catalyst substrates was pioneered by the United Kingdom Atomic Energy Authority at Harwell and Johnson Matthey, in collaboration with Resistalloy, which developed technology for producing thin strip [32]. This and related alloys, in addition to iron, chromium, and aluminum, contain low levels of elements such as yttrium (0.1-3.0%), thought to enhance the protective properties of the surface alumina layer. Alumina forms by the oxidation of bulk... 

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] ... 

A notable finding of the overall materials study was that there was a significant content of tritium in the secondary circuit components. This was over three orders of magnitude above the free issue level. Very marked partitioning of the tritium content away from the ferritic steel to the austenitic steel took place at the Evaporator 1 nozzle transition weld. The level on the austenitic steel side of the transition weld was three times higher than in the Superheater 3 vessel and Circuit 3 pipework austenitic steel and was at least 5000 times higher than in the adjacent... 

As ferritic steels, the materials Monit and Sea-Cure are highly resistant to stress corrosion cracking and remain free of cracking in the test in 40% CaCl2 solution at 373 K (100 °C) and under a load of up to 90% of yield point, even after 5,000 h. Also, in the test for stress corrosion cracking sensitivity in a solution containing H2S acc. to NACE Standard TM-01-77, the samples withstand a stress load at yield point level without cracking after the standard test period of 720 h. 

A special form of crevice corrosion was observed in crevices between superferrite and austenitic standard steels in condenser pipes. Corrosion products, which lower the pH levels in the crevice, cause depassivation and increased corrosion on the ferritic steel. This form of corrosion, also known as dissimilar metal crevice (DMC) corrosion, does not occur with the combination of superferrites and superaustenites [115]. 

Due to their structure, the austenitic-ferritic steels, e.g. DIN-Mat. No. 1.4462 (X2CrNiMoN22-5-3), show a high level of resistance to stress corrosion cracking in seawater at higher temperatures. 

Ferritic Cr steels (1.4000/ 403, 1.4003/ 410S, 1.4512/409) with a chromium content of 11-13% have the minimum chromium level of corrosion-resistant stainless steels. They are used for automobile construction, catalytic converters and containers (Tarutani and Hashizume, 1995). In the chemical industry the ferritic steel 1.4000 containing 13% Cr is often used for structured packing in washing columns. Parts for dishwashers and washing machines are usually made from 18% Cr steels (1.4016/430). 

The most important method is to control the amount of interstitial carbon and nitrogen to less than 50 ppm and 150 ppm, respectively. Lowering the carbon to 0.03% does not appreciably help, because of the lower solubility of carbon in ferrite. It has been shown for 18 Cr-2 Mo alloys that the maximum level of C -t- N is 60-80 ppm, and for 26 Cr-1 Mo steel, the level required is 150 ppm. Use ferritic steels in annealed conditions (788°C). 

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. 

Select a more crack-resistant alloy if other preventive measures fail to work. Use high nickel alloys or alloys containing very low levels of nitrogen and other impurities, if present. For instance, purified 16 Ni-20 Cr is not susceptible to cracking. Choose one of the lower nickel duplex stainless steels or a ferritic steel. 

The specific surface area of the coating on ferritic steel was studied against treatment time by cyclic voltametry. The results were confirmed by BET measurement. There was a rapid increase in the specific surface area from the first minutes of treatment which levelled out for longer treatment times ( > 30 min). 

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