Ferritic and martensitic stainless

Fig. 5. Metastable Fe—Ni—Cr "temary"-pliase diagram where C content is 0.1 wt % and for alloys cooled rapidly from 1000°C showing the locations of austenitic, duplex, ferritic, and martensitic stainless steels with respect to the metastable-phase boundaries. For carbon contents higher than 0.1 wt %, martensite lines occur at lower ahoy contents (43). A is duplex stainless steel, eg. Type 329, 327 B, ferritic stainless steels, eg. Type 446 C, 5 ferrite + martensite D, martensitic stainless steels, eg. Type 410 E, ferrite + martensite F, ferrite + pearlite G, high nickel ahoys, eg, ahoy 800 H,...

Hydrogen embrittlement can occur in carbon and low-alloy steels, in ferritic and martensitic stainless steels, and in duplex stainless steels. It is normally not a problem in either the austenitic stainless steels or nickel-based high alloys. Hydrogen can dissolve in a steel as a result of a number of phenomena (1) Corrosion creates nascent hydrogen, usually in the presence of a cathodic poison. 

Duplex stainless steels are susceptible to 885°F (475°C) embrittlement and to sigma-phase formation, and they are usually not selected for temperatures above 650°F (345°C). Because of their ferrite content, they are susceptible to low-temperature embrittlement. However, the duplex stainless steels tend to have relatively low brittle-ductile transition temperatures. The engineering codes typically require the duplex stainless steels to be qualified for low-temperature service by impact testing. They can be susceptible to hydrogen embrittlement, but are less susceptible than are the ferritic and martensitic stainless steels. 

There are four main classes of stainless steel (austenitic, ferritic, ferritic-austenitic (duplex) and martensitic), and within these, a variety of different grades. The names ferritic and austenitic follow from their structures ferrite (P-Fe) and austenite (y-Fe) lattices hosting the alloying elements. The presence of Cr promotes the formation of the ferrite structure, while the austenite lattice forms when Ni is introduced. While ferritic and martensitic stainless steels are magnetic, austenitic stainless steel is non-magnetic. Further additives to some stainless steels are molybdenum (which improves corrosion resistance) and nitrogen (which adds strength and improves corrosion resistance). 

After transformation, the alloys, otherwise resistant, become susceptible to hydrogen cracking also, ferritic and martensitic stainless steels on cold working tend to become more susceptible to hydrogen cracking. 

Ferritic and Martensitic Stainless Steels AISI403, 422, 630 AL6X-N, Sea-cure Superalloys... 

Ferritic and martensitic stainless steels are attractive for certain operating parts because of their hardness properties. The corrosion resistance of these materials is about the same as that of the austenitic stainless steels and. slightly better in a number of cases. Because certain alloys of this class do not appear to be susceptible to stress-corrosion cracking as are austenitic stainless steels, they remain potentially a very u.seful class of materials. 

These data are presumably valid also for ferritic and martensitic stainless steels. 

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