Delta-ferrite

These standards also outline requirements for surface finish, grain size, heat treatment, metallurgical cleanness, absence of delta ferrite and alloy segregation to ensure that besides having a well balanced chemistry the alloys shall be in the proper metallurgical condition to yield optimum mechanical and corrosion resistant properties. 

Hull F. C., Delta ferrite and martensite formation in stainless steels. Weld. J. Res. Suppl., 52 (1973) 193s-203s... 

Thier H., Delta-Ferrit und Heissrisse beim Schweissen chemisch bestandiger austenitischer Stable. Dtsch. Verlag fur Schweisstechnik (DVS) Dusseldorf 1976, 100-104... 

The presence of high-temperature delta-ferrite is inadmissible. 

All weld FZ microstructures were fully austenitic, except for the P-538 GTAW, which contained 6 to 7% delta-ferrite as welded and strain-induced... 

Kun] Kunze, J., Broz, P., Stloukal, L., Thermodynamic Analysis of the Delta Ferrite in the Systems Fe-Cr-N and Fe-Cr-Mn-N , Steel Research, 67(7), 279-284 (1996) (Phase Relations, Thermodyn., Experimental, 18)... 

Delta ferrite levels currently monitored during cladding operations. 

After the solution heat treatment the material has to be free of delta-ferrite. 

Duplex, austenitic stainless steels containing more than 5FN delta ferrite (weld metal, cast metal, weld deposit overlay), are not considered unstabilized since these alloys do not sensitize, that is form a continuous network of chromium-iron carbides. Specifically, alloys in this category are ... 

Cast stainless steel (delta ferrite 5FN to 33FN)... 

Singly and combined stainless steel weld filler metals (delta ferrite controlled to 5FN-20FN deposited)... 

The delta ferrite content of each lot and/or heat of weld filler metal used for welding of austenitic stainless steel code components shall be determined for each process to be used in production. Delta ferrite determinations for consumable inserts, electrodes, rod or wire filler metal used with the gas tungsten arc welding process, and deposits made with the plasma arc welding process may be determined by either of the alternative methods of magnetic measurement or chemical analysis described in Section III of the ASME Code. 

Delta ferrite verification should be made for all other processes by tests using the magnetic measurement method on undiluted weld deposits described by Section III of the ASME Code. The average ferrite content shall meet the acceptance limits of 5FN to 20FN for weld rod or filler metal. 

Weld deposited austenitic stainless steel with 5FN-18FN delta ferrite or NiCrFe alloy (equivalent to SB-168)... 

Table I lists the condition and composition of the four alloys prior to testing. Metallographic examination revealed traces of carbides in all alloys and in addition revealed approximately 11 delta ferrite in AM 350 (Fig. 1).

Fig. 1. Delta ferrite in AM 350 after 5% elongation (500X taken parallel to rolling direction, boiling stain etch, CrOg-lbg, H20(dist.)-145 ml, NaOH-80g green filter striated areas are...

Delta-ferrite (8-ferrite, bcc). Like alpha-ferrite, delta-ferrite consists of a solid solution of any element in body-centered cubic delta iron. In the case of carbon, its maximum solubility in delta-iron is only 0.1 wt.% at 1487°C. 

An alternative to the above is to use more weldable varieties of martensitic steels, such as the low-carbon nickel martensites. These have 12-17% Cr, 3-6% Ni and about 0.05% C. The low-carbon gives better weldability and the high-nickel prevents the formation of delta-ferrite. The development of these steels has been described by Irvine These steels are also... 

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