Structure and wear resistance of 20X13 steel ion-implanted with nitrogen at a high beam-current density

The “Expanded” Phases in the Low-Temperature Treated Stainless Steels: A Review

“…Expanded martensite has been reported to form in stainless steels which have a martensitic structure, i.e., martensitic [33,49,[64][65][66][67] and martensitic PH [62,65,[68][69][70][71] stainless steels.…”

“…This characteristic was observed for modified layers consisting of expanded martensite only [49,[68][69][70][71]89], and layers in which both expanded martensite and ε-Fe 2-3 N nitride [49,67,93], or expanded martensite and expanded austenite [62,65,69], or expanded martensite, expanded austenite, and ε-Fe 2-3 N [83] were detected. It should be noted that this featureless appearance may be due to the chemical etchings used, such as Marble's reagent [33] or Vilella's reagent [49,65,70,71,83], which are unable to etch Fe-based nitrides as well as N-rich phases, as was observed for the expanded austenite [11]. The microstructure of the modified layer can be better delin-eated, highlighting the martensite structure, by using acetic glyceregia [91], as showed in Figure 2b, or Kalling's reagent [92].…”

“…structure, even if the steel has a fairly high C content (see, for example, [101]). When N or C atoms are solubilized in the martensite structure, as a consequence of low-temperature treatment, a "tail" at lower diffraction angles is observed [33,60,69,77,94], thus an expansion of the lattice is hypothesized. The distinct shift of the peaks towards lower angles was reported as the interstitial content increases, hence the lattice expansion [64,69,70,77,102], even if it was not so marked as that observable for expanded austenite [103].…”

“…In the same period, many studies reported nitriding treatments carried out at temperatures comparable to those used for low alloy steels, i.e., at temperature of 500 • C or higher [25][26][27][28][29][30][31][32]. Lower temperatures were used in a few studies, but the formation of "expanded" phases was hardly recognized [25,29,33]. The term "expanded martensite" was used in 2000 by Kim et al [34] for the expanded phase observed in a nitrided AISI 420 martensitic stainless steel, while "expanded ferrite" was used later for the phase formed in treated martensitic [35], duplex [36], and ferritic stainless steels [37].…”