NiAl precipitation in delta ferrite grains of 17-7 precipitation-hardening stainless steel during low-temperature interstitial hardening

Abstract

a b s t r a c tInterstitial hardening via low-temperature carburization and nitridation has been studied in 17-7 precipitation-hardening stainless steel. During such interstitial hardening, nanometer-scale NiAl precipitates form in delta ferrite in the bulk, but not in the hardened surface layer. In the carburized ferrite grains, regions of NiAl stoichiometry could be identified, but cannot be identified as regions with a different crystal structure. In the nitrided grains, NiAl particles could neither be detected by corresponding changes in local composition nor by their crystal structure.Precipitation-hardening (PH) stainless steels are a family of stainless steels that combine high strength, ductility, and good corrosion resistance. They are widely used as structural materials in various industrial applications. Depending on the thermal history, alloys can contain variable amounts of austenite, ferrite, and martensite. Precipitation hardening in these steels is achieved by homogeneously dispersed fine precipitates formed during aging.PH stainless steels containing Ni and Al (like 17-7 and 13-8 Mo) can be age-hardened by the formation of NiAl particles (space group Pm 3m). The lattice parameter of stoichiometric NiAl at room temperature is 0.2887 nm, which is very close to the lattice parameter of the ferrite matrix (0.2881 nm in this study). The aging temperature of 17-7 PH stainless steel is above 773 K [1].Age hardening by NiAl was studied previously in several stainless steels. In the initial stage of NiAl precipitation, there are significant fractions of Cr, Mo, and Fe present in the precipitates [2-6]. As aging proceeds, the fractions of Ni and Al increases and the Ni-to-Al ratio approaches unity [2,3]. The NiAl precipitates are nanometer-sized, spherical, and coherent with the ferrite-or martensite matrix [6].Low-temperature carburization or nitridation are effective ways to improve surface properties and corrosion resistance of stainless steels [7][8][9][10][11][12][13]. For PH 13-8 Mo stainless steel, a significant increase of surface hardness and corrosion resistance was achieved after low-temperature carburization [13]. Interestingly, a thin carbidic layer formed on the carburized surface. However, there was no report of NiAl formation.Low-temperature carburization and nitridation have also been successfully applied to 17-7 PH stainless steel [14,15]. The ferrite phase in the alloy responds to such interstitial hardening in a very unique and unusual manner, leading to a weak-contrast appearance in conventional transmission electron microscopy (TEM) images. Nanometer-scale NiAl precipitates were observed in the ferrite grains in the bulk but absent in the weak-contrast grains [14].In the present work, we investigate the effect of lowtemperature carburization and nitridation on the formation of NiAl precipitates in the ferrite phase of 17-7 PH stainless steel. 17-7 PH stainless steel in condition A [1] was provided by AK steel in the form of 3 mm thick plates. The nominal composition is 17-19 at.% Cr, ...

“Colossal” interstitial supersaturation in delta ferrite in stainless steels: (II) low-temperature nitridation of the 17-7 PH alloy

“…3b. NiAl precipitates that form due to the nitridation heat treatment in ferrite grains have been discussed in a separate publication [15]. NiAl clusters cannot be identified by cluster analysis, suggesting that NiAl clusters that could have formed due to the nitridation heat treatment (see Figs.…”

Effect of microstructure on corrosion behavior of high strength martensite steel—A literature review

“…It is strengthened by adding hardening elements (copper, aluminum, titanium, etc.). The chromium content is generally 15wt%-17wt%, which always contains martensitic precipitation hardening stainless steel (such as 15-5PH, 17-4PH, 13-8Mo [4,[20][21]) and semi-austenitic precipitation hardening stainless steel (such as 0Cr17Ni7, AM355 [22][23]).…”

“…The high tensile strength (higher than 1380 MPa) and good ductility of high strength steels mainly derived from the complex heterostructure microstructure, such as martensite, precipitate and small amount of austenite. Simultaneously, the complex heterostructures have an important impact on the corrosion resistance, involving the alloy distribution and interface of heterogeneous [3][4][5][6].…”

Study on microstructure of NiAl matrix high temperature lubricating composites containing BaO/TiO2 metallic oxide

“…At present, the matrix material of high temperature lubricating composite is mainly Ni-based alloy, which is used below 1000°C and very difficult to meet the needs of industrial development [6,7]. NiAl intermetallic compound has high melting point (1638°C), high thermal conductivity, low density (5.86 g cm −3 ), as well as excellent corrosion and oxidation at high temperature [8][9][10][11][12][13]. Nevertheless, the NiAl-based high temperature composites usually suffer the severe wear at high temperature [14][15][16].…”