Low-Temperature Carburization of the Ni-base Superalloy IN718: Improvements in Surface Hardness and Crevice Corrosion Resistance

Abstract

Case-hardening'' of the Ni-base superalloy IN718 has been achieved by low-temperature gas-phase carburization. After carburization under optimum conditions, the hardened surface layer (the ''case'') has about twice the hardness of the core (HV of %800) and contains %12 at pct carbon in interstitial solid solution. This causes a lattice parameter expansion of %1 pct perpendicular to the surface and, because of the mechanical constraint provided by the noncarburized core below, develops a large biaxial surface compressive residual stress (%1.9 GPa) parallel to the surface. Microstructural studies and X-ray diffractometry reveal no carbide precipitates in the case. In agreement with this observation, low-temperature carburization does not compromise the ductility and actually improves the crevice corrosion resistance of the alloy.

Nanostructure Findings by Use of a Modified Preparation Method of TEM Samples Collected from the Topmost Surface Layer of Carburized Inconel-718 Superalloy

“…The model of colossal supersaturation with interstitial carbon atoms [3,4] of the surface grains crystal lattice, currently accepted as explanation of the SSs surface carburization effects, was automatically extended to the LT surface carburized IN-718 -the subject of this report. The predicted dilatation of matrix lattice that would normally happen in LT carburized IN-718, is denied by the anomaly [2] reported for the IN-718 carburized at 843K, i.e. an apparent contraction instead of dilatation of the crystalline lattice, occurring inside the topmost layer ≈ 2µm thick.…”

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The low-temperature (LT) gas atmosphere carburization processing, invented by Swagelok Co. for the surface hardness improvement of 316L austenitic stainless steel (SS), was applied to other SSs, corrosion resistant steels and superalloys [1] including [2]. The model of colossal supersaturation with interstitial carbon atoms [3,4] of the surface grains crystal lattice, currently accepted as explanation of the SSs surface carburization effects, was automatically extended to the LT surface carburized IN-718 -the subject of this report.

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Phase Transformations during Low Temperature Nitrided Inconel 718 Superalloy

“…Detailed information about microstructure of IN 718 superalloy after nitriding could be found in previous reports. 5) After nitriding, XRD patterns of the samples changed dramatically for different nitriding temperatures and times, indicating the strong impact of these parameters. Figure 6 shows different X-ray diffraction patterns of non-treated samples and nitrided IN 718 under different nitriding times at 425°C.…”

“…Most of the research was focused on plasma nitridization, gas nitridization, plasma carburization and gas carburization of different alloys. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] However, much less attention have been paid to nitridization of nickel based superalloys, in that such alloys are difficult to be nitrided. 20,21) This is due to the formation of passivated oxide layer on the surface, which impedes the infusion of alloying elements.…”

Phase instability of the topmost surface layer of Inconel-718 superalloy in low-temperature gas carburization atmosphere, revealed by means of (HR)(S)TEM

“…The surface microstructure modifications induced by the industrial treatments with the effect of enhancement of the surface engineering parameters in metallic alloys (hardness, wear, corrosion resistance) are utterly important. The recently invented, industrially successful surface hardening by carburization of 316L stainless steel (SS) in gas atmosphere at low-temperature (LT) [1] was extended with similar results to the Ni-Fe base Inconel-718 superalloy (IN-718) [3] and to several other alloys. The accepted model of surface microstructure modification due to processing is not based on full data about the modified topmost layer microstructure.…”