The feasibility of surface chromium enrichment by pack-cementation was assessed for different low chromium-containing cobalt alloys, in order to improve their resistance against high temperature. A binary Co-10Cr alloy, two ternary Co-10Cr-0.5C and Co-10Cr-1.0C alloys and two TaC-containing Co-10Cr-based alloys were elaborated by foundry for the study. 7.5h-long and 15h-long cementations at 1050°C, followed or not by a 75h-long heat treatment at 1200°C were performed on these alloys. Microstructure examinations performed using a Scanning Electron Microscope and concentration profiles using Electron Probe Micro Analysis -Wavelength Dispersion Spectrometry were realized in order to analyze the level of Cr-enrichment of the sub-surface region, with as studied criteria: the nature of the external Crenriched zone, the maximal chromium content on surface and the depth of chromium enrichment. The Cr-enrichment of the sub-surface succeeded for the Co-10Cr alloy and for the two tantalum-containing alloys, with the formation of an external metallic zone containing around 30wt.%Cr. In contrast the chromium carbides -containing alloys were effectively enriched in chromium in surface but in the form of a continuous chromium carbide layer which can induce other problems such as spallation and then possible fast oxidation of the denuded alloy. Finally it appeared that only the carbon-free alloys, and the alloys reinforced by carbides more stable than chromium carbides, are potentially able to be successful enriched in chromium in their sub-surface by pack-cementation.
Metallurgical and mechanical responses to the introduction of carbon and/or nitrogen have been investigated for alloys 16NiCrMo13 and 23MnCrMo5 through atmospheric pressure carbonitriding, carburizing and austenitic nitriding. Treatments were performed at 1173 K using CO − H 2 and/or NH 3 based atmospheres. As-quenched hardness is studied in terms of solid solution interstitial content, which is estimated from electron probe micro-analyses and thermodynamic simulations. This aims to quantify high-temperature precipitation of nitrides, which is accompanied by consumption of nitrogen in solid solution. Plots of as-quenched hardness versus the square root of the total content expressed in atomic fraction of interstitial elements, i.e. the sum of solution carbon and nitrogen, show the complementary character of these elements in determining the mechanical properties of the as-quenched alloys. Tempering of carbon-nitrogen martensite-for both carbonitriding and austenitic nitriding-led to lower hardness loss when compared to the same total content of interstitial carbon in martensite. Transmission electron microscopy analyses highlight precipitation of fine Fe 16 N 2 zones in the nitrogen-rich depth.
A high chromium content, typically 30wt.%, is generally considered as necessary for cobalt-based alloys to allow them resisting oxidation by hot gases. Cobalt alloys with a bulk poorer in chromium may become resistant against high temperature oxidation if they are enriched in chromium on surface. The aim of this second part of the work is to expose to air at 1200°C three low-chromium cobalt-based alloys (Co-10Cr, Co-10Cr-0.25C-4.4Ta and Co-10Cr-0.5C-8.7Ta in wt.%), previously enriched in chromium on surface by pack-cementation, in order to examine if the applied cementation treatments are enough to ensure a good behavior on long time. The initial surface enrichment in chromium of the Ta-containing alloys, characterized by a maximal content on surface of about 30wt.%Cr and a depth of enrichment of several hundreds of micrometers, gives them a good resistance to oxidation at 1200°C, despite the very low chromium content in the bulk. This is also due to the interdendritic TaC carbides which may facilitate outward diffusion of chromium during oxidation as they facilitated its inward diffusion during cementation. In contrast, the CoCr alloy, enriched in chromium in the same conditions did not display a so good oxidation behavior.
Nitriding of low-alloy steels is widely used for gears and bearings in aerospace industry. Some highly stressed surfaces require high nitrided depth, which involves long time/high temperature nitriding treatment. This study focused on identifying process parameter effects on hardness and residual stress profiles in a large range of nitriding time and temperature. We demonstrate that core and case can be considered as two materials, softening of which can be predicted by two tempering laws. In addition, a basic relationship was derived from the nitriding depth and a pseudo-diffusion coefficient, allowing prediction of the hardness profile. Residual stress results show that the diffusion controlled effect can also be used to predict residual stress profile affected depth. Likewise, the tempering controlled effect can be used to predict surface and maximum values of the residual stress profile.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.