# Several possible consequences of high temperature oxidation for the subsurface microstructure of carbides containing alloys

Patrice Berthod

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Patrice Berthod. Several possible consequences of high temperature oxidation for the subsurface microstructure of carbides containing alloys. Eurocorr 2008, Sep 2008, Édimbourg, United Kingdom. . hal-03887726

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Submitted on 7 Dec 2022

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# Several possible consequences of high temperature oxidation for the subsurface microstructure of carbides-containing alloys

Patrice BERTHOD

Nancy-Université
Université Henri Poincaré, Nancy Université

Laboratoire de Chimie du Solide Minéral, Université Henri Poincaré, Nancy Université
BP 239, Bd des Aiguillettes, 54506 Vandoeuvre lès Nancy, France

LCSM

Laboratoire de Chimie du
Solide Minéral [UMR 7555]

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Many refractory cast alloys are strengthened by carbides to achieve sufficient mechanical properties for high temperature applications. When they are oxidized at high temperature their subsurface microstructures are changing. Carbides generally disappear from surface over a depth increasing with temperature and time while, sometimes, carbides undergo modifications in an area separating carbidefree zone and bulk (coarsening, stoichiometry change, precipitation of new ones). In some cases, a change of matrix in iron alloys can also be encountered in the zone affected by oxidation, due to local chemical changes. The aim of this work is to list the types of subsurface modifications (concerning carbides only) induced by high temperature oxidation, and to attempt to correlate them to the identity of the base elements, the initial carbide density and the exposure temperature.

The subsurface modifications which were considered are illustrated below:

existence or not of an outer zone (1) in which carbides have disappeared, modification of the already interdendritic carbides, about their qualitative surface fraction or their apparent composition (2) and existence of new carbides precipitated in matrix in a zone separating the carbidefree zone and the unmodified bulk (3).

It appears that almost all alloys display a carbidefree zone ("cfz"). Its depth increases when T increases and decreases when the C content increases, and tends to be higher for Fe alloys than for Ni alloys, and higher for Co alloys than for Fe alloys.

About the coarsening of carbides (or their change of stoichiometry) in an area separating the cfz and the unaffected bulk, the Ni alloys concerned by these phenomena are mainly the ones with the low or medium C contents, and for the lowest T. This does not seem occurring in the Co alloys, and only at the highest T for the Fe alloys.

New secondary carbides precipitated in matrix, also in an area between the cfz and the bulk, in rare cases. This occurred in Fe30Cr-0.4C at 1200°C and for the Ni30Cr0.2C at 1000°C.

<table><thead><tr><th>Nickel alloys</th><th>T(°C)</th><th>0.2C</th><th>0.4C</th><th>0.8C</th><th>1.2C</th><th>1.6C</th><th>2.0C</th></tr></thead><tbody><tr><td>Carbide-free zone<br>(YES/no)</td><td>1200</td><td>190µm</td><td>←</td><td>YES</td><td>→</td><td>55µm</td><td></td></tr><tr><td></td><td>1100</td><td>80µm</td><td>←</td><td>YES</td><td>→</td><td>30µm</td><td></td></tr><tr><td></td><td>1000</td><td>15µm</td><td>←</td><td>YES</td><td>→</td><td>10µm</td><td></td></tr><tr><td>Coarsened carbides or<br>change of their nature<br>(YES/no)</td><td>1200</td><td>yes?</td><td>yes?</td><td>no</td><td>no</td><td>no</td><td>no</td></tr><tr><td></td><td>1100</td><td>yes?</td><td>yes?</td><td>yes?</td><td>no</td><td>no</td><td>no</td></tr><tr><td></td><td>1000</td><td>no</td><td>YES</td><td>yes?</td><td>yes?</td><td>no</td><td>no</td></tr><tr><td>Precipitated carbides<br>(YES/no)</td><td>1200</td><td>no</td><td>no</td><td>no</td><td>no</td><td>no</td><td>no</td></tr><tr><td></td><td>1100</td><td>no</td><td>no</td><td>no</td><td>no</td><td>no</td><td>no</td></tr><tr><td></td><td>1000</td><td>YES</td><td>no</td><td>no</td><td>no</td><td>no</td><td>no</td></tr></tbody></table>

<table><thead><tr><th>Cobalt alloys</th><th>T(°C)</th><th>0.2C</th><th>0.4C</th><th>0.8C</th><th>1.2C</th><th>1.6C</th><th>2.0C</th></tr></thead><tbody><tr><td>Carbide-free zone<br>(YES/no)</td><td>1200</td><td colspan="2">no carbide</td><td colspan="3">115µm→YES→105µm</td><td></td></tr><tr><td></td><td>1100</td><td colspan="2">300µm</td><td>←</td><td>YES</td><td>→</td><td>40µm</td><td></td></tr><tr><td></td><td>1000</td><td colspan="2">65µm</td><td>←</td><td>YES</td><td>→</td><td>20µm</td><td></td></tr><tr><td>Coarsened carbides or<br>change of their nature<br>(YES/no)</td><td>1200</td><td colspan="2">no</td><td colspan="3">**</td><td>no</td><td>no</td></tr><tr><td></td><td>1100</td><td colspan="2">yes?</td><td colspan="3">no</td><td>no</td><td>no</td></tr><tr><td></td><td>1000</td><td colspan="2">yes?</td><td colspan="3">no</td><td>no</td><td>no</td></tr><tr><td>Precipitated carbides<br>(YES/no)</td><td>1200</td><td colspan="2">no</td><td colspan="3">**</td><td>no</td><td>no</td></tr><tr><td></td><td>1100</td><td colspan="2">no</td><td colspan="3">no</td><td>no</td><td>no</td></tr><tr><td></td><td>1000</td><td colspan="2">no</td><td colspan="3">no</td><td>no</td><td>no</td></tr></tbody></table>

<table><thead><tr><th>Iron alloys</th><th>T(°C)</th><th>0.2C</th><th>0.4C</th><th>0.8C</th><th>1.2C</th><th>1.6C</th><th>2.0C</th></tr></thead><tbody><tr><td>Carbide-free zone<br>(YES/no)</td><td>1200</td><td colspan="2">no c.</td><td colspan="3">330µm→YES→40µm</td><td></td></tr><tr><td></td><td>1100</td><td colspan="2">195µm</td><td>←</td><td>YES</td><td>→</td><td>30µm</td><td></td></tr><tr><td></td><td>1000</td><td colspan="2">120µm</td><td>←</td><td>YES</td><td>→</td><td>15µm</td><td></td></tr><tr><td>Coarsened carbides or<br>change of their nature<br>(YES/no)</td><td>1200</td><td colspan="2">yes?</td><td>YES</td><td>no</td><td>YES</td><td>no</td></tr><tr><td></td><td>1100</td><td colspan="2">YES</td><td>no</td><td>no</td><td>no</td><td>no</td></tr><tr><td></td><td>1000</td><td colspan="2">no</td><td>no</td><td>no</td><td>no</td><td>no</td></tr><tr><td>Precipitated carbides<br>(YES/no)</td><td>1200</td><td colspan="2">YES</td><td>no</td><td>no</td><td>no</td><td>no</td></tr><tr><td></td><td>1100</td><td colspan="2">no</td><td>no</td><td>no</td><td>no</td><td>no</td></tr><tr><td></td><td>1000</td><td colspan="2">no</td><td>no</td><td>no</td><td>no</td><td>no</td></tr></tbody></table>

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The development of the cfz shows that almost all C quitted this part of subsurface, generally into gaseous oxidized species, but sometimes it diffused a little deeper in alloy where it promoted the carbide coarsening (or stoichiometry change for carbonricest carbides) or the precipitation of new carbides. Image analysis for specifying the carbides fractions versus the depth may lead to the distributions of C content, and then of the local solidus temperature, thanks to thermodynamic calculations (ThermoCalc).

The cfz with very low or zero C and the intermediate zone with a higher C content than in bulk, have their own solidus temperatures: a low one for the carbideenriched zone (when it exists) and a high one for the carbidefree zone, when compared to the bulk one.

<figure><img src="image_4.png" /></figure>

In addition to the development of oxides layer over the external surface and sometimes internal oxides growth, oxidation at high temperature promotes several subsurface microstructure modifications. Among them there are sometimes curious phenomena concerning deeper carbides, obviously induced by a not outward but inward diffusion of carbon released by disappearing carbides, but also a more frequent growing carbidefree zone. If the latter may be considered as not desirable for the alloy surface because of a decrease in hardness and probably in wear resistance, this carbidefree zone can be considered as an especially refractory skin which can prevent emergence of molten alloy areas in case of sudden significant increase of temperature in service.

<footnote>* no carbides were seen, ** idem, because fast oxidation</footnote>

<footnote>* no carbides were seen</footnote>