Low Temperature Surface Carburization of Stainless Steels Page: 53 of 119
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6001823K 10%
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1% C1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04
Time (h)
Figure 4.19. Time, temperature, transformation curves for M23C6 at varying carbon
concentrations.
The solubility of carbon in a body-centered cubic (BCC) matrix was evaluated using
CALPHAD-based thermodynamic modeling. For a binary Fe-carbon alloy at 750 K, the
solubility was 0.001 at. %, a factor of 680 less than that found in an equivalent FCC matrix.
With the addition of 30 wt % Cr to a BCC Fe matrix, the carbon solubility increased by a factor
of 4000, bringing the solubility to within a factor of 2.5 of that found in an equivalent FCC
matrix. Low-temperature carburization of PH 13-8 Mo was observed using XRD; the carbon
concentration in its case was comparable to that predicted by the thermodynamic modeling.
4.1.3.4 Ferrite/Austenite Stability
XRD analyses conducted on carburized 301 SS revealed that the ~ 50% austenite/50% ferrite
material completely transformed to austenite in the case-hardened layer. The CALPHAD-based
solution thermodynamic modeling has been expanded to allow evaluation of the relative stability
of ferritic and austenitic phases as a function of their carbon contents. Carbon is an austenite
stabilizer, and diffusion of carbon into the ferrite constituent of duplex SS provides the diving
force for the transformation of the ferrite to austenite at the carburization temperature.
Calculations have been performed with a model 301 SS alloy (17 Cr-7 Ni wt %) that have
established the carbon concentration range over which the alloy is in its two-phase (ferrite plus
austenite) field at 750 K, our standard carburization temperature. For the model 301 alloy, the
carbon concentration in the case must exceed 6 at. % to achieve complete transformation to
austenite. The measured carbon concentration in 301 SS specimens exposed to a single
carburization cycle was 8.6 at. %.39
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Collins, Sunniva R.; Heuer, Arthur H. & Sikka, Vinod K. Low Temperature Surface Carburization of Stainless Steels, report, December 7, 2007; United States. (https://digital.library.unt.edu/ark:/67531/metadc901012/m1/53/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.