Effect of Steel Composition on the Case and Core Fatigue Properties of Low Alloy Carburized Steels

In earlier posts, limited data indicated that the fatigue properties of the cores of carburized components do not vary significantly with the composition of low alloy steels.  There was evidence, however, that the fatigue properties of the cases of carburized components are influenced by steel composition.  Better case fatigue performance was exhibited by more highly alloyed steels.

To more completely examine this issue, case and core fatigue data were analyzed for three low alloy carburizing steels.  The three steel grades were SAE 4320, SAE 9310 and 20MnCr5.  SAE 4320 is a Ni-Cr-Mo steel, SAE 9310 is a high-nickel (3%) steel, and 20MnCr5 is a Mn-Cr low alloy steel.   Carburized cores were simulated by heat treating steel bars using a carburizing thermal cycle without the presence of carbon in the furnace atmosphere.  Carburized cases were simulated by through-carburizing fatigue specimens using a carburizing thermal cycle in a carbon-bearing furnace atmosphere.

The mechanical properties for the case and core simulations for the three steel grades are shown below.

Table Blog Post 24

Some inconsistencies were observed in the tensile properties in the case simulations due to low ductility and early failure.  The microstructures in the case simulations were martensite, and in the core simulations mixtures of martensite, bainite and ferrite were observed.

Since, in recent posts, it was noted that core fatigue properties varied with hardness resulting from changes in cooling rate following carburizing, core data for this analysis was obtained at constant hardness.

The strain controlled fatigue properties for the simulated carburized cores for all three steels are shown in Figure 1.  The data for SAE 4320 is given by Iteration No. 122, for SAE 9310 by Iteration No. 125 and for 20MnCr5 by Iteration No. 128.  It can be seen that the fatigue properties for the three steel grades all fall within a very narrow band.  Thus, at constant hardness, equivalent fatigue properties were observed for the carburized cores of all three steels.

No. 24 Fig. 1
Figure 1

Figure 2 shows the strain controlled fatigue properties for the simulated carburized cases for all three steels.  The data for SAE 4320 is given by Iteration No. 167, for SAE 9310 by Iteration No. 168 and for 20MnCr5 by Iteration No. 170. As can be seen the more highly alloyed SAE 9310 and SAE 4320 steels exhibit better fatigue performance than the 20MnCr5 steel.

No. 24 Fig. 2
Figure 2

Thus the data indicates that, while equivalent core fatigue properties can be obtained for carburized steels at various alloy levels and at constant hardness, applications requiring superior case performance require more highly alloyed steel grades.

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4 Responses to Effect of Steel Composition on the Case and Core Fatigue Properties of Low Alloy Carburized Steels

  1. Jan Papuga says:

    Hello,

    I wonder a bit, that all the curves related to the carburized case show a worse fatigue performance, when compared to the core material. Did I missed something in your short article? Which was the mode of the loading? Thanks for writing your opinion…

    Best regards

    Jan Papuga

    • The case properties generally demonstrate significantly less plasticity than the core properties. As a result, at low strain amplitudes, case properties will approach those of the core, sometimes even exceeding them due to higher hardness. At high strain amplitudes, however, due to the lack of plasticity, the case properties will generally be poorer than the core properties. The loading mode was axial.

      Response from Tom Oakwood, SMDI Program Manager Bar Steel Fatigue Sub-Committee

  2. Mary Starkey says:

    An interesting finding but not I think the whole story.
    The database shows some very different results for case simulations of the same alloy
    So for SAE8620 iteration 71 gives MUCH higher results than iteration 38
    Similarly for SAE4320 iteration 50 gives MUCH higher results than iteration 167. If you had used iteration 50 in the plot in your blog rather than iteration 167, it may have led to quite different conclusions.
    Clearly chemical composition is not the only factor affecting the fatigue properties of the case. There is insufficient metallurgical data in the reports (such as carburising conditions, presence / absence of Intergranular oxidation (IGO), presence / absence of free carbide, presence / absence of micro cracks; how much was machined off specimens and so on) to understand what may have caused the differences in the above examples. Has this been studied at all? As it stands there is a big question mark as to which dataset to use.

    • We’ve found that case properties exhibit more scatter than core properties, which are better behaved. This can be seen from the blog article. In some instances this has led to variability in test sets. The case behavior is probably the result of a nearly total lack of plasticity in the carburized cases. This in turn makes them vulnerable to the other variables you described in your comment, most of which we have not been in a position to evaluate. In general we continue to find that case properties exhibit higher sensitivity to composition in that more highly alloyed steels perform better than lower alloyed steels. Core properties on the other hand seem to show a dependence largely on hardness.

      Response from Tom Oakwood, SMDI Program Manager Bar Steel Fatigue Sub-Committee

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