Effect of Intergranular Oxidation on Axial Fatigue Test Bar Performance

Two 100 pound heats of vacuum melted 8695 steel were made for iterations 40 and 41. This chemistry represents the case of carburized 8620 steel. The two heats were forged into bar stock from which axial test bars were machined. In order to produce intergranular oxidation (IGO), both sets of test bars were carburized. Iteration 40 was copper plated prior to carburizing to prevent carbon diffusion and any associated IGO. Both sets of bars were polished prior to testing. Figure 1 shows the IGO present at the surface in iteration 41.

SMDI Blog 34 Figure 1Figure 1

Table 1 shows the mechanical properties and hardness for both iterations, and Figures 2 and 3 show the stress-strain curves. The static strength of iteration 40 without IGO is about 50% greater than iteration 41 with IGO. The elongation or ductility is also greater. It would seem possible that this difference could be related to the presence of IGO, however there is also a difference in how the samples were processed.

Table 1

SMDI Blog 34 Table 1

SMDI Blog 34 Figure 2
Figure 2

SMDI Blog 34 Figure 3
Figure 3

Figure 4 shows a comparison of the fatigue curves for iterations 40 and 41. In the low cycle portion of the curve, iteration 40 has better fatigue life. This is typical for a sample with higher strength. However, in the high cycle portion of the curve, iteration 41 has the better fatigue life. If IGO had a negative effect on performance this is where it would occur. It can therefore be concluded IGO is not detrimental to the fatigue life of these samples.

SMDI Blog 34 Figure 4.jpgFigure 4

 

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