In the last posting, the fatigue properties of surface induction hardened 1050 axle shafts were presented.  Strain life data showed that in the short life regime, the fatigue properties of the low hardness core were better than those of the case.  In the long life regime the fatigue properties of the high hardness case were superior to those of the core. 

Here we would like to look at additional data that has been developed on induction hardened SAE 1070 steel.  In this study, the case and core properties were simulated using 50mm diameter steel bars.  As-hot rolled bars were used to simulate an induction hardened core, and additional bars were through-induction hardened to simulate the high hardness case.

The mechanical properties and hardness obtained for the simulations of core and case were as follows:

   Location   Yield Str.     Tensile Str.   Red. in Area      BHN
                        (MPa)           (MPa)              %                           
   Core            520.0           659.0              36.2            280
   Case          1950.0          2069.0              2.3            613

The core exhibited a ferrite-pearlite microstructure and the high hardness case contained a mixture of martensite, bainite and a small amount of pearlite. 

The strain-controlled fatigue properties determined for both the case and the core are shown in Figure 1.

Figure 1

The strain-life curve for Iteration No. 36 shows the fatigue behavior of the core, and the strain-life curve for Iteration No. 37 shows the behavior for the high hardness case.  As was observed for the SAE 1050 steel in the previous post, the softer core exhibits better fatigue properties than the case in the short life (high strain amplitude) regime.  Conversely, in the long life regime (low strain amplitudes), the properties of the high hardness case are superior to those of the core.  A cross-over point occurs between 104 and 105 reversals.

Additional comparisons of the case and core of surface hardened steels will be included in future postings covering the properties of carburized low alloy steels.