Medium and high carbon bar steels are often used in the induction hardened condition for applications such as axles and shafts. Induction hardening is a surface hardening heat treatment whereby a component is rapidly heated for a short period of time in an induction coil and then quenched. This results in a high-hardness, wear resistant case and a softer core. Of interest are the comparative fatigue properties of the hardened case versus the softer core. AISI obtained axle shafts that had been hot forged, cold extruded and induction hardened. The steel grade was SAE 1050.
The mechanical properties and hardness obtained for the case and core were as follows:
Location Yield Str., MPa Tensile Str., MPa Red. in Area, % BHN
Core 460 828.5 34.1 220
Case 2100 2360 14.7 536
The core exhibited a ferrite-pearlite microstructure, and the high hardness case was 100% martensite. The differences in the mechanical properties and hardness of the case and core are (as might be expected) quite significant.
The strain-controlled fatigue properties determined for both the case and the core are shown in Figure 1.
Figure 1 (click here for larger image).
The strain-life curve for Iteration No. 4 shows the fatigue behavior of the core; the strain-life curve for Iteration No. 5 shows the behavior for the high-hardness case. In the short life regime (at high strain amplitudes), the softer core exhibits better fatigue properties than the case. In the long life regime where strain amplitudes are lower, the case shows superior fatigue properties to the core. A cross-over point can be seen at approximately 104 reversals.
High hardness is generally considered to result in better fatigue properties at long life, and this is confirmed in Figure 1. This data also shows however that at high strain amplitudes a high hardness exhibits a greater tendency toward crack initiation. This suggests that the case of induction hardened shafts may be vulnerable to crack formation under conditions where "spike" loading occurs.
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