Issue 77

M. V. Boniardi et alii, Fracture and Structural Integrity, 77 (2026) 405-420; DOI: 10.3221/IGF-ESIS.77.23

Consequently, at the end of the heat treatment, non-uniform or limited surface hardness may occur if the steel initially presents a coarse-grained ferritic-pearlitic structure, in which the pearlite is richer in carbon than the ferrite. For this reason, it is always better to surface-harden martensite in which the carbon is more homogeneously and uniformly distributed in the crystal lattice [21]. b. Great care must be taken to ensure a uniform hardening depth along the profile of the part. In addition to ensuring homogeneous strength, this result also allows for a uniform distribution of compressive residual stresses in the hardened surface layer, with beneficial effects on the fatigue resistance of the component [9,22]. c. If the treatment is performed correctly and the residual stresses on the surface are compressive, the fatigue fracture of an induction-hardened component shows initiation beneath the quenched layer. See two typical examples in Fig. 10. This morphology is valid for smooth parts or parts with limited notch effects; as the notch effect increases, the initiation will instead appear on the surface [23,24]. The subsurface initiation makes it clear that the depth of the quenched layer and the hardness of the core of the base material are the two parameters to control to improve fatigue resistance.

Figure 10: Schematic representation of the fatigue failure of two induction hardened steel shafts (left) C40 and (right) C50.

To evaluate the local fatigue strength of the surface-treated material, the aforementioned correlation between the ultimate tensile strength and the bending fatigue limit can be used [19] (valid only up to  FAb = 700-750 MPa) or, more simply, the formulas proposed in the literature can be used [15]. Of particular interest is the simple and effective correlation proposed by Nakonieczny [15], independent of any residual stresses present, valid for rotating bending stresses (R = -1) in the range 340 < HV < 900: (4) where HV i is the Vickers hardness measured where the local fatigue limit is to be determined. Fig. 11 shows the correlation between hardness and local bending fatigue limit. Case hardening Case hardening is the most studied hardening heat treatment for fatigue resistance. Many studies have focused on gears, as case hardening is the most widely used surface treatment to harden gears, improving both the bending fatigue resistance of the tooth root and the contact fatigue resistance. In the case of case-hardened layers, the fatigue fracture mechanism is very different from that observed in components with hardened surfaces: failure always initiates from the external surface, whether smooth or notched, with localized semi-elliptical fracture of intergranular or mixed intergranular/transgranular type [25–27]. Fig. 12 shows the schematisation of an intergranular initiation with stable transgranular propagation, as observed in a case-hardened AISI/SAE 4320 steel (similar to 17NiCrMo6-4) [27]. 2 1,98 0,0011 i i HV HV  FAb  

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