Section 4 Induction hardening

4.1 Induction hardening general comments

4.1.1 Generally, the hardness specification shall specify the surface hardness range, i.e. minimum and maximum values, the minimum and maximum extension in or through the fillet, and also the minimum and maximum depth along the fillet contour. The referenced Vickers hardness is considered to be HV0,5...HV5.

4.1.2 The induction hardening depth is defined as the depth where the hardness is 80 per cent of the minimum specified surface hardness.

Figure 3.4.1 Typical hardness as a function of depth

4.1.3 The arrows indicate the defined hardening depth. Note the indicated potential hardness drop at the transition to the core. This can be a weak point as local strength may be reduced and tensile residual stresses may occur.

In the case of crankpin or journal hardening only, the minimum distance to the fillet shall be specified due to the tensile stress at the heat-affected zone as shown in Figure 3.4.2 Residual stresses along the surface of a pin and fillet.

Figure 3.4.2 Residual stresses along the surface of a pin and fillet

4.1.4 If the hardness-versus-depth profile and residual stresses are not known or specified, one may assume the following:

The hardness profile consists of two layers (see Figure 3.4.1 Typical hardness as a function of depth. The arrows indicate the defined hardening depth. Note the indicated potential hardness drop at the transition to the core. This can be a weak point as local strength may be reduced and tensile residual stresses may occur):
- Constant hardness from the surface to the transition-zone;
- Constant hardness from the transition-zone to the core material.
Residual stresses in the hard zone of 200 MPa (compression);
Transition-zone hardness as 90 per cent of the core hardness unless the local hardness drop is avoided;
Transition-zone maximum residual stresses (von Mises) of 300 MPa tension.

4.1.5 If the crankpin or journal hardening ends close to the fillet, the influence of tensile residual stresses must be considered. If the minimum distance between the end of the hardening and the beginning of the fillet is more than three times the maximum hardening depth, the influence may be disregarded.

4.2 Local fatigue strength

4.2.1 Induction-hardened crankshafts will suffer fatigue either at the surface or at the transition to the core. The fatigue strengths, for both the surface and the transition-zone, can be determined by fatigue testing of full size cranks as described in Ch 2 Guidance for Evaluation of Fatigue Tests. In the case of a transition-zone, the initiation of the fatigue can be either subsurface (i.e. below the hard layer) or at the surface where the hardening ends.

4.2.2 Tests made with the core material only will not be representative since the tensile residual stresses at the transition are lacking.

4.2.3 Alternatively, the surface fatigue strength can be determined empirically as follows where HV is the surface Vickers hardness. The equation below provides a conservative value (in MPa), with which the fatigue strength is assumed to include the influence of the residual stress. The resulting value is valid for a working stress ratio of R = -1:

4.2.4 It must also be noted that the mean stress influence of induction-hardened steels may be significantly higher than that for QT steels.

4.2.5 The fatigue strength in the transition-zone, without taking into account any possible local hardness drop, shall be determined by the equation introduced in the Fatigue strength sub-Section of the applicable Rules.

For journal and respectively to crankpin fillet, the following applies:

where
	Y = D_G	and	X = R_G	for journal fillet
	Y = D	and	X = R_H	for crankpin fillet
	Y = D	and	X = D_O/2	for oil bore outlet

Note that the influence of the residual stress is not included in the equation above.

4.2.6 For the purpose of considering subsurface fatigue, below the hard layer, the disadvantage of tensile residual stresses must be considered by subtracting 20 per cent from the value determined above. This 20 per cent is based on the mean stress influence of alloyed quenched and tempered steel having a residual tensile stress of 300 MPa. When the residual stresses are known to be lower, a smaller value of subtraction shall be used. For low-strength steels the percentage chosen should be higher.

4.2.7 For the purpose of considering surface fatigue near the end of the hardened zone – i.e. in the heat-affected zone shown in the Figure 3.4.2 Residual stresses along the surface of a pin and fillet – the influence of the tensile residual stresses can be considered by subtracting a certain percentage, in accordance with Table 3.4.1 The influence of tensile residual stresses at a given distance from the end of the hardening towards the fillet, from the value determined by the above formula.

Table 3.4.1 The influence of tensile residual stresses at a given distance from the end of the hardening towards the fillet

I.	0 to 1.0 of the max. hardening depth:	20%
II.	1.0 to 2.0 of the max. hardening depth:	12%
III.	2.0 to 3.0 of the max. hardening depth:	6%
IV.	3.0 or more of the max. hardening depth:	0%