Fatigue limit of induction hardened railway axles

Abstract

A B S T R A C T A new surface induction hardening technology was designed for the purpose of increasing the resistance of railway wheelsets to fatigue damage. This paper gives a detailed presentation of the technological aspects of induction hardening of axles. The increased fatigue resistance in hardened surfaces compared with standard heat treatment of EA4T steel was verified using tensile test specimens, press-fitted wheel seat/axle joints at 1:3 scale and press-fitted wheel/axle joints at actual size. The 70% increase in the fatigue limit of induction hardened EA4T steel specimens compared with material subjected to standard heat treatment clearly demonstrates the effectiveness of this technology.A = parameter A 5 = ductility B = parameter C = parameter d = axle body diameter D = wheel seat diameter f = loading frequency f i = frequency HV30 = hardness i = test level M = bending moment q = notch sensitivity parameter R = aspect ratio R e = yield strength R fL = fatigue limit for the smooth specimen R fE = fatigue limit for notched specimen R m = tensile strength R 0 = the lowest level of stress ŝ = standard deviation W o = modulus of resistance of the test specimen section δ = difference in stress between neighbouring test levels Δ = parameter depending on A, B, C ε_ = strain rate I N T R O D U C T I O NDemands for higher reliability and safety of railway vehicles have brought intensive research of degradation processes in railway wheelsets 1-3 and new approaches to wheelset design. 4 Extremely high repeated contact stresses between wheels and rails cause rolling contact fatigue. 1,2 A very serious form of damage to wheel seats is fretting fatigue, caused by small-amplitude oscillatory movement of wheels in press-fitted seat/axle joints. 3

On the residual life assessment of high-speed railway axles due to induction hardening

“…Generally the maximum in-service stress amplitude is less pronounced than 200 MPa, which is far lower than allowable stress of full-scale axle used 34CrNiMo6 [17,22]. Such nature actually suggests that the axle is subjected to typical elastic deformation according to nominal stress approach.…”

“…In addition, the induction hardening can achieve a relatively small stress gradient and uniform hardness distribution, which can effectively prevent the nearsurface material from debonding. Such technological advantage throw more prospective action against fatigue crack initiation and casual propagation of full-scale axles [14,15,22].…”

“…To acquire the compressive stress field in terms of practical hardened layers and to assign corresponding material properties, all elements inside material layer were renumbered. This work proposed a novel unit pressure method (UPM) to rebuild the residual stress rather than through a complex thermo-elasto-plastic FEM simulation [14,22].…”

Fatigue crack growth behavior in gradient microstructure of hardened surface layer for an axle steel

“…A large number of studies suggested that the fatigue strength of a material is proportional to its tensile strength and the fatigue crack that results in final failure usually initiates at the surface of the material [1][2][3]. In order to upgrade the fatigue strength of engineering materials/components, investigators have used surface strengthening techniques in the applications of aviation, automobile and high-speed railway [4][5][6][7][8][9]. The methods of surface treatment, such as shot peening, nitriding and surface induction, produce a strengthened surface layer which always possesses a gradient feature of microstructure and mechanical behavior including residual stress from the surface to the interior of treated specimens or components.…”

“…Fajkoš et al [5] reported that the fatigue strength of an EA4T steel (25CrMo4) with surface induction treatment had an increase of 70% compared to that with standard heat treatment. Roland et al [7] and Yang et al [8] showed that a stainless steel with surface mechanical attrition treatment and a pure Cu with surface mechanical grinding treatment had higher fatigue strength than the original materials, especially in high cycle fatigue regime.…”