Fatigue Crack Initiation and Propagation in Laser‐hardened, Medium‐carbon Steel

Abstract

The results of investigations into fatigue crack behaviour of laser-hardened, medium-carbon steel specimens under reversed bending ( R = -1) are presented. Characteristic features of crack behaviour, such as crack source location, temporary crack arrest at the border between laser tracks and the matrix material, and discontinuous crack growth in the laser tracks are discussed. In order to explain the sites of primary crack sources, results of residual stress measurements are included as well as analytical descriptions of these stress distributions as a function of sample depth. A mechanism of cracking has been established on the basis of fractographic analyses of fracture surfaces. NOMENCLATURE c =carbon content in a steel E = Young's modulus f = martensite volume fraction K = stress intensity factor I = crack length N = number of cycles Tp = field of peak temperature in a sample V k , V t = specific volume fraction of martensite and austenite, respectively x = specimen longitudinal direction y = specimen transverse direction z = depth a = linear thermal expansion coefficient v = Poisson's ratio C T~, ub,, CTG = principal residual stresses uth = thermal residual stresses utr = phase transformation residual stresses u; =yield stress of austenite

Effect of Laser Beam Radiation on Fatigue Crack Propagation in Aisi 4150 Steel

“…The thermal cycle of laser processing is very short, resulting in minimum distortion of the work pieces. In addition, residual compressive stresses are developed at the HZ due to the volume increase associated with martensitic transformation [ 10, 12–15]. Singh et al [ 10] reported that an increase in fatigue life and an improvement in the fatigue limit were obtained for a 1045 carbon steel irradiated by laser.…”

“…Furthermore, the fatigue strength of grooved shaft specimens containing small cracks could be raised through laser hardening [ 11]. Kocanda and Natkaniec [ 14] have reported that the FCGRs of a laser‐hardened steel depended on the microstructures of the matrix, the HZ, laser track geometries and residual stresses in the surface layer. The effect of laser beam radiation can enhance the fatigue threshold and retard the crack growth [ 16].…”

Fatigue crack propagation in laser alloyed ductile cast iron surface

“…Some graphite nodules near the interface are surrounded by an intergranular failure facet band (marked by AA 0 at G2 and BB 0 at G4, respectively), which is typical in a martensite fracture fractograph. 2,3,23 This indicates that the graphite nodule in the heat affected zone is surrounded by a martensite shell formed by rapid cooling and carbon diffusion from the graphite nodule. 2,3 The crack initiated at the alloying zone or the heat affected zone is not easy to break through the martensite shell wrapping graphite nodule and rather would propagate through a bypass.…”

“…The former is attributed to the plastic deformation and the tear of the retained austenite in dendrites/cells, and the latter comes from the fracture of the martentsite and the cementite between dendrites. 23 Thus, the primary austenite dendrites/ cells in the laser alloying zone are effective to retard the fatigue crack propagation by the compressive stress and the crack closure mechanism.…”

“…After the laser beam irradiation, the fatigue threshold may be raised and the crack growth may be retarded. 23 It was reported that the laser surface hardening can reduce the FCGR effectively at low DK ranges. The presence of a residual compressive stress field ahead of the crack tip would decrease the fatigue crack growth rate and enhance the effect of crack closure.…”