Abstract
The fractography and mechanical behaviour of fatigue crack propagation in the heat-affected zone (HAZ) of AISI 4140 steel welded using the shielded metal arc process was analysed. Different austenitic grain size was obtained by normalizing performed at 1200 °C for 5 and 10 hours after welding. Three point bending fatigue tests on pre-cracked specimens along the HAZ revealed that coarse grains promoted an increase in fatigue crack growth rate, hence causing a reduction in both fracture toughness and critical crack length, and a transgranular brittle final fracture with an area fraction of dimple zones connecting cleavage facets. A fractographic analysis proved that as the normalizing time increased the crack length decreased. The increase in the river patterns on the fatigue crack propagation in zone II was also evidenced and final brittle fracture because of transgranular quasicleavage was observed. Larger grains induced a deterioration of the fatigue resistance of the HAZ.
Highlights
The lifetime of a component or structure under cyclical, alternating, repetitive or fluctuating loads is frequently considered to be determined by the crack initiation site, crack propagation and the final catastrophic fracture, which are allwell-identified stages of fatigue damage, as the core mechanism of degradation and failure in structural components[1]
The fatigue crack propagation tests provided the propagation rate as a function of the amplitude of the stress intensity factor (∆K) at the heat-affected zone (HAZ) for specimens normalized at 1200 °C for 5 and 10 hours and as-received condition
The effect of the increase in the austenitic grain size induced by the normalizing treatment on the fatigue behaviour was promotion of the fatigue crack propagation which brought a decrease in the critical crack size and fracture toughness with the consequent increase of the values of the m exponent
Summary
The lifetime of a component or structure under cyclical, alternating, repetitive or fluctuating loads is frequently considered to be determined by the crack initiation site, crack propagation and the final catastrophic fracture, which are allwell-identified stages of fatigue damage, as the core mechanism of degradation and failure in structural components[1]. Structures are said to be functionally static, in bridges for instance, which have the particular function of maintaining a static connection between shorelines These structures encompass manufactured components such as beams, tubes, plates, and sections, all of them mainly joined by welding and bolting. Service conditions involving traffic loads, collisions, temperature changes or even gusts of wind induce both static functional and cyclical loads on bridges
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