Effect of laser beam radiation on fatigue crack propagation in AISI 4150 steel

Abstract

The effect of laser beam radiation on fatigue crack growth in AISI 4150 steel was performed on compact-tension (CT) specimens, in which a composite region (CIR) comprised of the hardened zones (HZs) on the top and bottom surfaces and the base metal(BM) in the interior, was aligned either along or normal to the crack growth direction. The microstructure of the HZs consisted of martensite, while lower bainite was present in the 300 degrees C preheated laser-hardened specimens. When the crack propagated along the laser tracks (LTs), the fatigue crack growth rates (FCGRs) of the laser-hardened specimen were lower than those of the base plate, particularly at low Delta K ranges. On the other hand, for a crack propagating normally to the LTs, decelerated FCGRs in the regions preceding the CR and accelerated FCGRs within the CR itself were found. However, enhanced FCGRs in the CR were not found in preheated specimens with a bainite structure in the HZs as the crack grew normal to the LTs. The enhancement of FCGRs in the CR, which became more accentuated at high Delta K values, was closely related to an embrittled microstructure (martensite:) in the HZs.