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

The effect of laser beam radiation on fatigue crack growth in AISI 4150 ste el was performed on compact-tension (CT) specimens, in which a composite re gion (CIR) comprised of the hardened zones (HZs) on the top and bottom surf aces and the base metal(BM) in the interior, was aligned either along or no rmal to the crack growth direction. The microstructure of the HZs consisted of martensite, while lower bainite was present in the 300 degrees C prehea ted laser-hardened specimens. When the crack propagated along the laser tra cks (LTs), the fatigue crack growth rates (FCGRs) of the laser-hardened spe cimen 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, dec elerated 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 gr ew 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 m icrostructure (martensite:) in the HZs.