Fatigue Crack Growth Behavior of an Ultrahigh‐Strength Low‐Alloy Steel Fabricated Using Laser Powder Bed Fusion to Contain Layered Quenched/Tempered Martensite Microstructures

ABSTRACT This study investigates the influence of a characteristic microstructure in a high‐strength low‐alloy (HSLA) steel created using laser powder bed fusion (LPBF) additive manufacturing (AM) on fatigue crack growth behavior at room temperature. The characteristic microstructure consists of successive layers of quenched/untempered and tempered martensite formed owing to the cyclic heating and rapid cooling temperature gradients intrinsic to the layer‐by‐layer deposition. As the laser beam selectively melts a layer of metal powder, it tempers a layer of the material underneath the melt pool, creating the characteristic dual‐variant martensite microstructure. Specimens in as‐printed, quenched, and tempered conditions of the steel were tested to evaluate the extent to which these microstructural characteristics influence the fatigue crack growth behavior, including the crack propagation paths and rates. The results showed that the tempered layers enhance toughness and resistance to crack propagation, while the quenched layers exhibit higher brittleness and crack growth rates influenced by secondary cracks. The findings suggest that the control of AM processing parameters can optimize microstructure and mechanical properties of the steel, marking an advancement in the design of materials for demanding applications requiring enhanced fatigue resistance and toughness. The Paris law parameters were estimated using the measured data per specimen condition. A crack growth rate exponent of 2.24 and a crack growth rate coefficient of 1.12 × 10 −7 were established for the as‐printed material governing an intermediate crack growth behavior compared to the other material conditions. The as‐quenched martensite condition exhibited the highest fatigue crack growth rate, while the fully tempered martensite condition showed the lowest crack growth rate. The quality of fits and established parameters for the rate of fatigue crack growth are presented and discussed.