Study of monotonic and cyclic mechanical behavior and microstructural evolution at a high temperature of ASTM A297 HP steel modified with niobium

•Determination of mechanical behavior of materials used for oil cracking.•Study of niobium alloying on the elastoplastic/viscoelastic behavior.•The cyclic stresses with dwell times to precipitation of secondary carbides.•Fractography on monotonically fractured specimens and specimens after fatigue.•The material is prone to fracture in intergranular regions.