Microstructure, Defects, and Fatigue Response of High-Strength Tool Steels

Abstract

The martensitic tool steel family is designed for use in various working environments where they are subjected to repeatedly high mechanical loads. Despite the continuous upgrade of material´s microstructures through compositional development or processing techniques, defects remain a critical factor for the tool performance by leading to fatigue failure. The present study includes the investigation of six advanced high-strength tool steels. The materials include: (i) four cold-work tool steels (high-alloyed materials manufactured via powder metallurgy and further processed by hot isostatic pressing and forging) and (ii) two hot-work tool steels (one conventionally produced by ingot casting and forging and one produced by additive manufacturing). Fatigue tests were carried out at stress ratios of R = −1 and R = 0.1 using a servohydraulic machine in load control at 30 Hz frequency and/or an ultrasound machine in displacement control at 20 kHz frequency. Steel grades are compared based on their martensitic structures, fatigue initiation defect distributions, and fatigue strengths assessed using experimental results and a modified Murakami model. Within the cold-work group, the grade exhibiting a fine martensitic lath size distribution (0.71 μm) and the smallest defect size distribution (11 μm) demonstrated the highest fatigue performance (610 MPa). For the hot-work group, the two grades were of comparable fatigue strengths and defect size distribution (375 MPa and 42 μm for the conventional grade and 320 MPa and 37 μm for the additively manufactured grade), besides the finer martensite of the additively manufactured grade. The findings demonstrate that no single microstructural parameter is sufficient to ensure the highest fatigue resistance. Instead, overall performance is dictated by the combined balance of multiple interacting characteristics.