In the present study, the fatigue response of an additive manufactured H13 type hot-work tool steel is investigated across the High Cycle Fatigue (HCF) and Very High Cycle (VHCF) regimes. The primary focus encompasses the interpretation of fatigue strength models, the defect type analysis along with a detailed examination of crack initiation and growth mechanisms. Despite the tremendous development in AM technology, experimental data regarding advanced mechanical properties, and particularly fatigue behavior, are still limited. Here, microstructural analysis of a modified AMed H13 hot-work tool steel, a combination of HCF and VHCF testing methodologies implemented for the characterization of the fatigue behavior, as well as a thorough fractographic analysis of the fractured surfaces were performed. Results are compared with historical data of a conventionally ingot cast and forged grade to assess the influence of the AM process on the fatigue response of H13 hot-work tool steels. It proves to be comparable to the conventionally manufactured grade, showcasing the potential utilization of AM in the production of components used in high-demanding applications, and in hot work tooling applications. However, the type of critical defects identified in the AM grade was found to be process-induced, emphasizing the need to optimize process parameters to reduce both the number and size of defects and also to ensure component reliability and high performance in various industrial applications. 

High-nitrogen-chromium alloyed powder metallurgy (PM) tool steels offer many attractive features including high strength and corrosion resistance. The PM route offers various advantages such as advanced alloy composition, high homogeneity, and well-defined size distribution of hard phase particles. This study presents microstructure and mechanical properties of a PM Cr-Mo-V-N alloy. The conventional manufacturing route for this alloy is hot isostatic pressing (HIP) followed by hot working. To investigate the possibility of near-net-shape manufacturing, a comprehensive comparison of the performance was made between steels produced by as-HIPed and HIPed followed by hot working. Both steel types were heat treated in the same way to obtain martensitic matrix with limited retained austenite. In the present investigation, microstructure and phase analyses were performed by X-ray diffraction and scanning electron microscopy. Mechanical tests were carried out by hardness measurements and tensile fatigue tests in the very high cycle fatigue regime using ultrasonic fatigue testing. 

The present study aims to analyze the fatigue response and the crack growth rate of a high-nitrogen-chromium cold-work tool steel at the very high cycle fatigue regime. Detailed microstructural and fractographic analysis was conducted to examine the influence of the material´s highly alloyed martensitic microstructure on its fatigue response and crack growth damage at 20 kHz frequency. Fatigue testing was conducted under fully reversed cyclic loading (R= -1), while crack growth testing was performed in tensile-tensile cyclic loading (R= 0.1). The results revealed a fatigue strength of 691 MPa at 109 number of load cycles. Initiation points were of fish-eye type, i.e. inclusions of composition either directly connected to microstructure or pure oxides, and a fine granular area around the initiation defect. Regarding the crack growth behavior, the fractographic investigation revealed a strong influence of the stress intensity factor on the crack propagation microstructural features. A schematic interpretation of crack growth mechanisms as fine granular area formation, fish-eye crack growth, intra and trans martensite lath growth, and their relation to stress intensity levels were proposed.

The martensitic tool steel family is designed for various working environments. Their microstructure is continuously upgraded through composition development or processing techniques. These advancements introduce new types of defects, making fatigue response investigations critically important for industry.

The present study includes the investigation of six advanced high strength tool steels in terms of microstructure, common defects, and fatigue performance in high and very high cycle fatigue regimes. The materials include: i) cold work tool steels (high alloyed materials manufactured via powder metallurgy processed by hot isostatic pressing and forging) and ii) hot work tool steels (conventionally produced by ingot cast and forging or additive manufacturing). Steel grades are compared based on strengths, martensitic structures, defect distributions, and fatigue strength using experimental results and the Murakami model. The goal of the present study is to provide valuable insights to optimize martensitic tool steels for high and very high cycle fatigue applications.

Press hardening provides ultra-high strength steel components, typically boron steels, of complex geometries. In the process, the steel sheet is heated in a furnace to the austenitization temperature, transferred to the press, then simultaneously formed at high temperature and cooled in the die. Life limiting factors for the press hardening dies are mechanical fatigue, thermal fatigue, and wear. In the present case study two die segments were selected where critical damages were mechanical and thermal fatigue, respectively. The dies were made of a H13 type premium hot-work tool steel with complex heated die technology, die design integrating an advanced cooling system, for pressing automotive frame parts.

The first die failed due to mechanical loading with a crack initiated from the ejector pin area. The die design, the mechanical loads, the elevated temperature, and the tool steel crack resistance are main factors to consider. In the second die cracks initiated from an ejector pin hole, as well, due to thermal cycles causing alternating compressive and tensile stresses at the surface, which led to crack nucleation because of the accumulation of local plastic strain in the surface.