This study investigated the impact of stress ratio on the fatigue performance and fatigue crack initiation characteristics of PM-HIPed Inconel 625 in the very high cycle regimes. Ultrasonic fatigue tests, operating at a frequency of 20 kHz, were conducted on PM-HIPed Inconel 625 samples under stress ratios of R = -1 and 0.1 up to the ultimate fatigue life of 109 cycles. Detailed fractographic and microstructural analyses were conducted to identify the mechanisms of crack initiation. The results revealed that stress ratio played a critical role in the crack initiation process. At R = 0.1, cracks predominantly initiated at carbonitrides and non-metallic inclusions, with neighboring crystallographic facets assisting in the formation of microcracks. Conversely, at R = -1, crack initiation was driven by large gains and triple junctions. Microstructural characteristics resulting from the HIP process significantly influenced fatigue crack initiation. Prior particle boundaries were found to affect fatigue crack initiation behavior through the presence of large grains within the boundaries, as well as carbonitrides and non-metallic inclusions networks along the rim. The discussion explored fracture mechanics, fracture surface analyses, and associated microstructural properties to elucidate the observed phenomenon.

Additive manufacturing provides a unique opportunity to have freedom in design. Often it is necessary to perform post treatment processes on the printed component to adjust the mechanical properties. This is particularly true for martensitic H13 tool steels manufactured with laser-powder bed fusion, where repeated thermal cycles as a result of the addition of the new top layer, can give rise to columnar growth and microstructural inhomogeneities. Considering the unit size and texture of martensitic microstructure as directly influenced by the size of the prior austenite grains (PAGs), the focus of the investigation lies in the PAG size of the as-built (AB) and heat-treated conditions. Furthermore, four types of samples with different scan rotations (0 degrees, 45 degrees, 67 degrees, and 90 degrees) are investigated. The AB material displays a characteristic cell structure with cell boundaries enriched in alloying elements as well as nanosized carbides at triple junctions and intercellular regions. A key finding of this research is that the heat treatment gives rise to PAG size refinement as a result of recrystallization and pinning effect from carbide hindering the grain growth during austenitization.

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. 

Hot isostatic pressing (HIP) is a near-net shape powder metallurgy (PM) technique, which has emerged as an efficient technique, offering precise control over the microstructure and properties of materials, particularly in high-performance alloys. This technology finds applications across a wide range of industries, such as aerospace, automotive, marine, oil and gas, medical, and tooling. This paper provides an overview of powder metallurgy and hot isostatic pressing, covering their principles, process parameters, and applications. Additionally, it conducts an analysis of PM-HIPed alloys, focusing on their microstructure and fatigue behavior to illustrate their potential in diverse engineering applications. Specifically, this paper focuses on nickel-based superalloys and martensitic tool steels. The diverse microstructural characteristics of these alloys provide valuable insights into the PM-HIP-induced fatigue defects and properties.

This paper investigated the microstructure and fatigue behavior of PM-HIPed Inconel 625. The microstructure was composed of γ phase and (Mo, Nb) carbonitrides located mostly on prior particle boundaries. Despite the presence of these carbonitrides, the samples showed good tensile properties with high elongation. Two different surface conditions, pickled and machined, were used for high cycle fatigue testing under a four-point bending test. The results indicated that pickled samples had 6% lower fatigue strength (at 106 cycles) with three times higher standard deviation compared to the machined ones. Fatigue failure mechanisms were found to be dependent on surface conditions and showed different failure modes due to non-metallic oxide inclusions and surface defects in samples with machined and pickled surfaces, respectively. The effect of type, size, and location of defects, multiplicity of crack initiations, as well as surface roughness were analyzed and discussed.

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.

Top hammer drilling is a common method to drill holes in rock formations in mining and civil engineering applications. Failure of drilling machine components has a significant impact on the cost and period of the operation. Internal components of percussive hammers experience extreme loading conditions during their service life. The focus of the present case study is to characterize failure mechanisms of two cylindrical impact pistons subjected to impact loading. The investi-gated components were manufactured from two different steel grades, a surface hardened low alloyed high strength steel and a through hardened cold work tool steel.Failure of both pistons started with degradation of the impact surfaces in term of cavitation erosion and localized surface fatigue phenomena. Subsequently, chipping and removal of material from impact surfaces resulted in formation of semi-spherical holes and craters on both surfaces.Radial and hoop cracks started to develop from cavities on the impact surface. The radial cracks then propagated parallel to the impacting surface in the longitudinal direction of the piston. Once the cracks formed at the impact surface, the damage was controlled by impact fa-tigue. Fatigue beach marks were identified on the fracture surface of failed component.

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 microstructure evolution in both surface and bulk grains in a pure Fe-19Cr-12Ni alloy has been analyzed using electron backscatter diffraction after tensile testing interrupted at different strains. Surface grains were studied during in situ tensile testing performed in a scanning electron microscope, whereas bulk grains were studied after conventional tensile testing. The evolution of the deformation structure in surface and bulk grains displays a strong resemblance but the strain needed to obtain a similar deformation structure is lower in the case of surface grains. Both slip and twinning are observed to be important deformation mechanisms, whereas deformation-induced martensite formation is of minor importance. Since the stacking fault energy (SFE) is low, similar to 17 mJ/m(2), dynamic recovery by cross slip of un-dissociated dislocations is unfavorable. This reduces the annihilation of dislocations which in turn leads to a significant increase of low angle boundaries with increasing strain. The low SFE also favors formation of deformation twins which reduces the slip distance, leading to a hardening similar to the Hall-Petch relation. The combination of a low ability for cross-slip and a reduced slip distance caused by twinning is concluded to be the main reason for maintaining a high strain-hardening rate up to strains close to necking.

Plastic deformation of surface grains has been observed by electron backscatter diffraction technique during in situ tensile testing of a high-nickel austenitic stainless steel. The evolution of low- and high-angle boundaries as well as the orientation changes within individual grains has been studied. The number of low-angle boundaries and their respective misorientation increases with increasing strain and some of them also evolve into high-angle boundaries leading to grain fragmentation. The annealing twin boundaries successively lose their integrity with increasing strain. The changes in individual grains are characterized by an increasing spread of orientations and by grains moving towards more stable orientations with < 111 > or < 001 > parallel to the tensile direction. No deformation twins were observed and deformation was assumed to be caused by dislocation slip only.