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.

Near-net shape manufacturing using powder metallurgy (PM) and hot isostatic pressing (HIP) enables the production of fully isotropic components with control over microstructure and mechanical properties. This approach supports the manufacture of high-performance alloys such as nickel-based superalloys and tool steels, which are widely used in aerospace, energy, and tooling industries. PM-HIP technology can address many of the challenges associated with conventional processing and machining of these alloys.

This study investigated the nickel-based superalloy Inconel 625 and a high-nitrogen tool steel, both produced by PM-HIP. The focus was on understanding how manufacturing parameters and resulting microstructural features influence fatigue behavior under different fatigue life regimes. Detailed microstructural characterization and fatigue testing were carried out to evaluate alloy performance and identify fatigue crack initiation mechanisms. To further examine the potential of minimizing post-HIP treatment in near-net shape manufacturing, the surface properties and fatigue behavior of HIPed alloys with and without machining were assessed.

The findings showed that non-metallic inclusions acted as fatigue crack initiations in PM-HIPed Inconel 625 in both high cycle and very high cycle fatigue (VHCF) regimes, whereas large grains played a key role in fatigue crack initiation during VHCF, either individually, in conjunction with triple junctions, or by assisting carbonitrides. In PM-HIPed high-nitrogen tool steel, fatigue cracks initiated at non-metallic inclusions in both fatigue regimes, and at clusters of oxides and precipitates under VHCF conditions. These results highlight the importance of optimizing manufacturing parameters and post-processing steps based on the intended application.

This doctoral thesis studies Inconel 625 and high-nitrogen tool steel produced by near-net shape manufacturing using powder metallurgy (PM) and hot isostatic pressing (HIP). The goal is to characterize the microstructure and evaluate fatigue behavior of the alloys, with a focus on understanding how the manufacturing process and resulting microstructural features influence fatigue performance under different loading conditions and life regimes.

The study combines detailed microstructural analysis, fatigue testing, and fractography to assess the performance of the alloys and to identify the mechanisms responsible for fatigue crack initiation. In addition, the work evaluates the potential of near-net shape manufacturing to remove the need for post-HIP machining by comparing the surface properties and fatigue behavior of HIPed alloys with and without machining.

Nickel-based superalloys and tool steels are well-known high-performance alloys due to their extensive use in many different industries. Nickel-based superalloys have found their way into aircraft, aerospace, marine, chemical, and petrochemical industries owing to their excellent high-temperature corrosion and oxidation resistance. On the other hand, tool steels could provide a combination of outstanding corrosion and wear resistance. They can play an important role in cutting and wear applications and manufacturing plastic extrusion and food processing components.

Near-net shape manufacturing using powder metallurgy (PM) and hot isostatic pressing (HIP) can serve as an efficient manufacturing process to produce these alloys. This technology can successfully tackle conventional manufacturing challenges of highly alloyed materials i.e. segregation during the casting process or cracks during hot working processes of Ni-based superalloys, and carbide segregation and formation of large and irregularly shaped carbides in wrought and hot rolled tool steels. However, the presence of precipitates on prior particle boundaries (PPBs) in Ni-based superalloys, and metallurgical defects like non-metallic inclusions in both Ni-based superalloys and tool steels may affect the fatigue performance of these PM-HIPed products.

This licentiate thesis aims to investigate the microstructure and fatigue behavior of two PM-HIPed alloys i.e. Inconel 625 and high-nitrogen tool steel. The results confirm precipitation along PPBs in PM-HIPed Inconel 625; however, no effect was detected in the fractography studies of the high cycle fatigue samples, and tensile properties were comparable with wrought materials reported in the literature. On the other hand, the microstructure of PM-HIPed high-nitrogen tool steel displayed dispersed precipitates and no traces of PPBs. Moreover, in both cases, i.e. very high cycle fatigue of PM-HIPed high-nitrogen tool steel and high cycle fatigue of PM-HIPed Inconel 625, fatigue crack initiation was attributed to the presence of non-metallic inclusions, either individually or agglomerated with precipitates. This underscores the significance of the manufacturing process in fatigue performance. 

Near-net shape manufacturing using powder metallurgy (PM) and hot isostatic pressing (HIP) can serve as an efficient manufacturing process to produce high-performance alloys. Among the variety of engineering alloys, Nickel-based superalloys and tool steels stand out as well-known high-performance alloys, widely employed across diverse industries. PM-HIP technology can successfully address conventional manufacturing challenges associated with highly alloyed materials, such as segregation during the casting process or cracks during hot working processes of Ni-based superalloys, and carbide segregation and the formation of large and irregularly shaped carbides in wrought and hot rolled tool steels. However, the presence of precipitates on prior particle boundaries in Ni-based superalloys, and metallurgical defects like non-metallic inclusions in both alloys, may affect the fatigue performance of these PM-HIPed products.

The present study aims to assess two PM-HIPed alloys, namely Inconel 625 and high-nitrogen tool steel, with a comprehensive examination of their microstructure and fatigue properties. The objectives include examining the microstructural features introduced by the PM-HIP process and understanding how they influence fatigue failure mechanisms in these alloys.

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.

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.