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.

Fatigue response of metallic materials is considered of significant importance, particularly for high-demanding applications. It is proved that most of the engineering materials do not exhibit a conventional fatigue limit in the high cycle fatigue regime, but rather display a continuously decreasing stress-life response at even longer lifetimes. Consequently, investigations of the various failure mechanisms taking place are essential, especially at the high and very high cycle fatigue regimes.The development of new ultrasonic testing equipment made the fatigue testing beyond 10⁷ life cycles possible in a much shorter time, enabling testing with fatigue fractures at stress levels lower than the traditionally proposed “fatigue limit”. Nowadays, a classification of fatigue life regimes in Low Cycle Fatigue (LCF), High Cycle Fatigue (HCF) and Very High Cycle Fatigue (VHCF) is commonly used. The main reasons for this specific grading are: i) the need for safe design of components and ii) the fact that the failure mechanisms are particular in each of the LCF, HCF and VHCF regimes.The main goal of the present thesis is to address the fatigue response of high strength tool steels. Considering the novel alterations in composition and production methods in alloy development, materials of high-quality are continuously being introduced to the market; understanding the fatigue response of these materials is crucial for potential utilization across diverse industries and applications. The generation of fatigue experimental data, the analysis of the different types of fatigue initiation defects found in each material, as well as the investigation of the fatigue mechanisms occurring during cyclic loading are the main subjects analyzed throughout the present study. 

Utmattningsegenskaper hos metalliska material anses vara av stor betydelse, särskilt för krävande tillämpningar. Det är visat att de flesta konstruktionsmaterial inte uppvisar en konventionell utmattningsgräns i området för högcykelutmattning, utan snarare uppvisar ett kontinuerligt minskande spänning-livslängd förhållande med längre livslängder. Följaktligen är undersökningar av de olika brottmekanismer som äger rum väsentliga, särskilt vid utmattningsregimer med höga och mycket höga livslängder.Utveckling av nya ultraljudsutrustningar gjorde utmattningsprovning utöver 10⁷ livscykler möjliga på mycket kortare tid, vilket möjliggjorde provning med utmattningsbrott vid längre livstid och spänningsnivåer lägre än den traditionellt föreslagna "utmattningsgränsen". Nuförtiden används en indelning av utmattningsregimer i lågcykelutmattning (Low Cycle Fatigue LCF), högcykelutmattning (High Cycle Fatigue HCF) och utmattning vid mycket långa livslängder (Very High Cycle Fatigue VHCF). De huvudsakliga skälen till denna indelning är: i) behovet av säker design av komponenter, och ii) det faktum att brottmekanismerna är speciella i var och en av LCF-, HCF- och VHCF-regimerna. Det främsta målet med denna avhandling är att adressera utmattningsegenskaper hos höghållfasta verktygsstål. Med tanke på nya sammansättningar och produktionsmetoder i legeringsutvecklingen introduceras material av hög kvalitet kontinuerligt på marknaden; att förstå utmattningsresponsen hos dessa material är avgörande för potentiella användningar inom olika industrier och applikationer. Genereringen av experimentell utmattningsdata, analysen av de olika typerna av initieringsdefekter som hittats i varje material, såväl som undersökningen av utmattningsmekanismerna som uppstår under cyklisk belastning är huvudämnena som analyseras genom hela denna studie.