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.

The martensitic tool steel grades are 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. Also, a deeper analysis of the fatigue phenomena and mechanisms occurring during fatigue is attempted; characterization of fatigue initiation defects, the FGA, the fish-eye, as well as the crack propagation for the different microstructures are the main discussed subjects. The goal of the present study is to provide valuable insights to optimize martensitic tool steels for high and very high cycle fatigue applications.

De martensitiska verktygsstålen är utformade för olika arbetsmiljöer. Deras mikrostruktur förbättras kontinuerligt genom utveckling av sammansättning eller bearbetningstekniker. Dessa framsteg introducerar nya typer av defekter, vilket gör undersökningar av utmattningsbeteende mycket viktiga för industrin.Den aktuella studien omfattar undersökningen av sex avancerade högstyrka verktygsstål med avseende på mikrostruktur, vanliga defekter och utmattningsprestanda i hög- och mycket högcykliska utmattningsområden. Materialen inkluderar: i) kallarbetsverktygsstål (höglegerade material tillverkade via pulvermetallurgi, bearbetade genom varm-isostatisk pressning och smidning) och ii) varmarbetsverktygsstål (konventionellt producerade genom gjutning av göt och smidning eller additiv tillverkning). Stålgraderna jämförs baserat på hållfasthet, martensitiska strukturer, defektfördelningar och utmattningshållfasthet med hjälp av experimentella resultat och Murakami-modellen. Dessutom görs en djupare analys av de utmattningsfenomen och mekanismer som uppträder under utmattning; karakterisering av initieringsdefekter, FGA, fisköga samt spricktillväxt för de olika mikrostrukturerna är de huvudsakliga ämnen som diskuteras.Målet med den aktuella studien är att ge värdefulla insikter för att optimera martensitiska verktygsstål för tillämpningar inom hög- och mycket högcyklisk utmattning.

Οι μαρτενσιτικοί εργαλειοχάλυβες είναι σχεδιασμένοι για να ανταπεξέρχονται σε περιβάλλονταυψηλών απαιτήσεων. Η μικροδομή τους συνεχώς βελτιώνεται διαφοροποιώντας είτε τη χημικήσύσταση του εκάστοτε συστήματος, είτε τη μέθοδο παρασκευής τους, ή τα βήματα που ακολοθούνταικατά την κατεργασία τους. Οι βελτιώσεις αυτές οδηγούν μεν σε προηγμένες μικροδομές, εισάγουνωστόσο νέους τύπους ατελειών, οι οποίες επηρεάζουν σε μεγάλο βαθμό την απόκρισή τους σε κυκλικήφόρτιση. Για αυτό το λόγο, η μελέτη της συμπεριφοράς τους σε κόπωση κρίνεται αναγκαία υπό τοπρίσμα της συνεχούς αύξησης των απαιτήσεων από τη μεριά της βιομηχανίας για υλικά που θααντέχουν όχι μόνο υψηλότερα φορτία, αλλά και για περισσότερο χρόνο. Τα υλικά που μελετώνται στην παρούσα εργασία περιλαμβάνουν:  i) εργαλειοχάλυβες ψυχρής κατεργασίας (υψηλά κραματωμένα υλικά που παράγονται μέσω κονεομεταλλουργίας και επεξεργάζονται με θερμή ισοστατική πίεση και σφυρηλάτηση) και ii) εργαλειοχάλυβες θερμής κατεργασίας (παραδοσιακά παραγόμενοι μέσω χύτευσης και σφυρηλάτησης ή Additive Μanufacturing). Οι εξί μικροδομές συγκρίνονται με βάση τη μικροδομή, την αντοχή σε κόπωση πολύ υψηλού αριθμού κύκλων,  την κατανομή και τους τύπους των ατελειών, χρησιμοποιώντας πειραματικά αποτελέσματα και το μοντέλο Murakami.Επιπλέον, επιχειρείται βαθύτερη ανάλυση των φαινομένων και μηχανισμών κόπωσης που λαμβάνουν χώρα κατά τη διάρκεια της κυκλικής φόρτισης. Ο χαρακτηρισμός των ατελειών που οδηγούν στη δημιουργία ρωγμής λόγω κόπωσης, της περιοχής FGA, του fish-eye καθώς και τους μηχανισμούς διάδοσης ρωγμών για τις διαφορετικές μικροδομές αποτελούν τα κύρια θέματα συζήτησης.

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.