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.

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.

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. 

Very High Cycle Fatigue (VHCF) response of metallic materials is considered of significance importance, particularly for high- demanding applications. Since it is proved that most of the engineering materials do not exhibit a conventional fatigue limit, but rather display a continuously decreasing stress-life response at longer lifetimes. Consequently, the investigation of the various mechanisms are taking place during VHCF is essential. The primary object of the present effort is to explore and summarize recent developments and current status of the VHCF phenomenon in high strength alloys.

The development of the new ultrasonic machines made the fatigue testing beyond 107 life cycles possible in a very shorter time, leading to fatigue fractures at stress levels lower than the traditionally proposed “fatigue limit”. Nowadays, a classification between 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.

Ultrasound machines, also called piezoelectric, are resonance fatigue testing machines reducing the testing time at least 400 times compared to the conventional machines operating at frequencies up to 20 kHz.

The crack initiation stage is one of the most investigated subjects when it comes to VHCF. Usually, the fatigue initiates from a defect (inclusion, pore, grain boundary triple points), while up to 99% of fatigue life is consumed in that stage. Different models have been proposed in the literature regarding the evolution of events that contribute to the fatigue crack initiation and growth. Proposed models are such as the ODA, the polygonization, continuous grain refinement with local plasticity, the matrix fragmentation and the NCP numerous cyclic pressing. Several studies have assessed the different models and further enriched the knowledge in the VHCF field.

In the present literature review effort, the main VHCF mechanisms of crack initiation and growth as well as the proposed models are presented and analyzed. Moreover, the VHCF response and the recent experimental results referring to the most used engineering alloys, e.g. steels, Ti, Ni, Al and Mg alloys, are described, with a main attention to steels. The different testing parameters and the way they affect the VHCF response are also presented.

Finally, the literature review is concluded by presenting the new challenges and directions for future work in the field, especially under the light of the new low-carbon society.

Very High Cycle Fatigue (VHCF) response of metallic materials is considered of significance importance, particularly for high- demanding applications. Since it is proved that most of the engineering materials do not exhibit a conventional fatigue limit, but rather display a continuously decreasing stress-life response at longer lifetimes. Consequently, the investigation of the various mechanisms are taking place during VHCF is essential. The primary object of the present effort is to explore and summarize recent developments and current status of the VHCF phenomenon in high strength alloys. 

The development of the new ultrasonic machines made the fatigue testing beyond 107 life cycles possible in a very shorter time, leading to fatigue fractures at stress levels lower than the traditionally proposed “fatigue limit”. Nowadays, a classification between 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. 

In the present literature review effort, the main VHCF mechanisms of crack initiation and growth as well as the proposed models are presented and analyzed. Moreover, the VHCF response and the recent experimental results referring to the most used engineering alloys, e.g. steels, Ti, Ni, Al and Mg alloys, are described, with a main attention to steels. The different testing parameters and the way they affect the VHCF response are also presented. 

Finally, the literature review is concluded by presenting the new challenges and directions for future work in the field, especially under the light of the new low-carbon society. 

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. 

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.

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.