Rising global energy consumption and the increase in emissions of greenhouse gases (e.g. CO2) causing global warming, make need for more sustainable power generation. This could be accomplished by increasing the efficiency of the biomass-fired power plants, which is achieved by increasing the temperature and pressure. In addition, flexible generation of power is critical if only renewable power generation is to be achieved and this will increase the number of start-and stop cycles. Cyclic condition in a long-term high temperature environment is an operation process that such materials must withstand, in order to satisfy the needs for future power generation.

Commonly austenitic stainless steel are used for critical components of power plants. Because of future change in operating conditions, further investigations are needed to verify that the demands on safety for cyclic long-term usage is fulfilled. This work includes investigation of two commercial austenitic steels: Esshete 1250 and Sanicro 25. The materials were exposed to thermomechanical fatigue (TMF) in strain control under In-Phase and Out-of-Phase conditions and main testing temperature ranges of 100-650°C and 100-800°C respectively. Some of the specimens were pre-aged to simulate prolonged service condition. Mechanical test data were obtained and analysed in order to define the TMF performance of the investigated alloys. The differences in performance were discussed in relation to mechanical and microstructural characterization.

This work investigates two austenitic stainless steels, Sanicro 25 which is a candidate for high temperature heavy section components of future power plants and Esshete 1250 which is used as a reference material. The alloys were subjected to out-of-phase (OP) thermomechanical fatigue (TMF) testing under strain-control in the temperature range of 100 ∘C to 650 ∘C. Both unaged and aged (650 ∘C, 3000 h) TMF specimens were tested to simulate service degradation resulting from long-term usage. The scanning electron microscopy methods electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) were used to analyse and discuss active failure and deformation mechanisms. The Sanicro 25 results show that the aged specimens suffered increased plastic straining and shorter TMF-life compared to the unaged specimens. The difference in TMF-life of the two test conditions was attributed to an accelerated microstructural evolution that provided decreased the effectiveness for impeding dislocation motion. Ageing did not affect the OP-TMF life of the reference material, Esshete 1250. However, the structural stability and its resistance for cyclic deformation was greatly reduced due to coarsening and cracking of the strengthening niobium carbide precipitates. Sanicro 25 showed the higher structural stability during OP-TMF testing compare with the reference material.

The global energy consumption is increasing and together with global warming from greenhouse gas emission, a need for more environmentally friendly energy production processes is created. Higher efficiency of biomass power plants can be achieved by increasing temperature and pressure in the boiler section, this would increase the generation of electricity along with the reduction in emission of greenhouse gases e.g. CO₂. The generation of power must also be flexible to be able to follow the demands of the energy market and this results in a need for cyclic operating conditions with alternating output and multiple start-ups and shut-downs.

Because of the need for flexibility, higher temperature and higher pressure of future biomass power plants, the demands of improved mechanical properties of the materials used for the components are also increased. Properties like creep strength, maintained structural integrity, thermomechanical fatigue resistance and high temperature corrosion resistance are critical for materials used in the next generation biomass power plants. Highly alloyed austenitic stainless steels are known to possess such good high temperature properties and are relatively cheap compared to the nickel-base alloys, which are already used in high temperature cyclic conditions for other applications. The behaviour of austenitic stainless steels subjected to future biomass power plants operating conditions are not yet fully investigated.

This thesis presents research that includes investigations of the mechanical and microstructural behaviour during high temperature cyclic conditions of austenitic stainless steels. This is done using thermomechanical fatigue testing, dwell-fatigue testing and impact toughness testing at elevated temperatures. Material service degradation as an effect of microstructural evolution is investigated by ageing of some test specimens before testing. Microscopy is used to investigate the connection between the mechanical behaviour and the microstructural deformation- and damage mechanisms of the austenitic stainless steels after testing.

The results show that creep-fatigue interaction damage, creep damage and oxidation assisted cracking are present during high temperature cyclic conditions. In addition, ageing results in a less favourable microstructural configuration which negatively affects the resistance to high temperature damage mechanisms. An example of this is the lowering of impact toughness due to precipitation and coarsening of detrimental phases of some aged austenitic stainless steels. Moreover, TMF testing of aged austenitic stainless steels induce oxidation assisted cracking and an embrittling effect that cause significant cyclic life decrease. The creep-fatigue interaction behaviour during dwell-fatigue testing of two austenitic stainless steels generates various crack propagation characteristics. The higher alloyed material shows interchanging intra- and intergranular propagation with dynamic recrystallization, while the lower alloyed material shows propagation exclusively along the grain boundaries by the assistance of fatigue induced slip bands interaction with grain boundary precipitates.

The research of this thesis provides a deeper understanding of the structural integrity, deformation mechanisms, damage mechanisms and fracture mechanisms during high temperature cyclic conditions of austenitic stainless steels. Long term, this is believed to contribute to development of suitable materials used as components of future biomass-fired power plants to achieve sustainable power generation.

På grund av den ökande globala energikonsumtionen tillsammans med globaluppvärmning från växthusgasutsläpp, finns det ett behov av miljövänlig hållbar energiproduktion. Genom att öka tryck och temperatur hos biomassaeldade kraftverkens ångpannor kan energiproduktionen effektiviseras, vilket skulle bidra till både minskning av biogasutsläpp och ökad energiproduktion. Energiproduktionens flexibilitet har även blivit allt viktigare till följd av en mer osäker energimarknad innehållande ökande miljövänliga alternativ, som till exempel vind- och solenergi, med varierande utgående effekt beroende på väderomställningar. Detta innebär ett ändrat drifttillstånd i form av flera uppstarter och nedstängningar för biomassaeldade kraftverken som delvis kommer användas som uppbackningsalternativ.

De ökande kraven på biomassaeldade kraftverkens flexibilitet, temperatur och tryck ger ökande krav på de mekaniska egenskaperna hos materialen  som används för kritiska komponenter. Detta innefattar motstånd till termomekanisk utmattning, högtemperaturkorrosion, kryp och bibehållen strukturell integritet. Höglegerade austenitiska rostfria stål har visat potential att uppnå dessa krav och är dessutom billigare än andra alternativ som till exempel nickelbaserade superlegeringar som vanligtvis används för andra högtemperatursapplikationer med cyklisk drift. Dock har det inte helt utretts hur austenitiska rostfria stål beter sig vid cyklisk drift med förhöjd tryck och temperatur.

Forskningen i denna doktorsavhandling behandlar austenitiska rostfria ståls mekaniska och mikrostrukturella beteende under cykliska driftförhållanden vid hög temperatur. Detta undersöks med hjälp av termomekanisk utmattningsprovning, slagseghetsprovning och utmattningsprovning med hålltid. En del av provstavarna åldras innan mekanisk provning för att undersöka påverkan av mikrostrukturell utveckling över tid. Mikroskopi används för att undersöka kopplingen mellan de mekaniska egenskaperna och de mikrostrukturella deformations- och skademekanismerna.

De mekaniska och mikroskopiska undersökningarna tyder på att krypskador, interaktion mellan utmattning och kryp och oxidationsassisterad sprickbildning negativt påverkar livslängden för austenitiska rostfria stål under cykliskdrift vid hög temperatur. Åldring innan provning förändrar mikrostrukturens ursprungliga konfiguration vilket resulterar i minskat skydd mot livsreducerande deformations- och skademekanismer. Höglegerade austenitiska rostfria stål visar sig ha bättre högtemperatursbeteende jämfört med andra austenitiska legeringar på grund av dess förstärkta mikrostruktur som ökar motståndet för oxidation, kryp och plastisk deformation.

Sammanfattningsvis har denna forskning resulterat i ökad förståelse för det mekaniska och mikrostrukturella beteendet hos austenitiska rostfria stål under cyklisk drift vid hög temperatur. Långsiktigt kommer detta bidra till förbättrad utveckling av högpresterande material för biomassaeldade kraftverk, vilket gagnar övergången till en mer hållbar framtida energiproduktion.

Microstructural evolution related to the mechanical response from isothermal dwell-fatigue testing at 700 °C of two austenitic steels, Esshete 1250 and Sanicro 25, is reported. Coherent Cu-precipitates and incoherent Nb-carbides were found to impede dislocation motion, increase hardening and improving the high temperature properties of Sanicro 25. Sparsely placed intergranular Cr- and Nb-carbides made Esshete 1250 susceptible to creep damage and intergranular crack propagation, mainly from interaction of the carbides and fatigue induced slip bands. Dynamic recrystallization of the plastic zone at the crack tip appeared to affect crack propagation of Sanicro 25 by providing an energetically privileged path.

The increasing demands for efficiency and flexibility result in more severe operating conditions for the materials used in critical components of biomass power plants. These operating conditions involve higher temperature ranges, more pronounced environmental effects and cyclic operations. Austenitic stainless steels have shown to possess promising high temperature properties which makes them suitable as candidates for critical components in biomass power plant. However, their behaviour under such conditions is not yet fully understood. This work investigates three commercial austenitic alloys: Esshete 1250, Sanicro 25 and Sanicro 31HT. The alloys were subjected to in-phase (IP) thermomechanical fatigue (TMF) testing under strain-control in the temperature range of 100–800 °C. Both virgin and pre-aged TMF specimens were tested in order to simulate service degradation resulting from long-term usage. The results show that the pre-aged specimens suffered shorter TMF-life compared to the virgin specimens. The scanning electron microscopy methods electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) were used to analyse and discuss active failure and deformation mechanisms. The difference in TMF-life produced by the two testing conditions was attributed to an embrittling effect by precipitation, reduced creep properties and oxidation assisted cracking.

This work includes an investigation of two commercial austenitic steels: UNS S21500 (Esshete 1250) and UNS S31035 (Sandvik Sanicro (TM) 25). The materials were exposed to isothermal strain controlled fatigue with load controlled dwell time at maximum strain. The testing temperature used was 700 degrees C and the test cycles were performed in tension. Mechanical test data were obtained and analysed in order to define creep-fatigue damage diagrams at failure for the investigated austenitic alloys. During the given conditions, Sanicro 25 showed superior creep-fatigue life, suffered less amount of creep elongation for the same amount of strain amplitude and dwell times compared to Esshete 1250. Both alloys showed creep-fatigue interaction damage for specific test configurations.