The present work was conducted at Scania CV AB and has as main goal to produce a cast steel guideline. There is a great need to conduct a cast steel guideline, in order to help designers in their process to choose suitable materials and methods to produce lightweight components with higher performance. This work contains information related to mechanical properties, casting processes, castability, machinability, defect characterization, heat treatments, weldability and surface treatments of cast steels. This work was limited to cast steels which could be applied on two specific components of the truck, a bracket which is a structural component of chassis subjected to fatigue and a turbo manifold which is subjected to creep, oxidation, corrosion-, thermal- and mechanical- fatigue. A benchmark search was performed focused on these two components. A characterization of a cast stainless steel turbo manifold prototype was performed in the as-cast state, which included microstructural analyzes and hardness measurements. Besides this initial characterization, a set of heat treatments were conducted, in order to study the possibility to eliminate the initial grain boundary carbides. The main conclusions of this work are that cast steel has potential to be a material choice in many applications due to its wide range of properties. For structural parts, cast steel is advantageous compared with cast iron when for instance welding and high strength combined with high fracture toughness are requirements. For high temperature resistance components, cast steel or more precisely cast stainless steel, is advantageous for service at temperatures >750 ºC,besides its higher price. The annealing heat treatment followed by an aging treatment eliminated most of the grain boundary carbides and increase the hardness through a fine dispersion of carbides in matrix, which can also increase the creep resistance. / Examensarbetet utfördes på Scania CV AB med främsta målet att utarbeta en vägledande guide för gjutstål. Det finns ett stort behov av att sammanställa en guideline för gjutstål, med syfte att hjälpa konstruktörer i deras process att välja lämpliga material och metoder för att producera lättviktskomponenter med högre prestanda. Arbetet innehåller information om mekaniska egenskaper, gjutningsprocesser, gjutbarhet, skärbarhet, defektkarakterisering, värmebehandlingar, svetsbarhet, ytbehandling och mycket annat gällande gjutstål. Detta arbete var begränsat till gjutstål som kan tillämpas för två specifika komponenter i lastbilen, en konsol som är en strukturell komponent i chassit som utsätts för utmattning och ett turbogrenrör vilket är en komponent som utsätts för högtemperaturcykler. En benchmark utfördes med fokus på dessa två komponenter. Slutligen gjordes en karakterisering av turbogrenrörsprototypen i rostfritt stål, i det gjutnatillståndet, vilket inkluderade mikroanalyser och hårdhetsmätningar. Förutom den förstakarakteriseringen, utfördes en uppsättning värmebehandlingar för att undersöka möjligheten att eliminera de initiala korngräns-karbiderna. De viktigaste slutsatserna av detta arbete är att gjutstål har potential att bli ett materialval i många applikationer på grund dess breda egenskaper. Konstruktionsdelar i gjutstål är fördelaktiga jämfört med gjutjärn, till exempel vid svetsning och när hög hållfasthet i kombination med hög brottseghet är nödvändigt. För högtemperaturs-komponenter är gjutstål, eller mer exakt, gjutna rostfria stål fördelaktiga för service vid temperaturer >750 ºC, med undantag för dess högre pris. Glödgningsvärmebehandling följt av åldringsbehandling eliminerar de flesta av korngräns-karbiderna och ökar hårdheten genom en fin dispersion av karbider i matrisen, vilket också kan öka krypmotståndet.

Additive manufacturing technologies, particularly electron beam powder bed fusion (PBF-EB/M), are becoming increasingly important for the processing of intermetallic titanium aluminides. This study presents the effects of hot isostatic pressing (HIP) and subsequent two-step heat treatments on the microstructure and mechanical properties of the TNM-B1 alloy (Ti–43.5Al–4Nb–1Mo–0.1B) fabricated via PBF-EB/M. Adequate solution heat treatment temperatures allow the adjustment of fully lamellar (FL) and nearly lamellar (NL-β) microstructures. The specimens are characterized by optical microscopy and scanning electron microscopy (SEM), X-ray computed tomography (CT), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The mechanical properties at ambient temperatures are evaluated via tensile testing and subsequent fractography. While lack-of-fusion defects are the main causes of failure in the as-built condition, the mechanical properties in the heat-treated conditions are predominantly controlled by the microstructure. The highest ultimate tensile strength is achieved after HIP due to the elimination of lack-of-fusion defects. The results reveal challenges originating from the PBF-EB/M process, for example, local variations in chemical composition due to aluminum evaporation, which in turn affect the microstructures after heat treatment. For designing suitable heat treatment strategies, particular attention should therefore be paid to the microstructural characteristics associated with additive manufacturing.

Electron beam powder bed fusion (PBF-EB/M) has been attracting great research interest as a promising technology for additive manufacturing of titanium aluminide alloys. However, challenges often arise from the process-induced evaporation of aluminum, which is linked to the PBF-EB/M process parameters. This study applies different volumetric energy densities during PBF-EB/M processing to deliberately adjust the aluminum contents in additively manufactured Ti–43.5Al–4Nb–1Mo–0.1B (TNM-B1) samples. The specimens are subsequently subjected to hot isostatic pressing (HIP) and a two-step heat treatment. The influence of process parameter variation and heat treatments on microstructure and defect distribution are investigated using optical and scanning electron microscopy, as well as X-ray computed tomography (CT). Depending on the aluminum content, shifts in the phase transition temperatures can be identified via differential scanning calorimetry (DSC). It is confirmed that the microstructure after heat treatment is strongly linked to the PBF-EB/M parameters and the associated aluminum evaporation. The feasibility of producing locally adapted microstructures within one component through process parameter variation and subsequent heat treatment can be demonstrated. Thus, fully lamellar and nearly lamellar microstructures in two adjacent component areas can be adjusted, respectively.

The majority of UK’s intermediate level radioactive waste is currently stored in 316L and 304L austenitic stainless steel containers in interim storage facilities for permanent disposal until a geological disposal facility has become available. The structural integrity of stainless steel canisters is required to persevere against environmental degradation for up to 500 years to assure a safe storage and disposal scheme. Hitherto existing severe localised corrosion observances on real waste storage containers after 10 years of exposure to an ambient atmosphere in an in-land warehouse in Culham at Oxfordshire, however, questioned the likelihood occurrence of stress corrosion cracking that may harm the canister’s functionality during long-term storage. The more corrosion resistant duplex stainless steel grade 2205, therefore, has been started to be manufactured as a replacement for the austenitic grades. Over decades, the threshold stress corrosion cracking temperature of austenitic stainless steels has been believed to be 50-60°C, but lab- and field-based research has shown that 304L and 316L may suffer from atmospheric stress corrosion cracking at ambient temperatures. Such an issue has not been reported to occur for the 2205 duplex steel, and its atmospheric stress corrosion cracking behaviour at low temperatures (40-50°C) has been sparsely studied which requires detailed investigations in this respect. Low temperature atmospheric stress corrosion cracking investigations on 2205 duplex stainless steel formed the framework of this PhD thesis with respect to the waste storage context. Long-term surface magnesium chloride deposition exposures at 50°C and 30% relative humidity for up to 15 months exhibited the occurrence of stress corrosion cracks, showing stress corrosion susceptibility of 2205 duplex stainless steel at 50°C.The amount of cold work increased the cracking susceptibility, with bending deformation being the most critical type of deformation mode among tensile and rolling type of cold work. The orientation of the microstructure deformation direction, i.e. whether the deformation occurred in transverse or rolling direction, played vital role in corrosion and cracking behaviour, as such that bending in transverse direction showed almost 3-times larger corrosion and stress corrosion cracking propensity. Welding simulation treatments by ageing processes at 750°C and 475°C exhibited substantial influences on the corrosion properties. It was shown that sensitisation ageing at 750°C can render the material enhanced susceptible to stress corrosion cracking at even low chloride deposition densities of ≤145 µm/cm². However, it could be shown that short-term heat treatments at 475°C can decrease corrosion and stress corrosion cracking susceptibility which may be used to improve the materials performance. Mechanistic understanding of stress corrosion cracking phenomena in light of a comprehensive microstructure characterisation was the main focus of this thesis.