Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第19093号 / エネ博第317号 / 新制||エネ||64 / 32044 / 京都大学大学院エネルギー科学研究科エネルギー変換科学専攻 / (主査)教授 木村 晃彦, 教授 星出 敏彦, 教授 今谷 勝次 / 学位規則第4条第1項該当

The “475°C embrittlement” occurring in ferritic and duplex stainless steel is considered to be detrimental and it limits the application of ferritic and duplex stainless steel at elevated temperatures, i.e., above about 300°C . In this study, the effect from alloying elements Ni, Cu and Mn on 475°C embrittlement was examined based on microhardness measurement and Charpy V-notch tests as well as atom probe tomography (APT). It was found that, after aging for 10h, 3% Ni accelerates the ferrite decomposition dramatically, 5% Mn has minor effect and no effect of 1.5% Cu was seen. The hardness increase tested at 450°C and 500°C was consistent with the observations from APT. The embrittlement based on room temperature Charpy tests was observed mainly during the first 10h. The embrittlement in Fe-20Cr-3Ni alloy was attributed to ferrite decomposition, while the other three alloys may be influenced by other phenomenon as well. A clustering effect of Cu has been observed in Fe-20Cr-1.5Cu and it was supposed to contribute to the mechanical changes.

The “475°C embrittlement” occurring in ferritic and duplex stainless steel is considered to be detrimental and it limits the application of ferritic and duplex stainless steel at elevated temperatures, i.e., above about 300°C . In this study, the effect from alloying elements Ni, Cu and Mn on 475°C embrittlement was examined based on microhardness measurement and Charpy V-notch tests as well as atom probe tomography (APT). It was found that, after aging for 10h, 3% Ni accelerates the ferrite decomposition dramatically, 5% Mn has minor effect and no effect of 1.5% Cu was seen. The hardness increase tested at 450°C and 500°C was consistent with the observations from APT. The embrittlement based on room temperature Charpy tests was observed mainly during the first 10h. The embrittlement in Fe-20Cr-3Ni alloy was attributed to ferrite decomposition, while the other three alloys may be influenced by other phenomenon as well. A clustering effect of Cu has been observed in Fe-20Cr-1.5Cu and it was supposed to contribute to the mechanical changes.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第19093号 / エネ博第317号 / 新制||エネ||64(附属図書館) / 32044 / 京都大学大学院エネルギー科学研究科エネルギー変換科学専攻 / (主査)教授 木村 晃彦, 教授 星出 敏彦, 教授 今谷 勝次 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

Submitted by Angela Maria de Oliveira (amolivei@uepg.br) on 2018-12-21T10:53:57Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Giovanne Steudel.pdf: 9400807 bytes, checksum: 8a6ed5c0cfacf8380363dfd4584186c1 (MD5) / Made available in DSpace on 2018-12-21T10:53:57Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Giovanne Steudel.pdf: 9400807 bytes, checksum: 8a6ed5c0cfacf8380363dfd4584186c1 (MD5) Previous issue date: 2018-08-27 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O objetivo deste trabalho é avaliar os efeitos na cinética de transformação e nas propriedades mecânicas dos aços inoxidáveis ferrítico AISI 430 (16-18% Cr) e superdúplex SAF 2507 (24-26% Cr), quando submetidos a tratamentos térmicos de envelhecimento em 475°C, assim fragilizados pela formação da fase alfa linha. Este fenômeno é conhecido por “fragilização de 475°C” e ocorre na ferrita de aços inoxidáveis ferríticos e dúplex, e nos aços martensíticos envelhecidos entre 300°C e 550°C. O acompanhamento e análise da cinética de formação de α’ nos dois aços inoxidáveis, o ferrítico AISI 430 e superdúplex SAF 2507, foram feitas com base em ajustes matemáticos por regressão não-linear via sete modelos sigmoidais de aplicação geral, os quais serão comparados ao modelo JMAK de transformação de fases difusivas elaborada por Johnson, Mehl, Avrami e Kolmogorov. Amostras do aço ferrítico foram submetidas a tratamentos térmicos de solubilização em forno tubular em 1050 °C e 30 minutos, e em 1150°C durante 30 minutos para o aço dúplex. Tratamentos de envelhecimento posteriores em 475°C por diversos períodos foram realizados. Verificou-se a evolução da “fragilização de 475°C” e, indiretamente, a formação de α’, por ensaios de dureza Vickers, de tração e de impacto instrumentado Charpy. Análises microestruturais e fractográficas foram feitas em microscopias óptica e eletrônica de varredura (MEV-FEG); análises cristalográficas por difração de elétrons retroespalhados (EBSD); e as análises químicas por espectroscopia por dispersão de energia (EDS). O aço superdúplex, com aproximadamente 25,2% Cr, apresentou cinética de fragilização mais rápida do que o ferrítico, com cerca de 16,6% Cr. O modelo sigmoidal que mais se adequou aos dados de dureza de ambos os aços foi o de Boltzmann; o qual também apresentou maior versatilidade de informações e melhor ajuste e capacidade preditiva do que o modelo JMAK. As tendências para tempos de envelhecimento crescentes do aço AISI 430 foram o aumento dos limites de escoamento e de resistência à tração, com dados de tensão na ruptura oscilando em função do tempo de envelhecimento, porém a literatura aponta aumento também da tensão de ruptura. A mudança de comportamento de dúctil para frágil (clivagem) do aço AISI 430, em função do prosseguimento da “fragilização de 475°C”, foi mais acentuada nos corpos de prova de impacto, em relação aos de tração. / The goal of this work is to evaluate the effects in the transformation kinetics and in the mechanical properties of the AISI 430 (16-18% Cr) ferritic and the SAF 2507 (24-26% Cr) superduplex stainless steels, when they undergo aging heat treatments at 475°C, thus embrittled by the formation of the alpha prime phase. This phenomenon is known as “475°C embrittlement” and it occurs in the ferrite of ferritic and duplex stainless steels, and in martensitic stainless steels aged between 300°C and 550°C. The monitoring and analysis of the kinetics of α’ formation in the AISI 430 and SAF 2507 steels was done based on mathematical fitting by means of nonlinear regression of seven sigmoidal models of general application, whose were compared to the JMAK model of diffusive phase transformations elaborated by Johnson, Mehl, Avrami and Kolmogorov. Ferritic steel samples were submitted to solubilization heat tratments in a tubular furnace at 1050°C for 30 minutes, and at 1150°C during 30 minutes for the duplex steel. Posterior aging treatments will hold at 475°C by various periods. The evolution of the “475°C embrittlement” and, indirectly, the formation of α’, were verified by the way of Vickers hardness, tensile and Charpy instrumented impact tests. Microstructural and fractographic analyses were done by optical and scanning electron microscopies (FEG-SEM); crystallographic analyses by electron backscattered diffraction (EBSD); and chemical analyses by energy dispersive spectroscopy (EDS). The superduplex steel, containing approximately 25,2% Cr, showed a faster embrittlement kinetics than the ferritic steel, with about 16,6% Cr. The sigmoidal model that better fitted to the hardness data of both steels was the Boltzmann one; which also showed greater versatility of information and a better fit and predictive ability than the JMAK model. The trends for rising aging times of the AISI 430 steel were the increase in the yield and ultimate tensile stresses, having oscillation in the breaking stress in function of the aging time, yet the literature claims that there is also a increase in the breaking stress. The change in the mechanical behavior from ductile to brittle (cleavage) of the AISI 430 steel, in function of the following of the “475°C embrittlement”, was more severe in the impact test specimens, compared to the tensile specimens.

Slat-band chains are used as conveyors by the food industry, breweries and bottling plants. The operating conditions require abrasion resistance and strength which are at the limit of the capabilities of the current material of choice, cold worked type 430. In an unconventional way of strengthening this material, Mintek developed a process in which the cold worked material is aged between 450°C and 500°C. The present work aims to elucidate the strengthening mechanism, using type 430 stainless steel containing 16.42% Cr and 0.036% C, in the cold-rolled condition (38% reduction in area), with and without prior solution heat treatment. The Cr-rich precipitate α" may form in the 450°C to 500°C range (due to the miscibility gap in the Fe-Cr system), resulting in the increased hardness and lowered ductility. Mossbauer studies confirmed that the α", at this composition and temperature, forms through the process of nucleation and growth. Hardening due to α" precipitation was only observed after aging for 64 hours or more, however. After increasing the dissolved interstitial content by solution heat treatments (in the vicinity of 900°C), increases in Vickers hardness of 30-50 kg/mm2 could be obtained after only 8 minutes at 475°C. This hardness increase corresponds to an increase in tensile strength of more than 100 MPa. The increased hardness does not appear to be caused by strain aging, and presumably results from fine carbide or nitride precipitation. Solution treatment at 930°C also introduced some martensite (α') into the microstructure, which raised the hardness of the unaged cold worked material. Overaging of the carbide and nitride precipitates was observed at 475°C, but not at 450°C, probably due to the lower diffusion rates at the lower temperature. No averaging of the α" precipitates occurred, for aging times up to 2072 hours. Samples aged for selected periods of time at 475°C had low impact strengths - even well before the formation of α" - and revealed predominantly cleavage fracture with some ductile fracture areas, mostly at grain boundaries. Both impact strength and lateral expansion indicated that embrittlement accompanies the increased hardness obtained by aging. Calculation of critical crack lengths from the impact data, however, revealed that a maximum flaw length of 0.8 mm, for specimens solution treated at 880°C, could be tolerated before catastrophic failure. Since it is not expected that flaws of that size would exist in the as manufactured links, fatigue will probably determine the lifetime of the chains, although the lower K1c values indicate that less crack propagation will be tolerated before brittle fracture. During the aging treatment, the strength may be lowered by recrystallisation of the coldworked material. Transmission electron microscopy (TEM) revealed the start of recovery, but no recrystallisation. Some large precipitates (around lμm in diameter) were present. These were identified, through their diffraction patterns, as M23C6; these carbides were present in both aged and unaged material and hence represent precipitates which had not dissolved during the initial solution treatments. The α" precipitates- and the presumed newly formed nitride and carbide precipitates - were too fine for detection by TEM. Potentiodynamic testing of the treated material in a 0.5M H2SO4 solution indicated that, although the probable hardening mechanisms imply localised Cr depletion of the matrix, the general corrosion resistance and passivation behaviour were not affected. It is concluded that the strength of the chain may be increased markedly by short-term heat treatments at 475°C, with lowered toughness, but with no decrease in corrosion resistance. Martensite, work hardening, and precipitation of carbides and nitrides all contribute to the final strength, with α" formation only becoming significant after longer aging times. / Dissertation (MEng)--University of Pretoria, 2013. / Materials Science and Metallurgical Engineering / unrestricted

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.