The imminent emergence of the hydrogen fuel industry has resulted in an urgent mandate for very specific material testing. Although storage of pressurized hydrogen gas is both practical and attainable, demands for increasing storage pressures (currently around 70 MPa) continue to present unexpected material compatibility issues. It is imperative that materials commonly used in gaseous hydrogen service are properly tested for hydrogen embrittlement resistance. To assess material behavior in a pressurized hydrogen environment, procedures were designed to test materials for susceptibility to hydrogen embrittlement. Of particular interest to the field of high-pressure hydrogen in the automotive industry, austenitic stainless steels SUS 316 and 316L were used to validate the test programs. Tests were first performed in 25 MPa helium and hydrogen at room temperature and at -40°C. Tests in a 25 MPa hydrogen atmosphere caused embrittlement in SUS 316, but not in 316L. This indicated that alloys with higher stacking fault energies (316L) are more resistant to hydrogen embrittlement. Decreasing the test temperature caused slight embrittlement in 316L and significantly enhanced it in 316. Alternatively, a second set of specimens was immersed in 70 MPa hydrogen at 100°C until reaching a uniform concentration of absorbed hydrogen. Specimens were then loaded in tension to failure to determine if a bulk saturation of hydrogen provided a similar embrittling effect. Neither material succumbed to the effects of gaseous pre-charging, indicating that the embrittling mechanism requires a constant supply of hydrogen at the material surface rather than having bulk concentration of dissolved hydrogen. Permeation tests were also performed to ensure that hydrogen penetrated the samples and to develop material specific permeation constants. To pave the way for future work, prototype equipment was constructed allowing tensile or fatigue tests to be performed at much higher hydrogen pressures. To determine the effect of pressure on hydrogen embrittlement, additional tests can be performed in hydrogen pressures up to 85 MPa hydrogen. The equipment will also allow for cyclic loading of notched tensile or compact tension specimens for fatigue studies.

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O aço SAE/AISI 52100 é utilizado para a fabricação de rolamentos como também na fabricação de outras peças e dispositivos como cames de eixo comando de válvulas. Um dos problemas desta liga é a necessidade de revenimentos em temperaturas muito baixas para obtenção de alta dureza e para evitar o fenômeno da fragilização da martensita revenida (FMR), em detrimento da tenacidade. Foi projetada uma nova liga Fe-C-Mn-Si-Cr (AISI/SAE 52100 modificado com 1,74% de Si e 0,96% de Mn) baseada na mesma ideia do aço 300M em relação ao SAE/AISI 4340. O efeito do Si na cinética de precipitação da cementita retarda a fragilização da martensita revenida (FMR), além de aumentar a dureza e mantê-la com valores relativamente altos mesmo quando do revenimento em temperaturas mais altas. A proposta do presente trabalho foi comprovar a resistência desta nova liga frente à FMR e demonstrar a resistência ao amolecimento perante o revenimento (curvas de revenimento). Com o intuito de estudar o efeito do Si na dureza do novo aço, foram elaboradas curvas de revenimento medindo-se essa propriedade em amostras do novo aço e do aço comercial após têmpera em temperaturas de austenitização na faixa de 825ºC e 960ºC, seguida por tratamento criogênico em nitrogênio líquido (-196ºC) durante doze horas e revenidas em temperaturas na faixa de 250ºC a 500ºC. Foram obtidas durezas acima de 60HRC, sendo que o aço modificado chegou a tingir 68HRC, no estado temperado. Foi estudada também a resistência ao amolecimento da nova liga e do aço comercial submetendo amostras em temperaturas constantes na faixa de 350ºC a 450ºC, variando-se o tempo na faixa de uma a dez horas. O aço modificado, após 10 horas em 450ºC, apresentou ainda, dureza de 58HRC. Para determinação das propriedades mecânicas desta nova liga foram realizados ensaios de tração em amostras temperadas e revenidas, comparativamente ao aço SAE/AISI 52100 comercial. Para a realização do estudo da FMR, foram comparados resultados dos ensaios de impacto para o aço SAE/AISI 52100 comercial (0,25%Si) e modificado (1,74%Si). O aço modificado não apresentou o fenômeno da FMR. Foram analisados aspectos microestruturais por meio de microscopia eletrônica de varredura (MEV) e difração de raios-x. / The SAE/AISI 52100 steel is used for bearing manufacturing and automotive parts like camshafts lobes. A problem with this alloy is the need for low tempering temperature in order to obtain high hardness and to avoid the tempered martensite embrittlement phenomena, compromising the toughness. Based on the same idea as 300M steel regarding SAE/AISI 4340 steel, a new Fe-C-Mn-Si-Cr bearing alloy (AISI 52100 steel, modified with 1.74% Si and 0.96%Mn) was developed. The effect of Si on the kinetics of cementite precipitation leads to a higher temperature of tempered martensite embrittlement (TME) occurrence and keep high hardness values even when the steel is submitted to a higher temperatures tempering or for long time. The purpose of this work was to confirm the new alloy tempered martensite embrittlement (TME) resistance and to verify its softening resistance (tempering curves). Intending to investigate the Si effect on new steel hardness, hardness measurements were performed on modified and commercial steels samples after 825ºC 960ºC austenitization, twelve hours -196ºC cryogenic treatment and 250ºC 500ºC tempering. It was obtained hardness values over 60HRC and the modified steel presented 68HRC as quenched. The new alloy and commercial alloy softening resistance was studied by hardness measurement on samples submitted to 350ºC 450ºC constant temperature tempering in periods of time from one to ten hours. The Si alloyed steel presented 58HRC after 10 hours at 450ºC. For the mechanical characterization of the new alloy, tensile tests were performed in quenched and tempered samples. In the tempered martensite embrittlement study, impact tests results for commercial SAE/AISI 52100 (0.25%Si) and modified (1.74%Si) were compared. The modified steel presented no tempered martensite embrittlement. Microstructural aspects were studied by scanning electron microscopy and x-ray diffraction analysis.

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Orientador: Itamar Ferreira / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-12T16:02:28Z (GMT). No. of bitstreams: 1 Marcomini_JoseBenedito_M.pdf: 10388005 bytes, checksum: e970602f47c86edad59a5fdbb89fd097 (MD5) Previous issue date: 2008 / Resumo: Este trabalho tem como objetivo caracterizar e analisar o fenômeno da fragilização da martensita revenida no aço SAE 5160H, por meio de ensaios metalográficos, mecânicos, microscopia eletrônica de varredura e difratometria de raios-x. Corpos-de-prova do aço supracitado, laminado, foram austenitizados em duas temperaturas: 800ºC e 1050ºC, temperados em óleo a 80ºC, para obtenção de austenita retida e posteriormente, revenidos em 280ºC, 350ºC e 420ºC. Foram caracterizadas a microestrutura, as propriedades mecânicas e o micromecanismo de fratura em amostras nas condições laminado, temperado e temperado e revenido. Foi detectada a fragilização a 350ºC, temperatura na qual foi observada menor energia no ensaio de impacto, para ambas as temperaturas de austenitização, 800ºC e 1050ºC, em concordância com a literatura, porém com um micromecanismo de fratura misto: intergranular e alveolar, para a temperatura de austenitização de 1050ºC e micromecanismo intergranular para a temperatura de austenitização de 800ºC, como mostram as análises de microscopia eletrônica de varredura (MEV). Foi estudada, também a decomposição da austenita retida por análise de difração de raios-x nas amostras, nas varias condições cujos resultados mostram concordância com o previsto na literatura. Foi feita a análise por espectro de energia dispersiva (EDS acoplado ao MEV), com o intuito de verificar a influência de elementos como S e P neste fenômeno, porém, não foi detectada a presença destes elementos na superfície de fratura, por este método. Foi observado que existe uma concorrência de mecanismos para a fragilização da martensita revenida no aço SAE 5160H e que o fenômeno apresenta-se com maior intensidade para temperatura de austenitização mais alta. / Abstract: The purpose of this work is the characterization and analysis of tempered martensite embrittlement phenomena in SAE 5160 steel using metallographic and mechanical tests, scanning electron microscopy and X-Ray diffraction. The rolled SAE 5160 H steel samples were austenitized in two temperatures: 800ºC and 1050ºC, quenched in oil at 80ºC, to obtain retained austenite and tempered in three different temperatures: 280ºC, 350ºC and 420ºC. Mechanical properties, microstructure and fracture mechanism were analyzed in samples in the conditions: as rolled, quenched and quenched and tempered. The 350ºC tempered martensite embrittlement was detected by impact tests according to the references with an intergranular and dimples fracture mechanism for surface fracture of samples austenitized at 1050ºC and intergranular fracture mechanism for fracture surface of samples austenitized at 800ºC, as showed by scanning electron microscopy (SEM) analysis. The retained austenite decomposition was studied by X-Ray diffraction in all conditions samples and the behavior is in accordance with references. The X-Ray energy dispersive spectrum (EDS) was performed to study the influence of impurities like S and P in this phenomenon but no impurities were detected in fracture surface, by this method. It was observed that some mechanisms contribute to tempered martensite embrittlement in 5160H steel and the effects of this phenomenon are enhanced for higher austenitizing temperature. / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

The imminent emergence of the hydrogen fuel industry has resulted in an urgent mandate for very specific material testing. Although storage of pressurized hydrogen gas is both practical and attainable, demands for increasing storage pressures (currently around 70 MPa) continue to present unexpected material compatibility issues. It is imperative that materials commonly used in gaseous hydrogen service are properly tested for hydrogen embrittlement resistance. To assess material behavior in a pressurized hydrogen environment, procedures were designed to test materials for susceptibility to hydrogen embrittlement. Of particular interest to the field of high-pressure hydrogen in the automotive industry, austenitic stainless steels SUS 316 and 316L were used to validate the test programs. Tests were first performed in 25 MPa helium and hydrogen at room temperature and at -40°C. Tests in a 25 MPa hydrogen atmosphere caused embrittlement in SUS 316, but not in 316L. This indicated that alloys with higher stacking fault energies (316L) are more resistant to hydrogen embrittlement. Decreasing the test temperature caused slight embrittlement in 316L and significantly enhanced it in 316. Alternatively, a second set of specimens was immersed in 70 MPa hydrogen at 100°C until reaching a uniform concentration of absorbed hydrogen. Specimens were then loaded in tension to failure to determine if a bulk saturation of hydrogen provided a similar embrittling effect. Neither material succumbed to the effects of gaseous pre-charging, indicating that the embrittling mechanism requires a constant supply of hydrogen at the material surface rather than having bulk concentration of dissolved hydrogen. Permeation tests were also performed to ensure that hydrogen penetrated the samples and to develop material specific permeation constants. To pave the way for future work, prototype equipment was constructed allowing tensile or fatigue tests to be performed at much higher hydrogen pressures. To determine the effect of pressure on hydrogen embrittlement, additional tests can be performed in hydrogen pressures up to 85 MPa hydrogen. The equipment will also allow for cyclic loading of notched tensile or compact tension specimens for fatigue studies. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

In this study, the effects of stress and strain rate on cyclic deformation, secondary hardening, martensitic phase transformation, crack initiation, and fatigue behavior of type 304L stainless steel are examined. A series of load and strain controlled uniaxial zero and non-zero mean strain fatigue tests were conducted with varying frequencies in order to investigate the effect of loading rate on fatigue behavior. The volume fraction of martensite was quantified for several tests using x-ray diffraction and electron backscatter diffraction. The loading rates were found to have a direct effect on the microstructure and fatigue behavior of the alloy investigated. Adiabatic heating from an increased rate of loading was found to effect martensite formation which is a major contributor to the secondary hardening phenomena associated with many austenitic stainless steels under cyclic loading. Also affected by the microstructural changes were cyclic deformation, crack initiation, microstructurally small crack growth, and fatigue behavior.

AN ABSTRACT OF THE THESIS OFABHIYAN OLI, for the Master of Science degree in Applied Physics, presented on August 10, 2023 at Southern Illinois University Carbondale. TITLE: STUDY OF PHASE TRANSITION AND MAGNNETOCALORIC EFFECT FOR THE SYSTEM Ni-Mn-In-Bi MAJOR PROFESSOR: Dr. Saikat Talapatra We experimentally investigate the Heusler alloys Ni50Mn35In12Bi3 and Ni47Mn35In15Bi3 on their different magnetic properties: structural, magnetic, magnetocaloric and magnetotransport properties by using room-temperature X-ray diffraction (XRD), and magnetization measurements in the temperature interval of 10 -380K and field up to 5T. This alloys shows both high temperature austenite phase (AP) and martensite phase (MP). The alloy Ni47Bi3Mn35In15 crystallize in primitive Cubic structure with space group Fm-3m and Ni50Mn35In12Bi3 with the crystal structure of Tetragonal L21 type with space group I4-3m. Alloy Ni47Bi3Mn35In15 show two phase transition FOPT from Ferrimagnetic/AFM to FM and SOPT from FM to PM towards higher temperature and its result will be discussed here mainly. The martensitic transition (TM) takes place around 200K and Curie temperature (TC) 313K in presence of 100Oe field. The saturation magnetization (Ms) at 10K was found to be increasing at lower field and stabilized at higher field indicating ferromagnetic behavior. The Ni47Bi3Mn35In15 shows high magnetocaloric effects (ΔSM = -47.36 Jkg-1K-1) and Relative Cooling Power (RCP = 222.12 J/Kg) in the vicinity of its Curie temperature (TC =313K). Magnetotransport measurement is done by using a standard four-probe method from 10-380 K temperature in presence of zero field and 50 kOe field.

This master thesis was a part of an ongoing project at Research institutes of Sweden (RISE) and Chalmers University of technology, studying the formation of white layers (WLs) upon hard machining AISI 52100 steel. With a focus on the nanocrystalline microstructure of the machined steel, X-ray diffraction (XRD), white light interferometry (WLI), optical microscopy (LOM) and scanning electron microscopy (SEM) was utilized in the analysis of gathering an in-depth understanding of the WL formation mechanism. By introducing varying cutting parameters as part of the machining process, the effect of cutting speed and tool wear could be observed to directly impact the WL formation and could be linked to the thermomechanical contribution to the formation mechanism. Both thermal and mechanical WLs were observed and could be distinguished by the occurrence of dark layers in thermal WLs. The purpose of this thesis was to observe the influence of retained austenite (RA) on WL formation and from the XRD analysis the residual stress for different RA content could not be concluded. Furthermore, SEM concluded differences in the microstructure where a higher abundance of carbides was observed in the case of lower RA, a phenomenon most likely originating in the heat treatment process. Indications of facilitated mechanical WL formation for lower RA was observed but could not be deemed conclusive. The RA content could not be concluded to have an impact on the surface roughness nor the residual stress where variations were rather linked to the cutting parameters.

Le développement de matériaux d'ingénierie combinant ténacité et résistance à l'usure reste encore un défi. Dans le but de contribuer à ce domaine, cette thèse présente une étude de la carbonitruration des aciers 16NiCrMo13 et 23MnCrMo5. L'évolution cinétique des atmosphères à base d'hydrocarbures et d'ammoniac est étudiée numériquement, ainsi que le comportement local à l'équilibre et la cinétique de diffusion pour l'obtention de profils d'enrichissement des alliages traités. Les simulations sont confrontées à des mesures par chromatographie en phase gazeuse des produits de pyrolyse de l'acétylène et de décomposition de l'ammoniac, et aux réponses métallurgiques, par l'évaluation des profils de diffusion, des filiations de dureté et par l'identification des précipités formés par microscopie électronique en transmission. La dureté obtenue après trempe et traitement cryogénique évolue selon la racine carrée de la teneur en interstitiels en solution solide simulée à partir de la composition locale en utilisant des mesures des profils chimiques en carbone et en azote. Après revenu, les zones enrichies en azote montrent une tenue en dureté supérieure à celles obtenues avec la même teneur totale en carbone en solution, ce qui a été attribué après observation par microscopie électronique en transmission à une fine précipitation de nitrures de fer lors de cette dernière étape de traitement. Le bilan de matière des produits de pyrolyse montre que les principales espèces non détectées sont des radicaux fortement carbonés qui peuvent aussi donner lieu à la formation d'hydrocarbures polycycliques de haut poids moléculaire dans les zones froides du réacteur. À la pression atmosphérique et à basse pression l'établissement de conditions d'enrichissement en carbone à concentration constante est possible en utilisant de faibles pressions partielles d'acétylène dilué dans l'azote. La conversion atteinte par la pyrolyse de ce précurseur est pourtant importante à la température de traitement compte tenu du temps de séjour caractéristique du réacteur employé à la pression atmosphérique. La cinétique de décomposition de l'ammoniac étant beaucoup plus lente que celle des hydrocarbures légers, il a été possible de quantifier la vitesse de décomposition de cette espèce par unité de surface métallique exposée pendant la durée d'un traitement / The development of engineering materials combining both toughness and wear resistance is still a challenge. Aiming to contribute to this field of study, this thesis presents a study of the carbonitriding process of alloys 16NiCrMo13 and 23MnCrMo5. Kinetics of hydrocarbon- and ammonia-based atmospheres, as well as local equilibrium and diffusion kinetics for achieving the enrichment profiles, are studied by numerical simulation. These simulations are compared to chromatography measurements of gas phase pyrolysis products of acetylene and ammonia decomposition, and with metallurgical responses, where the comparison is made with evaluated diffusion profiles, hardness measurements and the identification of precipitates by transmission electron microscopy. Hardness after quench and cryogenic treatment depends on the square root of total solid solution interstitial content simulated by using local carbon and nitrogen compositions obtained experimentally. After tempering, the regions enriched in nitrogen show better hardness stability than those with same total carbon interstitial content, what was linked to a fine precipitation of iron nitrides observed by transmission electron microscopy. Mole balance of pyrolysis products show that the main non-detected species are high-carbon radicals, which may also lead to the formation of polycyclic aromatic hydrocarbons of high molecular weight at the reactor outlet. At both atmospheric and reduced pressures, constant concentration enrichment boundary conditions were established by using low partial pressures of acetylene diluted in nitrogen. Pyrolysis of this precursor attains high conversion rates at treatment conditions given the important residence time of the atmospheric pressure reactor. Ammonia decomposition kinetics being much slower than that of low molecular weight hydrocarbons, it was possible to identify the decomposition rate of this species over a metallic sample during a treatment