No description available.

No description available.

After hot rolling, the presense of segregation in the center of wire-rod can lead to a nonuniform transformation, resulting in bands of martensite in the microstructure. This is considered to be a defect, called center-martensite, because it can cause cracks and breaks during wire drawing. To identify the mechanism for the formation of center-martensite in wire-rod, the structure, macrosegregation and microsegregation in unworked billets were characterized. Based on measurements of secondary dendrite arm spacings, cooling rates during solidification were estimated. It appears that the macrosegregation of carbon and manganese in the billets manifests itself as the microsegregation in wire-rod, which is an agent in forming the center-martensite. Thus, electromagnetic stirring is proposed as a means to reduce the macrosegregation in the billet and, thereby, reduce the occurrence of center-martensite in wire-rod.

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.

Thesis. 1978. Sc.D.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / Sc.D.

Martensitic steels have become very important engineering materials in modern society. Crucial parts of everyday products are made of martensitic steels, from surgical needles and razor blades to car components and large-scale excavators. Martensite, which results from a rapid diffusionless phase transformation, has a complex nature that is challenging to characterize and to classify. Moreover the possibilities for modeling of this phase transformation have been limited, since its thermodynamics and kinetics are only reasonably well understood. However, the recent development of characterization capabilities and computational techniques, such as CALPHAD, and its applicability to ferrous martensite has not been fully explored yet. In the present work, a thermodynamic method for predicting the martensite start temperature (Ms) of commercial steels is developed. It is based mainly on information on Ms from binary Fe-X systems obtained from experiments using very rapid cooling, and Ms values for lath and plate martensite are treated separately. Comparison with the experimental Ms of several sets of commercial steels indicates that the predictive ability is comparable to models based on experimental information of Ms from commercial steels. A major part of the present work is dedicated to the effect of carbon content on the morphological transition from lath- to plate martensite in steels. A range of metallographic techniques were employed: (1) Optical microscopy to study the apparent morphology; (2) Transmission electron microscopy to study high-carbon plate martensite; (3) Electron backscattered diffraction to study the variant pairing tendency of martensite. The results indicate that a good understanding of the martensitic microstructure can be achieved by combining qualitative metallography with quantitative analysis, such as variant pairing analysis. This type of characterization methodology could easily be extended to any alloying system and may thus facilitate martensite characterization in general. Finally, a minor part addresses inverse bainite, which may form in high-carbon alloys. Its coupling to regular bainite is discussed on the basis of symmetry in the Fe-C phase diagram. / <p>QC 20120824</p> / Hero-m

No description available.

Thèse doctorat : Génie des Matériaux : INSA LYON : 2002. / Titre provenant de l'écran-titre. Bibliogr. p. 173-177.

Thèse doctorat : Génie des Matériaux : Villeurbanne, INSA : 2007. / Titre provenant de l'écran-titre. Bibliogr. p. 220-234.

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.