This thesis work was done in cooperation with Ovako Sweden AB in Hofors to better understand and improve cracking-related problems in bearing ring production and cooling after forging in particular. This goal was achieved by doing a literature search and executing cooling experiments in live production. Cracks have appeared after forging when heating and forming rings or in storage before reheating larger models. Hard, brittle centers were the origin of the cracks. The literature search suggested that connected martensite structures and residual stresses resulting from cooling rate, cooling gradients, austenite grain size, and segregation were problematic. A slower cooling rate, especially in the pearlite transformation zone, was identi ed as a possible solution. This hypothesis was tested by cooling on plates and under a box to reduce the cooling rate. Plates were ine ective, while the box experiments led to reduced hardness and less martensite. Isolation, the temperature under the box, and the placement of the forgings were critical factors. Forgings spending longer time at higher temperatures showed improved properties, supporting the hypothesis made. However, more tests need to be performed before making any  finnal conclusions.

No description available.

No description available.

The Thirteenth East Asia-Pacific Conference on Structural Engineering and Construction, September 11-13, 2013, Sapporo, Japan (EASEC-13)

To increase of bearing steels properties are research and development of surface modifications well above dedicated, in present. Basic requirements to the surface modifications are high wear resistance and good temperature resistance (origin by friction). Surface modifications can not negatively influence properties of bearing steels as a substrate. One of the possibilities can be used surface modifications used for high temperature applications. These are based on saturation of substrate surface layer by Al (and Si) elements. This work deals with preparing of Al and Al-Si diffusion layers from slurries on 100Cr6 bearing steel surface and study of elements interaction between coating and bearing steel as a substrate during proceeding of protective layers at high temperatures. To elements interaction study light microscopy, scanning electron microscopy equipped by energy dispersive microanalyses, glow discharge optical emission spectrometry and microhardness measurement methods were used. To the evaluation of layer thickness image analyses was used.

No description available.

<p dir="ltr">Rolling contact fatigue (RCF) is one of the leading causes of failure in critical tribological components such as rolling element bearings (REBs), gears, cam and followers, etc. This is especially paramount for advanced aerospace applications where REB components need to operate for billions of RCF cycles before routine maintenance or inspection is performed. The rolling motion between the rolling elements and raceway produces RCF, wherein a complex, non-proportional, alternating contract stress is applied over a small material volume. Moreover, the highly localized stress occurs on the same length scale as microstructural features such as carbides, inclusions, grain size, hardness gradients from carburization, surface roughness, thereby amplifying their effect on fatigue performance. Therefore, the objective of this dissertation is to investigate critical microstructural modifications and their effects on RCF performance via experiments and computational modeling.</p><p dir="ltr">Initially, an investigation was undertaken to investigate surface roughness effects on RCF. The surface roughness of various REBs was measured through optical surface profilometry and used to construct rough surface pressure distributions, which were then used in a continuum damage mechanics (CDM) finite element (FE) framework. The results demonstrated that life is reduced as lambda ratio decreases. It was also observed that a 2-parameter Weibull cumulative distribution function can describe the relationship between the near surface orthogonal shear stress concentration and ratio of surface failures.</p><p dir="ltr">Next, the enhancement to RCF life from grain size refinement of through hardened bearing steels was studied. To capture the effects of grain refinement, torsion stress-life data of various grain size were used in the RCF model. A predictive life equation for different grain sizes was constructed based on the exponential trend observed between grain size and life from the simulation data. The life equation was then used to calculate the quotient of RCF at two different grain sizes. This quotient was defined as the life improvement ratio and it was observed that this investigation’s ratios compared well with existing life improvement ratios from RCF experiments.</p><p dir="ltr">Hardness gradient is a common microstructural modification to improve RCF life of tribo-components. Variation of hardness gradients is prevalent in case hardened (i.e. case carburized) bearing materials. Therefore, the CDM-FE RCF model was modified to investigate the effects of various hardness gradient types and depths on fatigue life improvement. The simulation results enabled the identification of potentially optimal gradients aimed to mitigate manufacturing challenges and provided the foundation for the construction of a general fatigue life equation.</p><p dir="ltr">A fundamental study to understand the impact various common RCF failure criteria have on RCF life estimation was then conducted using computational modeling. To capture the variation of a material’s resistance to fatigue, the critical CDM damage parameters were assumed to follow a probabilistic distribution instead of a singular value. The CDM-FE model was modified to consider the shear reversal, the octahedral shear stress, the maximum shear stress, the Fatemi-Socie criteria, and the Dang Van multi-axial fatigue parameter as failure criteria. Simulation life results revealed that the CDM-FE model with shear reversal and Fatemi-Socie criteria best match empirical predictions from well-established RCF life theory. Notably, the Fatemi-Socie exhibited the best agreement over all operating conditions.</p><p dir="ltr">The next investigation focused on the cleanliness of aerospace-quality bearing steels. Torsion fatigue experiments established the stress-life (S-N) relation for three common aerospace quality bearing steels. The S-N data was later used to calibrate the RCF model’s damage equation, which considered the Fatemi-Socie criteria following conclusions from a previous investigation. Simulation results were observed to corroborate well with RCF experiments that were conducted for all three materials, while noting the simulations offered a significant time saving. As a result, a subsequent investigation focused on establishing the stress-life relationship for one of the aerospace quality bearing steels through a combined experimental and analytical approach. Good corroboration was observed between simulations and experiments at three contact pressures. This finding is particularly significant as it strengthens the reliability of computational RCF model as an efficient means to assess the RCF performance of bearing materials.</p><p dir="ltr">Furthermore, the detailed investigation on RCF performance of each critical microstructural modifications and their respective effect greatly improves the state-of-the-art. The findings emanating from the various investigations offer informed fatigue design recommendations that aid in the selection of rolling element bearings for critical tribological and aerospace applications.</p>

Ao longo deste trabalho foi proposta uma nova rota de tratamento térmico para o aço AISI 52100, principal material utilizado na fabricação de rolamentos. Esta nova rota visa refinar a microestrutura de carbonetos presentes no material, com o objetivo de otimizar as propriedades mecânicas. Os tratamentos térmicos realizados conduziram a uma diminuição no tamanho dos carbonetos quando comparados com a amostra tradicional. Para medir a tenacidade à fratura evitando a dificuldade experimental de se nuclear pré-trincas em materiais frágeis, foi utilizada a metodologia Chevron, como formalizada na ASTM E-1304(97). Algumas das amostras submetidas a nova rota apresentaram maior tenacidade à fratura quando comparadas a amostra tratada na rota convencional. O material tratado na nova rota de tratamento térmico foi avaliado também em ensaio de impacto, usando-se corpos de prova Charpy de secção reduzida, sem entalhe. Os resultados obtidos mostraram que o material com carbonetos mais finos apresentavam menor energia absorvida no impacto quando comparados com a amostra tratada na rota convencional. Este comportamento a princípio contraditório foi explicado pela maior presença de sítios de nucleação na amostra com carbonetos mais finos. A maior densidade de sítios nas condições do ensaio de impacto conduziu a uma intensificação da nucleação de trincas, levando a uma menor energia absorvida durante o ensaio de impacto; este efeito ocorre mesmo quando o mecanismo de nucleação de trincas envolve a nucleação e coalescimento de microcavidades. Procurou-se fazer uma correlação entre a morfologia das superfícies de fratura obtidas no ensaio Chevron, o tamanho do carboneto das amostras e o micromecanismo atuante. Para amostras com superfície de fratura com aspecto fibroso foi associado o micromecanismo de fratura controlado por deformação. Para amostras com superfície de fratura com aspecto intergranular foi associado o micromecanismo de fratura controlado por tensão. / This work proposes a new heat treatment route for AISI 52100 steel, the most used material for bearings. The new route is designed so as to refine the carbides distribution in material, with the aim of increasing the fracture toughness. In order to measure the fracture toughness a Chevron notch methodology was used, as described in ASTM E-1304(97). Obtaining a static pre-crack on brittle materials is difficult and expensive; the Chevron notch methodology allows avoiding this difficulty. The heat treatments reduced the carbide sizes as compared with conventionally heat-treated steels. Some samples treated by this new route presented an increase in Chevron notch fracture toughness. The materials treated by the new heat treatment route were also tested in impact using a subsize Charpy specimen, without notch. Materials with fine carbides presented less dissipated energy in impact test then the material conventionally treated. This behavior is associated with the increase in number of carbide particles, as second phase particles are known to behave as sites for microvoid initiation and growth. The higher density of such sites presented in the material with fine carbides enhance this mechanism, lowering the dissipated energy measured in impact test. A correlation between fracture morphology, carbide size and fracture mechanisms was attempted. Samples with fine carbides presents fibrous transgranular fracture morphologies with strain controlled fracture micromechanism while samples with coarse carbides presented intergranular type morphologies with stress controlled micro-mechanism.

Today, there is a high demand on clean steel for high performance materialproperties. Thus, steel producers try to deliver a steel product with the highestquality and cleanliness to the market. The number of parameters that affect thesteel cleanliness may vary depending on the required material properties of thefinal product. However, the non-metallic inclusion characteristics represent one ofthe most important parameters. More specifically, the composition, size, numberand morphology affect steel cleanliness. In this work, selected parameters affectingthe inclusion characteristics were studied using the following methods: i)thermodynamic calculations (including computational thermodynamiccalculations), ii) inclusion determinations using a cross sectional (CS) method (2Dinvestigations) and iii) inclusion determinations using an electrolytic extraction(EE) method (3D investigations). The computational thermodynamic calculations of the slag-steel and inclusion-steelequilibriums were carried out using the Thermo-Calc software. With the help ofthese calculations, the influence of the slag carryover on the top slag, aluminumcontent in steel and sulfur distribution ratio as well as predictions of stable phasesof inclusions were studied. In addition, inclusion determinations of tool steel gradesamples collected during various stages of the ladle treatment in a scrap-based steelplant were carried out using both 2D and 3D methods. Furthermore, inclusiondeterminations of bearing steel grade samples from a runner system after ingotcasting were performed using a 2D metallographic method (CS-method). Also, theINCAFeature software was used, when using cross sectional method, in order tocollect more statistics of the inclusion characteristics. It was found that slag carryover has a large influence on the composition of theactual top slag as well as the aluminum content in the steel as well as the sulfurdistribution ratio. In addition, steel and slag were found to be in “near”-equilibriumconditions, after the completion of the vacuum degassing operation. Furthermore,the composition of small-size inclusions in samples taken from tool steel was foundto be very scattered. Moreover, the composition of the large-size inclusions wasfound to be less scattered. Furthermore, closer to the top slag composition insamples collected after vacuum degassing. Finally, the accuracy of the inclusioncomposition determinations of tool steel samples using the electrolytic extractionmethod was found to be better than for the cross sectional method. The worseaccuracy of the CS-method is due to a considerable effect of matrix elements oninclusion composition. / QC 20100709

Ao longo deste trabalho foi proposta uma nova rota de tratamento térmico para o aço AISI 52100, principal material utilizado na fabricação de rolamentos. Esta nova rota visa refinar a microestrutura de carbonetos presentes no material, com o objetivo de otimizar as propriedades mecânicas. Os tratamentos térmicos realizados conduziram a uma diminuição no tamanho dos carbonetos quando comparados com a amostra tradicional. Para medir a tenacidade à fratura evitando a dificuldade experimental de se nuclear pré-trincas em materiais frágeis, foi utilizada a metodologia Chevron, como formalizada na ASTM E-1304(97). Algumas das amostras submetidas a nova rota apresentaram maior tenacidade à fratura quando comparadas a amostra tratada na rota convencional. O material tratado na nova rota de tratamento térmico foi avaliado também em ensaio de impacto, usando-se corpos de prova Charpy de secção reduzida, sem entalhe. Os resultados obtidos mostraram que o material com carbonetos mais finos apresentavam menor energia absorvida no impacto quando comparados com a amostra tratada na rota convencional. Este comportamento a princípio contraditório foi explicado pela maior presença de sítios de nucleação na amostra com carbonetos mais finos. A maior densidade de sítios nas condições do ensaio de impacto conduziu a uma intensificação da nucleação de trincas, levando a uma menor energia absorvida durante o ensaio de impacto; este efeito ocorre mesmo quando o mecanismo de nucleação de trincas envolve a nucleação e coalescimento de microcavidades. Procurou-se fazer uma correlação entre a morfologia das superfícies de fratura obtidas no ensaio Chevron, o tamanho do carboneto das amostras e o micromecanismo atuante. Para amostras com superfície de fratura com aspecto fibroso foi associado o micromecanismo de fratura controlado por deformação. Para amostras com superfície de fratura com aspecto intergranular foi associado o micromecanismo de fratura controlado por tensão. / This work proposes a new heat treatment route for AISI 52100 steel, the most used material for bearings. The new route is designed so as to refine the carbides distribution in material, with the aim of increasing the fracture toughness. In order to measure the fracture toughness a Chevron notch methodology was used, as described in ASTM E-1304(97). Obtaining a static pre-crack on brittle materials is difficult and expensive; the Chevron notch methodology allows avoiding this difficulty. The heat treatments reduced the carbide sizes as compared with conventionally heat-treated steels. Some samples treated by this new route presented an increase in Chevron notch fracture toughness. The materials treated by the new heat treatment route were also tested in impact using a subsize Charpy specimen, without notch. Materials with fine carbides presented less dissipated energy in impact test then the material conventionally treated. This behavior is associated with the increase in number of carbide particles, as second phase particles are known to behave as sites for microvoid initiation and growth. The higher density of such sites presented in the material with fine carbides enhance this mechanism, lowering the dissipated energy measured in impact test. A correlation between fracture morphology, carbide size and fracture mechanisms was attempted. Samples with fine carbides presents fibrous transgranular fracture morphologies with strain controlled fracture micromechanism while samples with coarse carbides presented intergranular type morphologies with stress controlled micro-mechanism.