No description available.

The present work reports the effect of heat input, cooling rate, parent plate deformation and restraining conditions on the formation and morphology of delta ferrite in welds on AISI 318L and 321 steels. The experiments were carried out on commercially produced plates in the following conditions: (i) as received condition (ii) further deformed by cold rolling. (iii) further deformed by hot rolling. The effect of heat input and cooling rates were examined using bead-onplate Submerged Arc welds on the same parent plate material. The parent plate condition was assessed using bead-on-plate metal Inert Gas (MIG) and Subm rged Arc butt welds. The results suggest that 1. The weld metal solidification proceeds epitaxially from the existing unmelted base metal. The weld exhibited surface marking i.e. deformation bands or close packed plane, in the austenite matrix, but not passing through delta ferrite phase. 2. The solidification substructure, the ferrite content, and morphology are influenced by tile (i) thermal stress induced during welding, (ii) parent plate chemical composition, (iii) the parent plate microstructural and deformation characteristics i.e. strain energy. 3. Rapidly cooled welds have lower ferrite content than welds produced with slower cooling rates. The randomly distributed elongeed ferrite with some lath type ferrite morphology was predominantly attributed with the welds produced with high cooling rates. 4. The ferrite is the first solidified phase to form and austenite is formed from the liquid rather than by solid phase transformation of primary ferrite to austenite. 5. The ferrite formation is a diffusion controlled phase transformation, the degree of its dendrites development depends upon the welding parameters i.e. heat input, cooling rates. 8. Two types of ferrite morphology were observed: (a) elongated type dendrites identified as vermicular by previous investigators and (b) the cellular type morphology which has not been classified in previous investigations.

No description available.

No description available.

Abstract This thesis studies controlling factors that govern transverse hydrogen cracking in high-strength multipass weld metal (WM). The experiments were concerned with heavy-restraint Y- and U-Groove multipass cracking tests of shielded-metal arc (SMAW) and submerged-arc (SAW) weld metals. Results of tensile tests, hardness surveys, weld residual stress measurements and microstructural investigations are discussed. The analytical phase comprised numerical calculations for analysing the interactions between crack-controlling factors. The objectives were: (i) the assessment of WM hydrogen cracking risk by defining the Crack-No Crack boundary conditions in terms of 'safe line' description giving the desired lower-bound estimates, and (ii) to derive predictive equations capable of giving reliable estimates of the required preheat/interpass temperature T0/Ti for the avoidance of cracking. Hydrogen cracking occurred predominantly in high strength weld metals of Rp0.2 ≈ 580-900 MPa. At intermediate strengths of Rp0.2 ≈ 500-550 MPa, cracking took place in the cases where the holding time from welding to NDT inspection was prolonged to 7 days. Low strength WMs of Rp0.2 ≤ 480 MPa did not exhibit cracking under any conditions examined. Cracking occurrence was, above all, governed by WM tensile strength, weld diffusible hydrogen and weld residual stresses amounting to the yield strength. The appearance of cracking vanished when transferring from 40 to 6 mm thick welds. The implications of the holding time were more significant than anticipated previously. A period of 16 hrs in accordance with SFS-EN 1011 appeared much too short for thick multipass welds. Interpass time and heat input showed no measurable effect on cracking sensitivity, hence being of secondary importance. Equations were derived to assess the weld critical hydrogen content Hcr corresponding to the Crack-No Crack conditions as a function of either weld metal Pcm, yield strength Rp0.2 or weld metal maximum hardness HV5(max). For the calculation of safe T0/Ti estimates, a formula incorporating: (i) WM strength as a linear function of either weld carbon equivalent CET or weld HV5(max), (ii) weld build-up thickness aw in the form of tanh expression and (iii) weld diffusible hydrogen HD in terms of a combined [ln / power law] expression was found descriptive.

No description available.

Made available in DSpace on 2014-06-11T19:27:13Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-10-27Bitstream added on 2014-06-13T19:35:05Z : No. of bitstreams: 1 musardo_gb_me_ilha.pdf: 3996445 bytes, checksum: ce4f6124da4b6648c41885a10eb3e368 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / No Brasil, a maior parte da energia elétrica disponível é gerada por usinas hidrelétricas as quais contam com grande número de turbinas hidráulicas. Durante sua operação estas turbinas sofrem sérios danos provenientes tanto de natureza mecânica como também hidráulica. Fenômenos de perda de massa e também trincas e rachaduras são alguns dos principais problemas que ocorrem devido a um fenômeno chamado de erosão cavitacional, que comumente é recuperado por soldagem. Sendo assim, grandes níveis de resistência à cavitação vêm sendo obtidos com a deposição de aços inoxidáveis com a presença de cobalto pelo processo de soldagem a arco com proteção gasosa. Neste trabalho, seis amostras feitas de aço-carbono ASTM A36, as quais foram extraídas de retalhos de turbinas, foram usadas como metal de base para as amostras. Usando um processo a arco elétrico com proteção gasosa (GMAW) em posição plana, duas camadas de aço AWS E70-S6 (1,2 mm de diâmetro) foi depositada em todas as amostras, e, somente em três das amostras foram depositadas duas camadas de amanteigamento com o arame de aço inoxidável AWS E309-T1 (1,6 mm de diâmetro) usando o mesmo processo, onde foi utilizado como proteção gasosa uma mistura de 75% de dióxido de carbono e 25% de argônio. A energia de soldagem nominal nestes casos foi de 0,5 kJ/mm. Por final, mais duas camadas de aço inoxidável com cobalto, liga resistente à cavitação foram depositadas por arco pulsado, com energia de soldagem nominal de 0,5 a 0,8kJ/mm, com uma mistura de gases de 98% de argônio e 2% de oxigênio como proteção. Secções transversais das amostras foram preparadas, devidamente lixadas e depois polidas com alumina 1,0μm, seguido de ataque químico moderado com Villela para observação de microestruturas. Foi feito o estudo de microdureza Vickers com carga padrão de 0,4 N e espaçamento regular (0,4 mm)... / Nowadays most of the power supply used in Brazil is provided for a large number of hydraulic turbines. During its operation in hydroelectric power plants, these turbines usually have been damaged either mechanical or hydraulic reasons. So, catastrophic cracking and loss-of-mass due to cavitation erosion are main problems which are commonly repaired by welding. Higher levels of resistance to cavitation erosion have been attained with surface deposition of a cobalt-alloyed stainless steel coating by gas-protected arc welding techniques. In the present work six plates, which were machined from blades of hydraulic turbines made with ASTM A36 carbon steel grade, were used as base metal. Using manual gas-metal arc welding (GMAW) in flat weld position two layers of AWS E70-S6 carbon steel (1.2 mm diameter) were deposited on all samples. Only 3 samples two buttering layers were deposited with AWS E309-T1 flux-cored wire (1.6 mm diameter) using same processing, where were applied a 75% carbon dioxide - 25% argon mixture as protection gas. The nominal heat input used in all layers was approximately 0.5 kJ/mm. At the last welding deposition another two cobalt-alloyed, cavitation resistant, stainless steel cladding layers were deposited under pulsed arc with a nominal heat input of 0.5 to 0.8kJ/mm, being used for them a protective gas mixture of 98% argon - 2% oxygen. Transverse sections of weld deposit were prepared according standard grinding method (up to 1200-grit SiC paper) and final mechanical polishing using 1.0μm alumina, followed by moderate etching in Villela reagent for microstructural observation. Vickers microhardness measurements were carried out at standard load (0.4 N) and regular spacings (0,4mm) from surface to base metal. Light microscopy (LM) was used to determine average size of inclusions, which was measured from digitalized images using a freeware image analysis ...(Complete abstract, click electronic access below)

[pt] Tradicionalmente, tanto os metais de solda, quanto os aço da classe Cr-Mo, são considerados susceptíveis ao fenômeno de fragilização ao revenido. O presente trabalho tem como objetivo avaliar metais de solda do tipo 2,25%Cr-1,0%Mo com diferentes teores de Mn (0,4% - 1,1%) obtidos pelo processo de eletrodo revestido submetidos aos tratamentos térmicos de alívio de tensões (TTAT), de step-cooling (TTSC) e TTAT seguido de TTSC. Verificou-se através de ensaios de impacto Charpy-V que o TTAT, promoveu um ganho de tenacidade e resistência para teores de Mn entre 0,9% - 1,1%, enquanto que a aplicação do TTSC não promoveu mudanças significativas na tenacidade para todos os teores estudados. Observou-se que, as variações microestruturais em função dos tratamentos térmicos aplicados para uma estrutura predominantementebainítica, detectáveis por microscopia eletrônica de varredura, foram pequenas para os teores de Mn estudados. A utilização dos índices tradicionais como o fator X , fator PE e fator J de susceptibilidade à fragilização se mostrou incompatível com as respostas obtidas nos ensaios de impacto dos metais de solda avaliados, sugerindo a necessidade de uma revisão dos fatores para a avaliação da tendência para a fragilização ou mesmo considerar que os materiais atualmente produzidos não sejam mais susceptíveis ao fenômeno fragilização ao revenido mesmo após tratamentos térmicos de alívio de tensões. / [en] Traditionally, both Cr-Mo class steels and the weld metals produced in joining such materials have been considered to be susceptible to the phenomenon of temper embrittlement. The current study evaluates 2,25%Cr-1,0%Mo type weld metals with varying Mn levels (0,4%- 1,1%), produced using coated electrodes with an SMAW process. The welded joints were subjected to three types of heat treatment; either stress relief annealing, or step cooling or stress relief annealing and step cooling. Charpy-V impact testing results indicated a gain in toughness and tensile strength for materials with Mn levels in the 0,9% to 1.1% range, and that the step cooling alone did not cause any appreciable changes in toughness for any of the Mn levels studied. Furthermore, scanning electron microscopy revealed that the bainitic microstructure of the materials studied was relatively little affected by the applied heat treatments. The values obtained for the traditional embrittlement indexes, X -Factor, PE-Factor and J-Factor were observed to be inconsistent and incompatible with the actual quantified impact behaviour of the weld metals investigated. This would suggest that a reevaluation of these traditional embrittlement indexes should be considered. The temper embrittlement susceptibility of steels of this type as they are currently manufactured may, in this way, come to be considered much lower than would otherwise be the case, even after post weld heat treatments.

Os aços Grau 91 vêm sendo amplamente utilizados como componentes para trabalhar em elevadas temperaturas nas indústrias de geração de energia, petroquímica e refinarias de petróleo em consequência da sua excelente resistência a fluência e corrosão em altas temperaturas. Apesar destas notáveis propriedades têm sido encontradas dificuldades na soldagem do aço Grau 91, visto que pode apresentar valores de dureza fora dos limites especificados e provocando redução de diversas propriedades mecânicas. Foram confeccionadas juntas tubulares do aço Grau 91, por meio de soldagem usando o processo TIG na raiz e o processo Eletrodo Revestido nas camadas de soldagem subsequentes. As juntas soldadas foram submetidas ao tratamento térmico póssoldagem (TTPS) a 760°C com diferentes tempos de patamar (1h, 4h e 8h) e posteriormente, as propriedades mecânicas foram analisadas para verificar o atendimento aos requisitos normativos, por meio da realização de ensaios de tração convencional e à quente (540°C), impacto Charpy, dureza Vickers e caracterização microestrutural. Os resultados mostraram que as propriedades mecânicas do aço Grau 91 sofreram alterações, como uma melhor resistência ao impacto e diminuição da dureza com o aumento do tempo de patamar de temperatura, indicando que é possível obter uma melhor combinação de resistência mecânica e tenacidade com o tempo de patamar maior do que 4 horas no TTPS nas condições utilizadas. / Grade 91 steels have been widely used as components to work at high temperatures in power generation plants, petrochemicals and oil refineries as a result of their excellent creep and corrosion resistance at high temperatures. Despite these remarkable properties, difficulties in welding Grade 91 have been encountered, as they may exhibit hardness values outside the specified limits and cause a reduction in various mechanical properties. Grade 91 tubular joints were made through welding by using the GTAW process at the root and the SMAW process in the subsequent welding layers. The welded joints were subjected to PWHT at 760°C with different tempering times (1h, 4h and 8h) and their mechanical properties were evaluated by performing conventional tensile, elevated-temperature tensile (540°C), Charpy V-notch impact toughness, Vickers hardness testing and microstructural characterization. Results indicated that mechanical properties of grade 91 steel change with increasing tempering time, and a better combination of strength and toughness can be reached with more than 4 hours of PWHT, under certain conditions.

Os aços Grau 91 vêm sendo amplamente utilizados como componentes para trabalhar em elevadas temperaturas nas indústrias de geração de energia, petroquímica e refinarias de petróleo em consequência da sua excelente resistência a fluência e corrosão em altas temperaturas. Apesar destas notáveis propriedades têm sido encontradas dificuldades na soldagem do aço Grau 91, visto que pode apresentar valores de dureza fora dos limites especificados e provocando redução de diversas propriedades mecânicas. Foram confeccionadas juntas tubulares do aço Grau 91, por meio de soldagem usando o processo TIG na raiz e o processo Eletrodo Revestido nas camadas de soldagem subsequentes. As juntas soldadas foram submetidas ao tratamento térmico póssoldagem (TTPS) a 760°C com diferentes tempos de patamar (1h, 4h e 8h) e posteriormente, as propriedades mecânicas foram analisadas para verificar o atendimento aos requisitos normativos, por meio da realização de ensaios de tração convencional e à quente (540°C), impacto Charpy, dureza Vickers e caracterização microestrutural. Os resultados mostraram que as propriedades mecânicas do aço Grau 91 sofreram alterações, como uma melhor resistência ao impacto e diminuição da dureza com o aumento do tempo de patamar de temperatura, indicando que é possível obter uma melhor combinação de resistência mecânica e tenacidade com o tempo de patamar maior do que 4 horas no TTPS nas condições utilizadas. / Grade 91 steels have been widely used as components to work at high temperatures in power generation plants, petrochemicals and oil refineries as a result of their excellent creep and corrosion resistance at high temperatures. Despite these remarkable properties, difficulties in welding Grade 91 have been encountered, as they may exhibit hardness values outside the specified limits and cause a reduction in various mechanical properties. Grade 91 tubular joints were made through welding by using the GTAW process at the root and the SMAW process in the subsequent welding layers. The welded joints were subjected to PWHT at 760°C with different tempering times (1h, 4h and 8h) and their mechanical properties were evaluated by performing conventional tensile, elevated-temperature tensile (540°C), Charpy V-notch impact toughness, Vickers hardness testing and microstructural characterization. Results indicated that mechanical properties of grade 91 steel change with increasing tempering time, and a better combination of strength and toughness can be reached with more than 4 hours of PWHT, under certain conditions.