Academic literature on the topic 'Ductile-to brittle transition temperature (dbtt)'

The small punch (SP) test is one of the small specimen test techniques, and standardization of the SP test method for evaluating the mechanical properties of metallic materials is in progress. In this study, the impact transition temperature of reactor pressure vessel steels (RPV) in nuclear power plants was estimated using the draft standard SP test method. The SP fracture energy (ESP) and normalized SP fracture energy (ENSP) of the RPV steels were evaluated at various temperatures, and their transition curves were derived and compared to the transition curve in the Charpy V notch (CVN) test. The SP transition region appeared at a much lower temperature range than that of the CVN owing to the size and notch effect. Ductile brittle transition temperature (DBTT) in the SP transition curve showed a linear relationship with DBTT and T41J in the CVN transition curve. The ductile to brittle transition behaviors of SP specimens were analyzed using fractographs and compared with the transition curves in ESP and ENSP. ENSP started to decrease at the temperature at which the SP ductile to brittle transition behavior occurred, and this means that the ENSP transition curves were in good agreement with transition behavior in the SP test. However, the ESP transition curves did not match transition behavior. Using DBTT in the ENSP transition curve is appropriate to estimate the CVNDBTT and T41J.

Ductile-to-brittle transition behavior of high-nitrogen 18Cr-10Mn-0.35N austenitic steels containing Ni and Cu was investigated by means of Charpy impact test and fractographic analysis. The commonly observed fracture mode of the specimens tested at -196 oC was transgranular cleavage-like brittle with flat facets occurring along {111} crystallographic planes, thereby leading to the occurrence of ductile-to-brittle transition. For all the steels investigated in the present study, the ductile-to-brittle transition temperature (DBTT) measured from Charpy impact tests was much higher by 90 to 135 oC than that predicted by empirical equation strongly depending on N content. The combined addition of Ni and Cu enabled the 18Cr-10Mn-0.35N steels to have the lowest DBTT, which could be explained by relatively high austenite stability and favorable effect of Cu as well as the absence of delta-ferrite.

The effect of intermetallic phases and grain size on ductile to brittle transition temperature of Aluminium-Iron alloy (Al–11% Fe) was investigated in this research work. An Izod impact testing method was adopted to study the DBTT in the temperature interval of 77 K to 373 K. The ductile-brittle transition points: fracture transition plastic (FTP), fracture-appearance transition temperature (FATT), impact energy transition temperature (IETT), fractional surface area of cleavage (brittle) and fibrous (ductile) fractures and grain size of the samples were also determined. The fracture toughness of Al-Fe alloy found decreasing with temperature in contrast to conventional materials. The fractographic investigation revealed that the microstructural changes play a major role in determining the fracture toughness of these alloys. Annealing of these samples slightly improved the fracture toughness as the spherical morphology of intermetallic particles resists the crack propagation.

Prolonged high temperature exposure of welded C-Mn steels is likely to cause microstructural changes leading to an inrease in the ductile-to-brittle transition temperature (DBTT) of the welded joint. Consequently, such degrading material properties should be quantified in view of establishing accurate component life prediction model. This study examined effects of isothermal aging on DBTT behavior of the heat affected zone (HAZ) in welded Type A516 Gr 70 steels. Microstructures of the as-received weld region revealed the presence of pearlite and ferrite in the base metal while upper and lower bainite are found in the HAZ and weld metal, respectively. Hardness measures for the weld metal region, HAZ and base steel are 172, 209 and 150, respectively. Aging at 420 oC, 500 hours lowers hardness value of the HAZ by 20 %. A series of Charpy impact tests on V-notched specimens are performed for as-received and thermally aged samples at 420 oC for 500, 800 and 1200 hours. Results showed that the absorbed impact energy displays a sigmoidal variation with test temperatures. DBTT ranges from -60 to 5 oC for HAZ while narrow range from -25 to 12 oC for weld metal region. Absorbed impact energy variations in samples aged for durations up to 800 hours display another saturation level over test temperatures between -30 to 10 oC. Fractographic analysis on HAZ fracture surface indicated brittle fracture at -60 oC while ductile failure dominated at 27.7 oC. A mix-mode fracture mechanism is displayed for test conducted at -38 oC.

The possible use of tungsten alloys as structural materials in future fusion reactor divertors strongly depend on their ductile-to-brittle transition temperature (DBTT). The present paper gives an overview on different rod and plate materials fabricated by PLANSEE. It is demonstrated that DBTT is clearly improved compared to commercially available standard materials. Moreover, the significant impact of the microstructure on fracture mode and on toughness is discussed in detail.

The characteristics of texture and microstructure of lean duplex stainless steels with low Ni content produced through hot rolling followed by annealing were investigated locally with electron backscatter diffraction and globally with neutron diffraction. Then, the ductile–brittle transition (DBT) behavior was studied by Charpy impact test. It is found that the DBT temperature (DBTT) is strongly affected by the direction of crack propagation, depending on crystallographic texture and microstructural morphology; the DBTT becomes extremely low in the case of fracture accompanying delamination. A high Ni duplex stainless steel examined for comparison, shows a lower DBTT compared with the lean steel in the same crack propagating direction. The obtained results were also discussed through comparing with those of cast duplex stainless steels reported previously (Takahashi et al., Tetsu-to-Hagané, 100(2014), 1150).

This paper presents the research results of the grain refining effect on the ductile-to- brittle transition temperature (DBTT) of commercial purity tungsten. The as-received tungsten was subjected to eight passes of equal-channel angular pressing (ECAP) at decreasing temperatures from 1300 to 1150 °C. According to optical and TEM microscopy the average grain size was refined considerably from ~80 μm to ~1 μm. The mechanical tensile tests, carried out at various temperatures for the tungsten samples, showed that DBTT decreased approximately 80 °C as a result of microstructure refinement by ECAP, at the same time the strength also increased 50-100 % by grain refinement. SEM observation of the fractures confirmed the mechanical testing results.

AbstractTemper embrittlement induced by segregation of metalloid solutes to grain boundary (GB) was evaluated by a shift of the ductile-brittle transition temperature (DBTT). DBTT was found to be linearly correlated with the amount of metalloid on the GB (Xgb) for both dynamic and static displacement rates (dδ/dt) in high and medium hardness steels. Recent first-principles calculations have determined the GB embrittling potency (Δep) of segregated Sb, Sn and P. In both high and medium hardness steels, the slope (α) of DBTT vs. Xgb was found to be linearly dependent on Δep regardless of the segregated solutes. In high hardness steels, the slope is independent of dδ/dt, while in medium hardness steels the α is dependent on dδ/dt. An Arrhenius plot of dδ/dt vs. the reciprocal DBTT was used to drive the thermal activation energy (Eact), which represents a barrier to plasticity. It was found that Eact correlates to a reduction in the GB fracture surface energy. The Eact depends strongly on GB decohesion in high hardness steels but only weakly depends on it in medium hardness steels.

In this study, the toughness of 11Cr ferritic stainless steel weld was evaluated by DBTT (Ductile-Brittle-Transition-Temperature) with the interstitial elements level. DBTT of the weld increased with increasing interstitial level due to the formation of martensite phase and solidsolution strengthening. Interstitial elements level should be limited by the adoption of back shielding gas during welding process because increased C+N level detrimentally affects the toughness of ferritic stainless weld. Adoption of Ar as back shielding gas lowered N content in the weld.

Charpy V-Notch impact tests of N1, N2 and N3 steels from 77K to 293K are possessed in this paper. With increasing the nitrogen concentration, the ductile to brittle transition temperature (DBTT) increases. The toughness of the tested steels decreases rapidly with decreasing the temperature. The change of fracture patterns of high nitrogen austenitic stainless steels is dimple → shallow dimple → mixture of quasi-cleavage facet and dimple → cleavage facet. Fracture facets with river patterns, with tear ridges, along annealing twin boundary and cross the annealing twin plane are observed in this investigation. Critical dislocation density of crack tips ρc=[6π(τp)2/(KIc)2]2 can affect ductile to brittle transition (DBT) behavior at cryogenic temperature. Deformation twinning is also frequently observed at cryogenic temperature. Crack forms along the coherent twin boundary between one twin and the matrix.

U disertaciji je izvršena karakterizacija mikrostrukture i mehaničkih osobinanelegiranog austemperovanogi nodularnog liva (ADI materijala), kao i uticajamikrostrukture na prelaznu temperaturu u intervalu od -196 do +100°C.Utvrđeno je da mehaničke osobine ADI-ja zavise od morfologije ausferitnemikrostrukture i količine zadržanog austenita, tj. parametara austemperovanja.Na osnovu mehaničkih osobina utvrđen je i opseg procesiranja u skladu sastandardima ASTM, ISO i EN. Zaključeno je da prelazna temperatura ADImaterijala zavisi od količine i stabilnosti zadržanog austenita. U višemtemperaturnom opsegu (iznad cca. -25°C) dominantna je količina zadržanogaustenita, dok na nižim temperaturama, stabilnost. Visoka obogaćenostugljenikom, stabilnog zadržanog austenita sprečava stvaranje martenzita naniskim temperaturama, a time i pojavu krtosti kod ADI-ja.
The object of this thesis was to characterize microstructure and mechanical propertiesof the unalloyed ADI material (Austempered Ductile Iron). In addition, the influence ofmicrostructure on the ductile to brittle transition temperature (DBTT) by Charpy impacttest in temperature interval from 196 to +100°C has been studied. The all propertiesobtained depend on the morphology of microstructure and the amount of retainedaustenite, i.e. on the austempering parameters. According to the mechanical propertiesand standard requirements (ASTM, ISO and EN) the processing window has beenproposed, also. It was found that DBTT is influenced by amount and stability of retainedaustenite. In upper temperature range (above cca. 25°C) the most influence factor onDBTT is amount of retained austenite, while at the lower temperatures the stability ismore prominent. Stability of high carbon retained austenite at lower temperaturesprevents transformation to martensite and thus the embrittlement of ADI.

Parmi les nombreuses nuances d’aciers inoxydables, les nuances ferritiques sont utilisées dans de nombreux secteurs grâce à leur résistance à la corrosion et à des propriétés spécifiques comme leur conductibilité thermique ou leurs propriétés ferromagnétiques. Leur teneur résiduelle en nickel en font des aciers économiques en comparaison des aciers austénitiques, plus massivement produits. Toutefois, l’utilisation de cette famille d’aciers en dessous de la température ambiante est limitée par sa fragilité mécanique, la moindre mobilité des dislocations dans le réseau cubique centré rendant l’écoulement plastique plus difficile aux basses températures. La précipitation et la taille des grains sont deux des paramètres microstructuraux qui influent sur la température de transition ductile/fragile des aciers ferritiques. L’objectif de cette thèse est d’étudier et de comprendre l’effet de ces paramètres sur cette température de transition et sur la rupture par clivage des aciers à matrice 100% ferritique contenant 18% de chrome et 2% de molybdène.Pour ce faire, trois coulées ont été élaborées avec la même teneur en carbone et en azote à laquelle s’ajoute, pour deux d’entre elles, du titane ou du niobium. La gamme de traitements thermomécaniques a été choisie pour fabriquer six microstructures distinctes pour lesquelles varient la taille des grains, la nature et la localisation de la précipitation et la teneur en carbone et en azote en solution solide. Ces microstructures sont caractérisées par microscopies optique et électroniques et par diffusion centrale des neutrons aux petits angles. La mise en place d’un essai de traction avec des éprouvettes axisymétriques entaillées permet de déterminer la contrainte de rupture par clivage de chaque microstructure à 20°C et à -40°C, puis de discuter des mécanismes qui régissent la rupture fragile.Les microstructures élaborées sont finalement classées en deux catégories. Pour les microstructures Ti, Nb et NbG, pour lesquelles l’ajout de titane ou de niobium permet d’éviter la précipitation de carbures et nitrures de chrome, la contrainte de rupture par clivage est dépendante de la taille et de la morphologie des particules intragranulaires. Les Ti(N,C), de 4 à 5 µm au maximum, dont la forme s’apparente à celle d’un cube, rendent la microstructure Ti plus fragile que la microstructure Nb caractérisée par des Nb(C,N), certes plus nombreux mais de plus petites dimensions. La taille des grains est un facteur de deuxième ordre sur la contrainte de rupture par clivage de ces microstructures comme cela est mis en évidence avec l’étude de la rupture fragile de la microstructure NbG, pour laquelle la taille des grains a été multipliée par 10 par rapport à celle de la microstructure Nb.Dans les microstructures CrP, Cr et Cr0, la présence ou non de carbone et d’azote en solution solide sursaturée et les précipitations intra et intergranulaires sont les conséquences des traitements thermiques choisis. Les précipités dans les joints de grains, même s’ils sont de petites dimensions (quelques dizaines à quelques centaines de nm), modifient significativement l’amorçage de la rupture par clivage. La température de transition ductile/fragile est, au premier ordre, dépendante de la limite d’élasticité des microstructures. La contrainte de rupture par clivage n’est plus dépendante de la taille des particules intragranulaires comme c’est le cas pour les microstructures Ti, Nb et NbG mais de la précipitation aux joints de grains et de la dépendance de la contrainte de friction de réseau à la solution solide (modèle de Smith).Au vu de ces résultats, des pistes pour limiter la fragilité des nuances ferritiques sont proposées
Among stainless steel grades, ferritic stainless steels are used in many applications thanks to their corrosion resistance and attractive properties such as their thermal conductivity and ferromagnetic properties. Their residual nickel content make them economical steels compared to austenitic steels, which are more heavily produced. However, the use of this family of steels is limited by its relatively low toughness at room temperature and below, coming from the lattice friction of the body centered cubic structure that hinders plastic flow. Precipitation and grain size are two microstructural parameters that influence the ductile to brittle transition temperature of ferritic steels. The objective of this thesis is to study and understand the effect of these parameters on the ductile to brittle transition temperature and on the cleavage fracture of ferritic stainless steels containing 18% chromium and 2% molybdenum.To do so, three castings were produced with the same carbon and nitrogen content. Titanium or niobium were added to two of them. Thermomechanical treatments were chosen to produce six different microstructures for which the grain size, the nature and the location of the precipitation and the carbon and nitrogen content in solid solution vary. These microstructures are characterized by optical and electron microscopies and small-angle neutron scattering. The implementation of a tensile test with notched round specimens allows the determination of the cleavage fracture stress of each microstructure at 20 ° C and -40 ° C and discuss the brittle fracture mechanisms.The microstructures developed are finally divided into two groups. For Ti, Nb and NbG microstructures, for which the addition of titanium or niobium prevents the precipitation of chromium carbides and nitrides, the cleavage critical stress is dependent on the size and morphology of the intragranular particles. The 4 to 5 µm long cubic Ti (N, C) make the Ti microstructure more brittle than the Nb microstructure characterized by more numerous but smaller Nb (C, N). The grain size is a second order parameter on the cleavage fracture stress of these microstructures as shown by the study of the brittle fracture of the NbG microstructure, for which grain size is ten times larger.In CrP, Cr and Cr0 microstructures, the presence, or absence, of carbon and nitrogen in solid solution and intra and intergranular precipitations are the consequences of the chosen heat treatments. The presence of precipitates in the grain boundaries, even if they are small (few dozens to few hundred nm), significantly modifies the initiation of cleavage fracture and the value of the ductile to brittle transition temperature. The ductile to brittle transition temperature depends in the first order on the yield stress of microstructures. The cleavage fracture stress is no longer dependent on the size of the intragranular particles as in Ti, Nb and NbG microstructures but on the precipitation of chromium carbides and nitrides at the grain boundaries and on the dependence of the lattice friction stress on the solid solution (Smith's model).In the light of these results, alternatives to limit the embrittlement of ferritic grades are suggested

Em projetos de oleodutos e gasodutos utilizam-se aços de alta resistência e baixa liga (ARBL), como o aço API 5L X80. Na soldagem multipasse destas tubulações, a zona afetada pelo calor (ZAC) do passe de raiz é submetida a um novo ciclo térmico pelos passes de soldagem subsequentes. Isto resulta em alterações nos valores das propriedades físicas. Nos aços ARBL, a ZAC de grãos grosseiros reaquecida intercriticamente (IC-ZACGG) pode se tornar uma zona frágil localizada, isto é, uma zona com maior dureza. Consequentemente, falhas estruturais podem ocorrer, ocasionando paradas não desejadas no transporte de fluidos. O objetivo deste trabalho é desenvolver uma metodologia baseada no modelo de fontes de calor distribuídas de Mhyr e Gröng, para avaliar o fluxo de calor na soldagem considerando as propriedades físicas dependentes da temperatura. Estender a aplicação desta ferramenta em soldagens multipasses para identificar sub-regiões da ZAC de um passe anterior sendo afetada pela ZAC de passes subsequentes. As isotermas simuladas foram validadas através de medições realizadas em macrografias de juntas soldadas. Os ciclos térmicos simulados foram validados através das temperaturas máximas atingidas e pelos tempos de resfriamento de 800 a 500 ºC (?t8-5) dos ciclos térmicos experimentais. Ao aplicar a metodologia proposta, foi possível delimitar com acurácia as regiões reaquecidas da ZAC e analisar os efeitos dos passes subsequentes em cada uma das sub-regiões da ZAC do passe de raiz. A IC-ZACGG na região do passe de raiz foi localizada, mas não se comportou como zona frágil devido à boa soldabilidade do aço API 5L X80 comprovada pelos ensaios de dureza e de tenacidade ao impacto Charpy-V.
In pipelines projects, the high strength low alloy (HSLA) steels are used, such as the API 5L X80 steel. During the multipass welding of these pipes, the heat affected zone (HAZ) of the root pass is subjected to a new thermal cycle by the subsequent welding passes. This results in changes in the values of the physical properties. In the HSLA steels, the intercritical reheated coarse-grained heat-affected zone (IR-CGHAZ) can become a local brittle zone, that is, a region with greater hardness. Consequently, structural failures could happen, causing undesired shutdowns in fluid transportation. The objective of this work is to develop a methodology based on the distributed heat sources model of Mhyr and Gröng, to evaluate the heat flux in the welding considering the temperature-dependent physical properties. Extend the application of this tool in multipass welds to identify HAZ subregions of a previous pass being affected by the HAZ of subsequent passes. The simulated isotherms were validated through measurements made on macrographs of welded joints. The simulated thermal cycles were validated through the maximum temperatures reached and the cooling times from 800 °C to 500 ºC (?t8-5) of the experimental thermal cycles. By applying the proposed methodology, it was possible to accurately delimit reheated HAZ regions and analyze the effects of subsequent passes in each of the root pass HAZ subregions. The IRCGHAZ in the root pass region was localized, but it did not behave as a brittle zone due to the good weldability of the API 5L X80 steel as proven by the hardness and Charpy-V impact tests.

Orientador: Marcelo dos Santos Pereira
Resumo: O ensaio de queda de peso DWTT (Drop Weigth Tear Test) é um método amplamente utilizado pela indústria de óleo e gás para determinar a capacidade de um material em impedir a propagação de uma trinca. Esse método foi desenvolvido pelo Battelle Memorial Institute, e é realizado em conformidade com a especificação API RP 5L3 "Práticas Recomendadas para a Condução de Testes de Queda de Peso". Com o desenvolvimento dos aços ARBL, o comportamento dos aços vem mostrando algumas particularidades resultantes do processamento termomecânico, e por esse motivo, podem não apresentar o mesmo comportamento à fratura que aços mais antigos, como delaminações ou inclusões não metálicas. Atualmente são propostos dois tipos de entalhe, sendo o tipo prensado, obtido pela estampagem de uma matriz na amostra, e o tipo Chevron, que deve ser usinado. A correlação entre ambos os entalhes pode ser realizada apenas para a análise da porcentagem da superfície dúctil da fratura. Outros tipos de correlação como energia absorvida para impacto, não são recomendados, uma vez que a concentração de tensão para o entalhe Chevron é muito maior, facilitando o rompimento da amostra, enquanto que o entalhe prensado demanda maior energia, uma vez que possui maior encruamento na região. No presente trabalho foram realizados os levantamentos de curvas de temperatura de transição dúctil e frágil (TTDF) do material base do tubo com dimensões de 762 mm x 38,1 mm de aço carbono com grau API X60, através da análise da porce... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: The Drop Weigth Tear Test (DWTT) is a method widely used by the oil and gas industry to determine the ability of a material to prevent the propagation of a crack. This method was developed by the Battelle Memorial Institute, and is performed in accordance with API RP 5L3 "Drop-Weight Tear Tests on Line Pipe" specification. With the development of ARBL steels, the behavior of steels has shown some particularities resulting from thermomechanical processing, and for this reason, they may not present the same fracture behavior as older steels, such as delamination or nonmetallic inclusions. Currently two types of notch are proposed, being the type pressed, obtained by the stamping of a matrix in the sample, and the type Chevron, that must be machined. The correlation between both notches can be performed only for the analysis of the percentage of the ductile surface of the fracture. Other types of correlation as energy absorbed for impact are not recommended, since the stress concentration for the Chevron notch is higher, facilitating the rupture of the sample, while the notched press demands greater energy, since it has greater hardening in the region. In the present study, the ductile and brittle transition temperature (TTDF) curves of the base material of the pipe with dimensions of 762 mm x 38.1 mm of carbon steel with API grade X60 were carried out, through the analysis of the percentage of ductile fracture resulting whose results were shown to be equivalent for both notch: ... (Complete abstract click electronic access below)
Mestre

The diploma thesis is aimed to assesment of the influence of the specimen orientation during the Small Punch Tests (SPT). Steel ČSN 41 0214, or its equivalent steel RFe80, W.-Nr. 1.104 was chosen for experiments. Resulting records from SPT were evaluated using already known correlations which are reported in literature. The correlation values were compared with values obtained by the standard tests like Charpy impact test and tensile test. The influence of axial, radial and tangential test specimens orientation is discussed in relation to Ductile-to-Brittle fracture behaviour of the steel.

Pt-aluminide (PtAl) coatings form an integral part of thermal barrier coating (TBC) systems that are applied on Ni-based superalloy components operating in the hot sections of gas turbine engines. These coatings serve as a bond coat between the superalloy substrate and the ceramic yttrium stabilized zirconia (YSZ) coating in the TBC system and provide oxidation resistance to the superalloy component during service at high temperatures. The PtAl coatings are formed by the diffusion aluminizing process and form an integral part of the superalloy substrate. The microstructure of the PtAl coatings is heavily graded in composition as well as phase constitution. The matrix phase of the coating is constituted of the B2-NiAl phase. Pt, in the coating, is present as a separate PtAl2 phase as well as in solid solution in B2-NiAl. The oxidation resistance of the PtAl bond coat is derived from the B2-NiAl phase. At high temperatures, Al from the B2-NiAl phase forms a regenerative layer of alumina on the coating surface which, thereby, lowers the overall oxidation rate of the superalloy substrate. The presence of Pt is beneficial in improving the adherence of the alumina scale to the surface and thereby enhancing the oxidation resistance of the coating. However, despite its excellent oxidation resistance, the B2-NiAl being an intermetallic phase, renders the PtAl coating brittle and imparts it with a high brittle-to-ductile-transition-temperature (BDTT). The PtAl coating, therefore, remains prone to cracking during service. The penetration of these cracks into the substrate is known to degrade the strain tolerance of the components. Evaluation of the mechanical behavior of these coatings, therefore, becomes important from the point of views of scientific understanding as well as application of these coatings in gas turbine engine components. Studies on the mechanical behavior of coatings have been mostly carried on coated bulk superalloy specimens. However, since the coating is brittle and the superalloy substrate more ductile when compared to the coating, the results obtained from these studies may not be representative of the coating. Therefore, it is imperative that the mechanical behavior of the coating in stand-alone condition, i.e. the free-standing coating specimen without any substrate attached to it, be evaluated for ascertaining the true mechanical response of the coating. Study of stand-alone bond coats involves complex specimen preparation techniques and challenging testing procedures. Therefore, reports on the evaluation of mechanical properties of stand-alone coatings are limited in open literature. Further, no systematic effort has so far been made to examine important aspects such as the effect of temperature and strain rate on the tensile behavior of these coatings. The deformation mechanisms associated with these bond coats have also not been reported in the literature. In light of the above, the present research study aims at evaluating the tensile behavior of free-standing PtAl coatings by the micro-tensile testing technique. The micro-tensile testing method was chosen for property evaluation because of its inherent ability to generate uniform strain in the specimen while testing, which makes the results easy to interpret. Further, since the technique offers the feasibility to test the entire graded PtAl coating in-situ, the results remain representative of the coating. Using the above testing technique, the tensile behavior of the PtAl coating has been evaluated at various temperatures and strain rates. The effect of strain rate on the BDTT of the coating has been ascertained. Further, the effect of Pt content on the tensile behavior of these coatings has also been evaluated. Attempts have been made to identify the mechanisms associated with tensile deformation and fracture in these coatings. The thesis is divided into nine chapters. Chapter 1 presents a brief introduction on the operating environment in gas turbine engines and the materials that are used in the hot sections of gas turbine engines. The degradation mechanisms taking place in the superalloy in gas turbine environments and the need for application of coatings has also been highlighted. The basic architecture of a typical thermal barrier coating (TBC) system applied on gas turbine engine components has been presented. The constituents of the TBC system, i.e. the ceramic YSZ coating, MCrAlY overlay as well as diffusion aluminide bond coats and, the various techniques adopted for the deposition of these coatings have been described in brief. Chapter 2 presents an overview of the literature relevant to this study. This chapter is divided into four sub-chapters. The formation of diffusion aluminide coatings on Ni-based superalloys has been described in the first sub-chapter. Emphasis has been laid on pack cementation process for the formation of the coatings. The fundamentals of pack aluminizing process, including the thermodynamic and kinetic aspects, have been mentioned in brief. The microstructural aspects of high activity and low activity plain aluminide and Pt-aluminide coatings have also been illustrated. The techniques applied for the mechanical testing of bond coats have been discussed in the second sub-chapter. The macro-scale testing techniques have been mentioned in brief. The small scale testing methods such as indentation, bend tests and micro-tensile testing have also been discussed in the context of evaluation of mechanical properties of bond coats. Since the matrix in the aluminide bond coats is constituted of the B2-NiAl phase, a description of the crystal structure and deformation characteristics of this phase including the flow behavior, ductility and fracture behavior has been mentioned in the third sub-chapter. In the fourth sub-chapter, reported literature on the tensile behavior and brittle-to-ductile-transition-temperature (BDTT) of diffusion aluminide bond coats has been discussed. In Chapter 3, details on experiments carried out for the formation of various coatings used in the present study and, their microstructural characterization, are provided. The method for extraction of stand-alone coating specimens and their testing is discussed. The microstructure and composition of the various coatings used in the present study are discussed in detail in Chapter 4. Unlike in case of bulk tensile testing, for which standards on the design of specimens exist, there are no standards available for the design of micro-tensile specimens. Therefore, as part of the present research work, a finite element method (FEM)-based study was carried out for ascertaining the dimensions of the specimens. The simulation studies predicted that failure of the specimens within the gage length can be ensured only when certain correlations between the dimensional parameters are satisfied. Further, the predictions from the simulation study were validated experimentally by carrying out actual testing of specimens of various dimensions. Details on the above mentioned aspects of specimen design are provided in Chapter 5. The PtAl coatings undergo brittle fracture at lower temperatures while ductile fracture occurs at higher temperatures. Further, the coatings exhibit a scatter in the yielding behavior at temperatures in the vicinity of BDTT. Therefore, the BDTT, determined as the temperature at which yielding is first observed in the stress-strain curves, may not be representative of the PtAl coatings. In Chapter 6, a method for the precise determination of BDTT of aluminide bond coats, based on the variation in the plastic strain to fracture with temperature, has been demonstrated. The BDTT determined by the above method correlated well with the variation in fracture surface features of the coating and was found representative of these coatings. In Chapter 7, the effect of temperature and strain rate on the tensile properties of a PtAl bond coat has been evaluated. The temperature and strain rate was varied between room temperature (RT)-1100°C and 10-5 s-1-10-1 s-1, respectively. The effect of strain rate on the BDTT of the PtAl bond coat has been examined. Further, the variation in fracture surface features and mechanism of fracture with temperature and strain rate are illustrated. The micro-mechanisms of deformation and fracture in the coating at different temperature regimes have also been discussed. The coating exhibited brittle-to-ductile transition with increase in temperature at all strain rates. The BDTT was strain rate sensitive and increased significantly at higher strain rates. Above BDTT, YS and UTS of the coating decreased and its ductility increased with increase in the test temperature at all strain rates. Brittle behavior occurring in the coating at temperatures below the BDTT has been attributed to the lack of operative slip systems in the B2-NiAl phase of the coating. The onset of ductility in the coating in the vicinity of BDTT has been ascribed to generation of additional slip systems caused by climb of dislocations onto high index planes. The coating exhibited two distinct mechanisms for plastic deformation as the temperature was increased from BDTT to 1100°C. For temperatures in the range BDTT to about 100°C above it, deformation was controlled by dislocations overcoming the Peierls-Nabarro barrier. Above this temperature range, non-conservative motion of jogs by jog dragging mechanism controlled the deformation. The transition temperature for change of deformation mechanism also increased with increase in strain rate. For all strain rates, fracture in the coating at test temperatures below the BDTT, occurred by initiation of cracks in the intermediate single phase B2-NiAl layer of the coating and subsequent inside-out propagation of the cracks across the coating thickness. Ductile fracture in the coating above the BDTT was associated with micro-void formation throughout the coating. The effect of Pt content on the tensile behavior of PtAl coating, evaluated at various temperatures ranging from room temperature (RT) to 1100°C and at a nominal strain rate of 10-3 s-1, is presented in Chapter 8. Irrespective of Pt content in the coating, the variation in tensile behavior of the coating with temperature remained similar. At temperatures below BDTT, the coatings exhibited linear stress-strain response (brittle behavior) while yielding (ductile behavior) was observed at temperatures above BDTT. At any given temperature, the elastic modulus decreased while the strength increased with increase in Pt content in the coating. On the other hand, the ductility of the coating remained unaffected with Pt content. The BDTT of the coating also increased with increase in Pt content in the coating. Addition of Pt did not affect the fracture mechanism in the coating. Fracture at temperatures below BDTT was caused by nucleation of cracks at the intermediate layer and their subsequent inside-out propagation. At high temperatures, fracture occurred in a ductile manner comprising void formation, void linkage and subsequent joining with cracks. The deformation sub-structure of the coating did not get affected with Pt incorporation. Short straight dislocations were observed at temperatures below BDTT, while, curved dislocations marked by jog formation were observed at temperatures above BDTT. The factors controlling fracture stress and strength in the PtAl coatings at various temperatures have also been assessed. The overall summary of the present research study and recommendations for future studies are presented in the last chapter, i.e. Chapter 9.

The effect of Laves phase (Fe2Nb) formation on the Charpy impact toughness of the ferritic stainless steel type AISI 441 was investigated. The steel exhibits good toughness after solution treatment at 850°C, but above and below this treatment temperature the impact toughness decreases sharply. With heat treatment below 850°C the presence of the Laves phase on grain boundaries and dislocations plays a significant role in embrittlement of the steel whereas above that temperature, an increase in the grain size from grain growth plays a major role in the impact embrittlement of this alloy. The toughness results agree with the phase equilibrium calculations made using Thermo–Calc® whereby it was observed that a decrease in the Laves phase volume fraction with increasing temperature corresponds to an increase in the impact toughness of the steel. Annealing above 900°C where no Laves phase exists, grain growth is found which similarly has a very negative influence on the steel’s impact properties. Where both a large grain size as well as Laves phase is present, it appears that the grain size may be the dominant embrittlement mechanism. Both the Laves phase and grain growth, therefore, have a significant influence on the impact properties of the steel, while the Laves phase’s precipitation behaviour has also been investigated with reference to the plant’s manufacturing process, particularly the cooling rate after a solution treatment. The microstructural analysis of the grain size shows that there is a steady increase in grain size up to about 950°C, but between 950°C and 1000°C there is a sudden and rapid 60 % increase in the grain size. The TEM analysis of the sample that was annealed at 900°C shows that the Laves phase had already completely dissolved and cannot, therefore, be responsible for “unpinning of grain boundaries” at temperatures of 900°C and higher where this “sudden” increase in grain size was found. The most plausible explanation appears to be one of Nb solute drag that loses its effectiveness within this temperature range, but this probably requires some further study to fully prove this effect. During isothermal annealing within the temperature range of 600 to 850°C, the time – temperature – precipitation (TTP) diagram for the Laves phase as determined from the transformation kinetic curves, shows two classical C noses on the transformation curves. The first one occurring at the higher temperatures of about 750 to 825°C and the second one at much lower temperatures, estimated to possibly be in the range of about 650 to 675°C. The transmission electron microscopy (TEM) analyses show that there are two independent nucleation mechanisms that are occurring within these two temperature ranges. At lower temperatures of about 600°C, the pertaining nucleation mechanism is on dislocations and as the temperature is increased to above 750°C, grain boundary nucleation becomes more dominant. Also, the morphology of the particles and the mis-orientation with the matrix changes with temperature. At lower temperatures the particles are more needle-like in shape, but as the temperature is increased the shape becomes more spheroidal. The effect of the steel’s composition on the Laves phase transformation kinetics shows that by lowering the Nb content in these type 441 stainless steels, had no significance effect on the kinetics on precipitation of the Laves phase. However, a Mo addition and a larger grain size of the steel, retard the formation of the Laves phase, although the optimum values of both parameters still need further quantification. The calculation made for the transformation kinetics of the Laves phase, using the number density of nucleation sites No and the interfacial energy, as the fitting parameters in this work, demonstrated a reasonable agreement with experimental results.
Thesis (PhD)--University of Pretoria, 2010.
Materials Science and Metallurgical Engineering
unrestricted

Abstract The toughness of a material is its ability to absorb energy in the form of plastic deformation without fracturing. It is thus a measure of both strength and ductility. This chapter describes the fracture and toughness characteristics of metals and their effect on component lifetime and failure. It begins with a review of the ductile-to-brittle transition behavior of steel and the different ways to measure transition temperature. It then explains how to predict fracture loads using linear-elastic fracture mechanics and how toughness is affected by temperature and strain rate as well as grain size, inclusion content, and impurities. It also presents the theory and use of elastic-plastic fracture mechanics and discusses the causes, effects, and control of temper embrittlement in various types of steel.

ABSTRACT The “petrological Moho” recognized in the Jurassic Vourinos Ophiolite (northern Greece) was the first “crust-mantle” boundary described within a fossil oceanic lithosphere. Early observations suggested a Cenozoic brittle-field block rotation of the petrological Moho transition area resulting in an oblique clockwise rotation of ∼100°, but a brittle fault system responsible for the mechanism of this rotation was never located. A modern interpretation of research dating from the 1960s to the present documents the occurrence of a diverse set of ductile structures overprinting this primary intra-oceanic feature. The following observations from our original “Moho” studies in the Vourinos complex are still pertinent: the contact between the upper mantle units and the magmatic crustal sequence is in situ and intrusional in nature; high-temperature intragranular ductile deformation (mantle creep at temperatures from around 1200 °C down to ∼900 °C) fabrics terminate at the crust-mantle boundary; the overlying oceanic crustal rocks display geochemical fractionation patterns analogous to crustal rocks in the in situ oceanic lithosphere. Since these original studies, however, understanding the mechanisms of ductile deformation and ridge crest processes have advanced, and hence we can now interpret the older data and recent observations in a new paradigm of oceanic lithosphere formation. Our major interpretational breakthrough includes the following phenomena: lower temperature, intergranular deformation of ∼900 °C to 700 °C extends from the upper mantle tectonites up into the lower crustal cumulate section; the origin of mineral lineations within adcumulate crustal rocks as remnants of ductile deformation during early phases of magmatic crystallization; syn-magmatic folding and rotation of the cumulate section; the tectonic significance of flaser gabbro and late gabbroic intrusions in the crustal sequence; and the relevance and significance of a cumulate troctolite unit within the crustal sequence. These observations collectively point to an important process of a ductile-field, syn-magmatic rotation of the Moho transition area. The most plausible mechanism explaining such a rotation is proto-transform faulting deformation near the ridge crest. By recognizing and distinguishing structures that resulted from such initial rotational deformation in the upper mantle peridotites of ophiolites, future field-based structural, petrographic, and petrological studies can better document the mode of the initiation of oceanic transform faults.

Abstract The irradiation embrittlement of reactor pressure vessel steels can be predicted using the ductile-to-brittle transition temperature (DBTT) shift obtained from Charpy impact tests. For the structural integrity assessment considering irradiation embrittlement, it is necessary to set margins for various uncertainties. It is important to understand what and how much factors contribute to the uncertainty. In the present study, a 34% credible interval value of Charpy DBTT at a 41J energy level (T41J) was evaluated by estimating the probability distributions of Charpy test data using Bayesian statistics. To fit the Charpy transition curves, a hyperbolic tangent with coefficients whose uncertainties depend on the test temperature was used. The probability distribution of T41J was estimated using Monte Carlo sampling and Bayesian inference. It was clarified that 34% of the credible-interval values of T41J before and after irradiation unchanged for base and weld metals when the number of specimens and test temperature were equivalent under un-irradiated and irradiated conditions. When the Charpy transition curve was determined by 12 specimens loaded in a surveillance test capsule, the estimated uncertainty of T41J was about 5 °C.

Fracture toughness from a CT specimen is used as a material constant for fracture evaluation, but it has a large constraint, which provides too conservative evaluation results. In ductile to brittle transition temperature (DBTT) region ferritic steel which is material of RPV has a large scatter and it becomes important to know the accurate scatter of an irradiated material because of less margin of RPV’s integrity after a long term operation. In this paper to establish a more precise fracture evaluation method in DBTT region for an irradiated RPV with a postulated surface flaw, fracture analysis procedures considering constraint effect, the Beremin model and damage mechanics model and a coupled model of these models were applied to the specimens with different constraints, which were 1/2TCT specimens and flat plate specimens with a semicircular flaw under tensile load. For evaluation of pure cleavage fracture of flat plate specimens, a Beremin model with plastic strain effect was applied with incorporation of plastic strain effect. Further, for ductile fracture, the local strain criterion of ASME Section VIII was applied to the specimens with different geometries and its applicability was discussed.

Three different coatings were studied in this work: vacuum plasma-sprayed NiCoCrAlYTa, electrolytically deposited NiCoCrAlYTa and Ni-Pt aluminide diffusion coatings. These three coatings were deposited on AM3 single crystal alloy. The tensile properties of coated single crystal test specimens were investigated. Ductile to Brittle Transition Temperatures (DBTT) were determined from tensile tests. All the coatings were examined before and after testing. All the tested coatings induce a ductile/brittle transition. Strain rate has a great influence on the transition temperature. The comparison between plasma-sprayed deposition and electrodeposition illustrates the strong influence of coating microstructure. In every case, NiCoCrAlYTa coatings were more ductile, and then less detrimental, than aluminide coatings.

Current defect assessment procedures based on fracture mechanics usually assume flaws to be infinitely sharp. While this assumption may be appropriate for fatigue cracks, for non-sharp flaws such as porosity, mechanical damage, or weld undercut, it can be an over-conservative assumption that can lead to pessimistic assessments of structural integrity and a significant underestimation of the true safety margin against fracture. Irwin studied notched and pre-cracked fracture toughness in the lower shelf region and suggested that notched KIc, which is linear-elastic plane-strain fracture toughness is proportional to the square root of notch radius ρ but is not continuous with pre-cracked KIc (i.e., when zero is substituted by ρ in the fitted KIc and ρ relationship, the obtained value differs from that of the pre-cracked specimen KIc). In contrast, Begley et al. conducted a similar study in the upper shelf region and suggested that JIc ∝ ρ In addition, they showed that notched JIc is continuous with pre-cracked JIc. Very few studies have been conducted on this topic in the ductile-to-brittle transition temperature (DBTT) region. In this study, the effects of notch acuity on notch fracture toughness in the lower shelf and DBTT regions were studied for 0.55% C steel JIS S55C with 0.5TSE(B) specimens. The notch sizes ρ were selected as 50, 150, and 375 μm. Fatigue pre-cracked specimens were also studied. The experimental results showed that notched KIc ∝ ρ1/2 but is not continuous with the pre-cracked KIc at a lower shelf temperature of −166 °C. The DBTT notch fracture toughness KJc ∝ ρ1/2 and is continuous with the pre-cracked KJc. By conducting elastic-plastic finite element analysis (EP-FEA), the mid-plane crack-opening stress distribution on the x1-axis, was shown that the scaled stress distribution at fracture load was identical for pre-cracked and notched specimens. Thus, notched and pre-cracked KJc has a reason to be continuous. The reason for notch size effect on Jc was explained as the difference in load for notched specimens to reach the stress level of the pre-cracked specimen.

In this paper, we demonstrate that a deterministic approach requiring only tensile test data for different temperatures has a possibility to predict the minimum fracture toughness for these temperatures. The material is assumed to be in the Ductile-to-Brittle-Transition Temperature (DBTT) region. The approach was based on one of the authors’ finding that the critical stress σ22c of the modified Ritchie-Knott-Rice criterion is correlated with the minimum fracture toughness and shows very small scatter and is specimen configuration independent. The criterion predicts onset of cleavage fracture of a material in the DBTT transition temperature region, when the crack-opening stress σ22 measured at a distance from the crack-tip equal to four times the crack-tip opening displacement δt exceeds a critical value σ22c. The proposed approach is expected to overcome some inconveniences which recent studies have reported to the Master Curve parameters vary with size and temperature.

This paper analyses the notch effect in ferritic-pearlitic steel S275JR in a range of temperatures within the material Ductile-to-Brittle Transition Zone (DBTZ). The notch effect is evaluated in terms of load-bearing capacity, apparent fracture toughness (modeled here using the Theory of Critical Distances) and fracture micromechanisms. The concept of Master Curve in notched conditions is also presented. To this end, experimental results obtained in S275JR notched specimens are presented, together with Scanning Electron Microscopy (SEM) fractographies. The analysis is performed at −50 °C, −30 °C and −10 °C, the material Transition Temperature (T0) being −26.1 °C, with the notch radii ranging from 0 mm (crack-type defects) up to 2.0 mm. The results show how the lower the temperature the larger the notch effect, and also that the evolution of both the load bearing capacity and the apparent fracture toughness is directly related to the evolution of fracture micromechanisms. Moreover, the proposed Master Curve in notched conditions has provided good predictions of the experimental results.

An Fe-Cr-W-V-Ti-N steel named SCRAM (super-clean reduced-activation martensitic) steel was designed for the first wall and blanket structure of fusion power plants. Compared with the Fe-Cr-W-V-Ta steel, TiN can precipitate first at 1650°C rather than TaC by the Thermal-cal Calculation. And we take vacuum induction melting (VIM) and electro-slag re-melting (ESR) together to manufacture the SCRAM steel, which can make the TiN fine and the steel pure. Mechanical properties and microstructures of SCRAM steels irradiated with single-beam (Fe) and sequential-beam (Fe plus He and Fe plus H) at 300°C were studied. The results show that, SCRAM steel can have better mechanical properties before and after irradiation while Ti was doped into the SCRAM steel. It has been reported that the precipitation formed in the steel has effect on the mechanical properties, irradiation properties and the ductile brittle transition temperature (DBTT). The effect of intermediate heat treatment on precipitation behavior and mechanical properties of SCRAM steel was investigated in order to obtain dispersed fine M23C6 carbides. The results indicated that MX carbonitrides precipitated first in the steel with intermediate heat treatment at 870°C rather than M23C6, which led to a decrease of carbon concentration in the supersaturated martensitic matrix and correspondingly a reduced volume fraction and mean size of M23C6. The intermediate heat treatment was beneficial to the mechanical properties, and proposed for reduction on the ductile brittle transition temperature (DBTT).

The fracture toughness KJc of the material in the ductile to brittle transition temperature (DBTT) range exhibits both test specimen thickness (TST) dependence and temperature dependence. Attention has been paid to the master curve (MC) method, which provides an engineering approach to address these two issues. Although MC is intended to be applied to arbitrary ferritic material whose yield stress is within the range of 275 to 825 MPa, the KJc value must be obtained to determine the material dependent reference temperature T0. The applicable range of MC method is restricted to T0 ± 50 °C. Previous studies indicate that additional pre-tests to obtain T0 are necessary; thus, there might be some unwritten requirement to the test temperature for the KJc temperature dependence prediction in MC method to work effectively. If testing must be conducted for the material of interest at some restricted temperature, a more flexible KJc temperature dependence prediction can possibly be obtained for a wide temperature range in the DBTT range, if the simplified and direct scaling (SDS) method, which predicts fracture “load” from yield stress temperature dependence proposed previously is applied. In this study, the SDS method was applied to two different steels: Cr-Mo steel JIS SCM440 and 0.55% carbon steel JIS S55C. Both tensile and fracture toughness tests were performed over a wide range of temperatures, specifically, −166 to 100 °C for SCM440 and −166 to 20 °C for S55C. The SDS method (i.e., fracture load is proportional to 1/(yield stress)) was initially validated for the specimens in the DBTT range. Finally, a simplified method was proposed and initially validated to predict the KJc temperature dependence, by applying the SDS using the EPRI plastic J functional form.

The in-service hardening and increase of the Ductile Brittle Transition Temperature (DBTT) of Reactor Pressure Vessel (RPV) steels are controlled at the atomic scale by the clustering of point defects with co-segregation of alloying elements like Cu, Mn, Ni, Si and P. The effort and strategy within the European Project PERFECT for developing advanced multi-scale numerical tools to predict this microstructure and the induced hardening as well as their experimental validation at the relevant scale will be presented. The first results obtained since January 2004, starting date of PERFECT, will be discussed and perspectives given.

Temper embrittlement is a common damage mechanism of pressure vessels in the chemical and petrochemical industry serviced in high temperature, which results in the reduction of roughness due to metallurgical change in some low alloy steels. Pressure vessels that are temper embrittled may be susceptible to brittle fracture under certain operating conditions which cause high stress by thermal gradients, e.g., during start-up and shutdown. 2.25Cr1-Mo steel is widely used to make hydrogenation reactor due to its superior combination of high mechanical strength, good weldability, excellent high temperature hydrogen attack (HTHA) and oxidation-resistance. However, 2.25Cr-1Mo steel is particularly susceptible to temper embrittlement. In this paper, the effect of carbide on temper embrittlement of 2.25Cr-1Mo steel was investigated. Mechanical properties and the ductile-brittle transition temperature (DBTT) of 2.25Cr-1Mo steel were measured by tensile test and impact test. The tests were performed at two positions (base metal and weld metal) and three states (original, step cooling treated and in-service for a hundred thousand hours). The content and distribution of carbides were analyzed by scanning electron microscope (SEM). The content of Cr and Mo elements in carbide was measured by energy dispersive X-ray analysis (EDS). The results showed that the embrittlement could increase the strength and reduce the plasticity. Higher carbide contents appear to be responsible for the higher DBTT. The in-service 2.25Cr-1Mo steel showed the highest DBTT and carbide content, followed by step cooling treated 2.25Cr-1Mo steel, while the as-received 2.25Cr-1Mo steel has the minimum DBTT and carbide content. At the same time, the Cr and Mo contents in carbide increased with the increasing of DBTT. It is well known that the specimen analyzed by SEM is very small in size, sampling SEM specimen is convenient and nondestructive to pressure vessel. Therefore, the relationship between DBTT and the content of carbide offers a feasible nondestructive method for quantitative measuring the temper embrittlement of 2.25Cr-1Mo steel pressure vessel.