L.H. de Almeida

Mechanistic Modeling of Dynamic Strain Aging in Austenitic Stainless Steels

Abstract On the basis of experimental data from tensile testing and review of the literature, the mechanisms of dynamic strain aging (DSA) in austenitic stainless steels have been evaluated. It is concluded that, in the lower temperature range of DSA, reorientation of carbon- or nitrogen-vacancy pairs occurs, with a preferential concentration where dislocations decelerate or are temporarily arrested. This mechanism is controlled by vacancy concentration and by vacancy diffusion to crystallographic defects. DSA is absent at intermediate temperatures because there are no regions with a higher concentration of pairs, owing to the higher mobility and greater equilibrium concentration of vacancies. At higher temperatures, where serrated flow is again observed, carbon or nitrogen atoms or carbon- or nitrogen-vacancy pairs have sufficient mobility to once more produce areas of higher point defect concentration where dislocations decelerate. At still higher temperatures, DSA disappears because of the increase in carbide precipitation.

Setting of commercial glass ionomer cement Fuji IX by 27Al and 19F MAS-NMR

OBJECTIVES To investigate the long term setting reaction in the glass ionomer restorative, Fuji IX, by monitoring the structural evolution of aluminium and fluorine species using (27)Al and (19)F MAS-NMR spectroscopy. METHODS Fuji IX cements aged from 5min up to 3 months were prepared according to the manufacturer instructions from the commercial material. The (27)Al and (19)F MAS-NMR studies were carried out on powders made after terminating the setting reaction. RESULTS (27)Al MAS-NMR results show conversion of aluminium from the glass phase, where it has coordination number four, Al(IV), into the cement matrix where it has a coordination number of six, Al(VI). At least two different Al(VI) species were detected at short ageing time cements. Assignment of these species is discussed and compared with the data from other sources. The possibility for a condensed aluminium species [Al(13)(OH)(24)(H(2)O)(12)](7+) to form is considered. The ratio of aluminium in the cement, Al(VI), to the remaining unreacted in glass has been evaluated by deconvolution of the spectra. Various theoretical ratios of aluminium species in the cement matrix to the unreacted ones remaining in glass have been estimated. The (19)F MAS-NMR spectra are identical for the glass and cements at the early times and contain a dominant signal assigned to Al-F-Sr(n). CONCLUSIONS The data confirms that the conversion of aluminium is a diffusion-controlled process at early stage less than 1h and it is largely complete between 1 and 6h. The comparison with the experimental data shows that the majority of aluminium cations do not form tricarboxylates but are coordinated with one or two carboxylic groups and other ligands. Insufficient amount of water and excess of glass in this cement formulation affect glass degradation mechanism.

The Effect of δ Phase on the Mechanical Properties of an Inconel 718 Superalloy

The relationship between the δ phase content and the mechanical properties of Inconel 718 superalloy is still uncertain in the scientific literature. This study investigated the effects of the amount of δ phase and of the grain size on the mechanical properties of an aged γ-matrix with γ′ and γ″ precipitates. The material in as-received condition in the form of a forged bar was solution-treated in different conditions and aged according to UNS7718 standard. The microstructures were characterized using optical, scanning, and transmission electron microscopy. Hardness and tensile tests were also conducted. After solution treatment, γ′ and γ″ phases are dissolved and δ phase volume fraction is reduced to a minimum amount only observed by TEM, resulting in an increase of the grain size and a decrease of hardness and strength. After aging, the precipitation of γ′ and γ″ occurs and the amount of δ phase increases. The volume fraction of δ phase varying from 0.30 to 1.38% and the grain size varying from ASTM 7 to 5 do not have a significant effect on the tensile properties and hardness.

Microstructural modifications induced by hydrogen in a heat resistant steel type HP-45 with Nb and Ti additions

Abstract Heat resistant stainless steel type HP45 modified by Nb and Ti additions, which is commonly used in chemical and petrochemical industrial plants, was submitted to hydrogen treatment at low and ultra-high pressures. In the low-pressure experiments, 0.1 Pa, hydrogen was supplied in a quartz tube and the material was heat-treated for 100 h at 1200 K. In the high-pressure experiments, 5 GPa, hydrogen was supplied in a NaCl container and the material heat-treated at 873 and 1073 K for 1 h. A coarsening of the M 23 C 6 (M=Fe, Cr, Ni) carbides, was observed in both cases, these being of a sharper shape for the low hydrogen pressure condition, compared with previously aged samples. In the high pressure experiments, where the atomic mobility is higher, a more intense coarsening of the carbides and the cracks associated with them was observed, which is deleterious for mechanical properties. The hydrogen-induced phase coarsening can be understood in terms of the vacancy formation that occurs under such conditions.

Analysis of Grain Boundary Character in a Fine-Grained Nickel-Based Superalloy 718

Abstract In the current work, sheets of superalloy 718 were processed via thermomechanical route by hot and cold rolling, followed by annealing below the δ phase solvus temperature and precipitation hardening to optimum strength. Grain boundary character distribution throughout the processing was mapped via EBSD and its evolution discussed. The results show that it is possible to process the alloy to a fine grain size obtaining concomitantly a considerably high proportion of special boundaries Σ3, Σ9, and Σ27. The precipitation of δ phase presented a strong grain refining role, without significantly impairing the twinning mechanism and, consequently, the Σ3, Σ9, and Σ27 boundary formations.

Failure of a Bucket-Wheel Stacker Reclaimer: Metallographic and Structural Analyses

The structural collapse of an iron-ore bucket-wheel stacker reclaimer at the beginning of operation was investigated by means of mechanical tests, microstructural characterization, and computational structural analysis. The mechanical failure was a consequence of a brittle fracture by cleavage. The crack followed the heat-affected zone of a welded joint connecting a rectangular hollow section member and a plate flange. The main factors contributing to failure were related with a combination of design-in and manufacturing-in factors like high load–strength ratio at the point of failure, local stress concentration as a result of geometry restrictions, and weld defects. This particular section was responsible for the load transfer between the front tie member and the boom extremity, and its failure was the main cause of the catastrophic failure of the equipment.

Transmission electron microscopy characterization of a Nb microalloyed steel for carburizing at high temperatures

The carbide particles in AISI 5115 steel modified with added 0.038 wt.% Nb were characterized by means of transmission electron microscopy after simulation of carburizing heat treatments at high temperatures. The advantage of a high-temperature carburizing treatment over the standard 930 °C heat treatment is the significant reduction in the processing time. However, higher carburizing temperature leads to general and abnormal grain growth, which must be avoided by grain-pinning precipitates. The carburizing heat treatment simulations were performed at 950, 1000, and 1050 °C for 1 and 2 h in samples with two prior microstructural conditions: hot rolled and spheroidized. Characterization of the precipitates regarding composition, morphology, size, and distribution was performed using carbon extraction replicas and thin foil specimens. The results confirm the existence of complex Nb carbides. Particle size distribution curves, as log-normal functions, are shown to be extremely important in the interpretation of grain boundary pinning, rather than the mean particle size. The spheroidized samples showed a susceptibility to abnormal grain coarsening.

Correlation between molten vanadium salts and the structural degradation of HK-type steel superheater tubes

HK steels are among the most used heat-resistant cast stainless steels, being corrosion-resistant and showing good mechanical properties at high service temperatures. These steels are widely used in reformer furnaces and as superheater tubes. During service, combustion gases leaving the burners come in contact with these tubes, resulting in corrosive attack and a large weight loss occurs due to the presence of vanadium, which forms low melting point salts, removing the protective oxide layer. In this work the external surface of a tube with dramatic wall thickness reduction was analyzed using light microscopy, scanning electron microscopy, and transmission electron microscopy. The identification of the phases was achieved by energy dispersive spectroscopy (EDS) analyses. The results showed oxides arising from the external surface. In this oxidized region vanadium compounds inside chromium carbide particles were also observed, due to inward vanadium diffusion during corrosion attack. A chemical reaction was proposed to explain the presence of vanadium in the metal microstructure.

Effects of Carbon and Nitrogen Levels on the Temperature Ranges for Serrated Flow in Austenitic Stainless Steel

ABSTRACT The ranges of occurrence of serrated flow as a function of temperature and strain rate are reported for two type 304 stainless steels having different carbon and nitrogen contents. Analysis indicates that the low temperature serrations are controlled by diffusion of C or N; their athermal termination depends on exhaustion of available interstitials. The higher temperature serrations depend on locking of dislocations by C-C pairs with interaction with Cr, while their termination is believed to relate to carbide formation.

Failure Analysis of a Copper Tube for Pneumatic System

Deoxidized-high-phosphorous (DHP) copper tubes are widely and successfully used in different applications as: water distribution, air-conditioning, solar heating, fuel, oil, and natural gas services and compressed air systems. In the present analysis, a failure of a copper tube composing a pneumatic system of a subway train was investigated by means of on-site examination, metallographic and fracture analyses. Results show that arcs coming from an exposed power cable lead to partial melting of a section of the tube, resulting in loss of pressure and interruption of the train.