Luiz Henrique de Almeida

Microstructural characterization of modified 25Cr-35Ni centrifugally cast steel furnace tubes

HP steels modified with Nb and Nb-plus-Ti, produced in the form of centrifugally cast tubes, were characterized by means of light optical microscopy, scanning electron microscopy (SEM) with secondary and backscattered electron imaging, and transmission electron microscopy (TEM) with energy-dispersive spectroscopy (EDS) and selected area diffraction. A complete description of the phases present in the as-cast, welded, and aged condition was made as a function of the modifying elements. The chromium carbide stoichiometry, secondary precipitation, NbC instability at elevated temperatures, the G-phase transformation, and the effect of Ti on this reaction are discussed. It is indicated that partial transformation of G-phase in the alloy modified with Ti is one of the explanations for the superior creep properties observed in this material.

Niobium additions in HP heat-resistant cast stainless steels

Abstract The microstructures of HP-type heat-resistant cast steels, having niobium additions up to 2 wt.%, were examined in the as-cast condition and after artificial aging at 700, 900, and 1100°C. Microstructural studies were conducted using scanning transmission electron microscopy and x-ray diffraction. Instability of NbC between 700 and 1000°C occurs in these alloys, with partial transformation to a nickel-niobium silicide, G phase. A time-temperature-transformation diagram has been constructed for the 1.97% Nb alloy. Creep tests were made on the as-cast and aged material. Niobium containing alloys aged at 700 and 900°C display superior creep properties to those of the as-cast material or material aged at 1100°C. The results of the creep tests have been related to the microstructural changes taking place upon aging.

Effects of Si content on the microstructure of modified-HP austenitic steels

Abstract The microstructures of two centrifugally cast modified-HP steels with 1.97 wt.% Nb and two different Si contents, 1.84 and 2.62 wt.%, were investigated using light and electron microscopy and x-ray diffraction. As noted in a previous study, the instability of the niobium carbide at elevated temperature is a particular feature of these alloys, exhibiting a partial transformation to a nickel-niobium silicide, G phase. The time-temperature-transformation diagram shows that the time to reach the nose of the transformation curve decreases with increase in silicon content. After aging at 900°C for 1000 h, the alloy with higher silicon content showed extensive secondary precipitation, and at teh same time the niobium carbide particles were almost completely transformed to G phase.

Role of Nb in modifying the microstructure of heat-resistant cast HP steel☆

Heat-resistant cast stainless steels have widespread uses in the petrochemical industry in pyrolysis and reformer furnaces; 25%Cr-20% Ni or 25% CR-33% NI high-carbon steels, designated HK and HP types, respectively, have traditionally been used, but requirements for higher productivity have raised the demand for improved performance. In the more recently developed steels, the Cr carbides normally present are replaced by carbides of Nb or Ti, or both, and the resulting alloys are termed modified HK or modified HP steels. The additions of Nb or Ti provide improved structural stability at high temperature in that their carbides are more stable than the normal secondary carbide precipitates of Cr, thus contributing to restraint of dislocation movement. Perhaps more important still is the eutectic modification caused by Nb additions that could restrict grain boundary sliding. This article reports on an investigation into the role of Nb in modifying the microstructure of HP steel in the as-cast and artificially aged conditions, for a series of alloys utilizing different levels of Nb.

Phase characterization in two centrifugally cast HK stainless steel tubes

Abstract The precipitates present in two HK steels after exposure at 890°C for some 60,000 h have been identified. In the standard HK steel, primary and secondary chromium carbides of M 23 C 6 type were observed. In the second alloy, having niobium additions, three precipitates were found, namely M 23 C 6 , NbC, and an intermetallic NiNbSi phase corresponding to G phase. It is suggested that the formation of the intermetallic is related to instability of the NbC with long-time exposure.

Effects of microstructural parameters on the mechanical properties of eutectoid rail steels

A study was made of the effects of microstructural variations, as produced by differences in heat treatment and the addition of Nb, on the mechanical properties of eutectoid pearlitic carbon steels. The pearlite interlamellar spacing increased with an increase in autenitizing temperature and, to a lesser extent, with the addition of Nb. The tensile strength depended strongly on microstructure, being higher for larger prior austensite grain size and for smaller pearlite interlamellar spacings. Ductility increased with a decrease in austenite grain size. In fatigue, the interlamellar spacing is an important variable with respect to crack initiation, the material with the largest spacing having the lowest resistance to initiation, despite the fact that it does not have the lowest yield stress. Fatigue crack propagation in stage II was found to be insensitive to variations in interlamellar spacing.

Athermal and temperature dependent behavior of serrated flow in an austenitic stainless steel

Abstract It has been shown that the occurrence of serrated flow during the high temperature tensile deformation of an austenitic stainless steel takes place over distinct temperature ranges, these depending on strain rate. The initiation of lower temperature serrated flow appears to correlate with vacancy diffusion, while its termination is thermal. The beginning of the high temperature serrated flow may be related to the diffusion of carbon or nitrogen, while its cessation is apparently related to chromium diffusion.

Maximisation of the ratio of microhardness to the Young's modulus of Ti-12Mo-13Nb alloy through microstructure changes.

Alloys for orthopaedic and dentistry applications require high mechanical strength and a low Youngs modulus to avoid stress shielding. Metastable β titanium alloys appear to fulfil these requirements. This study investigated the correlation of phases precipitated in a Ti-12Mo-13Nb alloy with changes in hardness and the Youngs modulus. The alloy was produced by arc melting under an argon atmosphere, after which, it was heat treated and cold forged. Two different routes of heat treatment were employed. Phase transformations were studied by employing X-ray diffraction and transmission electron microscopy. Property characterisation was based on Vickers microhardness tests and Youngs modulus measurements. The highest ratio of microhardness to the Youngs modulus was obtained using thermomechanical treatment, which consists of heating at 1000°C for 24h, water quenching, cold forging to reduce 80% of the area, and ageing at 500°C for 24h, where the final microstructure consisted of an α phase dispersed in a β matrix. The α phase appeared in two different forms: as fine lamellas (with 240±100 nm length) and massive particles of 200-500 nm size.

Hydrogen Diffusivity and Hydride Formation in Rich-Zirconium Alloys Used in Nuclear Reactors

Hydrogen gas permeation tests were performed on two Nb-modified Zr alloys used in the nuclear industry. The influence of the microstructure on hydrogen diffusivity in each alloy is discussed and a mechanism of hydride formation is presented. The hydrogen binding energy for different trap sites was calculated in the M5 alloy as well as hydrogen diffusivity value in the Zirlo alloy at 300oC, D = 2.5 x 10-13 m2/s.

Mechanical Characterization of Ti–12mo–13nb Alloy for Biomedical Application Hot Swaged and Aged

Beta titanium alloys were developed for biomedical applications due to the combination of its mechanical properties including low elasticity modulus, high strength, fatigue resistance, good ductility and with excellent corrosion resistance. With this perspective a metastable beta titanium alloy Ti-12Mo-13Nb was developed with the replacement of both vanadium and aluminum from the traditional alloy Ti-6Al-4V. This paper presents the microstructure, mechanical properties of the Ti-12Mo-13Nb hot swaged and aged at 500 °C for 24 h under high vacuum and then water quenched. The alloy structure was characterized by X-ray diffraction and transmission electron microscopy. Tensile tests were carried out at room temperature. The results show a microstructure consisting of a fine dispersed α phase in a β matrix and good mechanical properties including low elastic modulus. The results indicate that Ti-12Mo-13Nb alloy can be a promising alternative for biomedical application.