Papers by Keyword: Embrittlement

Abstract: One of the prime mechanical properties of any steel is impact strength, which may change dramatically due to the thermal treatments. The present investigation covers the influence of different thermally treated conditions on the impact strength of AISI 1020 steel. The thermal conditions included are: as rolled, oil quenched & tempered, air quenched, normalized and annealed condition. To evaluate the characteristics of the steel in different thermal conditions; optical microscopy, scanning electron microscopy, hardness testing and room temperature impact testing were carried out. Experimental results elaborated the existence of a narrow temperature range (250 ± 50 °C), which may decrease the impact strength drastically, when the steel is heat treated. In this temperature range toughness scavengers i.e., As, S, Sb etc. are precipitated on the grain boundary, which cause the noticeable decrease in impact strength. Optimum impact strength could be achievable where this critical temperature range was avoided.

Abstract: The influence of the parameters of the vacuum annealing mode on the thickness of the embrittled layers, formed on the surface of titanium as a result of its interaction with the residual gases of the vacuumed space, is studied. The thickness and structure of the layers were determined on samples made of VT6 alloy obtained from sheet metal with a thickness of 3 mm. Annealing of samples in the temperature range of 500-750 °C was performed with air dilution from 10 to 3∙10^-2 PA. The dimensions of the embrittled layers were determined by measuring the zone of brittle crack propagation in the fracture of the samples, and measuring the distance between the surface cracks in the embrittled layers, formed during bending deformation. To quantify the effect of vacuum annealing modes of sheet titanium alloy VT6 on the depth of the embrittled part of the formed oxide layer, it is proposed to use a parabolic relationship, characterized by the degree of growth and the constant of the embrittled layer. By processing experimental data, the effect of annealing time, temperature, and air dilution on the growth kinetics of the embrittled layers was established. Based on the obtained kinetic regularities of the growth of the embrittled layers, nomograms are constructed, to determine the size of the embrittled layer formed at the heating stage at different speeds up to the specified annealing temperature.

Abstract: X-ray structural studies have shown that with the increase of the life of the pipes, the surface layers of the metal adsorb a significant amount of gases (hydrogen, oxygen and sulfur), especially this is strongly manifested in the field of corrosion defects (ulcers, pitting). Also, it was found that in the process of long-term operation, the metal of the pipe walls is heavily flooded, which directly leads to its brittle and, as a consequence, increased hardness and reduced ductility. Such studies are unique in the subject of corrosion-mechanical resistance of metal and its degradation during long-term operation in the presence of alternating loads and at the same time, the aggressive environments of oil and gas fields.

Abstract: Beta titanium alloys have several attractive features; this has resulted in this group of alloys receiving much attention since 1980’s. Among the attributes which distinguish them for their superiority over other structural materials are (i) high strength to which they can be heat treated, resulting in high strength to weight ratio (ii) high degree of hardenability which enables heat treatment in large section sizes to high strength levels (iii) excellent hot and cold workability, making them as competitive sheet materials etc. The standard heat treatment consists of solution treatment in beta or alpha plus beta phase field followed by aging. However, certain aging treatments can render the materials in a state of little or no ductility; the designer has to be aware of this behaviour and has to keep away from such treatments while working with the materials. Such unfavourable aging treatments may adversely affect not only the static properties such as reduction in area and elongation in a tensile test, but also dynamic properties such as impact toughness. Results of fractographic studies are in line with those of mechanical testing. The authors would present the foregoing analysis, based primarily on the wide-ranging researches they carried out on beta titanium alloy Ti15-3 and to some extent data published by researchers on other grades of beta titanium alloys. An attempt is made to explain the mechanisms underlying the embrittlement reactions that take place in beta titanium alloys under non-optimal aging treatments.

Abstract: The stress corrosion cracking (SCC) sensitivity of aged Z3CN20.09M stainless steel (SS) was investigated by scanning electrochemical microscopy (SECM), nanoindenter and slow strain rate tensiometer (SSRT). The results indicated that the hardness and modulus of the samples increased with increasing aging time at 475 °C, which increased about 80% and 15% respectively after aging for 1000h. After immersed in NaCl+HCl solution (pH=2) for 7 h, the pitting corrosion point appeared of specimen aged for 1000h. While it was not found even for 24 h of as-received (AR) and aged for 500 h specimens at the same condition. The SCC sensitivity was increased about one time of the specimen aged for 1000 h tested in 300 °C high temperature water. The combined effect associated the embrittlement of mechanical properties and the decrease of corrosion resistance is regarded as the main reason for the increasing of SCC sensitivity.

Abstract: The complex research of influence of a structural condition of metal on resistance to corrosion destruction of the low-alloyed steel, operated in biocorrosion environments, is conducted. It is established that the ferritic component of a structure resists better in the environment modeling conditions of impact of biogenous gases on metal under the exfoliated anticorrosive covering. Ferrite – carbide mix, in the form of beynit, has a bigger resistance in the environment reproducing conditions of lack of biogenous factors. Existence in beynitny structure of a martensitic component leads to the largest resistance of biocorrosion in the environment modeling conditions of anaerobic corrosion with biogenous gases.

Abstract: The effect of long dwell time at elevated temperatures on two types of steels used for production of heavy forgings was evaluated. Microstructural changes and mechanical properties were monitored in temperature interval of 200 - 700 °C in order to avoid the possibility of grain boundary embrittlement during long dwell time at the processing temperature. Samples of the evaluated steels 26NiCrMoV14-5 and 22CrNiMoWV8-8 were being austenitized for 2 hours at temperature of 1200 °C and oil quenched. Subsequently the annealing at selected temperatures for 100 hours was applied. Tensile test, hardness and impact energy measurements were used for the evaluation. The results of mechanical testing, structural and fracture surface analyses indicate that for steels 26NiCrMoV145 and 22CrMoNiWV8-8 there exist a temperature interval of 300 - 400 °C and 500 600 °C respectively showing the toughness decrease.

Abstract: There are many welding methods which have been currently performed to prolong the life time of exhaust valve of marine engine from an economic point of view. In this study, one of these welding methods, plasma transferred arc (PTA) welding was performed at the base metal of Nimonic 80A which would be used as the material of exhaust valve with three kinds of filler metals such as Stellite 6, Inconel 625 and Inconel 718. The mechanical and corrosion characteristics were investigated with electrochemical and wear loss test methods. The Nimonic 80A as the base metal had a better corrosion resistance than those of the weld metals welded with three types of the filler metals. However, after post weld heat treatment, all these weld metals mentioned above exhibited better corrosion resistance compared to the base metal, shifting the corrosion potential to the noble direction, and pitting corrosion was more or less observed at the surface of Nimonic 80A after post weld heat treatment. In particular, Inconel 625 showed the best corrosion resistance among the filler metals after heat treatment. The wear ratio of Stellite 6 showed the lowest value due to its highest hardness among the filler metals, however, its ratio by cavitation test exhibited the highest value. It is considered that increasing of embrittlement with increasing the hardness was resulted in increasing the wear loss by cavitation test.

Abstract: The paper presents the results of the analysis of secondary phases formed during thermal exposure in creep resistant austenitic steels of SUPER 304H and HR3C types. These steels were worldwide used for construction of the superheaters (the heat exchangers) of supercritical and ultra-supercritical (USC) coal-fired power plants. In order to accelerate precipitation processes, the steels were isothermally aged at 675 °C for 20 000 h. The investigations of the precipitates were primarily focused on the occurrence of brittle phases. Changes in mechanical properties caused by occurrence of secondary phases were documented by tensile testing and measurement of the impact strength. For comparison reasons, the impact tests of the initial state of the steels were also performed. The results showed that long-term ageing had led to the significant decrease of the impact strength. Correlation between precipitation of secondary phase and measured values of mechanical properties is shown. The influence of brittle phases on long-term durability of the degraded steel regarding its insufficient impact strength has been discussed.

Abstract: The effect of interstitial and substitutional impurities on grain boundary (GB) cohesion in the series of B2-TiMe alloys is studied from first principles using pseudopotential approach. It is shown that the TiMe Σ5(310) symmetrical tilt GB cohesion is reduced by the segregation of hydrogen while it is increased due to boron or carbon segregation. We analyze also the combined and accumulation effect of interstitial B (C) and H impurities on the change of the Griffith work.