Papers by Keyword: Embrittlement

Abstract: Hot-dip process leading to creation of protective coating is one of the effective ways to protect metals from corrosion. It provides the way to obtain the coating with required physical, mechanical and chemical properties. However, contact of solid metal part with the liquid metal bath during this process may lead to changes in mechanical properties or, in extreme cases, to premature failure of the part. This is mainly due to the effect called LME. The author has researched the likelihood of occurrence of this phenomenon in parts subjected to metallization in zinc bath or in zinc bath with additions of tin or bismuth. The presented results are a continuation of these research and include an assessment of the possibility of LME occurrence in parts metallized in tin bath and subjected to external stress.

Abstract: A thermohydrogen process promoting metastable phase decomposition (THP-MD) treatment was performed on wrought Ti-6Al-4V to determine the effects of microstructure evolution on tensile ductility. Tensile ductility was affected by the nature of phase and morphology evolution in which dissolved hydrogen played a key role. Hydrogen reduced the beta transus and stabilised more beta phase at aging/tempering temperature. A reduced beta transus in a similar heat treatment resulted in a bimodal morphology (in non-hydrogenated samples) or a fully acicular morphology (in hydrogenated samples). It also reduced the volume fraction of alpha at aging/tempering temperature which resulted in the extensive enrichment of reduced alpha with aluminium (Al) during tempering. The increased Al content in the reduced alpha promoted ordering of the HCP lattice to the brittle titanium aluminide (Ti₃Al) phase. In addition to Ti₃Al embrittlement, the acicular morphology of Ti-6Al-4V tempered hexagonal martensite (ά) offers limited resistance to crack propagation. The highest degree of embrittlement was observed in prior hydrogenated samples because of the combined effect of the acicular morphology and Ti₃Al embrittlement.

Abstract: Two types of steels used for production of heavy forgings were selected for the experimental evaluation of the effect of long time dwell at elevated temperatures and cooling during heat treatment on their mechanical properties in order avoid the possibility of grain boundary embrittlement. Samples from evaluated steels 26NiCrMoV14-5 and 22CrNiMoWV8-8 were austenitized for 2 hours at temperature of 1200°C and oil quenched. Subsequently the annealing at temperatures (200 - 700)°C for 1 hour and 100 hours was applied. Selected mechanical properties, especially hardness and impact energy, were monitored. It was found that for steels 26NiCrMoV14‑5 and 22CrMoNiWV8-8 exist the temperature intervals (300 - 400)°C and (500 ‑ 600)°C respectively with the possible potential for toughness decreasing.

Abstract: Article presents the state of knowledge relating occurrences of liquid metals embrittlement. The results of experimental investigations of the LME phenomenon, reasons of its formation and influence of different parameters are described. In order to determine the basic mechanical properties of materials intended for research static tensile test was performed. Samples made of C66D and C70D steels with tensions stretching (500-800 MPa) at different values and kinds of loading during the hot dip metallization were investigated. Coating processes were made in zinc with 0.5% Bi addition and results were compared to zinc coating. The processes of hot dip metallization were done at 450°C temperature and immersion time 180 s. Coated samples were investigated by light microscope to specify possibility appear of LME effect.

Abstract: The article presents investigation of steel subjected to external stress during hot-dip metallization. The results of experimental investigations of the LME phenomenon, reasons of its formation and influence of different parameters are described. Samples made of C70D steel subjected to various loads inducing tensile stress (400-800 MPa) during hot dip metallization ina zinc bath with 2% tin addition were investigated. The processes of hot dip metallization were carried out at 450°C temperature and immersion time of 180 s. It can be observed that the stress value can affect the behaviour of metal during the hot dip metallization process. Coated samples were analysed on a light microscope to specify the possibility of the occurrence of LME. These studies provided the basis for the selection of samples and areas for further quantitative andqualitative analysis on a scanning electron microscope (SEM) with a microanalysis system. The analysis of cracks in three microareas revealed that the tin content in the front of a fracture is twice as big as it is in other measuring points. It may be related to crack propagation due to the phenomenon of LME, by the accumulation of the initiating element atoms and those propagating the fracture.

Abstract: Service life of VVER-type nuclear reactor is limited by decrease in brittle fracture resistance of reactor pressure vessel produced of low-alloy low-carbon steel under effect of irradiation and/or elevated temperatures. In this work fracture surfaces were studied by Auger-electron spectroscopy in order to estimate the contribution of intergranular embrittlement to the degradation of reactor pressure vessel steels under the influence of operating conditions. It was demonstrated that irradiation induced segregation leads to an increase of P content in grain boundaries that promotes intergranular brittle fracture on fracture surfaces. The similar effect but to a lesser degree was shown in the case of long-term temperature exposure. The grain boundary structure was examined and an effect of carbides located on the grain boundaries is supposed due to increased phosphorus segregation on carbide/matrix interface boundaries.

Abstract: The complexity of calibration procedure for determination of Weibull distribution parameters as well as necessity to perform FEM calculations is one of the shortcomings of conventional versions of Local Approach (LA). This report presents an attempt to ascertain interrelation between local and global characteristics of fracture of pre-cracked and notched specimens of reactor pressure vessel steels, high-strength steels etc. Criterion of cleavage fracture initiation ahead of a crack (notch) is formulated. Dimensionless parameter characterizing the effect of embrittlement by the crack or notch is proposed on the basis of this criterion. Based on the findings of low-temperature tests of pre-cracked or notched specimens of structural steels, it is proved that this parameter may be determined by both local and global characteristics.

Abstract: Tempering of alloy steels in the temperature range of 400-600 °C causes temper embrittlement i.e. decrease in notch toughness of the material and the nil ductility temperature is raised to room temperature and above. The fracture in temper-embrittled steel is intergranular and propagates along prior austenitic grain boundaries. The embrittlement occurs only in the presence of specific impurities, e.g. P, Sn, Sb and As. These elements have been shown to segregate along prior austenite grain boundaries during tempering. Similar type of temper embrittlement can occur in martensitic stainless steel (SS) if tempered in the temperature range of 450-600 °C. This paper reports a case of failure of components made from martensitic SS 420 due to temper embrittlement. These components were subjected to a temperature of 120 °C in the initial stages of service and had shown brittle fractures. Scanning electron microscopic examination of the fracture surface of both the components showed intergranular fracture. The microstructures of the failed components confirmed that the materials were in hardened and tempered condition. In addition, the microstructure revealed both intergranular corrosion (IGC) and intergranular cracking. The electron backscatter diffraction study also showed retained austenite in the first components material. The material undergoing IGC might be related to a wrong heat-treatment during fabrication and subsequent pickling procedures. To confirm this, a sample each from both the components was exposed to 5% nitric acid solution at 25 °C. The results showed very high corrosion rate and the attack was intergranular in nature. The failure of both the components was concluded to be due to wrong tempering treatment in the temperature range of 450-600 °C that cause grain boundaries to become susceptible to embrittlement and corrosion.

Abstract: Based primarily on microstructure, five stainless steel types are recognized: ferritic, martensitic, austenitic, duplex and precipitation-hardening. The major problem in ferritic stainless steels is the tendency to embrittlement, aggravated by various causes. During welding, control of heat input is essential and, in some cases, also a postweld heat treatment. The austenitic type is the easiest to weld, but two important issues are involved in the welding of these steels: hot cracking and formation of chromium carbide and other secondary phases on thermal exposure. The nature of the problems and remedial measures are discussed from a metallurgical perspective. Duplex stainless steels contain approximately equal proportions of austenite and ferrite. The article discusses the upset in phase balance during welding both in the weld metal and heat-affected zone and the formation of embrittling secondary phases during any thermal treatment. Martensitic stainless steels are susceptible to hydrogen-induced cracking. Welding thus involves many precautions to prevent it through proper preheat selection, postweld heat treatment, etc. In the welding of precipitation-hardening stainless steels, it is usually necessary to develop in the weld metal strength levels matching those of the base metal. This is achieved by applying a postweld heat treatment appropriate to each type of alloy.

Abstract: This paper summarizes the results of the studies on fracture mechanics characterisation of SS 316L(N) and its welds. The results presented include the fracture toughness and FCG properties of the base and weld materials at different temperatures. Influence of nitrogen content on the base material properties is discussed. Further, the effects of long-term ageing at different temperatures on the fracture and FCG behaviour of the welds are presented and discussed. The weld metal has been subjected to extended thermal ageing, and a detailed study has been undertaken to characterize the (i) FCG properties and (ii) quasistatic J-R curves for the indigenously developed SS 316(N) weld material at both ambient and service temperatures. The ageing conditions covered include the advanced ageing according to the RCC-MR design code, i.e, > 4000 h at 923 K and the low temperature ageing, i.e., 643-823 K the operating range for the SS 316L(N) components in PFBR. The results are discussed in detail in the light of microstructural changes taking place in the weld metal and their influence on the operating micromechanisms.