Microstructure influence on ductile to brittle transition temperature of ADI materials

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

An austempering study of ductile iron alloyed with copper

Austempered ductile iron (ADI) has proved to be an excellent material as it possesses attractive properties: high strength, ductility and toughness are combined with good wear resistance and machinability. These properties can be achieved upon adequate heat treatment which yields the optimum microstructure for a given chemical composition. In this paper the results of an investigation the austempering of ADI alloyed with 0.45 % Cu for a range of times and temperatures are reported. The microstructure and fracture mode developed throughout these treatments have been identified by means of light and scanning electron microscopy and X-ray diffraction analysis. It was shown that the strength, elongation and impact energy strongly depend on the amounts of bainitic ferrite and retained austenite. Based on these results, and optimal processing window was established

Microstructure and fracture mode of unalloyed dual-phase austempered ductile iron

Dual-phase ADI material microstructure consists of different amounts and morphologies of ausferrite and free ferrite, obtained by subjecting ductile iron to specific heat treatment. Its strength is lower compared to comparable ADI materials but exhibits a higher ductility, the major disadvantage of ADI. In the current study, an unalloyed ductile iron was intercritical austenitized in two-phase regions (??+??) at four temperatures from 840 to 780 ??C for 2 h and austempered at 400 ??C for 1 h to obtain dual-phase ADI with different percentages of free ferrite and ausferrite. Light and scanning electron microscopy was performed for the metallographic and fracture studies, respectively. Microscopy results were correlated to tensile testing results. The results indicated that, as the amount of ausferrite present in the matrix increases, higher values of strength and lower ductility are obtained. The fracture surfaces of dual-phase ADI microstructures with 22.8% of ausferrite in their matrix have regions of quasi-cleavage fracture around last-to-freeze zones, related to the presence of ausferrite in those areas. The specimens with the highest values of ausferrite of 86.8% among the dual-phase microstructure have a dominant quasi-cleavage type of fracture

Ballistic Behaviour of Austempered Compacted Graphite Iron Perforated Plates

In this study, the performance of austempered compacted graphite iron was evaluated to find its suitability as perforated plates used in add-on armour. Perforated compacted graphite plates were subjected to austenitisation at 900 degrees C for 2 h followed by austempering at 275 and 400 degrees C for 1 h. The basic plate was fixed at 400 mm away from the perforated plate and armour and then piercing incendiary projectile was shot from a distance of 100 m. It was observed that both 7 mm and 9 mm perforated plates austempered at lower temperature of 275 degrees C producing higher hardness and lower ductility were effective in fracturing the penetrating core, thereby significantly decreasing the chances of penetrating the basic plate

Ballistic Behaviour of Austempered Compacted Graphite Iron Perforated Plates

In this study, the performance of austempered compacted graphite iron was evaluated to find its suitability as perforated plates used in add-on armour. Perforated compacted graphite plates were subjected to austenitisation at 900 °C for 2 h followed by austempering at 275 and 400 °C for 1 h. The basic plate was fixed at 400 mm away from the perforated plate and armour and then piercing incendiary projectile was shot from a distance of 100 m. It was observed that both 7 mm and 9 mm perforated plates austempered at lower temperature of 275 °C producing higher hardness and lower ductility were effective in fracturing the penetrating core, thereby significantly decreasing the chances of penetrating the basic plate

Mikrostrukturni razvoj i degradacija mehaničkih svojstava legure HPNB posle jedanaest godina eksploatacije

In this work, the heat-resistant cast steel HP40Nb alloy, produced in the form of a centrifugally cast tube, which failed after 11.4 years service is investigated. The microstructure is examined using a scanning electron microscope (SEM), while the phases observed are analysed using an energy dispersive X-ray analyser system (EDS). Additionally, fractography is performed on the fractured surface of failed specimens. Mechanical properties of the tube are evaluated by using tensile and Charpy impact testing at room temperature (298 K). Main results indicate that the failed tube microstructure consists of an austenitic matrix and a continuous network of primary eutectic carbides of two types: one rich in Nb (bright particles), and one rich in Cr (dark particles). These carbides are NbC and complex M7C3 (M = Cr, Ni, Fe) type. The brittle intergranular fracture mode is explained by the presence of massive precipitation and coarsening of intergranular carbides at grain boundaries. Lower tensile properties of the in-service exposed tube might be correlated to the morphological, chemical, and distribution changes of precipitates during service at high temperature. Decreased values of hardness can be attributed to increase in grain size, as well as due to the decomposition of Cr-carbide during the heat treatment at temperatures above 1073 K.U ovom radu је ispitivana legura od livenog čelika HP40Nb otpornog na toplotu, proizvedena u obliku centrifugalno livene cevi, koja je otkazala posle 11,4 godina eksploatacije. Mikrostruktura je ispitivana primenom skenirajućeg elektronskog mikroskopa SEM, dok su uočene faze analizirane korišćenjem sistema energetski disperzivnog rendgenskog analizatora (EDS). Dodatno, fraktografija je urađena na površini loma oštećenih uzoraka. Mehanička svojstva su procenjena ispitivanjem na zatezanje i ispitivanjem energije udara po Šarpiju na sobnoj temperaturi (298 K). Glavni rezultati su pokazali da se mikrostruktura oštećene cevi sastoji od austenitne osnove i neprekidne mreže primarnih eutektičkih karbida dva tipa: jednog bogatog Nb (svetle čestice) i jednog bogatog Cr (tamne čestice). Oba karbida su tipa NbC i složeni M7C3 (M = Cr, Ni, Fe). Mehanizam krtog intergranularnog loma objašnjen je prisustvom masivnog taloženja grubih intergranularnih karbida na granicama zrna. Niža zatezna svojstva cevi izložene eksploataciji mogu biti u korelaciji sa morfološkim, hemijskim i distributivnim promenama taloga tokom eksploatacije na visokoj temperaturi. Smanjenje vrednosti tvrdoće može se pripisati povećanju veličine zrna kao i razlaganju Cr-karbida tokom termičke obrade na temperaturama iznad 1073 K

The Effect of Water Concentration in Ethyl Alcohol on the Environmentally Assisted Embrittlement of Austempered Ductile Irons

Austempered ductile iron (ADI) is an advanced cast iron material that has a broad field of application and, among others, it is used in contact and for conveyance of fluids. However, it is noticed that in contact with some fluids, especially water, ADI material becomes brittle. The most significant decrease is established for the elongation. However, the influence of water and the cause of this phenomenon is still not fully understood. For that reason, in this paper, the influence of different water concentrations in ethyl alcohol on the mechanical properties of ADI materials was studied. The test was performed on two different types of ADI materials in 0.2, 4, 10, and 100 vol.% water concentration environments, and in dry condition. It was found that even the smallest concentration of water (0.2 vol.%) causes formation of the embrittled zone at fracture surface. However, not all mechanical properties were affected equally and not all water concentrations have been critical. The highest deterioration was established in the elongation, followed by the ultimate tensile strength, while the proof strength was affected least

The Catalytic Effect of Vanadium on Sorption Properties of MgH2-Based Nanocomposites Obtained Using Low Milling Time

The effects of catalysis using vanadium as an additive (2 and 5 wt.%) in a high-energy ball mill on composite desorption properties were examined. The influence of microstructure on the dehydration temperature and hydrogen desorption kinetics was monitored. Morphological and microstructural studies of the synthesized sample were performed by X-ray diffraction (XRD), laser particle size distribution (PSD), and scanning electron microscopy (SEM) methods, while differential scanning calorimetry (DSC) determined thermal properties. To further access amorph species in the milling blend, the absorption spectra were obtained by FTIR-ATR analysis (Fourier transform infrared spectroscopy attenuated total reflection). The results show lower apparent activation energy (Eapp) and H2 desorption temperature are obtained for milling bland with 5 wt.% added vanadium. The best explanation of hydrogen desorption reaction shows the Avrami-Erofeev model for parameter n = 4. Since the obtained value of apparent activation energy is close to the Mg-H bond-breaking energy, one can conclude that breaking this bond would be the rate-limiting step of the process

Transition temperature and fracture mode of as-castand austempered ductile iron

The ductile to brittle transition temperature is a very important criterion that is used for selection of materials in some applications, especially in low-temperature conditions. For that reason, in this paper transition temperature of as-cast and austempered copper and copper-nickel alloyed ductile iron (DI) in the temperature interval from -196 to +150 degrees C have been investigated. The microstructures of DIs and ADIs were examined by light microscope, whereas the fractured surfaces were observed by scanning electron microscope. The ADI materials have higher impact energies compared with DIs in an as-cast condition. In addition, the transition curves for ADIs are shifted towards lower temperatures. The fracture mode of Dls is influenced by a dominantly pearlitic matrix, exhibiting mostly brittle fracture through all temperatures of testing. By contrast, with decrease of temperature, the fracture mode for ADI materials changes gradually from fully ductile to fully brittle

The standard processing window of alloyed ADI materials

In present paper the effect of austempering conditions and alloying elements on the microstructure, mechanical properties and processing window of Cu and Cu + Ni alloyed austempered ductile irons has been studied. A new concept of "standard" processing window was suggested as an alternative to the well-known "microstructure" processing window. This concept is based on the ability to achieve mechanical properties required by different standards, namely ASTM, ISO and EN. The results obtained show that alloying with Cu + Ni produced ductile grades of ADI, while alloying with Cu grades of higher strength. The standard processing window depends on the austempering parameters and alloying elements, as well as standard used. The ISO and EN standards give a larger standard processing window compared to the ASTM. It was also shown, that from the engineering point of view, determination of a standard processing window gives great advantage compared to the microstructure processing window where special methods and procedures have to be used