Papers by Keyword: Upper Bainite

Abstract: In the present study austempered ductile irons (ADI) with an upper and lower bainitic structure with nanosized particles of cubic boron nitride cBN (50nm) are studied. The austempering modes include austenization at 900^оС during an hour and isothermal retention at 280^оС from 0.5 to 6 hours and at 380^oС from 0.5 to 6 hours. The samples microstructure is investigated by means of an optical metallography, scanning electron microscopy and X-ray analysis. Tests for hardness, impact toughness and wear are performed. The influence of nanosized additives on the kinetic of the bainitic transformation and on the morphology of the obtained bainitic structure is studied. It is established that the changes of the bainitic cast irons microstructure under influence of the nanoparticles of cBN lead to the cast irons impact toughness and wear resistance increase. The research is aimed at expanding the scope for new ADI applications in the industry.

Abstract: This study was conducted to determine if austempered 4340 steel had different mechanical properties compared to step quenched 4340 steel. Tensile strength and impact energy was determined at room temperature under identical test conditions. The specimens were cut from a bar with 25 mm diameter and austenitized at 800°C for 60 min and followed by quenching at 430°C for the high austempering temperature to achieve the upper bainite morphology and at 360°C for the lower austempering temperature to achieve the lower bainite morphology. In the case of step quenched, the specimens were first austempered at 430°C and then austempered at 360°C to achieve the mixed structure of upper bainite and lower bainite morphology. The another set of specimens for step quenching, after austenitization were quenched to below M_s (martensite start temperature), followed by heating at 400°C to achieve the mixed structure of tempered martensite and lower bainite and 500°C to achieve the mixed structure of tempered martensite and upper bainite. It is also shown that the best combination of strength and ductility can be achieved by the mixed structure of tempered martensite and lower bainite that has been suggested in this investigation.

Abstract: In this paper, a modified up-quenching heat treatment method to the ASSAB 705M steel (ultra high strength steel) is proposed. A low alloy steel (0.33%C), was used to study the effect of isothermal austempering, successive austempering and modified up-quenching austempering heat treatment on the mechanical properties. The specimens, were cut from a bar with 25mm diameter and after achieving the best temperature and time of austenitizing, austenitized at for 60 min and followed by quenching at for the high austempering temperature to achieve the upper bainite morphology and at for the lower austempering temperature to achieve the lower bainite morphology. In the case of successive austempering, the specimens were first austempered at for different periods (500 sec and 60 sec) and then austempered at for 1000 sec to achieve the mixed structure of upper bainite and lower bainite morphology. The specimens selected for up-quenching, after austenitization were quenched to below ( ) for 120 sec. followed by heating at to achieve the mixed structure of tempered martensite and lower bainite and to achieve the mixed structure of tempered martensite and upper bainite for 1000 sec. All of the processes were performed in the salt bath furnaces. Experimental results are presented and the advantages of the modified method are discussed. As well, it is shown that the best combination of strength and ductility can be achieved by the proposed heat treatment method. This modified method, can offer techniques that simultaneously improve not only strength 12 %( compare with results of strength after other heat treatment methods), but also ductility 38 %( compare with results of ductility after other heat treatment methods). While, conventional heat treatment of ultra high strength steels (UHSS) cannot always meet the strict engineering requirements for improved strength and ductility simultaneously. It has been shown that the mixed structure of tempered martensite and lower bainite that has been suggested in this investigation offers a good combination of strength and ductility. The technical reason for this superiority returns back to the fact that it has increased dislocation density. As a result of the increment of the dislocation density, in the morphology, the inter lath carbide (e.g. cementite) decreases, and the intra lath carbide increases. This modified austempering is applicable to all the ultra high strength steels, has noticeable economic advantages because it is simple. Use of this modified austempering for heavy parts, leads to the lightness of heavy parts and combination of the thermomechanical methods with this modified austempering can yield even much more improvements.

Abstract: 4340 steel bars were austenitized at 850oC for 1 hour followed by heating at 700oC (ferrite and austenite region) for 90 min and quenching into a salt bath with different temperatures of 300, 350, 400 and 450oC. The steel bars were held for 1 hour at these temperatures before air cooling to room temperature. Various ferrite-bainite microstructures with 34% volume fraction of ferrite and different bainite morphologies were obtained. The results of SEM studies showed that by increasing the austempering temperature, the morphology of bainite varies from lower to upper bainite. According to the T-T-T diagram of the studied steel, the bainite transformation will not complete for the holding time of 1 hour at 400 and 450oC austempering temperatures and there can be 9 and 23Vol% of martensite at these temperatures respectively. Microstructural studies of specimens austempered at these temperatures showed complex microstructures of ferrite, bainite and martensite. Mechanical testing results showed reduction in yield and ultimate tensile strengths, hardness, uniform and total elongations and impact energy with increase of the austempering temperature from 300 to 400oC. But for dual phase steel austempered at 450oC, the yield and tensile strengths and hardness increased and a severe reduction in total elongation and impact energy was obtained. This brittle behaviour was related to martensite formation during cooling from this austempering temperature.

Abstract: The microstructure and fracture surface of bainite in JIS SK5 steel have been investigated using transmission electron microscopy (TEM) and electron back-scatter diffraction (EBSD) in this study. Specimens were austenitized at 880
for 30 min and then austempered at 300
and 400
for 1hr, as a result, lower bainite and upper bainite were formed, respectively. The lower bainite phase consists of plate-like bainitic ferrites and embedded cementite platelets, the cementite precipitated unidirectionally at an angle of 55 to 60 deg deviated from the long axes of the ferrites; in addition, the upper bainite phase consists of a parallel array of ferrite laths and discrete cementite layers sandwiched between them. Both the bainite structures have the same ferrite/cementite orientation relationship, which is close to that of the Bagaryatskii relation. The impact fractographs of lower and upper bainite structures exhibit brittle failure with cleavage facets, of which the size is correlated with the width of the bainite sheaves. Moreover, the crystallographic orientation of cleavage facets has been determined directly by EBSD. The results showed that the cleavage facet planes of lower bainite structure are close to the {001} plane of ferrite, and they were close to the {001}, {112} and {123} planes of ferrite for the upper bainite structure.

Abstract: This paper provides a detailed description of complex bainitic microstructures obtained during the recent development of low carbon linepipe steels with strengths in the range of X100 to X120. New experimental techniques based on a high resolution FEG-SEM and EBSD have been used to characterise and quantify the mixture of ultrafine bainitic ferrite and nanosize second phases in these steels. It was found that the occurrence of incomplete transformation generates new, previously unexplored bainitic microstructures with a wealth of microstructural features that is beyond classification based on conventional concepts. Clear differences in distributions of boundary misorientations and effective grain size were noted between upper, lower and granular bainites. Based on these results a new classification scheme and definition of bainite is proposed.