No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Effect of dual phase microstructure on the toughness of a Cr-Mo low-alloy plate steel
Abstract
Quench and tempered Cr-Mo plate steel was found not to contain a fully martensitic structure as a result of either slow slack quenching or insufficient hardenability at the cooling rates used. The resulting microstructure contains bainite in combination with martensite. It is necessary to examine the implications of bainite for the toughness of the final product. Loss of toughness arises primarily from the inter-lath carbides that typically form in upper bainite. An investigation into the effect of upper bainite in a martensitic matrix of a steel grade containing 0.17% C, 0.1% Cr, 0.54% Mo, and 20 ppm B indicated lower toughness than in a fully martensitic structure. © The Southern African Institute of Mining and Metallurgy, 2013.
... Les liens entre propriétés à rupture sur la courbe de transition et microstructures ont fait l'objet d'intenses recherches depuis les années 80, avec pour objectif notamment d'améliorer les performances des nuances caractérisées. Ces liens s'imposent comme outil incontournable pour dégager les entités microstructurales pertinentes qui gouvernent l'énergie à rupture dans les domaines de rupture ductile, de transition ductile-fragile et dans le domaine fragile [34] [35]. ...
... dans une zones située à environ 300µm du fond d'entaille, identifiée comme étant le voisinage de la zone d'amorçage probable lors de la rupture fragile, une hypothèse qu'on suppose également valide ici. 34, on observe une augmentation de la taille des cupules avec la température de revenu. Stüwe, 1980 [17] précise que la taille et la forme des cupules peuvent quantitativement refléter les propriétés à rupture des matériaux ductiles. ...
Cette thèse s’inscrit dans un projet dans lequel Ascometal souhaite améliorer le développement de nuances d’aciers à haute résistance destinées à la fabrication des pièces de forage offshore. Le projet vise notamment la prédiction, au moyen de la microstructure, de la courbe de transition ductile - fragile en résilience. Les objectifs de la thèse sont doubles à savoir : améliorer la compréhension du lien entre microstructure et propriétés à rupture, puis mettre au point un modèle prédictif de la résilience basée sur la microstructure. Nous avons adopté une démarche scientifique en quatre étapes: 1) la construction d’une base de données constituée de six microstructures. Les microstructures ont été caractérisées quantitativement à différentes échelles. 2) La caractérisation du comportement élastoplastique en variant la température d’essai, la vitesse de sollicitation, la direction de prélèvement des échantillons et le revenu. 3) La caractérisation du comportement à rupture au moyen des essais Charpy instrumentés dans une gamme de température allant de -196°C à +60°C. 4) L’établissement des liens quantitatifs entre chacune des microstructures et leurs propriétés à rupture. Dans certains cas, le recours aux outils numériques a été adopté pour accéder aux contraintes locales et les relier aux entités microstructurales. Les résultats ont permis d’aborder les discussions autour des effets : de la vitesse de sollicitation, du revenu, de la température d’essai et la direction de sollicitation sur le comportement élastoplastique des aciers de microstructure martensite et bainite supérieure. Pour les essais à rupture, l’analyse fractographique a permis d’accéder aux mécanismes de rupture dans les domaines de rupture ductile, de transition et de rupture fragile. Les observations de l’endommagement en post-mortem sur coupe polie ont permis de discuter l’effet du revenu sur l’endommagement ductile des aciers trempés-revenus à haute résistance. Au niveau du plateau ductile, un lien quantitatif est proposé entre la distance intercarbures et l’énergie totale à rupture, il permet d’expliquer l’augmentation de l’énergie à rupture. Au niveau du plateau fragile, l’approche locale de la rupture sur des éprouvettes axisymétriques entaillées est mise à contribution pour déterminer les contraintes critiques de clivage à -196°C. Un lien quantitatif a été établi entre la contrainte critique de clivage à -196°C et la taille des carbures. L’effet de la bainite supérieure sur la contrainte critique de clivage est également abordé. Enfin, un modèle global de la résilience basé sur la microstructure est proposé. Il s’appuie sur la distance intercarbures et la température de transition en cristallinité pour prédire la courbe de transition ductile-fragile des aciers à haute résistance de l’étude. Il semble enfin indispensable d’étendre l’approche proposée sur d’autres nuances de même grade afin de disposer d’une base de données plus large, ce qui devrait rendre le modèle global robuste.
... It is also of interest to understand how variations in microstructure though thickness due to autotempering or formation of non-martensitic transformation products may influence properties in large section components that may have significant variations in cooling rate through the thickness. There are few studies that systematically study autotempering effects on mechanical properties [3,4], and studies on the behavior of mixed microstructures, particularly of bainite and martensite, have shown both beneficial effects of lower bainite [5,6] in addition to martensite on toughness with little loss of strength and detrimental effects of upper bainite on both impact toughness and strength [6,7]. Two elements that are of particular interest as they are often added in small amounts to enhance properties are Mo and V. ...
The effects of Mo and V on impact toughness in martensitic steels tempered at low temperatures were investigated using three low-alloy medium-C steels. Previous examination of these alloys had identified differences in impact toughness without a clear cause. In this work, the Base alloy with a reduced Mo addition experienced a significant loss in hardenability leading to the formation of small fractions of bainite during quenching even at relatively high quench rates. The use of different quench media to simulate cooling rates throughout a heavy section demonstrated that the variation in previously reported Charpy V-notch impact absorbed energies was readily explained by some regions cooling fast enough to avoid bainite while others formed some small fraction of upper bainite leading to increased cleavage fracture and decreased impact toughness. Small amounts of bainite transformation were not detected by dilatometry or tensile properties. These results emphasize the importance of effective through-hardening and careful microstructure evaluation in alloys that are meant to maintain good toughness and strength in thicker sections.
The fracture of a grade HL steel sucker rod occurred after just nine days service in an oilfield of China. The failure cause was analyzed by direct-reading spectrometer, electronic universal testing machine, impact testing machine, micro-Vickers hardness tester, optical microscope and scanning electron microscopy in this paper. The results showed that folding defects and decarburization layer were formed on the surface of the wrench square, while numerous upper bainite formed in the center. The fracture cause of the failed sucker rod was folding defects and decarburization induced surface cracking under the combined effect of alternating stress and bending stress during the pumping operation. Furthermore, the brittle upper bainite accelerated the rapid propagation of the surface crack and finally resulted in the complete fracture failure of the sucker rod.
The quantitative analysis of substructure in the martensite/bainite mixed structure, which is obtained from low-carbon NiCrMoV steels under different cooling conditions, was made by means of optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscope (TEM), in order to research the effect on toughness. The test results indicate that the toughness of the steel is enhanced with the decrease in the packet and block size under the condition of the same prior austenite grain size mixed with different ratios of martensite and bainite while the lath width is about 0. 38 μm. The calculation shows that both the packet and block boundaries have the same hindering effect on crack extension. Furthermore, the effect of the block width on impact energy is much larger than that of the packet. Therefore, the block can be used as microstructural substructure to affect the toughness in low-carbon martensite steels, suggesting that the block size is “the effective grain size” for controlling toughness.
4340 steel bars were austenitized at 850°C for 1h followed by heating at 700°C for 90min and quenching into a salt bath at the temperature range of 300–450°C for 1h to obtain dual structures with 34vol.% fraction ferrite and various bainite morphologies. SEM studies showed that by increasing the austempering temperature, bainite morphology varies from lower to upper bainite. Tensile, impact and hardness tests revealed that increasing the austempering temperature from 300 to 400°C leads to a reduction in yield and ultimate tensile strength, hardness, uniform and total elongation and impact energy. But in dual phase steel produced by austempering at 450°C, yield and tensile strength and hardness increased and severe reduction in total elongation and impact energy obtained. Fractography of tensile specimens showed brittle behavior for this austempering temperature. Fatigue test results showed that fatigue limit decreases with increasing austempering temperature from 300 to 400°C. Finally, fractography studies showed cleavage fracture at the surface of fatigue specimens austempered at 400°C, which confirms the tendency to brittle behavior.
This paper addresses the mechanisms of cleavage fracture in the pressure-vessel steel A533B. Microstructures of single bainite, single auto-tempered martensite and a mixture of bainite-plus-martensite have been investigated. Upper-bainite microstructures exhibit a higher propensity for brittle cleavage fracture than do those of auto-tempered martensites. The K1c values of mixed microstructures are determined by the statistical distribution of the two phases and the range of the values is bounded by limits set by those for the single-phase microstructures. The results are explained in terms of the RKR model, which involves a local cleavage stress σF* and a distance ahead of the macrocrack tip, X, as two critical parameters. It is found that the carbides or carbide “colonies” act as critical microcrack nuclei, and hence play a key role in determining the fracture toughness, although packet boundaries in bainite may give rise to “pop-in” arrests in displacement-controlled tests.
The variation with tempering temperature of the mechanical properties of the bainite and the bainite/martensite duplex structure in a low-carbon highstrength steel has been investigated and compared with that of the martensite. Both the bainite and the duplex structure exhibited much lower impact transition temperatures than the martensite and this can be explained in terms of the unit crack path, which is the mean cleavage crack path.The 350° C embrittlement was more pronounced in the tempered martensite than in the tempered bainitic structures. Tempering at temperatures higher than 600° C greatly improved the toughness of each structure but decreased the strength. It is believed that this improvement in toughness by tempering is attributable to the ferrite matrix properties rather than the effect of the unit crackpath. The optimum combination of toughness and strength can be achieved by producing the tempered bainite/martensite duplex structure at all strength levels.
The Charpy impact energy of Al–Si–Cu AA319-type alloys was measured in terms of the total absorbed energy. The Charpy specimens were machined from end-chilled castings to incorporate the effect of cooling rate on the impact properties. Unnotched specimens were used to increase the accuracy of the measurements, and to emphasize the effect of microstructure. The influence of the microconstituents on the impact strength was investigated by adding various alloying elements (i.e. Sr, Fe, and P) to the AA319 base alloy, and applying two different heat treatments (T5, and T6). The results show that strontium-modification enhances the impact properties, so that the Sr-modified AA319 alloy exhibits the highest impact properties compared to the base, and other alloys at any given dendrite arm spacing (DAS). The impact energy increases with increase in cooling rate, while iron, and phosphorus additions have a detrimental influence due, respectively, to the formation of β-Al5FeSi, and phosphorus oxide particles during solidification. T6 treatment assists in the even distribution, and dissolution of the microconstituents (including the block-like CuAl2 particles) into the aluminum matrix. With more Cu available for strengthening during aging, the impact toughness is greatly enhanced. In the unmodified AA319 base alloy, crack initiation, and propagation occur mainly through Si-particle fracture, and the mechanism of void coalescence. In the Sr-modified, 1.2% Fe-containing 319 alloys, however, crack initiation takes place through fragmentation of β-Al5FeSi, Si, and CuAl2 or Cu2FeAl7 particles. Crack propagation occurs through cleavage of the β-Fe platelets, and fracture of the Cu-intermetallics, and brittle Si particles. Such samples exhibit very low impact energies.
Bainitic microstructures can be produced in a variety of steels either as a result of a deliberate attempt to achieve a particular
combination of strength and toughness or in response to welding during fabrication. In addition, such microstructures can
offer advantages in terms of their resistance to creep or fatigue deformation or susceptibility to hydrogen embrittlement.
The relationships among chemical composition, processing, microstructure, and the mechanical properties will be reviewed.
Particular emphasis will be placed on recent advances in alloy design. These developments rely on an improved understanding
of the mechanisms of bainitic transformation, and the relevance of recent research in this area to the design of new alloy
systems will be discussed. Bainitic structures which arise during welding can have a significant and sometimes detrimental
effect on the fracture toughness of the welded joint. The fracture toughness of bainitic microstructures in so-called “local
brittle zones” in the heat-affected zone and in weld metals and the importance of controlling the bainitic morphology will
be considered and the transformation mechanisms discussed. In summary, the aim of this review will be to indicate the prospects
for improved microstructural control of structure-property relationships in steels containing a significant proportion of
bainite.
Processes of formation of heterogeneous bainite-martensite structure in steels used for making oil and gas pipes are studied.
It is shown that the type of the formed bainite (upper or lower) affects the impact toughness and the corrosion resistance
of the steels.
Key wordsgas and oil pipelines–bainite-martensite structure–sparingly alloyed steel–corrosion
The aim of this study is to examine the relationship between fracture energy and the ductile area measured on the fracture surface. Instrumented Charpy tests and fracture toughness tests are performed in the transition temperature range, as well as at lower temperatures. Quantitative fractographic analyses of Charpy specimens reveal a certain proportion of ductile fracture even if the Charpy test is conducted at low temperatures, below the transition temperature. The ductile fracture area situated next to the notch is correlated to fracture energy for all temperatures. In the transition temperature range, fracture energy and the ductile area have a large scatter. Since the limiting event in the development of the ductile area is the initiation of cleavage, the maximum principal stress has been computed in different specimens using the finite element method. It has been shown that the propagating ductile crack does not increase the stress level, but does increase the probability of cleavage fracture through an expansion of the plastic volume where weak points can be found.
The paper presents the microstructural factors determining mechanical properties of bainite with a special attention given to strength, ductility and toughness. On this base the most important groups of bainitic steels are described and some modern trends in their chemical composition and industrial applications are indicated.