Ferritic-pearlitic Steel Research Articles

特集： 構造用鋼 フェライト・パーライト型非調質鋼の機械的性質に及ぼすＣ，Ｖ量の影響

The influence of carbon and vanadium contents on mechanical properties of ferrite-pearlite microalloyed steels was investigated by instrumented Charpy impact tests, tensile tests, rotating bending tests and microstructure observation. The following was obtained.(1) The crack initiation energy in impact test is not affected by carbon and vanadium contents.(2) The change in Charpy impact value is related to the crack propagation energy.(3) The optimum balance of carbon and vanadium contents for the crack propagation energy varies corresponding to the aimed hardness.(4) The fatigue limit of ferrite-pearlite steels is effectively improved by vanadium addition in comparison with carbon.

Influence of the microstructure of higher carbon steels on fracture mechanics properties

By varying the cooling rate in the region of the γ-α-transformation different parameters of pearlite microstructure, such as interlamellar spacing and pearlite colony size, were adjusted in a pearlitic steel with 0.79% C. Refinement of the microstructure improved the mechanical properties and the fracture mechanical properties, expressed by the JR-curve. Partial spheroidization of pearlite was achieved by deformation just after the pearlite formation (at the deformation temperature of 600°C). Investigations on ferritic-pearlitic steels with 0.19, 0.35 and 0.45% C, thermomechanically treated in such a way, showed a significant influence of the change in pearlite morphology on the fracture properties, especially with a pearlite fraction larger than about 80%. Durch Veränderung der Abkühlgeschwindigkeit im Bereich der γ-α-Umwandlung wurden in einem perlitischen Stahl mit 0,79% C verschiedene Gefügeparameter – wie Lamellenabstand und Perlitkoloniegröße – eingestellt. Eine Verfeinerung des Gefüges führte sowohl zur Steigerung der mechanischen Eigenschaften als auch zur Verbesserung der bruchmechanischen Eigenschaften, wiedergegeben durch die JR-Kurve. Eine teilweise Einformung des Perlits wurde durch eine Umformung nach der Perlitumwandlung bei einer Umformtemperatur von 600°C erreicht. Untersuchungen an nach diesem Verfahren thermomechanisch behandelten ferritisch-perlitischen Stählen mit 0,19, 0,35 und 0,45% C zeigten einen signifikanten Einfluß der Perlitmorphologie auf die Brucheigenschaften, besonders ab einem Perlitanteil von ca. 80%.

Low-cycle fatigue properties of microalloyed medium carbon precipitation hardening steels in comparison to quenched and tempered steels

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Prediction of creep deformation in ductile two-phase alloys by a continuum mechanics model

The creep deformation of the ductile two-phase alloys was analysed on the basis of the continuum mechanics model which incorporated the Θ projection concept proposed by Evans and Wilshire. The calculated creep curves were compared with the experimental ones in ferrjte-pearlite steels. It was found that the continuum mechanics model was able to predict the whole creep deformation process of the ductile two-phase alloys from the onset of creep loading to the final rupture, if the creep-deformation and creep-rupture data of the individual phases which constituted the two-phase alloys were known. A “steady-state creep” in the ductile two-phase alloys was predicted by the continuum mechanics model to occur, even if the constituent phases did not have the inherent steady-state creep. This was caused by the internal stresses arising from the creep-strength difference between second-phase and matrix in the two-phase alloys. This steady-state creep was observed in ferrite-pearlite steels during creep at 873 K. The predicted rupture life on the basis of the continuum mechanics model was correlated well with the experimental results in ferrite-pearlite steels, although the former was somewhat shorter than the latter under higher creep stresses. The continuum mechanics model was able to apply to the life prediction and the creep-strength design of the ductile two-phase alloys.

The Improvement of Toughness of Low-Carbon Ferrite-Pearlite Steels by Intercritical Annealing - A Review

The Improvement of Toughness of Low-Carbon Ferrite-Pearlite Steels by Intercritical Annealing - A Review

Aspects of fatigue design of marine structures: Prabhu, S.N. Conf. Fatigue and Fracture in Steel and Concrete Structures, ISFF '91 Madras, India, Dec 1991 Vol 2 (Oxford & IBH Publishing Co. Pvt. Ltd, New Delhi, India, 1991) 1301–1318

The primary focus of this work is to investigate the sensitivity of cyclic waveform, frequency (f), load level and microstructure on the corrosion-fatigue crack growth rate (CFCGR) in modern normalised-rolled (NR) and thermomechanical control process (TMCP) ferrite-pearlite steels in the Paris Region of the da/dN vs. ΔK log-log plot. Constant amplitude sinewave (si) and trapezoid waveform (generally referred to here as hold-time (h-t)) were used under frequencies of 0.2 Hz, 0.3 Hz and 0.5 Hz and stress ratio of 0.1. Comparison is also made between the crack path in the S355 TMCP steel under si and h-t in seawater (SW). The role of microstructure in retarding or accelerating fatigue crack growth in SW is also discussed. Experimental results showed that the CFCGR corresponding to the si is higher than that of the h-t for all the load levels and frequencies examined. It was observed that reduction in the f and fatigue load level increased the CFCGR for the h-t but had little effect on the si. Generally, f in the range 0.2–0.5 Hz had little effect; and for a given f an increase in load led to a reduction in the CFCGR, in the Paris Region (PR) for both si and h-t in SW. Under both si and h-t, the CFCGR in the TMCP steels (e.g. S355G8 + M, S355G10 + M) is lower than that of the normalised steels (e.g. S355J2 + N). Metallurgical analyses on the fractured surface of corrosion-fatigue specimens show that the main active crack tip blunting process is the primary factor controlling the CFCGR of steel at high stress intensity factor range (SIFR) and low f in SW. The results obtained from this study have been discussed in terms of the potential impact on the structural design and integrity of offshore wind turbine foundations.

Calculated evaluations of the threshold characteristics for ferritic-pearlitic steels

On the basis of modern physical concepts of the process of formation of the prefailure zone and rules of the change in microcleavage resistance during deformation within the limits of the deformation theory of plasticity relationships of the threshold criteria of fracture to the standard mechanical properties σt, σ0.2, and ΨK were obtained analytically. It was shown that the resistance to crack advance in ferritic-pearlitic steels is determined by the strain hardening exponent. A method of analytical determination of the constant λ relating the two basic threshold characteristics Kth and δth was developed.

Laser surface alloying of plain carbon steels

Deposition and alloying of metallic powders containing WC on a ferritic-pearlitic plain carbon steel have been investigated. Metallic powder was injected into the melted surface layer of the samples under Argon atmosphere during the laser irradiation. The sample surface was previously graphitizing treated to prevent energy loss by reflection. For comparison, irradiated samples, without powder alloying, were also analysed. The coating layer is mainly composed of alloyed α-Fe; γ-austenite and non-magnetic M3C carbide are also present. Partial dissolution of WC and the preliminary graphitizing treatment are the causes of the rather high carbon concentration in γ-austenite. As it is to be expected this concentration decreases with increasing distance from the external surface of the coating layer.

Relationship between the structure and ductile fracture resistance of ferritic-pearlitic structural steels

The nucleation in structural steels of regions of ductile failure whose main micromechanism is the formation, development, and coalescence of microvoids is usually linked with nonmetallic inclusions, and the cracking resistanceδcdwith their dimensions, form, composition, number and the nature of distribution.

Calculation of threshold and critical characteristics of ferritic-pearlitic steels

The unambiguous dependence of the micromechanisms of failure on the parameters of the structure of structural steels, established, makes it possible to develop models linking the micro- and macrocharacteristics of failure resistance, However, this cannot be carried out without taking into account special features of the evolution of the structure at high plastic strains because the processes of failure and occurrence of a specific failure micromechanism are determined to a large extent by the parameters of the structure formed during deformation and not by the parameters of the initial structure. The aim of this work is to attempt, on the basis of data on the parameters of the initial structure and standard mechanical characteristics, to develop a model which would make it possible to calculate threshold K{sub th} the critical K{sub Ic} stress intensity factors. 12 refs., 2 figs., 1 tab.

Characterization of grain-boundary configuration and fracture surface roughness by fractal geometry and creep-rupture properties of metallic materials

Grain-boundary configuration in heat-treated specimens and fracture surface roughness in creep-ruptured specimens of several kinds of metallic material were quantitatively evaluated on the basis of fractal geometry. Correlations between the fractal dimension of grain boundary, that of fracture surface profile, the creep-rupture properties and the fracture mechanisms of the alloys are discussed. In heat-resistant alloys, the fractal dimension of a nominally serrated grain boundary was always larger than that of a straight grain boundary in the same alloy. The relative importance of the ruggedness of grain boundaries was estimated by the fractal dimension difference between these two grain boundaries. There was a quantitative relationship between the increase of the fractal dimension of the grain boundary and the improvement of rupture ductility and rupture strength owing to grain-boundary serration in the alloy. A similar correlation was also found between the increase in the fractal dimension of the fracture surface profile and the improvement of the creep-rupture properties, since in some cases the fractal dimension of the fracture surface profile was correlated with that of the grain boundary. Both grain boundary and fracture surface profile were assumed to exhibit a fractal nature between one grain boundary length (upper bound) and an interatomic spacing (lower bound). In carbon steels with ferrite-pearlite structure, according to the increase in pearlite volume fraction, the rupture ductility decreased and the fracture mechanism changed from transgranular fracture in pure iron and low-carbon steels to intergranular fracture at ferrite-pearlite grain boundaries in medium-carbon steels, and further to intergranular fracture at pearlite grain boundaries in high-carbon steels. The correspondence between the fractal dimension of the grain boundary and that of the fracture surface was confirmed in ruptured specimens of ferrite-pearlite steels when the grain boundary was the fracture path.

Arrest and propagation of a brittle crack in structural steels. Report 2

The authors describe a laboratory method of determining the fracture toughness of structural steel in the crack propagation stage KID. The KID — a dependences were obtained for three structural ferritic-pearlitic and bainitic steels at different temperatures. The KID — a dependences are analyzed from the viewpoint of microstructural features of failure of these materials.

Cyclic crack resistance of ferritic-pearlitic steels and their weld joints

The cyclic crack resistance of ferritic-pearlitic steels and their weld joints is determined. On the basis of an analysis of our own and literature data a method of evaluation of the endurance of welded structures in the area of multicycle fatigue is proposed. The endurance limit σe, the parameter †Kth *, and other parameters have been introduced into this method. The concepts of “repairless structures” and “safely damaged structures” are proposed.

Fatigue crack propagation in weathering weldable steel KT 315 Si

Complex loading conditions are typical for structural materials, especially for steels. Fatigue crack propagation measurements during bend loading were carried out with different load ratios on a newly developed weathering steel KT 315 Si. This steel indicates an improved corrosion resistance in comparison with the weathering steel KT 315. Scanning electron microscopy of the fracture surfaces and quantitative microstructure analysis by means of light microscopy were applied to elucidate relations between microstructure and mechanical properties. An important result of the investigation is that the steel KT 315 Si reveals a lowered mean stress sensitivity of the fatigue threshold value ΔK th in comparison with that of other ferritic-pearlitic steels of the same strength level

Low cycle of dual phase steeels produced by different cooling rates and a ferrite-pearlite steel: Mediratta, S.R., Ramaswamy, V., Singh, V. and Rama, Rao, P. Scr. Metall. Mater. Apr. 1990 24, (4), 793–797

The effect of inclusions in rail steels on fatigue-crack initiation was investigated. A comparative study of fatigue-crack initiation in four-point bending specimens obtained from the rail head perpendicular to the rail direction was carried out. Four rail steels with different properties and inclusion type and distribution were considered. Acoustic emission was used to detect the onset of crack initiation during fatigue testing. Following testing, detailed microstructural observation and composition analysis were carried out by scanning electron microscopy to characterize the crack-initiation sites. The results reveal that crack initiation occurs at elongated inclusions. It was observed that there are three different mechanisms for initiation of fatigue cracks. They are: (1) microcrack initiation in a local deformation band ahead of the inclusion, (2) interfacial surface decoherence, and (3) the inclusion breaks and acts as a crack. The difference in the fatigue properties among the four rail steels was explained by these three fatigue-crack initiation mechanisms.

Evaluation of the Minimum Fatigue Crack Length for Application of the Small-Crack Growth Law.

In the present study, the influences of microstructures of material on the characteristics of small-crack growth were investigated and the minimum crack length to which the small-crack growth law (dl/dN ∝ l) is applicable was evaluated using a ferrite-pearlite steel, a ferrite-martensite steel and a quenched and tempered carbon steel. The crack growth is influenced markedly by grain boundary, pearlite structure and martensite structure at the initial stage of crack growth, and the influences decrease with the increase in crack length. Even in the process in which crack growth is influenced by the microstructures, however, the mean value of crack growth rate is estimated by extrapolating the crack growth rate in the region where the crack growth is mainly controlled by the mechanical parameter: that is, the small-crack growth law is applicable in most regions of the crack growth process.

Effect of the welding thermal cycle and heat treatment on the structure and properties of the heat affected zone metal of welded joints in 09G2SBF quenched and tempered ferriticpearlitic steel

Effect of the welding thermal cycle and heat treatment on the structure and properties of the heat affected zone metal of welded joints in 09G2SBF quenched and tempered ferriticpearlitic steel

Fretting fatigue strength analysis of Ti6Al4V in air: Maruyama, N., Sumita, M. and Nakazawa, K. Tetsu-to-Hagane (J. Iron Steel Inst. Japan) Feb. 1990 76, (2), 262–269 (in Japanese)

Monotonic and cyclic plastic zone sizes were measured in a medium strength ferrite-pearlite steel (BM 45) tested in fatigue at 25 Hz at room temperature. Two methods were applied: microhardness and the recently developed ‘fatigue in compression’ technique. The results obtained are discussed in terms of accuracy and reliability.The retardation effect due to overloads was also studied in the same material and is illustrated experimentally as a dadN vs ΔK curve. This effect emphasizes the importance of an accurate evaluation of both the size and shape of the overall plastic zone. The shape and dimensions of the cyclic plastic zones seem to indicate that in ductile metals the steady state of fatigue crack growth occurs under plane strain conditions.

Stress enhancement of fatigue susceptibility of porous coated Ti6Al4V implants: an elastic analysis: Messersmith, P.B. and Cooke, F.W. J. Biomed. Mater. Res. May 1990 24, (5), 591–604

Monotonic and cyclic plastic zone sizes were measured in a medium strength ferrite-pearlite steel (BM 45) tested in fatigue at 25 Hz at room temperature. Two methods were applied: microhardness and the recently developed ‘fatigue in compression’ technique. The results obtained are discussed in terms of accuracy and reliability.The retardation effect due to overloads was also studied in the same material and is illustrated experimentally as a dadN vs ΔK curve. This effect emphasizes the importance of an accurate evaluation of both the size and shape of the overall plastic zone. The shape and dimensions of the cyclic plastic zones seem to indicate that in ductile metals the steady state of fatigue crack growth occurs under plane strain conditions.

Rupture strength of structural steels after thermomechanical treatment

1. A significant improvement in the set of in-service properties of the commercially smelted ferritic-pearlitic steels under investigation is achieved as a result of TMT in the intercritical temperature interval (regulated rolling at 800°C) and accelerated cooling; this is associated with the formation of mixed ferritic-martensitic and ferritic-bainitic-martensitic structures, which ensure a simultaneous improvement in strength, plasticity, impact strength, cold resistance, and crack resistance (fracture toughness). 2. Even a negligible change in the alloying system of monotypic ferritic-pearlitic structural steels (an increase in carbon content from 0.15 to 0.18% and alloying with 0.2% of Mo in lieu of 0.33% of Cu) leads to a significant change in the character of the fine crystalline structure during TMT in the intercritical temperature interval, and, consequently, to a different degree of hardening and resistance to brittle fracture for this class of steels.