Showing papers on "Pearlite published in 2007"

Effect of cooling rate on the microstructure and mechanical properties of Nb-microalloyed steels

Abstract: We describe here the effect of cooling rate on the microstructure and mechanical properties of Nb-microalloyed steels that were processed as structural beams at three different cooling rates. Nb-microalloyed steels exhibited increase in yield strength with increase in cooling rate during processing. However, the increase in the yield strength was not accompanied by loss in toughness. The microstructure at conventional cooling rate, primarily consisted of polygonal ferrite-pearlite microconstituents, while at intermediate cooling rate besides polygonal ferrite and pearlite contained significant fraction of degenerated pearlite and lath-type ferrite. At higher cooling rate, predominantly, lath-type (acicular) or bainitic ferrite was obtained. The precipitation characteristics were similar at the three cooling rates investigated with precipitation occurring at grain boundaries, on dislocations, and in the ferrite matrix. The fine scale (∼8–12 nm) precipitates in the ferrite matrix were MC type of niobium carbides. The microstructural studies suggest that the increase in toughness of Nb-microalloyed steels with increase in cooling rate is related to the change in the microstructure from predominantly ferrite-pearlite to predominantly bainitic ferrite.

Effect of cooling rate on the microstmcture and mechanical properties of Nb-microalloyed steels

Abstract: We describe here the effect of cooling rate on the microstructure and mechanical properties of Nb-microalloyed steels that were processed as structural beams at three different cooling rates. Nb-microalloyed steels exhibited increase in yield strength with increase in cooling rate during processing. However, the increase in the yield strength was not accompanied by loss in toughness. The microstructure at conventional cooling rate, primarily consisted of polygonal ferrite-pearlite microconstituents, while at intermediate cooling rate besides polygonal ferrite and pearlite contained significant fraction of degenerated pearlite and lath-type ferrite. At higher cooling rate, predominantly, lath-type (acicular) or bainitic ferrite was obtained. The precipitation characteristics were similar at the three cooling rates investigated with precipitation occurring at grain boundaries, on dislocations, and in the ferrite matrix. The fine scale (∼8-12nm) precipitates in the ferrite matrix were MC type of niobium carbides. The microstructural studies suggest that the increase in toughness of Nb-microalloyed steels with increase in cooling rate is related to the change in the microstructure from predominantly ferrite-pearlite to predominantly bainitic ferrite.

Influence of the pearlite fineness on the mechanical properties, deformation behavior, and fracture characteristics of carbon steel

Abstract: Specific features of plastic deformation and tensile failure of a plain carbon (C = 0.62%) pearlitic-ferritic steel with various pearlite fineness have been investigated. It is shown that the steels with coarse lamellar pearlite and fine lamellar pearlite have similar strain-hardening coefficients, but the relative elongation of the former steel is higher. Deformation results in a uniform dislocation distribution in the fine pearlite and in the formation of a cellular substructure in the coarse pearlite. It is established that the fine pearlite undergoes plastic deformation and ductile failure as a single structure, while the coarse pearlite exhibits a structure discontinuity upon deformation. A model of microplastic pearlite deformation and the initial stage of macroplastic pearlite deformation is proposed. It is established that the strain-hardening coefficient of pearlite at the initial deformation stage does not depend on its dispersity. A size effect, which manifests itself in the dependence of the dislocation structure formed in the ferrite interlayers on their thickness, is shown to be characteristic of pearlite deformation.

Effects of molybdenum and vanadium addition on tensile and charpy impact properties of API X70 linepipe steels

Abstract: This study is concerned with the effects of V and Mo addition on tensile and Charpy impact properties of API X70 linepipe steels. Twelve kinds of steel specimens were produced by varying V and Mo additions and rolling conditions. The addition of V and Mo promoted the formation of acicular ferrite (AF), banitic ferrite (BF), and martensite-austenite (MA) constituents, while suppressing the formation of polygonal ferrite (PF) or pearlite (P). The tensile test results indicated that the tensile strength of the specimens rolled in the two-phase region increased with the addition of V and Mo, while the yield strength did not vary much in these specimens except the water-cooled specimens, which showed the increased yield strength with addition of Mo. The tensile strength of specimens rolled in the single-phase region followed by water cooling increased with increasing V and Mo contents. The yield strength, however, did not vary much with increasing V content or with addition of Mo to the low-V alloy. In these specimens, a substantial increase in the strengths was achieved only when Mo was added to the high-V alloy. The specimens rolled in the single-phase region had higher upper-shelf energy (USE) and lower ductile-brittle transition temperature (DBTT) than the specimens rolled in the two-phase region, because their microstructures were composed of AF and fine PF. According to the electron backscatter diffraction (EBSD) analysis data, the effective grain size in AF was determined by crystallographic packets composed of a few fine grains having similar orientations. Thus, the decreased DBTT in the specimens rolled in the single-phase region could be explained by the decrease in the overall effective grain size due to the presence of AF having smaller effective grain size.

Microstructural Features of Austenite Formation in C35 and C45 alloys

Abstract: The microstructural evolution during continuous heating experiments has been studied for two C-Mn steels with carbon contents in the range 0.35 to 0.45 wt pct using optical microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). It is shown that the formation of the austenitic phase is possible in pearlite as well as in ferrite regions. Thus, a considerable overlap in time of ferrite-to-austenite and pearlite-to-austenite transformations is likely to occur. Another observation that was made during the experiments is that, depending on the heating rate, the pearlite-to-austenite transformation can proceed in either one or two steps. At low heating rates (0.05 oC/s), ferrite and cementite plates transform simultaneously. At higher heating rates (20 oC/s), it is a two-step process: first ferrite within pearlite grains transforms to austenite and then the dissolution of the cementite lamellae takes place. Several types of growth morphologies were observed during the experiments. The formation of a finger-type austenite morphology was noticed only for low and intermediate heating rates (0.05 oC/s and 20 oC/s), but not for the heating rate of 300 oC/s. The formation of this fingertype austenite occurs on pearlite-ferrite grain boundaries and coincides with the direction of cementite plates. The carbon inhomogeneities in the microstructure affect the formation of martensitic/bainitic structures on cooling.

Predictive modeling of laser hardening of AISI5150H steels

Abstract: This paper presents accurate predictive modeling of the laser hardening process in terms of laser operating parameters and initial microstructure without the need of any experimental data. The model provides the diagrams that are useful for predicting hardness profiles, optimizing practical process parameters and assessing the potential of laser hardening for different steels. It is shown that the hardness and depth of the hardened layer in hypoeutectoid steels (carbon wt% The model combines a three-dimensional transient numerical solution for a rotating cylinder undergoing laser heating by a translating laser beam with a kinetic model describing pearlite dissolution, carbon redistribution in austenite and subsequent transformation to martensite by utilizing the feedback from the CCT diagram. In order to validate the thermal model and assert the accuracy of temperature predictions the temperature was measured using an infrared camera and a good agreement between the predicted and measured temperatures is shown. Results are presented as processing maps, which show how the case depth and hardness depend on input operating parameters. The good agreement between the measured and predicted hardness profiles ascertains the accuracy of the thermal-kinetic model developed for AISI5150H steels.

Stress shielding and fatigue crack growth resistance in ferritic–pearlitic steel

Abstract: The effect of pearlite morphology on stage IIb (Paris regime) fatigue crack growth behavior in ferritic–pearlitic steel was investigated. Networked and distributed pearlite structures were prepared. Constant-Δ K fatigue crack growth tests were performed in situ in a scanning electron microscope. The results revealed that a distributed pearlite structure had better fatigue crack growth resistance than a networked pearlite structure. From the in situ observations, the distributed pearlite structure indicated a tortuous crack path, which induced crack interlocking as well as crack closure. For the networked pearlite structure, some crack branching was found on the crack path. The crack growth curves for the two microstructures, plotted using the effective stress intensity factor range Δ K eff , where crack closure behavior is taken into consideration, did not coincide. The crack growth curves plotted using the crack tip effective stress intensity factor range Δ K eff,tip , where crack tip shielding behavior as well as crack closure are taken into consideration, successfully coincided on one line.

Sliding wear response of cast iron as influenced by microstructural features and test condition

Abstract: This study analyzes the effects of material and test parameters on the sliding wear characteristics of a cast iron. Material parameters involved microstructural characteristics in terms of the shape of graphite phase and changing fraction of the matrix constituents like ferrite and pearlite. Factors related to sliding wear conditions selected for the study were applied pressure and test environment in terms of the presence of an oil lubricant and two different concentrations of graphite particles suspended in the oil. The microstructure of the cast iron in one case revealed spheroids of graphite in a matrix of mainly ferrite with a limited quantity of pearlite; this material would be referred to as spheroidal graphite iron (SGI) in the subsequent text. On the contrary, flakes of graphite were observed and the matrix comprised mainly pearlite plus a limited quantity of ferrite in the other case to be referred to as gray cast iron (GCI) now onwards. Better wear resistance of GCI in dry, oil and oil plus 5% graphite lubricated conditions than SGI was attributed to the predominant effect of harder and stronger matrix and flaky graphite having large surface area in the GCI while a reversal in the trend in the oil plus 10% graphite lubricant mixture was thought to be due to the predominantly adverse effect of the increasing concentration of the suspended graphite particles in the lubricant mixture. The wear rate versus pressure plots revealed single slope in dry condition for both the varieties of the cast iron. A similar trend was also noticed for the GCI in oil plus 5% graphite lubricant mixture. Three slopes were noticed in case of the GCI tested in oil. In remaining cases, two slopes were observed. The wear rate increased with pressure. Testing the samples in oil lubricated condition led to improved wear resistance than during dry sliding. Addition of graphite particles to the oil lubricant caused still better wear resistance. Presence of 5% graphite in the oil lubricant improved the wear behaviour to the maximum extent while a further increase in the graphite content to 10% in oil tended to produce a reverse effect but it was still superior to that in the oil-only environment. Frictional heating increased with intermediate sliding distance. The rate of increase was high initially followed by a lower rate of increase at longer test durations (intermediate sliding distance). In some cases, the rate of increase became larger once again towards the end of the tests. The frictional heating increased with pressure at a low rate initially followed by a higher rate of increase beyond a specific pressure in general. Less frictional heating was observed in the presence of oil while addition of graphite to the oil proved still better, 5% graphite producing minimum frictional heating. The observed response of the samples has been substantiated further through the characteristics of wear surfaces, subsurface regions and debris particles and operating wear mechanisms discussed.

Microstructure and mechanical properties of 4% cobalt and nickel alloyed ductile irons

Abstract: In the study, 4% cobalt and nickel elements were added to ductile iron (DI) to investigate the effects of the alloying elements on both its microstructure and its mechanical properties. The results show that 4% cobalt alloyed ductile iron (4% Co-DI) not only increases the nodule count and ferrite content in the microstructure, but also improves the mechanical properties such as yield strength, tensile strength, impact toughness and fracture toughness ( K IC ) as compared to unalloyed DI. In contrast, with 4% nickel addition, there is a decreased nodule count and increased pearlite content in the matrix. However, this material (4% Ni-DI) has the highest hardness and strength but the lowest ductility and toughness among all the ductile irons. In the case of microstructure, all kinds of iron have a sufficient nodularity up to 90% and the sequence of the nodule count is 4% Co-DI (150 nodules/mm 2 ) > DI (106 nodules/mm 2 ) > 4% Ni-DI (85 nodules/mm 2 ). Moreover, it appears that cobalt plays an important role in stabilizing the ferrite phase, whereas 4% nickel accelerates the formation of pearlite in ductile iron.

New insights into crystallographic correlations between ferrite and cementite in lamellar eutectoid structures, obtained by SEM–FEG/EBSD and an indirect two-trace method

Abstract: Four different ferrite/cementite orientation relationships (ORs) in near-eutectoid steel are derived using SEM–FEG/EBSD (scanning electron microscopy–field emission gun/electron back-scatter diffraction) and an indirect two-trace method. They show a common feature of close-packed plane parallelism between ferrite and cementite. Their crystallographic compatibility with habit planes shows a variety of possible habit planes and excludes the existence of the exact conventional Bagaryatsky and Pitsch–Petch ORs. Each of these new ferrite/cementite ORs is correlated with a different edge-to-edge matching condition between austenite and pearlitic ferrite, and between austenite and pearlitic cementite, and possesses specific morphological features. The present results may give deep insight into the crystallography of pearlitic transformation and provide useful information for materials design through interface tailoring in steels.

Investigation of orientation gradients in pearlite in hypoeutectoid steel by use of orientation imaging microscopy

Abstract: For steels and related ferrous alloys containing pearlite, the precise microstructural characterization of pearlite is of great importance since it significantly contributes to the mechanical properties of the material. The yield strength of pearlite depends on its interlamellar spacing. Refining that spacing entails an increase in strength which follows a HallPetch type relation [1,2]. Beyond the contribution of the interlamellar spacing, the strength of pearlite was reported to depend as well on the prior austenite grain size and on the pearlite colony size although these effects were considered to be small [3]. In contrast to these findings Ray and Mondal [4] observed that the strength of pearlite in hypoeutectoid steels does not follow a Hall-Petch type relation. They reported that the strength of pearlite in the hypoeutectoid steels varied considerably even when the interlamellar spacings in the pearlite colonies are almost constant. They attributed that variation in the strength of the pearlite in the hypoeutectoid steels to the influence of hydrostatic stresses exerted by the presence of proeutectoid ferrite in the material. Besides the morphological parameters such as interlamellar spacing, prior austenite grain size, and pearlite colony size, other microstructural factors such as the amount of elements in solid solution and the dislocation density in the ferrite have been commonly taken into account when evaluating the strength of pearlite. The influence of the substructure and of the crystallographic texture of the pearlite, however, have not been sufficiently considered in that context yet. The contributions of these influencing factors were up to now only evaluated on the basis of regression analysis by which possible scattering of the strength of the pearlite constituents due to the presence of microstructural heterogeneities was simply averaged over the whole pearlite volume rather than studied in detail. It is a common observation though that the lamellar structure in a pearlite colony is not always homogeneous, but rather contains various types of substructures even in the astransformed state. Puttick [5] has identified several kinds of growth faults in the lamellae of the pearlite colonies. These include linear discontinuities in the cementite lamellae; branched crystallization; deviations of lamellar orientation; rod-like growth; and growth of round cementite inclusions. Bramfitt and Marder [6] have also studied the substructural faults of pearlite colonies. They showed that areas with higher dislocation densities and extended dislocation substructures in the pearlitic ferrite were sometimes associated Investigation of Orientation Gradients in Pearlite in Hypoeutectoid Steel by use of Orientation Imaging Microscopy

Wire rod excellent in wire-drawing workability and method for producing same

Abstract: Disclosed are a wire rod and a method therefor. The wire rod is excellent in wire-drawing workability, insusceptible to wire break in spite of an increase in wire-drawing rate, and reduction of area, and capable of extending a die life by suppressing die wear. The wire rod is made of steel containing C: 0.6 to 1.1 %, Si: 0.1 to 2.0%, Mn: 0.1 to 1 %, P: not more than 0.20 %, S: not more than 0.20 %, N: not more than 0.006 %, Al : not more than 0.03 %, and O: not more than 0.003 %, the balance including Fe, and unavoidable impurities. Further, the wire rod comprises a pearlite structure wherein an area ratio of a second-phase ferrite is not more than 11.0 %, and a pearlite lamellar spacing is not less than 120 µm.

The mechanical properties and corrosion behaviors of ultra-low carbon microalloying steel

Abstract: The continuous cooling transformation (CCT) microstructure evolution in low carbon Mo–Nb–Cu–B microalloying steel (LCS) was investigated by thermo-simulation test. It was found that the full multi-phase of intermediate transformation products could be obtained at cooling rate greater than approximately 2 °C/s. In order to get bainitic microstructures, the thermo-mechanical control process (TMCP) was worked out according to CCT microstructure characteristics. After TMCP, 750 MPa grade steel twice over that of weathering steel 09CuPCrNi (WS) with ferrite and pearlite structures could be achieved. The corrosion behaviors of LCS and WS were also studied by wet-dry cycle corrosion test. The results showed that LCS had higher corrosion resistance than WS, attributed to suppression of the formation of pearlite.

A Study of Diffusion- and Interface-Controlled Migration of the Austenite/Ferrite Front during Austenitization of a Case-Hardenable Alloy Steel

Abstract: Migrating austenite/ferrite interfaces in the ferrite regions of an alloy steel, containing 0.20 wt pct C, 0.87 wt pct Mn, and 0.57 wt pct Cr, with a ferrite/pearlite microstructure have been observed during austenitization by a high-temperature confocal scanning laser microscope in order to determine the mechanisms of transformation. The samples were subjected to isothermal (790 °C to 850 °C) and nonisothermal (0.5 °C to 20 °C/s) temperature profiles. The kinetic rates extracted from the observations were compared to models for long-range diffusion-controlled and interface reaction-controlled migration. The transition between the two mechanisms was found to occur at T 0, which defines the temperature and composition at which a partitionless phase transformation is possible. Occurrence of the partitionless, interface-controlled transformation was confirmed by an analysis of carbon distribution and microstructure before and after a sample was subjected to a particular thermal profile. The mobility of such interfaces was found to be in the range 1.6·10−13 to 2·10−12 m4·J−1·s−1, which is consistent with previous studies on interface-controlled migration of the reverse reaction, α to γ, during cooling of dilute substitutional iron alloys. The diffusion-controlled migration, at temperatures below T 0, was found to occur in two stages: an initial stage, at which the growth rate can be predicted by a semi-infinite diffusion model; and a second stage, at which the growth slows more rapidly, possibly due to the overlap of diffusion fields.

A computational study of austenite formation kinetics in rapidly heated steels

Abstract: Surface hardening of steels involves rapid austenitization and subsequent quenching of the surface. The resulting extent of hardening largely depends on the rate of austenitization of the surface under the applied high heating rates. In the present work the kinetics of austenite formation in Fe–C alloys during rapid, non-isothermal heating conditions, characterized by high heating rates and short austenitization periods, were studied by means of computational simulation. Austenitization of lamellar pearlite/proeutectoid ferrite microstructures was simulated by assuming two kinetically distinct stages: i) dissolution of lamellar pearlite followed by ii) dissolution of proeutectoid ferrite. The two stages were simulated by two corresponding 1-D diffusion models employed in series. Numerical solution of the resultant moving-boundary diffusion problems provide calculated results regarding the dependency of vol. fraction austenite on thermal cycle parameters and on initial microstructural features of the steel. Analysis of calculated results showed that the vol. fraction of pearlite transforming to austenite during pearlite dissolution depended on maximum temperature, dwell time and pearlite interlamellar spacing. A functional relationship between these variables, consisting of a thermodynamic and a kinetic term, was established. On the other hand, the total vol. fraction of austenite forming in the steel, after both stages of austenitization, was found to follow a typical sigmoidal kinetic behaviour.

Mechanical properties and fracture upon static tension of the high-carbon steel with different types of pearlite structure

Abstract: The hardness and mechanical properties of the U10 steel (1.03 wt % C) with pearlite structures that were formed by isothermal decomposition at temperatures of 650°C (coarse-lamellar pearlite) and 500°C (fine-lamellar pearlite) as well as upon subsequent annealing of fine-lamellar pearlite at a temperature of 650°C for 10–300 min have been studied upon tensile tests. Fractures of the steel with different types of pearlite structure have been examined using scanning electron microscopy. The interrelation between the mechanical properties and the structural features and character of fracture has been analyzed for this steel with pearlite structures differing in the dispersity, morphology, and defect structure of cementite, and in the levels of solid-solution strengthening and microdistortions of the ferrite-constituent lattice.

Mechanical properties of as cast microalloyed steels containing V, Nb and Ti

Abstract: Tensile, hardness and room temperature Charpy V notch impact tests were used to evaluate the variations in the mechanical properties of a low carbon cast steel containing combinations of vanadium, niobium and titanium in the as cast condition. Tensile and hardness test results indicate that good combinations of strength and ductility can be achieved by microalloying additions. Based on the TEM studies, random and interphase fine scale microalloy precipitates play a major role in the strengthening of the microalloyed heats. However, the presence of titanium leads to some reduction in the strength of the microalloyed heat. Coarse TiN particles can be responsible for this behaviour. On the other hand, microalloying additions significantly decrease the impact energy and lead to the dominance of cleavage facets on the fracture surfaces. It seems that heterogeneous nucleation of microalloy carbonitrides on dislocations along with coarse ferrite grains and pearlite colonies has triggered the brittle frac...

Precipitation behavior of M2N in a high-nitrogen austenitic stainless steel during isothermal aging

Abstract: The precipitation behavior of M 2 N and the microstructural evolution in a Cr-Mn austenitic stainless steel with a high nitrogen content of 0.43mass% during isothermal aging has been investigated using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The aging treatments have led to the decomposition of nitrogen supersaturated austenitic matrix through discontinuous cellular precipitation. The precipitated cells comprise alternate lamellae of M 2 N precipitate and austenitic matrix. This kind of precipitate morphology is similar to that of pearlite. However, owing to the non-eutectoidic mechanism of the reaction, the growth characteristic of the cellular precipitates is different from that of pearlite in Fe-C binary alloys. M 2 N precipitate in the cell possesses a hexagonal crystal structure with the parameters a = 0.4752nnm and c = 0.4429nm, and the orientation relationship between the M 2 N precipitates and austenite determined from the SADP is [0110] M2N //[101]γ, [2110] M2N //[010]γ,

Microstructure and properties of carbide free bainite railway wheels produced by programmed quenching

Abstract: A kind of carbide free bainite steel was used to produce railway wheels. The alloy design, manufacture process, microstructure and behaviours are summarised. The novel steel is distinctively superior in excellent combination of strength and toughness compared with the traditional pearlite type wheel steels. The details of the microstructure of novel steel have been investigated in nanoscale.