Pearlitic Cementite Research Articles

Reverse Austenite Transformation Behavior of Equal Channel Angular Pressed Low Carbon Ferrite/Pearlite Steel

Reverse austenite transformation characteristics of ultrafine grained ferrite/pearlite low carbon steel were examined by using a dilatometry with a heating rate of 10°C/s, and compared with those of coarse grained counterpart. Ultrafine grained steel was prepared by equal channel angular pressing at 500°C, resulting in a microstructure consisting of ultrafine ferrite grains and pearlite with partially dissolved pearlitic cementite. Reverse austenite transformation start and finish temperatures of ultrafine grained steel were lower than those of coarse grained steel. Reverse austenite transformation of coarse grained steel occurred with the two serial stages of pearlite→austenite followed by proeutectoid ferrite→austenite. In the case of ultrafine grained steel, reverse austenite transformation was manifested by the three serial stages of carbon-supersaturated ferrite→austenite, pearlite→austenite, and proeutectoid ferrite→austenite in the order. The formation of carbon-supersaturated ferrite in ultrafine grained steel is associated with carbon dissolution from pearlitic cementite during equal channel angular pressing. The effect of equal channel angular pressing on reverse austenite transformation temperatures can be explained in terms of the difference in shear stress driving transformation between two steels under assumption that the nature of carbon-supersaturated ferrite in ultrafine grained steel is similar to that of conventional martensite.

Ultrafine grained steels processed by equal channel angular pressing

Recent development of ultrafine grained (UFG) low carbon steels by using equal channel angular pressing (ECAP) and their room temperature tensile properties are reviewed, focusing on the strategies overcoming their inherent mechanical drawbacks. In addition to ferrite grain refinement, when proper post heat treatments are imposed, carbon atom dissolution from pearlitic cementite during ECAP can be utilized for microstructural modification such as uniform distribution of nano-sized cementite particles or microalloying element carbides inside UFG ferrite grains and fabrication of UFG ferrite/martensite dual phase steel. The utilization of nano-sized particles is effective on improving thermal stability of UFG low carbon ferrite/pearlite steel but less effective on improving its tensile properties. By contrast, UFG ferrite/martensite dual phase steel exhibits an excellent combination of ultrahigh strength, large uniform elongation and extensive strain hardenability.

X-Ray Study of Microstresses in Lamellar Pearlite

The material studied was commercial pearlitic steel deformed by cold drawing. Internal stresses appearing due to the plastic interaction of cementite and ferrite phases of the lamellar pearlite (interphase stresses) were examined in order to clarify the mechanisms controlling plastic behavior of the lamellar pearlite. It was shown that, while the interphase stress level is high at relatively small drawing strain, it is significantly reduced with true strain increasing up to e = 2. The high level of the interphase stress is restored after annealing of the heavily drawn wire and its repeated straining. This result is considered to support a suggestion that the reduction of interphase stress under severe cold drawing is caused by a transformation of the cementite lamellae substructure.

Ultrafine grained ferrite–martensite dual phase steels fabricated via equal channel angular pressing: Microstructure and tensile properties

Ultrafine grained (UFG) ferrite–martensite dual phase steels containing different amounts of vanadium were fabricated by equal channel angular pressing (ECAP) and subsequent intercritical annealing. Their room temperature tensile properties were examined and compared to those of a coarse grained counterpart. The formation of UFG martensite islands of ∼1 μm was not confined to the former pearlite colonies but they were uniformly distributed throughout UFG ferrite matrix. A diffusion analysis showed that this specific microstructure may result from dissolution of carbon atoms from pearlitic cementite and their concurrent diffusion into UFG ferrite during ECAP, making the average carbon content reach the equilibrium content to form austenite during subsequent intercritical annealing. The strength of UFG dual phase steels was much higher than that of the coarse grained counterpart, but uniform and total elongations were not degraded. More importantly, the present UFG dual phase steels exhibited extensive rapid strain hardening unlike most UFG materials. The addition of vanadium slightly increased the strength and elongation of the present UFG dual phase steels, but it was found that excessive vanadium addition did not lead to further improvement of their mechanical properties. An excellent combination of strength, elongation and strain hardening of the present UFG dual phase steels was explained in terms of their specific microstructural features.

Failure analysis of diesel engine rocker arms

This paper presents a failure analysis of two diesel engine rocker arms used in trucks, which failed in service. The fracture occurred at the hole of the rocker arm shaft in two cases. Beach marks and fatigue steps can be observed on the fracture surface. Multiple-origin fatigue is the dominant failure mechanism. A detailed metallurgical investigation was conducted on the failed rocker arms, and compared with a new one. The failed rocker arms present general metallurgical characteristics that the spheroidization of cementite in pearlite appears in all the matrix structure, and a banded structure was observed in the crack origin region. The appearance of the granulated pearlite makes the hardness of the material decrease so as to reduce the fatigue strength of the rocker arms. A normalizing treatment test was performed on the material of the failed and new rocker arms. The formation of a well-distributed lamellar pearlite structure, the increase in hardness, and the disappearance of the banded structure indicate that the unsuitable normalizing technology was responsible for the microstructure defects.

A Mössbauer Spectral Study of the Hull Steel and Rusticles Recovered from the Titanic

The recent recovery of steel from the Titanic has permitted a 295 K conversion electron Mossbauer spectral study of the Titanic hull plate steel oriented with the gamma-ray direction either perpendicular or parallel to the microstructural banding directions. The two spectra reveal virtually identical average orientations of the magnetization close to the plane of the plate. The hyperfine parameters are virtually identical to those of α-iron, a finding which agrees with the chemical analysis which reveals at most 0.21 wt% carbon corresponding to 3 wt% of cementite in pearlite. A 4.2 to 295 K transmission Mossbauer spectral study of the rusticles reveals small particles of geothite undergoing superparamagnetic relaxation with a blocking temperature of ca. 300 K. In addition approximately two percent of the Mossbauer spectral absorption area corresponds to a quadrupole doublet with hyperfine parameters typical of green rust. The identified iron containing components in the rusticles agree with the powder X-ray diffraction results which reveal the predominant presence of small particles of poorly crystallized goethite and traces of quartz and green rust. An average size of 20±5 nm for the goethite particles is obtained from both the average hyperfine field and the broadening of the X-ray diffraction peaks. The magnetic anisotropy constant of the goethite particles deduced from the hyperfine field and the particle size is 8×103 J/m3.

Mechanical Properties of Cementite and Fabrication of Artificial Pearlite

The deformation behavior of cementite was studied using eutectoid steels with pearlitic and spheroidite structures and fine grained bulk cementite prepared by sintering the mechanically alloyed powders. Cementite lamellae in deformed pearlite exhibited inhomogeneous slip, thinning by deck-of-cards slip, homogeneous bending, fragmentation, cleavage fracture and cracking by shear bands. Complete dissolution of cementite lamellae and spheroidal cementite was also observed in specimens deformed at high strain rates. The polycrystalline bulk cementite is brittle and fractures within elastic limit when deformed below 573 K. However, large plastic deformation was observed when compressed at high temperatures (over 773 K) and low strain rates (10 -5 ~ 10 -3 s -1 ). The artificial pearlite was successfully fabricated by hot compression starting from the

Effect of carbon content on mechanical properties of fully pearlitic steels

The effects of microstructural features, such as interlamellar spacing, prior austenite grain size (AGS), and carbon content, on mechanical properties have been investigated for fully pearlitic steels containing 0.52 and 0.82 wt-%C. Interlamellar spacing and prior AGS had significant influence on strength and ductility of eutectoid steels. While interlamellar spacing and prior AGS had a significant influence on the ductility of eutectoid steels, the carbon content showed a stronger effect on ductility than interlamellar spacing and prior AGS for fully pearlitic steels with various carbon contents. The carbon content in steels controls the discontinuous nature, the thickness, and the volume fraction of cementite in pearlite. Therefore, hypoeutectoid steels show the superior reduction of area than eutectoid steels in all tested ranges of interlamellar spacing, in spite of the various prior AGS.

Microstructure evolution in pearlitic steels during wire drawing

Major processes affecting microstructure of a drawn pearlitic wire including lamellae thinning, changes in inter-lamellar interface and metallographic and crystallographic texture, plastic flow localization, and dynamic strain aging were characterized. Heavily drawn pearlite represents a nano-composite with thickness of ferrite and cementite lamellae decreasing during wire drawing. Volume fraction of inter-phase interfaces is comparable with that of pearlitic cementite and they are associated with high elastic stresses. Stretching and rotation of pearlite colonies result in their alignment with the wire axis. This is accompanied by increase in a local ductility at the true strain below from 1.5 to 2 and then decrease at higher strain levels. Development of a strong crystallographic texture causes anisotropy in mechanical properties. Targeted observations of plastic flow at the same region showed two systems of localized shear bands and provided information on their development. A dramatic decrease in elongation to failure in wires after drawing is linked to the existence of the localized shear bands. Dynamic strain aging increases strength and degrades ductility of drawn wires.

Annealing behavior of submicrometer grained ferrite in a low carbon steel fabricated by severe plastic deformation

The annealing behavior of submicrometer grained ferrite in a low carbon steel processed by equal channel angular pressing (ECAP) was investigated. In particular, the effect of the ECAP strain on the annealing behavior of submicrometer grained ferrite was studied by examining the microstructural changes of the samples processed by two different ECAP strains, 4 and 8, during static annealing. Contrary to the general belief that more severely worked materials exhibit faster kinetics of microstructural evolution related to annealing, i.e. recovery, recrystallization and grain growth, the steel subjected to higher ECAP strain exhibited more sluggish recovery and recrystallization kinetics. The more sluggish recovery and recrystallization kinetics with increasing the ECAP strain is associated with the carbon dissolution from pearlitic cementite during ECAP. The grain growth kinetics of submicrometer grained ferrite was analyzed by the non-ideal grain growth law. The grain growth exponent was about 0.2 and the apparent activation energy for grain growth was 170–180 kJ mol −1. Neither value was influenced by the ECAP strain.

2833 鉄鋼材料のマイクロ機械加工における結晶依存性

MEMS is getting a lot attention in processing micro devices. But, these processing is limited to mass production and structure. So, micro machining is reconsidered because that is efficient way to process three-dimentional shape, to make complex structure. The propose of this investigation is to analyze the effect of material's inhomogeneity in carbon steel and size effect on the shape, accuracy of dimension, machinability. Micro scratching test is one of the methods to analyze these phenomena. This paper reports on the results of micro scratching conduct on the S25C and discussion on the deformation mechanism of cementite in pearlite.

Ultrafine Grained Low Carbon Steels Fabricated by Equal Channel Angular Pressing: Microstructures and Tensile Properties.

Equal channel angular pressing (ECAP) was conducted on the two grades of low carbon steel, with or without vanadium, in order to produce an ultarfine grained structure. As a result, the ferrite grains were re-fined from 30 µm to 0.2–0.3 µm. The strength of the ECAPed steels increased remarkably, over twice of the strength of the steels before ECAP. A series of static annealing experiments showed that the increment of ECAP strain and the dilute addition of microalloying element such as vanadium were very effective on enhancing thermal stability of the ultrafine gained low carbon steels produced by ECAP in terms of microstructure and tensile properties. This enhanced thermal stability resulted from; (a) presence of excessive carbon content in the ferrite matrix by carbon dissolution from pearlitic cementite during ECAP; (b) preservation of high dislocation density due to addition of vanadium, providing the effective diffusion path for dissolved carbon atoms; (c) precipitation of excessive carbon as the form of nano-sized cementite particles during subsequent annealing and its effect on suppressing grain growth.

Kinetics and Crystallography of Intragranular Pearlite Transformation Nucleated at (MnS+VC) Complex Precipitates in Hypereutectoid Fe-Mn-C Alloys.

Kinetics and crystallography of intragranular pearlite nucleated at the surface of (MnS+VC) complex precipitate were studied in hypereutectoid Fe-Mn-C steels. The incoherent MnS embedded in the austenite does not act as a strong nucleation site of pearlite unless the transformation time is prolonged. The intragranular pearlite transformation is promoted effectively by the addition of vanadium (V). EPMA analysis showed that the intragranular pearlite nucleates on the (MnS+VC) complex precipitate in the V-added alloy. As the transformation temperature decreases, the intragranular pearlite formation occurs more frequently. A single intragranular pearlite is composed of several colonies, indicating that multiple pearlite colonies nucleate on a (MnS+VC) complex precipitate for intragranular pearlite transformation. There is no specific orientation relationship (OR) between ferrite in intragranular pearlite and austenite matrix while there is a specific OR (Pitsch-Petch OR) between pearlitic ferrite and cementite in the intragranular pearlite.

Deformation and dissolution of spheroidal cementite in eutectoid steel by heavy cold rolling

The microstructural evolution of a spheroidised Fe-0.85C wt- steel during heavy cold rolling and post-aging was studied, with the emphasis on cementite. A new morphology of cementite, wavy thin bands nearly parallel to the rolling plane, was found in the specimens subjected to rolling reductions >90. Slip on 001theta planes was observed in the deformed cementite particles. By X-ray diffraction, it was found that the initial spheroidal cementite particles dissolve partially in a solid solution of ferrite by deformation, in a manner similar to that of the lamellar cementite of pearlite. Greater strain age hardening was observed for the spheroidised steel than for a similarly treated pearlitic Fe-0.85C steel. The strain age hardening peak was observed at 573 K for the spheroidite structure, which is 150 K higher than was observed for the pearlite structure. The activation energy for age hardening was estimated to be 122 kJ mol1, which is attributed to carbon atom diffusion from cementite to dislocations in ferrite.

Microstructural stability of ultrafine grained low-carbon steel containing vanadium fabricated by intense plastic straining

Two grades of low-carbon steel, one containing vanadium and the other without vanadium, were subjected to equal channel angular pressing (ECAP) at 623 K up to an effective strain of ∼4. After equal channel angular pressing, a static annealing treatment for 1 hour was undertaken on both pressed steels in the temperature range of 693 to 873 K. By comparing the microstructural evolution during annealing and the tensile properties of the two steels, the effect of the addition of vanadium on the thermal stability of ultrafine-grained (UFG) low-carbon steel fabricated by intense plastic straining was examined. For the steel without vanadium, coarse recrystallized ferrite grains appeared at annealing temperatures above 753 K, and a resultant degradation of the strength was observed. For the steel containing vanadium, submicrometer-order ferrite grain size and ultrahigh strength were preserved up to 813 K. The enhanced thermal and mechanical stabilities of the steel containing vanadium were attributed to its peculiar microstructure, which consisted of ill-defined pearlite colonies and ultrafine ferrite grains with uniformly distributed nanometer-sized cementite particles. This microstructure resulted from the combined effects of (a) the preservation of high dislocation density providing an effective diffusion path, due to the effect of vanadium on increasing the recrystallization temperature of the steel; and (b) precipitation of fine cementite particles at ferrite grain boundaries through the enhanced diffusion of carbon atoms (which were dissolved from pearlitic cementite by severe plastic straining) along ferrite grain boundaries and dislocation cores.

Microstructural evolution in a commercial low carbon steel by equal channel angular pressing

In the present investigation, both macroscopic and microscopic microstructural changes in a plain low carbon steel with a mixed structure of ferrite and pearlite were examined during equal channel angular pressing . During equal channel angular pressing, the sample was rotated 180°around its longitudinal axis between the passages. By repeating the pressing up to the accumulated effective strain of ∼4, ferrite grains with a submicron size of 0.2–0.3 μm were obtained. Optical observation revealed that hard pearlite phase deformed macroscopically in a similar manner to soft ferrite phase due to a considerable capability of pearlitic cementite for plastic deformation under the present equal channel angular pressing conditions. An examination of micrographs taken by transmission electron microscopy showed that the microstructural change in ferrite phase was similar to those in single phase Al solid solution alloys in that the non-equilibrium configurations of grain boundary were observed. In pearlite phase, the morphological change of cementite lamellar plates was represented by the existence of either severely necked fragments parallel to each other or curled wavy fragments. Microstructural evolution in a plain low carbon steel subjected to equal channel angular pressing was discussed by comparing it to that of equal channel angular pressed Al solid solution alloys and heavily cold drawn pearlitic steel wire, which exhibited similar microstructural change.

Abnormal ferrite in hyper-eutectoid steels

The microstructural characteristics of ultra-high carbon hyper-eutectoid Fe–C and Fe–C–Cu experimental steels have been examined after isothermal transformation in a range just beneath the eutectoid temperature. Particular attention was paid to the formation of so-called “abnormal ferrite”, which refers to coarse ferrite grains which can form, in hyper-eutectoid compositions, on the pro-eutectoid cementite before the pearlite reaction occurs. Thus it is confirmed that the abnormal ferrite is not a result of pearlite coarsening, but of austenite decomposition before the conditions for coupled growth of pearlite are established. The abnormal ferrite formed on both allotriomorphic and Widmanstätten forms of pro-eutectoid cementite, and, significantly, it was observed that the pro-eutectoid cementite continued to grow, despite being enclosed by the abnormal ferrite. Under certain conditions this could lead to the eventual formation of substantially reduced amounts of pearlite. Thus, a model for carbon redistribution that allows the pro-eutectoid cementite to thicken concurrently with the abnormal ferrite is presented. The orientation relationships between the abnormal ferrite and pro-eutectoid cementite were also determined and found to be close to those which have been reported between pearlitic ferrite and pearlitic cementite.

Visualization of carbon in steels using energy filtering TEM

Energy filtering TEM [Reimer L (1984) Transmission Electron Microscopy, Physit of Image Formation and Microanalysis, pp. 391–393, (Springer, Berlin)] i potentially capable of visualizing the distribution of a particular element a the nanometre scale. However, visibility of the energy-loss image depend on the combination of the atomic species of interest and the matrix. In thi paper we discuss experimental conditions required for obtaining carbo: ‘mapping’ in steels. Three types of common carbides cementite, (Fe3C), (Ti, Nb)C and M23C6, in both thin foil and extraction replica specimens wer examined in a high voltage high resolution electron microscope equippe with a post-column energy filtering system. The peak to background (P/B ratio at the C–K edge is the determining factor for obtaining a carbon image Carbon mapping was relatively easy for cementite in pearlite when th electron beam specimen geometry was such that the carbide was present a through the foil thickness. In contrast, in the case of M23C6 in a stainles steel, carbon imaging was possible when the carbide was extracted but nc when it was embedded in the matrix. Based on the statistic theory of Berge and Kohl [(1993) Optik 92: 175–193], we estimated the minimum carbid thickness tmin and the maximum total foil thickness as a function of the Mi type carbide thickness for obtaining the carbon image. The total foil thicknes to should be thinner than about 44 nm when the MC carbide, whose thicknes is 5 nm, is to be imaged with the C–K edge.

Boundary characterisation of X65 pipeline steel using analytical electron microscopy

Analytical electron microscopy was used to characterise grain boundaries (GBs) and interphase boundaries (IBs) of X65 pipeline steel. There was no segregation of P or S at the proeutectoid ferrite GBs. This indicates that contrary to literature expectations, P and S are unlikely to be involved in the mechanism of SCC of pipeline steels. There was Mn segregation at IBs between pro-eutectoid ferrite and pearlitic cementite, and desegregated from the IBs between pro-eutectoid ferrite and pearlitic ferrite. This pattern of Mn segregation is attributed to diffusion in the process zone ahead of the pearlite during the austenite to pearlite transformation and diffusion in the IBs between the proeutectoid ferrite and pearlite. A new mechanism was proposed for pearlite formation. A GB carbide first forms at an α : α GB, and then grows along the α/γ interface. Subsequently pearlite initiates from this interface carbide.

Measurement of grain boundary composition for X52 pipeline steel

Analytical electron microscopy was used to measure the composition of grain boundaries (GBs) and interconstituent boundaries (IBs) of X52 pipeline steel using specimens about 40–60nm in thickness. All elements of interest were examined with the exception of carbon. With this caveat, there was no segregation at proeutectoid ferrite GBs. This indicated that the commonly expected species S and P are not responsible for preferential corrosion of GBs during intergranular stress corrosion cracking of pipeline steels. Manganese was the only species measured to segregate at the IBs. Manganese segregated to the IBs between proeutectoid ferrite and pearlitic cementite, and desegregated from IBs between proeutectoid ferrite and pearlitic ferrite. The pearlitic cementite was Mn rich. There was no Mn segregation at the IBs between pearlitic ferrite and pearlitic cementite. The pattern of Mn segregation could be explained in terms of diffusion in the process zone ahead of the pearlite during the austenite to pearlite transformation and diffusion in the IBs between the proeutectoid ferrite and pearlite.