Showing papers on "Pearlite published in 2003"

Decomposition of cementite in pearlitic steel due to plastic deformation

Abstract: The available experimental data and hypotheses concerning cementite decomposition during the cold work of pearlitic steels are reviewed. The results of studies performed using thermomagnetic analysis, Mossbauer spectroscopy, internal friction and APFIM are used to discuss the mechanism governing cementite decomposition. The following features of this phenomenon seem to be important: (i) the fraction of the decomposed cementite increases with the refining of the initial pearlitic structure, i.e. with the increase of the ferrite–cementite interfacial area; (ii) the decomposition effect saturates as strain increases; (iii) carbon–dislocation interaction in ferrite and MeC bonding in cementite have a strong influence on cementite decomposition. The conclusion is made that cementite decomposition is controlled by the transfer of carbon atoms from cementite to dislocations accumulated near the interface during deformation. This is because the binding enthalpy between carbon atoms and dislocations in ferrite exceeds the solution heat of cementite. Some relevant effects of cementite decomposition in practice are discussed.

Continuous cooling transformation of undeformed and deformed low carbon pipeline steels

Abstract: The continuous cooling transformation (CCT) behaviors of three low carbon pipeline steels containing the different carbon and alloy additions such as Mn, Nb, V, Ti and/or Mo were investigated in the undeformed and deformed conditions, respectively. The corresponding static (without hot deformation) and dynamic (with hot deformation) CCT diagrams were constructed, which almost involved the formation curves of bainitic ferrite, acicular ferrite, polygonal ferrite, and pearlite. It was found that with the exception of V, the aforementioned alloy additions played a significant role in suppressing the formation of polygonal ferrite and promoting the formation of acicular ferrite. Furthermore, hot deformation could also strongly promote the formation of acicular ferrite, that is, the temperature zone of acicular ferrite transformation was enlarged from 400-600 degreesC in the static CCT diagrams to 450-700 degreesC in the dynamic CCT diagrams. The corresponding cooling rate range of acicular ferrite transformation was significantly increased, and the island constituents in acicular ferrite became finer due to hot deformation. (C) 2003 Elsevier Science B.V. All rights reserved.

Microstructure-Level Force Prediction Model for Micro-milling of Multi-Phase Materials

Abstract: A mechanistic model for the micro-endmilling process is developed that explicitly accounts for the different phases while machining heterogeneous materials. It is shown that frequencies in the cutting force signal higher than those that can be explained by the kinematics of the process can be explained by considering the multiple phases in the material. Experiments are performed on two compositions of ductile iron, pure ferrite and pearlite workpieces. These experiments show that the nature of the variation in the ductile iron cutting force signals can be attributed to the mixture of the phases. Additionally, simulation studies show that the frequency component of the variation is related to the spacing of the secondary (ferrite) phase and the magnitude of this component is determined by the size of the secondary phase particles.

Deformation-induced phase transitions in a high-carbon steel

Abstract: The methods of Mossbauer spectroscopy, X-ray diffraction analysis, electron microscopy, magnetostructural analysis and durometry were used to examine structural and phase transitions in a high-carbon pearlitic steel of the formula Fe–1.37mass% C (U13) during cold deformation (300 K) by compression shear in Bridgman anvils. The dissolution kinetics of a cementite having different morphologies was established. A supersaturated solid solution of carbon in BCC iron, an austenite with a high carbon concentration, and metastable e - and χ -carbides were formed during dissolution of the pearlite. Strengthening of the U13 steel depended on the formation of solid solutions of carbon in iron and precipitation of dispersed metastable carbides facilitating pinning of dislocations.

Machining Simulation of Ductile Iron and Its Constituents, Part 2: Numerical Simulation and Experimental Validation of Machining

Abstract: To understand the influence of cast iron microstructure on its machinability, a numerical model that simulates machined material on a microstructure scale was developed. This microstructure-level model assembles individual constituents into a composite material based on microstructural composition, grain size, and distribution. Extensive experimentation was performed to determine strain, strain rate, temperature, and load history dependent material properties. The purpose of this work is to validate the microstructure-level model on machining of ductile iron and two of its constituents: pearlite and ferrite. Orthogonal cutting experiments were conducted of the three materials. The measured chip morphology and machining forces were compared with the model predictions, and a good correlation between them was found.

Stress corrosion cracking fracture mechanisms in rock bolts

Abstract: Rock bolts have failed by Stress Corrosion Cracking (SCC). This paper presents a detailed examination of the fracture surfaces in an attempt to understand the SCC fracture mechanism. The SCC fracture surfaces, studied using Scanning Electron Microscopy (SEM), contained the following different surfaces: Tearing Topography Surface (TTS), Corrugated Irregular Surface (CIS) and Micro Void Coalescence (MVC). TTS was characterised by a ridge pattern independent of the pearlite microstructure, but having a spacing only slightly coarser than the pearlite spacing. CIS was characterised as porous irregular corrugated surfaces joined by rough slopes. MVC found in the studied rock bolts was different to that in samples failed in a pure ductile manner. The MVC observed in rock bolts was more flat and regular than the pure MVC, being attributed to hydrogen embrittling the ductile material near the crack tip. The interface between the different fracture surfaces revealed no evidence of a third mechanism involved in the transition between fracture mechanisms. The microstructure had no effect on the diffusion of hydrogen nor on the fracture mechanisms. The following SCC mechanism is consistent with the fracture surfaces. Hydrogen diffused into the material, reaching a critical concentration level. The thus embrittled material allowed a crack to propagate through the brittle region. The crack was arrested once it propagated outside the brittle region. Once the new crack was formed, corrosion reactions started producing hydrogen that diffused into the material once again.

Steel sheet for hot press having excellent hot workability, and automotive member

Abstract: PROBLEM TO BE SOLVED: To provide a steel sheet for hot press having ≥980 MPa tensile strength, high workability, high formability and further excellent hot formability and also to provide an automotive member using the steel sheet. SOLUTION: The steel sheet for hot press having excellent hot formability has the following characteristics: after hot press is applied to the steel sheet for hot press which has a composition consisting of, by mass, 0.05 to 0.40% C, ≤2.0% Si, 0.01 to 4% Mn, ≤0.1% P, ≤0.05% S, 0.005 to 2% Al, ≤0.01% N, 0.1 to 3%, in total, of either or both of Mo and Nb, and the balance Fe with inevitable impurities and also has a microstructure composed of ferrite and pearlite or of ferrite, cementite and pearlite, the microstructure contains martensite as a phase with a maximum area ratio and tensile strength after hot press is ≥980 MPa. COPYRIGHT: (C)2005,JPO&NCIPI

Mecanical properties of cementite and fabrication of artificial pearlite

Abstract: 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

Model for estimation of transformation kinetics from the dilatation data during a cooling of hypoeutectoid steels

Abstract: The purpose of this paper is to depict a method for quantitatively relating the dilatation curves to the proeutectoid ferrite and the pearlite transformation kinetics during a continuous cooling of the hypoeutectoid steels. If the volume fractions of phases can be quantitatively related to the dilatation data, the dilatometric technique may be efficiently applicable to the investigation of the phase transformation kinetics. In the present work, a model directly extracting the information of the proeutectoid ferrite and the pearlite transformation from the dilatation curve for the hypoeutectoid steels was developed. The volume fraction of the phases was calculated from the dilatation curve by using the linear thermal contraction coefficients, the lattice parameters and the compositions of phases under the paraequilibrium condition. The proposed model was based on the carbon enrichment of the austenite and the difference in unit volume of phases during the phase transformation. The model was applied to the determination of the phase transformation kinetics from the dilatation curves and verified by comparing the model results with the experimental results of the hypoeutectoid steels. It was shown that there were excellent agreements between the calculation results and the experiments results.

Process for producing high tensile hot-dip zinc-coated steel sheet of excellent ductility and antifatigue properties

Abstract: A hot rolled plate of a composition comprising 0.05 to 0.20 mass% of C, 0.3 to 1.8 mass% of Si, 1.0 to 3.0 mass% of Mn and the remainder consisting of Fe and inevitable impurities, having a composite texture consisting of ferrite main phase and second phase wherein the second phase comprises 80 vol.% or more of bainite and the remainder consisting of any one or two or more of martensite, retained austenite and pearlite and wherein the ratio of plate-thickness-direction average length to average length of the second phase is 0.7 or more, as a starting material, is subjected to cold rolling, given heat treatment, hot dip zinc coating and cooling operations. As a result, there can be obtained a high tensile hot-dip zinc-coated steel sheet exhibiting a ductility that is satisfactory for use as a material of automobile parts and also excellent antifatigue properties.

Magnetic Evaluation of Microstructures and Strength of Eutectoid Steel

Abstract: Microstructures and strength of variously heat treated eutectoid steel were evaluated by magnetic property measurements. Isothermal transformation, continuous cooling or spheroidization heat treatment was performed to produce various microstructures. Microstructural parameters (phase, pearlite interlamellar spacing), mechanical properties (fracture strength) and magnetic parameters (coercivity, remanence, hysteresis loss, saturation magnetization) were measured to investigate the relationships among these parameters. Coercivity and remanence were observed to be high in order of martensite, pearlite and ferrite phase. The linear decrease of coercivity and remanence with the interlamellar spacing and the linear increase of those with fracture strength of pearlitic eutectoid steel were found. Coercivity and remanence were suggested as potential magnetic parameters for discriminating phases and quantitatively assessing the pearlite interlamellar spacing as well as strength of eutectoid steel.

Machinable austempered cast iron article having improved machinability, fatigue performance, and resistance to environmental cracking and a method of making the same

Abstract: A machinable austempered cast iron article has improved strength, machinability, fatigue performance, and resistance to environmental cracking. A method of making the machinable austempered cast iron article includes austenitizing an iron composition having a substantially pearlitic microstructure in an intercritical temperature range of between 1380° F. and 1500° F. This produces a ferritic plus austenitic microstructure. The ferritic plus austenitic microstructure is quenched into an austempering temperature range of between 575° F. and 750° F. within 3 minutes to prevent formation of pearlite. The ferritic plus austenitic microstructure is then austempered in the austempering temperature range of between 575° F. and 750° F. to produce a microstructure of a continuous matrix of equiaxed ferrite with islands of austenite. Finally, the microstructure of the continuous matrix of equiaxed ferrite with islands of austenite is cooled to ambient temperature to produce the machinable austempered cast iron article.

Low-Stress Abrasive Wear Behaviour of a 0.2% C Steel: Influence of Microstructure and Test Parameters

Abstract: A low (0.2%) carbon steel has been subjected to heat treatment to form varying quantities of ferrite plus martensite in its microstructure. This was achieved by holding the samples in the two-phase (ferrite plus austenite) region at three different temperatures (750, 780, and 810 °C) for a specific duration followed by quenching in ice water. In another exercise, the steel was also subjected to annealing treatment by austenitizing at 890 °C followed by furnace cooling for comparison purposes. The samples were subjected to low-stress (three-body) abrasion tests using an ASTM rubber wheel abrasion test apparatus at different wheel speeds (150, 273 and 400 rpm corresponding to linear speeds of 1.79, 3.26 and 4.78 m/s respectively) for different sliding distances at a fixed load of 49 N. Crushed silica sand particles of size ranging from 212 to 300 μm were used as the abrasive medium. The wear rate of samples decreased progressively with sliding distance until a (nearly) steady-state condition was attained. This was considered to be due to abrasion-induced work hardening of subsurface regions as well as the greater tendency of protrusion of the harder martensite/pearlite phase at longer sliding distances, thereby providing greater resistance to wear. Decreasing wear rate with increasing treatment temperature 750–810 °C could be attributed to the greater volume fraction of the hard martensite phase in the samples containing ferrite plus martensite. The lower wear rate observed in the case of the samples containing ferrite plus martensite over the annealed ones comprising ferrite and pearlite was attributed to the higher bulk hardness of the former. Increasing linear speed from 1.79 to 3.26 m/s led to an increase in wear rate. This could be attributed to greater tendency of the abrasive particles to create deeper scratches and scouping (digging). A reduction in wear rate with a further increase in the linear speed from 3.26 to 4.78 m/s could be due to a change in the mechanism of wear from predominantly sliding to rolling of the abrasive particles in view of the increased plastic deformability characteristics of the specimens due to higher frictional heating. The present investigation clearly suggests that it is possible to attain a desired combination of bulk hardness and microstructure (consisting of ferrite plus martensite) leading to optimum abrasion resistance in low-carbon steels. The quantity of the two phases in turn could be varied by suitably controlling the heat-treatment temperature.

A model for austenitisation of hypoeutectoid steels

Abstract: In the field of phase transformations in steels, much attention has been paid to the transformation of austenite into diverse product phases but, until recently not much work has been done on the formation of austenite during heating. There are few published models dealing with the transformation of eutectoid or hypoeutectoid steels with a starting microstructure which is a mixture of ferrite and pearlite. The aim of the present work was to use phase transformation theory to develop a model for austenite formation which takes into account the chemical composition and microstructure of the steel studied, and thermal history experienced. Classic nucleation theory and diffusion-controlled growth equations are used to determine the progressive transformation of the different phases into austenite. A phase transformation model with sound physical basis as the one presented in this work can be used to determine the effects of various parameters in the reaction involved, like microstructure (grain size, pearlite spacing), composition, heating rate and others. Another direct application of this model is the generation of CHT (continuous heating transformation) diagrams for specific steels, which are a useful reference in research, as well as in many industrial processes.

Dynamic recrystallization of ferrite in a low carbon steel with different minor microstructures

Abstract: The microstructure evolution during dynamic recrystallization and the deformation characteristics of a low carbon steel within the temperature range of 600-700℃ and a strain rate range of 10-3 to 10 s-1 have been investigated by means of thermal-compression simulation test, SEM, TEM and EBSD. In order to understand the effect of microstructure on dynamic recrystallization behaviour, two different microstructures were adopted, which consist of ferrite plus pearlite and ferrite plus fine cementite particles respectively. The results show that dynamic recrystallization takes place in these two cases. However, in the microstructure with pearlite higher strain is needed to start recrystallization and reach the steady state, comparing with the microstructure with fine cementite particles. The measurement confirms that ferrite with smaller grain size in steady state can be obtained in microstructure with ferrite and cementite, which indicates a more significant effect of separately distributed fine cementite particles on promoting recrystallization nucleation and hindering the grain growth than that of pearlite.

Spheroidization of Low Carbon Steel Processed by Equal Channel Angular Pressing

Abstract: Spheroidization behavior of cementite in a low carbon steel processed by the equal channel angular pressing technique was investigated The effects of annealing temperature and time, and accumulated strain on the morphology of cementite and mechanical properties were studied The results indicated that the application of the severe plastic deformation can improve the kinetics of spheroidization significantly In this study, the enhanced spheroidization kinetics was discussed in terms of carbon dissolution from cementites and defects induced in cementites by the severe plastic deformation In addition, the softening of the steel after the spheroidization treatment was evaluated

Effect of copper on structure formation in cast iron

Abstract: The Fe – C – Cu system has great practical importance due to the use of copper in some kinds of steel and cast iron. However, the content of copper in the known alloys is not always optimum from the standpoint of combination with other alloying elements and carbon. In this connection, the effect of copper on the processes of structure formation in cast iron is analyzed with the use of known and newly plotted fragments (sections and projections) of the phase diagram of the Fe – C – Cu system. Specifically, the solubility of copper is determined in the main phases and structural components (ferrite, austenite, pearlite). The conditions of formation of the copper-bearing phase are investigated.

An attempt to establish the variables that most directly influence the austenite formation process in steels

Abstract: The aim of this work is to evaluate the influence of heating rate and initial microstructure on the anisothermal formation of austenite. In this sense, the start (Ac 1 ) and finish (Ac 3 ) temperatures of austenite formation have been determined on dilatometric curves obtained at various heating rates in steels with ferrite and/or pearlite initial microstructures. As it was expected, Ac 1 and Ac 3 temperatures rises linearly with heating rate, except for steels with a pure ferrite initial microstructure where the Ac 1 temperature is almost insensitive to heating rate over the range studied. Experimental results in steels with a pearlite and ferrite-pearlite initial microstructures also show that the elevation of the critical temperatures with heating rate is quite sensitive to the morphology of pearlite. It seems that the higher the heating rate is, the stronger the influence of morphology on the critical temperatures are. This experimental study and the knowledge of the mechanisms that control the austenite formation process have allowed to establish the variables that most directly influence this reaction in steels with pearlite and ferrite-pearlite initial microstructures. Those are the heating rate and the two parameters that characterise the morphology of pearlite, the mean true interlamellar spacing and the edge length of the pearlite colonies interface in pearlitic steels, together with the volume fraction of pearlite and the mean free distance of pearlite in ferrite plus pearlite initial microstructures. Likewise, two equations have been proposed for the determination of the start (Ac 1 ) and (Ac 3 ) finish temperatures of austenite formation as a function of those variables.

Microstructure and grain boundary microanalysis of X70 pipeline steel

Abstract: Grain boundaries (GBs), particularly ferrite: ferrite GBs, of X70 pipeline steel were characterized using analytical electron microscopy (AEM) in order to understand its intergranular stress corrosion cracking (IGSCC) mechanism(s). The microstructure consisted of ferrite (alpha), carbides at ferrite GBs, some pearlite and some small precipitates inside the ferrite grains. The precipitates containing Ti, Nb, V and N were identified as complex carbo-nitrides and designated as (Ti, Nb, WC, N). The GB carbides occurred (1) as carbides along ferrite GBs, (2) at triple points, and (3) at triple points and extending along the three ferrite GBs. The GB carbides were Mn rich, were sometimes also Si rich, contained no micro-alloying elements (Ti, Nb, V) and also contained no N. It was not possible to measure the GB carbon concentration due to surface hydrocarbon contamination despite plasma cleaning and glove bag transfer from the plasma cleaner to the electron microscope. Furthermore, there may not be enough X-ray signal from the small amount of carbon at the GBs to enable measurement using AEM. However, the microstructure does indicate that carbon does segregate to alpha : alpha GBs during microstructure development. This is particularly significant in relation to the strong evidence in the literature linking the segregation of carbon at GBs to IGSCC. It was possible to measure all other elements of interest. There was no segregation at alpha : alpha GBs, in particular no S, P and N, and also no segregation of the micro-alloying elements, Ti, Nb and V. (C) 2003 Kluwer Academic Publishers.