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The effect of heat treatment parameters (the temperature of 850 – 930°C and the cooling rate of 1 – 20 K/min) on the microstructure and mechanical properties of carbon steel with 1.95 wt.% C is studied. The temperatures of the phase transformations in the steel are determined. X-ray diffractometry is performed. A heat treatment mode preventing the formation of ledeburite in the structure of the steel and promoting a uniform distribution of carbide particles in the matrix is suggested.

Heat treatments are frequently used to modify the microstructure of cast irons according to experimental parameters. Among these, laser surface engineering (LSE) has become relevant for being a highly localized treatment with rapid heating and cooling of the irradiated area resulting in minimal distortion of the workpiece. This work presents and experimentally validates a thermo-metallurgical model able to predict the phase transformations occurring during the LSE treatment of nodular cast iron when it is subjected to different laser beam powers and scanning velocities. For this purpose, an experimental characterization of the thermal history and final microstructure is performed for several operating scenarios. In particular, significant changes in the microstructure can be seen at high powers and low scanning velocity where the matrix is transformed into ledeburite and martensite. The final phase volume fractions predicted by the proposed model along the depth of the sample are compared with the corresponding experimental measurements. The results obtained in the simulation are in good agreement with the experimental measurements. This work highlights the use of our model to be systematically applied for the design and optimization of LSE treatments on cast irons.

The structure and mechanical properties of an ultrahigh-carbon steel modified with amorphous ferrosilicon with a low melting point are studied. The methods of x-ray diffraction, optical and scanning electron microscopy are used to study the effect of the amorphous modifier on the structure of eutectic ledeburite in the cast steel. Tests for tensile strength and impact toughness are conducted. It is shown that the mechanical properties increase as a result of the modification without subsequent heat treatment.

In this study, the effect of microstructure evolution, especially that involving the proeutectoid carbides, on hardness distribution of ledeburite steel containing 8% chromium content was investigated at different cooling rates after hot deformation. The continuous cooling transformation was carried out using a fully automatic transformation measuring apparatus under cooling rates in the range of 0.03-80 °C/s. The transformation temperatures, Ac1, Ac3 and Ms, were measured by dilatometry. A nanoindentation test was employed to evaluate the mechanical properties of the cooled specimens. The evolution of microstructure indicated that the proportion trend of the precipitated carbides was dependent on the cooling rates. In addition, a mixture of pearlite and proeutectoid carbides was observed under slow cooling rates in the range of 0.03-0.2 °C/s, for which only a martensite transformation took place in austenite, and the proeutectoid carbides precipitated negligibly at cooling rates greater than or equal to 10 °C/s. The results also showed that the nanohardness significantly decreased and then slowly increased to a stable value with increasing cooling rates, which was similar to the content trend of the carbides that precipitated during the cooling process.

The deformation behavior of cementite in drawn pearlitic steel and spheroidal cementite steel, which have hypereutectoid composition, was investigated by X-ray diffraction using synchrotron radiation. A detailed analysis of diffraction peak profiles reveals that the deformation behavior strongly depends on the shape of cementite in steel. The unit cell volume of the cementite in the drawn pearlitic steel compressively and elastically deforms by 1.5 to 2 pct of the initial volume at the early stage of drawing, whereas that in the drawn spheroidal cementite steel is compressed by 1 pct of the initial volume even at a large true strain. The cementite in the drawn pearlitic steel fragments into small pieces with increasing the true strain, and these pieces change to amorphous cementite. The dislocation densities of the cementite in the drawn pearlitic steel and in the drawn spheroidal cementite steel are estimated to be ~1013/m2 before drawing and ~1014/m2 after drawing. Although the large strain is induced in the cementite by drawing, the maximum strain energy in the cementite is too small to contribute to the dissolution of the cementite.

It is detected that in some specimens of Damascus steel part of the excess cementite is of unusual origin, in contrast to excess phases of secondary cementite, ledeburite cementite, and primary cementite in ironcarbon alloys. It is revealed that a morphological feature of separate particles of cementite in Damascus steel includes anomalously large excess carbides in the form of irregular octahedral and prisms. It is shown that angular carbides form within the original metastable ledeburite colonies, and therefore they are called “eutectic.” It is established that unalloyed materials of the carbide class acquire Damascus properties steel properties during isothermal exposure on annealing, which leads to thermal separation of colonies of metastable ledeburite, to limitation of newly formed eutectic carbides, and to their subsequent coalescence. It is revealed that some sorts of Damascus steel, being within the field of white cast iron with respect to carbon content, do not contain broken ledeburite within their structure. It is shown that the pattern of carbide inhomogeneity entirely consists of eutectic of angular carbides of non-etching triangular-prismatic morphology.

A Cr-Ni-Mo overlayer was deposited on the surface of compacted graphite iron (CGI) by the plasma transferred arc (PTA) alloying technique. The microstructure of Cr-Ni-Mo overlayer was characterized by optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), and X-ray diffractometer (XRD). Results show that the cross-section consists of four regions: alloying zone (AZ), molten zone (MZ), heat affected zone (HAZ), and the substrate (SUB). The microstructure of AZ mainly consists of cellular γ-(Fe, Ni) solid solution, residual austenite and a network of eutectic Cr7C3 carbide while the MZ area has a typical feature of white cast iron (M3C-type cementite). The martensite/ledeburite double shells are observed in the HAZ. With decreasing the concentration of Cr-Ni-Mo alloys, the fracture mode changes from ductile in the AZ to brittle in the MZ. The maximum hardness of the AZ (450 HV0.2) is lower than that of the MZ (800 HV0.2). The eutectic M3C and M7C3 carbides increase the microhardness, while the austenite decreases that of the AZ.

International audience; The austenitization from a spheroidized microstructure during intercritical annealing was studied in a Fe-0.1C-3.5Mn alloy. The austenite grains preferentially nucleate and grow from intergranular cementite. The nucleation at intragranular cementite is significantly retarded or even suppressed. The DICTRA software, assuming local equilibrium conditions, was used to simulate the austenite growth kinetics at various temperatures and for analyzing the austenite growth mechanism. The results indicate that both the mode and the kinetics of austenite growth strongly depend on cementite composition. With sufficiently high cementite Mn content, the austenite growth is essentially composed of two stages, involving the partitioning growth controlled by Mn diffusion inside ferrite, followed by a stage controlled by Mn diffusion within austenite for final equilibration. The partitioning growth results in a homogeneous distribution of carbon within austenite, which is supported by NanoSIMS carbon mapping.

The effect of final rolling temperature and cooling process on the microstructure of 1.0C-1.5Cr bearing steel was studied, and the relationship between the microstructure parameters and subsequent spheroidization annealing was analyzed. The results indicate that the increase of water-cooling rate after hot rolling and the decrease of final cooling temperature are beneficial to reducing both the pearlite interlamellar spacing and pearlite colony size. Prior austenite grain size can be reduced by decreasing the final rolling temperature and increasing the water-cooling rate. When the final rolling temperature was controlled around 1103 K (830 °C), the subsequent cooling rate was set to 10 K/s and final cooling temperature was 953 K (680 °C), the precipitation of grain boundary cementite was suppressed effectively and lots of rod-like cementite particles were observed in the microstructure. Interrupted quenching was employed to study the dissolution behavior of cementite during the austenitizing at 1073 K (800 °C). The decrease of both pearlite interlamellar spacing and pearlite colony size could facilitate the initial dissolution and fragmentation of cementite lamellae, which could shorten the spheroidization time. The fragmentation of grain boundary cementite tends to form large-size undissolved cementite particles. With the increase of austenitizing time from 20 to 300 minutes, mean diameter of undissolved cementite particles increases, indicating the cementite particle coarsening and cementite dissolution occuring simultaneously. Mean diameter of cementite particles in the final spheroidized microstructure is proportional to the mean diameter of undissolved cementite particles formed during partial austenitizing.

The influence of cementite precipitation and dissolution on the formation of the carbide-free, ultra-fine-grained, ferrite + austenite microstructure of medium manganese steel was analyzed. During heating to the intercritical temperature, cementite nucleates at low-angle lath martensite boundaries, austenite subsequently nucleates at ferrite/cementite boundaries, and the cementite is gradually replaced by the growing austenite grains. The intercritical austenite carbon is therefore due to cementite dissolution, rather than carbon partitioning between ferrite and austenite.

The effect of substitutional element aluminum (Al) on the cementite growth from austenite phase in Fe–C–Al alloy was simulated using ThermoCalc–Dictra software. The simulation is based on diffusional reactions of multi components in the system. The distribution of elements across the interface of cementite and austenite phases and cementite fraction as a function of time for different Al concentrations in Fe–C–Al system were evaluated. It has been found that the presence of Al retards the growth of cementite which attributes to the negligible solubility of Al in the cementite phase.

A medium Mn steel with the martensitic microstructure was rapidly heated at a high rate of 300 K/s to the intercritical temperature of 923 K (650 °C) and then isothermally hold for 5 minutes. Although cementite should dissolve above the Ae1 temperature due to the constraint of thermodynamics, it is surprising to find cementite particles after such intercritical annealing (IA), whose Mn contents and sizes are even up to 30 wt pct and 60 nm, respectively. Numerical simulations have been performed to reveal the mechanism responsible for this new observation. They indicate that a small nucleus of cementite in martensite could rapidly grow to the observed size before austenitization takes place during IA. Such a rapid growth is driven by the rapid partition of C from martensite into cementite. It is then concluded that the precipitation of cementite during the IA of martensitic Mn-alloyed steel appears inevitable no matter how high the heating rate is. Moreover, the growth kinetics of cementite depend on the composition and size of its neighboring martensitic phase, rather than those of cementite nucleus.

The nature of change in the amount and morphology of iron dendrites and excess phases in relation to the degree of high-carbon alloy melt supercooling is considered. It is revealed that in pure iron-carbon alloys by changing the degree of melt supercooling it is possible to obtain steel with a carbon content as in white cast iron, or white cast iron with a carbon content as in steel. It is shown that before forging Damascus steel the structure is ideal with a pearlite matrix, without secondary free cementite, and with excess ledeburite phase. Features of ferrite and ledeburite shape change with thermomechanical treatment (forging) are studied. It is detected that during deformation heating for forging there is ledeburite recrystallization into a more stable phase of eutectic carbide. Limits are determined for transformation of pure high-carbon alloys in carbide class steel of the Damascus type.

In this work, an in situ synthesized TiC-reinforced metal matrix composite (MMC) coating of approximately 350–400 µm thickness was fabricated on a gray cast iron (GCI) substrate by plasma transferred arc (PTA) surface alloying of Ti–Fe alloy powder. Microhardness tests showed that the surface hardness increased approximately four-fold after the alloying treatment. The microstructure of the MMC coating was mainly composed of residual austenite, acicular martensite, and eutectic ledeburite. Scanning electron microscopy (SEM) and X-ray diffraction analyzes revealed that the in situ TiC particles, which were formed by direct reaction of Ti with carbon originally contained in the GCI, was uniformly distributed at the boundary of residual austenite in the alloying zone. Pin-on-disc high-temperature wear tests were performed on samples both with and without the MMC coating at room temperature and at elevated temperatures (473 K and 623 K), and the wear behavior and mechanism were investigated. The results showed that, after the PTA alloying treatment, the wear resistance of the samples improved significantly. On the basis of our analysis of the composite coatings by optical microscopy, SEM with energy-dispersive X-ray spectroscopy, and microhardness measurements, we attributed this improvement of wear resistance to the transformation of the microstructure and to the presence of TiC particles.

Cementite dissolution kinetics in austenite was investigated in a hypereutectoid steel alloy during continuous heating. The quantitative change in cementite volume fraction as a function of thermal history was determined from dilation curves by using the martensite start temperature to calculate prior austenite carbon content. Two characteristics of the cementite dissolution kinetics were found: (1) the cementite dissolution rate increased with time regardless of heating rate due to the increased surface area of cementite particles, and (2) the rate of cementite dissolution was strongly affected by heating rate. An empirical equation combining the effects of cementite volume change and heating rate is proposed to describe cementite dissolution kinetics. A continuous heating transformation diagram for hypereutectoid steels was obtained and compared with the DICTRA simulations and metallographic analyses.

A new steel with high Cr and low W, Mo contents for forged cold work roll was designed based on the composition system of traditional high-speed steel roll. The Fe-C isopleths of the steel and the mass fraction of equilibrium phases versus temperature were calculated by Thermo-Calc, and the effects of different alloying elements (W, Mo, Cr, V) on austenite, ferrite, and carbides (MC, M6C, M7C3, M23C6) were also established to optimize the composition and structure. The designed and optimized specimens were both quenched at 1100 °C and then tempered twice at 560 °C. The hardness and wear resistance of the samples were measured. The microstructures of quenched tempered and forged specimens were studied. The results show that ferrite crystallization, peritectic reaction, austenite crystallization, and the precipitation of MC, M6C, M7C3, M23C6 occur during equilibrium solidification process. The alloying elements W, Mo mainly affect the precipitation of M6C, while Cr affects the precipitated region and mass fraction of M7C3. Higher V content widens the high-temperature region of the peritectic reaction and results in a large amount of MC precipitation. The optimized composition (wt.%) for cold work roll steel is 1.30-1.35%C, 9-10%Cr, 2.5-3.0%Mo, 0.5-1.0%W, 2.5-3.0%V, 0.5-0.6%Mn, 0.5-0.6%Si. The hardness of the steel after quenching and tempering is 60.8 HRC and weight loss after 120 min is 6.2 mg. This meets the requirement of hardness and wear resistance requirements for cold work roll. The ledeburite in the optimized steel disappears after forging and the carbide network break into a large amount of tiny blocky ones dispersed in the matrix without cracks, which shows a good forgeability of the steel and rationality of the optimized composition.

A number of electric flash butt rail welds fractured vertically through the cross section of the rail. The fractures initiated in the web near the bending neutral plane in a region where high-carbon-content liquid was identified to have been produced during the welding process, resulting in a brittle cementite (ledeburite) network structure that reduced the load carrying capacity of the weld. Fracture initiation near the neutral plane was encouraged due to longitudinal stresses in the continuously welded rail from thermal contraction at colder temperatures. Additionally, residual stresses induced by flash butt welding could have contributed to fracture initiation.

The objective of this investigation is to study the influence of vanadium (5.0wt%–10.0wt%) and chromium (0–9.0wt%) on the microstructure and hardness of Cr-V-Mn-Ni white cast irons with spheroidal vanadium carbides. The alloys’ microstructural features are presented and discussed with regard to the distribution of phase elements. The structural constituents of the alloys are spheroidal VC, proeutectoid cementite, ledeburite eutectic, rosette-shaped carbide eutectic (based on M7C3), pearlite, martensite, and austenite. Their combinations and area fraction (AF) ratios are reported to be influenced by the alloys’ chemical composition. Spheroidized VC particles are found to be sites for the nucleation of carbide eutectics. Cr and V are shown to substitute each other in the VC and M7C3 carbides, respectively. Chromium alloying leads to the formation of a eutectic (γ-Fe + M7C3), preventing the appearance of proeutectoid cementite in the structure. Vanadium and chromium are revealed to increase the total carbide fraction and the amount of austenite in the matrix. Cr is observed to play a key role in controlling the metallic matrix microstructure.

Two hypereutectic cast irons (5.01 pct Cr and 5.19 pct V) were cast and the polished surfaces of test pieces were deep-etched and analyzed via scanning electron microscopy. The results show that graphite lamellae intersect the cementite and a thin austenite film nucleates and grows on the cementite plates. For both compositions, graphite and cementite can coexist as equilibrium phases, with the former always nucleating and growing first. The eutectic carbides grow from the austenite dendrites in a direction perpendicular to the primary plates.

The surface of nodular cast iron (NCI) with a ferrite substrate was rapidly remelted and solidified by plasma transferred arc (PTA) to induce a chilled structure with high hardness and favorable wear resistance. The effect of scanning speed on the microstructure, microhardness distribution, and wear properties of PTA-remelted specimens was systematically investigated. Microstructural characterization indicated that the PTA remelting treatment could dissolve most graphite nodules and that the crystallized primary austenite dendrites were transformed into cementite, martensite, an interdendritic network of ledeburite eutectic, and certain residual austenite during rapid solidification. The dimensions of the remelted zone and its dendrites increase with decreased scanning speed. The microhardness of the remelted zone varied in the range of 650 HV0.2 to 820 HV0.2, which is approximately 2.3–3.1 times higher than the hardness of the substrate. The wear resistance of NCI was also significantly improved after the PTA remelting treatment.

Brittle pro-eutectoid cementite that forms along prior-austenite in hypereutectoid steels is deleterious to mechanical properties. The optimum process parameters which suppress the formation of pro-eutectoid cementite in hypereutectoid steels with carbon content in the range of 0.8%–1.3% in mass fraction, were investigated. Pro-eutectoid cementite formation is effectively hindered by increasing the deformation temperature and decreasing the amount of strain. Transformation at lower temperatures close to the nose of the cooling-transformation diagram also reduces the tendency of the formation of pro-eutectoid cementite. Control of prior-austenite grain size and grain boundary conditions is important. Due to larger number of nucleation sites, finer prior-austenite grain size results in the acceleration of transformation to pro-eutectoid cementite. However, large prior-austenite and straight boundaries lead to less nucleation sites of pro-eutectoid cementite. The cooling rate and carbon content should be reduced as much as possible. The transformation temperature below 660 °C and the strain of 0.5 at deformation temperature of 850 °C are suggested.

The possibilities and special features of formation of two-phase austenite-carbide structure in decomposition of supersaturated austenite are studied. The effect of alloying is considered for manganese steels, cast irons, and chilled nickel-molybdenum high-strength cast iron with globular graphite.

The warm compression tests were performed on the eutectoid steel to investigate the evolution of cementite morphology. Several processing parameters, such as temperature, strain rate and reduction, were changed to analyze the effect of each parameter on spheroidization of cementite. The results showed that the warm compression promoted the fragmentize and the spheroidization of lamellar cementites. When the specimen was compressed with reduction of 50% at 700 °C and in the strain rate of 0. 01 s−1, the excellent spheroidized cementite was obtained. The mechanism of fragmentation and spheroidization of lamellar cementites during compression was discussed by using transmission electron microscope. The formation of spheroidized cementite was related to the time of compression process. The fragmentize of lamellar cementites was due to the extension of sub-grain boundary in the cementite. The spheroidization of cementite depended on the diffusion of carbon atoms at the tip of bended and breakup cementite.

The main purpose of the studies was to establish the effect of the tempering temperature on the changes of carbide and retained austenite contents. It has been found that the course of changes of the coercive force of hardened steel after austenitizing is qualitatively similar to the hardness changes. That means in practice it is possible to determine the 217H12WF steel hardness based on the measurement of the coercive force.

The possibility is demonstrated of improving the microstructure of cast high-speed steel R6M5-Sh by use of thermal cycling treatment (TCT) as an intermediate production operation between preliminary isothermal annealing and final hardening with triple tempering. As a result of TCT there is a reduction in the amount of ledeburite eutectic, size of the carbide network, carbide inhomogeneity, and a change in carbide size and composition. Rational treatment regimes are determined.

Fracture mechanisms are considered for high-speed steels of ledeburite (R6M5) and hypereutectoid (11M5F) classes. It is shown that formation of critical length cracks for hypereutectoid steels occurs as a result of not only elastic, but also significant plastic strain, and therefore work for crack generation is significantly greater for steel 11M5F than for R6M5. A relationship is established between strain and work for failure with static bending on one hand, and impact strength on the other.

The isothermal and nonisothermal tempering of martensite in dual-phase (DP) steels was investigated mainly by analytical transmission electron microscopy, and the effect on softening behavior was studied. The isothermal tempering resulted in coarsening and spheroidization of cementite and complete recovery of laths. However, nonisothermal tempering manifested fine quasi-spherical intralath and platelike interlath cementite, decomposition of retained austenite, and partial recovery of laths. The distinct characteristic of nonisothermal tempering was primarily attributed to the synergistic effect of delay in cementite precipitation and insufficient time for diffusion of carbon due to rapid heating that delays the third stage of tempering. The finer size and platelike morphology of cementite coupled with partial recovery of lath resulted in reduced softening in nonisothermal tempering compared to severe softening in isothermal tempering due to large spheroidized cementite and complete recovery of lath substructure. The substitutional content of precipitated cementite in nonisothermal tempering was correlated to the richness of particular steel chemistry. Softening resistance during nonisothermal tempering was related to DP steel chemistry, i.e., Cr and Mn content. Fine cementite and less decomposed martensite in rich chemistry confer high resistance to softening compared to leaner chemistries, which indicated severe decomposition of martensite with coarser cementite.

The effect of silicon on the spheroidization of cementite in hypereutectoid high carbon chromium bearing steels has been investigated on the basis of microstructural analysis and thermodynamic calculations. The results showed that an increase of silicon content in high carbon chromium bearing steels retards the spheoridization of cementite. The thermodynamic calculations revealed that the shrinkage of the austenite phase field in bearing steels with increasing silicon content gave rise to an increase of volume fraction of cementite at an annealing temperature, possibly resulting in incomplete spheroidization. Furthermore, due to the low solubility of silicon in cementite, an increase of silicon content can raise the activity or chemical potential of carbon atoms in austenite at the austenite/cementite interfaces. Consequently, the difference in chemical potential of carbon atoms at the interfaces would be reduced with increasing silicon content, causing a decrease of the driving force for their diffusion from cementite to austenite.

Information on the diffusional transformation products of austenite in high-carbon steels is reviewed and supplemented with new microscopic studies. A comparison with transformation products in low-carbon steels indicates that there is a symmetry with pearlite in the middle, where ferrite and cementite are equal partners, and with acicular ferrite or cementite on each side. They both form with a surface relief, and at lower temperatures, each one is the leading phase in a eutectoid microstructure, bainite and inverse bainite, respectively. However, there is an asymmetry because at low temperatures bainite appears in high-carbon steels but inverse bainite never appears in low-carbon steels. At a constant high carbon content, there is another kind of symmetry, which is related to temperature. At intermediate temperatures the eutectoid reaction results in spherical nodules in which the cementite constituent originates from Widmanstatten plates. It turns spiky at both higher and lower temperatures with the leading phase in the spikes being cementite at higher temperatures and ferrite at lower temperatures. In the first kind of symmetry, there is an abrupt change among the three reaction products; in the second kind of symmetry, there is a gradual change. Accepting that all the eutectoid microstructures form by diffusion of carbon, one may explain the existence of both symmetries by the variation of the ratio of the supersaturations of ferrite and cementite with carbon content and with temperature.

The effect of prior microstructures on the behavior of cementite particles in conjunction with microstructural changes of the matrix during subcritical annealing was investigated by changing the initial microstructures into ferrite + coarse pearlite, ferrite + fine pearlite, bainite, and martensite, in medium carbon steels. While the coarsening of cementite particles in martensite proceeded rapidly with the growth of large cementite particles at boundaries with the dissolution of smaller particles within martensite laths, the coarsening rate of cementite particles in bainite was found to be much slower than that in martensite. This could be attributed to the thermal stability of cementite particles, the smaller amount of carbon in solution, and the lower driving force for solute diffusion due to the uniform size distribution of cementite particles in bainite. The controlling coarsening kinetics in medium carbon steels with ferrite-pearlite, bainite and martensite, were found as boundary diffusion, diffusion along dislocation, a combination of boundary diffusion and diffusion along dislocation, respectively.

Cr-Mo diffusion layer on carbon steels was prepared by double glow plasma surface metallurgy and treated by ultra-saturated carbonization, quenching and tempering, the excellent wear resistance was obtained. The results indicated that the hardness of surface alloying was up to 1300 Hv 0.025 . The carbides in the alloyed layer were fine and dispersed without coarse eutectic ledeburite. The microstructure of the composite layer indicated that the dimension of surface carbides was less 1μm. The abrasion experimental results showed that the average relative wear resistance of modified-steels was 10.3 times as that of T10 steel by quenching and low tempering. The alloy layer possessed a very strong ability resistance to tempering, and its surface hardness exceeds 595 Hv 0.025 after tempering at 690°C for 4h.

Eutectic polyhedration of the diagram is presented and special features of structure formation in high-manganese irons are analyzed. It is shown that eutectics of two kinds (alloyed ledeburite and a eutectic based on M7C3 carbides), which are in peritectic equilibrium with the melt, can be formed and be accompanied by a special austenite-carbide component having a pearlite-like structure. The quasi-eutectic and the M7C3-bearing eutectic in heat-treated high-manganese irons have a special morphology with composite arrangement of carbides in austenite. It is shown that additional alloying of manganese irons with silicon might be a promising direction. Silicon stabilizes the M7C3 carbide in its competition with cementite and ensures the possibility of fabricating austenitic mottles with such carbide. Mottles alloyed with manganese and silicon and having globular graphite, M7C3 carbides and metastable austenitic matrix are characterized by high wear resistance, good antifriction properties, and a combination of high mechanical properties.

Neutron diffraction measurements have revealed that cementite peaks disappear in a pearlite steel with drawing and that the residual intergranular stresses are generated. The diffraction profiles in a heavily drawn specimen suggest the tetoragonality with a small c/a in the ferrite matrix. Although cementite was hardly observed in the heavily drawn specimen, its c/a value determined by neutron diffraction and mechanical behavior are quite different from those of as-quenched martensite. The changes in hardness and c/a with annealing or tempering were also different between heavily drawn pearlite and marteniste. Hence, most of carbon atoms do not exist inside the ferrite lattice in the drawn pearlite and multi-scaled heterogeneous plastic deformation in pearlite seems to affect the asymmetry in the diffraction profile. Fracture behavior and hardness change with tempering is different in the two microstructures.

In this work, we studied the wear behavior of a low-Cr white cast iron (WCI) modified with ferrotitanium-rare earths-bismuth (Fe-Ti-RE-Bi) up to 2%. These additions modified the eutectic carbide structure of the alloy from continuous ledeburite into a blocky, less interconnected carbide network. The modified structure was wear tested under pure sliding conditions against a hardened M2 steel counter-face using a load of 250 N. It was observed that wear resistance increased as the modifier admixture increased. The modified structure had smaller more isolated carbides than the WCI with no Fe-Ti-RE-Bi additions. It was observed that large carbides fracture during sliding, which destabilizes the structure and causes degradation in the wear behavior. A transition from abrasive to oxidative wear after 20 km sliding occurred for all alloys. In addition, the modified alloys exhibited higher values of hardness and fracture toughness. These results are discussed in terms of the modified eutectic carbide microstructure.

The present work studies the effect of small additions of a mixture of Ti, rare earths (RE), and Bi on the eutectic solidification of a low-Cr white cast iron (WCI) commercially designed as Ni-Hard Class I Type B according to the ASTM A532. For this purpose, systematic additions of a mixture of ferrotitanium-rare earths-bismuth (Fe-Ti-RE-Bi) up to 2% were made to a low-Cr WCI. By means of these additions, a modified carbide structure was obtained. Eutectic carbides changed from a highly interconnected ledeburite structure to more isolated, finer blocky structure. This microstructural change caused variations in the mechanical behavior of the WCI. The decrease in size and reduced connectivity of the eutectic carbides increased fracture toughness as well as wear resistance under dry sliding conditions. The effect of the admixture on the microstructure and mechanical properties is discussed in terms of the segregation and refining effects of the elements that comprise the mixture.

The effect of spinodal austenite-carbide layering on formation of groups in liquid iron and stability of its microheterogeneous structure are considered.

A modified Bridgman technique was used to grow a nickel-alloyed gray cast iron. During the experiments, morphological modifications (MMs) were induced by the change in chemical composition of the melt in front of the solid/liquid interface. This was achieved by means of an “in-situ” addition of commercial modifiers, maintaining the thermal-gradient-to-growth-rate ratio. Optical and scanning electron microscopy (SEM) and X-ray diffraction were used to characterize the resulting microstructures. From the analysis of the evolution of the microstructures during our unidirectional experiments, correlated with results of other researchers, a new explanation for MMs observed in cast iron is proposed; it covers the whole range of morphologies from flake to nodular graphite. The explanation emphasizes the following points. (1) Graphite always grows like sheet crystals. (2) The MM of flake cast iron starts when the triple-contact line between austenite-graphite-liquid disappears, provoking the separation of austenite from graphite during eutectic growth. (3) Vermicular and nodular morphologies arise from the compacting action of solid austenite and liquid, respectively, exerted upon sheet crystals.

1. Liquid-phase laser sintering based on Rapid Prototyping technology can be successfully used on powder steel 3Kh3F12, the structure of which has a high concentration of a low-melting carbide eutectic. No low-melting additives are needed in this case. 2. Optimization of the parameters of the laser fusion of powder steel for the duration of the production process makes it possible to obtain small compact specimens (5 mm in diameter and 30 mm in length). 3. Sintered specimens of steel 3Kh3F12 have a microstructure of ledeburite and martensite with 15% vanadium carbides and 16% residual austenite. 4. Despite the high cooling rates (100–1000°C/sec), multicomponent segregation along grain boundaries is seen in the specimens after the laser treatment.

Commercial flake graphite cast iron substrate was coated with titanium powder by low pressure plasma spraying and was irradiated with a CO2 laser to produce the wear resistant composite layer. The macro and microstructural changes of an alloyed layer with the traveling speeds of laser beam, the precipitate morphology of TiC particulate and the hardness profile of the alloyed layer was examined. From the results, it was possible to composite TiC particulate on the surface layer by direct reaction between carbon existed in the cast iron matrix and titanium with thermal sprayed coating by remelting and alloying them using laser irradiation. The cooling rate of the laser remelted cast iron substrate without a titanium coating was about 1 × 104 K/s to 1 × 105 K/s in the order under the condition of this study. The microstructure of the alloyed layer consisted of three zones; the TiC particulate precipitate zone (MHV 400–500), the mixed zone of TiC particulate + ledeburite (MHV 650–900) and the ledeburite zone (MHV 500–700). TiC particulates were precipitated as a typical dendritic morphology. The secondary TiC dendrite arms were grown to a polygonized shape and were necking. Then the separated arms became cubic crystal of TiC at the slowly solidified zone. In the rapidly solidified zone near the fusion boundary, however the fine granular TiC particulates were grouped like grapes.

Semi-solid casting of M2 high speed steel ingots was investigated by inclined slope pre-crystallization method. Effects of casting temperature and slope length on the microstructure of M2 HSS ingots were investigated. M2 cast ingots of non-dendritic primary austenite and fine eutectic ledeburite network carbide structure were obtained, with the casting temperature, slope length and angle of 1480 °C, 500 mm and 60° respectively. Meanwhile, the microstructure of cast samples was quantitatively assessed by Image tool software. Results show that optimum mean equivalent diameter of primary austenite crystal grain is 50.8 µm, shape factor is 0.83, and mean thickness of network carbide is 5.21 µm.

1. Eutectic graphite of different morphologies is observed in cast hypoeutectic iron depending on the cooling rate. After hardening at atmospheric pressure we distinguished rosette, “twisted” flake, and point graphite, and after hardening under pressure we observed pseudoeutectic and fine-flake graphite. 2. Pressure promotes considerable deceleration of segregation of graphite independently of the cooling rate. The appearance of cementite and needle ledeburite in the structure of hypoeutectic cast iron under the action of pressure is connected with superposition of the stable and metastable transformation ranges and a change in the thermodynamic equilibrium between graphite and cementite in the metal. 3. Pressure applied to the hardening casting increases the proportion of bound carbon that segregates in the form of compact graphite inclusions in graphitization annealing, thus promoting an increase in the mechanical properties of the metal.

This study investigates the effects of high-energy electron-beam (1.4 MeV) irradiation on surface hardening and microstructural modification in a gray cast iron currently used for a diesel engine cylinder block. The gray cast-iron samples were irradiated in air using an electron accelerator. Afterward, their microstructure, hardness, and wear properties were examined. The original microstructure, which contained graphite flakes in a pearlitic matrix, was changed to martensite, ledeburite, and retained austenite, along with complete or partial dissolution of the graphite. This microstructural modification occurred only when the surface was irradiated with an input-energy density over 1.1 kJ/cm2, and it greatly improved the surface hardness and wear resistance. In order to investigate the complex microstructures, thermal analysis and simulation testing were also carried out. The results indicated that the irradiated surface was heated to the austenite-temperature region and then quenched to room temperature, which was enough to obtain surface hardening through martensitic transformation. The thermal analysis results matched well with the microstructures of the thermally simulated samples.

Special features of plastic deformation and fracture of excess cementite segregated under the conditions of high-temperature carbon supersaturation of steel are studied by the methods of light and scanning electron microscopy. It is shown that regularly oriented flakes of excess cementite with a lamellar structure are formed within the former austenite grains. The cementite flakes oriented along the, rolling plane fracture by a brittle mechanism forming numerous fragments. The behavior of free cementite under the conditions of cold rolling depends on the orientation of the flakes in the specimen. The mutual shift of the layers allows coarse cementite formations to deform under the action of compressive stresses without cracking.

Experiments are presented which show that the eutectoid transformation in steel can occur by two different modes for temperatures just slightly below A 1. In the normal mode, the transformation product is lamellar pearlite. The second mode occurs if the austenite contains cementite particles or nuclei with a spacing on the order of a few microns or less. In this case, the transformation product consists of spheroidal cementite particles in a ferrite matrix. This second mode is here called the divorced eutectoid transformation (DET), after recent work by Sherby and co-workers. A literature survey shows that the faster kinetics of the DET over lamellar pearlite in the presence of inhomogeneous austenite was established before 1940, but has received little attention. The inhomogeneities are generally small cementite particles. Experiments show that the DET does not occur by a shell of one phase (ferrite) forming around the other phase of the eutectoid (cementite), as is the case in divorced eutectic growth. Rather, a fairly planar austenite/ferrite front simply advances into the austenite, with no apparent effect on its shape being produced by the cementite particles. A first-order kinetic model is presented for the growth velocity as a function of undercooling below A 1 and is compared to the velocity vs undercooling for lamellar pearlite. The simple model indicates that the velocity of the divorced mode should be faster than the lamellar mode at low undercooling for cementite nuclei distributed in the austenite with spacings less than a few microns. This result is consistent with the experimental data.

The effects of high-energy electron beam irradiation on surface hardening and microstructural modification in a ductile cast iron (DCI) roll are investigated in this study. The DCI roll samples were irradiated by using an electron accelerator (1.4 MeV), and then their microstructures and hardnesses were examined. Upon irradiation, the unirradiated microstructure containing graphites and the tempered bainite matrix was changed to martensite, ledeburite, and retained austenite, together with the complete or partial dissolution of graphites. This microstructural modification improved greatly the surface hardness due to transformation of martensite whose amount and type were determined by heat input during irradiation. In order to investigate these complex microstructures, a simulation test including thermal cycles of abrupt heating and quenching was carried out. The simulation results indicated that the irradiated surface was heated up to about 1100 °C to 1200 °C and then quenched to room temperature, which was enough to obtain surface hardening through martensitic transformation. Thermal analysis of the irradiated surface layer was also carried out using a finite difference method to understand the surface hardening of the DCI roll and to compare with the simulation test results.

The objective of this study is to clarify the fracture characteristics of high-speed steel (HSS) rolls in terms of microstructural factors such as matrix phase and primary carbide particles. Three HSS rolls with different chromium contents were fabricated by centrifugal casting, and the effect of the chromium addition was investigated through microstructural analysis, fracture-mechanism study, and toughness measurement. The hard and brittle primary carbides, as well as the eutectic carbides (ledeburites), were segregated in the intercellular regions and dominated overall properties. Observation of the fracture process revealed that these primary carbides cleaved first to form microcracks at low stress-intensity factor levels and that the microcracks then readily propagated along the intercellular networks. The addition of chromium to a certain level yielded microstructural modification, including the homogeneous distribution of primary carbides, thereby leading to enhancement of fracture toughness of the HSS rolls.

Several studies of the ledeburite eutectic (Fe-Fe{sub 3}C), in pure Fe-C alloys have shown that it has a lamellar morphology under plane front growth conditions. The structure of ledeburite in white cast irons, Fe-C-is, consists of a rod morphology. It is generally not possible to produce plane front growth of Fe-C-Si eutectic alloys in the Fe-Fe{sub 3}C form, because at the slow growth rates required for plane front growth, the Fe{sub 3}C phase is replaced by graphite. By using small additions of Te, the growth of graphite was suppressed, and the plane front growth of the ledeburite eutectic in Fe-C-Si alloys was carried out with Si levels up to 1 wt pct. It was found that the growth morphology became a faceted rod morphology at 1 wt pct Si, but in contrast to the usual rod morphology of white cast irons, the rod phase was Fe{sub 3}C rather than iron. It was shown that the usual rod morphology only forms at the sides of the two-phase cellular or dendritic growth fronts in Fe-C-Si alloys. Possible reasons for the inability of plane front directional solidification to produce the usual rod morphology in Fe-C-Si alloys are discussed. Also, data are presented onmore » the spacing of the lamellar eutectic in pure Fe-C ledeburite, which indicates that this system does not follow the usual {lambda}{sup 2}V = constant relation of regular eutectics.« less

In order to modify the continuous network of eutectic cementite normally found in low alloy white cast irons into a dispersive distribution, strategies of controlling the morphology of eutectic cementite by additives are discussed. Qualitative arguments are presented and applied to the development of a complex modifier REAINTi. With the addition of this modifier to low carbon white cast irons nearly one fifth of the original eutectic cementite network can be modified into blocky particles in the plane of polish, and one third into isolated irregular ones. The experimental data shows that Ti(CN) may act as an effective nucleant or a grain refiner for eutectic cementite. The modification effects of rare earth elements are attributed to their abilities to purify the melt, promote divorced eutectic growth and refine the microstructure. Aluminium contributes to the modification by its purification effect.

Pure Fe-3.5 wt% C alloy was surface melted using a cw CO 2 laser and the microstructure of the laser tracks was investigated by scanning and transmission electron microscopy. Structure of the melted zones consisted of dendrites of partially transformed primary austenite and of very fine lamellar ledeburite. The secondary dendrite arm spacings were indicative of a cooling rate of ∼10 5 K s -1 , in good accord with calculations based on the model of a moving Gaussian beam. Using methods of quantitative metallography the volume fraction of dendrites within the melted zone and the volume fractions of the carbide and ferrite phases in the decomposed ledeburite were estimated. These data were combined with the results of a quantitative X-ray diffraction phase analysis (see Part II) and compared with the equilibrium phase diagram. It was found that the volume fraction of dendrites was near the equilibrium value while the volume content of cementite in the rapidly solidified structures was considerably higher than predicted from the equilibrium phase diagram.

As-cast ductile cast iron with an as-machined shiny metal surface was remelted with a high-power (1 kW) pulsed Nd:YAG laser using both single- and multipass overlap melt tracks. Changes in the microstructure of the underlying laser melted track caused by the transient overlap heating during multipass overlap remelting process were studied. The rapidly solidified metastable ledeburite structure of the underlying laser melted track was found to be rapidly graphitized during overlap remelting. The graphitized zone consists of a fully graphitized zone containing extremely fine graphite nodules and a partially graphitized zone containing extremely fine graphite nodules and undissolved cementite. The overlap ratios of the melt tracks were shown to have no noticeable influence on either the graphitized microstructure and the size of the graphitization zones. This newly observed rapid graphitization phenomenon is preliminarily discussed in terms of the microstructural characteristics of the rapidly solidified ductile iron and the unique heating behavior of pulsed laser beam to material.