Fine Pearlite Research Articles

Three-body abrasive wear of fine pearlite, nanostructured bainite and martensite

The abrasive wear of three metallurgical structures with radically different hardnesses has been investigated for the same steel. The particular steel concerned is a recent innovation capable of generating extremely fine distributions of crystals. The austenite in the alloy nevertheless has the capability of uniformly transforming into extremely fine pearlite, nanostructured bainite, and plate martensite. It is found that although the abrasion rates and wear coefficients are not very different for the three states, the mechanisms of abrasion are quite different. We report detailed characterisation experiments together with comparisons with commercially available steels subjected to identical tests.

고탄소강의 구상화속도에 미치는 초기 미세조직, 냉간압연 및 온도의 영향

Abstract The spheroidization behavior of cementite in a SK85 high carbon steel was investigated in this study. Fine and coarse pearlite microstructures were obtained by appropriate heat treatments according to the TTT diagram of SK85 high carbon steel. oHot rolled plates of SK85 steel were austenitized at 800 C for 2 hrs and then put directly into a salt bath at either 570 o C or 670 C to obtain a fine pearlite (FP) structure and a coarse pearlite (CP) structure, respectively. Cold rolling was subsequently conducted on those specimens with reduction ratios from 0.2 to 0.4. Spheroidization heat treatments were conducted at the subcritical temperatures of 600 and 720 o C for 1 to 32 hrs to elucidate the effect of initial microstructures, heat treatment temperature, and cold reduction ratios on the cementite spheroidization rate. Spheroidization proceeded with fragmentation of cementite plates, spheroidization of the cementite platelets, and coarsening consecutively. Mechanical fragmentation of cementite by cold rolling expedited the rate of spheroidization. The spheroidization rate of FP was much more rapid than that of CP and the spheriodization rate increased with increases in the cold reduction ratio.

Investigation on the behaviour of medium carbon and vanadium microalloyed steels by hot forging test

Two medium carbon steel grades were used in the present investigation. One of them was microalloyed with vanadium. Both steel grades were subjected to a controlled closed die forging followed by cooling in sand, air or oil mediums. Final microstructures and mechanical properties were evaluated by optical microscopy, scanning electron microscopy, hardness and tensile tests. The results indicated that the microstructures of all close die forging and cooling conditions are dominated by ferrite and pearlite phases with different morphologies and grain sizes according to both chemical composition and cooling rate. Oil quenching leads to a formation of relatively fine ferrite and pearlite in medium carbon steel (MC) or martensite in medium carbon microalloyed steel (MC–MA). Relatively fine ferrite, pearlite and martensite increase strength but decrease ductility. The cooling rate has a remarkable effect on the microstructure and mechanical properties at room temperature.

Parameters optimization of low carbon low alloy steel annealing process

A suitable match of annealing process parameters is critical for obtaining the fine microstructure of material. Low carbon low alloy steel (20CrMnTi) was heated for various durations near Ac temperature to obtain fine pearlite and ferrite grains. Annealing temperature and time were used as independent variables, and material property data were acquired by orthogonal experiment design under intercritical process followed by subcritical annealing process (IPSAP). The weights of plasticity (hardness, yield strength, section shrinkage and elongation) of annealed material were calculated by analytic hierarchy process, and then the process parameters were optimized by the grey theory system. The results observed by SEM images show that microstructure of optimization annealing material are consisted of smaller lamellar pearlites (ferrite-cementite) and refining ferrites which distribute uniformly. Morphologies on tension fracture surface of optimized annealing material indicate that the numbers of dimple fracture show more finer toughness obviously comparing with other annealing materials. Moreover, the yield strength value of optimization annealing material decreases apparently by tensile test. Thus, the new optimized strategy is accurate and feasible.

Microstructure of Thixoformable Hypoeutectic Cast Iron

The use of a specially designed hypoeutectic cast iron as a potential raw material for the thixoforming process is described in this paper. Thixoforming technology normally uses aluminum-silicon alloys such A356 and A357 as raw materials. Iron-based alloys are less common, despite the lower cost of the raw material. The paper describes the semi-solid behavior and corresponding final microstructure of a hypoeutectic gray cast iron after thixoforming tests. The Fe-2.6wt%C-1.5wt%Si alloy was prepared via conventional casting in sand molds. Samples were heated to the semi-solid state at 1160 and 1180oC and held at these temperatures for 0, 30, 90 and 120s, and then subjected to compression tests. Two-platen compression tests were carried out in an instrumented eccentric press in order to determine the semi-solid behavior. The holding time in the semi-solid range simulates the industrial heating process that is time-controlled rather than temperature-controlled. The semi-solid behavior indicated that the semi-solid cast iron behaves like aluminum-silicon alloys, presenting a stress of up to 24MPa under 80% strain and a corresponding apparent viscosity of up to 1.5*105 Pa.s at 1180oC. The final microstructure after compression testing was essential in determining the material’s morphological evolution. Tests revealed that heating up to the semi-solid range followed by thixoforming changes the material’s graphite morphology from type A to B (or E), but does not significantly affect the interdendritic arm spacing between graphite lamellae. The resulting structure is composed of fine graphite and pearlite.

Enhancement of mechanical properties by changing microstructure in the eutectoid steel

The different microstructures of eutectoid steel were analyzed with SEM and its corresponding room-temperature tensile tests were carried out. The results show that the ultrafine (α+θ) duplex structure consisting of ferrite matrix (α) with grain size of about 1μm and dual-size distributed cementite particles (θ) could be formed by hot deformation of undercooled austenite and subsequent annealing. Moreover, the mixed microstructures consisting of polygonal ferrite grains with size of about 2μm and fine pearlite colonies could be also obtained by this process under different conditions. Although the yield strength and total elongation of ultrafine (α+θ) duplex structure increase markedly, its tensile strength decrease obviously comparing to that of lamellar pearlite. The lower work-hardening rate at the beginning of uniform plastic deformation is probably responsible for tensile strength decrease in the ultrafine (α+θ) duplex structure. Moreover, the tensile strengths and total elongations of mixed microstructures are both larger than that of ultrafine (α+θ) duplex structure and lamellar pearlite due to their well work-hardening capabilities.

Thixoformability of hypoeutectic gray cast iron

Hypoeutectic gray cast iron of composition Fe–2.6wt%C–1.5wt%Si, specifically developed as a thixoformable raw material, was prepared via conventional casting in sand molds. Samples were heated to the semi-solid state at 1160 and 1180°C and held at these temperatures for 0, 30, 90 and 120s to study their morphological evolution. Tests reveal that heating to the semi-solid range changes the morphology of the graphite from A to B type, but does not significantly affect interdendritic arm spacing between graphite lamellae. The resulting structure is composed of fine graphite and pearlite. Two platen compression tests were also carried out in an instrumented eccentric press to examine the material's semi-solid behavior at the same temperatures and holding times. The tests indicated that the semi-solid cast iron behaves like aluminum–silicon alloys, presenting a stress of up to 24MPa under 80% strain and a corresponding apparent viscosity of up to 1.5×105Pas at 1180°C.

Microstructural modifications and changes in mechanical properties during cyclic heat treatment of 0.16% carbon steel

In this work an annealed 0.16 wt% carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (6 min) holding at 910 °C (above Ac3 temperature) followed by forced air cooling. A typical microstructural development, not so common for low carbon hypoeutectoid steel, was observed. The main features involved were: (i) a substantial grain refinement (mainly at initial stage), (ii) generation of dislocations (at initial stage) and its annihilation (at later stage) and (iii) generation of grain boundary network of cementite and cementite cluster owing to divorced eutectoidal reaction. In low carbon steel, the presence of large proportion of grain boundary areas promoted grain boundary diffusion of carbon during short-duration holding. This phenomenon finally led to the generation of grain boundary cementite network and cluster through divorced eutectoidal reaction. Unlike high carbon steel, the contribution of divorced eutectoidal reaction to spheroidization was not so significant. For higher cycles (5–8 cycles) substantial presence of grain boundary cementite and cementite cluster deteriorated the strength property. However, quite a high strength (UTS = 455 MPa) was achieved for this low carbon steel with two cycles of heat treatment due to fine ferrite grain size, high dislocation density and adequate proportion of fine lamellar pearlite in the microstructure.

Mechanisms of extrusion wear of medium carbon steel

This study investigated extrusion wear mechanisms and transition of wear mechanisms of medium carbon steel by using a pin-on-disc configuration and micro-analyzing the wear surfaces. In an extrusion wear condition, layers of material on the contact surface of the tested pin were extruded continuously from the interface. Under such a condition, the oxides, mainly Fe3O4 and FeO, on the pin contact surface were very thin at about 2 μm, and likely generated during cooling in air after the wear test. The microstructure of the pin very close to the contact surface was fine pearlite, which was transformed from austenite at the end of the wear test. Compared with the microstructures of the same medium carbon steel cooled in air from a melting state, the debris collected from extrusion wear tests had a fine pearlite structure instead of coarse pearlite and ferrite. Thus, the real contact temperature in extrusion wear did not reach the melting point of medium carbon steel. Such changes of microstructures cannot be found in other types of wear.

Microstructural Observation of Ni-WC Surface Alloyed Carbon Steel

EPS (expanded polystyrene spheres) templates with thickness of a 10 mm coating on a surface that is mainly consisted of Ni-WC particles, are prepared. Carbon steels containing 0.45C%wt are then cast in the templates using V-EPC process (vacuum expandable pattern casting), forming the surface alloyed steels. The microstructures are observed and analyzed using optical microscope, SEM and EDS. Macrostructural observation showed that the surface alloyed zone is dense and there is no obvious defects, such as gas bubbles, occluded gas holes and delimitation. Microstructural investigations indicate that the samples from top to bottom are obviously consisted of three different zones, i.e., the top alloyed zone, the interim transitional zone and the bottom matrix zone. Ni-WC particles are totally decomposed during the molten steel infiltration. The microstructures in the surface alloyed zone are consisted of small amounts of fine WC+W2C carbides, large amounts of M3C+M7C3 carbides and dendritic matrix. There is a fine pearlite strip at the top of the transitional layer. Small amounts of carbide particles within the matrix grains and net carbide precipitation in the grain boundaries at the transitional zone can be observed.

Tribological Properties of Rail Steel in Straight Moderately Loaded Sections of Railway Tracks

Tribological Properties of Rail Steel in Straight Moderately Loaded Sections of Railway Tracks The paper describes the examination results of tribological properties of three types of steels used and suggested for rails manufacturing. The tests concentrated on loads, sliding and rolling speeds similar to those occurring in real conditions i.e. rolling track operation. Average loaded conditions were assumed and applied at straight railway track sections. Slight track declivity and high/low speeds of locomotive were considered. "Amsler" stand was used for laboratory tests. Three types of steels i.e. two pearlitic steels: WHT - without heat treatment, HT - with heat treatment (with the microstructure of fine pearlite) as well as one bainitic steel suggested for rail production have been tested. The measurements of wear, hardness, friction coefficient as well as structural changes at surface layers of the tested rollers have been performed during the test.

Microstructural Observation of SiC Surface Alloyed Carbon Steel

EPS (expanded polystyrene spheres) template with a 5 mm coating on a surface that is mainly consisted of SiC particles are prepared. Carbon steels are then poured in the templates using V-EPC (Vacuum Expandable Pattern Casting) process. The microstructures are observed and analyzed using optical microscope, SEM and EDS. Macrostructural observation showed that the surface alloyed zone is dense and there are no obvious defects, such as gas bubbles, occluded gas holes and delimitation. Microstructural investigations indicated that the samples from top to bottom are obviously consisted of three different layers, i.e., the surface compound layer, the interim transitional layer and the bottom matrix layer. SiC particles are totally decomposed during infiltration. The microstructures in the surface compound layer are consisted of a large amount of graphite flakes, carbides, Si rich ferrites and dendritic pearlites. There is a fine pearlite strip at the top of the transitional layer and small amounts of carbide particles within the matrix grains and net carbides at the grain boundaries can be also observed in this layer.

Effect of silicon on the wear resistance of high-carbon steels with the structures of isothermal decomposition of austenite during friction and abrasive action

The hardness and wear resistance during sliding and abrasive friction of 80S2 (0.83% C, 1.66% Si) and U8 (0.83% C) steels subjected to the isothermal γ → α decomposition in the temperature range 330–650°C and additional 5-min annealing at 650°C are compared. The optimum decomposition temperature is found to be 550°C. At this temperature, fine lamellar pearlite with the maximum hardness and wear resistance as compared to other pearlitic and bainitic structures forms in the silicon steel. The silicon-alloyed fine lamellar pearlite of 80S2 steel is found to have high hardness and abrasive wear resistance as compared to the similar structure in plain U8 steel; however, this pearlite has no advantages in the wear resistance under conditions of sliding friction on a steel plate. Silicon alloying of the bainitic structures in the eutectoid steel leads to a noticeable decrease in the wear resistance during sliding friction and abrasive action. Friction oxidation is shown to negatively affect the abrasive wear resistance of the silicon steel.

Structural features of the behavior of a high-carbon pearlitic steel upon cyclic loading

The structural evolution upon high-cycle fatigue (tension with the magnitude of stress in a cycle below the macroscopic yield stress) of the hypereutectoid steel U10 (1.03 wt % C), in which pearlite of different morphology (fine-lamellar, coarse-lamellar, and partially spheroidized pearlite) was formed, has been investigated by scanning and transmission electron microscopy. Based on the fractographic analysis, features of fracture of these structural states have been considered. At a significant distance (10 mm) from the fatigue fracture, features of structural transformations caused by cyclic loading have been revealed. It has been shown that upon high-cycle fatigue in the steel U10 with structures of lamellar and partially spheroidized pearlite, substantial structural changes occur, namely, fragmentation and partial dissolution of cementite plates, and in fine pearlite, additionally, spheroidizing of cementite and polygonization of the ferritic component are observed. A dependence of the character of fracture on the type of structure formed upon fatigue loading has been established. In the steel with a nonequilibrium structure of unannealed fine pearlite, an enhanced elasticity modulus, as compared to other more stable structures (coarse-lamellar, annealed fine, and spheroidized pearlite), and a reduction in the magnitude of the elasticity modulus under the action of cyclic loading have been found. It has been established that the structural changes in fine pearlite of laboratory specimens of the steel U10 upon cyclic tension are qualitatively similar to those in a railroad wheel of the steel 65G under the service conditions.

Fatigue characteristics of small radius pipe fabricated by pipe bending with induction local heating

Bending applying local induction heating is an efficient way to produce bended pipes of various radii with lower cost and to provide better quality product. In this process, it is very important to control the thickness of the pipe to ensure its performance. The lower thickness usually turned out in outer section of bended pipe, even though controlling the thickness carefully by reverse moment. As the radius of pipe is smaller, especially, the thickness reduction is larger. The reduced thickness becomes a cause of deterioration of properties and confidence of bended pipe. To verify confidence of bended pipe, the fatigue properties were investigated for the bended pipes with small radius of the curvature of 1.5 ∼ 2.0DR. Two bended sections, which were extrados and intrados region, were compared with as-received section about microstructure, hardness, high cycle fatigue and residual stress. Carbon steel showed better fatigue characteristics at bended section than at as-received pipe. Improvement of fatigue strength resulted from both refinement of ferrite grain size and the achievement of a relatively fine pearlite by both plastic deformation and rapid-heating-cooling during pipe bending applying local induction heating. Hardness increased over 20% and fatigue endurance limits of deformed section were enhanced over 10%.

Environment-conscious Cu Free Sintered Steels (Report 2) Effect of Sintering Temperature

Fe-Cu-C sintered steels are widely used for automobile parts. In case of reusing these sintered steels as the raw materials of wrought steels, it becomes to be a problem that Cu contamination deteriorates hot workability of wrought steels. In order to recycle the scrapped sintered steels, Cu free sintered steels are strongly required. We developed new Fe-Mn-Si-C sintered steel as a candidate material to the conventional Fe-Cu-C sintered steels. The developed sintered steel shows superior mechanical properties compared with the conventional sintered steels by adding Mn and Si elements to the Fe-Mn-Si ternary master alloy (FeMS) powder.In this study, the influence of sintering temperature and also particle size of FeMS powders on the mechanical properties of the new Fe-Mn-Si-C sintered steels were investigated. The tensile strength is improved by not only rising sintering temperature and also adding the fine FeMS powder. In particular, this improvement is considered to be resulted from the microstructural changes as follows. The addition of the fine FeMS powder works to increase amount of the fine pearlite phase in whole of the microstructure and also inhibit the precipitation of the martensite phase at the Mn and Si enriched regions.

Microstructure and mechanical properties of V–Ti–N microalloyed steel used for fracture splitting connecting rod

A new kind of V–Ti–N high strength microalloyed medium carbon steel has been developed, which is used for fracture splitting connecting rod. In this article, the characteristics of this carbon steel and its production process were studied. The microstructure, precipitated phases and their effects on mechanical properties were investigated by optical microscope, SEM, and TEM. The results showed that the steel was constituted of ferrite and pearlite. By reducing the finish rolling temperature and accelerating the cooling rate after rolling, microstructure with fine grain ferrite and narrow lamellar space pearlite could be obtained in V–Ti–N microalloyed medium carbon, and a large number of precipitated phases distributed over ferrite. These led the tensile strength to be more than 1000 MPa, yield strength (YS) more than 750 MPa. The impact fractograph showed typically brittle fracture characteristic.

Effect of heat treatment on microstructure and mechanical properties of high boron white cast iron

The effect of different tempering temperatures on the microstructure and mechanical properties of high boron white cast iron after air quenching was studied. The experimental results indicate that the high boron white cast iron comprises a dendritic matrix and interdendritic boride M 2B that distributed in the form of continuous network and the matrix is composed of fine pearlite at as-cast condition. After austenitising at 920 °C and air quenching, the matrix of high boron cast iron is changed from pearlite to lath-type martensite, and the morphology of boride is still kept in the form of continuous network. After tempered at 200 °C or a higher temperature, a secondary precipitation with a size of tens of nanometers is found in the matrix, and the size and the amount of this secondary precipitation increase with the increasing of tempering temperature. TEM analysis shows that the secondary precipitates have a crystal structure of M 23(B,C) 6. With the increasing of tempering temperature after air quenching, the hardness decreases, and the impact toughness firstly increases to a maximum value and then decreases. The optimum structure that has a combination of high hardness and high impact toughness can be obtained when the tempering temperature is lower than 400 °C.

Microstructure and mechanical properties of high boron white cast iron with about 4 wt% chromium

The microstructure and mechanical properties of high boron white cast irons with about 4 wt% chromium before and after treating with rare earth magnesium alloy were studied in this article. The experimental results indicate that the cast irons comprise a dendritic matrix and interdendritic eutectic borides M2B and M′0.9Cr1.1B0.9 that distributed in the form of continuous network in as-cast condition. The matrix is made up of fine pearlite in the alloys with and without modification, but the grain size of the matrix is decreased greatly after modification. After water quenching at 1,303 K and tempering at 473 K, the matrix of the alloy mostly changes to lath-type martensite. For the alloy without modification the boride morphology remains almost unchanged after heat treatment. And a secondary precipitation of M23(C,B)6 compound appears in the central region of dentritic matrix grains. The morphology of the eutectic borides is changed to the form of isolated blocks after heat treatment and there is only little intragranular M23(B,C)6 particles in the matrix are found in the alloy modified with rare earth magnesium alloy. The modification by rare earth magnesium alloy can refine the primary austenite and the eutectic borides. Combined with a high austenitizing temperature the modification can improve the morphology of the borides which results in the improvement of toughness and tensile strength.

Spheroidization Behavior of Cementite in a High Carbon Steel

The effect of initial microstructure, cold reduction ratio, and annealing temperature on the spherodization rate of SK85 high carbon steel sheet was investigated. High carbon steel sheet fabricated by POSCO was soaked at 800oC for 2 hr in a box furnace and then treated at 570oC for 5 min in a salt bath furnace followed by water quenching to obtain a fine pearlite structure. Cold rolling was conducted on the sheets of fine pearlite by reduction ratios of 20, 30, and 40 % and heat treatment for spheroidization was carried out at 600 and 720oC for the various time intervals from 0.1 to 32 hrs. Area fraction of spheroidized cementite was measured with an image analyzer as a function of cold reduction ratios and duration times.