Pearlitic Matrix Research Articles

Formation of Cu-rich Nanoprecipitates in Cu Containing Pearlitic SGI

The presence of Cu-rich nanoprecipitates in a pearlitic spheroidal graphite (ductile) cast iron alloyed with 0.82 wt%Cu was studied. The size and distribution of the precipitates were examined by transmission electron microscopy at different locations of the pearlitic matrix. Some areas were nearly free from precipitates, while other regions showed precipitates at the cementite and ferrite lamellae and at the ferrite/cementite interface. Calculation of the thermodynamic equilibrium under stable and metastable conditions using Thermo-Calc led to the identification of the conditions controlling the formation of Cu-rich nanoprecipitates along three different stages depending on the Cu concentration. Together with a differential scanning calorimetry test and elemental diffusional calculations, thermodynamic predictions supported the observation of Cu-rich precipitates despite the low concentration of Cu of the alloy investigated and allowed the authors to explain the observed heterogeneity in the distribution of precipitates as resulting from the heterogeneous distribution of Cu in the alloy caused by microsegregation during solidification. The knowledge gained is relevant for the design of strengthening strategies in SGI based on the dispersion of Cu-rich nanoprecipitates.

Investigation and Prediction of Abrasive Wear Rate of Heat-Treated HCCIs with Different Cr/C Ratios Using Artificial Neural Networks

In this study, artificial neural networks (ANNs) technique was used in the prediction of abrasive wear rate of high-Cr white cast irons (HCCIs) after subcritical heat treatment at different temperatures. High Cr WCI alloys with different compositions were tested at sliding speed of 1.04 m s−1 under the normal load of 30 N and different sliding distances of 500, 1000 and 1500 m. The abrasive wear rates obtained from wear tests were used in the formation of the data sets of the ANN. A multilayer perceptron model has been constructed with back-propagation algorithm using the input parameters of load, tempering temperature, and Cr/C ratio. The output parameter of the model is abrasive wear rate. Experimental results showed that abrasive wear rate of high-Cr WCI was significantly increased with the increasing of Cr/C ratio. High-Cr WCI alloys with higher volume fraction of carbides and structures with martensitic matrix at lower Cr/C ratio exhibited lower abrasive weight losses. The increasing of both sliding distance and load increases the abrasive weight losses. The HCCI-2 alloy exhibited the lower abrasive weight losses as compared with the other alloys in both as-cast and heat-treated conditions. In addition, the abrasive weight losses for all investigated alloys with different Cr/C ratios after destabilization heat treatment are lower than alloys in the as-cast state. This is may be due to the presences of stronger martensitic matrix structure rather than austenitic or pearlitic matrix structures. Correlation coefficients between the experimental data and outputs from the ANN established the feasibility of ANNs to effectively model and predict the wear rate of high Cr WCI. From the sensitivity analysis, it is concluded that the tempering temperature had the most influence on the wear rate, while the applied load and Cr/C ratio had a small influence on the wear rate of high-Cr WCI.

Effect of interaction between cementite and pearlite on two-body abrasive wear behaviors in white cast iron

The wear interaction of cementite and pearlite in the white cast iron (WCI) was investigated using the two-body abrasive wear test under contact loads of 20, 35, and 50 N. The wear behavior, wear surface morphology, sub-surface structure, and wear resistance were evaluated using X-ray diffraction, microhardness testing, and nano-indentation. The results indicated that when the Cr content was increased from 0 to 4 wt%, there was a significant increase in the microhardness (H) and elasticity modulus (E) of the cementite. This yielded a 15.91%- and 23.6%-reduction in the degree of wear resistance and surface roughness, respectively. Moreover, no spalling and breaking of cementite was observed with increasing Cr content during the wear process, indicating improved wear resistance of the bulk cementite. In addition, the hard phase (cementite) and tough matrix (pearlite) composite structure exhibited a good protective and supporting effect. Thus, it was concluded that the interaction mechanism of the wear phase contributed to the reduction of the wear weight loss of the composite during the wear process. The contribution of the interaction between the hard wear-resistant phase and the tough phase in WCI to the wear resistance decreased with increasing hardness of the pearlite matrix.

Effect of spheroidization of cementite in ductile cast iron

The research aims to provide an alternative to austempering treatment of ductile cast iron with a simple and cost-effective heat-treatment process. This goal was accomplished by applying a simple one-step spheroidization heat treatment to the as-cast ductile iron, which would normally possess a coarse pearlitic microstructure to a significant extent. Spheroidization experiments involving isothermal holding below the lower critical temperature (A1) were conducted followed by standard mechanical testing and microstructural characterization for an experimental ductile iron. After improving the spheroidization holding time at a given temperature, the work shows that the ductility and toughness of an as-cast ductile iron can be improved by 90% and 40%, respectively, at the cost of reducing the tensile strength by 8%. Controlled discretization of the continuous cementite network in pearlitic matrix of the ductile iron is deemed responsible for the improved properties. The work also shows that prolonged holding time during spheroidization heat treatment leads to degradation of mechanical properties due to the inhomogenous microstructure formation caused by heterogeneous decomposition and cementite clustering in the material. The main outcome of this work is the demonstration of ductile cast iron’s necking behavior due to spheroidization heat treatment.

Extrema of micro-hardness in fully pearlitic compacted graphite cast iron

ABSTRACT For improving the mechanical strength of cast irons, a fully pearlitic matrix is sought by alloying with tin, copper and manganese. This applies in particular to the compacted graphite irons considered here. While average macro- and micro-hardness values were similar for the four fully pearlitic alloys under study, it was found that high copper and low tin content showed some very high maximum micro-hardness values, significantly higher than the corresponding mean. Such values may be seen as hard points in the material that would certainly impair machinability. Discussion of the reasons for these extreme values opens directions for further studies.

ISO 185/JL/225

Abstract ISO 185/JL/225 is an intermediate-tensile-strength gray cast iron that has a predominantly pearlitic matrix, and a tensile strength of 225–325 MPa (33-47 ksi), when determined on test pieces machined from separately cast, 30 mm (1.2 in.) diameter test bars. Compared with the lower strength gray cast iron grades, ISO 185/JL/225 contains lower carbon and silicon contents, while still maintaining excellent thermal conductivity, damping capacity, and machinability. This datasheet provides information on composition, physical properties, tensile properties, and compressive strength as well as fatigue. It also includes information on heat treating. Filing Code: CI-73. Producer or source: International Organization for Standardization (ISO).

Learning from Failure Investigation of Worm Wheel Gearbox in a Wire Rod Mill

Failure of gearbox has always incurred huge loss to the industries due to interruption in operation and consequent repair or replacement cost. This study presents the investigation of a gearbox failure in wire rod mill of a steel plant. Visual and macroscopic examination of the fracture surfaces was carried out to understand the characteristics of failure. Keyway on the shaft and the key were found to be damaged non-uniformly suggesting the movement of key during service before final failure of the shaft. Ratchet marks with smooth fracture surface of shaft indicated fatigue failure. Chemical analysis confirmed the material of shaft, key and hub as EN 24, 45C8 and gray cast iron, respectively. Microstructural observation of the shaft showed inclusions in banded tempered martensitic structure. No inclusions were found in pearlitic matrix of the key, and hub revealed graphite flakes in pearlite matrix. Hardness of all three components and mechanical properties of the shaft were found to be satisfactory. Fractography of the shaft using scanning electron microscopy revealed striations, thus confirming the mode of failure as fatigue. Misfit of the key caused fracture of the shaft in keyway area during service and subsequent failure of the gearbox.

Manufacture material characteristic analysis of original and alternative auto parts. Case Study: Brake Disc

All powered vehicles have a braking system adopted according to the type of vehicle and work for which they are designed. In Ecuador, there is a great variety of auto parts that are proposed as alternative replacement, varying in cost and quality. This research aims to quantitatively analyze the characteristics of the brake disc material between original and alternate auto parts. This research analyzes three brake disc samples (A, B, C) from different manufacturing sources, using the ASTM E415-08 spectrometry test and the metallography test under the ASTM A247-16A standard. The spectrometry detects 15 elements in each sample, the carbon equivalent of sample A is 18% and 20% higher in relation to sample B and C, which significantly influences the microstructure. The metallographic tests indicate a pearlitic matrix in each sample, however, there is a different distribution and size of the graphite flakes. According to the SAE J431 standard, each auto part meets the material conditions for the brake disc function.

Cast a different iron: Grey and mottled cast iron production in early China

Cast iron smelting has been the dominant technique for iron production in Central Plains China since the 5th century BC. Objects made of grey or mottled cast iron have been identified in metallographic studies, yet the techniques employed to make them or the underlying motivations for producing such materials have not been studied in detail. Here we present analyses of mold-cast objects excavated at two civilian cemeteries in north-eastern Xi’an, Shaanxi Province, which are dated to the late Warring States period (ca. 3rd century BC). Metallographic examinations show that the majority of these products are made of mottled or grey cast iron, with flake graphite on the pearlitic matrix, indicating a systematic and purposeful production of materials that clearly required production techniques different from those involved in the making of white cast iron. Based on a comprehensive discussion, we argue that the production of such materials was an intentional process based on controlling the solidification process during casting, which allowed a good balance between acceptable quality and low cost in the manufacture of certain types of artefacts.

Microstructural modification of a static and dynamically solidified high chromium white cast iron alloyed with vanadium

The objective of this study is to analyze the effect of dynamic solidification and vanadium additions in a range from zero to 6.42 ​wt% to a high chromium white iron with the purpose of refining the microstructure and to improve hardness. The alloys were characterized by optical and electron microscopy, EDS and X-ray diffraction and complemented with phase quantification undertaken by image analysis and XRD data. Bulk hardness was measured for different casting thicknesses in both as-cast and after a destabilization heat treatment at 900 ​°C for 45 ​min. The results show that V addition produces a refining effect on the M7C3 carbide as well as the VC carbide formation which can nucleate as primary carbide for V contents higher than 4.75 ​wt%. Dynamic solidification of the experimental irons produced a notable decrease in the size of the carbide phase and increased its volume fraction. This solidification technique also produced a transformation of the iron matrix of the base alloy from predominantly austenitic to a fully pearlitic matrix, while for the 6.42 ​wt% V alloyed iron a notably smaller, slightly elongated and isolated VC carbides embedded in a highly martensitic matrix were observed. This microstructural modification resulted in the maximum harness values obtained in the as-cast conditions. After the destabilization heat treatment, a softening of the microstructure of the V alloyed irons was observed due to the C depletion of the matrix inhibiting the martensitic transformation; conversely, the base alloy experienced a considerably increase in hardness as a result of the high amount of precipitated carbides embedded in a highly martensitic matrix. The results of the present study show that, V alloying and the dynamic solidification technique can be used as effective ways to refine the carbide phase obtaining high hardness levels in the as-cast condition, avoiding this way heat treatments.

ISO 185/JL/200

Abstract ISO 185/JL/200 is an intermediate-tensile-strength gray cast iron that has a predominantly pearlitic matrix, and a tensile strength of 200–300 MPa (29–44 ksi) when determined on test pieces machined from separately cast, 30 mm (1.2 in.) diameter test bars. Compared with the lower strength gray cast iron grades, 185/JL/200 contains lower carbon and silicon contents, while still maintaining excellent thermal conductivity, damping capacity, and machinability. This datasheet provides information on composition, physical properties, tensile properties, compressive and bend strength as well as fatigue. It also includes information on casting and heat treating. Filing Code: CI-70. Producer or source: International Organization for Standardization.

Effect of chromium-induced (Fe, Cr)3C toughness improvement on the two-body abrasive wear behaviors of white cast iron

An as-cast chromium white cast iron alloy with chromium (Cr) content of 0 wt%, 1 wt%, 2 wt%, 3 wt%, and 4 wt% was prepared via metallurgical smelting and casting processes. The effects of Cr content on the toughness, hardness, and microstructure of (Fe, Cr)3C carbides was evaluated. The results revealed that the microstructure of the as-cast alloys is mainly composed of bulk carbide and the pearlite matrix. The carbide in the alloy without added Cr, i.e., eutectic Fe3C, was characterized by a continuous network structure. With the addition of Cr atoms, the Fe3C-type carbide was transformed into (Fe, Cr)3C, leading to a gradual increase in the fracture toughness of the carbide. This may have resulted from the incorporation of Cr into Fe3C and the consequent crystal-structure transformation of the carbide. When the Cr content of the carbide was lower than the saturation level (<2 wt%), the microhardness of the carbide in the as-cast alloy increased significantly. This resulted from an increase in the number of strong (Fe–Cr) metallic bonds in the carbide. However, for Cr content exceeding 2 wt%, the microhardness increased only slightly. Changes in the mechanical properties of the carbides resulted in improved wear resistance of the materials and changes in the wear mechanism of the carbides. At low Cr content, the wear failure of the carbide was characterized mainly by brittle fracture and peeling. With improvement in the toughening performance, microcutting became the main wear failure mechanism of the carbides and exerted a significant blocking effect on the silicon oxide abrasive.

Morphology and mechanisms of cavitation damage on lamellar gray iron surfaces

Engine parts in contact with liquids may suffer cavitation erosion damage. Understanding its mechanisms in realistic operating environments is necessary for improvements in the service life and durability of materials for heavy-duty diesel engines. This work illustrates the cavitation erosion behavior of a cylinder liner material with a follow-up of detailed, high-magnification SEM images of damaged sites on the surface. The cylinder liner encloses the piston and combustion chamber in the engine of large trucks and its material of choice is usually a lamellar gray cast iron, its microstructure consisting of flake-shaped graphite, a pearlitic matrix and some steadite. Testing was carried out using an ultrasonic vibratory apparatus, and the liquid of choice was a commercial engine coolant composed of water, glycol and inhibitors. Based on observations of tested surfaces, a sequence of damage patterns is proposed as an explanation of the material’s cavitation erosion behavior. Initiation consists of: chipping at graphite cluster centers, graphite flake removal, pitting along graphite flakes and direct matrix pitting. Development consists of: evolution of chipped spots into matrix-damaging pits, radial pit expansion, pit merging and surface roughening. It can be concluded that presence and morphology of graphite are critical to the cavitation erosion behavior of LGI.

Microstructural Adjustment of the Degenerated Graphite Layer in Ductile Iron for Targeted Evaluation on the Fatigue Properties

Graphite degeneration is the most prominent microstructural defect in the casting skin of ductile iron. Induced by either sulfur or oxygen contained in the molding material, its occurrence can be observed in a large range of iron castings, having substantial negative influences on the mechanical properties, especially fatigue resistance. Previous investigations predominately focused on the influence of the degenerated graphite layer, while accompanying changes of the iron matrix were neglected. The superposition of these effects hinders the evaluation of casting skin defects in state-of-the-art design of ductile iron components. The presented solution utilizes an experimental procedure, which enables the production of specimens with specific, individual microstructural configurations in the casting skin. This approach is based on the application of a modified sand core coating. By adding sulfurizing additives to the coating and adjusting its binder content and viscosity, a predominately homogeneous degenerated layer of 0.5 mm thickness was obtained. Using specific heat treatment steps assured a fully ferritic or pearlitic iron matrix, isolating the degenerated layer as the main microstructural defect. Fatigue testing of these specimens will further enable the numerical evaluation of the fatigue resistance in dependence of the casting skin microstructure.

Study on Tensile Creep Behavior of 12Cr1MoV Alloy Steel under High-Temperature Alkali Metal Salt Environment for Solar Thermal Power Generation

The heat exchange tubes of solar thermal power generation work in molten salt environment with periodic temperature change. In order to reveal the tensile creep behavior of 12Cr1MoV pipeline steel under high-temperature alkali metal salt environment, the tensile creep behavior of 12Cr1MoV alloy under different applied load and reaction temperature in high-temperature alkali metal chloride salt environment was studied. The results show that the deformation of 12Cr1MoV alloy in 600°C, NaCl-35%KCl mixed salt environment is mainly controlled by diffusion creep; with the increase of stress, the creep life of 12Cr1MoV alloy decreases. The creep fracture mechanism of 12Cr1MoV alloy in 600°C, NaCl-35%KCl mixed salt environment is intergranular ductile fracture; the increase of temperature will enhance the activation and oxidation of the chlorine atoms, thereby accelerating the corrosion of the base metal and increasing the spheroidization speed of the pearlite matrix, and the creep deformation rate of the alloy increases with increasing temperature.

Micromechanism of Damage of the Graphite Spheroid in the Nodular Cast Iron During Static Tensile Test

This work was focused on two particular phenomena contributing to a damage process of nodular cast iron under tensile stress: Internal destruction of graphite nodule and debonding at graphite/matrix (G-M) interface. The G-M debonding was analyzed depending on the phase characteristics of the metal matrix and with the increase in the distance of the observation field from the main crack surface. Typical morphological effects of decohesion in the graphite-matrix microregions related to an internal structure of graphite nodule were revealed and classified. The obtained results of the microscopic observations suggest that the path of both types of internal cracks in the graphite nodule passed through areas of weakened cohesion. Detailed microscopic observations allowed revealing some additional phenomena associated with G-M debonding along the G/M interface. In the most ductile of the tested alloys, with ferritic and ausferritic matrix, the G-M debonding was preceded by the formation of a layer of shifted graphene plates in the external envelope of the spheroid. In the alloys of polyphase pearlitic and ausferritic matrix, the revealed morphology of the G-M interface suggests that G-M debonding might be delayed by the interaction with some phase components as cementite lamellae and austenite plates.

Effect of Antimony on Graphite Growth in Ductile Iron

Antimony is used in ductile cast iron melts to avoid graphite degenerations like exploded graphite and chunky graphite. However, the effect of Sb in the melt on the crystallization of ductile cast iron is still unknown, therefore the aim of our investigations. The current study presents thermodynamic calculations showing that at the beginning of the eutectic solidification, Mg3Sb2 is formed. Furthermore, the amount of Sb increases in the austenite and decreases in the remaining melt during solidification. An experiment was conducted, in which a higher amount of Sb than usual was added to a ductile iron melt. In the microstructure of the Sb-containing sample, a higher graphite precipitate count was measured than in the reference sample without Sb. The graphite spheroids of the Sb-containing sample show an almost perfect roundness and a very smooth surface. However, lamellar outgrowths from the graphite spheroids were also observed. Using scanning electron microscopy with energy-dispersive X-ray spectroscopy, it could be shown that Sb-containing phases have been formed, which are located in the pearlitic matrix and also in graphite spheroids. As a result of spectroscopic investigations, it can be assumed that Sb-containing phases act as nucleant for graphite nucleation. Further investigations by Auger electron spectroscopy showed that a thin layer of Sb at the interface between graphite and matrix has been formed. It seems that this layer constricts the diffusion of carbon to the spheroidal graphite and acts as a surfactant favoring a layer-by-layer growth of the graphite spheroids.

Relationship between mechanical and metallurgical properties with machinability when drilling high-strength cast irons

The search for greater energy efficiency in internal combustion engines has led to the development of high-strength cast irons. The superior properties of these materials do, however, lead to lower machinability, making the final product less competitive. This paper aims to investigate, in the drilling process, the correlations between machinability and both mechanical and metallurgical properties of high-strength cast irons. Three grades of gray cast irons were evaluated: the standard grade 250 and two high-strength grades 300, one with refined graphite and other with molybdenum addition. The compacted graphite cast iron grade 450 was also tested for comparisons, as this material is also a promising candidate for such application. The output variables considered were the tool life, with analysis of the tool wear mechanisms, thrust force, torque, and quality of machined holes (roughness, roundness, and cylindricity). For correlation with the machinability, the materials were characterized according to the interlamellar spacing and microhardness of the pearlite matrix, the number of eutectic cells, and distribution of the manganese sulfide (MnS) inclusions. Test results generally showed a close correlation between the machinability and the mechanical and metallurgical properties of the materials, especially between the gray cast irons.

Conventional high-strength nodular graphite iron as a substitute for austempered ductile iron (ADI)

Cast iron materials are becoming more widely used for large and complex components in mechanical engineering and drive technology. As a consequence of steadily increasing requirements, high-strength nodular graphite irons are in great demand. In order to fully exploit the potential of these materials, an in-depth characterization of their mechanical properties is necessary. This allows for reduced safety margins. In this study, fatigue tests and accompanying microstructural characterization were performed comparatively on conventional nodular graphite iron EN-GJS-700-2. The material was characterized by very fine nodular graphite. The material has been cast into patterns of differently sized planetary carriers. Fatigue specimens were prepared from the cast components and tested in order to determine the fatigue limit. Microstructural characterization and fractography including optical and scanning electron microscopy were carried out. As a key result, the correlation between mechanical and microstructural properties reveals the importance of fine and nodular graphite accompanied by the absence of microporosities or other casting defects in order to achieve high fatigue strengths. Moreover, a minor influence of the (pearlitic or ausferritic) matrix structure, in which the graphite is embedded, was determined. Consequently, the fatigue strength of conventional high-strength nodular cast iron may be rendered adequate by improving the graphite quality and eliminating casting defects. By understanding the correlations between mechanical properties and its microstructure, a precise load-carrying capacity can be obtained. Thus, the accurate design process of a cast component may avoid additional alloying elements and the expansive heat treatment in a salt bath that is necessary for ADI. The implementation of these findings can enhance the efficiency of cast components while reducing the costs.

Effect of Matrix Microstructure on Abrasive Wear Resistance of Fe–2 wt% B Alloy

Two-body abrasive wear tests of Fe-B alloys with various matrix microstructures were performed using a pin-on-disc tribometer at a normal load of 3 N. The wear behavior was analyzed using scanning electron microscopy (SEM) and color 3D laser scanning microscopy. The results show that the Fe–2 wt% B alloy is mainly composed of a metallic matrix, M2B and M23(C, B)6. A pure pearlitic matrix occurs at a cooling rate of 0.05 °C/s, and a pure martensitic matrix forms at a cooling rate above 0.3 °C/s. Compared to the pearlitic matrix, the martensitic matrix can better support the M2B against fracture and provides higher abrasion resistance for the Fe–2 wt% B alloy. Moreover, with an increase in sliding distance, the abrasion resistance of Fe–2 wt% B alloy decreases slightly at first and then decreases rapidly. To be exact, the M2B is steadily scraped off until the critical sliding distance of 6.06 m is reached, after which the neighboring M2B fractures, leading to high material removal.