Austempered Cast Iron Research Articles

Evaluation of Corrosion Behavior of Different Grades of Cast Iron Insodium Chloride Solutions

In this work, the corrosion behavior of different grades of cast iron in 3.5% and 5% of NaClsolution was evaluated. The samples used in this work are; Grey cast iron (GI), ductile cast iron(DI), austempered ductile cast iron (ADI), intercritically austempered cast iron (IADI) and Ni-Resist cast iron. The study was carried out using the Open- Circuit technique (OPC),Potentiodynamic polarization (PP), and electrochemical impedance spectroscopy (EIS)measurements and complemented by Scanning electron microscopy (SEM) and Energydispersive X-ray analysis (EDAX). The results obtained showed that the austempering heattreatment and nickel addition improves the corrosion resistance of cast iron. The order ofcorrosion resistance in NaCl solution is as follows: Ni-Resist > ADI > IADI > DI > GI.

Elevated temperature thermal conductivities of some as-cast and austempered cast irons

ABSTRACT The aim of this study was to provide insight on thermal conductivity of three cast iron groups, namely lamellar, compacted and spheroidal graphite irons at elevated temperatures up to 673 K (400°C) in as-cast and austempered states. Austempering treatments increased mechanical properties of all the studied materials while decreasing thermal conductivity across the line. The effects of austempering on conductivity were lower for grey and compacted graphite iron than for spheroidal graphite irons. The results indicate that heat treating can be a viable option in increasing cast iron performance in thermally stressed applications. One ferritic low-silicon spheroidal graphite iron surpassed lamellar graphite iron in conductivity at elevated temperatures, while high-silicon spheroidal graphite irons exhibited low conductivities.

AUSTEMPERING HEAT TREATMENT STUDY OF CAST DUCTILE IRON: ANALYSIS OF MECHANICAL AND MICROSTRUCTURAL PROPERTIES, ACCORDING TO THE A897M STANDARD SPECIFICATIONS FOR AUSTEMPERED DUCTILE IRON CASTINGS

The objective of this work was to develop heat treatment parameters of an austempered cast iron alloy ASTM 897 / A 897M - 1400/1100/1, aiming at the production of a truck spring support. The austempered nodular cast iron, known by the acronym ADI - Austempered Ductile Iron - is a class of nodular cast iron that, after austempered thermal treatment, increases significantly its mechanical properties and tenacity (Machado, 2007). Mechanical and metallographic tests demonstrated the great influence that the level of microshrinkage has on the elongation and mechanical resistance of the material. Generally, tensile tests demonstrate high elongation due to minimal presence of microshrinkage and segregations in the metallic matrix of the material, as well as to the presence of austenite with high carbon retained in the ADI matrix. Analyzes were performed to determine if the mechanical properties required by ASTM 897 / A897M were achieved. Within this standard, four degrees can be obtained. The degree of interest in this study was 1400/1100/1, which is the grade requested by the company, so that the truck spring support can be put into service. Tensile, Charpy and optical microscopy tests were carried out.

Effect of Ti as a trace element on mechanical properties and microstructural evolution in direct austempered ductile cast iron

Abstract In this study, the effect of Ti as a trace element on acicular ferrite formation in direct austempered ductile cast irons was investigated. A step-block shape sample was used and the melt was poured into a sand mold at about 1450 °C. After cooling in the mold, the melt was hot knocked out at approx. 900 °C and then medium cooling to 350 °C was applied to the step-block shape sample. Afterwards, isothermal treatment was conducted in a commercial furnace at this temperature for 1 h, and then the sample was air-cooled down to room temperature. It was found that Ti as a trace element has an important effect on the nucleation of beneficial phases such as acicular ferrite in ductile irons which can result in increasing of mechanical properties. The microstructural examination confirmed that Ti actually acts as nucleation site for acicular ferrite. Moreover, the results show that in direct austempered ductile cast iron samples contain 0.25 wt.-% Ti, an increase from 820 to 950 MPa in tensile strength properties can be seen. Furthermore, the impact test results show that with an increase of tensile strength, the impact toughness does not decrease, but rather remains constant.

Initial Assessment of Abrasive Wear Resistance of Austempered Cast Iron with Vermicular Graphite

Abstract The work compares the abrasive wear resistance of cast iron containing vermicular graphite, measured in the as-cast state and after austempering carried out at 290°C, 340°C, or 390°C. Theexaminations were performed by means of the T-01M tribological tester using the pin-on-disc configuration. Specimens used for examinations were taken from the end tabs of the tensile specimens, these being cut out of the test walls of the double-leg keel block test castings. Examinations proved that the austempering process increases the abrasive wear resistance of vermicular cast iron by several times as compared with the as-cast material. A tendency for a slight decrease in abrasive wear with an increase in austempering temperature can be stated. The coefficient of friction took a little higher values for cast iron after thermal treatment than for the as-cast material. The work was completed with roughness examination by means of electron scanning microscopy.

Dry rolling-sliding wear of austempered cast iron

In the present investigation, the dry rolling-sliding wear of two austempered ductile irons (ADIs), characterized by different hardness values, was investigated. The tests were carried out using disks with 40 mm diameter and 10 mm height. The applied load was in the range between 50 and 500 N. Since the initial Hertzian pressures were quite large because of the line contact, a plasticity dominated wear with the formation of delamination was obtained. Therefore, the treatment cycle of some specimens was conducted with the aim of obtaining a mild wear during rolling-sliding through the formation of a surface oxide layer. As a comparison, the dry rolling-sliding wear of a gas nitrided steel was also investigated. The results show that the wear coefficients of the specimens with the oxide layer on their surface was actually mild. However, the specimens produced without such a layer displayed even lower wear coefficients. Although wear was by delamination, wear coefficients were still low because of the action of graphite, which was able to squeeze on the surface during sliding, thus reducing adhesional forces. In addition, a mechanically mixed layer formed on the sliding surfaces, and this provided an additional increase in the wear resistance. Because of this, the material with lower matrix hardness displayed a greater wear resistance. The wear resistance of the nitrided steel was found to be determined by the outer part of the compound layer, and it was found to be lower than that displayed by the two ADI.

Rolling Contact Fatigue of Austempered Cast Irons with Different Graphite Shapes

Rolling Contact Fatigue of Austempered Cast Irons with Different Graphite Shapes

Estudo comparativo de ferros fundidos nodulares temperados e austemperados

This work aims to evaluate the mechanical properties of three different materials, relating the tests results with the metallurgical features of the samples. The first and the second material were an austempered ductile cast iron (austempering temperatures of 260 and 3000C, respectively). The third material was a quenched and tempered ductile cast iron. Impact, tension and hardness tests were employed in the mechanical characterization of the materials. In the metallurgical characterization, optical microscopy and image analysis were employed. It was observed that in the impact tests, the austempered ductile iron in the temperature range of 3000C was the material with higher toughness. In this material, low hardness allied to the high retained austenite level (21%), were the main factors acting in the higher impact resistance of the 3000C austempered cast iron. Key words: ADI, mechanical properties, retained austenite.

Influence of graphite nodularity on microstructure and processing window of 1.5% Ni–0.3% Mo austempered cast iron

The effect of graphite nodularity on microstructure and processing window of austempered cast iron with four-different levels of graphite nodularity and a composition of 3.6% C, 2.2% Si, 0.2% Mn, 1.5% Ni, 0.3% Mo was studied. Microstructural observations and measurements of the retained austenite content and hardness, are reported as functions of austempering time at 440 and 300 °C after austenitising at 900 °C. The results of the experimental investigation indicate that graphite nodularity has considerable effect on the microstructure. After 8 min austempering time at 440 °C austempering temperature and 20 min austempering time at 300 °C austempering temperature, the amount of retained austenite (Xγ) increases with the graphite nodularity. The microstructural investigation and hardness test investigation indicate that the graphite nodularity has a significant effect on the time t 1 corresponding to the end of stage I (process window), whereas decreasing the graphite nodularity delays the end of stage I reaction. The starting time of reaction II, t 2 is determined from the variation of Xγ with austempering time, it is indicated that lower graphite nodularity accelerates the start of stage II reaction.

Effect of graphite morphologies on the tribological behavior of austempered cast iron

Three different cast irons, gray, nodular and compact iron, with different chemical compositions were subjected to austempering to investigate the effects of graphite morphology, composition and austempering on tribological behavior. The austempering process was carried out at 375 °C for different periods to achieve optimum mechanical properties. The austempered cast iron specimens with optimum mechanical properties were used for wear tests. For comparison, wear tests were also carried out on a pearlitic gray iron specimen made from railway brake shoes. Sliding wear tests were performed in a block-on-ring wear tester with the test materials rubbing under dry atmospheric condition against a ring made from a railway wheel at speeds of 250 and 950 rpm with nominal stress of 2 MPa. A non-contacting capacitance transducer with a computer interface was used to measure the coefficient of friction of the specimens. The impact test results show that austempered nodular iron with 105 J/cm 2 has the highest impact energy while austempered gray iron with 7–9 J/cm 2 shows the lowest. The optimum impact energy of the austempered compact cast iron was 14–22 J/cm 2 as a function of chemical composition. The wear test results indicate that austempered cast iron shows superior wear resistance than pearlitic gray iron, particularly at the lower speed. For austempered cast iron specimens, compact iron shows the highest wear resistance while gray and nodular cast iron have lower wear resistance.

Influences of graphite shapes on wear characteristics of austempered cast iron

Dry slip–rolling contact wear test and wet slip–rolling contact fatigue test of several austempered cast irons with various graphite shapes were conducted by using a Nishihara-type wear-testing machine. The influences of the graphite shapes on the wear and fatigue characteristics of cast iron were investigated. Changing graphite shape from spheroidal to flake was found to result in a considerable increase in wear loss in both dry and wet conditions. Decrease in graphite nodularity results increase in wear loss at the initial wear stage. It is because lower graphite nodularity results in a shorter average matrix distance between graphites and also a larger stress concentration factor at graphite tips. The pitting phenomenon is easier to occur in sample with lower graphite nodularity.

Monte Carlo Simulations of Mössbauer Results in Fe–C Austenite

The Mossbauer signal of Fe–C austenite is difficult to interpret because different Fe–C configurations give rise to different hyperfine interactions. Two competing models for the distribution of carbon atoms in the fcc austenite lattice have been proposed to account for Mossbauer spectra. Both models assume a repulsive C–C interaction of different magnitude and different Fe–C configurations. From the Mossbauer spectra, the abundance of the different iron sites could be determined. That abundance may also be calculated with Monte Carlo simulations. In this work, using the two controversial models, we compare the relative fractions of the Fe–C configurations obtained from Mossbauer spectra of vermicular austempered cast iron with Monte Carlo simulation.

Current Status of Austempered Cast Irons

Current Status of Austempered Cast Irons

Evaluation of Fatigue Limit of Spheroidal Graphite Cast Iron

The fatigue limits of various spheroidal graphite cast irons were determined under completely reversed plane bending condition. The relationships between the fatigue limit and microstructural factors such as graphite nodule diameter and matrix hardness were clarified, and the prediction equations of the fatigue limit of the irons were determined. It is observed that the fatigue limit strongly depends on maximum nodule diameter presumed by using extreme statistics, because fatigue cracks initiate from larger graphite nodules just below the specimen surface prior to others. The fatigue limit can be significantly improved by the hardening matrix only in the specimens with a fine nodule size. Austempered cast iron has a higher fatigue limit than other irons although having the same matrix hardness. As a consequence of the quantitative analysis, the prediction equations of the fatigue limits of spheroidal graphite cast irons are given.

Shot Peening Induced Phase Transformations and Residual Stresses Measurements on Austempered Cast Irons by XRD

Shot Peening Induced Phase Transformations and Residual Stresses Measurements on Austempered Cast Irons by XRD

Relationships between microstructure and properties of unalloyed compacted graphite cast irons

Austempered cast irons have been the subject of much attention in recent years because of their excellent mechanical properties. The hardness, ultimate tensile strength and dynamic elastic modulus are presented for a commercially available unalloyed compacted iron (C.E. 4.31) and correlated with different matrix microstructures (as-cast, ferritized, normalized and austempered). For this study, two isothermal temperatures for the austempering treatment were chosen: 400°C and 300°C. The influence of a ferritizing treatment prior to normalizing and austempering has been evaluated, the results indicating that no advantages are obtained with this additional treatment. The influence of microstructure on properties and on the resulting fracture surfaces in tensile tests are discussed.

Friction and wear behavior of high silicon bainitic structures in austempered cast iron and steel

The sliding wear behavior of various microstructural combinations obtained by austempering ductile cast iron (3.7 wt.% C, 2.54 wt.% Si, 0.45 wt.% Mn) in the upper bainitic temperature range of 370 °C as a function of time was investigated. The wear behavior of high silicon steel (2.5 wt.% Si, 1 wt.% Ni) austempered at 420 °C for various times was also studied. Sliding wear tests were performed in a pin-on-disk wear tester with the test material rubbing under dry atmospheric conditions against a flat hardened toolsteel disk at a speed of 1.15 m s−1 and loaded with normal loads of 10.5 and 19.6 N. It was found that the wear resistance of austempered steel was much higher than that of austempered cast iron.The effect of graphite on friction and wear was studied using the high silicon steel austempered to provide the same microstructures as in ductile cast iron with the exception of graphite. The increased wear rate of cast iron was attributed to the surface-weakening produced by the presence of graphite. The effect of retained austenite on wear behavior was investigated in terms of work hardening and the deformation-induced transformation of retained austenite to martensite.The worn surfaces of austempered cast iron and steel were studied by scanning electron microscopy. Surface cracks and extensive deformation were observed on the worn surfaces of austempered cast iron but there was no indication of cracking on the austempered steel surfaces.

Erosion behavior of high silicon bainitic structures: II: High silicon steels

The erosion behavior of two austempered high silicon (2.5 wt.% Si) steels, one containing 1 wt.% Mn and the other 1 wt.% Ni, has been compared with the mechanical property variations after austempering for various times in the upper bainite temperature range (420 °C). For both steels the erosion resistance is a maximum for austempering times near the end of stage II. The erosion resistance correlates directly with the tensile toughness and with the percentage elongation divided by the percentage reduction in area and inversely with the hardness in all three stages of austempering. It is shown that the strain-hardening coefficient peaks at the minimum erosion rate and that the high strain-hardening character of the retained austenite in these austempered steels is beneficial to erosion resistance. Comparison with the austempered cast irons in Part I of this study shows that the graphite nodules of the cast irons have no significant effect upon erosion rate.