A practical method for fatigue limit prediction in ductile cast irons

A simple, reasonably accurate method was proposed for fatigue limit prediction in ductile cast irons (DCIs) containing small defects, employing an easy‐to‐use prediction equation. A technique was also presented for the estimation of the statistical effects of complex structural discontinuities, as characterized by graphite and casting defects. The validity of these approaches was confirmed by the experimental results obtained via rotating‐bending fatigue tests on six DCIs, with varying distributions of graphite nodule size and different matrix structures.     

Wear and oxidation behaviour of high-chromium white cast irons

High-chromium white cast irons can attain very hard microstructures composed of intergranular chromium carbides dispersed in a tempered martensitic matrix. Two heat-treated high-chromium white cast irons with different carbon contents were studied in this research work using compression at 500 °C, and wear and oxidation tests at 500 and 600 °C. The hot compression behaviour of these products is excellent, with high strengths and significant ductility. They also exhibited good wear and oxidation properties. Fracture micromechanisms are discussed in relation to the chemical composition and microstructure of the two alloys.     

Characterization of the microstructure of high-chromium cast irons using Mössbauer spectroscopy

The finish grinding process for cement materials uses ball mills with differently sized grinding balls. Because the grinding takes place through the impact and friction of the balls, balls with a high wear resistance are desirable and the materials of choice are the high-chromium white cast irons. This article examines the behavior of these cast irons when subjected to various heat treatments. The emphasis is on the dependence of the microstructure properties on the chemical composition of the alloys and the heat treatment parameters. The content of retained austenite and that of carbon in the martensite phase were determined by the use of Mössbauer spectroscopy. The results verify that a higher proportion of retained austenite with low carbon content and a martensite with higher hardness produce a material with good fracture toughness and resistance to abrasive wear.     

Identification of Cu-rich precipitates in pearlitic spheroidal graphite cast irons

This investigation studies the partitioning of Mn and Cu – and Si– between ferrite and cementite in the pearlite of two alloys containing similar Mn and Si concentrations, but different Cu contents, keeping the Cu concentration within the usual industrial limits. The results confirm the partitioning of all the elements between ferrite and cementite during pearlite growth. Furthermore, the presence of nanometric Cu-rich precipitates was detected for an alloy with typical Cu contents for achieving pearlitic structures in spheroidal graphite cast irons. This might be linked to the microsegregation of Cu developed during the solidification step. These precipitates have not been reported before and cannot be ignored as they can affect the transformation kinetics and the mechanical properties of the alloy.     

Effect of copper on boride layer of boronized ductile cast irons

GGG-50, GGG-60 and GGG-80 ductile cast irons containing 0.01, 0.3 and 0.98 wt% copper, respectively, were boronized in a salt bath and then analyzed using optical microscopy, scanning electron microscopy and X-ray diffraction analysis. Increasing copper concentration in ductile cast irons resulted in formation of mono-phase boride layer (Fe₂B), decreased Si-ferrite zone and hindered the growth discontinuous graphite between boride layer and matrix.     

Optimization of machining variables in turning hard cast irons with PCBN-inserted tools

The paper presents empirical relations between the turning performance characteristics, process variables, technical restrictions, and machining costs. The authors of the paper have performed calculations of cutting conditions and cutting edge geometry for polycrystalline cubic boron nitride inserts, analyzed the influence of cast iron hardness and technical restrictions on the assessment parameters.     

The role of graphite morphology and matrix structure on low frequency thermal cycling of cast irons

Low frequency thermal cycling tests were carried out on four types of cast iron (viz., austempered ductile iron, pearlitic ductile iron, compacted/vermicular graphite iron and grey cast iron) at predetermined ranges of thermal cycling temperatures. The specimens were unconstrained. Results show that austempered ductile iron has the highest thermal cycling resistance, followed by pearlitic ductile iron and compacted graphite iron, while grey cast iron exhibits the lowest resistance. Microstructural analysis of test specimens subjected to thermal cycling indicates that matrix decomposition and grain growth are responsible for the reduction in hardness while graphite oxidation, de-cohesion and grain boundary separation are responsible for the reduction in the modulus of elasticity upon thermal cycling.     

Microstructural evolution and thermal fatigue resistance of grey cast iron

In the present work, thermal fatigue in grey cast iron has been investigated by means of a numerical and an experimental approach. Temperature gradients were generated within the material by means of a testing rig specifically designed for the experiments. The temperature gradients were responsible for the formation of severe stress fields that led to the failure of the specimens after a fairly low number of cycles. Crack growth was monitored during the tests, and the microstructure and hardness of samples were analysed after failure and compared with those of untested alloy. The repeated thermal cycles at peak temperatures of 600, 700, and 800°C led to important microstructural alterations of cast iron and to a drop in material hardness. The pearlite lamellae lost their original shape and became more fragmented. Oxygen‐rich regions surrounding the graphite flakes were produced by microgalvanic corrosion mechanism.     

Relationship between microstructure,hardness and corrosion resistance in 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons

The microstructures, hardness and corrosion behavior of high chromium cast irons with 20, 27 and 36 wt.%Cr have been compared. The matrix in as-cast 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons is pearlite, austenite and ferrite, respectively. The eutectic carbide in all cases is M₇C₃ with stoichiometry as (Cr_3.37, Fe_3.63)C₃, (Cr_4.75, Fe_2.25)C₃ and (Cr_5.55, Fe_1.45)C₃, respectively. After destabilization at 1000 °C for 4 h followed by forced air cooling, the microstructure of heat-treatable 20 wt.%Cr and 27 wt.%Cr high chromium cast irons consisted of precipitated secondary carbides within a martensite matrix, with the eutectic carbides remaining unchanged. The type of the secondary carbide is M₇C₃ in 20 wt.%Cr iron, whereas both M₂₃C₆ and M₇C₃ secondary carbides are present in the 27 wt.%Cr high chromium cast iron. The size and volume fraction of the secondary carbides in 20 wt.%Cr high chromium cast iron were higher than for 27 wt.%Cr high chromium cast iron. The hardness of heat-treated 20 wt.%Cr high chromium cast iron was higher than that of heat-treated 27 wt.%Cr high chromium cast iron. Anodic polarisation tests showed that a passive film can form faster in the 27 wt.%Cr high chromium cast iron than in the 20 wt.%Cr high chromium cast iron, and the ferritic matrix in 36 wt.%Cr high chromium cast iron was the most corrosion resistant in that it exhibited a wider passive range and lower current density than the pearlitic or austenitic/martensitic matrices in 20 wt.%Cr and 27 wt.%Cr high chromium cast irons. For both the 20 wt.%Cr and the 27 wt.%Cr high chromium cast irons, destabilization heat treatment gave a slight improvement in corrosion resistance.     

On the fatigue damage micromechanisms in Si‐solution–strengthened spheroidal graphite cast iron

Graphite nodules in spheroidal graphite cast iron (SGI) play a vital role in fatigue crack initiation and propagation. Graphite nodules growth morphology can go through transitions to form degenerated graphite elements other than spheroidal graphite nodules in SGI microstructure. These graphite particles significantly influence damage micromechanisms in SGI and could act differently than spheroidal graphite nodules. Most of the damage mechanism studies on SGI focused on the role of spheroidal graphite nodules on the stable crack propagation region. The role of degenerated graphite elements on SGI damage mechanisms has not been frequently studied. In this work, fatigue crack initiation and propagation tests were conducted on EN‐GJS‐500‐14 and observed under scanning electron microscope to understand the damage mechanisms for different graphite shapes. Crack initiation tests showed a dominant influence of degenerated graphite elements where early cracks initiated in the microstructure. Most of the spheroidal graphite nodules were unaffected at the early crack initiation stage, but few of them showed decohesion from the ferrite matrix and internal cracking. In the crack propagation region, graphite/ferrite matrix decohesion was the frequent damage mechanism observed with noticeable crack branching around graphite nodules and the crack passing through degenerated graphite elements. Finally, graphite nodules after decohesion acted like voids which grew and coalesced to form microcracks eventually causing rapid fracture of the remaining section.     

Structure-property relationships in polyimide molecular composites

Rigid rod-like polyimide (PI_R) / flexible-chain polyimide (PI_F) molecular composites, with 20, 40, 50, 60 and 80 wt. % of the rigid component, were prepared and their structure and molecular dynamics were investigated, in relation to those in pure PI_R and PI_F. SAXS, DSC, DMA, TSDC and laser-interferometric creep rate spectroscopy (CRS) techniques were used for characterization of nanostructure, glass transition and sub-T _g relaxations at temperatures from 100 to 600–650 K. All the experiments indicated pronounced deviations from additivity in both nanostructure and dynamics of molecular composites. Mixing of the PI_R and PI_F components led in particular to a smaller nanostructure, down to formation of the nanoscale-homogeneous composite. Changes of the dynamic characteristics in two opposite directions and arising of large dynamic heterogeneity around T _g were observed as a result of confinement/constraining effects. Received: 19 November 2001 / Reviewed/Accepted: 7 December 2001     

Round‐robin prediction of the fatigue limit of a ring of spheroidal graphite cast iron

A round‐robin investigation has been performed, where stress analysts from eight different organisations carried out a total of 11 predictions of the expected fatigue limit of a diametrically loaded cast ring subjected to fluctuating tensile or compressive loading. Whereas geometry, load parameters, and type and quality of material (spheroidal graphite cast iron EN‐GJS‐600‐3) had been prescribed, the participants were free to use computational tools and models, and fatigue assessment models and data of their own choice. The objectives of the investigation were to compare the 11 predictions (i) among themselves, and (ii) with a posteriori experimental fatigue limits determined by means of stair‐case testing. The fatigue limit predictions showed coefficients of variation of as large as 25%. Even for a group of analysts from a single organisation, the coefficients variation were around 15%. Fatigue tests gave mean fatigue limits 60% (tensile loading) and 30% (compressive loading) above the a priori predictions. Possible reasons for the large deviations between single predictions and for their conservatism have been proposed. It seems that design engineers (i) make use of the available room for interpretation of models and data, and (ii) have an unconscious tendency to make conservative assumptions. Only if models and data for fatigue assessment are prescribed in great detail, can the ‘scatter’ among fatigue limit predictions be expected to decrease below 15–25%. Improved ‘absolute’ predictions would require more accurate fatigue data.     

Investigation into mechanical properties of austempered ductile cast iron (ADI) in accordance with austempering temperature

This work concerns mechanical properties of an austempered ductile cast iron (ADI). Samples alloyed with copper and molybdenum were austenitized at 910 °C for 90 min and subsequently austempered in a salt bath over a range of temperatures from 350 °C to 410 °C to obtain favorable mechanical properties such as tensile strength, elongation, and fracture toughness. Those properties were compared from various austempering heat treatments.     

Influence of chunky graphite on mechanical and fatigue properties of heavy-section cast iron

This research work aimed to find out the influences of the different amounts of chunky graphite on mechanical and fatigue properties of GJS 800 ductile cast iron. Chunky graphite has been a problem of heavy section thick-walled ductile cast iron components. Chunky graphite is branched and interconnected as a network within eutectic cells and has been observed to form in thermal centres of heavy ductile cast iron sections during solidification. This research work proved that chunky graphite in the microstructure decreases the ultimate tensile strength, the elongation to fracture and fatigue life significantly, but does not influence on the yield stress of ductile cast iron GJS 800 substantially. Low nodular count and nodularity rate also decrease the fatigue life of ductile cast iron, and the difference of fatigue life of specimens containing chunky graphite or having low nodular count and nodularity rate is not large. Influence of the amount of chunky graphite on fatigue life increases until 20% chunky graphite content, and above that the fatigue life does not decrease substantially.     

An energetic approach in thermomechanical fatigue for silicon molybdenum cast iron

Abstract

The purpose of this paper is to define a low cycle fatigue criterion in order to predict the failure of engineering structures. The major problem in defining a predictive fatigue criterion is that it should be applicable for structures submitted to complex multiaxial thermo-mechanical loadings but should be identifiable from simple experiments on specimens. After a short critical review of the principal criteria used in low cycle fatigue it will be shown that the dissipated energy per cycle permits a correlation of isothermal and anisothermal results obtained on silicon molybdenum cast iron in the case of specimens and also on structures.     

High‐temperature low‐cycle fatigue behaviour of a cast Al–12Si–CuNiMg alloy

The low‐cycle fatigue behaviour of a cast Al–12Si–CuNiMg alloy, with a high content of Si, is investigated at 200, 350 and 400 °C. The fatigue test results show that the alloy exhibits symmetrical hysteresis loops, moderate cyclic softening and higher fatigue resistance at higher temperature. The fracture surface analysis reveals that more tear ridges are formed at higher temperature, which strongly affect the fatigue resistance. Furthermore, evaluation of the material fatigue resistance using an energy‐based Halford–Marrow model indicates that the material's ability to absorb and dissipate plastic strain energy is enhanced as temperature increases.     

Fatigue crack nuclei in austempered ductile cast iron

ABSTRACT Short fatigue crack nuclei in austempered ductile cast iron have been studied using optical microscopy, scanning electron microscopy, atomic force microscopy and X‐ray microtomography and by electron backscatter diffraction analysis. Fatigue cracks nucleate at graphite nodules and shrinkage microporosity. The crack nuclei are arrested and retarded by barriers in the microstructure, by either blocking of slip at boundaries or owing to the requirement for tilt and twist of the stage I crystallographic crack at grain boundaries. These observations indicate that both the size of the defects, such as graphite nodules and microporosity, and the size of the prior austenite grains control the largest crack nucleus that can develop, and hence determine the component fatigue limit.     

Influence of microstructure of (α+β) Ti-6.2.4.6 alloy on high-cycle fatigue and tensile test behaviour

Tensile and gigacyclic fatigue behaviour of Ti–6Al–2Sn–4Zr–6Mo alloy were investigated as a function of lamellar primary α- and β-transformed microstructures. Three thermomechanical processes (TP1, TP2, TP3) were selected to produce different combinations of microstructural features on two slightly different compositions of the alloy (A and B). Ultrasonic fatigue tests were performed in air and liquid nitrogen at a frequency of 20 kHz ( R = −1, T  = 300 and 77 K), giving fatigue tests up to 10⁹ cycles. Microstructural features and the fracture initiation dependence on the primary α lamellar phase were observed by SEM and/or characterized by quantitative image analysis. It has been found that the microstructure of alloy B produced by TP1 represents a better compromise between resistance to initiation and resistance to microcrack growth. Quicker initiation occurs in coarser α-platelets (TP2, alloy B), and the continuous partially transformed β matrix appears to effectively decrease the tensile and HCF resistance. The bimodal structure (TP3, alloy B) has the best resistance at room temperature, but the presence of a coarse globular phase decreases this fatigue resistance at low temperature.     

Computation and application of local solidification times of grey cast iron cast in metallic moulds

Heat conduction equations applicable to the solidification of grey cast iron cast in moulds of the same material were solved for the cases of plate and cylindrical shaped castings made by pouring the metal at different temperatures into moulds of different wall thicknesses preheated to different temperatures, when the heat transfer coefficient at the casting-mould interface was assumed to have different values. An explicit finite difference method was used to solve the equations with the aid of a digital computer. Local solidification times at different nodal points were determined from the solutions and a relationship between the local solidification time and the location in a ‘casting’ was established. The application of local solidification times for predicting the microstructure and estimating the ultimate tensile strength of these “castings” cast in metallic moulds has been demonstrated.     

玻璃模具用球墨/蠕墨复合铸铁的研究

介绍了用稀土和Ｋ－Ｎａ复合变质处理而形成的球墨／蠕墨复合铸铁玻璃模具材质的组织,力学性能及热特性,探讨了影响这些性能的因素,试用表明,这种材和于玻璃具有着良好的使用性能和较高的经济价值。