The effect of triaxial stress field on intermediate temperature embrittlement of ferritic spheroidal graphite cast irons

The effect of varying triaxial stress field, by elongating the spheroidal graphite, on intermediate temperature embrittlement of ferritic spheroidal graphite cast irons has been investigated. The spheroidal graphite cast irons which have a maximum ductility at 40 pct reduction can eliminate the intermediate temperature embrittlement and its dependence on triaxiality. The flow stress of ferritic spheroidal graphite cast irons can be expressed by the following equation as the function of shape ratio (β) and the ratio of half mean free path to equatorial radius (α) at 673 K:     

The effect of triaxial stress on ductility and fracture morphology of ferritic spheroidal graphite cast iron

Four ferritic spheroidal graphite (SG) cast irons of different graphite nodule interparticle spacings were prepared for a tensile experiment. The results indicate a decreasing ductility with increasing nodule spacing. Transition from pure dimple to a mixed mode of dimple and cleavage occurs when the nodule spacing reaches a critical value. The data of the S.G. cast iron possessing the largest nodule spacing reveal that the ductility is increased by raising the superposed hydrostatic pressure. The mixed mode of dimple and cleavage fractures of this SG cast iron, which occurs under normal pressure, is replaced by pure dimple fracture under all the hydrostatic pressure conditions. These results are analyzed in this article in terms of the triaxiality ratio at the ferrite center region. The higher the local triaxiality ratio, the lower the ductility and the larger the chance for cleavage fracture to occur.     

The effect of phosphorus segregation on the intermediate-temperature embrittlement of ferritic,spheroidal graphite cast iron

The objective of this article is to study the effect of phosphorus segregation on the fracture modes of the intermediate-temperature intergranular embrittlement which occur in ferritic, spheroidal graphite cast iron. The specimens were quenched from 820 °C and 500 °C during the furnace-cooling period of ferritization annealing in order to vary the degree of phosphorus segregation, then deformed in tension at various temperatures between 20 °C and 520 °C with a constant crosshead speed of 0.01 mm/s. These two kinds of specimens were also fractured by impact at about -50 °C in the vacuum chamber of a scanning Auger microscope in order to analyze the phosphorus segregation and compare the fracture modes. The results show that the fracture mode of the intermediate-temperature embrittlement is influenced by the history of heat treatment prior to tension. When the specimens were held at 500 °C and quenched from this temperature, the fracture was intergranular. However, the specimens quenched from 820 °C revealed cleavage fracture with cracks propagating radially from a central region with magnesium-rich particles. Identified by transmission electron microscopy (TEM), the particles were MgO. Grain-boundary segregation of phosphorus in the specimen held at 500 °C was confirmed by Auger analysis of the impact fracture surface. Segregation of phosphorus must play an important role in the fracture mode of the intermediate-temperature intergranular embrittlement.     

High-temperature tensile deformation and thermal cracking of ferritic spheroidal graphite cast iron

Ferritic spheroidal graphite (SG) cast irons of different silicon contents were used to study the tensile behavior in the temperature range of 500 °C to near Ac ₁. The thermal-cracking behavior under cyclic heating to various temperatures from 650 °C to 850 °C was also explored. According to the tensile data, the temperature dependence of the flow stress is concave upward, and that of the elongation is concave downward with drastic descent after reaching the maximum. The temperature range of ascending stress and descending elongation is above Ac ₁, in which the eutectic cell-wall region transforms to austenite. Intergranular fracture with serious ductility loss can take place at 500 °C, if the silicon content is too high (3.9 wt pct in this test). This brittle phenomenon can be eliminated through microstructure refining. As to the thermal-cycling test, it indicates that the thermal cracking occurs through intergranular fracture. Whereas the susceptibility to thermal cracking increases with increasing silicon content, it can be reduced by refining the microstructure. Unlike the cast irons heated above Ac ₁ with phase transformation, the heating temperature of about 750 °C leads to the most severe thermal cracking. In addition, the specimens heated in air have lower thermal-cracking resistance than those heated in a neutral salt bath.     

Effect of pearlite on the vibration-fracture behavior of spheroidal graphite cast irons under resonant conditions

This investigation examined the effect of pearlite on the vibration-fracture behavior of spheroidal graphite (SG) cast irons under resonant conditions. The experimental materials can be divided into four groups, according to their pearlite content. They are (1) a fully pearlitic matrix, (2) a bull’s-eye structure, (3) a colony-type pearlitic structure, and (4) a fully ferritic matrix. Experimental results indicated that the variation of pearlite content significantly affects the initial deflection amplitude. Increasing the amount of pearlite in the matrix leads to a lower logarithmic decrement and, thus, reduces the deflection amplitude. Moreover, the distribution and amount of pearlite considerably influence the crack-propagation mode. The specimen with a bull’s-eye structure exhibits the highest resonant-vibration fracture resistance. This can be attributed to the lower initial deflection amplitude caused by the ferrite rim around graphite particles and the better crack-propagation resistance of the surrounding pearlite.     

Eutectic cell wall morphology and tensile embrittlement in ferritic spheroidal graphite cast iron

Four ferritic spheroidal graphite (SG) cast irons (3.4C-3.9Si, 3.5C-2.7Si, 3.5C-2.0Si, and 2.0C-2.1Si, in wt pct) were chosen to study the eutectic cell morphology and the related tensile embrittlement. For the three test materials with high carbon concentration, the results indicate that the average eutectic cell wall size and the amount of cell wall inclusions decrease with decreasing silicon concentration. Compared to the test material with 3.5C-2.0Si, which has many inclusions dispersed in the ferrite matrix, the one with 2.0C-2.1Si shows larger average eutectic cell wall size and larger degree of inclusion clustering in the cell walls. Tensile embrittlement may occur in two temperature ranges, namely, the intermediate low-temperature range at the end of the upper elongation shelf and the intermediate high-temperature range of around 400 °C. In both cases, the brittle cracks are initiated preferentially at the inclusion clusters in the eutectic cell wall region. Hence, the two tensile embrittlements occur only for the materials with a larger degree of inclusion clustering in the eutectic cell walls.     

Effect of nodularity on resonant vibration fracture behavior in upper bainitic and ferritic cast irons

This work examines resonant vibration fatigue fracture behavior. According to those results, the region I section of the D-N curve (deflection amplitude vs number of vibration cycles) declined with decreasing graphite nodularity or a changing of the matrix to a bainitic structure. Resonant vibration fatigue cracks were often initiated at graphite particles on the surface and grew into the body of the specimen. Initiation and propagation of these cracks were promoted by lowering the graphite nodularity, but no significant difference in deflection amplitude was observed with a bainitic or ferritic matrix microstructure. Bainitic cast iron speciments proved to have a higher initial deflection amplitude and would simply drop almost without displaying region I as the cycle numbers of vibration increased.     

On the microstructure of graphite spherulites in cast irons by TEM and HREM

The microstructure of graphite spherulites (G.S.) in nodular cast irons doped with either Ce or (Mg + Ce) has been extensively studied using transmission electron microscopy (TEM), μ-diffraction and lattice images techniques. The results show that the section of a well-developed G.S. consists of fan-like areas formed from platelets with sizes ranging from several to tens nm radial and hundreds nm circumferential. The [001] direction of the graphite platelets within each fan-like area tends to parallel to the radius of the G.S. but spreads about ± 10° among them. 002 Lattice images of graphite reveal the arrangement of graphite based plane layers, such as branching, bending and stages with intercalary layers besides dislocations and faults. These structural characteristics of G.S. have shown that {001} planes of graphite did grow predominatly through the formation of stages by intercalary layers of ceriumoxide and ceriumoxysulfide, then the G.S. takes its final morphology.     

A kinetic model of the γ → α + Gr eutectoid transformation in spheroidal graphite cast irons

The eutectoid transformation of austenite in spheroidal graphite cast iron can follow one of two paths: (a) transformation to a mixture of ferrite and graphite or (b) transformation to pearlite. The extents to which the two reactions occur determine the relative amounts of ferrite and pearlite in the microstructure and, hence, the properties of the iron. In this paper, the kinetics of the γ → α+ Gr reaction is studied, and a model is developed to predict the isothermal transformation rates. The transformation occurs at a rate determined by the rate of carbon diffusion. The diffusion of carbon through ferrite, as well as through austenite, has been considered. The model predicts that the volume fraction of austenite transformed isothermally increases with increasing number density of graphite spheroids. Predictions of the model are compared with data available in literature.     

Study on the relationship between intermediate-temperature embrittlement and triaxial stress induced by the ellipsoidal graphite in hot-rolled ferritic spheroidal graphite cast iron

The intermediate-temperature embrittlement of a hot-rolled ferritic spheroidal graphite cast iron was studied with the consideration that triaxial stress is induced by the ellipsoidal graphite particles of three unequal axial radii. The graphite shape was changed by various rolling reductions, and the tensile tests were performed at 673 K. The results show that the elongation and flow stress are independent of rolling reduction, and intergranular fracture occurs in all specimens. In the plasticity analysis, the triaxiality ratio (σ _m σ_eq) at a point in the ferrite matrix center can be expressed in terms of graphite shape ratio (b _d ) and graphite interparticle spacing (2a _d ) as σ _m /σ_eq = 1/3 +a _d /(2b _d ) where σ _m is the hydrostatic tensile stress and σ_eq is the equivalent stress. Accordingly, the average triaxiality ratio in the matrix center region is independent of rolling reduction and greater than one, a result that is consistent with the fact that the elongation is about constant, and all specimens undergo intergranular fracture. Moreover, the rolling reduction independent flow-stress behavior can be rationalized by the analytical result that the average σ _m /σ_eq is unchanged with rolling reduction, where σ _z is the internal stress along the tensile direction.     

Effect of carbon content and ferrite grain size on the tensile flow stress of ferritic spheroidal graphite cast iron

The effects of varying carbon content and ferrite grain size on the tensile flow stress of ferritic spheroidal graphite cast iron have been investigated. The flow stress of this material can be expressed by the following equation as a function of graphite volume fraction (V_g) and ferrite grain size (d_f) without an influence of graphite nodule diameter: Φ = K₃(l - k₁V_g)({Φ₀} + k₂d_f). The relationship between tensile stress (Φ) and the volume fraction of graphite (V_g) obtained by the analysis of plastic deformation can be approximated by the above equation. It is apparent from the results that the flow stress of ferritic spheroidal graphite cast iron is influenced appreciably by ferrite grain size and the triaxial stress field developed in the ferrite matrix between graphite nodules. Formerly Graduate Student, Faculty of Engineering, Hiroshima University     

Fracture toughness and crack growth rate of ferritic and pearlitic compacted graphite cast irons at 25 ‡C and 150 ‡C

This research studied the ambient (25 ‡C) and intermediate (150 ‡C) temperatures plane strain fracture toughness(K _Ic ) and crack growth rateda/dN vs stress-intensity variation (δK) behaviors of compacted graphite (CG) cast irons in an atmospheric environment. As-cast ferritic irons with different percentages of compacted graphite (vermicularity) were produced by using insufficient amounts of spheroidizer. Irons with pearlitic matrix were obtained by heat treating the as-cast structure. The results of fracture toughness testing indicated that (1) for the same matrix, CG irons with higher vermicularity yielded lowerK _Ic values, but their values were still much higher than those of gray (flake graphite) cast iron; (2) for the same vermicularity, CG irons with pearlitic matrix exhibited higher fracture toughness values than those of ferritic matrix; (3) at intermediate temperature (150 ‡C), the influence of vermicularity and matrix on fracture toughness is the same as at ambient temperature, except that theK _Ic values were all a bit lower (1 to 8 pet). From crack growth ratevs stress-intensity variation experiments, the Paris equationda/dN = C(δK) _n was derived, where a smaller value of indicates better crack growth resistance of materials. Compacted graphite cast irons with pearlitic matrix and/or greater vermicularity rendered highern values and, thus, inferior crack growth resistance. At elevated temperature, then values were all lower, indicating that the crack growth resistance was improved. Formely a Graduate Student, Department of Materials Engineering, Tatung Institute of Technology.     

The intergranular microstructure of cast Mg-Zn and Mg-Zn-rare earth alloys

The solidification path and microstructure of cast Mg-9Zn and Mg-8Zn-1.5MM (misch metal) alloys have been investigated by a combination of thermal analysis and analytical electron microscopy. The addition of 1.5 wt pct MM had a strong influence on the as-cast microstructure with the introduction of new “ternary” interdendritic phases and structural modification of known binary phases. The temperature ranges for formation of these phases from the melt were identified, their crystal structures determined, and their compositions analyzed. Products from eutectoidal decomposition of the interdendritic phase in the binary Mg-9Zn alloy were also identified.     

Mechanism of bainitic transformation in compacted graphite cast irons

Samples of unalloyed compacted graphite cast iron (CGI) have been thermally treated to obtain a bainitic structure. Heat treatments consisting of various holding times at two different austenitizing temperatures and two different austempering temperatures have been carried out followed by metallographic observations of the resultant structures by scanning electron microscopy (SEM). The formation of bainite is discussed in terms of the temperature difference (undercooling) between the austenitizing temperature and austempering temperature, and the subsequent development of the phases present to bainite is related to the carbon concentration gradient caused by compacted graphite acting as a sink for carbon. This favors the final stages of the transformation. A hypothesis for the bainitic transformation mechanism in CGI's is thus proposed.     

Dissipative processes during cyclic hardening of spheroidal graphite cast iron

The hardening of spheroidal graphite cast irons alloyed by vanadium via chemical dispersion has been studied. Strain hardening is experimentally found to effectively occur in a cast iron with a relatively low vanadium content (0.5 wt %). Dissipative processes in cast irons with dispersed vanadium carbide inclusions are analyzed. An increase in the degree of dispersion of vanadium carbide inclusions is shown to favor more complete dissipation of the supplied energy into heat and decreases the level of damage in the material.     

Study of the austempering transformation kinetics in compacted graphite cast irons

In this work, the phases involved in the transformation austenite→ferrite+high-carbon austenite in cast irons were assessed by means of Mössbauer spectroscopy and hardness measurements for samples austempered at five different temperatures between 573 and 673 K with two Mn contents. The C content in the high-carbon austenite was found to have a dependence of t ^0.40±0.05 on the austempering time t, which evidences that diffusion of carbon has taken place. The kinetic parameter k determined using the Johnson-Mehl-Avrami equation has a maximum of 3.9×10^?3 (s^?1) at ≈623 K and corroborates that Mn slows the transformation rate.     

Study of microsegregation buildup during solidification of spheroidal graphite cast iron

This work presents an attempt to describe the complex relationship between the development of the solidification microstructures and buildup of microsegregation in spheroidal graphite (SG) cast irons by coupling an experimental investigation and a modeling approach. Experimental characterization of microsegregation in cast iron was made by means of point counting microanalysis along a grid. With this method, the differences of silicon distribution in alloys solidified in the stable system, the metastable system, or in both systems were clearly evidenced. The distribution of manganese in alloy solidified in the stable system was also investigated. It has been, in particular, observed that alloys solidified in the stable (respectively, metastable) system present significant negative (respectively, positive) segregation of silicon, and that alloys solidified in both systems are much less segregated. The solidification path of these alloys has been conveniently reproduced by means of predictions made with a physical model accounting for the nature of the alloy, either hypoeutectic or hypereutectic, and for the sensitivity to temperature and composition of the partition coefficient of alloying elements.     

The role of interphase boundary adsorption in the formation of spheroidal graphite in cast iron

The interphase boundaries in gray and ductile cast iron were studied with a scanning Auger microprobe (SAM). Sulfur and oxygen were found to be adsorbed at the flake/ metal interfaces in the gray iron, while the nodule/metal and intercrystalline graphite interfaces in the ductile iron were free of foreign elements. The only magnesium detected in the magnesium modified ductile iron was combined with phosphorus and sulfur as a compound. A model is presented which proposes that Fe−C−Si eutectic alloys in the absence of surface active impurities (such as in vacuum casting of high purity materials) produce nodular graphite due to the inherent instability of the graphite/melt interface. The sulfur and oxygen always present in commercial alloys adsorb at the graphite/melt interface, effectively “stabilizing” the active sites on the graphite basal planes, and preventing spherulitic growth. The purpose of modifiers is to getter these impurities.     

Effect of bainite transformation and retained austenite on mechanical properties of austempered spheroidal graphite cast steel

Austempered ductile iron (ADI) has excellent mechanical properties, but its Young's modulus is low. Austempered spheroidal graphite cast steel (AGS) has been developed in order to obtain a new material with superior mechanical properties to ADI. Its carbon content (approximately 1.0 pct) is almost one-third that of a standard ADI; thus, the volume of graphite is also less. Young's modulus of AGS is 195 to 200 GPa and is comparable to that of steel. Austempered spheroidal graphite cast steel has an approximately 200 MPa higher tensile strength than ADI and twice the Charpy absorbed energy of ADI. The impact properties and the elongation are enhanced with increasing volume fraction of carbon-enriched retained austenite. At the austempering temperature of 650 K, the volume fraction of austenite is approximately 40 pct for 120 minutes in the 2.4 pct Si alloy, although it decreases rapidly in the 1.4 pct Si alloy. The X-ray diffraction analysis shows that appropriate quantity of silicon retards the decomposition of the carbon-enriched retained austenite. For austempering at 570 K, the amount of the carbon-enriched austenite decreases and the ferrite is supersaturated with carbon, resulting in high tensile strength but low toughness. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.