Influence of stepped austempering process on the fracture toughness of austempered ductile iron

This research studied the effect of a two-step austempering process on the fracture toughness of ductile iron and compared it to that of the conventional upper- and lower-ausferrite austempered ductile irons (ADIs). The results showed that such a two-step austempering heat-treatment process yielded a fracture-toughness value equivalent to that of the upper-ausferrite ADI, while the hardness was maintained at the level of lower-ausferrite ADI. This provided a unique combination of high toughness with good hardness (strength) properties for the ADI with a two-step austempering. Optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural features.     

Dependence of fracture toughness of austempered ductile iron on austempering temperature

Ductile cast iron samples were austenitized at 927 °C and subsequently austempered for 30 minutes, 1 hour, and 2 hours at 260 °C, 288 °C, 316 °C, 343 °C, 371 °C, and 399 °C. These were subjected to a plane strain fracture toughness test. Fracture toughness was found to initially increase with austempering temperature, reach a maximum, and then decrease with further rise in temperature. The results of the fracture toughness study and fractographic examination were correlated with microstructural features such as bainite morphology, the volume fraction of retained austenite, and its carbon content. It was found that fracture toughness was maximized when the microstructure consisted of lower bainite with about 30 vol pct retained austenite containing more than 1.8 wt pct carbon. A theoretical model was developed, which could explain the observed variation in fracture toughness with austempering temperature in terms of microstructural features such as the width of the ferrite blades and retained austenite content. A plot of K _IC ² against σ _y (X _γ, C _γ)^1/2 resulted in a straight line, as predicted by the model.     

Influence of casting size and graphite nodule refinement on fracture toughness of austempered ductile iron

Casting size affects the solidification cooling rate and microstructure of casting materials. Graphite nodules existing in the structure of ductile iron are an inherent and inert second phase that cannot be modified in subsequent heat-treatment processing. The matrix and the fineness of the second phase undoubtedly have some impact on the fracture toughness of the as-cast material, as does the subsequent heat treatment, as it alters the microstructure. This research applied austempering heat treatment to ductile iron of different section sizes and graphite nodule finenesses. The influence of these variables on the plane strain fracture toughness (K _IC ) of the castings so treated was compared to that of the as-cast state. Metallography, scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural observation.     

The effect of testing temperature on fracture toughness of austempered ductile iron

The effect of testing temperature (− 150 °C, 25 °C, and + 150 °C) on the fracture toughness of austempered ductile iron (ADI) was studied. Specimens were first austenitized at 900 °C for 1.5 hours and then salt-bath quenched to 360 °C or 300 °C, for 1, 2, or 3 hours of isothermal holding before cooling to room temperature. The resulting matrices of the iron were of upper-ausferrite and lower-ausferrite. It was found that raising the testing temperature to 150 °C from ambient improved the fracture toughness by 18, 30, and 7 pct for the as-cast/lower-ausferrite ADI/upper-ausferrite ADI, respectively. Lowering the testing temperature to −150 °C produced a decrease of −15, −35, and −48 pct. Optical microscopy, X-ray diffraction analysis, and scanning electron microscopy (SEM) fractography were applied to correlate the toughness variation with testing temperatures.     

Mechanical behaviour of an austempered ductile iron

Austempering of a ferrite-pearlitic grade of ductile iron was carried out to assess the potential use of the material for crank shaft application reported. A commercial material was austempered at 340°C to realize the properties. The austempered ductile iron gave good strength although the ductility values were lower. The material developed had complete ausferritric structure free of pearlite. The various phase constitution and phase transformation associated with the treatment and during mechanical deformation was examined. Using XRD analysis the volume fraction of the austenite in the matrix was estimated. The various aspects of processing a commercial cast iron during ausetmpering, the phase transformation, microstructural evolution have been examined along with the property of the material. The mechanical behaviour of the material and the scope for further improvement is discussed.     

Neutron diffraction study of austempered ductile iron

Crystallographic properties of an austempered ductile iron (ADI) were studied by using neutron diffraction. A quantitative phase analysis based on Rietveld refinements revealed three component phases, α-Fe (ferrite), γ-Fe (austenite), and graphite precipitate, with weight fractions of 66.0, 31.5, and 2.5 pct, respectively. The ferrite phases of the samples were found to be tetragonal,14/mmm, with ac/a ratio of about 0.993, which is very close to the body-centered cubic (bcc) structure. The austenite phase had C atoms occupying the octahedral site of the face-centered cubic (fcc) unit cell with about 8 pct occupancy ratio. A strong microstrain broadening was observed for the two Fe phases of the samples. The particle sizes of the acicular ferrite phase were studied by using small angle neutron scattering. The analysis suggested a mean rod diameter of 700 A. The scattering invariant predicts a ferrite volume fraction consistent with the powder diffraction analysis. A textbook case of nodular graphite segregation, with average diameters ranging from 10 to 20 μm, was observed by optical micrography.     

Development of high toughness in austempered type ductile cast iron and evaluation of its properties

In order to increase the toughness of austempered ductile cast irons, we attempted to strengthen the fracture initiation sites such as graphite-matrix interfaces and eutectic cell boundaries in a way of the microsegregation of alloying elements. For instance, the retained austenite which is stable under external stresses may be introduced preferentially into these sites by the addition of Ni, which segregates to a graphite periphery and of Mn, which partitions mainly to eutectic cell boundaries. Following this concept, the effects of various austempering processes on toughness are also in-vestigated. The cast iron alloying with Ni and Mn shows the best fracture toughness when it is heat-treated by either QB' or B' process; here, the QB' means the oil-quenching from an austenite γ phase range followed by austempering from a ferrite α plus γ range and the B' means austempering from a (α + γ) range. In the newly developed iron, there is a mixed microstructure composed of the ferrite, bainitic ferrite, and austenite. Abnormal elongation due to the TRIP effect in the austenite phase is found to have occurred at about 198 K. Moreover, it is shown that this TRIP effect may be caused by the formation of deformation twins.     

淬火温度对含碳化物等温淬火球墨铸铁耐腐蚀磨损性能的影响

通过等温淬火获得含碳化物等温淬火球墨铸铁(carbidic austempered ductile iron,简称CADI),并分别在中性、酸性和碱性腐蚀介质中进行腐蚀磨损实验,研究淬火温度对CADI在不同腐蚀介质中耐腐蚀磨损特性的影响,并与低铬铸铁进行对比。结果表明:在酸性和碱性介质中,CADI的质量磨损随等温淬火温度升高先增加,然后再减少;在中性介质中,CADI的质量磨损随等温淬火温度升高而逐渐增加;CADI在酸性介质中的耐腐蚀磨损性能相对较差;CADI在不同p H值溶液中的耐磨损性能均优于低铬铸铁,是一种优良的耐腐蚀磨损材料。     

Evaluation of static and dynamic fracture toughness in ductile cast iron

Crack extension behavior and fracture toughness of ductile cast iron were examined by three-point bend tests, where various detection methods of crack initiation under static and dynamic loading conditions were adopted. Loading on the specimens was interrupted at various displacement points, and the final fracture surfaces of the specimen were observed via scanning electron microscopy (SEM). Crack-tip opening displacement (CTOD) obtained under the dynamic loading condition was smaller than that under the static loading condition in ferritic ductile cast iron, and CTOD additionally decreased with increasing pearlite content in the matrix. The relationship between J (ΔC) obtained by the compliance changing rate method and J(R) established by the intersection of the crack extension resistance curve and the theoretical blunting line varied with pearlite content. The average value of .J(ΔC) and J(R), that is J (mid), was proposed to define the fracture toughness of ductile cast iron; J (mid) was considered to be a reasonable measure for the fracture toughness of ductile cast iron, irrespective of loading condition and the pearlite content in the matrix.     

Effects of carbides on fatigue characteristics of austempered ductile iron

Crack initiation and growth behavior of an austempered ductile iron (ADI) austenitized at 800 °C and austempered at 260 °C have been assessed under three-point bend fatigue conditions. Initiation sites have been identified as carbides remaining from the as-cast ductile iron due to insufficient austenization. The number of carbides cracking on loading to stresses greater than 275 MPa is critical in determining the failure mechanism. In general, high carbide area fractions promote coalescence-dominated fatigue crack failure, while low area fractions promote propagation-dominated fatigue crack failure. Individual carbides have been characterized using finite body tessellation (FBT) and adaptive numerical modeling (Support vector Parsimonious Analysis Of Variance (SUPANOVA)) techniques in an attempt to quantify the factors promoting carbide fracture. This indicated that large or long and thin carbides on the whole appear to be susceptible to fracture, and carbides that are locally clustered and aligned perpendicular to the tensile axis are particularly susceptible to fracture.     

An analysis of retained austenite in austempered ductile iron

Data from the literature have been analyzed to understand aspects of the retained austenite in austempered ductile irons, especially its relationship with the transformation mechanism of bainite. The final and initial carbon concentrations in austenite, and C _γ ⁰ , respectively, are important in determining the maximum extent of reaction, and hence, the amount of austenite and and bainitic ferrite and C _γ ⁰ data have been expressed in terms of chemical compositions and reaction temperature, with reasonable agreement between experimental and predicted results. It is demonstrated that, in connection with the lever rule, the calculated and C _γ ⁰ values can be employed to predict the microstructural constituents of austempered ductile irons.     

Influence of the amount and morphology of retained austenite on the mechanical properties of an austempered ductile iron

High Si contents in nodular cast irons lead to a significant volume fraction of retained austenite in the material after the austempering treatment. In the present work, the influence of the amount and morphology of this phase on the mechanical properties (proof stress, ultimate tensile strength (UTS), elongation, and toughness) has been analyzed for different austempering conditions. After 300 °C isothermal treatments at intermediate times, the austenite is plastically stable at room temperature and contributes, together with the bainitic ferrite, to the proof stress and the toughness of the material. For austenite volume fractions higher than 25 pct, the proof stress is controlled by this phase and the toughness depends mainly on the stability of γ. In these conditions (370 °C and 410 °C treatments), the present material exhibits a transformation-induced plasticity (TRIP) effect, which leads to an improvement in ductility. It is shown that the strain level necessary to initiate the martensitic transformation induced by deformation depends on the carbon content of the austenite. The martensite formed under TRIP conditions can be of two different types: “autotempered” plate martensite, which forms at room temperature from an austenite with a quasi-coherent epsilon carbide precipitation, and lath martensite nucleated at twin boundaries and twin intersections.     

Correlation of microstructure and fracture toughness in high-chromium white iron hardfacing alloys

A correlation is made of microstructure and fracture toughness in hypereutectic high-chromium white iron hardfacing alloys. In order to investigate the matrix effect of these alloys, in particular, four different matrices such as pearlite, austenite, and a mixture of pearlite and austenite were employed by changing the ratio of Mn/Si, while the total volume fraction of carbides was fixed. The hardfacing alloys were deposited twice on a mild steel plate by the self-shielding flux-cored arc-welding method. Fracture toughness was increased by increasing the volume fraction of austenite in the matrix, whereas hardness and abrasion resistance were nearly constant.In situ observation of the fracture process showed that cracks initiated at large primary carbides tended to be blocked at the austenitic matrix. This suggested that fracture toughness was controlled mainly by the amount of austenite in the matrix, thereby yielding the better toughness in the hardfacing alloy having the austenitic matrix. Considering both abrasion resistance and fracture toughness, therefore, the austenitic matrix was preferred for the high-chromium white iron hardfacing alloys.     

Hydrogen-assisted ductile fracture in porous iron

The present paper has studied the effect of porosity and hydrogen on the deformation and fracture properties in porous iron using double notched specimen tests and quantitative metallography analyses to characterize void growth. The presence of hydrogen produced the greatest hardening effect in the sample with a porosity of 0.037. It was found that while the effect of hydrogen on the reduction in area was greatest in the specimen with a porosity of 0.037, its effect on the fracture energy did not depend on the porosity. The ductile fracture surface morphology remained the same in all the specimens without and with hydrogen except for the sample with porosity of 0.003 where hydrogen induced some quasi-cleavage fracture. The quantitative metallography analysis of voids in the region near the fracture surface and below the unfractured notch has demonstrated that hydrogen causes the void growth to increase prior to plastic instability. The effect of hydrogen on the void growth was found to be slightly enhanced as the porosity was increased. The microscopic process of hardening induced by hydrogen is presented. The mechanism for hydrogen-assisted void growth is discussed in light of a recently developed model using a dislocation theory.     

球化剂稀土含量对大断面球铁石墨组织及低温冲击韧性的影响

选用3种具有不同Ce含量的球化剂FeSiMg6RE2、FeSiMg8RE3和FeSiMg8RE5对铁液进行盖包法处理,球化处理后浇注成180mm×180mm×200mm的球铁试块。通过室温拉伸、光学显微镜(OM)、扫描电子显微镜(SEM)、能谱分析仪(EDS)和低温冲击试验机等分析手段,研究了稀土元素Ce对大断面低温球铁的石墨组织、力学性能和低温冲击韧性的影响。结果表明:用稀土含量低的球化剂FeSiMg6RE2进行处理,试样的石墨组织细小均匀,石墨形态好,低温冲击韧性高,冲击功平均值约为14.5J;用稀土含量高的球化剂FeSiMg8RE3和FeSiMg8RE5分别进行处理,试样中出现了大面积碎块状石墨等变异组织,低温冲击韧性差,冲击功平均值分别约为6.5和5.1J。能谱分析结果表明:高稀土含量球化剂处理的球铁试样,石墨组织晶界上发生了稀土元素Ce的富集,其质量分数分别约为0.36%和0.42%,这是造成铸件中碎块状石墨等变异组织产生的根本原因,严重恶化了铸件的力学性能。     

Investigation on grinding wear behaviour of austempered ductile iron as media material during comminution of iron ore in ball mills

An attempt has been made to assess the grinding wear behaviour of austempered ductile iron (ADI) as media material in comminution of Kudremukh haematite iron ore in a ball mill. Spheroidal graphite (S.G) iron balls were austenitised at 900°C for one hour and austempered at 280°C and 380°C for different time durations. These materials were characterized by measuring hardness, carrying out X-ray diffraction analysis, studying microstructures using scanning electron microscope (SEM). Grinding wear behaviour of ADI was assessed during wet grinding at different pH of the mineral slurry. The wear resistance of ADI was compared with that of forged En 31 steel balls under similar grinding conditions. It was found that ADI balls austempered at 280°C for 30 minutes which contains lower bainite registered superior wear resistance. It was also noted that the wear resistance of ADI was more at higher pH range of the slurry.     

Effects of microstructure on fracture toughness of a high-strength low-alloy steel

Fracture toughness and ductile-brittle transition behavior were measured for a copper-bearing HSLA steel. The value ofK _lc for cleavage failure was independent of heat treatment, whileJ _lc for ductile failure decreased monotonically with increasing strength level. With both failure modes, fracture appears to be controlled by cracking of sulfide inclusions. The decrease in ductile-failureJ _lc is caused by decreased work-hardening rates that suppress cleavage and facilitate void coalescence. Both higher austenitizing temperature and quenching rate after austenitization influence the ductile/brittle transition temperature, either through grain-size and precipitate refinement or through an increase in the resistance of the steel to shear failure. Formerly Graduate Student, The Ohio State University     

Effect of microstructure on plane-strain fracture toughness of aisi 4340 steel

Commercially available AISI 4340 steel has been studied to determine the effect of transformation structures on plane-strain fracture toughness (K _IC). Martensitic and bainitic steels with wide variation in the prior austenitic grain size, and steels having two different mixed structures of martensite and bainite were investigated. Microstructures were examined by optical and transmission electron microscopy. Fracture morphologies were characterized by scanning electron microscopy. The significant conclusions are as follows: in a martensitic or lower bainitic steel in which well-defined packets were observed, the packet diameter is the primary microstructural factor controllingK _IC. The steel's property is improved with increased packet diameter. If the steel has an upper bainitic structure, the packet is composed of well-defined blocks, and the block size controls theK _IC property. When the steel has a mixed structure of martensite and bainite, the shape and distribution of the second phase bainite have a significant effect on theK _IC property. A lower bainite, which appears in acicular form and partitions prior austenite grains of the parent martensite, dramatically improves theK _IC in association with tempered martensite. If an upper bainite appearing as masses that fill prior austenite grains of the parent martensite is associated with tempered martensite, it significantly lowers the K_IC.