Hydrogen-assisted ductile fracture in spheroidized 1518 steel

Hydrogen was introduced into smooth and notched tensile specimens of spheroidized 1518 steel by electrocharging under controlled galvanostatic conditions. The role of hydrogen during void nucleation and growth was investigated by identifying two different void nucleation modes. Hydrogen promoted void nucleation at average-sized carbide particles by reducing the critical interfacial strength, σ_C, from 1200 to 1000 MPa (using a dislocation model). The stress-induced hydrogen segregation to the particle interfaces during deformation was estimated to explain the hydrogen-reduced σ_C. Void growth in both longitudinal and lateral directions was enhanced by internal pressurization of hydrogen. In order to better quantify such hydrogen-enhanced void growth, the internal hydrogen pressure inside a void was calculated on the basis of thermodynamics. The final void coalescence stage was analyzed by assuming that the void nucleation rate follows a normal distribution.     

Hydrogen-assisted ductile fracture in spheroidized 1520 steel: Part II. Pure bending

The effects of hydrogen on ductile fracture were investigated in a spheroidized steel similar to AISI 1520, containing negligible amounts of P and S, using notched bend bar tests. The effects of hydrogen were exhibited either in the form of enhanced plastic instability along characteristic slip traces in mode II or of enhanced, strain-controlled, local crack tip processes. The contri-bution of enhanced plastic instability, however, was only apparent under conditions in which flow localization also occurred without hydrogen. The role of plastic instability near the crack tip was found to be dominant in these bend bar tests, whereas it was small or negligible in previous tests in axisymmetric tension. Microstructural effects were rationalized in terms of a critical, local concentration of hydrogen. The intrinsic effect of hydrogen appeared to be the enhancement of strain-controlled fracture processes. Formerly with Carnegie Mellon University.     

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.     

Influence of microstructure on fracture toughness of austempered ductile iron

An investigation was carried out to examine the influence of microstructure on the plane strain fracture toughness of austempered ductile iron. Austempered ductile iron (ADI) alloyed with nickel, copper, and molybdenum was austenitized and subsequently austempered over a range of temperatures to produce different microstructures. The microstructures were characterized through optical microscopy and X-ray diffraction. Plane strain fracture toughness of all these materials was determined and was correlated with the microstructure. The results of the present investigation indicate that the lower bainitic microstructure results in higher fracture toughness than upper bainitic microstructure. Both volume fraction of retained austenite and its carbon content influence the fracture toughness. The retained austenite content of 25 vol pct was found to provide the optimum fracture toughness. It was further concluded that the carbon content of the retained austenite should be as high as possible to improve fracture toughness.     

Four-dimensional observation of ductile fracture in sintered iron using synchrotron X-ray laminography

Synchrotron X-ray laminography was used to examine the time-dependent evolution of the three-dimensional (3D) morphology of micropores in sintered iron during the tensile test. 3D snapshots showed that the networked open pores grow wider than 20?µm along the tensile direction, resulting in the internal necking of the specimen. Subsequently, these pores initiated the cracks perpendicular to the tensile direction by coalescing with the surrounding pre-existing microvoids or with the secondary-generated voids immediately before fracture. Topological analysis of the barycentric positions of these microvoids showed that they form the two-dimensional networks within the ～20?µm of radius area. These observations strongly indicate that the microvoid coalescence could occur on shear planes formed close to the enlarged open pores or between closed pores by strain accumulation and play an important role in the crack initiation.     

Hydrogen-assisted deformation and fracture behaviors of Al 8090

In the present study, the hydrogen embrittlement (HE) and the hydrogen-assisted fracture (HAF) behaviors of electrochemically hydrogen-charged Al 8090 with different specimen orientations and aging practices were examined using smooth bar and single-edge notch (SEN) specimens. It was found that the cathodic hydrogen charging substantially reduced the tensile ductility and the resistance to fracture of Al 8090. The susceptibility to HE and HAF of Al 8090 was strongly dependent on specimen orientations and aging practices. Hydrogen attack was the most significant along the grain boundaries, and, consequently, T-S oriented SEN and T-oriented smooth-bar specimens showed the highest susceptibility among the orientations studied. The susceptibility to HE and HAF decreased from underaging (UA) temper to overaging (OA) temper. It is speculated that the formation and development of a precipitate-free zone (PFZ) along the grain boundaries, rather than the change in slip planarity with prolonged aging is responsible for the reduced susceptibility to HE and HAF. Further studies are, however, required to confirm this notion.     

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.     

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.     

Impact fracture toughness of porous iron and high-strength steels

The impact fracture toughness of sintered iron and high-strength sintered steels, with densities between 7.0 and 7.25 g/cm³, have been investigated by means of instrumented impact testing on fatigueprecracked as well as 0.17-mm-notched specimens. Experimental results show that the fracture behavior is controlled by the properties of the resisting necks at the crack/notch tip. The materials with impact yield strengths of up to 700 MPa display an increase in fracture toughness as the yield strength is increased. These materials undergo continuous yielding during loading, and ductile fracture takes place once the critical plastic strain is attained within a large process zone. A process-zone model, physically consistent with the fractographic observations, correctly rationalizes their impact fracture toughness. The materials with higher impact yield strengths display an impact curve which is linear up to fracture and are characterized by a fracture toughness which is independent of the yield strength. For these materials, the process zone reduces to the first necks at the crack/notch tip, and fracture takes place once the local applied stress-intensity factor reaches the fracture toughness of the matrix.     

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.     

Modeling of local strains in ductile fracture

The concept of dividing microvoid coalescence (MVC) ductile fracture into three constituent processes, nucleation, growth, and coalescence, is discussed, with emphasis on needs for additional analytical and experimental work. Statistical and stochastic aspects of the problem are presented. Recent work on modeling of local strains during ductile fracture, and particularly as components of fracture toughness, is summarized and discussed in light of current knowledge of ductile fracture. Such local strain modeling is especially attractive because it permits micromechanisms of fracture to be explicitly included in the fracture model. This paper is based on a presentation made at the symposium “Stochastic Aspects of Fracture” held at the 1986 annual AIME meeting in New Orleans, LA, on March 2-6, 1986, under the auspices of the ASM/MSD Flow and Fracture Committee.     

Cavity formation from inclusions in ductile fracture

The previously proposed conditions for cavity formation from equiaxed inclusions in ductile fracture have been examined. Critical local elastic energy conditions are found to be necessary but not sufficient for cavity formation. The interfacial strength must also be reached on part of the boundary. For inclusions larger than about 100? the energy condition is always satisfied when the interfacial strength is reached and cavities form by a critical interfacial stress condition. For smaller cavities the stored elastic energy is insufficient to open up interfacial cavities spontaneously. Approximate continuum analyses for extreme idealizations of matrix behavior furnish relatively close limits for the interfacial stress concentration for strain hardening matrices flowing around rigid non-yielding equiaxed inclusions. Such analyses give that in pure shear loading the maximum interfacial stress is very nearly equal to the equivalent flow stress in tension for the given state of plastic strain. Previously proposed models based on a local dissipation of deformation incompatibilities by the punching of dislocation loops lead to rather similar results for interfacial stress concentration when local plastic relaxation is allowed inside the loops. At very small volume fractions of second phase the inclusions do not interact for very substantial amounts of plastic strain. In this regime the interfacial stress is independent of inclusion size. At larger volume fractions of second phase, inclusions begin to interact after moderate amounts of plastic strain, and the interfacial stress concentration becomes dependent on second phase volume fraction. Some of the many reported instances of inclusion size effect in cavity formation can thus be satisfactorily explained by variations of volume fraction of second phase from point to point. This work has been presented in part orally at the Third International Conference on Fracture in Munich, Germany April 1973.     

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.     

Brittle to ductile transition in cleavage fracture

In many cleavable solids, cleavage cracks can propagate at steady state by laying down a trail of dislocations emanating from the crack tip and lying on planes inclined to the crack front. These crack-tip initiated dislocations produce shielding at the crack tip that reduces both the crack tip tensile stresses and the shear stresses on the inclined planes. They also blunt the crack. Cleavage cracks, can nevertheless, still propagate under appropriately increased stress intensity conditions to keep the crack tip tensile stress constant. A condition is reached in the propagation of such slightly blunted cracks where a small increment in temperature or a decrement in the crack velocity permits the nucleation of a new set of dislocations that produce additional shielding and blunting which tip the balance against the crack-tip tensile stresses. This results in a transition from brittle cleavage to ductile behavior. The steady state specific plastic work that can just be tolerated by a propagating cleavage crack before it catastrophically blunts is calculated to be only of the order of 10% of the specific surface energy. Although most geometrical details of the dislocation emission process are adequately modeled, the calculated brittle to ductile transition temperatures are found to be more than an order of magnitude higher than those that have been experimentally measured. This discrepancy is a result of the present inadequate methods of modeling activation configurations by considering the dislocation loop radius as the only activation parameter, while proper modeling of such configurations must consider also the Burgers shear displacement of the loop as an activation parameter. Such two parameter analyses, however, require accurate information on interlayer atomic shear resistance profiles for specific crystals which are presently not available. The analysis furnishes ready explanations of the toughening effects of so-called “ductilization” treatments and embrittling effect of aging and dislocation locking, as well as the relatively large difference between the lowest levels of toughness between fracture in polycrystals and in single crystals.     

Cavity formation from inclusions in ductile fracture

The previously proposed conditions for cavity formation from equiaxed inclusions in ductile fracture have been examined. Critical local elastic energy conditions are found to be necessary but not sufficient for cavity formation. The interfacial strength must also be reached on part of the boundary. For inclusions larger than about 100Å the energy condition is always satisfied when the interfacial strength is reached and cavities form by a critical interfacial stress condition. For smaller cavities the stored elastic energy is insufficient to open up interfacial cavities spontaneously. Approximate continuum analyses for extreme idealizations of matrix behavior furnish relatively close limits for the interfacial stress concentration for strain hardening matrices flowing around rigid non-yielding equiaxed inclusions. Such analyses give that in pure shear loading the maximum interfacial stress is very nearly equal to the equivalent flow stress in tension for the given state of plastic strain. Previously proposed models based on a local dissipation of deformation incompatibilities by the punching of dislocation loops lead to rather similar results for interfacial stress concentration when local plastic relaxation is allowed inside the loops. At very small volume fractions of second phase the inclusions do not interact for very substantial amounts of plastic strain. In this regime the interfacial stress is independent of inclusion size. At larger volume fractions of second phase, inclusions begin to interact after moderate amounts of plastic strain, and the interfacial stress concentration becomes dependent on second phase volume fraction. Some of the many reported instances of inclusion size effect in cavity formation can thus be satisfactorily explained by variations of volume fraction of second phase from point to point.     

石墨球对系列冲击温度下球墨铸铁断口作用机制

 为了明确低温用球墨铸铁材料断裂微观机理,针对石墨球对系列温度球墨铸铁冲击断口演变过程的作用机制进行研究。采用SEM、激光共聚焦显微镜等手段系统分析了不同温度下石墨球对球墨铸铁冲击断裂过程的影响。定量断口分析结果表明,与冲击功随温度的变化一致,断口表面粗糙度Sa和空穴扩张比Rc/R0(韧窝与石墨球半径之比)均随温度的下降呈明显降低趋势。随着冲击试验温度的降低,由韧性断裂逐渐过渡到脆性断裂,这种断裂机制的变化导致断面粗糙度指数剧烈下降,空穴扩张比值趋近于1。冲击过程中裂纹总是在石墨 基体界面处发生开裂并沿着石墨 基体界面不断扩展,因此实际生产过程中应该注重改善石墨球与基体界面处组织状态。     

Studies on induction hardening of gray iron and ductile iron

The effect of induction case hardening of a gray cast iron (FG 260) and SG iron (600/3) as a function of applied induction power has been studied. The influence of various operating parameters on the penetration depth has been analysed. The case depth as a function of applied power and the associated changes in microstructure has been investigated. The case depth of SG iron was found to be twice than the gray iron due to higher resistivity of the material and increase in depth of penetration. Both hardness and the depth of penetration increased with increase in applied power associated with martensitic case formation. The surface hardness of both the irons varies between 600 to 800 VHN. The core microstructure in both the irons displayed pearlitic matrix. In the case of SG iron, the nodule size, sphericity and nodularity have reduced in the induction hardened case compared to the core.     

As cast acicular ductile aluminum cast iron

 The effects of 2.2% Ni and 0.6% Mo additions on the kinetics, microstructure and mechanical properties of ductile aluminum cast iron were studied in the as-cast and tempered conditions. Test bars machined from cast to size samples, were used for mechanical and metallurgical studies. The results showed that the addition of Ni and Mo to the base iron produces an upper bainitic structure resulting in an increase in strength and hardness. The same trend was shown when the test bars tempered from 300oC in the range of 300 to 400oC. The elongation increased with increasing the temperature from 300 to 400oC. The carbon content of the retained austenite also increased with increasing temperature. The results also showed that the kinetics, microstructure and mechanical properties of this iron are similar to Ni-Mo alloyed silicon ductile iron.     

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.