Fracture toughness of austempered chilled ductile iron

The structure and properties of ductile iron are highly dependent on the solidification mechanism and chills are used to promote directional solidification to get sound castings. A series of fracture toughness experiments were carried out involving austempered chilled ductile iron containing 3.42% C, 1.8% Si and other alloying elements. By using copper chills of different thickness, the fracture toughness of varying the chill rate was also examined. The fracture toughness tests were carried out using three-point bend specimens, each with a chevron notch, as per ASTM E 399 1990 standards. It was found that austempered chilled ductile iron is highly dependent on the location on the casting from where the test samples are taken and also on the Ni and Mo content of the material. Chill thickness, however, also affects the fracture toughness of the material.     

Fracture toughness of austempered chilled ductile iron

Abstract

The structure and properties of ductile iron are highly dependent on the solidification mechanism, and chills are used to promote directional solidification to obtain sound castings. A series of fracture toughness experiments was carried out, involving austempered chilled ductile iron containing 3·42%C, 1·8%Si, and other alloying elements. By using copper chills of different thicknesses, the fracture toughness with varying chill rate was examined. Fracture toughness tests were carried out using three point bend specimens, each with a chevron notch, according to ASTM Standard E399 : 1990. It was found that the fracture toughness of austempered chilled ductile iron is highly dependent on the location in the casting from where the test specimens are taken and also on the nickel and molybdenum contents of the material. Chill thickness, however, also affects the fracture toughness of the material.     

Experimental and numerical assessment of fracture toughness of dual-phase austempered ductile iron

The effects of the microstructure topology on the fracture toughness of dual-phase austempered ductile iron are studied in this paper by means of finite element modelling and experimental testing. To this end, specimens with matrix microstructures ranging from fully ferrite to fully ausferrite were studied and the preferential zones and phases for crack propagation were identified in every case. The effectiveness of the ausferrite phase as a reinforcement of the ferritic matrix via the encapsulation of the brittle and weak last-to-freeze (LTF) zones was confirmed. The toughening mechanism is consequence of the increment in the crack path longitude as it avoids the encapsulated LTF zones. Besides, the presence of small pools of allotriomorphic ferrite increase the crack propagation resistance of the ausferrite-ferrite matrices.     

Fractographic analysis of austempered ductile iron

This investigation involves a systematic study of the fracture surfaces of two grades of austempered ductile iron (ADI) broken under quasi‐static, dynamic and cyclic loading conditions. The study used electron microscopy, optical microscopy and image post‐processing. The results show that the predominating fracture mechanism in ADI upon impact loading changes from quasi‐cleavage to ductile (with little areas of cleavage facets) as the testing temperature increases. Noticeably, even at the lower temperatures tested, the fracture surface of ADI shows clear signs of ductile fracture mechanisms. In particular, graphite nodule cavities suffer marked plastic deformation. Fracture after bending tests at room temperature was characterized by a mix of quasi‐cleavage facets, deformation of the contour of nodular cavities and microvoid coalescence. In the case of fatigue fracture at room temperature, the fracture surfaces show a flat appearance which has notorious differences with those reported for other loading conditions, but the typical fatigue striations were not found. The particular features identified on the fatigue fracture surfaces can be used to identify fatigue failures. It was also shown that the determination of the direction of main crack propagation by using the experimental methodology proposed earlier by the authors is applicable to ADI fractured by impact and quasi‐static loads. The results provide information potentially useful to fractographic analyses of ADI, particularly in samples that fail in service under unknown conditions.     

Corrosion fatigue of austempered ductile iron

Corrosion fatigue (CF) behavior has been investigated for an austempered ductile iron (ADI) by conducting systematic fatigue tests at 20 Hz, including both high-cycle fatigue (HCF, S-N curves) and fatigue crack growth (FCG, da/dN-K curves), in air, lubrication oil and several aqueous environments. Results showed the HCF resistance of ADI was dramatically reduced by the given aqueous media, in particular, to a greater extent with a decrease in pH value. However, the given room-temperature aqueous solutions did not exert significantly detrimental effects on the Stage II crack growth compared with an atmospheric environment but an increase in solution temperature caused enhanced Stage II crack growth. Among the given variables of the bulk environment, pH had the greatest influence on HCF response while temperature had the most influence on the FCG of long cracks. In addition, SAE 10W40 lubrication oil provided an inert environment to remove the corrosive effect and enhance the CF resistance of ADI. The overall comparisons indicated the environmental effects would generate more influence on Stage I cracking than on Stage II cracking for the given ADI.     

Fracture toughness characterization of austempered ductile iron produced using both conventional and two-step austempering processes

The aim of the present study is to characterize mainly fracture toughness as well as the other mechanical properties of austempered ductile iron produced using both single-step and two-step austempering processes. The effect of alloying with Ni and Mo has been investigated. Austempering heat treatment was conducted at temperatures between 260 °C and 390 °C. Plane strain fracture toughness was evaluated for each material and heat treatment condition. It was found that two-step austempering process resulted in improving the fracture toughness of the material, while maintaining reasonable levels of strength. Alloyed samples showed higher fracture toughness than un-alloyed ones.     

Fracture behavior and mechanism in austempered ductile iron

The fracture behavior of copper-alloyed austempered ductile iron (ADI) was studied using metallography and fractography of selected samples. Three different grades of ADI were developed by austenitization at 900 °C for 60 min, followed by austempering for 60 min at either 270, 330, or 380 °C. The variation in austempered microstructure was determined by scanning electron microscopy of metallographically prepared samples, and structural parameters such as volume fraction of austenite, carbon content, and bainitic needle width were determined from the X-ray diffraction of powdered samples. The effect of austempering temperature on these structural parameters and on hardness, 0.2% proof stress, ultimate tensile strength (UTS), percent elongation, and impact strength was also studied. The fracture behavior under tensile and impact loading was determined by examination of the fractured surfaces and transverse cross sections near the fracture surface. The hardness, 0.2% proof stress, and UTS decrease and the impact energy increases as the austempering temperature is increased, and the morphology of the bainitic structure changes from lower to upper.     

Study of wear behaviour of austempered ductile iron

An investigation was carried out to examine the influence of austempering temperature on microstructural parameters and the wear behaviour of austempered ductile iron. Ductile iron was austenitised at 900 °C for 30 min and austempered for 2 h at 260, 280, 300, 320, 350, 380 and 400 °C. Resulting microstructures were characterised through optical microscopy and X-ray diffraction. Wear test was carried out using a pin-on-disc machine with sliding speed of 289 m min⁻¹. Coarse ausferrite microstructure exhibited higher wear rate than fine ausferrite microstructure. At high austempering temperature large amounts of austenite was instrumental in improving the wear resistance through formation of deformation induced martensite. Study of the wear surface under scanning electron microscope showed that, under dry sliding condition, wear occurred mainly due to adhesion and delamination. Wear rate was found to be dependent on the yield strength, austenite content and its carbon content.     

Multiphase matrix structure of unalloyed austempered ductile iron

A multiple phase structure characterised by a mixture of lenticular prior martensite (PM), fine needle bainitic ferrite and film retained austenite (RA) of an unalloyed ductile iron is developed. The designed austempering consists of initial rapid quenching to 210, 200 and 180°C respectively and finally austempering at 220°C for 240 min. The optimum mechanical properties, with a tensile strength of 1330?MPa, an elongation of 3.1% and 422HB, can be achieved by controlling the volume fraction of PM to 12.3% and the RA content to 18.1%. This is mainly attributed to PM that can accelerate the subsequent bainitic transformation and promote refinement of multiphase colonies.     

Metallography of bainitic transformation in austempered ductile iron

Abstract

An unalloyed nodular cast iron has been used to investigate the development of microstructure on heat treating in the bainite temperature region. Specimens were austenitised at 900°C for 1·5 h, then austempered for 1, 2, or 3 h at 250,300, and 350°C, respectively, and examined by light, transmission electron, and scanning electron microscopy. Experimental results indicate a microstructure consisting of a stable, highly enriched, retained austenite with one of two lower bainitic ferrite morphologies. One of these morphologies is carbide free acicular ferrite for specimens austempered at 350°C for 1 h and the other is bainitic ferrite in which carbide is distributed within the ferrite produced by different heat treatment conditions. Austempering at 350°C for 2 h and at 300°C for 1 and 2 h resulted in the formation of transition carbides in bainitic ferrite platelets. The η carbide was formed at 350°C for 2 h by precipitation from a bainitic ferrite supersaturated with carbon. By contrast, ? carbide was associated with austempering at 300°C for 1 and 2 h and precipitates either on the austenite twin/bainitic ferrite boundaries or within the bainitic ferrite. The fracture mode of tensile and impact specimens in the austempered condition was fully ductile compared with as cast specimens, which had mixed fracture characteristics.

MST/1646     

Formation of strain-induced martensite in austempered ductile iron

The present work has been taken up to study the influence of microstructure on the formation of martensite in austempered ductile iron. Ductile iron containing 1.5 wt.% nickel and 0.3 wt.% molybdenum was subjected to two types of austempering treatments. In the first, called as conventional austempering, the samples were austempered for 2 h at 300, 350 or 400 °C. In the second treatment, called as stepped austempering, the samples were initially austempered at 300 °C for 10, 20, 30, 45 or 60 min. These were subsequently austempered for 2 h at 400 °C. Tensile tests revealed considerable variation in the strain-hardening behaviour of the samples with different heat treatments. In the case of samples subjected to conventional austempering, it was found that strain-hardening exponent increased with increasing austempering temperature. In the case of samples subjected to stepped austempering, increased strain hardening was observed in samples subjected to short periods of first step austempering. Study of the microstructures revealed that increased strain hardening was associated with the formation of strain-induced martensite. There was a greater propensity for the formation of strain-induced martensite in the samples containing more of blocky austenite. Retained austenite in the form of fine films between sheaths of ferrite was relatively more stable. Studies revealed that the morphology, size and carbon content of the retained austenite were important parameters controlling their tendency to transform to martensite.     

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.     

Dry sliding wear of low alloyed austempered ductile iron

Abstract

A multiple low alloyed ductile iron with 0.8 wt-%Ni and 0.25 wt-%Mo was austempered in single and two step processes at 300 and 400°C for 120 min. Specimens were used to study the effect of austempering conditions on the wear behaviour of this material. Sliding wear tests were carried out using a pin on disc apparatus, the tes tmaterials rubbing under dry atmospheric conditions against a surface of hardened steel (55 HRC) at speeds of 0.6, 0.7 and 1.0 m s^-1 and normal loads of 15.82 and 22.84 N. Test durations were 30, 60, 90 and 120 min. Scanning electron microscopy was used to examine the worn surfaces of test specimens. It was found that two step austempered specimens exhibited wear resistance that was higher than that of specimens austempered at 400°C, and almost as high as that of specimens austempered at 300°C. These two step austempered specimens, moreover, gave the highest impact energy and showed the best combination of mechanical properties. During two step austempering, the first stage reaction (formation of ausferrite) was completed in the intercellular area before the undesired second stage reaction (precipitation of carbides) had started in the eutectic cells. The two step treatment resulted in a duplex structure: upper and lower bainitic ferrite without formation of carbides. This structure was responsible for the improvement of mechanical properties and the good wear resistance. The results show that a well balanced choice of smaller additions of multiple alloying elements can reduce the negative effects of segregation and resulting structural inhomogeneity. MST/5472     

Transformation kinetics of unalloyed and high Mn austempered ductile iron

Abstract

The kinetics of bainitic transformation was studied in unalloyed and 1%Mn alloyed ductile irons. The samples were subjected to different austempering heat treatment cycles in dilatometry equipment. The results showed the effect of Mn on delaying the start and end of bainitic transformation, supposedly because of Mn segregation to the primary austenite grain boundaries. Decreasing austempering temperature led to precipitation of carbides inside the lower bainitic ferrite plates and therefore, more dilatation during the bainitic reaction. The dilatometric results were also used to calculate n and k in the Johnson-Mehl-Avrami equation and to derive the time-temperature-transformation diagrams. X-ray results were used to calculate the quantitative amount of precipitated carbides within lower bainitic ferrite and the dilatation during austempering.     

Microstructural stability of austempered ductile iron after sub-zero cooling

Abstract

The present article investigates the stability of the retained austenite, present in austempered ductile iron (ADI) after cooling at sub-zero temperatures, considering that the austenite could transform into martensite when austempered parts are exposed to low temperatures or stresses or strains. Optical microscopy with oblique illumination, X-ray diffraction techniques and microhardness tests were used to analyse the transformation of the austenite on samples with different austempering thermal cycles. The results indicated that the martensitic transformation took place mainly at the unreacted austenite present at the last to freeze areas of samples austenitised and austempered at the highest temperatures. On the other hand, the reacted austenite, present in the bulk of all the investigated samples, remains unchanged after cooling. Tensile tests were performed in order to evaluate the influence of the martensitic transformation, promoted by the sub-zero cooling, on strength and ductility.     

Modelling microstructural evolution and mechanical properties of austempered ductile iron

Abstract

Austempered ductile iron (ADI) is finding an ever increasing worldwide market in the automotive and other sectors. It offers a range of mechanical properties superior to those of other cast irons, and shows excellent economic competitiveness with steels and aluminium alloys. The aim of the present research is to develop a generic model that will enable the producers of ADI to optimise their product in terms of microstructure and mechanical properties, hence minimising the need for expensive and exhaustive experimental trials and reducing alloy development lead times.     

A study on microstructure and toughness of copper alloyed and austempered ductile irons

In this study, ductile irons with and without 1 wt% copper alloy were austempered to become austempered ductile irons (ADIs). Microstructure, impact toughness, and fracture toughness were evaluated to determine how both the copper alloying and austempering treatments influenced the toughness properties of ductile irons. The results show that, because copper increases the retained austenite content in ADI, the Cu-alloyed ADI has better impact toughness and fracture toughness (K_IC value) than does the unalloyed one. In particular, the impact toughness and the fracture toughness of ADI could be efficiently improved by treating the Cu-alloyed ductile iron at a higher austempering temperature (360 °C) to obtain more retained austenite in its microstructure.     

Mechanical behaviour of graphite in fracture of austempered ductile iron

Abstract

An investigation was carried out to examine the mechanical behaviour of graphite in the fracture of austempered ductile iron (ADI) by in situ tensile testing with an SEM. The results indicate that the graphite in ADI cannot be regarded as voids with no strength because graphite–matrix (G–M) interface cracking and the internal fracture of graphite were observed. Under tensile testing, microcracks always initiated at and propagated along the G–M interface first. Graphite nodules do not cause micronotch stress concentration thermselves in advance of the G–M interface cracking. The propagation of interfacial cracks along the G–M interface resulted in crack deflection. When the main crack propagated to a graphite nodule whose G–M interface had cracked and formed a void, it was obviously blunted. Graphite–matrix interface cracking occurred ahead of a propagating crack. The G–M interfaces and graphite nodules have a certain strength.     

Phase transformation in austempered ductile iron by microjet impact

Specimens of ductile irons austempered at 320 °C and 360 °C for 2 h were conducted with ultrasonic vibration treatment. Microstructures were characterized using optical (OM) and scanning electron microscopy (SEM) and X-ray diffraction. Vickers hardness was also measured to assess the variation of hardness affected by the microstructure. Experimental results show that specimens after cavitation erosion progressively developed phase transformation induced by plastic deformation. Some stringer-type austenite was found to precipitate carbides, while some island-like austenite was transformed into martensite. If the microstructure led to deplete carbon within the island-like austenite, this austenite readily transformed to martensite due to stress induced by microjet impact. If austenite was rich in Mn, the transformation from austenite to martensite would be retarded. The observations then indicate the coexistence of martensite and precipitated ferrite particles within the colony of island-like austenite. With increased cumulative ultrasonic time, microhardness values were enhanced both in areas near graphite and in intercellular regions. In addition, the elastic strain-energy density estimated from a microjet impact was lower than the theoretical value of homogeneous nucleation of martensite nucleus. Clearly, the transformed martensite was induced by heterogeneous nucleation.     

Role of pre-formed martensite on transformation of austempered ductile iron

A commercial ductile iron is treated by a novel austempering process to obtain a good combination of strength and ductility. The samples are austenitised at 890°C for 10 min, then quenched into patented quenching liquid, and austempered in an electric furnace at 220°C for 5, 10, 30, 60, 240 and 600 min, respectively, finally air cooled. The bending test and the tensile test are conducted and microstructural features are analysed on the austempered ductile iron. The optimum mechanical property is achieved at 220°C for 240 min. Main reason for high strength and ductility is the formation of a fine structure consisting of multiple phases of pre-formed martensite and lath bainitic ferrite with film retained austenite.