Fatigue of austempered ductile cast iron

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2012

The effect of reducing the austempering time on the high-cycle fatigue behavior of austempered ductile iron (ADI) has been investigated by subjecting samples to rotating bending and fatigue crack propagation tests. Starting from the same cast iron, two distinct types of ADI were austempered at the same temperature of 360 1C but for different austempering times, which resulted in two materials whose main difference was the content of carbon in austenite. The first sample was austempered for 1.5 h, while the second was austempered for only 0.6 h. The austenitizing cycle (900 1C, 1.5 h) was the same for both ADIs. Therefore, we investigated the influence of the mechanical stability of austenite in the initiation and propagation of fatigue cracking. Reducing the austempering time increased the fatigue life and did not affect the mechanical properties or the rate of fatigue crack propagation. Reducing the austempering time increased the time to fatigue crack nucleation in ADI in addition to its economic and environmental benefits.

Journal of materials …, 1994

Rotating bending fatigue tests were carried out on austempered ductile iron containing 1.5 wt% nickel and 0.3 wt% molybdenum. The ductile iron was austenitized at 900 or 1050~ and then austempered at 280 or 400~ for different lengths of time to obtain different ...

ISIJ International, 2013

In the present study the fatigue strength of austempered ductile irons with dual matrix structures (ADI with DMS) has been studied for an unalloyed ductile cast iron. For this purpose, specimens were intercritically austenitized (partially austenitized) in two phase region (α + γ) at various temperatures (810°C, 820°C and 830°C) for 20 minutes and then quenched into salt bath held at austempering temperature of 315°C and 375°C for 120 minutes and then air cooled to room temperature to obtain various ausferrite volume fractions and their morphologies. Conventionally austempered specimens (austempered from 900°C) with fully ausferritic matrix and unalloyed as cast specimens having ferrit + pearlite structures were also tested for a comparison. Rotating bending fatigue test were carried out an the experimental results showed that, in ADI with DMS, volume fraction of ausferrite and continuity of ausferritic structure along intercellular boundaries play important role in determining fatigue strength. The fatigue strength of these specimens increases with increasing ausferrite volume fraction. The fatigue strength was correlated with the ausferrite volume fraction and high carbon austenite and its carbon content. Conventionally austempered specimens exhibited much greater fatigue strength than ADI with DMS specimens.

Fatigue <html_ent glyph="@amp;" ascii="&amp;"/> Fracture of Engineering Materials and Structures, 2000

The growth of short fatigue cracks was investigated in an austempered ductile cast iron (wt% 3.6C, 2.5Si, 0.6Mn, 0.15Mo, 0.3Cu), austenitized at 870 °C and then austempered at 375 °C for 2 h. At stress amplitudes close to the fatigue limit endurance limit of 10 7 cycles, subcritical crack nuclei initiated at graphite nodules. The crack nucleus decelerated and arrested after propagating a short distance. The position of an arrested crack tip was characterized using an electron backscatter diffraction technique, demonstrating that short fatigue cracks in austempered ductile cast iron (ADI) can be arrested by boundaries such as those between ausferrite sheaves or packets and prior austenite grains. Refinement of the prior austenite grain size decreased the size of subcritical crack nuclei. It is proposed that the arrest and retardation of short crack nuclei are controlled by the austenite grain size and graphite nodule size. This determines the fatigue endurance limit.

Metals

Alloyed Ductile iron, austenitized at 840 • C for 30 min in a special sealed austempering furnace, was austempered for 30 min in molten salt mixture at 4 trial temperatures of 300 • C, 320 • C, 340 • C and 360 • C. Tensile strength, yield strength, percentage elongation and impact energy were evaluated for the as-cast and austempered samples. Microstructures were investigated using microscopy, coupled with analyzing software and a scanning electron microscopy. The specific wear of samples was tested using pin-on-disc wear testing machine. X-ray diffraction was performed to calculate the amount of retained austenite present in the ausferrite matrix. As-cast microstructure consists of ferrite and pearlite, whereas austempered ductile iron (ADI) contains a mixture of acicular ferrite and carbon enriched austenite, called "ausferrite". Hardness and strength decreased, whereas ductility and impact strength improved with an increase in the austempering temperature. XRD analysis revealed that the increase in austempering temperature increased the retained austenite content. A decrease in wear resistance with austempering temperature was observed. Modified Quality Index (MQI) values were envisaged, incorporating tensile strength, elongation and wear resistance. MQI for samples austempered at 340 • C and 360 • C showed a better combination of properties. About an 8% reduction in energy consumption was gained when the heat treatment parameters were optimized.

2018

Austempered ductile cast irons (ADI) have received great attention in last years because their combined properties of good ductility, high strength and fracture toughness, good fatigue strength, good wear properties and low production cost. Such combination of properties can be reached because of their microstructures consist of a mixture acicular ferrite (bainite), residual austenite with a high carbon content and nodular graphite. In this work, the effect of austempering heat treatment on the microstructure of a commercial alloy to produce three different grades of ADI, with different strength level, is analyzed. Microstructure characterization has been performed using techniques of optical microscopy, scanning electron microscopy and x-ray diffraction. Mechanical properties were evaluated from tensile and impact tests at room temperature. In addition, the residual stress due to heat treatment was evaluated. The results of this study show that there is a strong relationship betwee...

International Journal of Fatigue, 1999

The aim of this work is to study the propagation of fatigue cracks in Austempered Ductile Iron (ADI). Threshold and Propagation fatigue regimes are analyzed. Nominal and Effective Threshold stress intensity factors were measured for five different ADI matrix microstructures, and related to its mechanical properties. It was found that the effect of the microstructure is minor, and can be attributed to the different crack closure contribution of each matrix microstructure. The effective threshold values measured are greater than those reported for steels. The crack-nodule interaction was modeled numerically. Based on boundary element modeling results, a crack propagation mechanism was proposed. This mechanism is in good agreement with the observed crack behavior during propagation, and can be used to explain the low propagation rates and high effective threshold values measured in ADI when compared with those of steels.

Mechanics &amp; Industry, 2015

This paper treats the effect of alloying elements on the modification of the microstructure and properties of austempered ductile iron. The basic cast iron was elaborated in an induction furnace, its solidification shows a ferrite-pearlitic structure. This cast iron was alloyed with Mn, Ni, Mo and V. The samples were cast in the form of cylindrical bars of 22 mm in diameter and 300 mm in length. They have undergone a bainitic heat treatment type. Microstructures were characterized by optical microscopy, "SEM" and "XRD". Tensile strength, hardness, microhardness, resilience and wear were determined and correlated with the microstructure. In the treated condition, metallographic study shows that the structures were formed of bainitic ferrite and feathery upper ausferrite. The results of mechanical tests and wear show that the cast irons studied achieve superior properties.

Materiali in Tehnologije, 2020

Austempered ductile iron (ADI) belongs to the heat-treated class of ductile iron. The heat treatment consists of austenitization and the tempering process. The microstructure of an ADI sample is ausferrite consisting of acicular ferrite, carbon-saturated austenite and a graphite phase in the shape of nodules. The corrosion properties of ADI samples depend on the microstructure constituents and stability of the microstructure. In this paper, the influence of ausferrite microstructure decomposition on the corrosion properties of the ADI samples are presented. During the research, Tafel curve extrapolation and potentiodynamic polarization were used. It was found that the ausferrite microstructure decomposition very strongly affected the general corrosion behavior of the ADI samples. Keywords: ausferrite, decomposition, general corrosion, austempered ductile iron Duktilna nodularna siva litina izdelana s postopkom "austemperiranja", je posebna vrsta duktilne litine. Austempering (izraz se je udoma~il tudi pri nas) je posebna vrsta toplotne obdelave, s katero izbolj{amo lastnosti litine. To je postopek, pri katerem so postopki austenitizacije, hitrega ohlajanja in popu{~anje, zdru`eni v eno operacijo. Mikrostruktura austemprane litine je sestavljena iz acikularnega (igli~astega) ferita, z ogljikom nasi~enega austenita in grafitne faze v obliki nodul (kroglic). Korozijske lastnosti tak{ne litine so odvisne od mikrostrukturnih sestavin in stabilnosti mikrostrukture. V~lanku avtorji opisujejo vpliv termi~nega razpada ausferitne mikrostrukture na korozijske lastnosti litine. Med postopkom raziskovanja so avtorji uporabili Tafelove ekstrapolacijske krivulje in tehniko potenciodinami~ne polarizacije. Avtorji ugotavljajo, da termi~ni razpad ausferitne mikrostrukture zelo mo~no vpliva na splo{ne korozijske lastnosti duktilne nodularne sive litine.

Journal of Materials Sciences and Applications, 2018

This study aimed at evaluating the effects of castings dimensions on the microstructure and mechanical properties of austempered ductile iron (ADI). Ductile iron that conforms to ASTM A536 65-45-12 grade was produced, cast into Y-block, machined into section thicknesses ranging from 5 to 25 mm, and isothermally heat treated at 300°C and 375°C austempering temperatures to produce ADI. Thereafter, the microstructure and mechanical properties were characterized. The microstructures were characterized using Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) method. Their strength and hardness were also evaluated in accordance with ASTM standard procedures. The microstructure revealed significant coarsening of ausferrite as the section thickness increases with 375°C austempering temperature coarser. The mechanical test results indicated that strengths and hardness value decreases with increase in section thickness while the percentage elongation and impact strength increases with it. The study concluded that the structure and mechanical properties of ADI strongly depends on the castings dimensions.