Abstract: High silicon content in nodular cast iron leads to a large amount of retained austenite (γ) during isothermal transformation in the bainitic zone by inhibiting the precipitation of carbides. The presence of γ phase results in high toughness, with an optimum fracture toughness KIc of about 85 MN m−3/2 for a 0·2% proof stress of 1000 MN m−2 being observed at 30% retained austenite. The presence of martensite after transformation for short times and of coarse carbides after transformation for long times greatly reduces the toughness. For lower bainite having austenite volume fractions less than 30%, optimum fracture toughness is obtained when the fracture is predominantly transgranular ductile. For austenite volume fractions greater than 30% in the upper bainite region, γ⇛α′ (martensite) transformation induced plasticity occurs, leading to superior toughness compared with conventional cast irons. In this case the fracture, although predominantly ductile, also contains some intergranular zones due to ...

Abstract: Sand castings are removed from their molds before cooling and reheated in a heat treating booster furnace. The castings are transferred from the booster furnace to a rotary drum filled with agitation media. As the castings pass through the media drum, they are simultaneously cleaned of sand particles and down-quenched by the agitation media. The system is particularly suited for austempering cast iron parts.

Abstract: In the austempering of ductile cast iron, it is necessary to alloy the metal to prevent pearlite transformation. The addition of alloying elements delays the pearlite transformation and at the same time, causes a similar delay in the beginning of the stage I reaction. In general, alloying addition levels greater than a specified amount are not desirable in the conventional austempering process due to the decrease in toughness and ductility. In this experiment, 1% manganese alloyed ductile iron was used in the successive-stage, High-Low Austempering Temperature Process (HLAT Process) in different time combinations of high and low temperature austempering to improve the toughness of high manganese ductile cast iron. The impact strength provided by the HLAT Process is a function of high temperature austempering time as well as low temperature austempering time. The impact strength at the best process combination is much higher than conventional high manganese ADI and well comparable to low alloy aust...

Abstract: A method for producing a selectively surface hardened cast iron part includes the steps of (a) heating the part to a desired austempering temperature of between about 450° F. and about 800° F. until the entire cast iron part possesses the desired austempering temperature substantially uniformly throughout it; (b) heating only the surface of the cast iron part to an austenitizing temperature of between about 1500° F. and about 1800° F. by immersing the cast iron part in a molten lead or tin bath until a desired thickness of an austenite layer is formed on the surface of the cast iron part, without substantial heating of the interior of the cast iron part; (c) quenching the surface-heated cast iron part in a non-liquid quenching bath atmosphere, i.e. a gaseous atmosphere maintained at the desired austempering temperature, for a time adequate to transform the surface austenite layer to an ausferritic structure; and (d) cooling the cast iron part before bainite is formed in the heat-treated surface layer. In this manner, only the heat-treated surface layer of the cast iron part is hardened, because it is quenched from both sides simultaneously from two sources: self-quenching by the interior of the part, and external quenching by the austempering atmosphere. Typical heating times in step (a) are between about 10 minutes and about 10 hours, and in step (b) are between about 3 seconds and about 10 minutes. The quenching time in step (c) is typically between about 15 minutes and about 8 hours.

Abstract: PURPOSE:To facilitate a countermeasure to high critical pressure in a leaf spring hose band and to suppress its thermal settling and brittle cracking by subjecting a steel sheet having a specified compsn. in which the content of Si and P is regulated to forming and thereafter executing austempering treatment to form its structure into a uniform bainitic one. CONSTITUTION:The compsn. of a hot rolled or cold rolled steel sheet is formed of, by weight, 0.30 to 0.70% C, 0.002 to 0.015% SolAl and the balance Fe with inevitable impurities. This steel sheet is formed into a leaf spring hose band and is thereafter subjected to austempering treatment of heating to the temp. range of 800 to 900 deg.C and rapidly cooling and holding to 240 to 400 deg.C to form a uniform bainitic structure. If required, 0.005 to 0.10% Ti and 0.0003 to 0.0020% B are incorporated into the compsn. of the above steel sheet. This leaf spring hose band is excellent in brittle cracking resistance on the strength level of >=150kgf/mm .

Abstract: PURPOSE:To enhance the cooling power of a fluidized bed and to facilitate austempering by transforming a material to be treated heated to an austenite region by the fluidized bed with gaseous He as the fluidizing gas into bainite. CONSTITUTION:A spheroidal graphite cast iron member 2 heated to 800-900 deg.C is charged into the fluidized-bed furnace 1 of an austempering device. Gaseous He from a gaseous He cylinder 8 accelerated by a turbofan 10 is introduced into the furnace 1 through a pipeline 9. The gaseous He blown up from a diffuser plate 12 forms a fluidized bed along with alumina powder 11 and cools the member 2. The gaseous He is passed through a filter 14, discharged from the upper part of the furnace 1 and cooled by a heat exchanger 15. The gaseous He is circulated in this way to quench the member 2 to the bainite transformation temp. of about 250-400 deg.C. The furnace 1 is kept at this bainite transformation temp. by energizing a heater 16, as required, and bainite transformation is carried out.

Abstract: Time-temperature-transformation (TTT) diagrams within the medium temperature range of some alloy steels have usually been found to be composed of three separate kinds of C-curves for low-carbon alloy steels. The microstructure associated with one of three kinds of C-curves is a granular structure which is nonbainite, upper bainite, or lower bainite, depending on the higher, middle, or lower location of the C-curve in the TTT diagrams. On the other hand, only two types of C-curves can be established for the alloy steels of middle and high carbon content, and the category of microstructures (upper bainite and lower bainite) corresponding to each C-curve also depends on the location of the C-curve. Meta-upper bainite, or alternatively, carbide-free bainite, and meta-lower bainite and/or the mixture of them can often be obtained in some alloy steels containing silicon. Each type of microstructure still possesses its own overall transformation activation energy, Q*, and morphological exponent, n. A considerably detailed analysis has suggested some differences among the transformation mechanisms of granular structure, upper bainite, and lower bainite.

Abstract: The isothermal transformation products of austenite over a wide range of temperatures and times in the bainitic range in a 0.2 wt.% C-1.5 wt.% Mn steel have been studied by transmission electron microscopy in order to characterise the bainitic microstructures in low-carbon low-alloy steels. Widmanstatten ferrite has formed with alternate layers of austenite (martensite) as a transition product at 600 and 500-degrees-C that has finally transformed on further isothermal transformation to either pearlite (at 600-degrees-C) or upper bainite (at 500-degrees-C). This type of transformation product was referred to as Bl bainite by earlier investigators, but on the basis of the present investigation it is concluded that such ferrite-austenite (martensite) structures are not bainitic as this is not the final transformation product either at 600 or 500-degrees-C. Both upper bainite and lath-type lower bainite are formed at 450-degrees-C while the transformation product has been only lath-type lower bainite at 400-degrees-C.

Abstract: PURPOSE:To manufacture high toughness cast steel by casting molten cast steel having a relatively low Si content with a metal mold and forming a uniform fine bainite-retained austenite mixed structure by austempering. CONSTITUTION:Molten cast steel consisting of 0.3-0.7wt.% C, 1.8-3.0wt.% Si and the balance essentially Fe is cast with a water-cooled copper mold at >=24 deg.C/sec cooling rate and a uniform fine bainitic ferrite-retained austenite mixed structure nearly free from graphite, untransformed massive austenite and free cementite is formed by austempering (heating for austenitizing at 900 deg.C for 1hr and heating for bainite formation at 400 deg.C for 1hr) to obtain cast steel having superior strength and toughness.

Abstract: A unique method is proposed for the preparation of a body of an austempered ductile cast iron having a gradient of the mechanical property within the body by subjecting a body of a nodular graphite cast iron to an isothermal transformation treatment for austempering at a temperature in the range from 250° to 450° C. while the body has a temperature difference between two points or between two surfaces. The temperature difference can be produced by bringing the two points or two surfaces into contact with melts of a salt kept at different temperatures. A two-compartment salt-bath apparatus therefore is disclosed.

Abstract: In order to improve fatigue strength of ductile cast iron, effects of austempering from the (α+γ) phase region (partial austempering) and/or the (γ) phase region (full austempering) were investigated under axial load and rotating bending load. The results of tension, hardness and fatigue tests were compared with those of as-cast material. Although the tensile strength and hardness of fully austempered material were greatest in all of the materials used in this study, the greatest fatigue limit was obtained in partially austempered material. The reasons were discussed based on the Vickers hardness, the defect size and the threshold stress intensity range.

Abstract: PURPOSE:To obtain machine element parts improved in the properties of impact value from stock with a thickness higher than that of the conventional article by subjecting the stock with a specified thickness or below of a boron steel contg. specified amounts of B and Ti to austempering treatment. CONSTITUTION:The stock with <=11mm sheet thickness of a boron steel cocntg. 0.40 to 0.65% C, 0.0005 to 0.0030% B and 0.020 to 0.050% Ti is subjected to austempering treatment. By the use of the baron steel having the above components, the region B at the time of the austempering treatment deviates to the long time side, so that the austempering treatment for the stock of a sheet with a thickness higher than that of the conventional one can be executed in the region A.

Abstract: PURPOSE:To improve the machinability of high strength austempered ductile cast iron by subjecting spheroidal graphite case iron to heat treatment in a decarburizing atmosphere, further to austenitizing at specific temp, to holding at the isothermal transformation temp, and then to air cooling CONSTITUTION:Spheroidal graphite case iron is heat-treated in a decarburizing atmosphere It is preferable to regulate the decarburizing atmosphere and the decarburizing treatment time to 900-950 degC and 3-5hr, respectively By the above procedure, the surface of the spheroidal graphite case iron is formed into decarburized state Further, austenitizing is applied to the above at 850-950 degC It is preferable to regulate the austenitizing time to 30-90min The above decarburizing treatment can be done prior to or simultaneously with austenitizing treatment By the above procedure, the surface of the spheroidal graphite case iron can be formed mainly into ferrite matrix Subsequently, the cast iron is held at an isothermal transformation temp of 300-400 degC for 30-90min and air-cooled By this method, the ferrite in the decarburized layer is subjected to pearlite transformation and the matrix is transformed from austenite into bainite structure, by which the machinability of the austempered ductile cast iron can be improved

Abstract: PURPOSE:To provide a steel member having high coefficient of linear expansion while securing rigidity by regulating the composition of the steel member to be austempered by specifying and increasing C content and Si content. CONSTITUTION:This heat treated steel member is produced by regulating C content and Si content in the steel member to 0.7-1.2wt.% and 1.5-2.5wt.%, respectively, performing austempering treatment to form the structure into a mixed structure of 30-60vol.% retained austenite and bainite, and regulating the coefficient of linear expansion to >=15X10 / deg.C. The coefficient of linear expansion cannot be increased to >=15X10 / deg.C when the amount of retained austenite is lower than the lower limit, and strength becomes insufficient when it exceeds the upper limit. When C content is less than the lower limit, hardenability becomes insufficient and the application of this member to large- sized parts is made difficult and also retained austenite becomes unstable, and, when it exceeds the upper limit, the problem of the precipitation of free carbon is taken place. Further, carbide precipitation inhibiting power becomes insufficient when Si content is less than the lower limit, and, when it exceeds the upper limit, the precipitation of free carbon is facilitated and also carbide precipitation is inhibited, and retained austenite stabilizing effects can be saturated.

Abstract: PURPOSE:To improve the machinability of austempered spheroidal graphite cast iron without deteriorating the mechanical properties and to obtain high- quality cast iron with high productivity. CONSTITUTION:When spheroidal graphite cast iron is austempered, the cast iron is held at 650-800 deg.C for >=15min or passed through 650-800 deg.C at <=5 deg.C/min heating rate during heating for austenitizing and it is continuously heated to the austenite range and subjected to prescribed austempering. The machinability of the cast iron is improved.

Abstract: PURPOSE:To improve the fatigue strength of a steel member mixed with relatively large amounts of C and Si by improving its internal hardness. CONSTITUTION:This is a steel member increased in internal hardness by forming retained austenite in a steel member contg., by weight, 0.7 to 1.2% C and 1.5 to 2.5% Si by austempering treatment and subjecting the above retained austenite to nitriding treatment at 450 to 490 deg.C for melting to form its structure into a mixed one of cementite and ferrite and its manufacturing method.

Abstract: PURPOSE:To improve the wear resistance of a member of steel where relatively large amounts of C and Si are added. CONSTITUTION:The steel member is obtained by subjecting a member of a steel having a composition containing, by weight, 0.6-1.2% C and 1.0-2.5% Si to graphite precipitation treatment before or after austempering treatment. In the case of the steel member prepared by performing graphite precipitation treatment after austempering treatment, the inner part has a mixed structure of bainite and retained austenite and the surface part has a pearlite structure where graphite is dispersed. On the other hand, in the case of the steel member prepared by exerting graphite precipitation treatment before austempering treatment, the inner part and the surface part have mixed structure of bainite and retained austenite and graphite is dispersed in the surface part.

Abstract: PURPOSE:To improve the microstructure of a flat spring and to reduce set-up time in austempering heat treatment. CONSTITUTION:The austempering heat treatment for a flat spring consists of a process wherein a flat spring stock is subjected to low temp. austempering treatment at the prescribed temp. 300 deg.C to 320 deg.C and then to tempering. By low temp. austempering treatment, cooling capacity can be increased and microstructure can be improved, and further, the sheet thickness capable of austempering treatment can be increased. Moreover, the necessity of changing austempering treatment temp. according to sheet thickness can be obviated, and as a result, set-up loss can be removed.

Abstract: In order to estimate and improve the fatigue reliability of spheroidal graphite iron (SGI) with high tension and toughness, the influence of the matrix structure on the fatigue limit was examined through fractography, with a special focus on austempered ductile iron (ADI) with the bainitic ferrite-retained austenite dual structure. Under the fatigue test of tension-tension loading, R=0.05, it was found that ADI is superior to the SGI with all-pearlite matrix in terms of the fatigue limit and also the ratio of fatigue limit/tensile strength. The fractography showed the fatigue crack initiated at the microporosity in both of the above mentioned SGIs with the matrix structure of high tension. Therefore, it could be understood that bainitic ferrite-retained austenite dual structures had lower notch sensitivity to the microporosity of casting defects than the full pearlite matrix.

Abstract: The effects of Cu, Hi and Mo alloying elements, austempering temperature and test temperature on static fracture toughness of bainitic ductile iron have been described. Test results have been introduced in the general diagram of the ultimate fracture toughness. High resistance to brittle fracture under static load (KIC = 70-90 MPa.m172) allows the manufacturing of heavily loaded parts of bainitic ductile iron which up to now have been produced from high strength structural steels.

Abstract: This paper studies the effects of austenitizing temperature, austempering temperature and isothermal transformation time on the short fatigue crack growth behavior of nodular cast irons. Differences between short and long fatigue crack behavior are compared. The influence of stress ratio is also investigated. Fracture surfaces are observed through a scanning electron microscopy to understand the fracture mechanism. The results show that: (1) For nodular cast iron with lower bainitic matrix, the short fatigue crack growth rate increases with higher austempering temperature, while with the upper bainitic matrix, it behaves contrarily. (2) The nodular cast iron austenitized at 900°C for 1 hr and then austempered at 350°C for 30 min-4 hr have better resistance to short fatigue crack growth. (3) The short fatigue crack growth rate of austempered nodular cast iron increases with stress ratio.

Abstract: Non-classical bainite microstructures can develop during continuous cooling of low-carbon alloy steels. These differ from classical upper and lower bainite developed by isothermal transformation. Two non-classical bainite microstructures were produced in a 3Cr-1.5Mo-0.25V-0.1C steel using different cooling rates after austenitizing--water quenching and air cooling. The carbide-free acicular bainite formed in the quenched steel had a lower ductile-brittle transition temperature (DBTT) than the granular bainite formed in the air-cooled steel. With increasing tempering parameter, the DBTT of both decreased and approached a common value, although the final value occurred at a much lower tempering parameter for the quenched steel than for the air-cooled steel. The upper-shelf energy was similarly affected by microstructure. These observations along with similar observations in two Cr-W steels indicate that control of the bainite microstructure can be used to optimize strength and toughness.

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, ɛ carbide ...

Abstract: To understand the controlled-rolled granular bainite structure, a series of high-strength low-alloy steels was specially designed and investigated. The effects of the chemical composition, the finish-rolling temperature, and the cooling rate on the amount of martensite/austenite (M/A) constituent formation in the as-rolled alloy steels were evaluated. It was found that for steels containing the same addition of Nb (0.045 wt.%), the granular bainite transformation was retarded with an increased carbon content ranging from 0.021 to 0.056 wt.%. As the carbon content was raised, more niobium combined with carbon to form carbide, and the austenite therefore became niobium-depleted. This effect significantly influenced the granular bainite transformation for the steels studied. It is also shown that the effects of Mn and Mo on the formation of second phases are different from that of carbon. These alloying elements tend to promote the M/A constituent and depress pearlite formation. As to the effect of rolling temperature, it is shown that as the finish-rolling temperature is lowered, the larger strain accumulated in austenite enhances polygonal ferrite formation along the previous austenite grain boundaries, and reduces the amount of M/A constituent. Furthermore, based on a thermodynamic analysis, it is indicated that the granular bainite transformation terminates as the carbon content of austenite reaches the TO curve. The critical carbon content increases with the decrease of the granular bainite transformation temperature. It was also found that the amount of M/A constituent formation decreases with the increase in cooling rate. This result is not consistent with that reported by Shiga et al., Tetsu-to-Hagane, 68 (1982) A227 and Bufalini et al., Accelerated cooling of steel,

Abstract: The superior combination of attainable properties has caused austempered spheroidal graphite cast iron to emerge as a new class of cast iron. The mettalurgy of austempered spheroidal graphite cast iron is presented with the purpose of clarifying tne mechanical properties. It is emphasized that segregation of alloying elements can cause an iron to behave differently than might be expected