Abstract: The primary goal of this investigation was to create austempered ductile cast iron (ADI) with a fully ferritic microstructure without compromising its mechanical properties. This was achieved by applying a novel heat treatment process. This process consists of austempering and subsequent isothermal tempering below the inter critical ( A 1 ) temperature of ductile cast iron. Ductile cast iron samples were initially austenitized at 927 °C (1700 °F) for 2 h and then austempered at three different temperatures 260 °C (500 °F), 316 °C (600 °F) and 385 °C (725 °F) and finally isothermally tempered at 484 °C (900 °F) for 2 h. This resulted in a fully ferritic microstructure. The effect of this tempering on the physical and mechanical properties of the material was examined and compared with conventionally processed ADI. Tests results show that when ADI is austempered at 260 °C (500 °F) and subsequently tempered at 484 °C (900 °F) it has significantly better mechanical properties than the samples initially austempered at other two temperatures (i.e. 316 °C (600 °F) and 385 °C (725 °F)).

Abstract: Several kinetic models for bainite transformation have been widely applied in industry and research. The majority of these models, that do not consider the effect of cementite precipitation during bainite transformation, were validated in high silicon bainitic steels in order to avoid the interference of cementite precipitation during bainite formation. In this work, displacive models for bainite transformation have been validated in bainitic steels with different silicon content with the aim of evaluating their applicability on steels where cementite precipitation may play an important role on bainite formation. It has been found that these models fail in the calculus of the maximum volume fraction of bainite of lean silicon steels, but lead to a reasonable accuracy in high silicon steels. This is not surprising since cementite formation reduces the carbon content in the residual austenite, stimulating the formation of a further quantity of ferrite. Likewise, an imprecise estimation of the nucleation rate of bainite must be the reason for the poor correlation in the predictions of the bainite transformation kinetics in high silicon steels. This entails a better treatment of autocatalytic nucleation, still unresolved issue in the bainite transformation kinetics theory. [doi:10.2320/matertrans.47.1492]

Abstract: In the present study, an unalloyed ductile iron containing Fe–3·50C–2·63Si–0·318Mn–0.047Mg (wt-%) were intercritically austenitised (partially austenitised) in two phase region α+γ at various temperatures of 795, 805, 815 and 830°C for 20 min and then quenched into salt bath held at austempering temperature of 365°C for various times to obtain different ausferrite volume fractions (AFVFs). Results showed that dual matrix structure containing proeutectoid ferrite, new ferrite (also called epitaxial ferrite) and ausferrite (bainitic ferrite+high carbon austenite, which is retained or stabilised austenite) has been developed. Within each of the austempered series in α+γ temperature range, new ferrite volume fraction increased with increasing intercritical austenitising temperature (ICAT). Although, transforming percentage of new ferrite from parent austenite present at ICAT increased with decreasing ICAT. Some specimens were also conventionally austempered from 900°C for comparison. The new ferrite ...

Abstract: The isothermal bainite formation of austenite with and without pre-strain has been investigated in an alloyed eutectoid steel of Fe–0.80C–0.27Si–0.61Mn–0.15V (wt.%). It is found that the isothermal decomposition of deformed austenite within the regimes of bainite and finer pearlite is significantly promoted as the incubation period is remarkably shortened for small strain (e.g. 2%) and almost vanishes for strain larger than 5%. According to the TTT curves obtained for deformed (2%) and non-deformed austenite, B s (B s for isothermal bainite formation) for deformed austenite is about 10 °C higher than that for the latter. On the basis of the diffusion mechanism of bainite formation, the experimental results are explained by taking account of defects brought about by plastic deformation.

Abstract: Common TRIP (TRansformation Induced Plasticity)-aided steels contain roughly 0.15 mass% C, 1.5 mass% Si and 1.5 mass% Mn. The high Si contents in conventional CMnSi TRIP-aided steels are known to cause low ductility levels in the as-cast condition and give rise to galvanizing problems which is an essential challenge limiting their use in automotive applications. Partial substitution of the Si by Al leads to improved galvanising properties, but a loss in strength. The effects of substituting the Si and Al partially by P was therefore studied in detail with a special attention to the processing conditions in the hot dip galvanizing and continuous annealing processes. The addition of P was found to result in a higher amount of retained austenite which was more resistant to decomposition at longer austempering times compared to non-P alloyed TRIP steel. A synergetic effect of Si and P was observed as the increase in tensile strength per mass% Si was five times larger than expected for solid solution strengthening. The volume fraction of retained austenite was found to depend very much on the annealing cycle, with long austempering times resulting in lower amounts of retained austenite (6 to 12 vol%), while short austempering times resulted in higher retained austenite contents (12 to 20 vol%).

Abstract: The micro mechanical properties, stability and three-dimensional (3D) morphology of intragranular ferrite was studied in a low carbon microalloyed steel utilizing nano indentation, three-dimensional reconstruction techniques along with LEO1 450 scanning electronic microscopy. The mixed microstructures of intragranular ferrite, granular bainite and lath or plate-like bainite was obtained in a Nb–Ti microalloyed steel, which was water cooled immediately after relaxation for a fixed time as hot deformation ended. The elastic modulus and hardness increased in the sequence of intragranular ferrite, granular bainite and lath or plate-like bainite by approximately 15 GPa and 0.6 GPa, respectively. On the contrary, the tempering treatment of the specimens showed that the stability of the mixed microstructures decreased in the sequence of intragranular ferrite, granular bainite and lath or plate-like bainite. The results of stability, elastic modulus and hardness indicated that intragranular ferrite was formed prior to bainite transformation. The intragranular ferrite thus effectively sectioned the prior austenite into many small zones and thus the bainite transformed at lower temperatures was restricted in the small zones. It is likely that the formation of lath or plate-like intragranular ferrite prior to bainite transformation played an important role in the refinement of mixed microstructures.

Abstract: Austempered ductile iron (ADI) is an austemper-treated ductile iron containing acicular ferrite and high-carbon austenite constituents in its microstructure. It is well known that the traditional surface treatment at high temperature is not available to treat ADI because of the austempering temperature in the range of Ms ∼ 450 °C. This study utilized electroless nickel (EN) and cathodic arc deposition (CAD) technologies with lower processing temperature to treat ADI and then evaluated the availability of applying the EN and CAD-DLC duplex coatings on ADI. Characteristics of the resultant coatings such as roughness, hardness, and adhesion were analyzed and microstructures of ADI before and after surface treatment were observed. Also, polarization curve test was carried out for further understanding the effects of the coatings on the corrosion resistance of ADI. The results showed that microstructures of ADI did not deteriorate after EN and CAD surface treatments. Moreover, both the EN and CAD-DLC coatings were identified to be amorphous type and they could be well deposited on the ADI substrate. The surface of CAD-DLC-coated specimen was rougher than that of EN-coated specimen due to the globular particles deposited by CAD process. In the case of hardness, it showed that the duplex coated DLC/EN-ADI had the highest hardness (1312 HV0.05), followed by DLC-ADI (1088 HV0.05), EN-ADI (409 HV0.05) and then uncoated ADI (396 HV0.05). In the performance of corrosion resistance, all coated specimens were better than that of the uncoated one in 3.5 wt.% NaCl aqueous solution, and the sequence was DLC/EN-ADI > EN-ADI > DLC-ADI > ADI. Also, corrosion resistance of uncoated ADI was better than that of uncoated as-cast iron due to ADI's unique microstructure. It implied that the austempering treatment could improve the corrosion resistance of ductile iron in NaCl solution.

Abstract: The morphology and mechanical properties of upper bainite formed isothermally at 400 °C for different holding times in a 1.83 wt.% silicon steel have been investigated by optical metallograph, X-ray diffraction and transmission electron microscopy (TEM). In the early stage of upper bainitic transformation, lathlike bainite whose individual lath ferrite is separated by the thin film type of retained austenite is obtained. As the isothermal holding times are increased, the blocky region consisting of retained austenite and martensite is also found. The stability of retained austenite in lathlike upper bainite is studied in relation to the isothermal treatment times, and the heat treatment conditions. The results show that an optimum combination of strength and ductility is attributed to the formation of bainitic ferrite (BF) and a large amount of thin film carbon-enriched retained austenite in the upper bainite.

Abstract: A technology for manufacturing the gear with modular 'austenite plus residual bainite' iron and used for the diesel engine includes such steps as making iron mould, making sand mould, smelting in electric furnace, nodulizing, pouring, annealing, ultrasonic inspection, primary machining, semi-fine machining, hobbing gear, iso-quenching, blasting pills, fine machining, grinding teeth, marking, washing, and applying antirust oil. It has high mechanical performance and strength.

Abstract: Bainites can be of two types, upper and lower bainite. The microstructure of upper bainite consists of fine plates of ferrite, each of which is about 0.2 μm thick, and about 10 μm long. The plates grow in clusters called “sheaves.” The individual plates in a sheaf are often called the “sub-units” of bainite. They are usually separated by low-misorientation boundaries or by cementite particles. Lower bainite has a microstructure and crystallographic features, similar to those of upper bainite. The major distinction is that cementite particles precipitate inside the plates of ferrite. Bainite forms at somewhat higher temperatures, where the carbon can escape out of the plate within a fraction of a second. The bainitic reaction has several of the recognized features of a nucleation and growth process. “Granular bainite” is a term used to describe the bainite that occurs during continuous cooling transformation. There are significant differences in the tempering behavior of bainite and martensite, the most prominent being that there is little carbon in solid solution in bainite. Consequently, bainitic microstructures are much less sensitive to tempering, since there is hardly any loss of strength due to the removal of the small quantity of dissolved carbon. Major changes in strength occur only when the bainite plate microstructure coarsens or recrystallizes into one consisting of equi-axed grains of ferrite. Minor changes in strength are due to the cementite particle coarsening and a general recovery of the dislocation substructure. Bainitic steels containing strong carbide-forming elements tend to exhibit secondary hardening phenomena rather like those observed in martensitic steels, which depends on the precipitation of fine alloy carbides.

Abstract: The microstructure and mechanical properties of austempered high silicon cast steel pro and after treating with a modifier containing titanium, vanadium, and rare earth metals (so-called Ti-V-RE modifier) and austempered at different temperatures are investigated. The results show that the dendritic austempered structure and the blocky retained austenite are reduced after treating with the Ti-V-RE modifier. The modification can obviously improve the mechanical properties of austempered high silicon cast steel. The austempering temperature at which the optimum impact toughness is obtained shifts from about 320 °C for the steel unmodified to about 360 °C for the steel modified. High impact toughness is obtained in austempered high silicon cast steel high silicon cast steel when the retained austenite amount is about 15 to 25 pct for the modified steel and 20 to 35 pct for the unmodified steel.

Abstract: A high-strength steel sheet plated with zinc by hot dipping which has excellent processability. It can be produced using a production line which includes zinc plating by hot dipping and in which a sufficient austempering time after annealing is not secured, without necessitating a special structural control as a pretreatment. The high-strength steel sheet plated with zinc by hot dipping contains 0.05-0.3 mass% carbon, up to 1.4 mass% (including 0 mass%) silicon, 0.08-3 mass% manganese, 0.003-0.1 mass% phosphorus, up to 0.07 mass% sulfur, 0.1-2.5 mass% aluminum, 0.1-0.5 mass% chromium, and up to 0.007 mass% nitrogen, provided that silicon + aluminum ≥ 0.5 mass%, with the remainder consisting of iron and unavoidable impurities. The steel sheet has a volume content of residual austenite of 3% or higher, and residual austenite grains have an average aspect ratio of 2.5 or lower.

Abstract: Tensile tests were conducted in order to study the effects of nitrogen on the mechanical properties of the 0.2C–1.5Mn–1.5Si–0.04Al–(0.003–0.015)N steels often annealing at 800–830°C, followed by the austempering at 400–450°C. The results show that both tensile strength and elongation increase and the balance of tensile strength×elongation are improved upon nitrogen addition to the TRIP steel. It was found that the density of AlN precipitates increases with addition of nitrogen. The volume fraction of retained austenite increases due to AlN precipitation, because the precipitates retard the transformation of austenite during cooling and austempering. As a larger volume fraction of austenite remained after annealing has been transformed to martensite during deformation, elongation as well as strength has increased in the nitrogen added steel. It is also observed that the average grain size of ferrite and bainite decreases because of AlN precipitation that hinders grain growth. The refinement of ferrite and bainite by AlN precipitates also contributes to the increase in the strength of nitrogen added steels.

Abstract: A new model for the overall kinetics of the bainite transformation has been validated experimentally. The new model, presented in the 1st part of this work, is based in the displacive mechanism for bainite transformation. Thus, the bainite transformation kinetics of three medium carbon-high silicon steels has been studied. Results show that the model, which does not consider the effect of carbide precipitation in the bainite kinetics, predicts with a high degree of agreement the time evolution of bainitic ferrite volume fraction, even when lower bainite is present at the microstructure. Data from two additional medium carbon-high silicon steels, frequently reported in the literature, have been also used for reinforcing the validation, obtaining, again, a high agreement between the kinetic results for bainite transformation predicted by the model and those obtained experimentally. (doi:10.2320/matertrans.47.2473) In the first part of this study, a new model for the kinetics of the bainite transformation has been proposed. The model is based on the displacive mechanism for bainite transforma- tion. An important characteristic of this new model is the complete separation between the kinetics of both nucleation events of bainitic ferrite subunits, in austenite grain bounda- ries and at subunits previously formed. This distinction is based in a geometrical conception of the development of the transformation and has led to the elimination of the autocatalysis factor, an obscure parameter used in former kinetics models. The model correctly predicts the effect of carbon and other alloying elements such manganese and cobalt on the transformation kinetics, as it was shown in the first part of this work. The precipitation of cementite between the subunits of bainitic ferrite during bainitic transformation can be sup- pressed by alloying the steel with about 1.5 mass% silicon, which has very low solubility in cementite and greatly retards its growth from austenite. 1-3) The carbon that is rejected from the bainitic ferrite enriches the residual austenite, thereby stabilising it down to room temperature. Consequently, an isothermal transformation in the range of the bainite trans- formation leads to a microstructure consisting of bainitic ferrite separated by carbon-enriched regions of austenite. Since the cementite precipitation between the plates of bainitic ferrite plates has not been considered in the modelling, steels with high silicon content are most adequate for the validation of the proposed model. However, cementite precipitation within bainitic ferrite plates (lower bainite), that can occur even in high silicon steels, has not been considered in the model. In this sense, the study of lower bainite kinetics is interesting in order to evaluate the reliability of the model predictions in the event of lower bainite formation. It has been shown in the 1st part of the study that if no other reaction interacts with the successive nucleation and growth of subunits of bainitic ferrite, the incomplete reaction phenomenon, by means of the T 0 0 curve, provides a method for the estimation of the maximum volume fraction of bainitic ferrite that can be formed, at a given temperature, in a steel, vbmax. Considering a steel with a nominal carbon content �

Abstract: Austempering heat treatment practice helps in achieving high strength with good ductility and toughness by evolving a predominantly bainitic microstructure in the steel. Further, austempering practice can be modified to facilitate formation of some amount of ferrite and pearlite along with bainite which in turn enhances the ductility with some lowering of strength in the steel. The actual strength and ductility depends upon the relative amount of ferrite, pearlite and bainite present in the austempered steel. Present study was taken up to understand the influence of austempering parameters like austenitising temperature and cooling rate on the microstructure of an austempered steel with 0.4% carbon and 1.4% manganese. Laboratory simulation studies on conventional austempering showed that a microstructure comprising predominantly bainite with up to 4% ferrite was evolved which imparted a tensile strength of 120 kg/mm 2 and elongation of 4% (at 150 mm gauge length) to the austempered steel. On the other hand, when the steel was subjected to modified austempering practice which included austenitising at different temperatures for a period of 2 min followed by controlled air cooling and quenching in a lead bath at 450 °C, variety of microstructures were obtained. Particularly, when the steel was austenitised at 870–900 °C, a microstructure comprising uniformly distributed fine pearlite and bainite in a matrix of ferrite was formed. This microstructure resulted in tensile strength of 95 kg/mm 2 and elongation of 9% (at 150 mm gauge length). Based on the results achieved in this study, a relationship was established between tensile properties and the volume fraction of ferrite formed during austempering of steel.

Abstract: The production of wood-pellets involves fractioning of the raw material, e.g. the sawdust and cutter shavings, into a homogeneous particle size before pressing through a ring die pelletizer. Th ...

Abstract: The carbon partitioning during bainite transformation was studied using a diametrical dilatometer. The relationship of Ms temperature as a function of carbon content was determined for 4317 type steels. Austempering experiments were performed in which the Mstemperature was measured as a function of austempering holding time and temperature. The minimum values of carbon content in the residual austenite were determined from measured Ms temperature values and the relationship to carbon content. The results are compared to the T0 carbon composition calculated by ThermocalcTM for each austempering temperature assuming paraequilibrium. Good agreement is found between the calculated T0 composition and the final austenite composition.

Abstract: The austempering after hot rolling in hot rolled Si-Mn TRIP (transformation-induced plasticity) steels was investigated. The mechanism of TRIP was discussed through examination of the microstructure and the mechanical properties of this kind of steel. The results showed that the strain-induced transformation to martensite of retained austenite occurs in hot rolled Si-Mn TRIP steels. The sample exhibited a good combination of ultimate tensile strength and total elongation when it was held at the bainite transformation temperature after hot deformation. The stability of retained austenite increases with an increase in isothermal holding time, and a further increase in the holding duration resulted in the decrease of stability. The mechanical properties were optimal when holding for 25 min, and tensile strength and total elongation reached the maximum values (774 MPa and 33%, respectively).

Abstract: The microstructure and mechanical properties of a GCr18Mo steel were investigated. The martensite/lower bainite (B L /M) duplex structure with various B L volume fractions ( f B L ) or that full of B L were produced by austenitizing at 870 °C, quenching in nitrates at 230 °C and holding for different times. The steels with such structures exhibit higher strength–toughness than that of full martensite. When f B L is approximately 37.5% obtained by holding for about 60 min, the duplex-structured steel gives rise to the best combination of mechanical properties.

Abstract: Microstructure observations and measurements of retained austenite content are reported as a function of austempering time at temperatures of 440, 370, and 300°C after austenitising at 900°C. The results of the experimental investigation indicate that at early stages of austempering (less than ∼8 min), compacted graphite irons usually show higher amounts of retained austenite than spheroidal graphite irons. Compacted graphite irons are characterised by larger iron/graphite interface areas. After the early stages of austempering, the rate of retained austenite increase is higher for spheroidal graphite irons.

Abstract: Transformation-induced plasticity (TRIP) steel is a relatively new type of automotive steel known for its combination of high-strength and high ductility which was developed in the 1990s. 20Mn2SiVB steel is a kind of TRIP steel with low-carbon and low-alloying contents and high-strength. Specimens of a tested 20Mn2SiVB steel austenitized at 920 °C and austempered at 420 °C in a salt bath at different time are investigated. The microstructure obtained is studied by means of optical microscopy, scanning electron microscopy and X-ray diffraction. The results show that bainitic ferrite precipitates at the boundary of the austenite first, and with the prolongation of the isothermal time, the amount of bainitic ferrite increase. Then the ferrite decollates the austenite grain and lath-shaped bainitic ferrite with little island-shaped austenite forms during the holding time. The microstructure contains carbide-free bainite, granular bainite, retained austenite and martensite in the process of bainite transformation. Tensile test of the different treated specimens indicates that a better comprehensive property can be gained after austenized at 920 °C following austempered at 420 °C for 5 min, a certain TRIP effect can be also obtained under this condition.

Abstract: The predicted processing window has been used to select temperature–time parameters for austempering of a 3·39C–2·28Si–0·48Mn–0·13Mo–0·23Cu–0·05Ni–0·034Mg (wt-%) ductile iron. Comparison of a single step austempering with two step high–low temperature regime has been carried out. The results have shown that with increasing austempering temperature from 300 to 380°C, the morphology of ausferritic ferrite changes from lenticular or lath to feathery, accompanied by an increase in the volume fraction of high carbon retained austenite and carbon concentration in it. This microstructural evolution leads to an increase in ductility and impact energy, but a decrease in strength and hardness after high temperature treatment. All samples have satisfied the mechanical properties of ASTM 897M:1990. Two step high–low (380–340°C) temperature austempering produced microsturctures similar to those seen in high temperature single step processes but with significant time savings. The improvement in the mechanical p...

Abstract: An as-cast carbidic ductile iron is provided, having a pearlitic matrix with 5-50% by volume carbides and high wear resistance properties. The as-cast carbidic ductile iron is produced without an austempering heat treatment step. The as-cast carbidic ductile iron preferably includes a carbide stabilizing element and a spheroidizing agent.

Abstract: The effect of graphite nodularity on microstructure and processing window of austempered cast iron with four-different levels of graphite nodularity and a composition of 3.6% C, 2.2% Si, 0.2% Mn, 1.5% Ni, 0.3% Mo was studied. Microstructural observations and measurements of the retained austenite content and hardness, are reported as functions of austempering time at 440 and 300 °C after austenitising at 900 °C. The results of the experimental investigation indicate that graphite nodularity has considerable effect on the microstructure. After 8 min austempering time at 440 °C austempering temperature and 20 min austempering time at 300 °C austempering temperature, the amount of retained austenite (Xγ) increases with the graphite nodularity. The microstructural investigation and hardness test investigation indicate that the graphite nodularity has a significant effect on the time t 1 corresponding to the end of stage I (process window), whereas decreasing the graphite nodularity delays the end of stage I reaction. The starting time of reaction II, t 2 is determined from the variation of Xγ with austempering time, it is indicated that lower graphite nodularity accelerates the start of stage II reaction.