Abstract: The influence of temperature and time of austempering on a nodular cast iron austenitised for 30 min at 900°C has been investigated using optical and transmission electron microscopy. The carbon content of the austenite has been evaluated by measuring the lattice parameter via X -ray diffraction. The structure consisted of bainite containing retained austenite, the amount of which increased, and the carbon content of which decreased, with increasing austempering temperature. No carbides were detected in the materials austempered at 300 and 370°C for up to 2 h, which contained only bainite and austenite. After austempering at 410°C and cooling to room temperature a certain amount of martensite was always detected. After long times at this temperature the austenite decomposed to ferrite and complex carbides. Martensite formation seems to be promoted by the decrease in the carbon content of austenite due to the precipitation of ɛ carbide homogeneously within it. During long austempering treatments at...

Abstract: The present invention relates to the field of insert bits (10) for use with powered screwdrivers. The formed bits (10) may be treated to have a desired microstructure by either an austempering process or by the three step heating, quenching, tempering process. The bits (10) may also be electro-polished. Austempering provides for improved function and increased fatigue life and electro-polishing provides for improved function and increased fatigue life and torque capacity. The bits (10) may have reduced diameter midportions (30), particularly for use with impact drivers.

Abstract: An unalloyed nodular cast iron, in which the conventional addition of silicon has been replaced by aluminium, has been studied after heat treating in the bainite temperature range. Specimens were austenitised at 950°C for 2 h and then austemperedfor times up to 5 h at either 300 or 400°C. At 400°C the microstructure producedfor austempering times up to 3 h was typical of austempered ductile iron, consisting of carbide free upper bainitic ferrite together with a stable, high carbon enriched retained, reacted, austenite. For longer times, transition carbides are precipitated, initially η carbide in the ferrite, and after 5 h, χ carbide at the austenite/ferrite interfaces by decomposition of the high carbon austenite. Austempering at 300°C produced e carbide in a lower bainitic ferrite together with stable, high carbon retained austenite, for all heat treatment conditions: χ carbide precipitated at the ferrite/austenite interfaces after 5 h austempering. The fracture behaviour of samples austempered ...

Abstract: Samples of unalloyed silicon and aluminium spheroidal graphite cast iron have been studied in the austempered condition. Austempering times of up 3 h at 400°C for Al SG and 1 h at 350°C for Si SG gives a typical ADI microstructure consising of carbide-free banitic ferrite and stable, high carbon enriched, retained austenite. This has an attractive combination of elongation and strength. For longer austempering times transition carbides are precipitated in the bainitic ferrite, η-carbide in the upper bainitic range, i.e.400°C for Al SG and 350°C for Si SG, and ϵ-carbide in the lower bainite range. Increasing amounts of transition carbide reduce the ductility and produce a mixed model of fracture. For longer austempring times X-carbide is precipitated at the ferrite/austenite boundaries leading to a more brittle fracture mode.

Abstract: Measurements of 0·2% proof stress, ultimate tensile strength, elongation, and impact energy are presented for low Mn, Cu and low Mn, Ni–Cu irons for various austempering conditions and related quantitatively to microstructural features. It is shown that the formation of more than 20% pearlite in the austempered structure causes an unacceptable reduction in mechanical properties. The pearlite affects ductility in upper bainitic irons and strength in lower bainitic irons. The 0·2% proof stress is shown to be dependent on the bainitic ferrite particle size and a Hall–Petch relationship exists between proof stress and particle size. The impact energy of irons austempered within the processing window is shown to increase with increasing volume fraction of retained austenite and then to reach a saturation level. The saturation level is discussed in terms of the occurrence of a transformation induced plasticity mechanism.MST/1900

Abstract: There is extensive work on the fracture of steel with ferrite or/and austenite structure, but little on crack propagation in austempered ductile iron (ADI) whose microstructure also comprises austenite and ferrite (in the form of bainitic ferrite) but with graphite nodules in the matrix. Because of its good combination of wear resistance and toughness, and its low density and low cost (compared with forge steel), ADI has been widely used for various kinds of engineering components, such as gears, crankshafts, vehicle components, sprockets, cutting and digging tools etc. The matrix of ADI can withstand a certain amount of deformation before fracture during tensile or impact testing; for example, the elongation of ADI (grade 1050/700/7 to ASTM Standard) can reach 7--10% during tensile testing. However, the graphite nodules in the matrix cannot deform and hence are barriers to matrix deformation and give rise to crack initiation. In addition, carbides may precipitate in the bainitic ferrite laths or at the ferrite/austenite interfaces and these may also influence the fracture of ADI and produce characteristic features.

Abstract: A new class of ductile iron is formed by the hot isostatic pressing of a ductile iron casting, followed by austempering of the ductile iron casting. Hot isostatic pressing can be carried out at a pressure in the range of 10,000 to 17,000 psi at a temperature above 1600° F., and usually in the range of 1850° F. to 2050° F. Austempering of the material is carried out by heating to the austenitizing temperature (about 1500° F. to 1800° F.), maintaining the austenitizing temperature for a suitable time period, and rapidly cooling to an austempering temperature (about 400° F. to 750° F.) to form ausferrite within the sample.

Abstract: Mechanical property measurements are described for a ductile iron alloyed with Mn, Mo, and Cu in the fully austempered condition, andfor irons with various amounts of pearlite introduced by isothermal transformation at 550°C after austenitising at 920°C for 120 min and before austempering at 370°C for 60 min. The ultimate tensile strength, 0·2% proof strength, elongation, impact energy, and hardness all decrease as the amount of pearlite in the structure increases. A smaller amount of pearlite can be tolerated in the alloyed iron compared with an unalloyed iron before itfails to satisfy the standard.MST/2041

Abstract: A relatively new family of ferrous alloys provides low-cost castings with close tolerances and high strength

Abstract: Measurements of UTS, 0.2% proof stress, elongation impact energy and thermal conductivity are presented for low-Mn, 1% Cu compacted graphite iron as a function of austempering time, austempering te...

Abstract: Addition of Mn for preventing pearlite formation in austempered ductile iron causes intercellular segregation which may lead to precipitation of stable eutectic carbide. SEM-EDX and TEM-EDX were us...

Abstract: PURPOSE:To obtain a steel member excellent in pitting resistance and fatigue strength in which sufficient pitting resistance and spalling resistance can be secured even in a steel member applied with a high bearing pressure. CONSTITUTION:Steel stock contg., as essential components, 0.7 to 1.0% C, 0.9 to 1.5% Si and 1.0 to 3.5% Cr is subjected to a high carbon carburizing treatment to dispersedly form carbides on the surface part of the steel stock, and after that, the steel stock is subjected to austempering treatment. By the high carbon carburizing treatment, the hardening of the surface part is attained to improve its pitting resistance. Moreover, by the austempering treatment, its toughness and fatigue strength are improved over the surface part and the inner part to improve its spalling resistance. Since this steel stock contains C, Si and Cr by the same prescribed amounts, by the hardness of carbides, bainite and martensite and compressive residual stress generated by the martensitization owing to the strain induced transformation of retained austenite, its fatigue strength can be improved.

Abstract: Silicon-modified 4340 (Si-4340) and 4340 steels have been studied to determine the effects of silicon addition on austempered structure and mechanical properties of ultrahigh strength low alloy steels. As a result of austempering Si-4340 steel in the bainitic temperature region (593-673K), the retention of a large amount of austenite (12-25vol.%) was observed in the carbon-free upper-bainitic ferrite region. Compared to the conventionally quenched and tempered steel (CQT), the austemper of Si-4340 steel increased fracture toughness (KIC) at 623K and below, owing to the increased Charpy impact energy and percent elongation, while the strength decreased. Compared to the CQT and 4340 steel austempered in the same temperature region, the austemper of the steel had detrimental effects on the KIC and other relevant mechanical properties at 643K and above. The results are described and discussed in terms of metallographic observations, X-ray measurements and fractography.

Abstract: The morphology of continuously cooled and isothermally transformed bainite structures formed in a Cr-Mo-V rotor steel has been studied using transmission electron microscopy. The samples were austenitised at 955-degrees-C for an hour followed by air cooling to room temperature. The isothermal transformation reaction was carried out at 450-degrees-C for up to 100 000 s. The microconstituents observed are predominantly lower bainite with very small amount of upper bainite and martensite (formed from untransformed austenite due to water quenching). Analysis of the selected area diffraction patterns confirm that the carbide in bainite is orthorhombic cementite and the orientation relationship between ferrite and cementite is consistent with that of Bagaryatskii. The carbide particles in isothermally transformed bainite are coarser than those of continuously cooled bainite. Tempering one hour at 670-degrees-C of continuously cooled steel samples exhibited formation of fine spheroidal MC type carbides. In addition tempering leads to the enrichment of prior austenite grain boundaries by cementite particles. Tempering ten hours at 670-degrees-C exhibited microstructures almost identical to those observed in one hour tempering.

Abstract: Mechanical properties of austempered ductile iron can be guaranteed if a good knowledge of its metallugical structure is obtained. In this paper, the effects of small Mo additions to austempered ductile iron on its metallurgical structures as a function of section size are examined. From the results, it is found that region and segregation of Mo in region are increased with increase in the section size.

Abstract: : Plates of Austempered Ductile Iron (ADI) containing, in mass pct, 3.7 C. 2.7 Si, 0.97 Ni and 0.27 Mn were heat treated to ASTM 897 Grade 1 and 3 tensile properties and subjected to tensile testing at temperatures between -80 and 120 deg C. Room temperature yield and ultimate tensile strengths met specifications but values of elongation were lower than expected. The retained austenite contents contributed to lowered yield strengths during low temperature testing and increased ultimate tensile strengths and ductilities during high temperature testing. These changes in properties were related to the effects of temperature on the strain-induced transformation of austenite to martensite. The lower than expected ductility of the ADI specimens was related to microporosity, differences in graphite nodule distributions, differences in strain hardening related to austenite stability, and the ductile-to-brittle fracture transition of the matrix ferrite with decreasing temperature. Ductile iron, Austempering, Mechanical properties, Austenite transformation.

Abstract: PURPOSE:To predict structure distribution and hardness distribution in a material to be treated in a short time by collating the temp. hysteresis in each part of the material to be treated, determined by means of heat transfer analysis, with a CCT curve to predict the duration of transformation in each part of the material to be treated and computing the amount of isothermal transformation occurring in each part of the material to be treated. CONSTITUTION:In the austempering treatment for steel or cast iron, the temp. hysteresis in each part of the material to be treated is determined by means of heat transfer analysis by using a numeric value computing method and this temp. hysteresis is collated with the CCT curve, by which the duration of transformation in each part of the material to be treated is predicted. As to this duration, the amount of isothermal transformation occurring in each part of the material to be treated is computed in the form of the sum total of each trace amount of isothermal transformation DELTAf1, occurring at transformation temp., Tt1, for each trace duration, DELTAtsec and obtained on the basis of a TTT curve and the structure in each part of the material to be treated is obtained on the basis of the resulting value, by which the structure distribution after treatment is predicted. Further, the average transformation temp., TA, for each duration, DELTAtsec, is computed and the hardness in each part of the material to be treated is obtained on the bases of the resulting value, by which hardness distribution after treatment can be predicted.