Induction surface hardening is a process often used in industrial applications to efficiently increase the lifetime of components. Recently, this process has been enhanced with the inductive short time austempering process, creating a martensitic-bainitic microstructure. It is well-known that in homogeneous mixed microstructures, an optimally adjusted volume fraction of bainite can significantly increase the lifetime of the components even further. Regarding inductive short time austempering, there is a lack of knowledge in characterizing and differentiating graded microstructures, which occur due to the temperature gradients within the process. Therefore, three methods were investigated: the analysis of the grayscale profile of metallographic sections, the hardness profile and the full width at half maximum (FWHM) profile from the intensity curve (rocking curve) of the X-ray diffraction pattern. These methods were initially applied to homogeneous structures and evaluated. The findings were then transferred to graded microstructures. Finally, the graded microstructures could be differentiated both via the hardness profile and the FWHM value, while the grayscale analysis only allowed qualitative statements to be made. It became evident that both the volume fractions and their structure are crucial for subsequent mechanical characterization. Since the martensitic microstructure is easier to identify, it serves as a reliable reference for evaluating the mixed microstructure. In summary, these findings offer the foundation for further characterization of graded martensitic-bainitic mixed microstructures.

Carbide free bainitic microstructures can be developed via different thermal processing routes, and the details affect the scale and morphology of the microstructural constituents. In this study, bainitic microstructures are formed by either a controlled cooling process or an austempering process to evaluate the relationship between microstructure and mechanical properties in a 0.2C - 2Mn - 1.5Si - 0.8Cr steel containing small amounts of Nb, Ti, B, and N, and the results are compared to a 4140 steel processed via quenching and tempering. The resulting microstructures are characterized with scanning electron microscopy. When compared to microstructures produced via austempering, microstructures produced with a controlled cool exhibit an increased variety of transformation products, specifically regarding size and distribution of martensite-austenite constituents within a lath-like bainitic ferrite matrix. Nanoindentation testing shows that different transformation products exhibit significantly different local hardness. In all (primarily) bainitic conditions tested for these materials, the martensite/austenite constituent exhibits the highest hardness, followed by the lath bainitic ferrite/retained austenite constituent. Granular bainite and coarse bainitic constituents exhibit the lowest relative hardness in the conditions where they are observed.

AISI 52100 is a high carbon alloy steel typically used in bearings. One hardening heat treatment method for AISI 52100 is austempering, in which the steel is heated to above austenitizing temperature, cooled to just above martensite starting (Ms) temperature in quench media (typically molten salt), held at that temperature until the transformation to bainite is completed and then cooled further to room temperature. Different austempering temperatures and holding times will develop different bainite percentages in the steel and result in different mechanical properties. In the present work, the bainitic transformation kinetics of AISI 52100 were investigated through experiments and simulation. Molten salt austempering trials of AISI 52100 were conducted at selected austempering temperatures and holding times. The austempered samples were characterized and the bainitic transformation kinetics were analyzed by Avrami equations using measured hardness data. The CHTE quench probe was used to measure the cooling curves in the molten salt from austenitizing temperature to the selected austempering temperatures. The heat transfer coefficient (HTC) was calculated with the measured cooling rates and used to calculate the bainitic transformation kinetics via DANTE software. The experimental results were compared with the calculated results and they had good agreement.

This paper evaluates the influence of niobium additions on the wear behavior of high-silicon steel, representative of the advanced high strength steels used in the automotive industry. It describes the alloy compositions of the test samples used, the heat treatments to which they were subjected, and the tests that were subsequently performed. It also interprets test results and outlines key findings.

The purpose of this work is to incorporate boriding and austempering treatments in a single thermal cycle and assess its effect on two high strength bainitic steels. The combined process, called boro-austempering, is a promising alternative to increase the surface wear resistance of advanced high strength steels as shown in the test results presented.

This paper will present the advantages and disadvantages of quenching media options like HPGQ (high pressure gas quenching), Oil and Press Quenching, Austempering (salt) for steel, ADI (austempered ductile iron) and aluminum to achieve certain targets relating to automotive component heat treating. Each heat treating/quenching process provides unique solutions for automobile designers and plant engineers. However, there likely is no single process or material that provides all of the answers that one would desire. Therefore, what process or combination of processes will satisfy the overall need? Detail will be discussed that outlines how OEM’s and heat treaters can and do take advantage of a particular hardening process.

Austempered Ductile Cast Iron (ADI) has emerged as an important engineering material in recent years. It has a combination of high strength, good ductility, good fatigue strength and fracture toughness. Because of these excellent properties, it is now extensively used in many structural applications such as automotive components, earth moving machineries etc. An investigation was carried out to develop ADI with a nano scale microstructure. This was achieved by high temperature deformation and subsequent austempering of ductile cast iron. The effect of processing parameters such as deformation temperature, strain rate, austempering temperature and austempering time on microstructural features such as volume fraction of phases, size and distribution of phases were examined.

Austempering is an alternative hardening process that has been operating under the radar of manufacturers and designers for decades, primarily because its application is directed to fasteners having a cross-section of 12.7 mm or less. The primary goal of austempering is to create an extremely tough microstructure throughout the part’s mass or cross-section, a typical requirement for fasteners. This paper provides an overview of austempering processes and applications.

Usually bainitic microstructures exhibit good toughness and austempering is typically the preferred heat treatment when toughness is the primary requirement of the component. Several reports have shown such characteristics when compared to tempered martensite. High carbon steel may exhibit brittle characteristics but it is a good steel with respect to mechanical properties and wear resistance. The objective of this study was to compare the impact properties of AISI O1, a high carbon tool steel as VND in Brazil. This was done by comparing Charpy impact strength under different heat treatment cycles. Tempered martensite and bainite was obtained at 350°C after holding at temperature for 20, 40, and 60 minutes. Since hardness influences impact behavior, comparative studies were performed at the same surface hardness level. Results show a low absorbed energy for the austempered samples which for this temperature is independent of the holding time.

Ductile cast iron can be heat-treated to obtain a significant property improvement austempering, resulting in Austempered Ductile Iron (ADI). Performance can be further improved by using boronized surface layers which are capable of reaching high hardnesses (2100 HV). In this work, samples of nodular cast iron alloyed with copper, copper-nickel and copper-nickel molybdenum were borided in a salt bath (borax + aluminum) at temperatures 850, 900 and 950 °C for 2 and 4 hours. After these treatments, the samples were directly austempered from the boriding temperature in salt baths at temperatures of 240, 300 and 360°C (boroaustempering) which avoided the need for a subsequent reheating for such processing. The boriding treatment produced uniform layers with thicknesses in the range 35-130 micrometers and hardness in the range from 1300 to 1700 HV.

Boriding thermochemical treatment produces layers with high hardness which improves the tribological performance of ductile cast iron while the austempering treatment improves the mechanical performance of the substrate. In this work, samples of the ductile cast iron alloyed with copper, copper-nickel and copper-nickel-molybdenum were borided in a salt bath (borax + aluminum) at temperatures of 850, 900 and 950°C during 2 and 4 hours. The data for the layers obtained were used to determine the diffusion coefficients and activation energies of this process. The results of the calculated diffusion coefficients were similar to those obtained by the direct measurements of the layer thicknesses. For the sample alloyed with Cu or Cu-Ni the activation energy obtained was 141.27 kJ/mol, and for the sample alloyed with Cu-Ni-Mo the value was 212.98 kJ/mol. The statistical parameters and the correlation coefficients (R) showed satisfactory agreement.

Austempering, which produces a bainitic microstructure, is widely recognized as the preferred heat treatment when toughness is the primary requirement for a component. The literature contains numerous studies comparing the mechanical properties of tempered martensite with bainite of equivalent hardness. The majority of these findings demonstrate superior properties in bainitic microstructures. Bainite exists in two forms: upper bainite, which forms during isothermal transformation at higher temperatures (typically around 500 °C), and lower bainite, which develops at lower temperatures (approximately 350 °C, depending on the specific alloy steel). Lower bainite exhibits hardness values similar to tempered martensite and is preferred in applications where elastic characteristics are desired.

In the paper new phenomena are discussed which were discovered during investigation of the intensive quenching processes. It is shown that in many cases film boiling is prevented completely during quenching of steel parts in cold liquids, especially in water salt solutions. In this case, the part surface temperature drops almost immediately to the liquid boiling point at the beginning of the quench and then maintains at this level for a relatively long time, i.e. a so called self - regulated thermal process is established. A simple equation for determining the duration of the self - regulated thermal process is proposed. Thermal waves are generated during an immersion of steel parts into a cold liquid and after the self regulated thermal process is completed. The thermal waves move in opposite direction from where the cooling process starts. The self - regulated thermal process was used to develop an original intensive quenching technology (IQ-2 process). It can be a basis for developing other new technologies such as an austempering and a martempering in cold liquids under pressure. Discovered effects of thermal waves can be used for determining a duration of the self - regulated thermal process and for reconstructing an existing theory on the double electrical layer. Practical examples of calculations of the duration of the self-regulated thermal process are provided in the paper.

Austempered irons and steels offer the design engineer alternatives to conventional material/process combinations. Depending on the material and the application, Austempering may provide the following benefits to producers of powertrain components like gears and shafts: ease of manufacturing, increased bending and/or contact fatigue strength, better wear resistance and enhanced dampening characteristics resulting in lower noise. Austempered materials have been used to improve the performance of powertrain components in numerous applications for a wide range of industries, from gears and shafts to clutch plates and crankshafts. This paper focuses on Austempered solutions for powertrain applications with an emphasis on gear and shaft solutions.