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.

Increasing power density and rotational speed pose significant challenges for transmission design, especially in the aerospace and electro mobility sectors. Due to increased energy input and reduced heat dissipation, higher operating temperatures occur in high performance gears. At higher temperatures, the hardness and microstructure of conventional bearing and gear materials are affected by annealing effects, which can reduce the load capacity of these components. Therefore, increased operating temperatures can only be considered if the components are made of special heat-resistant, high-performance material systems. Heat treatment is essential to achieve the required performance. Today, high performance gears are typically case hardened to achieve the best performance in service. Due to the meta-stable properties of martensite and retained austenite, especially for low alloy case hardening steels, the microstructure can degrade in service if the temperature equals or exceeds the previous tempering. As a result, the hardness and performance of the components will decrease. Alternative steel grades with increased alloy content can mitigate but are in most cases more expensive. Therefore, an increase in temperature resistance through heat treatment of the low-alloy steels would be of increased interest. To achieve a more stable microstructure state, new heat treatments and alternative microstructures must be considered. This presentation will address the tempering behavior of martensitic and bainitic microstructures under long-term thermal stress above typical tempering conditions at 210 °C for up to 200 hours. The microstructure degradation and hardness change are shown.

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.

Flash Bainite Processing employs rapid thermal cycling (<10s) to strengthen commercial off the shelf (COTS) steel sheet, plate, and tubing into advanced high strength steel (AHSS). In a continuous process, induction technology heats a narrow segment of the steel cross section in just seconds to atypically high temperatures (1000-1300°C). Quenching substantially immediately follows. Flash Processing optimizes the inherent heterogeneity of steelmaking to form a complex intra-granular multi-chemistry, mixture of bainite, martensite, and other morphologies. Carbon migration and carbide dissolution are controlled by limiting time in the austenitic range. Unlike conventional heat treating, homogeneity is intentionally avoided. Flashing steel such as AISI1010 (1100MPa UTS), AISI1020 (1500MPa UTS), AISI1330 (1800MPa UTS), and 13Cr (1800MPa) has shown excellent room temperature stampability. Flash 1500 has room temp stamped into complex shapes including B-pillars and automotive seat components. Flash 1800 has shown <2T bends in complex geometry from 1 to 2mm thick 400x2000mm sheets.

This paper reports on a study to investigate the creation of a mixed microstructure consisting of proeutectoid ferrite, bainite, and austenite in a medium carbon high silicon steel. The microstructure was produced through the use of a continuous cooling process at a moderate cooling rate from the austenitizing temperature range to room temperature. The investigation also examined the influence of this microstructure on the mechanical properties of the material. Test results indicate that the developed steel has better mechanical properties compared to commercially available dual phase steels.

Flash Bainite Processing employs rapid thermal cycling (<10s) to strengthen commercial off the shelf (COTS) steel sheet, plate, and tubing into AHSS. In a continuous process, induction technology heats a narrow segment of the cross section in just seconds to atypically high temperatures (1000-1300°C). Quenching substantially immediately follows. Flash Processing utilizes inherent heterogeneity of steel creating multi-chemistry, complex mixtures of approximately 20% bainite and 80% martensite. Carbide dissolution and carbon migration are controlled by limiting time in the austenite temperature range. Unlike conventional heat treating to create advanced high strength steels (AHSS), homogeneity is intentionally avoided and non-equilibrium conditions are created. The leanest prior ferritic regions transform to bainite while prior pearlite forms martensite. A 7-10% higher yield/tensile strength product results with the beneficial ductility of the bainite constituent. Flashed AISI1010 (1100MPa UTS) and AISI1020 (1500MPa UTS) have shown exceptional room temperature stamp-ability to 0T/1T bend radii. The 1500MPa formability of 3G-AHSS is achievable. Flash Bainite offers simultaneous weight and cost savings.

The bainitizing potential for new forging steels and heat treatment gas quenching have both gained a growing interest in the automotive industry. The bainitizing process, particularly within the lower bainite range, aims at providing an improved ductility with strength above martensite level. Previous investigations demonstrated the ability of controlled gas quenching for bainitizing of a stepped shaft from the forging heat. In the present work, quenching and machining process steps have been combined to investigate the bainitizing potential of the specimen during machining at elevated temperatures (hot machining). Therefore, gas jet quenching has been experimentally evaluated and the derived heat transfer coefficient distributions have been implemented into heat treatment simulations. Bainitizing strategies featuring various quenching field configurations can be operated based on simulations and time-temperature-transformation diagrams for high-strength ductile bainite (HDB) steel grade. Selected strategies have been tested within a turning machine adapted for controlled heat treatment using a gas quenching field.

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.