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Microstructural Characteristics of Metals
Published in Vladimir B. Ginzburg, Metallurgical Design of Flat Rolled Steels, 2020
Austempering is a heat treatment that is used to obtain a lower bainite microstructure which is similar in strength and superior in ductility to those of tempered martensite [3]. This heat treatment includes quenching to the desired temperature in the lower bainite region, usually in a molten-salt bath and holding the workpiece at this temperature until the transformation to bainite is completed (Fig. 1.41). After austempering, the workpiece is either quenched or air cooled to room temperature. The workpiece can also be tempered to a lower hardness level if required.
Common Heat Treatment Practices
Published in Bankim Chandra Ray, Rajesh Kumar Prusty, Deepak Nayak, Phase Transformations and Heat Treatments of Steels, 2020
Bankim Chandra Ray, Rajesh Kumar Prusty, Deepak Nayak
Austempering is a hardening treatment in which austenite transforms isothermally to lower bainite and is used to reduce distortion and cracks in high-carbon steels. It involves heating the steel to above the austenitizing temperature, quenching in molten salt bath held at a particular temperature above Ms point and within the bainitic range (300°C–400°C), and then keeping the steel at this temperature to let austenite transform completely to lower bainite. After full transformation, steel is removed out of the bath and cooled in the air up to the room temperature. A schematic illustration of the austempering process superimposed on the TTT diagram is shown in Figure 11.13. The equalization of temperature throughout the cross section of the part before bainite formation minimizes the stresses developed during austempering, which are negligible compared to stresses developed during hardening. Steel to be austempered should have adequate hardenability to avoid pearlite formation. TTT diagram is also useful for determining the suitability of given steel for austempering. The rate of cooling should be higher than the upper critical cooling rate, i.e., care must be taken such that there is no austenite to pearlite transformation. Steels having considerable soaking time for the bainite transformation are not suitable for this treatment.
Heat Treatment of Steels
Published in P. C. Angelo, B. Ravisankar, Introduction to Steels, 2019
Austempering is designed to produce bainite in carbon steels. Bainite is similar to pearlite having ferrite and cementite but with different morphology. There are two types of bainite; lower and upper. The hardness and strength of bainite are comparable to hardened and tempered martensite with better ductility, toughness, and uniform mechanical properties than tempered martensite. In addition, austempered products do not require tempering.
Microstructure and mechanical properties of manganese-alloyed austempered ductile iron produced by novel modified austempering process
Published in Cogent Engineering, 2022
Ananda Hegde, Sathyashankara Sharma, Rajarama Hande, Gurumurthy B M
Austempered Ductile Irons (ADI) are widely used in automobile industry and agriculture equipment due to its excellent strength to weight ratio. However, the mechanical properties of the ADI are dependent on the type of austempering heat treatment and its parameters. It is important to obtain the optimum combination of strength and impact energy in ADI. “Austempering” is a heat treatment process which improves the strength and hardness of the ductile iron without compromising in toughness. Manganese addition helps increasing the hardenability of the ADI. Hence, it is essential to select the appropriate type of austempering heat treatment with optimum addition of manganese, so that ADI acquires required properties. This article reports the effect of novel two step austempering treatment on the mechanical properties of ADI alloyed with manganese addition. This study helps to upgrade the knowledge regarding the assessment of manganese alloyed ADI produced by a novel method.
The mechanical properties of austempered 1.75Ni-1.5Cu-0.5Mo alloyed PM steels
Published in Canadian Metallurgical Quarterly, 2023
Ahmet Güral, Onur Altuntaş, Nurullah Sarıçiçek
After this period, a dispersed coarse bainitic formation in the microstructure was observed with further increasing austempering time (Figure 2(d)). According to the Ni and Mo element distribution mapping in the samples, the distribution of nickel over larger areas than molybdenum is clearly seen. The wider distribution of nickel in iron than molybdenum is thought to be due to its greater solubility and higher diffusion rate. Ni-rich areas especially contribute to toughness resistance. If locally high concentrations are achieved due to partial dissolution, Ni-rich areas may become locally austenitic [35,36]. Molybdenum, on the other hand, showed a block distribution as its solubility in iron is very limited compared to nickel.
Study on the influential process parameters in machining the austempered ductile iron
Published in Materials and Manufacturing Processes, 2018
K. M. Kumar, P. Hariharan, B. Ramesh
Austempererd ductile iron (ADI) is the latest development within the region of ductile iron or spheroidal graphite iron. It is a heat treated form of as-cast ductile iron. ADI is produced by a two stage heat treatment process of austenitizing and austempering. The austempering heat treatment process was developed with the intent of improving the strength and toughness of ferrous alloys. It offers a range of mechanical properties superior to those of other forms of cast iron and shows excellent economic competitiveness with steel and aluminum alloys. In recent years, Austempered Ductile Iron is very commonly used by many industries in view of its high strength, hardness and toughness. Due to the enhanced mechanical properties, an ADI material finds its presence in automotive, rail and heavy duty applications [1]. Machining before heat treatment presents no considerable difficulty, but machining after heat treatment is a complex process. The reason behind taking up the Austempering process ahead of machining process is that the change in volume during the subsequent heat treatment after machining will result in distortion and loss of dimensional accuracy. These inaccuracies are to be curtailed in the fabrication of the intricate parts to be used in any machinery or engines. The extent of dimensional loss and distortion depends on the component geometry and pre-austempering microstructure. The machining of ADI in its austempered condition is highly desirable because it can yield tight tolerances and surface finish generally required. Moreover it saves machining time and reduces cost. The reduction in machinability of ADI is due to the development of stress during the transformation from austenite to martensite [2]. This problem arises as a result of its high strength and wear resistance. Several works [2345] report the ability of ADI to be machined with various cutting tools.