Abstract
Thermomechanical treatment (TMT) describes the effect of thermal and mechanical conditions on the microstructure of materials during processing and offers possible integration in the forging process. TMT materials exhibit a fine-grained microstructure, leading to excellent mechanical properties. In this study, a two-step TMT upsetting process with intermediate cooling is used to demonstrate possibilities for a process-integrated treatment and corresponding properties. A water–air-based cooling system was designed to adjust different phase configurations by varying the target temperature and cooling rate. Four different thermal processing routes and four combinations of applied plastic strains are investigated in standardized mechanical tests and metallographic analyses. The applied TMT results in a finely structured bainitic microstructure of the investigated tempering steel AISI 4140 (42CrMo4) with different characteristics depending on the forming conditions. It can be shown that the demands of the standard (DIN EN ISO 683) in a quenched and tempered state can be fulfilled by means of appropriate forming conditions. The yield strength can be enhanced up to 1174 MPa while elongation at break is about 12.6% and absorbed impact energy reaches 58.5 J without additional heat treatment when the material is formed after rapid cooling.
Highlights
Forged components made from alloyed tempering steel are commonly used for parts with demands on safety and durability for example in the aviation industry or in automotive applications.These components must be both lightweight and highly resistant to dynamic loads [1]
The resulting mechanical properties are clearly stated in summarizing diagrams for both parameters, so this paper gives a complete overview of the results of the entire test matrix
The investigations on different thermal and mechanical process parameters during the process-integrated TMT of tempering steel AISI 4140 proved the significant influence of this processing on the microstructure and the mechanical properties of components
Summary
Forged components made from alloyed tempering steel are commonly used for parts with demands on safety and durability for example in the aviation industry or in automotive applications.These components must be both lightweight and highly resistant to dynamic loads [1]. Forged components made from alloyed tempering steel are commonly used for parts with demands on safety and durability for example in the aviation industry or in automotive applications. The parts’ properties are influenced by surrounding processes such as heat treatment and by input parameters such as material composition [2] The combination of these factors is summarized as thermomechanical treatment (TMT) and leads to excellent properties, which could not be achieved by the individual processes if they are performed separately. The excellent strength and toughness of TMT parts results from the fine-grained microstructure [3] This type of heat treatment enables integration into the forging process chains, which can lead to considerable savings in time and costs [4]. They are already widely used in the manufacturing of semi-finished products, for example in the sheet metal sector [5] or extrusion processes [6]
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