What is it about?
In the process of thermoplastic processing, bismuth stainless steel is prone to crack, leading to a high reject rate and even failure in production. In order to solve this problem and provide a feasible process for the actual production, the true stress-strain curves of the material were obtained by hot compression experiments. Because the thermal simulation system cannot automatically consider the temperature and stress variation of the specimen caused by the heat of plastic deformation and the friction between the specimen and the tool, the experimental errors were carefully corrected. A kind of exponential function is found to be suitable for describing the modified stress-strain relationship of the invested steel. Based on the dynamic material model (DMM) and Murty criterion, the thermal processing diagram of the free-cutting austenitic stainless steel was established, and the stable and unstable regions of the hot working of the free-cutting austenitic stainless steel were given. Rolling tests showed that under the condition of the selected stable zone, no crack occurred in the section of the rolled piece. Microscopic analyses showed that the microstructure distribution of rolled parts by the non-instability zone process is relatively uniform, and there were fewer spherical and fusiform inclusions.
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Why is it important?
Because the thermal simulation system cannot automatically consider the temperature and stress variation of the specimen caused by the heat of plastic deformation and the friction between the specimen and the tool, the experimental errors were carefully corrected.
Perspectives
The plastic deformation heat makes the material stress recorded in the thermal simulation compression experiment lower than the actual stress of the sample. Under the experimental conditions in this paper, the numerical deviation was more than 10%. The friction between the specimen and the indenter led to uneven deformation, thus making the recorded material stress higher than the specimen's actual stress. The experimental error should not be ignored and it is necessary to correct it. The mean value of stress after the correction was still higher than 5% of the stress value before the correction. The hot working maps constructed according to the dynamic materials model and Murty instability criterion are of great reference significance for the selection of the thermoplastic forming process of bismuth-containing austenitic stainless steel. After rolling with the improved process parameters, the roll has no cracks, and the forming effect is good. The microstructure distribution of rolled parts was uniform in processes designed according to the stable zones of the hot working map. However, there were more spherical or fusiform inclusions in the microstructure of the cracked rolling parts, mainly MnS and bismuth precipitates. These inclusions embrittle the material and lead to cracking in rolling.
shengde hu
Wuhan University of Science and Technology
Read the Original
This page is a summary of: Thermal Deformation Behavior of Bismuth-Containing Stainless Steel, Journal of Testing and Evaluation, January 2024, ASTM International,
DOI: 10.1520/jte20230744.
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