Abstract
SUS304 stainless steel is characterized by combined tensile and compression testing, with an emphasis on flow stress at higher strain and temperature. The plastic deformation behavior of SUS304 from room temperature to 400 °C is examined and a general approach is used to express flow stress as a closed-form function of strain, strain rate, and temperature; this is optimal when the strain is high, especially during automatic multi-stage cold forging. The fitted flow stress is subjected to elastothermoviscoplastic finite element analysis (FEA) of an automatic multi-stage cold forging process for an SUS304 ball-stud. The importance of the thermal effect during cold forging, in terms of high material strength and good strain-hardening, is revealed by comparing the forming load, die wear and die stress predictions of non-isothermal and isothermal FEAs. The experiments have shown that the predictions of isothermal FEA are not feasible because of the high predicted effective stress on the weakest part of the die.
Highlights
In the coming era of electric and hydrogen cars, autopart industries must innovate in various ways
We explored the flow stress behaviors of SUS304 using both tensile tests at room temperature and compression tests at temperatures and strains ranging from 100 to 400 ◦ C and 1/s to 20/s, respectively
Cold forging of an SUS304 ball-stud was evaluated with emphasis on the forgeability
Summary
In the coming era of electric and hydrogen cars, autopart industries must innovate in various ways. Cold forging of stainless steels [9,10,11,12,13] is very attractive because it is difficult to lubricate optimally designed dies These have been long-standing difficulties for application engineers and researchers of materials. For the ESW materials [7], the Bauschinger effect reduces die stress by 8%, and lowers the forming load; these must not be neglected during the cold forging of high-strength materials because die fatigue life decreases exponentially as die stress increases. 90% of energy consumption is dissipated as thermal energy (via viscous heating) This increase in temperature drastically decreases flow stress, at lower temperatures. Decreases in flow stress caused by viscous heating must be considered when evaluating the forgeability of high-strength materials. We evaluate the feasibility of automatic multi-stage cold forging of an SUS304 ball-stud both numerically and experimentally, with an emphasis on flow stress characterization
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