การยืดตัวที่อุณหภูมิสูงของเหล็กกล้าเครื่องมือทำงานเย็นชนิด AISI L3 และ AISI O1

Abstract

Hot tensile tests were carried out to investigate hot ductility behavior and critical temperature range (ΔT) between zero strength temperature (ZST) and zero ductility temperature (ZDT) of tool steel grades AISI L3 and O1. The test specimens solidified from melting at cooling rate of 0.5℃/s and 3℃/s to the test temperature in the range between liquidus and 900℃. The specimens were then strained to failure at strain rate of 2x10⁻³/s and 2x10⁻²/s. Quenching test was carried out in order to investigate the microstructure occurred during hot tensile test. The microstructure that appeared in quenched specimen was then identified with SEM-EDX. It was found that the critical temperature range of the steels solidified at the slow or the fast cooling rate, widened with increasing strain rate. The specimens which solidified at slow cooling rate when tensile strained to failure at low or high strain rate, had wider critical temperature range than the specimens that solidified at fast cooling rate. For steel grade AISI L3, tensile test at high and low strain rate (after solidified at the fast cooling rate) revealed the hot ductility of 35-65%RA between 900-1250℃. Whereas, the hot ductility of the samples which solidified at slow cooling rate and pulled at slow strain rate were markedly higher than those pulled at higher strain rate between 900-1200℃. Precipitation of manganese sulfide in the interdendritic region resulted in deterioration of hot ductility of steel grade AISI L3. For steel grade AISI O1, tensile tests of specimens solidified between 900-1100℃ at slow cooling rate and low strain rate yielded higher hot ductility than those specimens pulled at high strain rate. The hot ductility of samples at fast cooling rate and low strain rate showed no difference to those samples of hot tested at high strain rate. Nevertheless, slow cooling rate affected hot ductility more than fast cooling rate for tensile tests at both high and low strain rate. Carbide precipitates of vanadium, tungsten and chromium along austenite grain boundaries influenced the ductility loss of steel grade AISI O1.