ABSTRACT

This study was undertaken to understand the effects of two-step austempering treatment on an AISI 9254 high silicon steel towards tailoring the properties as desired while simultaneously employing the benefits of high and low austempering temperatures. The samples were initially austenitized at 850°C for 20 min, followed by austempering in a salt bath at the temperatures of 250–270–290°C for 20 min during the first stage. Subsequently, a second stage austempering was carried out by raising the temperature of the salt bath to 300°C at an average heating rate of 0.5°C/min, and the samples were kept in the salt bath for achieving a total austempering time of 120 min including the heating time. A conventional single-stage austempering was also conducted for comparison purposes, in which the austenitization temperature, the austempering temperatures and total time (stage I and stage II, i.e. 120 min) were kept the same for the benchmark samples. In the characterization studies, tensile test, hardness test, XRD analysis, optical microscope and field emission-scanning electron microscope (FE-SEM) equipped with EBSD detector were utilized. The findings of this study indicated that lowering the austempering temperature resulted in refining the structure with a decrease in the amount of austenite. According to the carbon content analysis through XRD patterns, the two-step austempering processes appeared to have considerably increased the carbon content of the austenite irrespective of austempering temperature. The best ultimate tensile strength (U.T.S) of 2194 MPa was achieved in the conventionally austempered sample at the lowest temperature of 250°C, while the best yield strength (Y.S.) of 1753 MPa was reached in the stepped austempered sample initially at 250°C followed by 300°C. In general, two-step austempering process led to a higher yield strength while affecting the ultimate tensile strength and total elongation depending on the austempering temperature.

摘要

本研究旨在了解两步等温淬火处理对 AISI 9254 高硅钢的影响，以根据需要定制性能，同时利用高和低等温淬火温度的优点。样品最初在 850°C 奥氏体化 20 分钟，然后在第一阶段在 250–270–290°C 温度的盐浴中等温回火 20 分钟。随后，以0.5℃/min的平均加热速率将盐浴温度升至300℃进行第二阶段等温回火，并将样品保持在盐浴中以实现总等温回火时间120分钟，包括加热时间。为了进行比较，还进行了传统的单阶段奥氏体回火，其中奥氏体化温度，基准样品的等温回火温度和总时间（第一阶段和第二阶段，即120分钟）保持相同。在表征研究中，采用了拉伸试验、硬度试验、X射线衍射分析、光学显微镜和配备EBSD检测器的场发射扫描电子显微镜（FE-SEM）。这项研究的结果表明，降低等温回火温度可以细化组织，同时减少奥氏体含量。根据通过 XRD 图谱进行的碳含量分析，无论等温回火温度如何，两步等温回火工艺似乎都显着增加了奥氏体的碳含量。传统等温淬火样品在最低温度 250°C 下实现了 2194 MPa 的最佳极限拉伸强度 (UTS)，阶梯式等温淬火样品最初在 250°C，随后在 300°C 下达到了 1753 MPa 的最佳屈服强度 (YS)。一般来说，两步等温淬火工艺可带来更高的屈服强度，同时影响极限拉伸强度和总伸长率，具体取决于等温淬火温度。