The objective of this work is to investigate the phase transformation during friction stir processing of dual-phase 600 steel. Friction stir processing is a microstructure modification process through high strain rate deformation at high temperature. The material undergoes appreciable microstructural changes, which reflects in the refined mechanical properties. The type of metallurgical phases and their fractions have direct impact on the mechanical properties. A coupled 3D thermo-mechanical and phase transformation model is developed to predict the temperature history, plastic deformation and phase transformation during friction stir processing. The modified Johnson–Mehl–Avrami Kolmogorov and Koistinen–Marburger equations are used to model the diffusional and non-diffusional phase transformation, respectively. Friction stir processing is performed with 1.4-mm-thick dual-phase 600 steel and pinless tool. Friction stir tool rotation frequency is varied across friction stir-processed samples, at constant traverse speed. Electron back-scattered diffraction and optical microscopy are used to analyse the phases and microstructure. Model predicts the maximum bainite and martensite volume fractions at stir zone centre. Further, more bainite and martensite formation is estimated at higher tool rotation speeds. These predictions compare well against the experimental observations. Numerical results suggest that the ferrite in the stir region is the untransformed ferrite during heating. During cooling, very small austenite grain size restricts the amount of austenite transformation to ferrite. The phases present in the stir region upon FSP are significantly affected by the amount of carbon content and the initial phases present in the steel.

中文翻译：

---

双相600钢搅拌摩擦加工过程中的相变

这项工作的目的是研究双相 600 钢搅拌摩擦加工过程中的相变。搅拌摩擦加工是在高温下通过高应变率变形进行的微观结构改性过程。材料经历了显着的微观结构变化，这反映在精细的机械性能上。冶金相的类型及其分数对机械性能有直接影响。开发了一个耦合的 3D 热机械和相变模型来预测搅拌摩擦加工过程中的温度历史、塑性变形和相变。修改后的 Johnson-Mehl-Avrami Kolmogorov 和 Koistinen-Marburger 方程分别用于模拟扩散和非扩散相变。摩擦搅拌加工是用 1.4 毫米厚的双相 600 钢和无销工具进行的。摩擦搅拌工具旋转频率在摩擦搅拌处理的样品中以恒定的移动速度变化。电子背散射衍射和光学显微镜用于分析物相和微观结构。模型预测搅拌区中心的最大贝氏体和马氏体体积分数。此外，估计在较高的工具旋转速度下会形成更多的贝氏体和马氏体。这些预测与实验观察结果相得益彰。数值结果表明，搅拌区的铁素体是加热过程中未相变的铁素体。在冷却过程中，非常小的奥氏体晶粒限制了奥氏体向铁素体的转变量。