Super long-range diffusion of carbon during proeutectoid ferrite transformation

Abstract

In order to explore the possible diffusion distance of carbon during proeutectoid ferrite transformation, a slow cooling test of low carbon steel was carried out under vacuum of the thermal simulator. The microstructure and thermal expansion curve were discussed and the carbon concentration inside the sample was measured. The ferrite layer of about 450 µm thickness was obtained without pearlite on the surface of the sample in the microstructure. The thermal expansion curve shows that the ferrite layer without pearlite is formed during the local phase transformation, which is followed by the global transformation. The carbon concentration in the core of the sample (0.061%) is significantly higher than that of the bulk material (0.054%). All results show that carbon has long-range diffusion from the outer layer to the inner layer of the sample. The transformation is predominantly interface-controlled mode during local transformation, and the interface migration rate is about 2.25 µm/s.

摘要

为研究先共析铁素体相变过程中碳原子可能的扩散距离,在热模拟的真空环境下进行了低碳钢 的缓冷试验,分析了金相组织和热膨胀曲线,并测量了试样芯部的碳含量。观察金相组织发现试样的 表面获得了 450 pm厚的铁素体层,该铁素体层中不含珠光体。热膨胀曲线显示不含珠光体的铁素体 层形成于全局相变之前的局部相变过程中。试样芯部的碳含量(0.061%)显著高于基体材料的碳含量 (0.054%)。结果表明,碳从试样表层到试样芯部发生了超长程扩散。局部相变主要为界面控制模式, 界面迀移速率大约为2.25 (im/s。