Abstract The objective of this investigation is to investigate the retained austenite deformation and transformation behavior during nanoindentation of 18CrNiMo7-6 steel after a high-carbon case-hardening process. We conducted a high-carbon carburizing treatment that led to approximately 1.2 mass% carbon at the surface. Further heat treatment consisted of austenitization, quenching in oil, and low-temperature annealing. To transform large amounts of retained austenite into martensite and to stabilize the existent retained austenite, some specimens were cryogenically treated in liquid nitrogen. The resulting microstructures were analyzed as a function of carbon content in the gradient by means of scanning electron microscopy with energy dispersive X-ray spectroscopy and electron backscatter diffraction analysis. Nanoindentation was used to study locally the mechanical behavior and stability of retained austenite at the carburized surface. To better analyze the deformation behavior during indentation, atomic force microscopy was applied to measure the topography of individual indentation imprints and the deformation pattern around the indents. The results show that nanoindentation is suitable for mechanically induced phase transformation of retained austenite into martensite, which leads to pronounced pop-ins in the load-displacement curves at specific critical loads. The occurrence of pop-ins, and, thus the phase transformation is highly dependent on the local microstructure of the carburized specimens. The main influencing factors are the local amount of retained austenite in the deformed region and, predominantly, the crystallographic orientation of retained austenite. (001)-Oriented retained austenite is highly metastable, whereas (111) orientations show mostly stable austenitic deformation behavior with only isolated mechanically induced retained austenite transformation during indentation. Cryogenic treatment transformed larger amounts of retained austenite into martensite. However, even after cryogenic treatment, (111)-oriented retained austenite could undergo mechanically induced transformation into martensite. Furthermore, we used the progression of hardness as a function of indentation depth and topography images to analyze the extent of retained austenite transformation and the complex compound deformation/transformation behavior of preexisting martensite and retained austenite.

中文翻译：

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18CrNiMo7-6高碳渗碳处理后残余奥氏体局部变形及相变行为

摘要 本研究的目的是研究 18CrNiMo7-6 钢在高碳表面硬化工艺后纳米压痕过程中的残余奥氏体变形和转变行为。我们进行了高碳渗碳处理，导致表面碳含量约为 1.2 质量％。进一步的热处理包括奥氏体化、油淬和低温退火。为了将大量残余奥氏体转化为马氏体并稳定存在的残余奥氏体，一些试样在液氮中进行了低温处理。通过具有能量色散 X 射线光谱和电子背散射衍射分析的扫描电子显微镜，分析所得的微观结构作为梯度中碳含量的函数。纳米压痕用于局部研究渗碳表面残余奥氏体的力学行为和稳定性。为了更好地分析压痕过程中的变形行为，应用原子力显微镜来测量单个压痕印记的形貌和压痕周围的变形模式。结果表明，纳米压痕适用于残余奥氏体向马氏体的机械诱导相变，这会导致在特定临界载荷下载荷-位移曲线中出现明显的突进。爆裂的发生以及相变高度依赖于渗碳试样的局部微观结构。主要影响因素是变形区局部残余奥氏体的数量，主要是 残余奥氏体的结晶取向。(001) 取向的残余奥氏体是高度亚稳态的，而 (111) 取向显示出大部分稳定的奥氏体变形行为，在压痕过程中只有孤立的机械诱导残余奥氏体转变。低温处理将大量残余奥氏体转化为马氏体。然而，即使经过深冷处理，（111）取向的残余奥氏体仍可能发生机械诱导转变为马氏体。此外，我们使用硬度随压痕深度和形貌图像的变化来分析残余奥氏体转变的程度以及预先存在的马氏体和残余奥氏体的复杂复合变形/转变行为。而 (111) 取向显示大部分稳定的奥氏体变形行为，在压痕过程中只有孤立的机械诱导残余奥氏体转变。低温处理将大量残余奥氏体转化为马氏体。然而，即使经过深冷处理，（111）取向的残余奥氏体仍可能发生机械诱导转变为马氏体。此外，我们使用硬度随压痕深度和形貌图像的变化来分析残余奥氏体转变的程度以及预先存在的马氏体和残余奥氏体的复杂复合变形/转变行为。而 (111) 取向显示大部分稳定的奥氏体变形行为，在压痕过程中只有孤立的机械诱导残余奥氏体转变。低温处理将大量残余奥氏体转化为马氏体。然而，即使经过深冷处理，（111）取向的残余奥氏体仍可能发生机械诱导转变为马氏体。此外，我们使用硬度随压痕深度和形貌图像的变化来分析残余奥氏体转变的程度以及预先存在的马氏体和残余奥氏体的复杂复合变形/转变行为。低温处理将大量残余奥氏体转化为马氏体。然而，即使经过深冷处理，（111）取向的残余奥氏体仍可能发生机械诱导转变为马氏体。此外，我们使用硬度随压痕深度和形貌图像的变化来分析残余奥氏体转变的程度以及预先存在的马氏体和残余奥氏体的复杂复合变形/转变行为。低温处理将大量残余奥氏体转化为马氏体。然而，即使经过深冷处理，（111）取向的残余奥氏体仍可能发生机械诱导转变为马氏体。此外，我们使用硬度随压痕深度和形貌图像的变化来分析残余奥氏体转变的程度以及预先存在的马氏体和残余奥氏体的复杂复合变形/转变行为。