工具钢深冷处理残余奥氏体演变、残余应力分布及变形行为的多场耦合数值研究

Deep cryogenic treatment (DCT), an supplementary process to conventional heat treatment usually involves cooling the part to a liquid nitrogen temperature around -196 ℃, has been widely accepted as an important improving and strengthening technology of productions. Many investigations suggested that this technique can not only evidently reduce the amount of retained austenite and create a favorable redistribution of residual stress, but also efficiently improve the physical and mechanical properties of the various materials such as wear, abrasion, corrosion and fatigue resistance. However, because DCT is a very complex process involving thermal, metallurgical and mechanical phenomena, utilized the traditional research approaches, it is still difficult, even impossible to completely and accurately predict the dynamic distribution regular of temperature, microstructure and stress-strain fields during DCT. Therefore, in this work, taking tool steel as the research object, establishing the phase transition kinetic mathematical model of DCT, and combining with the low temperature physical parameters obtained from experimental test and the boiling heat transfer coefficient acquired by an solution of empirical formula, a multi-physical field coupling numerical model based on metallo-thermo-mechanical coupled theory is built to reproduce the DCT process of a tool steel specimen immersed in the liquid nitrogen bath. It helps to completely and accurately reveal the underlying relationship among retained austenite evolution and the formation, evolution and distribution of residual stress, as well as the distortion behaviour in a tool steel specimen during DCT, and then provide a theoretical guidance to further evaluate the microstructure and properties of tool steel and make the more efficient and reasonable cryogenic treatment procedure.

工具钢淬火过程辅助以深冷处理的方式是近年来为业界广泛认可的产品改性及强化的重要工序。研究表明，深冷处理不仅能够显著消除残余奥氏体，产生表面残余压应力，而且可以改善组织均匀性，提高材料的耐磨性、耐蚀性和抗疲劳性能。然而，深冷处理是一个相当复杂的过程，采用传统的研究手段和方法已不能完整、准确地分析深冷处理过程中工件温度场、组织场以及应力/应变场的动态演变规律。因此，本课题拟以工具钢为研究对象，建立深冷处理相变动力学数学模型，实测材料低温物性参数，求解深冷处理沸腾换热模型，基于金属-热-力耦合理论框架，建立深冷处理多物理场耦合数值分析模型，全面揭示深冷处理过程中工件内残余奥氏体的动态演变规律，残余应力的形成、演变与分布特征，以及工件变形三者之间的内在联系，以期为深冷处理工序设计和优化提供借鉴，为深冷处理的组织控制提供理论依据。

深冷处理作为常规热处理工艺的延续，不仅能够显著消除工具钢的残余奥氏体，产生表面残余压应力，而且可以改善组织均匀性，提高材料的耐磨性、耐蚀性和抗疲劳性能。然而，鉴于深冷处理是的复杂性，采用传统的研究手段和方法已不能完整、准确地分析深冷处理过程中试件温度场、组织场以及应力场的动态演变规律。因此，本课题以SDC99冷作模具钢为研究对象，基于金属-热-力耦合理论，在考虑材料低温物性参数、沸腾换热系数随温度的非线性变化以及相变潜热的情况下，采用有限元方法建立了SDC99冷作模具钢C型环试样深冷处理的数值分析模型，详细研究了试样在淬水冷却和深冷处理过程中温度场、组织场和应力场的动态演变规律，预测试样残余奥氏体、残余应力和硬度的分布状态，并辅以XRD分析和硬度测试对数值结果进行验证，确保数值模型的可靠性和准确性。同时，基于已建立的深冷处理多物理场耦合数值分析模型，全面揭示C型环试样深冷处理过程中各场的耦合状态和演变规律，尤其是残余奥氏体的动态演变规律和机制，以及残余应力的形成、演变与分布特征，为深冷处理的组织控制提供理论依据和借鉴；系统探讨了淬火和深冷处理过程中试样的偏心度、浸没方式和冷却行为等不同深冷处理工艺条件下工件内残余奥氏体的演变和残余应力的分布状态的内在联系，并辅以实验验证，以期实现对深冷处理工艺的控制和优化，达到提高工具使用寿命的目标。