耐热合金21-4N楔横轧成形的高温损伤机制与控制研究

Heat resistant alloy 21-4N is a kind of typical steels used in valve. It possesses good thermal strength, but internal failure easily occurs during cross wedge rolling due to its poor thermo-plasticity and narrow hot deformation temperature range, which has restricted the application of cross wedge rolling in valve precision forming process. So, quantitative prediction and effective control of damage in 21-4N forming during cross wedge rolling are becoming key problems to be solved. This project considers both the evolution of microstructure and deformation characteristics of cross wedge rolling, studies the damage mechanism of 21-4N forming by cross wedge rolling, then models a coupled constitutive equations for heat-transfer, deformation and damage and establishes a proper damage criteria for heat resistant alloy 21-4N forming in cross wedge rolling. User-defined subroutine is going to be programmed, and then we get a new damage prediction method of FEM in cross wedge rolling of heat resistant alloy 21-4N. Based on FEM, damage nucleation and evolution affected by technological parameters variation in cross wedge rolling of heat resistant alloy 21-4N will be studied, and we will provide a new method for damage control and parameters selection. The achievements of this project will have an important practical value of quality control and die parameters optimization in cross wedge rolling of heat resistant alloy. This study also has a great scientific significance in presenting a new research method for damage prediction in cross wedge rolling.

耐热合金21-4N是一种典型的气门用钢，热强性好，但热塑性差、热变形温度范围窄，在楔横轧成形中容易出现心部疏松,制约了楔横轧气门精密成形工艺的应用。如何定量地预测21-4N楔横轧成形的高温损伤并有效控制缺陷产生已成为亟待解决的关键基础问题。本项目拟结合21-4N成形的微观结构变化和楔横轧变形特征，揭示21-4N楔横轧成形的损伤机制，建立热-力-损伤耦合模型，制定21-4N楔横轧的损伤判定标准；编制有限元用户子程序，建立21-4N楔横轧高温损伤的有限元预测平台；阐明各工艺参数对21-4N楔横轧成形中的损伤形核与演变的独立及耦合影响机制，提出21-4N楔横轧成形的高温损伤控制方法和工艺参数选择方法。研究成果对楔横轧耐热合金的质量控制和模具参数优化具有重要的实用价值，研究方法对楔横轧工艺的损伤预测具有重要的科学意义。

气门是汽车发动机的关键零件之一，工作时处于高温、高压、高腐蚀性的环境中，对性能的要求高。传统的气门成形工艺是电热镦粗后模锻，由于电热镦粗是个自由变形过程，存在产品质量不稳定、生产效率低等问题。楔横轧制坯后模锻是气门成形的一种新工艺，与传统工艺比较，具有生产效率高、产品质量稳定可控等优点。. 耐热合金21-4N是一种典型的耐热奥氏体气门钢，具有良好的热强性和耐腐蚀性，但其热塑性差，热变形温度范围窄，楔横轧成形时易出现中心孔洞和疏松等问题，影响最终产品的质量。. 项目开展耐热合金21-4N楔横轧成形过程的高温损伤与控制等基础科学问题研究，分析了温度、应变速率和变形量等对21-4N高温变形行为的影响，基于内变量方法和遗传优化算法，建立并求解了21-4N的高温变形统一本构模型和热-力-损伤耦合模型；编制了21-4N本构方程的有限元用户子程序，建立了21-4N楔横轧成形的有限元分析模型；分析了21-4N楔横轧成形各阶段的应力应变状态和台阶的成形过程，在楔横轧展宽段，轧件主要受两向拉应力和一向压应力作用，由于轧件的旋转，呈现两向压应变和一向拉应变；研究了21-4N楔横轧成形的微观组织变化，揭示了21-4N楔横轧成形的高温损伤机制，21-4N加热后未溶碳化物M23C6和晶界析出氮化物Cr2N是损伤的萌生源，反复的剪应力和拉应力是损伤扩展的主要因素；研究了成形角、展宽角和断面收缩率等对21-4N楔横轧成形的损伤影响规律，大的成形角有利于轧件损伤的控制，成形角大于30°时轧件心部质量较好，断面收缩率控制在35%-50%有利于保证轧件的心部质量，展宽角对21-4N楔横轧成形的损伤影响规律不明显。研究结果可应用于气门楔横轧制坯的模具设计和参数优化上。