论文总字数：28809字

摘 要

奥氏体不锈钢在工业生产中成为一种不可或缺的材料是因为在很大的温度范围下它们具有优良的耐腐蚀性、成型性、相容性以及很好的韧性。但是，与其他钢种相比，其表面强度和硬度一般大大限制了它的应用范围。近年来发展起来的低温渗碳技术被证明是奥氏体不锈钢表面强化最有效的方法之一，该技术能在提高其表面强度的同时兼顾其耐腐蚀性。为了模拟渗碳后的奥氏体不锈钢中碳浓度沿深度方向的分布，本文建立了基于应力引起的和浓度相关的碳扩散的动力学模型。此模型认为碳的扩散系数取决于碳浓度且由渗入的碳原子引起的应力反过来也将影响碳的扩散行为。对316L奥氏体不锈钢在470℃下进行不同时间的低温表面渗碳处理，并用扫描电子探针微量分析仪（EMPA）测量沿深度方向的碳浓度，将实验结果与模型计算结果对比，验证模型的有效性。结果表明，碳扩散模型的计算结果与实验结果具有高度一致性，说明应力和浓度相关的扩散系数对低温渗碳的碳浓度分布有重要的影响。渗碳后的材料表面将产生较大的压缩残余应力，且呈梯度分布，同时压缩应力梯度也会加速碳原子的扩散。虽然压缩残余应力的存在不是引起总的碳扩散系数显著增长的主要因素，但低温表面渗碳过程中应力梯度是碳扩散过程不可忽视的一个驱动因素。所建立的模型仅适用于描述奥氏体不锈钢在低温表面渗碳时未发生碳化物析出的碳扩散。

关键词：奥氏体不锈钢 低温表面渗碳 碳扩散 应力引起的扩散 浓度相关的扩散系数

Numerical analysis of stress-induced and concentration-dependent carbon diffusion in low-temperature surface carburization of austenitic stainless steel

ABSTRACT

Due to their good corrosion resistance, formability, compatibility and excellent toughness in a large temperature range, austenitic stainless steels are the indispensable materials in the industrial production .However, they have only modest surface strength and hardness compared to other steels，limiting their implications in many fields. Low-temperature surface carburization (＜500℃) has been demonstrated to be one of the most effective techniques for improving the surface hardening of austenitic stainless steel , while the good corrosion resistance is maintained . A kinetic model based on stress-induced and concentration-dependent carbon diffusion was developed for simulating the carbon concentration-depth profile of carburized austenitic stainless steel. The model considers that the diffusivity of carbon is dependent of carbon concentration and the stress induced by diffusion of the dissolved carbon atoms can affect the diffusion behavior in turn. 316L austenitic stainless steel was treated by low-temperature surface carburization at 470 ℃ for different times, and the carbon concentration along the depth direction was measured by scanning electron probe micro-analyzer (EPMA). The results show that in carburized austenitic stainless steel, the predicted carbon concentration-depth profile based on stress-induced and concentration-dependent carbon diffusion model is in good agreement with experimental results, which indicates that the stress and concentration-dependent diffusivity play important roles in carbon diffusion. As a result of carburization, large compressive residual stress is generated and gradiently distributes in the carburized layer, meanwhile, the diffusion of carbon atoms can be accelerated by compressive residual stress gradient. Although, the compressive residual stress is not the dominant reason for total carbon diffusivity increases significantly with increasing carbon concentration, the stress gradient as the next driving force cannot be ignored during low-temperature surface carburization process. The proposed model can be used to describe the carbon diffusion in ASS during low-temperature surface carburization without obvious carbides precipitation.

Keywords: Austenitic stainless steels；Low-temperature surface carburization；Carbon diffusion； Stress-induced diffusion；Concentration-dependent diffusivity

目 录

摘要 I

ABSTRACT II

第一章 绪论 1

1.1引言 1

1.2 奥氏体不锈钢表面强化方法 2

1.2.1强化方法简介 2

1.3低温表面渗碳技术 2

1.3.1奥氏体不锈钢低温表面渗碳机理与发展 3

1.3.2 低温离子渗碳 3

1.3.3低温气体渗碳 5

1.4奥氏体不锈钢低温表面渗碳中碳扩散的数值分析的研究现状 6

1.5 研究内容及方法 7

1.5.1研究内容 7

1.5.2 研究方法 8

第二章 低温超饱和气体渗碳实验与结果分析 9

2.1引言 9

2.2低温超饱和气体渗碳 10

2.2.1低温超饱和气体渗碳基本原理 10

2.2.2 表面活化预处理 10

2.2.3 低温表面超饱和气体渗碳过程 11

2.3 实验装置 12

2.3.1 供气系统 14

2.3.2 压力控制系统 14

2.3.3 流量控制系统 14

2.3.4 气体混合系统 15

2.3.5 加热系统 15

2.3.6 其他辅助系统 16

2.4 渗碳工艺及流程 17

2.4.1 实验材料与试样制备 17

2.4.2 表面活化预处理流程 18

2.4.3 表面气体渗碳流程 18

2.5 表面特性测试 18

2.6 实验结果与分析 19

2.6.1渗碳层显微形貌 19

2.6.2表面物相分析 20

2.6.3 表面碳浓度分布 21

2.6.4 表面残余应力分布 21

2.7 本章小结 22

第三章 低温超饱和气体渗碳数值分析 23

3.1 引言 23

3.2 应力引起的和浓度相关的扩散模型的建立 23

3.3 模型结果与讨论 26

3.4 本章小结 28

第四章 总结与展望 29

4.1 总结 29

4.2 工作展望 29

参考文献 31

致谢 34

第一章 绪论

1.1引言

不锈钢凭借优良的耐腐蚀性能和综合性能奠定了其在工业生产中的地位。由于不锈钢中Cr元素的含量比较高，其表面能形成一层很薄的且能自我修复的Cr₂O₃氧化膜，将钢基质与外界腐蚀介质隔开，从而起到了抵抗腐蚀的作用。

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