论文总字数：22121字

摘 要

为了研究奥氏体不锈钢低温超饱和气体渗碳层的硬度分布规律，开展了AISI316L奥氏体不锈钢低温超饱和气体渗碳（LTGC）试验，通过光学显微镜（OM）、残余应力测试仪（XRD）、电子探针显微分析仪（EPMA）测量了经低温超饱和气体渗碳处理的奥氏体不锈钢表面渗碳层的厚度、表面残余应力及沿深度方向的碳含量分布。使用显微硬度计及纳米压痕仪，分别沿渗碳层深度方向测量渗碳层的硬度分布曲线。主要工作和结论如下：

（1）使用实验室自制的低温气体渗碳炉对AISI316L奥氏体不锈钢表面进行低温超饱和气体渗碳处理。并对渗碳层厚度、表面残余应力及沿深度方向碳含量进行测量。结果表明，经低温超饱和气体渗碳处理后，奥氏体不锈钢表面含碳量显著增高，约2.4 wt.%；由于基体的约束，表面产生了极大的压缩残余应力，约-1.9 GPa。金相及碳含量分析结果都表明渗碳层厚度约为30 μm。

（2）采用显微硬度计（25g试验力）沿奥氏体不锈钢渗碳层深度方向测量了渗碳层硬度分布。结果表明：渗碳层表面硬度最高，约980 HV。显微硬度随着距渗碳试样表面距离的增加而降低。约距表面30 μm位置达到最低，与基体保持一致。

（3）通过纳米压痕仪在316L奥氏体不锈钢低温气体渗碳层不同深度进行不同压痕位移下的压痕试验。然后采用应变梯度塑性理论模型对不同位移下的纳米硬度进行回归分析，获得了渗碳层不同深度位置真实的纳米硬度。结果表明，纳米硬度值对压痕试验最大位移产生一定依赖关系（Indentation Scale Effect, ISE），即纳米硬度随着压痕最大位移的增加而逐渐减小，回归分析结果可以发现，压痕的真实纳米硬度较试验结果均有所下降,且随着距试样表面距离的增加，ISE越来越明显。最表面纳米硬度值下降约10%，而基体位置却高达35%。

（4）通过将纳米压痕试验回归结果与显微硬度计测试结果相比发现：当渗碳层深度在15μm范围以外时，纳米硬度约为显微硬度的10倍关系。当渗碳层深度在15μm范围以内时，上述关系开始不成立，且有增大趋势。

关键词：低温气体渗碳 纳米压痕 回归分析 尺度效应

Study on Nano Hardness of Austenitic Stainless Steel in Low Temperature Gas Carburizing Layer

ABSTRACT

The AISI316L austenitic stainless steel low temperature supersaturated gas carburization (LTGC) test was carried out in order to study the law of hardness distribution of austenitic stainless steel in low temperature supersaturated gas carburizing layer. The thickness of the carburized layer on the surface of austenitic stainless steel treated with low temperature supersaturated gas carburizing, the surface residual stress and the carbon content distribution along the depth direction was measured by optical microscopy (OM), residual stress tester (XRD) and electron probe micro analyzer (EPMA). The hardness distribution curve of carburized layer was measured along the depth direction of carburized layer by nanoindentation tester and microhardness tester. The main conclusions are as follows:

(1) Low-temperature super-saturated gas carburizing treatment of 316L austenitic stainless steel surface was carried out by laboratory homemade low-temperature gas carburizing furnace. The thickness of the carburized layer, the surface residual stress and the carbon content in the depth direction were measured. The results reveal that the carbon content on the surface of austenitic stainless steel is significantly increased by about 2.4 wt% after the low temperature supersaturated gas carburizing treatment. Due to the matrix constraints, the surface has a great compressive residual stress, about -1.9 GPa. Metallographic and carbon content analysis results show that the carburized layer thickness is about 30 μm.

(2) The hardness distribution of carburized layer was measured by microhardness tester (25 g test force) along the depth direction of carburized layer of austenitic stainless steel. The results show that the hardness of the surface of carburized layer is the highest, about 980 HV. With the increase of the distance from the surface of the carburized specimen, the micro hardness decreases gradually, and the position is about 30 μm from the surface, which is consistent with the matrix.

(3) In order to obtain the true nano-hardness of the low temperature gas carburizing layer at different depth positions, The indentation tests with different indentation displacements at different depth positions of austenitic stainless steel carburized layer was carried out by nanoindentation tester and regression analysis of nano-hardness under different displacement by strain gradient plasticity theoretical model. The results show that the nano-hardness has a certain dependence on the maximum displacement of the indentation test(Indentation Scale Effect,ISE), that is, the nano-hardness decreases with the increase of indentation maximum displacement. The results of regression analysis show that the true nano-hardness of indentation decreases with the experimental results, and ISE is more and more obvious as the distance from the sample surface increases. The nano-hardness of the surface decreased by about 10%, while the matrix position is as high as 35%.

(4) By comparing the results of regression analysis of nanoindentation test with the results of the microhardness test, when the depth of the carburized layer is outside the range of 15μm, the nano-hardness is about 10 times the microhardness. When the depth of the carburized layer is within the range of 15 μm, the above relationship is not established and there is an increasing tendency.

Key Words: Low temperature gas carburizing; Nanoindentation; Regression analysis; Scale effect

目 录

摘要………………………………………………………………………………………………I

ABSTRACT……………………………………………………………………………………II

第一章 绪论……………………………………………………………………………………1

1.1 研究背景…………………………………………………………………………………1

1.2 奥氏体不锈钢低温气体渗碳技术(LTGC）的研究现状………………………………1

1.2.1 表面低温气体渗碳强化层微观结构研究………………………………………1

1.2.2 低温气体渗碳层硬度分析………………………………………………………3

1.2.3 低温气体渗碳层残余应力的分析………………………………………………3

1.2.4 低温气体渗碳对材料机械性能的影响…………………………………………3

1.3 纳米压痕技术……………………………………………………………………………6

1.3.1 纳米压痕试验原理………………………………………………………………6

1.3.2 纳米压痕技术分类………………………………………………………………6

1.3.3 纳米压痕技术在低温气体渗碳层中的应用……………………………………6

参考文献 ……………………………………………………………………………………7

第二章 奥氏体不锈钢低温气体渗碳层纳米硬度试验结果及讨论……………10

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