固体渗铬层组织结构及性能稳定性研究

Structure and performance stability of chromized coating by pack cementation

摘要:

采用固体粉末法在GCr15钢表面进行渗铬处理，研究了不同渗铬时间下渗层的结构和性能，并研究了自然时效对渗层结构、力学性能的影响。采用扫描电子显微镜及EDS能谱仪、X射线衍射仪研究了时效前后渗层组织结构的变化，采用显微硬度仪、纳米压痕仪、洛氏硬度计分析了时效前后渗层表面硬度、截面微区硬度、与基体的结合强度。结果表明：渗层主要由Cr2N、(Cr,Fe)23C6和(Cr,Fe)7C3相组成，渗层外表面为不连续的氮化铬晶粒分布在疏松的碳化铬中，随着渗铬时间的增加，氮化铬晶粒长大；渗层下表面为碳化铬层，由外向内Cr逐渐减少，Fe逐渐增加，形成了梯度过渡层结构。渗层表面显微硬度在1492HV0.05~1698HV0.05之间，经过1年自然时效后表面显微硬度稍有降低；截面纳米硬度表明，随着Cr含量逐渐减少，纳米硬度也相应下降。洛氏压痕结果表明，压痕周围存在放射性裂纹及不同程度的渗层剥落现象；经过1年自然时效后压痕周围只存在放射性裂纹，渗层与基体结合强度提高。

Abstract:

Chromized coatings were fabricated using a conventional pack-cementation method. Effect of chromizing time, as well as natural ageing on the structure and performance was investigated. The phases and microstructure were characterized by scanning electron microscopy (SEM) with EDS and X-ray diffraction (XRD). The microhardness of the surface, the nanohardness of cross-section, the adhesion strength of the coatings were characterized by Vickers microhardness tester, nano-indenter and Rockwell-C tester. The results show that the chromized coatings were composed of Cr2N, (Cr,Fe)23C6 and (Cr,Fe)7C3. In the top surface, the discontinous Cr2N crystal grains distributed in the loose chromium carbides. The Cr2N crystal grains growth larger as the chromizing time increased. In the sublayer, the phase was composed of chromium carbides. The gradient structure was formed in the chromized coatings with Cr content decreased from surface to substrate and the contrary trend for Fe content. The microhardness of the surface was between 1492HV0.05~1698HV0.05 and it decreased slightly after 1 year natural ageing. The nanohardness reduced with Cr content decreasing in cross-section of chromized coatings. The Rockwell-C test results show that there were some radial cracks and spalls around the indentation crater. However, only radial cracks were presented around the indentation craters after 1 year natural ageing, indicated the improvement of adhesion strength of the coatings to substrate.