激光熔覆-渗氮复合改性38CrMoAl钢及耐腐蚀性能

Modification and Corrosion Resistance of 38CrMoAl Steel by Laser Cladding-Ion Nitriding

目的增加38CrMoAl渗氮钢海洋环境下耐腐蚀性能。方法采用激光熔覆设备和等离子体渗氮炉,在38CrMoAl表面先后进行FeCoCrNiAl0.5高熵合金(HEA)涂覆和离子渗氮处理,同时对激光熔覆-渗氮复合处理的38CrMoAl进行机理和耐蚀性分析。结果复合处理38CrMoAl表面生成厚度约为0.6mm高熵合金(HEA)+0.35μm氮化高熵合金(HEAN)改性层,氮化HEA层组织结构由HEA的单一FCC结构转变为复相(FCC+AlN+CrN)结构,基体、HEA和氮化HEA层的平均硬度分别为400、225、1164HV,复合处理后38CrMoAl盐雾试验下6 d后表面开始出现蚀坑,而未经过任何处理的38CrMoAl表面2 d后即出现明显的腐蚀现象。结论激光熔覆-氮化复合处理能够有效改善38CrMoAl的耐腐蚀性能,中间的软韧型HEA层不仅增加了侵蚀性离子的扩散壁垒,还可作为高硬度表层和基体之间的缓冲介质而避免表面脆化。

To enhance the wear and corrosion resistance of 38CrMoAl nitriding steel,the nitriding high-entropy alloy(HEA)/HEA layer was prepared on the surface of 38CrMoAl high strength steel by laser cladding and subsequent ion nitriding technology.The microstructure,hardness and electrochemical properties of nitriding-HEA coating,HEA layer and substrate were measured by experiments.The influence of hardness and corrosion resistance on the surface following compound treatment involving laser cladding and ion nitriding was investigated.The results demonstrated the formation of a strengthening layer on the surface of 38CrMoAl steel,comprising the HEA and nitriding-HEA layer with respective thicknesses of 0.6 mm and 0.35μm.The crystal structure of the intermediate HEA layer formed after laser cladding was a single face-centered cubic(FCC)structure,and the surface crystal structure of nitriding-HEA layer was transformed into a multiphase structure(FCC+AlN+CrN)after subsequent nitriding treatment.The average hardness of the substrate,HEA and nitriding-HEA layers was 400HV,225HV and 1164HV,respectively.The HEA layer with lower hardness could avoid surface embrittlement caused by thick modified layer and high hardness,and was expected to improve the impact resistance of the material as a buffer medium between the substrate and the surface layer.Salt spray tests were carried out on 38CrMoAl steel after laser cladding and subsequent nitriding combined treatment under 5%NaCl solution with pH=3.5 for various duration(2 days,4 days,6 days,8 days and 10 days).The results revealed that the composite treatment led to the appearance of corrosion pits on the steel surface after 6 days,which further developed into irregular corrosion pits after 10 days.In contrast,untreated 38CrMoAl exhibited extensive formation of corrosion products on its surface within just 2 days of salt spray exposure and complete corrosion as the duration increased.In addition,the scanning Kelvin probe(SKP)test results of 38CrMoAl steel section after laser cladding-nitriding composite treatment showed that the voltaic potential of the substrate was significantly lower than that of the reinforced layer.The above results showed that the combined treatment of laser cladding and subsequent ion-nitriding could increase the diffusion barrier of erosive ions by increasing the thickness of the strengthened layer.The intermediate soft and tough HEA layer was used to avoid surface embrittlement phenomenon,which significantly improved the corrosion resistance of 38CrMoAl steel.Similar results could be found in the polarization curves of composite treated 38CrMoAl steel after salt spray tests of different periods.In this research,laser cladding combined with nitriding treatment technology is used to effectively improve the surface hardness and corrosion resistance of 38CrMoAl steel.The research content lays a theoretical foundation for the application of high-entropy alloy coating to equipment,and provides experience and support for the study of improving the impact resistance and service life of equipment as a soft and tough layer.The development of this technology can provide a theoretical basis for the surface strengthening technology of 38CrMoAl steel or other high strength steel,and ensure the sustainable development of equipment life.