涡流电迁移加速金属表面合金化研究

Ultra supercritical steam power plants, which have higher efficiency, lower coal-consumption and CO2-release, are an important target of clean coal technology. The reliability of this technology is limited by the supercritical steam oxidation of the steels for boiler components. To improve the water vapor resistance of those steels, an effective way is to make the steel possess alumina-forming ability by surface aluminizing. However, it is hard to applying traditional aluminizing techniques to large components such as boiler tubes, due to the inherent defects of the traditional aluminizing techniques. In this research, a novel fast surface alloying technique which uses eddy current-induced electromigration to enhance the diffusion kinetics is introduced in the slurry aluminizing process. The effects of eddy current-induced electromigration on the diffusion kinetics, microstructure, chemical and phase composition of the aluminized layer in boiler steel will be studied. Furthermore, the mechanical properties and supercritical steam oxidation-resistance of the aluminized layer will be investigated. This research will provide theoretical basis for the development of the new technique of fast surface alloying for metallic materials, especially for components with large scale.

超超临界火力发电可提高热效率、降低煤耗和CO2排放，是洁净燃煤发电技术的主要方向。锅炉耐热钢管道超临界水蒸汽氧化问题制约其可靠性。耐热钢表面合金化一定含量的Al以达到Al2O3膜形成能力，可显著提升耐热钢的抗水蒸汽氧化性能。传统热扩散渗铝方法的固有缺点难以在诸如锅炉管道等大型构件上实施。本研究将涡流电迁移引入料浆渗铝过程以探索快速高效的新型金属表面合金化方法。本项目拟以耐热钢料浆法渗铝为例，深入探讨涡流电迁移对渗铝动力学、渗层组织结构、成分和相组成及演化的影响规律和机制,并探究该渗铝层的力学和抗超临界水蒸气氧化性能。为发展大型构件快速表面合金化新技术提供理论指导。

超超临界洁净燃煤发电已成为我国火电主流。锅炉运行参数和发电热效率的提高，对锅炉高温段耐热钢管道内壁抗超临界水氧化性能的要求也显著提高。传统具备氧化铬形成能力的耐热钢在超临界水中易出现灾难性氧化。对耐热钢管内壁渗铝使其具备氧化铝形成能力是提高其抗超临界水氧化的有效途径，然而传统渗铝方法存在效率低、加工困难或相容性等问题。因此，研究出适用于耐热管材内壁的新型高效渗铝技术十分紧迫。项目负责人提出利用涡流电迁移加速金属表面合金化的思想，以达到对管道类构件表面进行超高速可控渗铝的目的。结果表明，对试样通入脉冲电流，利用电流自身的焦耳热及表层涡流电迁移效应，10min内（~900oC）在涂有渗铝料浆的耐热钢表面可生长出塑韧性较好的厚度~35µm的FeAl或FeCrAl渗层。电迁移效应改变了扩散模式，促进了铁原子的外扩散，名义扩散系数是传统方法的十倍以上；而要获得类似渗层，使用传统方法需要数小时的时间和更高的温度，然而长时间高温处理将严重恶化基体的力学性能。对于实际尺寸的耐热钢管，传统工艺仅加热升温阶段都需要数小时，而该技术依然仅需数分钟实现超高速渗铝。此外，实验表明，更高的电流密度下，在更短的时间内（5min）可获得塑性固溶态FeCrAl渗层，渗层厚度可达~35µm，这是传统工艺无法实现的，形成这种固溶态渗层有利于避免传统渗铝涂层脆性开裂的问题。实验表明，高温水蒸汽环境下，该方法制备的FeAl或FeCrAl渗层均表现出良好的Al2O3膜形成能力。涡流电迁移加速渗铝技术有望用于解决超临界锅炉耐热钢管的超临界水氧化问题。此外，由于低成本、易操作，该技术还可应用于制备化工管道类部件的高温防护涂层。