激光熔覆NiCrBSi涂层/TP347HFG基材互扩散对基材组织及硬度的影响

Effect of Laser Cladding NiCrBSi Coating/TP347HFG Substrate Interdiffusion on Substrate Microstructure and Microhardness

摘要:

激光熔覆技术已在电站锅炉高温腐蚀磨损防护领域获得一定规模的应用，然而近年来频发激光熔覆后锅炉受热面管长期服役开裂爆管的问题，为探究激光熔覆NiCrBSi涂层的长期服役安全性问题，本研究在TP347HFG炉管表面采用激光熔覆系统制备了NiCrBSi涂层，并置于650、750、850和950 ℃进行高温2000 h时效。研究激光熔覆NiCrBSi涂层长期服役过程中元素互扩散对基材组织和硬度的影响规律，为后续安全性评价提供理论依据。采用SEM和EDS表征了TP347HFG材料表面的组织及成分变化规律；采用显微硬度计测试了TP347HFG基材硬度变化规律。结果表明：长期服役过程中涂层中的B、C等元素向TP347HFG基材中发生了扩散，靠近界面附近的基材中，晶粒内部和晶界部位均产生了高Cr碳化物、硼化物析出相。随着服役温度的升高，扩散深度越深，析出相长大明显，TP347HFG基材表面硬度增加。锅炉管靠近界面处50 μm的硬度随着服役温度的升高发生变化。650 ℃高温服役后，最高硬度为254 HV_0.2；750 ℃高温服役后，最高硬度为299 HV_0.2；经850 ℃高温服役后，最高硬度为417 HV_0.2；950 ℃高温服役后，最高硬度降到376 HV_0.2。

Abstract:

Laser cladding technique has been applied on power station boiler tube against high-temperature corrosion and wear. However, in recent years, tube burst and cracking occurred frequently during long-term service after laser cladding. To explore the long-term service safety of laser cladding NiCrBSi coating, TP347HFG tube with NiCrBSi coating was prepared with a laser cladding system. These tubes were heat treated at 650, 750, 850, and 950 ℃ for 2000 h, separately. The influence of interdiffusion of elements between laser cladding NiCrBSi coating and TP347HFG tube on substate microstructure and microhardness was studied, which could provide a theoretical basis for subsequent safety evaluation. The evolution of microstructure and composition on the surface of TP347HFG were characterized by scanning electron microscope and energy dispersive X-ray spectroscope. The microhardness changes of TP347HFG substrates were tested by a microhardness tester. The results show that during the long-term service, the elements B and C in the coating diffuse into the TP347HFG substrate. In the substrate near the interface, carbides and boride phases with high Cr contents are precipitated in the interior and boundary of grains. With the increase in service temperature, the deeper the diffusion depth, the greater the size of the precipitation phases. The surface microhardness of the TP347HFG substrate increases with the increase in service temperature. The microhardness of the boiler tube at 50 μm near the interface changes with the increase in service temperature. The maximum hardness is 254, 299 and 417 HV_0.2 after high-temperature service at 650, 750 and 850 ℃, respectively. The microhardness of this position drops to 376 HV_0.2 after high-temperature service at 950 ℃ .