ABSTRACT

Welding is one of the major assembly processes employed in most common aeronautical applications. However, the consequences of the thermal cycle might be detrimental to the material’s fatigue resistance depending strongly on the resulting microstructure. The 4340 steel is known to present temper embrittlement, which greatly reduces its fatigue life and weldability. This work proposes plasma nitriding as an alternative solution to tempering as a post-weld treatment and investigates its effects on material’s hardness and fatigue life. This analysis was complemented by microstructural and fracture surface characterization. Plasma nitriding resulted in the formation of a double layer consisting of a smaller white micrometric nitride layer at the surface and a more diffuse sub-superficial region which led to better fatigue behaviour when compared with the conventional tempering treatment. The achieved higher surface hardness might also be interesting in other applications such as those involving high erosion and wear.

摘要

焊接是最常见的航空应用中采用的主要装配工艺之一。然而，热循环的后果可能会对材料的抗疲劳性产生不利影响，这在很大程度上取决于所产生的微观结构。众所周知，4340钢会出现回火脆化，这会大大降低其疲劳寿命和可焊性。这项工作提出等离子渗氮作为焊后处理回火的替代解决方案，并研究其对材料硬度和疲劳寿命的影响。该分析得到微观结构和断裂表面表征的补充。等离子渗氮导致形成双层，该双层由表面上较小的白色测微氮化物层和更扩散的亚表面区域组成，与传统回火处理相比，这导致更好的疲劳行为。所获得的更高表面硬度在其他应用中也可能很有趣，例如涉及高侵蚀和磨损的应用。