Plasma nitriding effects in fatigue behaviour of the welded 4340 steel

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.