The investigation of cleaning chemicals on the effects of surface nitriding process at gas nitriding

Fuel injection technology should improve fuel combustion and evaporation as well as be resistant to high pressure in order to enhance current characteristics of diesel engines, and provide minimizing the exhaust emissions, combustion noise, along with maximizing the fuel conservation. New generation injection systems, which are being developed for more efficient combustion, have to possess high pressure strength. Gas nitriding process has an important role in order to improve the injector's performance in terms of the the resistance to fatigue and high pressure. Besides, the fact of being able to increase the corrosion resistance on nitrided injector bodies is experienced [1] Cleaning has an essential place before heat treatment processes, since residual films from cleaning chemicals together with ineffective cleaning create cloudy surfaces, local passivation, and unnitrided zones, too, after gas nitriding. In the literature, there are some researches which the cleaning chemicals have effects on nitriding. In the ınvestigations of Surtec Company, the effects of two commercial cleaning chemicals were searched, and different effects on gas nitriding were observed. The thicknesses of diffusion and compound layers of those parts were measured. The fact that, cleaning chemical named Surtec 042 increasing compound layer thickness, cleaning the surface well, hence activating the surface, though, the cleaning chemical named Surtec 101 decreasing compound layer thickness has been reported. Within the scope of this thesis, along with aforementioned cleaning chemical Surtec 042, the other surface cleaning chemicals which are used in RBTR have been investigated, and earthly effects to the gas nitriding of the cleaning chemicals which were used before gas nitriding process. With the aid of obtained consequences, it has been aimed to build a database for the factory. Moreover, the effect of nitriding duration has been searched within this thesis. The samples cleaned with various cleaning chemicals have been subjected to gas nitriding during 1 hour, 3 hour, and 5 hour, consequently, both the effect of cleaning chemicals and the duration are tried to be observed. In this thesis, a promising material, 50CrMo4 (DIN 1.2344) that is being planned to be used in the bodies of new generation common rail injectors has been used. The real CR injector bodies were not chosen to be used in order to reduce the cost and provide easiness which is about the difficulty of analysis. Gas nitriding process was made at the temperature of 520 oC and the nitriding potential (KN) of 2 for the whole samples. After nitriding process, the thicknesses of diffusion and compound layers along with the hardness measurements were realized. Therefore, it is to be aimed of seeing the effects of different cleaning chemicals on nitriding. Samples were cleaned with different cleaning chemicals, though, at the same conditions (65 oC, 10 min.) at the trials which were performed in order to investigate whether the cleaning chemicals have effects on nitriding process or not. The hardnesses of the samples after nitriding were measured with micro hardness device, the diffusion layer and compound layer thicknesses were taken with optical microscope. Besides that, designating of which phases formed at nitrided samples was settled with XRD, and microstructures of nitrided cases were analyzed with scanning electron microscope. SEM-EDS was done so as to determine the composition of the elements on the surface after nitriding process. As a result of conducted studies, it has been seen that cleaning chemicals gave approximate values about diffusion layers' thicknesses and hardness values. Furthermore, the fact that only Surtec 042 being lead to a thicker compound layer in comparison with the other cleaning chemicals, yet, another cleaning chemical named Gardoclean T5441 being give rise to just an exact opposite effect were revealed. That diference could be interpreted as of whether the surface cleaned more active or passive. With increasing duration of nitriding process except for the cleaning chemicals, the case of directly proportional relationship at the hardness values and the thickness of both compound and diffusion layers of the samples were put forth. Although, Surtec 042 yielded the thickest compound layer, because of the consequences indistinguishable from each other, the other cleaning chemicals are said not to give rise to a negative effect on nitriding, except for Gc T5441.