FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 2 5 8 in nitrogen to an estimated 18 at.% in its peak value at a depth of approximately 0.33 to 0.5 µm. It stays at a level of about 8 to 9 % through the 12 µm of the tested thickness of the case depth. It should also be noted that the oxygen level near surface exceeds 20 at.% and diminishes rapidly in the first 0.5 µm. Elevated carbon and hydrogen could also be detected in the first 0.5 µm of the nitrided layer. Presence of oxygen in the near-surface area of the 17-4 PH steel is nothing new. This steel in the un-nitrided condition adsorbs significant amount of oxygen on the surface and this is typical for all other types of stainless steels in the passive condition[5,6]. However, because oxygen is not accurately determined via GDS, its content at the surface has to be treated as of qualitative nature only. More accurate readings are at deeper locations. Oxygen concentration in the un-nitrided sample showed a similar trend. This data should also be treated as of the qualitative nature only. What is interesting here is that the presence of oxygen in the steel in these experiments reaches more significant depth than in the naturally passivated steel, suggesting that this gas is still present in the chamber when the sample achieves final temperature. Therefore, nitrogen and oxygen as well as carbon and hydrogen diffuse into the steel together at the same time although maximum ultimate depth is reached only by nitrogen. KINETICS 3D graphs of case depth as a function of both time and temperature were created. Figure 6 illustrates the complex character of the relationship. However, when temperature is treated as constant, the relationship has a parabolic character. When time is treated as a constant, the relationship has an exponential character, as clearly seen in Fig. 6. CONCLUSIONS These studies demonstrated how to overcome the main issues related to obtaining a nitride layer of a specific depth in 17-4 PH stainless steel using the plasma method. Knowing the identified relations will lead to higher reproducibility in future runs of nitride 17-4 PH components. This study also lays the groundwork to examine other processing parameters such as pressure and gas composition. To further understand the effectiveness of nitride 17-4 PH in field applications, studies into both corrosion resistance and wear could be investigated[11,13]. A study into how different alloys perform under time vs. temperature studies could lead to a better understanding of alloying elements and their effects on nitriding results. Processing has to be done in the vacuum system with exceptionally good leak rate and well controlled temperature of the treated objects. ~HTPro Fig. 4 —Young’s modulus (green line), force penetration depth (blue) and nano hardness (purple) of the near-surface area of the 17-4 PH sample, ion nitrided at 513°C for 15 hrs. X scale represents distance from the surface in nm[12]. Fig. 5 —GDS analysis of the near-surface area of the 17-4 PH sample ion nitrided at 513°C for 15 hours, showing nitrogen, oxygen, and carbon. Fig. 6 —Total case depth in the 17-4 PH samples nitrided at various temperatures and various length of time in the two different views. Case determined “as etched with Marbles” depth. Coefficient of coordination r2 = 0.993. The fit equation is: lnz = –1.4321+0.5251·lnx-724691/y2, (mm). 8
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