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 6 The nitriding of stainless steels is a valuable process, as when it is done correctly, it can greatly increase both the surfacemechanical properties as well as corrosion resistance. However, the nitriding of stainless steels requires special care due to the problems relating to activation of the surface of the components when conventional methods such as gas and salt bath nitriding are used[1-11]. Hydrogen chloride admixture is often used as a surface activator[10] in these atmospheres. This may have side effects on the equipment. Therefore, in many situations the preferred method for nitriding stainless steels (SS) is plasma nitriding. Themost common questions asked by engineers about nitriding are those related to kinetics of the process and properties of the layers. Plasma nitriding, also referred to as ion nitriding, is very efficient in eliminating native chromium oxides present on the SS surface. However, kinetics of the process must be under control to achieve the desired results. To demonstrate some of those challenges, a series of ion nitriding tests was performed using sandblasted samples of 17-4 PH steel. The samples were treated at a temperature range of 482° to 557°C (900° to 1034°F) with a nitriding time up to 132 hours and constant gas composition at constant processing pressure[1]. Changes in the core hardness of the nitrided samples were analyzed. The results show that microhardness stayed at a high level PRACTICAL ASPECTS OF PLASMA NITRIDING KINETICS FOR 17-4 PH STAINLESS STEEL Plasma nitriding is very effective in removing the passive layer of chromium oxide formed naturally on the surface of stainless steel, but controlling the kinetics is key for optimal results. E. Rolinski,* A. Springer,* and M. Woods Advanced Heat Treat Corp., Monroe, Michigan in the nitride layer and then fell abruptly at the interface layer/substrate[1]. Nano-hardness measurements were performed on the treated samples and the hardness, as well as the Young’s modulus of the nitrided layer were determined[12]. Glow discharge spectroscopy was performed on the near-surface regions of the sample and an elevated level of oxygen was found in the distance close to the surface. Case depth was determined viametallographic examination, using Marble’s and Nital etching, as well as via microhardness curve determination. A three-dimensional relationship was developed for the mathematical prediction of the case depth dependent on processing time and temperature based off of this data. EXPERIMENT The 17-4 PH 1.25-in. diameter bars were aged at 593°C (1100°F) at a general heat-treating facility. Incoming hardness of the steel was 371 HV1, determined by an average *Member of ASM International 7 (a) Fig. 1 —Hardness profile in the ion nitrided 17-4 PH sample. Nitriding parameters: temperature 513°C and soak time 15 hrs. Total case depth was 0.0735 mm and the etched zone measured 0.063 mm. The following curve fit equation was used: [LgstcDoseRsp] y=a+b/(1+x/c)d, r2 = 0.998, confidence interval = 95%. Fig. 2 —View of an individual indentation after slight etching from the traverse of Fig. 1. 6
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