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 7 of five readings. Oxides were present on the surface of the samples after the treatment. Therefore, all the samples were sand blasted 5minutes before each nitriding test. The plasma nitriding was carried out in a cold-wall vessel with a very good leak rate. Plasma density was in the range of 2 W/cm2. The typical leak rate was found to be: 0.52 torr·in3/ sec or 0.000012 (1.2 x 10-5) mbar·L/s. The processing pressure was arbitrarily selected at 3.2 mbar (2.4 torr). A gas concentration of 10% nitrogen and 90% hydrogen was used to avoid formation of the excessive compound zone or carbonitrides network. Ramping rates were the same for all samples, about 27°C/hr for the last ramp, prior to soaking. Processing temperatures were selected to allow covering the full range of interest for the case depth and for the temperature range typically applied for processing various industrial parts. The aim was to quickly calculate the required processing time for the specified case depth and temperature using interpolation rather than extrapolation method for future runs. CASE DEPTH DETERMINATION The case depth was initially analytically determined from the microhardness traverse with the TableCurve2D software using the common definition of the total case depth as the core hardness + 50 HK0.05. A typical hardness curve using the above method is shown in Fig. 1. It is also worth mentioning that the core hardness of the sample nitrided at 557°C dropped from the initial 371 HV1 to 310 HV1. This demonstrates that severe over-aging processes took place in the steel during this short exposure of the sample at such a high nitriding temperature. Based on the spread of the core hardness data, it cannot be excluded that the samples were not from the same heat-treating batch. This however, has a secondary meaning to the total case depth formation in the steel. As can be seen from the microhardness curve, it is extremely difficult to precisely measure hardness in the transition zone between the case and the matrix because of the abrupt change of the steel properties in this zone (Fig. 2). Therefore, measurements of the case depth in this steel may be more accurate using optical methods. For this study, it was possible and appropriate to measure the case in the samples etched with Marbles and Nital reagents, as shown in Fig. 3. NANO HARDNESS MEASUREMENTS Due to customer interest in mechanical properties, such as Young’s modulus and hardness of the nitrided layer in 17-4 PH steel, nanoindentation and GDS studies were performed at an outside source[12]. These tests were performed on a flat sample prepared in a special way by polishing prior to nitriding for the purpose of the test. The results of this study are presented in Fig. 4. As can be seen from the graphs, Young’s modulus, as well as the nano hardness of the layer, is lower near the surface of the sample than at the depth of 240 nm (0.24 µm), where these values seem to stabilize. Nano hardness in the plateau is about 18,000 MPa, which is equivalent to about 1835 HV. This value is quite high; however, it has to be treated as the true hardness value, more precisely determined than the Knoop value of about 1200 to 1300 HK0.1; a typical result of microhardness testing, which usually is not free of cracking at such high hardness level. GLOW DISCHARGE SPECTROSCOPY (GDS) MEASUREMENTS As can be seen from the distribution of the elements in the near-surface region in Fig. 5, the sample is enriched 8 Fig. 3 —Photomicrograph of the case formed in the 17-4 PH sample. Nitriding parameters: temperature 513°C and soak time 15 hrs. Observed at 400x, etched with a) Marble’s b) and Nital. Note a sharp drop of hardness at the transition zone (the case/ matrix). 7
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