In manufacturing processes involving diffusion (of C, N, S, etc.), the evolution of the layer depth is of the utmost importance: the success of the entire process depends on this parameter. Currently, nitriding is typically either calibrated using a “post process” method or controlled via indirect measurements (H2, O2, H2O + CO2). In the absence of “in situ” monitoring, any variation in the process parameters (gas concentration, temperature, steel composition, distance between sensors and furnace chamber) can cause expensive process inefficiency or failure. Indirect measurements can prevent process failure, but uncertainties and complications may arise in the relationship between the measured parameters and the actual diffusion process. In this paper, a method based on noise and fluctuation measurements is proposed that offers direct control of the layer depth evolution because the parameters of interest are measured in direct contact with the nitrided steel (represented by the active electrode). The paper addresses two related sets of experiments. The first set of experiments consisted of laboratory tests on nitrided samples using Barkhausen noise and yielded a linear relationship between the frequency exponent in the Hooge equation and the nitriding time. For the second set, a specific sensor based on conductivity noise (at the nitriding temperature) was built for shop-floor experiments. Although two different types of noise were measured in these two sets of experiments, the use of the frequency exponent to monitor the process evolution remained valid.
The paper opens a new direction in the research field of the diffusion process by proposing as a measurement system the low-frequency electronic noise -named the Barkhausen method. Experimental research performed has confirmed that measurement of electronic noise can be sensitive enough to reveal the structural modification as a result of nitrogen diffusion in the superficial layers. The above statement can be confirmed only through microstructure analysis (SEM) of diffusion layers obtained at different times of nitriding and through XRD tests that illustrate the growth of the nitrogen and iron compounds in the same layers. With these values it is possible to confirm whether there is correspondence between Barkhausen noise diagrams and diagrams of superficial layer modifications. The authors present their experimental attempts, in an attempt to verify that Barkhausen noise measurement is a non-destructive control method with viability and future in any type of diffusion layer investigation.
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