Using Noise and Fluctuations for In Situ Measurements of Nitrogen Diffusion Depth

Abstract

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.

Barkhausen Noise Emission in Hard-Milled Surfaces

“…Magnetic Barkhausen noise (MBN) is a promising non-destructive technique that could be potentially adapted for fast and reliable surface monitoring. An increasing number of studies can now be found in which MBN is investigated as a function stress state [ 1 , 2 , 3 , 4 ], dislocation density [ 5 , 6 ], carbides or nitrides size and distribution [ 7 , 8 ], presence of non-ferromagnetic phases [ 9 ], etc. However, this technique is most widely employed for monitoring ground surfaces in real industrial applications.…”

A Novel Approach to Eliminate the Effect of External Stress on Interdiffusivity Measurement

“…The diffusion couple (or diffusion multiple) technique is the most frequently used method to determine the interdiffusion coefficients and/or kinetic coefficients in the gas, liquid and solid states [ 16 , 17 , 18 , 19 ]. Literature surveys [ 20 , 21 , 22 , 23 ] indicate that most of the related publications available in the literature focus on the measurement of interdiffusivities in a single phase of the binary or ternary alloys with the aid of several reliable calculation methods (i.e., the Boltzmann-Matano method for binary systems [ 24 ], or the newly developed pragmatic numerical inverse method for binary, ternary and multicomponent systems [ 25 , 26 ]) together with the scientific error propagation method [ 27 , 28 ]. It should be noted that the so-called pragmatic numerical inverse method was augmented by fully considering the variations of interdiffusion flux into the calculation procedures [ 26 ].…”