Combined Micro-Hardness and Eddy Currents Applied to the Study of Steel Decarburizing

Abstract

Carbon is the most important element going in the steels composition since it largely contributes to their mechanical performance. During the heat treatment of steels, a part of atoms of carbon could be removed from the superficial zone of the steel, which is the result of the atmospheric oxygen attraction by forming gaseous carbon monoxide. This phenomenon, called decarburizing, can change microstructure in a large extent and, as a result, mechanical properties of the steel. This could lead to unsuitable properties for the service life of mechanical parts. In order to prevent such part from breaking in service, it is necessary to know the affected zone by the decarburizing process. For this purpose, the most used technique is a visual observation by optical microscopy of a cross section of the material. This technique is often associated to the hardness profile obtained by Vickers or Knoop indentations in the same section. Depending on the material and on the specimen preparation, some discrepancies are often observed between the two informations. On the other hand, these techniques require a long preparation and cautious interpretation. Then we propose here to use eddy current testing, which is widely employed to detect fatigue cracks, to give some information about changes in microstructure and to examine carburizing of steel from both mechanical and microstructural point of view. For carburizing, eddy current has been successfully connected to the increase of micro-hardness near the surface. This last result has particularly motivated us to use eddy current to analyze decarburizing. In the present work, different durations in the furnace at a temperature of 920°C before oil quenching were performed on 54SiCrV6 (SAE 92V45) steel in order to obtain various morphologies of the decarburized zone. We show that eddy current test may be used to qualify the level of decarburizing since, after a convenient Fourier transformation, three components of the amplitude of the signal are directly linked to the duration of the heat treatment. Associated to optical and hardness measurements, it is also shown that the eddy current signal is well related to the decarburizing depth.

A diffusion driven carburisation combined with a multiaxial continuum creep model to predict random multiple cracking in engineering alloys

“…Figure 6) showing estimated failure strain, crack growth and carburisation depth versus predicted rupture times (see Figure 10b). Figure 1: carburised examples of a) a high Cr steel [7] showing compacted carburised grains and b) Surface and sub-surface microstructural intergranular damage and cracking due to carburisation for 316H steel AGR surveillance samples [14] a) b)…”

“…For metals, different degradation processes [6][7][8][9][10][11][12][13] can result in a time growth relationship that is linear, cubic, parabolic or logarithmic. Only the parabolic case, being the most realistic for engineering components, is considered in this paper.…”

“…Mechanisms such as creep cracking [1][2][3][4][5] and oxidation/carburisation/nitriding [4][5][6][7][8][9][10][11][12][13] have in common a 'rate dependent' damage inducing component. It has been shown that three main categories of damage rate dependencies exist in the oxidation processes.…”

“…The extreme case is linear or accelerating rates of empirical laws damage which can occur at extreme temperatures and environments. The second and third can be described by logarithmic and parabolic relationship [6][7][8][9][10][11][12][13] which essentially describe the rate of damage as continually decreasing with time. In some cases the process can deplete or harden the surface and/or can develop a self-healing/protection mechanism, depending on the type of alloy, environment and temperature and oxidation mode which prevents further surface damage.…”

“…The oxidation damage rate processes can be modelled using oxidation rate processes [4][5] as well as fundamental diffusion or oxidation laws which describe and quantify the rate of damage for different materials [6][7][8][9][10][11][12][13]. When oxidation/carburisation is dominant at the surface the degradation could harden the surface layer which could in turn affect creep damage development.…”