·Table of Contents ·Materials Characterization and testing | Evaluation of Fatigue State of Ferromagnetic Steels by Magnetic MethodsS. Chifan, R.Grimberg, A. Savin, A. Andreescu, R. SteigmannNational Institute of R&D for Technical Physics, 47 Mangeron Blvd, 6600, Iasi, ROMANIA V. Palihovici Technical University "Gh. Asachi", 61-63 D. Mangeron, 6600, Iasi, ROMANIA Contact |
Fig 1: Microstructure of SAE 4130 steel without fatigue(x100) | Fig 2: Installation for measuring the incremental magnetic permeability |
The sample was subjected to the simultaneous action of two magnetic fields: a triangular one with the frequency f=0.1 Hz created by a U shaped Fe-Si magnetizing yoke which ensures the sample remagnetization according to the major loops, and a sinusoidal field with the frequency ranging between 1 kHz and 10 kHz inducing minor reversible loops superimposed on every point of the major hysteresis loop. The incremental permeability represents the slope of the minor loop resulting from reversible phenomena (figure 3).
Fig 3: The basic principle used to determine the coercive force by the incremental magnetic permeability method | Fig 4: The correlation between the coercive force determined by hysteresis loop method and the one estimated via incremental magnetic permeability |
Fig 5: The saturation induction and coercive force vs. the number of fatigue cycles for150 MPa and 135 MPa loads |
As a general tendency, one notes for both loads a decrease in saturation induction and an increase in coercive force with the increase in degree of fatigue characterized by the number of cycles.
The saturation magnetic induction and the coercive force of the samples were the same after 0.5x10^{6} cycles at 150 MPa load and 1.5´10^{6} cycles at 135 MPa load. The size of the mosaic blocks became about 200 Å, which shows an increase in the number of dislocations, and the 2^{nd} and 3^{rd} order microstrains became 0.38´10 ^{-3}. The saturation induction decrease due to carbon loss in the ferrite grains caused the decrease of specific magnetization of ferrite grains and implicitly of the whole sample. The ferrite concentration increased to 15.2% (figure 6).
Fig 6: The microstructure of SAE 4130 steel after 0.5´10^{6}cycles (150 MPa) and after 1.5´10^{6} cycles (135 MPa), (´100) | Fig 7: The microstructure of SAE 4130 steel after 0.5´ 10^{6} cycles (150 MPa) and after 1.5´10^{6} cycles (135 MPa), (´600). The carbon appearance on the ferrite grains can be noticed |
The increase of the coercive force results from the occurrence of carbon on the inside of the ferrite grains, as can be seen in figure 7.
After 10^{6} cycles at 150 MPa load and 2.5´ 10^{6} cycles at 135 MPa load the crack array characteristic to fatigue appears. The mosaic block are about 670 Å while the 2^{nd} and the 3^{rd} order microstrains are virtually zero. The ferrite concentration increases to 20.4% (figure 8).
Fig 8: The microstructure of SAE 4130 steel after 10^{6} cycles at 150 MPa load and 2.5´10^{6}cycles at 135 MPa load, (´100) | Fig 9: The microstructure of SAE 4130 steel after 10^{6} cycles (150 MPa) and after 2.5´10^{6} cycles (135 MPa), (´600). The amplification of carbon diffusion can be noticed. |
The absence of microstrains and, by implication, internal stresses is due to their relaxation as the result of the increasing number of cycles. Following the carbon diffusion into the boundary of ferrite grains, the saturation induction decreases and the coercive force increase (figure 9).
When the fatigue microcrack arrays appear, the saturation magnetization and the coercive force of the samples subjected to the two loads are practically the same, which suggests that these magnetic parameters can be used to predict the fatigue state.
The coercive force was determined by the nondestructive method of incremental magnetic permeability which can therefore be used as a method for fatigue state prediction.
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