Test and study of electric spark carbon deposits inside cracks during line cutting sampling Zheng Jianjun, Tian Xiangju, Hao Jianying, Dang Shuzhen (School of Materials, Taiyuan University of Science and Technology, Taiyuan 030024) The analysis of the objects was carried out. It is considered that these black coverings are pulsed currents during discharge, and the oily liquid medium fission products (mainly carbon black) are coated with carbon deposits formed inside the cracks.
When a batch of cracked samples was subjected to failure analysis, it was found that there was a large amount of black covering in the crack of the sample after the warp cutting sample, which caused a lot of inconvenience in the analysis of the crack surface. The sample material is 35 steel, and the crack appears at the intersection of the thick and thin sections of the member. Although there is a rounded transition here, after quenching and tempering, a certain number of products are cracked, and some cracks are penetrated.
1 Inspection Macroscopic inspection of the cut sample, along the crack direction can see a number of white spots due to spark discharge (see). Scanning electron microscopy of the crack surface revealed that there were more black coverings on the crack surface (see). These black coverings were similar in appearance to other places, but the edges were smoother than others. The black covering was examined by X-ray energy spectrometer and confirmed to be a large amount of carbon (see). Its composition (mass fraction) is shown in Table 1. Crack macro photographs Table 1 Chemical composition of black covering 2 Results and discussion From the above experimental results It can be seen that the black cover on the crack surface is carbon black. The lubricant used in the spark cutting process is the EDS spectral liquid medium of the black cover in the oil crack fracture energy of the five/keV SEM image, and the oily liquid lubricant medium easily penetrates directly into the crack and remains.
Electric discharge machining is the use of a pulse spark discharge between two electrodes to generate a large amount of thermal energy, melting, evaporating and throwing electrode materials to achieve the purpose of processing. Due to the small area of ​​the discharge point, the energy density can reach 105 ~ 107W / mm2, and the discharge energy is mostly absorbed by the workpiece electrode. 111. The high discharge energy accumulated instantaneously causes partial discharge at the crack of the sample, so that the oily liquid medium in the crack Fission occurs, and the product of fission is mainly carbon black.
The carbon black formation mechanism is as follows. As can be seen from 2, as the temperature increases, the moisture in the residual emulsion begins to evaporate (about 100 ° C, point A) and the hydrogen dew point (the temperature at which the hydrogen reaches saturation) rises. At the beginning of cooling, the dew point drops to -60*Q and then the residual emulsion begins to decompose (point B in the middle, about 300 is as follows: the hydrocarbon first reacts with oxygen in the water vapor, between the narrow slits of the crack Forming C0. As the temperature increases, the long chain begins to crack, and the atomic group is formed. See the reaction formula (2): the hydrogen molecules are easily diffused between the narrow slits of the crack, and the open ends of these atoms are in a hydrogen saturation state, forming a volatile Light hydrocarbons (some of the carbon remains on the crack surface, and at around 700 *C, hydrogen reacts directly with carbon deposits to form methane.
Due to the high energy of the instantaneous accumulation during the online cutting process, the overall temperature field is still low under the action of the cooling medium. Therefore, only the reactions of the formula (1) and the formula (2) occur, and hydrogen does not directly react with the carbon deposit to form methane.
The relationship between the mass of the carbon layer and the energy allocated to the electrode between the pole and the workpiece during discharge is 111: quantity, g * proportional coefficient, energy generated by a g/J PT single pulse discharge to form a stable discharge channel and transmitted to the medium and the workpiece, J is known from equation (3), the mass Wt of the carbon deposit layer is proportional to the energy Pt transmitted to the medium and the workpiece when a single pulse discharge forms a stable discharge channel, and Pt is large with a pulse duration of 111, which can also be said. The mass Wt of the carbon layer is proportional to the pulse duration.
3 Conclusion From the above analysis and discussion, it can be seen that when using wire cutting sampling, the cooling medium will continuously penetrate into the through cracks, and these cooling media will fission to form carbon deposits during the discharge of the pulse current.
The purpose of this study is to understand the carbon deposition phenomenon when using wire cutting processing, and to uncover the illusion caused by carbon deposition, so that it can analyze and judge timely and accurately when performing failure analysis of products or parts. Not bewildered by some illusions.
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