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Testing of Hardening Parameters of Metal Products undex Plastic Deformation by Barkhausen's Effect MethodV.Shaternikov, V.Filinov, N.Averin
Moscow State Academy of Instruments Engineering and Computer Science (Moscow).
The figure 1 shows the block diagram of the device. It includes a remagnetizing unit (RU), a primary transducer (PT), a measuring unit (MU). The remagnetizing unit is used for the primary transducer feeding by the linear changing current with period T and for the forming of the strobe pulse with the Q duration. Triangular from current generator of RU consists of integrator I, Shmid's flip-flop 2, power amplifier 3, embraced by the current back negative connection from a resistor R. Remagnetizing current frequency is regulated by period changing in the integrator I. The circuited of RU strobe pulse formation includes a threshold element 4 and a single shot multivibrator 5, forming a strobe pulse.
The stress changing V at the second input of the threshold element 4 makes possible to vary a strobe pulse temporal position t on the testing product hysteresis loop.
The operation of MU includes the averaging current EMP signal accordingly from primary transducer 12 and piezotransducer 13, amplifiers 6 and 11. This process occurs in the period duration, determined by strobe pulse multivibrator 5. The choice of the measuring parameter is performed by means of the switch "S". In the first position of the switch measuring channel of EMP outbursts of Barkhausen's jumps turns on. This channel includes the threshold element 7 and analogue key 8. The threshold element 7 transforms EMF of BJ, excessing the C selection level (C is given by the V stress) into impulse sequence of equal value93 and the key 8 opens in the period of strobe pulse operation of multivibrator 5. Changing of temporary position t in detection of bending current meanings of EMF outbursts of BJ or AN takes place during the remagnatizing period. In the second position of the switch the block 9 for measuring EMF intensity from BJ or AN turns on. In this block a signal detects, averages, transforms into the digital form and reflects on the indicator 10. During the measuring average remagnetizing characteristics EMF from BJ or AN the strobe pulse duration equals the half cycle of remagnetizing.
The possibility of wide range remagnetizing measurement of a testing product taking into account parameters of frequency or current remagnetizing amplitude is an advantage of this device. In the combination with regulated C selection level this advantage anables to use the device in the preliminary researches and on testing stages. The device is equipped by a set of passing and strapping transducers.
There are two stages in the method of using this device. Experimental researches are beld at the first stage. The analyze of correlation relationships of EMF characteristics from BJ or AN in different regimes of remagnetizing and measurement with the control of product is the aim of these researches. The calibration diagram defining the scale of the device is developed at this stage. At the second stage the device readings are putting in correspondence with the testing product parameters. High alloyed steels of marten site structure hardened by shot blasting and diamond grinding are the object of these researches. The main hardening factors are the following: the air pressure in the pistol - "P", the period of hardening - "T", the shot blasting diameter - "D".
The distribution of the remagnetizing period T of arrange EMF from BJ e(tj) current meaning of EMF out bursts of N(c,t) BJ in the C selection level with changing factors of shot blasting are presented in the figure 2, 3. It's evident that e(t) changes ambignitily: it decreases in low hardening degrees and increases in high ones. It should be explained by the fact that at the beginning of hardening the increased defects of crystal structure and the residual stresses influence upon the readings of EMF from BJ. The maximum meaning of residual stresses, the influence of structural - phrasal changes, connected with the milling of the initial structure increase with higher degree of hardening and at deeper level of penetration in to the metal. The carbides released in this process concentrate in the surface layer. The mentioned factors of deformation influence on EMF from BJ differently and at high degree of deformation they change the curves at fig.2
At the same time the number of EMF outbursts from BJ is determined by deformation degree.
The average number of signal outbursts increases in spite of the falling of intensity of BMP from BJ. It confirms the process of initial structure milling of the metal and joined with is the process of domain texture milling in large degrees of hardening .
The complex character of informative EMF parameters from BJ changes does not always allow to use average meanings for testing during the remagnetizing period. The increasing of reliability during testing can be achieved by using current EMF from BJ e ( t j ) meanings and the number of outbursts N(C, t j ) in the selection level. So on the curves of figures 2,3 the sections of t j , gating can be distinguished, at which the monotonous dependence of e ( t j ) and N(C, t j ) from the degree of shot blasting hardening is observed.
Inner loading diagram analysis in SPD developed by X-ray structural analysis method and by measuring residual stresses with the help of diffractometer showed that the value of shot blasting degree from high-alloy steels should be estimated by the maximum reading of G max"
|Fig 3: Distribution of EMF outbursts of BJ N(C, t j ) for different shot blasting regimes alone the remagnetizing period||Fig 4: stresses G max : I-tj/Tr= 0.45; 2-tj/Tr=0.5;3-tj/Tr= 0.85|
The using of current meanings of informative parameters, measured on the hysteresis loop t j, allows to get the dependence with different slope and linearity. Analysing the dependences of figure 4 it is evident that curve 3 greater sensitivity tog max , correspondent to the "limit" or-realization EMF from BJ, i.e. the gating section tj, remoted from the coercitivity force of product material. At the same time curve 2 has greater nearity.
The high-alloy steels like Fe-Ni-W-Co having a necessary complex of strength and plastic properties are widely used in machine-building. Steel hardening is carried on by martensite quenching with the following aging. High level of inner stresses in combination with technological stresses can lend to port destruction and assemblies from this steel [f2].
The calibration dependences of effective EMF meanings from BJ,Vmn and remagnetizing acoustic noizes from stresses , Van , were developed according to the results of mechanical tests. These dependences were used for scale calibration of AFS device. The dependences of remagnetizing current I p of a coercimeter KIFM, meanings' of BEM Vmn and V an from are given at the figure 5
|Vmn,Van, condit values|
Fig 5: Effective stress dependence of magnetizing Vmn and acoustic Van noizes remagnetizing current of a coercimeter from applied stresses
|Fig 6: Dependence of stress magnetizing noizes Vmn from stresses G during tine of rings|
These meanings were received during a deffinite period of heating
The dependence Vmn from G is given at figure 6.
This dependence was determined during the testing of steel rings by beating with the same period of duration. The difference between the results of measuring by AFS device was not more than +-6%.
The fig.5 ahows that the dependence V from G is linear up to 0,6-0,7 8 p G0,2 changes into nonlinear. This fact proves the sensitivity of BEM to stable micro-deformational processes in steel, taking place in the region of elastic stresses before changing into plastic region. The dependence of Van from G shown at fig.6, is linear up to G =0,3 G 0,2 sensitivity to stresses of remagnetizing acoustic noizes Van and coercivity force Ip is lower than magnetizing noizes Vmn.
The results of researches, on stress connections in steel and the BEM parameters were used in the technology development for 100% testing of shell stress conditions in production process.The shell destruction were observed in their production and storage. The high level of inner stresses is the reason of these destructions. The existence of regions with stress change' along the forming shell and also the danger combination of inner and applied stresses during shell assembly and production are considered to be the reasons of destruction process. The regions with high level of tension were determined by scanning with the help of an induction transducer along the forming shell with the maximal readings of AFS device. The following shell testing in 20% sulfuric acid solution shows the crack formation in these regions, when the level of inner stresses was more than critical.
The typical distribution of V signals along a forming cylinder billet, produced by cold rolling with the following dressing is presented at fig.7.
|Fig 7: Distribution of magnetizing vmn and acoustic van noizes during magnetizing along a billet|
It is stated that the deformation of the surface layer during dressing is the reason of inner stress irregularity in a billet and also in the device AFS V. To avoid the inner stress irregularity the mechanical of a billet should be carried at the depth not more than 2 Omm. The parameter V didn't change practically along the billet length. Soit can be used for determination of the average stress level in the billet.
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