PENETRANT TESTING: ABILITY EXTENSION AND SENSITIVITY EVALUATION

Transcription

1 PENETRANT TESTING: ABILITY EXTENSION AND SENSITIVITY EVALUATION Migoun N. P. Institute of Applied Physics of National Academy of Sciences of Belarus, Minsk, Belarus Introduction. Several ways of the increasing of penetrant testing (PT) efficiency and ability extension are described in the paper. The first one is the application of testing object heating at various stages of PT process. One more effective way to improve PT process can be realized by the optimization of various parameters of the process for prescribed product family with the use of image processing system. The third possibility to extend PT possibilities is a working out of special technology and means for revealing defects at the surfaces with comparatively high roughness. Practically important problem is also a development of suitable test panels for the evaluation of PT materials sensitivity level. Equipment. New test panels with several cracks of various dimensions at the same surface (fig.1) were developed recently at the Institute of Applied Physics (IAPh) and applied both for the evaluation of sensitivity level of penetrants and for researches described in the paper [1]. Fig.1. Test panels (B 1, B 2 and B 3 ) with different cracks openness: B 1 depth l 01 l 02 = µm, openness h 1 = 1,5 µm, h 2 = 2,5 µm; B 2 - l µm, l µm, l µm, h 1, h 2, h 3 1,5 µm; B 3 l 01, l 02, l µm, h 1, h 1, h 12 = 1-11 µm. There are two principal advantages of new technology test panels production. Firstly, it gives a possibility to produce the cracks with approximately similar depth but substantially different openness (fig.1, panel B 1 ). On the other hand, it is possible to obtain the cracks of different depths with approximately the same openness (fig.1, panel B 2 ). Secondly, the cracks in our test panels are formed at a test surface in such a way that they don t touch a lateral panel s face. The widely used test panels corresponding to EN ISO have lateral surface with a U-type form cracks outlet. As a result at penetrant s application stage during the procedure of the determination of product family sensitivity one may obtain overestimated result. Our test panels are free of such a deficiency. More than 20 years ago the image processing system (IPS) with corresponding software was proposed for the first time in BAM [2] for quantitative evaluation of PT results. The IPS represents an effective instrument for quantitative analysis of defects indications. As the device for recording images of test surface, the IPS allows detecting and automatically recording defect s indications of small size and poor contrast. The important advantage of such system is the opportunity of carrying out a fast and reliable quantitative assessment of product families quality. But the software used in [2] didn t provide adequate results of PT evaluation for some practically important cases. Later principally upgraded software based on another algorithms was developed in IAPh and successfully applied with the IPS by two teams from IAPh and BAM for quantitative evaluation of PT results [1,3-10]. During the last 10 years our software allows performing a reliable quantitative evaluation of defect s indications by its optical and geometrical characteristics. Simple coefficients for a complex evaluation of geometrical and optical characteristics of defect s indications revealed on the test surface were proposed and used in our investigations. The criterion V of a quantitative estimation of visibility of defects indications on the basis of their geometrical and optical parameters is proposed. It allows decreasing the subjective factor and increasing an efficiency of the estimation of penetrant materials sensitivity. The criterion V includes both a brightness and quantity of all pixels belonging to discontinuity indications and a brightness of surrounding background. Special algorithm for the threshold processing, which takes into

2 account an image sensing at a different contrast between visible indication pixels and the background was worked out and effectively used later in our researches. Fig.2 illustrates a workability of our software for typical images if discontinuities indications in the cases of their low values of the brightness and the contrast comparing with corresponding background characteristics. a b Fig. 2. Threshold processing of the indications in the cases of their low brightness and contrast comparing with a background: a - background and indication brightness are correspondingly 93 and 116; b - background and indication brightness are correspondingly 36 and 63. Basic results. The increasing of PT efficiency by testing surface warming can be achieved both before penetrant application stage and during development stage. Maximal effect we get using the heating for both PT process stages. The essence of PT sensitivity increasing by the warming of testing surface before a penetrant application consists in the following. The pressure of a gas (an air, as a rule) locked in defects cavities is one of the major factors limiting defects filling by penetrant. Preliminary heating of a test object before penetrant application to the temperature exceeding initial by T 2 = C (depending on demanded method sensitivity and test object temperature stability) leads to the reduction of air quantity in available surface defects in the object. At the subsequent penetrant application and cooling to initial temperature it causes the reduction of closed air pressure, allowing penetrant to fill defects to greater depth. Thus at the subsequent development process defects with smaller width will be revealed. Sensitivity increasing by the heating a test object during development process takes place due to the fact that a pressure of an air locked in defect s cavity is increasing that leads to larger and faster penetrant displacement from a defect and as a result the sensitivity and the productivity are increasing as well. The optimal results can be reached using combined heating of tested objects both at the stage of penetrant application and at the stage of defects development. Using the hydrodynamic model of defects development [11] we got the formulas describing dependence of PT sensitivity on the temperature of preliminary heating [8]. The efficiency of increasing the sensitivity and productivity of penetrant testing based on the using of thermal effects upon tested objects over various stages of penetrant testing was described in this paper. а b c Fig.3. Defects indications revealed by fluorescent PT using the heating at several technological stages: a conventional testing procedure (without heating): T 1 = 21 0 C; b - preliminary heating of test panel before penetrant application: T 2 = 49 0 C; c - combined heating of the panel at stages of penetrant and developer application: T 2 = 49 0 C, T 3 = 45 0 C. Defects openness: 6, 5, 3, 2, 1 µm (from left to right).

3 Fig.3 illustrates experimental result of fluorescent PT in the case of consequent heating of the test panel at stages of penetrant (to T 2 ) and developer application (to T 3 ). The analysis of obtained theoretical and experimental data shows that heating of tested surface at the stages of penetrant and developer application provides revealing smaller-size defects, which are not revealed using conventional testing procedure of testing (fig.3), that increase method sensitivity. Besides the duration of development process is significantly reduced, that positively affects the method productivity as well. The heating during development process possesses one more useful ability: at low ambient temperatures it also decreases the duration of development stage by several times and, moreover, provides substantially brighter and contrast indications. In wintertime at low air temperature duration of drying of a suspension developer layer applied on a test surface, often exceeds tens of minutes. It strongly reduces the productivity of the test that can be the critical factor when an effort is substantial. We studied the influence of ambient temperature upon developing characteristics and the effect of warm airflow upon defects detectability [5,6,9]. Several types of modern widely used product families, which provide high productivity and sensitivity of penetrant testing at low ambient temperatures, were defined. The technological regime with use of warm airflow of a test surface was developed. It both substantially reduces the duration of development stage and also provides essentially brighter and more contrast defects indications. B, relative unit 0,40 0,30 0,20 T = -5 C T = 0 C T = 18 C warm blowing a 600 t, s D1 D2 D3 D4 D T = -5 C T = 0 C T = 18 C warm blowing Fig. 4. The values of average indications brightness B (a) and duration t (b) of developer layer drying for various wet developers (D1-D5). b

4 Obviously on the basis of the obtained results it is possible to solve a problem of penetrant testing productivity increase at low ambient temperatures. The possibility of quantitative evaluation of PT results enables to improve a testing process by the optimization of principal parameters of various stages of PT procedure. Applying the characteristics of main testing stages (penetrant application, penetrant excess removal and developer application), which don t correspond to their values for optimal regimes we can get significant decrease in sensitivity. We studied the influence of various parameters of the main stages of PT process upon detectability of defects. Optimum values of the main characteristics of various technological stages for some widely used product families were defined. Principal results were presented in our papers [4-7]. The problem of reliable quantitative evaluation of penetrant systems quality is an actual one because of its practical importance. The market of the penetrant systems is increasing every year. A sensitivity of the penetrant systems is one of the main criteria for a customer. It was established in our experiments that repeating the procedure of type testing in accordance with ISO EN , in many cases one can obtain rather variable results for the sensitivity level. Several years ago the principal factors influencing upon the result of penetrant system type testing were established [3-5]. The values of the duration of such penetrant inspection technological stages as penetrant and developer applications may be considerably lower than the values recommended by ISO EN for the type testing. At the same time the character of the procedure s realisation of excess penetrant removal and developer application strongly influences upon the type testing results. Some recommendations for obtaining more reliable results of the determination of penetrant system sensitivity were worked out and proposed in [12,13]. Later a special set up for excess penetrant removal intended for the determination of penetrant systems sensitivity level was developed at the Institute of Applied Physics (fig.5). This set up ensures a high repeatability of main characteristics of excess penetrant removability (residual background brightness, a level of penetrant washing out from a crack s cavity) during several procedures for the same test panel. The repeatability is provided by the following requirements. The test panel must be fixed at prescribed position in the bath (fig.5). Then using regulable spatial orientation of a nozzle (appropriate angle α) at prescribed pressure of washing liquid jet we ensure the same level of excess penetrant washing-off from the test panel. Optimal values of the angle α and the duration of the washing at prescribed pressure drop should be determined experimentally for prescribed penetrant system. Fig.5. The set up for excess penetrant removal.

5 Penetrant testing methods can be applied under the condition that the roughness of tested surface is lower than the certain critical value. Many industrial objects have comparatively rough surface therefore the application of penetrant testing of their surfaces in these cases is impossible or at least problematic. To overcome this difficulty preliminary processing of the surface with the aim to decrease a roughness can be applied. But in the most cases a mechanical surface treatment leads to inadmissible decrease (or even closing) of defect opening. The influence of several kinds of surface treatment upon defects shape and form was recently studied in [10]. Special methods and equipment for decreasing of surface roughness at keeping defects opened were described in this paper. Some results of researches, which enable the application of penetrant testing for rough surfaces were presented in this paper as well. The researches of the influence of steel surfaces high-speed treatment upon PT sensitivity have been conducted at the Institute of Applied Physics. Several kinds of high-speed hand-operated treatment were applied: rotary treatment by steel brush, abrasive treatment by abrasive disk and high-speed milling. The process of electrochemical treatment (ECT) in environmentally friendly aqueous NaCl solutions was used as well. The important feature of ECT is dissolution of metal surface layer without plastic forming and without power impact upon the metal [14]. Let us consider some conclusions of our investigations, which enable the application of penetrant testing for rough surfaces. The main results of the researches the influence of various kinds of surface treatment upon PT sensitivity are followings [10]. The use of rotary treatment by steel brush before consequent PT procedure is inexpedient, because it results in about full closing of defects smaller than 9-10 µm in used specimens. High-speed hand-operated milling can be used only in special cases (cusped teeth of tungstencarbide mill with positive values of front angle and cutting speed ~15 m/s) before consequent PT to detect only comparatively deep defects (with a depth over than µm). a b c Fig. 6. Steel specimen containing the longitudinal welded seam with a thermal crack: a luminescent PT after the abrasive treatment to Ra 1-1,4; b - luminescent PT after the abrasive treatment up to Ra 10-20; c - luminescent PT after the abrasive treatment to Ra and ECT. The abrasive treatment application with a use of finish grinding is the most effective procedure to ensure the consequent PT only being provided with consequent electrochemical treatment in aqueous NaCl solutions. The effect of ECT on the results of the penetrant testing of the steel specimen containing a longitudinal welded seam and a thermal crack is illustrated by the Fig.6. Test surface of the specimen was treated by a diamond disk to Ra and consequent luminescent PT was performed (Fig.6,a). Then the transverse abrasive treatment was applied to test surface to Ra and followed by luminescent PT (Fig.6,b). As is seen from the photographs, it is impossible to detect a crack during the luminescent PT, because of the high background level of the penetrant luminescence. The repeated ECT and luminescent PT of the specimen (Fig.6c) made it possible to visualize the crack.

6 The results briefly described above concerning the ways of the increasing of PT sensitivity and ability extension demonstrate essential potential of penetrant testing methods for their more effective and wider application. References 1. Migoun N., Gnusin A., Delenkovsky N. New means for penetrant families sensitivity determination. Metrology and Instrument-Making (in Rus.), 2007, 2, p Stadthaus M., Thomas H.-M. Anwendung eines bildverarbeitungssystems zur ermittlung der anzeigenerkennbarkeit beim magnetpluver- und eindringverfahren. Materialprufung, 1988, v.30, N2, p Migoun N., Prokhorenko P., Gnusin A., Stadthaus M., Thomas H.-M. On the reliability of quantitative evaluation of penetrant systems quality. In: Review of Progress in Quantitative NDE, American Institute of Physics, New York, 2002, v.21b, p Migoun N., Prokhorenko P. On the optimal value of developer layer thickness. In: Proceedings of 8th European. Conf. on NDT, Barcelona, Spain, 2002, CD, 7p. 5. Migoun N., Volovich I. The way of penetrant testing technology development. Science & Innovations (in Rus.), 2005, N2, p Migoun N., Gnusin, А., Volovich I. Some Possibilities of Enhancing the Efficiency of Penetrant Inspection. Rus. Journal of NDT, 2005, v.41, N 7, p Migoun N., Gnusin A., Stadthaus M., Jaenisch G.-R. New potentials of penetrant testing. In: Proceedings of the 9th European Conf. on NDT, Berlin, Germany, 2006, CD, 6 p. 8. Migoun N., Gnusin, А., Volovich I. Increasing of the efficiency of penetrant testing using thermal effects / Proceedings of VI Minsk International Heat & Mass Transfer forum, [Electronic resource]. Minsk, Belarus, 2008, CD, 10p. 9. Migoun N., Delenkovsky N. Penetrant testing development. Journal of Engineering Physics and Thermophysics, 2009, v.82, N4, p Migoun N., Delenkovsky N. The Ways of Penetrant Testing Applicability for Rough Surfaces. In: Proceedings of 17th World Conf. on NDT, Shanghai, China, Oct., 2008, CD, 5p. 11. Prokhorenko P., Migoun N., Stadthaus M. Theoretical Principles of Liquid Penetrant Testing. Berlin, DVS-Verlag, 1999, 252 p. 12. Stadthaus M., Thomas H.-M., Baugatz J., König W., Migoun N.P., Prokhorenko P.P., Gnusin A.B. On the optimal value of developer layer thickness. Reproducibility problem for the procedure of type testing of liquid penetrants. In: Proceedings of 8th European. Conf. on NDT, Barcelona, Spain, 2002, CD, 4 p. 13. Migoun N.P. Problem of Revising New International Standards of Penetrant Testing. Rus. Journal of NDT, 2003, v.39, N 6, p Delenkovsky N. Electrochemical Treatment of Welded Seams before Penetrant Testing, Rus. Journal of NDT, 2007, v.43,n 5, p