RU2097758C1 - Method of magnetic-tape test of articles with small- curvature surface of magnetically soft steels - Google Patents

Abstract

FIELD: nondestructive test. SUBSTANCE: when article under test is magnetized along with magnetic tape positioned on its surface by permanent magnet with field H_O, use is made of tapes with starting field H_B not less than 250-300 A/cm. Tape with maximum value of saturating field H_S is selected of all suitable tapes. Value H_O is selected equal to value H_B, and article quality is determined by results of magnetogram reading. EFFECT: higher test results. 3 dwg

Description

 The invention relates to magnetographic inspection of products with a surface of small curvature and welds with removed reinforcement from soft magnetic steels (with a coercive force of less than 10 A / cm).

A known method of magnetographic control [1, 2] where a magnetic tape is placed on a product, it is magnetized once together with the product in the direction of the magnetic field in the transverse length of the tape, the magnitude of which H _o is determined from empirical tables that take into account the magnetic properties of the tape, the thickness of the product and other factors the tape removed from the product is scanned line by line in the direction transverse to its length by the induction head, the signal of the head is fed to an oscilloscope and the presence of defects is judged by the image on the screen. The method is used to control products, welds, etc., with a thickness of up to 20 25 mm and should ensure the detection of defects in 5 10% or more of the wall thickness.

The disadvantage of this method is the difficulty of choosing a value of H _o optimal for identifying all possible defects. Indeed, a value of H _o sufficient to detect external defects (ND) does not allow the detection of deep-lying internal defects (VD); in turn, in the fields H _o sufficient to detect VD, the ND signal can decrease sharply. In practice, they are guided by the identification of the smallest unacceptable defects on the product’s internal wall for the tape, moreover, artificial gaps serve as such defects [2]
There is a method of choosing the optimal value of H _o according to the magnitude of the magnetic induction in the metal of the product, corresponding to the maximum derivative of the magnetic permeability of the material by magnetic induction [3, p.30] The disadvantage of this method is the choice of the value of H _o from the point of view of optimal field induction in the metal, and not taking into account the magnetic properties of the tape; there is no way to select the latter. As a result, the defect field will be created, but due to the wrong choice of tape, it may not be recorded or it may not be recorded optimally.

 A known method of magnetographic control, where in order to reduce the noise of the tape and improve the accuracy of the control, the tape laid on the product is magnetized together with the product until the tape is saturated in one direction, and then in the opposite direction to a field value equal to the value of the relaxation coercive force of the tape [4] of technical essence, the method is closest to the essence of the claimed invention and is accepted by us as the closest analogue.

The method has disadvantages: it requires double magnetization of the product, which reduces the performance of the control; it does not indicate the magnetic characteristics of the tape, and for low-coercive tapes, the field H _o corresponding to the value of the relaxation coercive force of the tape may not be sufficient to detect VD.

The aim of the invention is to increase the efficiency of magnetographic inspection of products with a surface of small curvature and welds with removed reinforcement from soft magnetic steels by setting the optimal standard value of the magnetizing field H _o and reducing the intrinsic noise of the tape.

The problem is achieved in that how the controlled product is magnetized together with a magnetic tape laid on its surface with a constant magnetic field H _o , and then the quality of the product is determined by the results of reading the magnetogram. However, unlike the prototype, tapes with a starting field H _{b of} at least 250-300 A / cm are used, of all suitable tapes, the tape with the highest saturation field H _{s is} selected, and the value of H _{o is} chosen equal to the value of H _b .

The essence of the invention lies in the fact that the effectiveness of any method increases if the signal of the defect increases, the noise decreases and the relationship between the signal amplitude and the size of the defect becomes clearer. The effectiveness of the magnetographic control is determined, first of all, by the magnetic properties of the applied tapes and the properties of the initial fields of various defects. Our studies [5] revealed a number of significant factors shown in FIG. 1-3. It turned out, firstly, that recording on a tape creates a component H _{x of the} original field, which is mostly tangential to the plane of the tape, above the product, moreover, three parameters shown in FIG. 1b with solid lines of the approximated amplitude characteristic (i.e., the dependence of the residual magnetization M _rx on the value of the component H _{x of the} applied field; the actual experimentally measured dependence is shown in dashed lines in Fig. 1b) of the tape: the starting field H _b , i.e. the field in which the residual magnetization begins to appear; tape saturating field H _s ; width (H _b , H _s ) of the linear portion.

Secondly, the value of H _xmax (Fig. 1 a) for various defects, products and values of H _o can vary from tens to thousands of A / cm and depends on H _o differently: for strained pressure type cracks, the curve H _xmax (H _o ) in shape it repeats the main magnetization curve (for magnetically soft steels with a coercive force less than 10 A / cm already in fields H _{o of the} order of 50 A / cm, the defect field is almost saturated and reaches thousands of A / cm near the metal surface); for VD there is a certain threshold value (the greater, the deeper the defect lies), the value of H _o , at which the defect field is fixed on the surface, and for round VDs of the type of lack of penetration, the dependence Hxmax (H _o ) is almost linear, the defect field quadratically depends on the discontinuity cross-sectional area and for small VDs, it can reach values sufficient for detecting a tape only at H _o of hundreds A / cm.

Thirdly, the value of the useful signal of the tape (it can be characterized, for example, by the largest value of the tangential component of the recording field H _{rmax of} Fig. 1, d) depends not only on the characteristics of the tape and the value of H _{rmax of the} initial defect field, but also on the entire topography of the defect. In particular, the tape, due to the physical features of its operation, turned out to be a differential converter, i.e. of two defects with an equal value of H _xmax and a different degree of localization of the initial field, a stronger signal gives a defect on the tape with a greater localization of the field (in other words, the tape is much more sensitive to small external cracks than to internal large defects with equal values of H _xmax ).

In FIG. 1 schematically shows for two values of the field H _{o the} sequence of transformation of the distribution of the tangential component H _{x (x) of the} initial field of the outer 1 and inner 2 extended defects tape with the characteristic given in FIG. 1b, into the distribution of the tangential remanent magnetization M _{rx (x) of the} tape in FIG. 1c, creating a recording field with the distribution of the tangential component H _{rx (x)} as in FIG. 1 g

It turned out [5] that for all defects the tape signal begins to fall if the value of H _{rmax is} greater than the saturation field H _s , i.e. for signal growth, the highest possible value of H _s is desirable. For ND, however, an important factor is not only the saturation of the tape, but also the influence of the lateral minima H _{xmin of the} initial field (Fig. 1 a): if they are recorded (field H _o H ''), then there is a sharp drop in the tape signal, so and the appearance of powerful interference signals that impede the recognition of a defect (Fig. 1 g, curve 1 ''); if not recorded (field H _o H '<H _b ), then interference signals are absent, and the useful signal grows (Fig. 1 g, curve 1'). The initial VD field has no lateral minima, and the ratio of the values of H _o , H _b , H _xmax and H _s plays a decisive role.

 Two options are possible here (the case of a rounded VD is considered).

1. If, for H _o <H _{b, the} value of H _{xmax is} less than the width of the linear interval (H _b , H _s ), then with increasing H _o the tape signal at first rapidly grows both due to more and more complete recording of the initial defect field on the tape and beyond account of the growth of the latter (case H _o = H 'for defect 2 in Fig. 1 a); as soon as the condition H _o > H _b starts to be satisfied, the initial field is recorded completely and growth continues only by increasing H _xmax , until the condition H _xmax > H _{s is} fulfilled and the growth is replaced by a decrease in the signal (case H _o -H '' in FIG. . 1 a).

2. If, for H _o <H _{b, the} value of H _{xmax is} greater than the width of the linear interval (H _b , H _s ), then with an increase in H _{o, the} initial increase in the signal of the tape will quickly be replaced by its decrease due to saturation of the tape.

Thus, the value of H _o must satisfy at least two conditions: be large enough to create a noticeable field from small VD, exceed the value of H _b tape to optimize the detection of ND and reduce its own noise to zero. The tape should have the maximum width of the linear section (H _b , H _s ).

Existing tapes have a relatively narrow linear section (at best 300-400 A / cm) and relatively little different starting values (H _b for different tapes varies from about 50 to 300 A / cm) and saturating fields (for different tapes H _s varies from about 300 to 700 A / cm). Since the value of H _xmax for ND and large VDs can reach thousands of A / cm [5], the fields of such defects saturate the tape, and the requirement H _s > H _{xmax has} to be abandoned, limiting ourselves to choosing the tape with the maximum H _s . A signal drop due to saturation of the tape makes it difficult to estimate the size of the defect (this is one of the problems of magnetographic defectometry), but the detection of the defect remains reliable. On the other hand, in experiments [5] at H _o 250-300 A / cm VD at a depth of 20-25 mm at a size of 5-10% of the thickness (cylindrical holes with a diameter of 1-2 mm were used) give a fairly clear signal on all existing tapes.

So it remains to choose a tape with H _{b of the} order of 250-300 A / cm and a maximum value of H _s , and then magnetize in a field H _o , approximately equal to H _b . These will be the optimal conditions for detecting defects under the indicated conditions (welds with removed reinforcement and products with a surface of small curvature from soft magnetic steels up to 20-25 thick with the detection of defects of 5-10% or more of the wall thickness).

In practice, the implementation of the method does not cause difficulties, since the control technique is standard, and the novelty lies in the choice of tape and the value of H _o . Selected tape with the value of the starting field H _b of the order of 250-300 A / cm and the maximum value of H _s (H _b and H _s are known from reference value). The controlled product, together with the magnetic tape laid on its surface, is magnetized with a constant magnetic field H _o equal to the value of H _b ; for measuring H _o , for example, Hall sensors are used. The tape is read in one of the known ways.

Example. The advantages of the proposed magnetographic inspection method are illustrated in FIG. 2 and 3, which show the dependences of the tape signal on the field H _o for different tapes and defects in flat samples up to 40 mm thick made of mild steel. The defects were located along the tape and had a length of up to 150 mm; the field H _o acted transversely to the tape. The value of H _rxmax was measured by a linear sensor that responds to the tangential component of the magnetogram field (we used a flux probe of the corresponding design).

In FIG. Method 2 is used to identify three IDs in the form of rectangular slits with a width of 0.25 and a depth of 2, 7 and 10 mm (curves 1, 2 and 3, respectively) with three tapes: a I-4701 with H _b = 50 and H _s = 300 A /cm; b "Type 2" with H _b = 250 and H _s = 600 A / cm; in A-3901-35H with H _b = 300 and H _s = 700 A / cm. In FIG. 3, the same three tapes reveal VD in the form of a cylindrical hole with a diameter of 2 and 5 mm (curves 1 and 2, respectively) at a depth of 10 mm from the upper plane of the sample. It can be seen that the largest signal from both the ND and the VD while satisfying the condition of the most distinct dependence of the signal on the size of the defect is obtained using tape A-3901-35H and the field H _o = H _b = 300 A / cm. The noise of the A-3901-35H tape in the starting field is naturally absent.

 The proposed method really allows you to increase the efficiency of the magnetographic control of products with a surface of small curvature and welds with removed reinforcement from soft magnetic steels. Assess the economic effect before the implementation of the method is not possible.

Claims (1)

The method of magnetographic control of products with a surface of small curvature from soft magnetic steels, which consists in the fact that the controlled product is magnetized together with a magnetic tape laid on its surface with a constant magnetic field H ₀ , and then the quality of the product is determined by reading the magnetogram, characterized in that use a tape with a starting field H _in not less than 250 300 A / cm and a maximum value of the saturating field H _s , and the value of H _{0 is} chosen equal to the value of H _in .

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