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Problems of welded joints quality control and possibility of metal magnetic memory method using A.A. Doubov, E.A. Demine Energodiagnostika Co. Ltd. Contact

The problems of non-destructive testing (NDT) of quality of ferromagnetic and austenitic steels welded joints, shortcuts of modern defectoscopy means for NDT, feasibility of MMM application are reviewed. The main problem at NDT of welded joints is the definition of places of coincidence of continuity defects of metal with concentration of welding stresses and with stresses from operation loads. It is shown that the applying of MMM allows to solve this problem and method has a number of other advantages in comparison with known means of NDT, in particular, as a method of the express - control.

The quality control of welded joints consists of two known problems:

1. 1. Non-destructive testing of metal continuity.
2. 2. Estimation of structural - mechanical and stress states.

In practice, and in the theory, these two problems are solved, as a rule, separately and independently.

At a non-destructive testing of metal continuity the problem of scientifically based definition of the sizes of defects which are permissible and invalid in exploitation is not resolved till now. Besides widely spread on practice domestic and foreign instrumentation of a ultrasonic inspection does not allow to determine precise geometrical parameters of a defect. In a non-destructive testing there is also a problem of the express - control of new welded joint on a degree of a contamination by continuity defects (on any design shape of a welded joint and without preliminary preparation of a surface).

For the second problem the main difficulty at NDT of welded joints is impossibility of unequivocal definition of maximum stress concentration zones (SCZ), in which ones the development of damages is most possible at exploitation. About today this problem is solved on the basis of composite idealized calculations as volumetric or plane problem of a theory of elastic strength with set of allowances and idealized factors.

The complex problem on definition of a degree of hazard of continuity defects at their coincidence with zones of welding stresses concentration and with stresses from operation loads practically is not solved.

There were proposals, in particular in MVTU works, to create a technique of a tentative estimation of hazard of technological defects in welded joint by method of stress concentration calculation for continuity defects of welds. However, allowing large variety of the design shapes of welded joints, thicknesses of metal, kinds, quantity, sizes and arrangement of continuity defects at welding, actual stress distribution in a welded assembly, such problem was not resolved.

Known on today methods and the instrumental means of a non-destructive inspection do not allow to decide operatively, and furthermore completely, any of listed problems, and, specially, in version of the express - control. On the other hand, the state of the art of methods and welded joint NDT means has a number of objective shortcuts, such as:

• Locality of a monitoring facilities;
• Large labour input during the control of extended segments;
• Directivity of the control only on detection of already developed defects;
• Large difficulties, and sometimes impossibility of the control of welded joints with angular welds;
• High inaccuracy at the control of single (local) defects;
• The impossibility of definition of hazard degree of a defect in connection with impossibility of the unequivocal solution of problem - is the detected defect located in SCZ or not?

Extending the said, it is possible to draw a conclusion, that the looking up and definition of a weak point in a unified overall system "stress concentration - defect" remains an actual problem of a non-destructive inspection of weld joints, as at their manufacturing, i.e. directly after welding, and during their exploitation.

In this connection MMM, according to our reckoning, introduces a large outlook and capabilities, first of all, as a unique method of the express - control and, as a method of a complex estimation of welded joint by integral physical properties conditioned by magnetoelastic and magnetomechanical effects.

The formation of magnetic (domain) pattern in welded joints takes place at a cooling of metal in a magnetic field of the Earth at passing through a Curie point (768° C) simultaneously to a crystallization. On arising defects of welding the clusters of fastening of domains with cropping of a weld as magnetic fields of dissipation (MFD) will be derivated. Thus, by reading of MFD, which ones are formed during welding, we are granted a capability to execute an estimation of weld actual condition. It is known, that the permissible technological defects of welding which are not falling in a zone of concentration of operation loads, do not introduce hazards to a reliable operation of a construction. At the same time, the small-sized defect which is permissible by the norms, and even located outside sensitivity of conventional methods of verification, falling in a zone of a cyclic operation load, introduces large hazard to formation and development of welded joint damage.

MMM gives an integral (complex) estimation of welded joint quality in combination of technological defects, residual welding stresses and stress concentration conditioned by a design and operation loads. In Fig.1 the scheme and example of the control of butt joints of tubes are shown.

From the figure the sharply different condition of two identical by a design and know-how of manufacturing welds is visible. The results of the control shown on Fig.1a testify to a satisfactory state of weld. The distribution H_p with both parties of weld has practically identical mark with some displacement concerning a tube axis. The distribution of the field H_p shown on Fig.1b characterizes a nonsatisfactory state of weld with strongly pronounced zones of residual stresses concentration (SC). In these zones the maximum gradient of H_p between channels is fixed. In specified on fig. 1b SC zones at the additional ultrasonic testing defects in a root of weld are revealed hardly is lower extreme allowable on norms (BCH.012-88).

Distribution of magnetic field Hp from two sides of the weld characterizing it's satisfactory condition.	Unsatisfactory condition of weld with stress concentration zones (SC).
Fig 1: Scheme tube welded joints control with two-channel device of type TSC-1M: Dlb - base distance between sensors.

In Fig.2 the distribution of MFD along a weld of two laminas and on definite spacing interval from a weld is shown.

Fig 2:

From Fig.2 it is evident, that in this case distribution of MFD images permanent distortion (trough) of laminas conditioned by their welding. The example submitted on Fig.2 demonstrates an opportunity of use metal magnetic memory for an estimation of warp of welds and influence of conditions of fastening of weld joints on formation of residual stresses.

On Fig.3 the distribution of the field H_p fixed along perimeter of weld joint of pipes (Æ 160´8 mm, steel 3) is submitted with both parties of weld on zones of thermal influence.

Fig 3: Stress-strained state of welded joint of tube (Æ 160´8 mm, steel 3) by the testing results of metal magnetic memory

Here on Fig.3 are designated in figures (from 0 up to 8) points in which have carried out measurements of diffraction angle of X-rays from two parties of weld on zones of thermal influence. Was used diffraction-meter which had small-sized X-ray tube with anode (Fe-Cr-Cu), total capacity 2,5-5,0 W, with high specific brightness (size of focus of 0,4´1,0 mm) and positional sensitive detector. The angle of simultaneous registration of the detector is 2q=43° that has allowed to reduce time of an exposition up to two minutes and to ensure high radiating safety without special means of protection. The area of measurement was ~3 mm² in each point. It is known, that the X-ray method consists that the position of the centre of gravity of interference maximum at X-ray diffraction on a crystal lattice of a material is connected to inter-plane distance d and length of a wave of X-ray radiation l. This connection is shown by the equation Wulf-Bragg: 2d×sinq=nl. At n=1 (case of the greatest intensity of interference maximum) this equation presents as follows:

q = arcsin (l/2d)

The deformation of a crystal lattice is expressed by dependence:

e = (d-d₀) Ð d₀,
where d₀ and d - inter-plane distance accordingly before and after deformation.

The deformation causes displacement of the centre of gravity of X-ray interference maximum of given crystallography plane on value determined by the ratio:

Dq=e×tgq₀ or e=Dq.ctgq₀,
where q₀ - diffraction angle appropriate non-deformed state of a material.
At acceptance of an assumption about flatly stressed state in a surface layer of a product the determination of sum of main stresses s ₁+s ₂ on a surface of a researched sample on data of the X-ray method according to the generalized Hooke's law is made under the equation:
where E and m - accordingly the Young's modulus and Poisson's constant of a researched material, e - deformation in a direction normal to a researched surface, q _c - coordinate of the centre of gravity of X-ray interference maximum appropriate to average distance between crystallography planes of stressed material perpendicular direction of survey, q_co - coordinate of the centre of gravity of X-ray interference maximum appropriate to average distance between crystallography planes of non-stressed material perpendicular direction of survey (is determined depending on a material and length of a wave of radiation).

Equivalent (equally dangerous) the stress s _eq according to the second theory of durability is determined for flatly stressed state under the equation:

s _eq = m(s₁+s₂).
On Fig.4 the s_eq distributions are shown.

Fig 4 : Stress-strained state of welded joint of tube (Æ 160´8 mm, steel 3) by the testing results of X-ray diffraction-meter: 1, 2, 3 - points of stress measurements; results of stress measurements appropriate to the field H2; results of stress measurements appropriate to the field H1

At comparison Fig.3 and Fig.4 is possible to make the following conclusions.

1. The areas of a different mark of intensity of magnetic field H_p (A/m) correspond to areas of a different mark of stresses s (MPa). In this case a mark the plus of a field H_p corresponds to a mark a minus of s and on the contrary. At another (opposite) arrangement of a tube joint at welding in relation to a magnetic field of the Earth the field H_p will have an opposite mark.
2. The zones of change of a mark of a field H_p (A/m) correspond to zones of change of a mark of s (MPa). Some divergence of these zones is connected to methodical divergences of method of metal magnetic memory and X-ray method. At the control by the method of magnetic memory was measured normal component of magnetic field H_p, and at the X-ray control were measured normal and tangential components of magnetic field H_p and s_eq.
3. The areas with the maximum values of stresses s (total on absolute size at the left and to the right of welded joint) coincide with the areas of maximum values of field H_p and with zones of the maximum divergence of two channels of measurements, distributed with two sides of a joint.
4. Is received qualitative similarity of distribution of residual stresses along perimeter of welded joint according control results by the metal magnetic memory and X-ray method.
5. Necessity of development of a technique for determination of stress value is revealed by combined practical use of both methods.
6. It is necessary to continue comparative researches and measurements for an establishment of more precise correlation dependences of value and mark of stresses s with parameter H_p.Distribution of magnetic field H_p

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