The metal magnetic memory method
Dr., Professor A.A. Dubov
For the first time the author introduced the concept "the magnetic memory of metal" in 1994. Before that this concept was not used in the technical literature. The following terms and concepts were known: "the magnetic memory of the earth" in archeological investigations, "the magnetic memory" in sound recording and "the effect of shape memory" conditioned by the oriented internal stresses in metal products.
Based on the established correlation of dislocation processes with the physics of magnetic phenomena in the metal of products, the concept "the magnetic memory of metal" was introduced and a new diagnostic method was developed. Similar to the effect of shape memory, "the magnetic memory of metal" - is the effect of the magnetic memory of metal strain conditioned by the oriented internal stresses. The uniqueness of the metal magnetic memory (MMM) method consists in the fact that it is based on application of the effect of occurrence of high metal magnetization in the areas of large strains of structural elements’ metal due to the exposure to working loads. At the same time there is no artificial source of magnetization, except for the weak magnetic field of the earth, in which we all exist.
Many of us also observed the effects of occurrence of high metal magnetization, for example, in case of cutting of some metallic product with a hack-saw, or at the end of the screwdriver after its influence on the screws, as well as in places of friction at contact of metallic products (for instance, of gear wheel teeth). Occurrence of anomalous magnetization can be observed on the metallic wire in the area of its cyclic strain. When we break the wire by its cyclic bending in different directions, our fingers feel the wire heating in the maximum strain site. And if we carry out some measurement in this place using a magnetometer, we shall record the increase of the metal’s magnetization. Oscillation damping – absorption of the energy of mechanical oscillations (for example, of turbine blades) is accompanied by generation of the magnetic energy and, accordingly, by the growth of the metal’s residual magnetization. The list of the observed in practice cases of the products metal magnetization without the source of the artificial magnetic field can be continued further.
The phenomenon of strong magnetization of boiler pipes metal in places of their damaging, discovered in the 70-s of the last century by the Chief of "Volgogradenergo" metals laboratory O.V. Philimonov, should be attributed to the history of occurrence and development of the metal magnetic memory method as a new direction in diagnostics. The discovered phenomenon roused the interest of many power engineering experts, including me. At that time I worked for the production service of "Mosenergo" and dealt with the problems of boiler pipes reliability assurance. At that time an assumption was made about the possibility to use the phenomenon of operated pipes self-magnetization in order to detect potential damages. And occurrence of high magnetization was presumably explained by the action of cyclic strains and stresses due to working loads.
In case of confirmation of this assumption there opened a unique possibility to detect by the residual magnetization of metal in the location of stress concentration - the source of damages occurrence and development - by means of readout of the magnetic information presented to the researcher by the boiler piping itself.
In connection with this circumstance the author of this paper, jointly with the experts of the Institute of Metals Physics of the Russian Academy of Sciences (Yekaterinburg), arranged and carried out special laboratory and industrial researches aimed at investigation of the phenomenon of boiler pipes magnetization in conditions of their operation. The results of these investigations are described in the dissertation and monograph by A.A. Dubov "Diagnostics of boiler pipes using the magnetic memory of metal" (Moscow: Energoatomizdat, 1995, 112 p.).
As a result of the carried out investigations it was demonstrated that the reason of anomalously high magnetization of individual segments of boiler pipes is the magnetoelastic effect, which is known in the physics of magnetic phenomena.
Fig.1 shows the scheme of appearance of the magnetoelastic effect causing the growth of residual magnetization (М), for instance, in any place of the structure there is a cyclic load Δσ and an external magnetic field Н0 (for example, the field of the earth), then the residual magnetization ΔМσ growth takes place in this location. Upon the load relief the reversible component disappears, and only the irreversible component of the residual magnetization (ΔМσ irr) remains. Due to the magnetoelastic effect the pipes are as though "self-magnetizing" in the zones of stress concentration due to working loads. In the course of further industrial investigations it was established that practically all units of metal components and structures are subject to "self-magnetization".
The phenomenon metal components and structures "self-magnetization" is fought against everywhere by means of their periodic demagnetization (shipbuilding, power engineering, ball bearing and other industries). It would be sufficient to give as an example of struggle against self-magnetization, as a "harmful" phenomenon, the well-known story of struggle against the mines combated by our scientists at the beginning of the World War II against Germany. At that time, for example, occurrence of strong magnetization of hulls was discovered due to impacts of waves, especially after the storm. And then this phenomenon of hulls magnetization under exposure to cyclic stresses due to wave impacts in conditions of the weak magnetic field of the earth was first investigated under the supervision of academician A.P. Alexandrov. This phenomenon was explained by the action of the magnetoelastic effect. Then, in order to fight against the magnetic mines, special demagnetizing devices for periodic demagnetization of hulls were developed.
Since then the natural phenomenon of metal components and structures "self-magnetization" is everywhere fought against by means of their periodic demagnetization.
Upon investigating of this phenomenon on the example of boiler pipes and other units, it was suggested for the first time to use it for the purposes of engineering diagnostics, and the new diagnostic method was called "the metal magnetic memory method".
Based on experimental investigation of the metal magnetic memory phenomenon, a number of practical inspection techniques and specialized modern instruments were developed, allowing registration of magnetic anomalies location sites, corresponding to stress concentration zones (SCZs), being the sources of damages, in the quick-inspection mode without any preparation of the inspection object (IO) (in some cases through the layer of paint or insulation) by scanning along the surface (for example, of a long pipeline). Obtaining such an information source like a proper magnetic field is not possible under any conditions of artificial active magnetization in operated structures. Such information is formed and can be obtained only in a weak external field like the magnetic field of the earth in loaded structures, when the strain energy exceeds the external magnetic field energy by at least an order of magnitude. It was demonstrated in practical investigations that MMM may be applied both for equipment operation (in the monitoring mode) and after relief of working loads, during the repairs. By virtue of "magnetodislocation hysteresis" the magnetic texture formed under the effect of working loads somehow "congeals" after their relief. Thus, there is a unique opportunity to perform assessment of equipment stress-strain state and detect zones of maximum metal damage by measuring this information using specialized instruments. Application of MMM for assessment of metal structural life and survivability looks very promising since this method unites potential opportunities of non-destructive testing, materials science and fracture mechanics.
The unique opportunities of the MMM method reveal themselves in cases of finding out the reasons of failure of various structural elements of buildings, constructions, bridges, etc. For example, at failure of one of the piers supporting the roof in aqua park (Moscow, 2005) or at failure of the so-called "finger" carrying the heavy load in a complex structure of the new skating rink building (Moscow, 2007), the arouse some arguments between the metallographers (and, accordingly, between the manufacturing plant) and the designers of these structures. In these cases it is impossible to assess the actual stress-strain state (SSS) using the design methods with application of current design norms and relying on the theory of metals’ resistance, which does not take into account the structure inhomogeneity. If in these cases of occurred failures of structures (and best of all – not waiting for failure to occur) there was a possibility of their inspection using the MMM method and the appropriate scanning devices, then it would be possible to assess the actual SSS of any structural element by the diagram of the self-magnetic field distribution. SSS assessment using the MMM method has an integral nature, taking into account all the factors: design peculiarities, structural inhomogeneity of each element, factory, installation and operational factors. Now the designer has the opportunity to see the work of his "brainchild".
Due to the use of the metal magnetic memory effect during the diagnostics of metal components and structures it became possible to fulfill many problem tasks. First of all, this is prediction of metal components’ reliability and lifetime. Based on the 100% inspection of structures, the MMM method and the appropriate instruments allow detecting all potentially dangerous areas (SCZs) and revealing the reasons for their occurrence. Using the program software, these zones are classified by the degree of their danger. It allows timely conducting the repairs or replacement of individual elements and granted prolongation of the IO’s lifetime, at least till the next overhaul and/or inspection.
Welding exists in the world for more than 100 years, and the most important factor determining reliability of a welded joint - distribution of residual welding stresses - is so far not controlled due to the lack of NDT methods suitable for wide practical application. The use of the metal magnetic memory effect allows solving this problem. Reading out of the residual magnetization, formed naturally during welding with subsequent metal cooling, provides a unique possibility to integrally assess the actual weld state: to detect welding defects simultaneously with the distribution residual welding stresses.
Formation of the magnetic (domain) texture in welded joints occurs simultaneously with crystallization during metal cooling in the weak magnetic field of the earth (or of the workshop) and its passing through the well-known Curie point (760-770°С for carbon steel grades). In conditions, when the energy of thermal strains and stresses is by order higher than that of the weak magnetic field, the residual magnetization distribution in the weld metal is conditioned by the appropriate distribution of residual stresses (RS).
An amazing fact! The physical Curie point (Pc) and the effects of magnetization disappearance, when the metal is heated above the Pc and, on the contrary, of high magnetization appearance when the metal is cooled below the Pc have been known to experts as far back as since 1895, when the French scientist Pierre Curie first discovered this physical effect. However till date this effect has not been used in the everyday practice, for example, for quality assessment of products at engineering plants.
In the course of industrial investigations we established that the magnetic memory of metal on ferromagnetic products (and in some cases on the products made of a paramagnetic material) reflects their structure and process history. During fabrication of any products (melting, forging, punching, heat treatment, welding) when they are cooled below the Pc in a weak magnetic field of the earth (or of the workshop) the magnetic texture is formed naturally. Upon studying the distribution of the natural magnetization on a large number of new products after various technological processes at manufacturing plants, a number of practical techniques for inspection of the products’ quality was developed. And the unique possibilities of the metal magnetic memory application for testing of the effectiveness of products manufacturing technologies (quality inspection of casting, heat treatments, etc.) were revealed.
It should be noted that nowadays there is no 100% quality inspection of products for structural inhomogeneity at engineering plants both in Russia and abroad. According to the statistics, it is known that approximately up to 20% of the new products (pipes, rails, shafts, etc.) are put in operation with unacceptable metal defects. Application of the metal magnetic memory effect during the quality inspection of products at the manufacturing plants will allow carrying out rapid sorting of products and not to let the products with metal defects and process manufacturing defects be put in operation. Equipment and instrument-computer complexes using the metal magnetic memory effect during the inspection are considerably simpler and cheaper as compared to the available ultrasound-based equipment or equipment using artificial magnetization.
At present the magnetic NDT methods applied at manufacturing plants use artificial magnetization of products. At that the natural magnetization (the magnetic memory of metal – the most valuable information!) is removed by means of preliminary demagnetization.
What keeps from wider implementation in practice of the new direction in the engineering diagnostics based on the use of the metal magnetic memory effect today? Here it is appropriate to quote the words of the German poet and thinker Johann Goethe (28.08.1749 – 22.03.1832): "If somebody points out something new … people reject it as hard as they can; they behave as if they do not hear or cannot understand, speaking about the new opinion with contempt, as if it was not worth the effort spent on investigation or attention at all, and, thus, the new truth has to wait for a long time until it manages to pave its way".
Appearance of the first publications about the magnetic memory method was at first met in the scientific circles with indulgent indifference: "mister is a little bit lost". However further results of original experimental investigations and practical diagnostics using the MMM method, the quantity and quality of which grew rapidly, also rapidly gave rise to transition from indulgent bewilderment to aggressive indignation in the circles of scientists and experts both in the field of magnetism and diagnostics. And it is no wonder: many results, obtained using the magnetic memory method, contradicted the established during many decades’ ideas about magnetism and, first of all, the concepts like magnetoelasticity and magnetization. Moreover, the necessity to explain the multiple results of practical diagnostics using the metal magnetic memory effect revealed the "white spots" existing till date in the theory of magnetism. As it turned out, the basic provisions of the theory of magnetic phenomena in ferromagnetic materials were developed in the 30-s - 40-s of the last century without application of modern conclusions and achievements of quantum physics and theories of dislocations in the metal.
Consideration of magnetism development in the historical aspect made it possible to understand that some results could not be used yet and some were already "rejected" by the theory of magnetism formed.
For these reasons the following questions remained unanswered:
what the starting point is and how the process of self-magnetization develops in the volume of a ferromagnetic;
what restricts the domain growth and what its sizes are in absence of external exposures;
what the three-dimensional shape of the domain is, whether its dimensions are interrelated and if yes, then how;
how in conditions of spontaneous self-magnetization the domains form and group in the volume of a ferromagnetic; whether "closing" domains really exist as some auxiliary formations;
what actually the boundaries between the domains are, since the quantum physics proved the impossibility of arbitrary spatial position of the atom’s magnetic moment vector within the lattice. Consequently, the hypothesis about the gradual smooth turning of the magnetic moment vector in the transition layer - the interdomain boundary - is wrong;
why there is no symmetry in the processes of self-magnetization during tension and compression of polycrystalline ferromagnetic specimens;
why even an ideal monocrystal of a ferromagnetic still initially shows the magnetic anisotropy;
what physically determines the amount of the residual magnetization;
what the criterion of magnetic fields division into weak and strong is;
whether the magnetic field of the earth influences the process of self-magnetization;
what the influence of weak magnetic fields on the process of magnetization variation at cyclic loads is;
why during the specimens loading their magnetization increases abruptly at transition to the plastic strain region;
how magnetization and the domain structure, determining it, are related to dislocations and their clusters;
why in a loaded ferromagnetic there occur local magnetic fields, the orientation of which is not related to external magnetic fields, and what determines their spatial direction;
whether the boundary between weak and strong magnetic fields depends on the state of the environment (of the magnetized material).
Thus, one has to state that, despite the large number of the obtained by the present time in theoretical and experimental investigations results and conclusions, developing and supplementing the basic provisions of the domain structure formation, the theory of the domain structure cannot be considered completed.
With the purpose of searching for the answers to the formulated question and explaining the phenomenon of the magnetic memory of metal some theoretical and experimental investigations were performed. The results of these are reflected in the book by V.T. Vlasov and A.A. Dubov "Physical bases of the metal magnetic memory method" (Moscow: ZAO "TISSO", 2004, 424p.).
The carried out investigations resulted in obtaining of the answers to many above-mentioned questions. It was demonstrated that several physical effects underlie the phenomenon of the magnetic memory of metal. Besides the known till date magnetoelastic effect, a new, not studied before effect of magnetoplasticity - the process of a ferromagnetic object’s self-magnetic field formation in conditions of plastic strain - was revealed. The direct experimental proof, confirming the considerable increase of the dislocations density in stress concentration zones, was obtained during the specimens tensile testing using the specialized magnetometers and investigation of the dislocation structure using the electronic microscope. The rules of the magnetomechanical effect, showing itself at the macro level in the product’s volume, were investigated.
Based on the analysis of experimental investigations of various industrial objects using the MMM method and the analysis of the reasons of low effectiveness of the existing stress control methods, the contradictions between the diagnostic results and the formed ideas about the characteristics of internal stresses were revealed.
The bases of the physical theory of the "strain - failure" process were developed. This theory will ensure objective effectiveness evaluation of various stress-strained state inspection methods, strength calculations and equipment life prediction. It will allow providing scientific grounding of defects admissibility norms and the degree of their hazard in non-destructive testing, as well as more effective solution of other problems of fracture mechanics.
And, of course, in conclusion it is necessary to quote a well-known phrase: "Practice is a criterion of truth".
The new direction in engineering diagnostics based on the use of the metal magnetic memory phenomenon, which was born in Russia, has been developed practically and theoretically for more than 30 years. The method and the appropriate inspection instruments are used at more than 1000 enterprises of Russia. Besides Russia, the method became widespread in 44 countries of the world.
The inspection technology based on the MMM method was brought to the level of National and International Standards. In November 2007 International Standards were published:
ISO 24497-1:2007(E). Non-destructive testing. Metal magnetic memory. Vocabulary.
ISO 24497-2:2007(E). Non-destructive testing. Metal magnetic memory. General requirements.
ISO 24497-3:2007(E). Non-destructive testing. Metal magnetic memory. Inspection of welded joints.
GOST R ISO 24497-1-2009, GOST R ISO 24497-2-2009 and GOST R ISO 24497-3-2009 were put in effect in 2009 under the Federal Agency for Technical Regulation and Metrology Decrees No.499-st, 586-st and 587-st, respectively.
Since 1996 a Certification Center of Energodiagnostika Co. Ltd. has been operating in Moscow. By the present time more than 2600 Russian and about 900 foreign experts were trained.