The examination of metals is used in judicial practice, in the investigation of the causes of accidents, as well as for making responsible technical decisions in the manufacture of various objects. The peculiarity of metals and alloys is that they well “retain” the parameters of previous states that arose under the influence of external factors (mechanical loads, thermal stress, and others). This property allows you to set the conditions preceding any event, and to identify a causal relationship.
Tasks
All tasks of metallurgical expertise can be combined into 3 large groups:
- Classification - determination of belonging to any type (type of metal or alloy, grade, field of application; field from which the material was extracted; compliance with the requirements of the drawing and other technical documentation).
- Individual identification: determination of the whole in parts (by signs of elemental composition and structure, traces of processing and other parameters), belonging to the general production batch, establishing the source - equipment or manufacturer; group - identifying the identical properties of metals and alloys by particular signs (characteristic impurities, crystal structure, mechanical properties, nature of destruction, surface condition: oxidation, corrosion, scratches and others).
- Diagnostic: establishing the fact of interaction with a metal part, the causes and features of the destruction process, determining the manufacturing technology, the type of devices on which the metal product was made, identifying deviations from the requirements of the technical regulation.
The objects
The following micro and macro objects can serve as objects of examination of metal products:
- steel parts, billets, wire and wires, metal hoses, cables, pipes;
- reflow on wires and other metal products;
- details of cars and technological equipment destroyed as a result of the accident;
- knives (factory and homemade) and other edged weapons;
- splinters or parts of explosive devices;
- solders;
- jewelry, precious metals and native gold;
- traces of metallization;
- household items.
Stages
Examination of metals and alloys is carried out according to the following characteristics determination scheme:
- the nature of the material (density, ferromagnetism, color, hardness, metallic luster, electrical conductivity, interaction with acids);
- geometric and structural features (size, shape, the presence of metallic and non-metallic coatings);
- production technology and equipment;
- operating conditions (wear, corrosion and others);
- fracture features (fusion, deformation, shock or static loads, erosion, a combination of several factors);
- classification type and scope;
- microstructure;
- physical and mechanical properties, chemical composition.
Depending on what questions are raised before the examination, the construction of the examination methodology depends. Since some methods of study require the destruction of the studied object, it is necessary to clearly establish the purpose of the study.
Forensic examination of metals
Forensic examinations are conducted to address the following most common issues:
- determination of the brand of metal from which the instrument of crime is made;
- identification of the type of gold - native or industrial;
- the presence of metal particles at another object;
- determining the authenticity of a coin or medal, as well as their age;
- identification of the type and duration of exposure to the metal;
- the reasons for the destruction of a metal part or assembly (to develop a conclusion in emergency situations, fires);
- determination of the identity of the part and the main metal part / structure, as well as the method by which they were separated and others.
Optical and scanning microscopy
Microscopic examination of surfaces is done at the initial stage of metallurgical examination. An examination of metals and alloys reveals features that have formed during the manufacturing and operation of parts and assemblies. To carry out this work, raster and transmission electron microscopes are used.
One type of such examination is fractography — a microscopic study of fracture surfaces to establish causal relationships between signs of damage and external or internal factors that cause fracture.
Chemical composition study
The most common method for studying the chemical composition of metals and alloys is emission spectral analysis (ESA), which is based on the characteristics of the spectrum of emission of atoms of a substance in the optical range of electromagnetic waves. Light radiation is excited by a spark discharge between the studied metal object and the auxiliary electrode.
This method allows you to determine the chemical element in the presence of its smallest amount (up to 10 -4 %), which is used in the analysis of trace amounts on both metallic and non-metallic contacting material. This technique is also used in the examination of the quality of the metal to identify microimpurities that are not regulated by the technical conditions for the manufacture of this object.
Nondestructive and classic research methods
If it is necessary to conduct a study with a non-destructive method, then it is done using x-ray fluorescence analysis. This technology involves exposing the material to x-rays that cause fluorescence in the surface layer. Each atomic number of the periodic system of chemical elements is characterized by its own wavelength. This technique helps to determine both qualitative and quantitative composition.
Examination of metals can be carried out in classical ways. They take more time and require a significant amount of sampling. These methods include:
- high-quality chemical reactions;
- colorimetric analysis;
- polarography;
- conductometry (measurement of the electrical conductivity of solutions) and others.
Microstructure study
Structural metallography is used to determine the nature and number of phases in the examination of non-ferrous metals, steel, cast iron and other metals. The study is carried out by powder x-ray diffraction. The sample is crushed into powder, a monochromatic x-ray beam is sent to it and an image in the form of rings on a photographic film rolled around the object under study is obtained. The presence of a certain texture is revealed by the change in the intensity of the lines.
To study powder X-ray diffraction patterns, special X-ray diffractometers are used. With their help, you can also get information about the crystalline structure of the metal. In practical terms, this is used to determine the type of heat treatment.
The determination of the phase composition is necessary to identify the general parameters of the parts of the studied object. Such a study, for example, allows you to get information about what happened first - a short circuit or a fire (copper contacts), whether a lamp with a tungsten filament burned at the time of its destruction and solve other issues.
To detect thin films of oxides, carbides, chlorides, sulfides and other salts on the surface of metal products, the electron diffraction method is used. It is based on the diffraction of the flow of moving electrons, the wavelength of which is less than that of x-ray radiation. The analysis of metals and alloys is carried out in vacuum electron diffractors, which can also give shadow electron-optical images.
Mechanical properties
To determine the physico-mechanical properties of metals and alloys, the following test methods are used:
- tensile / compression (determination of tensile strength, elasticity, yield strength and other characteristics);
- to bend;
- hardness and microhardness;
- heating to find the temperature of phase transformations, thermal conductivity and linear expansion.
Such methods are usually used in combination when studying the causes of accidents and the destruction of metal parts and assemblies.
Expert opinion
After the examination of metals, the specialist issues a conclusion that contains the following information:
- general data (name of the research work, start and end date, place of the examination, grounds for its execution, information about the expert, questions posed);
- methods used and general scientific laws on the basis of which the work was done;
- regulatory documents and other sources;
- data obtained during the study of the object;
- conclusions (or reasons why it is impossible to give a definite answer).
Examples
The following can be cited as examples of expert conclusions:
- destruction of the part occurred as a result of the action of a static bending load acting simultaneously;
- node failure was two-stage: after a single dynamic load, a crack formed, which as a result of fatigue stresses increased by 80%; the object was divided into parts under the influence of a static tensile load in the area weakened by this crack;
- the destruction of the vessel occurred along the weld, which was weakened by a defect obtained during welding (lack of penetration of the weld root), at the time of operation, the object no longer had the necessary strength;
- structural failure was simultaneous, with a rivet connection, the reason is the influence of dynamic load, the value of which exceeds the strength inherent in the design documentation.
From cases of judicial practice illustrating the application of the examination of metals, the following can be cited: a metal fragment was found in the body of the murdered. It was necessary to establish which of the 3 knives he was part of. Spectral analysis showed that the fragment coincided in composition with one of the knives. According to the microstructure, the piece of metal differed from all the blades, but during the examination it was proved that the blade was sharpened on an electric sharpener without cooling, as a result of which these changes occurred.
Laboratories
Independent examination of metal can be carried out by laboratories with state accreditation. The certificate of accreditation should indicate in which field of activity a permit for research has been obtained, test methods and regulatory documents on the basis of which they are carried out, as well as the protocol of the meeting of the accreditation commission.
In the Moscow region, such services are provided by Metal Expertise, the Interregional Center for Expertise and Assessment (ICEC), the Federation of Forensic Experts and other organizations.