Failure Analysis of Metallic Materials - Scanning Electron Microscopy (SEM) Applications

Metallic materials are materials with properties such as luster, ductility, easy conductivity, and heat transfer. They are generally classified into two types: ferrous and nonferrous metals. Ferrous metals include iron, chromium, manganese, etc. [1]. Among them, steel is the basic structural material and is called the "skeleton of industry". So far, steel still dominates the composition of industrial raw materials. Many steel companies and research institutes use the unique advantages of SEM to solve production problems and assist in the development of new products. SEM with corresponding accessories has become a favorite tool for the steel and metallurgical industry to conduct research and identify problems in the production process. With the increase in SEM resolution and automation, the application of SEM in material analysis and characterization is becoming more and more widespread [2].

Failure analysis is a new discipline that has been popularized by military enterprises to research scholars and enterprises in recent years [3]. Failure of metal parts can lead to degradation of workpiece performance in minor cases and even life safety accidents in major cases. Locating the causes of failure through failure analysis and proposing effective improvement measures is an essential step for ensuring the safe operation of the project. Therefore, making full use of the advantages of scanning electron microscopy will make a great contribution to the progress of the metallic materials industry.

01 SEM Observation of the Tensile Fracture of Metals

Fracture always occurs at the weakest point in the metal tissue and records much valuable information about the whole process of fracture. Therefore, the observation and study of fracture have been emphasized in the study of fracture. The morphological analysis of the fracture is used to study some basic problems that lead to the fracture of the material, such as the cause of fracture, the nature of the fracture, and the mode of fracture. If the fracture mechanism of the material is to be studied in depth, the composition of macro-areas on the fracture surface is usually analyzed. Fracture analysis has now become an important tool for failure analysis of metallic components.

Figure 1. CIQTEK SEM3100 Tensile Fracture Morphology

According to the nature of the fracture, the fracture can be roughly divided into brittle fracture and ductile fracture. The fracture surface of a brittle fracture is usually perpendicular to the tensile stress, and from the macroscopic point of view, the brittle fracture consists of a glossy crystalline bright surface; while the ductile fracture usually has a tiny bump on the fracture and is fibrous.

The experimental basis of fracture analysis is the direct observation and analysis of the fracture surface's macroscopic morphology and microstructural characteristics. In many cases, the nature of the fracture, the location of the initiation, and the crack extension path can be determined using macroscopic observations. However, microscopic observation is necessary to conduct a detailed study near the fracture’s source and analyze the fracture cause and fracture mechanism. And because the fracture is an uneven and rough surface, the microscope used to observe the fracture should have the maximum depth of field, the widest possible magnification range, and high resolution. All these needs have led to the wide application of SEM in the field of fracture analysis. Figure 1 shows three samples of tensile fracture by low magnification macroscopic observation and high magnification microstructure observation: sample A fracture is river flower-like (Figure A), which is a typical brittle fracture feature; sample B macroscopic no fibrous morphology (Figure B), microstructure no tough nest appears, which is a brittle fracture; sample C macroscopic fracture consists of glossy facets. Therefore, the above tensile fractures are all brittle fractures.

02 SEM Observation of Inclusions in Steel

The performance of steel depends mainly on the chemical composition and organization of the steel. Inclusions in steel mainly exist in the form of non-metallic compounds, such as oxides, sulfides, nitrides, etc., which cause the uneven organization of the steel. Moreover, their geometry, chemical composition, and physical factors not only reduce the cold and hot workability of steel but also affect the mechanical properties of the material [4]. The composition, number, shape, and distribution of nonmetallic inclusions have a great influence on the strength, plasticity, toughness, fatigue resistance, corrosion resistance, and other properties of steel. Therefore, nonmetallic inclusions are indispensable items in the metallographic examination of steel materials. By studying the behavior of inclusions in steel, and using the corresponding technology to prevent further formation of inclusions in steel and reduce the inclusions already present in the steel, is of great importance to the production of high purity steel and to improve the performance of steel.

Figure 2. Inclusion Morphology

Figure 3. Energy Spectrum Surface Analysis of TiN-Al2O3 Composite Inclusions

In the case of inclusions analysis shown in Figure 2 and Figure 3, the inclusions were observed by scanning electron microscopy, and the inclusions contained in the electrical pure iron were analyzed by energy spectroscopy, which showed that the inclusions contained in the pure iron were oxide, nitride and composite inclusions.

The analysis software that comes with the SEM3100 has powerful functions to measure directly on the sample or directly on the picture for any distance and length. For example, by measuring the length of the electrical pure iron inclusions in the case shown above, it can be seen that the average size of Al2O3 inclusions is about 3 μm, TiN and AlN sizes are within 5 μm, and the size of composite class inclusions does not exceed 8 μm. These tiny inclusions play a pegging role for the magnetic domains within the electrical pure iron, which will affect the final magnetic properties.

The source of oxide inclusions Al2O3 may be the deoxidation products of steel making and secondary oxides of the continuous casting process, the morphology in steel material is mostly spherical, a small part is irregularly shaped. The morphology of inclusions is related to their components and a series of physicochemical reactions occurring in the steel. When observing inclusions, we should not only observe the morphology and composition of inclusions but also pay attention to the size and distribution of inclusions, which requires statistics in many aspects to judge the level of inclusions comprehensively. SEM has advantages in the observation and analysis of individual inclusions, such as inclusions causing cracking of workpieces for failure analysis. Large particles of inclusions are often found at the source of cracking, and it is important to study the size, composition, quantity, and shape of the inclusions. The analysis can be used to locate the cause of the failure of the workpiece.

03 SEM for the Detection of Harmful Precipitation Phases in Steels

The precipitated phase is the phase precipitated when the temperature of the saturated solid solution is reduced, or the phase precipitated during aging of the supersaturated solid solution obtained after solid solution treatment. The relative aging process is a solid state phase change process, is the second phase particles from the supersaturated solid solution precipitation desolvation and nucleation growth process. The precipitated phase has a very important role in steel, its strength, toughness, plasticity, fatigue properties, and many other important physical and chemical properties have an important impact. Reasonable control of the steel precipitation phase can strengthen the steel properties. If the heat treatment temperature and time control are not appropriate, it will lead to a sharp decline in metal properties, such as brittle fracture, easy corrosion, etc.

 Figure 4. CIQTEK SEM3100 Electrotechnically Pure Iron Precipitation Phase Backscatter Diagram

Under a certain accelerating voltage, since the yield of backscattered electrons basically grows with the increase of the atomic number of the specimen, the backscattered electrons can be used as an imaging signal to display the atomic number lining image, and the distribution of chemical components on the surface of the specimen can be observed within a certain range. The atomic number of Pb is 82, and the backscattered electrons yield of Pb are high in back-scatter mode, so Pb appears bright white in the image.

The hazards of Pb in iron and steel materials are as follows because Pb and Fe do not generate a solid solution, which is difficult to remove in the smelting process, and it is easy to polarize at the grain boundaries, forming low melting point eutectic crystals to weaken the grain boundary bonding, so that the hot processing performance of the material is reduced. The possible sources of Pb precipitation in electrotechnical pure iron are the Pb contained in the raw materials of ironmaking and the trace Pb contained in the alloying elements added during smelting. if used for special purposes, the possibility of adding to the smelting process is not excluded, the purpose is to improve the cutting and machining properties.

04 Conclusions

Scanning electron microscopy as a microscopic analysis tool can be a variety of forms of observation of metal materials, can be a detailed analysis of various types of defects, metal materials failure of the causes of comprehensive positioning analysis. With the continuous improvement and enhancement of SEM functions, SEM is able to perform more and more tasks. It not only provides a reliable basis for research to improve material properties, but also plays an important role in production process control, new product design, and research.

Figure 5. CIQTEK SEM3200

References

[1] Zhang Yunchuan. Common problems and solution measures for metal material testing [J]. Digital User, 2018, 24(052):67. 

[2] Guo Libo, Li Peng, Wu Qiang, et al. Application of scanning electron microscopy and energy spectrum analysis in steel metallurgy[J]. Physical Testing,2018,36(1):30-36. 

[3] Chen Nanping, Gu Shouren, Shen Wanci, et al. Failure analysis of mechanical parts [M]. Beijing: Tsinghua University Press,2008,15-17. 

[4] Cheng Xiaofang, Hu Yu. Exploration of inclusions analysis methods in steel[J]. Metal Products,2006, 032(004):52-54.