The metals and alloys structure distinguishes by: the macrostructure, the microstructure and the fine structure. There are three methods for their investigation:
- Macroscopic analysis
- Microscopic analysis
- X-ray diffraction analysis and X-ray flaw detection
The macroscopic analysis.
Macrostructure is the metals and alloys structure that can be visible with small magnifications by a magnifying loop (up to 30 times max). The macrostructure investigated by macro analysis. Metals are non-transparent substances and their structure investigated in the fracture or in specially prepared samples (macrosections). The sample cuts from a certain location in a certain plane, depending what examine (casting, forgings, stamping, rolling, welded or thermally processed details) and what needs to identify (primary crystallization, structural heterogeneity, defects breaking the metal continuity). Therefore, the samples cut from one or more ingot places (or billet, or detail) in the longitudinal and transverse directions. The sample (template) surface leveled on the emery wheel and then ground. The template etched in special reagents after grinding. The reagents differently dissolving the structural components and irritate the defects.
- the fracture type (brittle, viscous);
- the size, shape and location of grains and cast metal dendrites;
- the ingots and castings defects(shrink sleeves, gas bubbles, cracks);
- the defects breaking a metal continuity (shrinkage porosity, gas bubbles, shells, cracks);
- the metal chemical heterogeneity created by crystallization processes or thermal and chemical-thermal treatment;
- fibers arrangement in forged and stamped billets;
- the cracks during pressure treatment or heat treatment, defects in welds.
The microscopic analysis.
The microanalysis is a more subtle method for investigation of the metals structure and defects. In this case, metals structure study occur at large magnifications with the metallographic microscope help.
Microscopic analysis — is the surface study by light microscopes, where an increase in 50-2000 times allows detecting structure elements up to 0.2 microns in size.
The metallographic microscope treats the metal in reflected light (the main difference from a biological microscope, where an object viewed in transmitted light). A much larger increase can be obtained with an electron microscope, where the light beams replaced by an electron flux. In this case achieves an increase of up to 100,000 times.
The electron flux passes through the research object. Received image is the result of unequal electron scattering by the object. There are indirect and direct research methods.
The imprint object researches by an indirect method. Imprint object is a quartz cast or carbon cast (replica) reflects the microsection relief to prevent secondary radiation (it distorts the image).
The thin metal foils 300 nm thickness research on the lumen obtain by the direct method. The foils obtain directly from the research metals.
The image creates by the electrons secondary emission radiated by surface, where the primary electrons flux falls on it and continuously moves along this surface. The metal surface directly investigate in the raster microscopes. Raster microscopes resolution is lower than translucent microscopes resolution.
Microsections prepared for studying the microstructure, also.
- The grains size, shape and arrangement;
- The separate structural components of alloy (they allows to determine the annealed carbonaceous steels chemical composition);
- The heat treatment quality (for example, the hardening penetration depth);
- Various defects (overheating, decarburization, the nonmetallic inclusions presence).
X-ray diffraction analysis and X-ray flaw detection
X-rays have the same nature as light rays. X-rays are electromagnetic oscillations with a wavelength of 2 х10-7 sm to 10-9 sm (the light rays length from 7,5 х10-5 sm to 4 х10-5 sm).
X-rays obtain in x-ray tubes as a result of electron deceleration during collide with a metal surface. In this case, the electrons kinetic energy converted into X-rays energy.
X-ray diffraction analysis based on the atoms ability to reflect X-rays in the crystal lattice. Reflected rays leave a spots or rings group on a photographic plate (roentgenogram). The nature of their location determines the type of crystal lattice, as well as the distance between atoms (positive ions) in the lattice.
X-ray translucence based on the X-rays ability to penetrate into the substance depth. Due to this, occurs possibility to see various internal defects of metal (shrink shells, cracks, and welding defects) on the on the radiogram without metal products cutting.
Methods for detecting defects in the material based on the X-rays possibility to partially absorbed during passing through the metal. In this case, the metal product loose parts (areas with defects) absorb the rays weaker than the metal product dense parts (solid metal). This leads to the areas with defects will have dark or light spots at the solid metal background on the radiogram.
Modern x-ray machines allow to scan the steel products to a 60-100 mm depth.
The gamma rays used to detect defects in high-thickness metal products. The gamma rays nature is similar to X-rays, but their wavelengths shorter. Due to the gamma rays large penetrating power, they can scan steel parts with up to 300 mm thickness.