Material Characterization & Non-Destructive Testing Methods
Metallurgical Optical Microscope
A metallurgical optical microscope, also known as a metallographic microscope, is a specialized type of optical microscope designed for the examination and analysis of opaque materials, such as metals, ceramics, and polymers. It is a valuable tool in materials science, metallurgy, and other fields where the microstructure of materials needs to be studied.
Working Principle
The working principle of a metallurgical optical microscope is based on the reflection and refraction of light as it interacts with the specimen. Unlike standard light microscopes, which transmit light through transparent samples, metallurgical microscopes are designed to illuminate and view samples that do not transmit light.
Key Components and Their Functions:
- Illumination: Metallurgical microscopes typically use reflected light illumination. The light source, often a halogen or LED lamp, directs light onto the surface of the specimen.
- Objective Lens: The objective lens is specially designed for metallurgical microscopy. It has a short working distance (the distance between the objective lens and the specimen) to accommodate the thickness of the sample. It can also have various magnification powers, typically ranging from 2x to 100x.
- Eyepiece: A standard eyepiece is used to observe the image formed by the objective lens.
- Beam Splitter: In many metallurgical microscopes, a beam splitter is used to direct some of the reflected light to the eyepiece for visual observation while allowing the rest of the light to be captured by a camera for digital imaging and analysis.
- Microscope Body/Stand: This houses the light source and other optical components.
- Stage: The stage is where the specimen is placed for observation. It often has mechanical controls for precise movement in both the X and Y axes.
- Illumination System: The illumination system usually consists of a light source, condenser lenses, and a diaphragm to control the intensity and angle of the illumination.
Spark Test
- Spark testing can be considered a macro examination as the inspection is carried out by the unaided eye. This test is useful for sorting ferrous materials according to their chemical composition by observing the spark struck by the steel when brought in contact with a rotating abrasive wheel, such as a grinder.
- Spark testing is used because it is quick, easy, and inexpensive. Moreover, test samples do not have to be prepared in any way, so often, a piece of scrap is used. Spark testing is most often employed in tool rooms, machine shops, heat treating shops, and foundries.
- When steel is brought in contact with a rotating abrasive wheel, due to friction, steel particles become loose. These hot particles move away from the wheel in a trajectory called “carrier lines.”
- Hot steel particles following carrier lines contain carbon. Carbon in these particles reacts with oxygen in the air to form CO2. This reaction develops internal pressure within the particle, causing them to explode. This explosion is expressed as a burst. Bursts for different steels vary in color, number, shape, and intensity.
- The basis of the test is that different metals give off sparks or particles, each having a different trajectory and form.
Spark Characteristics by Material Type
- Wrought Iron: Wrought iron sparks flow out in straight lines. The tails of the sparks widen out near the end, similar to a leaf.
- Mild Steel: Mild steel sparks are similar to wrought iron’s, except they will have tiny forks, and their lengths will vary more. The sparks will be white in color.
- Medium-Carbon Steel: This steel has more forking than mild steel and a wide variety of spark lengths, with more near the grinding wheel.
- High-Carbon Steel: High-carbon steel has a bushy spark pattern (much forking) that starts at the grinding wheel. The sparks are not as bright as the medium-carbon steel ones.
- Cast Iron: Cast iron has very short sparks that begin at the grinding wheel.
Sulphur Print Test
It is a method to detect sulphur/sulphides in steel and cast irons.
Method
- The process includes grinding flat surfaces, smoothing with emery paper, and cleaning the surface for dirt and grease.
- The next step is to soak pieces of silver-bromide paper into a 3% solution of Sulphuric Acid / HCl in water for 2 minutes and remove excess solution with blotting paper.
- Next is to place sensitized surfaces in contact with one edge of the prepared surfaces for a contact time of 1-2 minutes.
- A corner of the paper is lifted to examine the intensity of the brown stain of silver sulphide.
- The paper is removed, rinsed in water for 3 minutes, and then immersed in a 20% hypo solution (Na2S2O3 / Sodium thiosulphate) for 5 minutes. It is then rinsed in water for 20 minutes and then dried.
Reaction of Sulphur Print Test
The sulphuric acid reacts with the sulphide inclusions and forms hydrogen sulphide. This again reacts with the bromide paper to form dark silver sulphide. This indicates the distribution of the sulphide inclusions (figure) in the material. The print is secured, watered, dried, and can thus be archived.
Hume-Rothery Rules
Formation of substitutional solid solutions between two metals is governed by a set of rules known as Hume-Rothery rules:
- Size difference between the atoms of the solute and the parent metal should be less than 15%.
- The electronegativity difference between the metals should be small (minimum chemical affinity to each other).
- The solubility of a metal with higher valence in a solvent of lower valence is more compared to the reverse situation, e.g., Zn is much more soluble in Cu than Cu in Zn.
- For complete solubility over the entire range of compositions, the crystal structures of the solute and the solvent must be the same.
Dye Penetrant Test (DPT)
Dye penetrant inspection (DPI), also called liquid penetrant inspection (LPI) or dye penetrant testing (DPT), is a widely applied and low-cost inspection method used to check surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). The penetrant may be applied to all non-ferrous materials and ferrous materials.
Working Principle
DPI is based upon capillary action, where surface tension fluid flow penetrates into clean and dry surface-breaking discontinuities. The penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed, and a developer is applied. The developer helps to draw penetrant out of the flaw so that an invisible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending on the type of dye used – fluorescent or non-fluorescent.
Advantages
- It is a very sensitive method, capable of finding extremely fine flaws.
- It can be used on magnetic and non-magnetic metals, some plastics, and glass.
- Small objects, with awkward shapes, can be inspected.
- A power supply is not needed for some methods of penetrant testing.
- The method requires no great skill and is easy to understand.
- Lots of small articles, in batches, can be examined using automated systems.
Limitations
- Can only detect defects open to the surface.
- Preparation, before testing, can be time-consuming and costly.
- The method takes time and can rarely be completed in less than 30 minutes.
- The method cannot normally be applied to painted objects.
- It is messy.
Applications
- LPI is used to detect casting, forging, and welding surface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.
- For inspection of tanks, vessel piping, driers, and reactors in the petrochemical and chemical industries.
- To inspect tools and dies like drill bits, pipes, casting, forging, and other drilling equipment.
- In aerospace industries, components like rotor disks, blades, aircraft, turbines, and welding joints are inspected by DPT.
Magnetic Particle Testing (MPT)
Working Principle
Magnetic particle testing (MT) is an NDT method that utilizes the principle of magnetism. The material to be inspected is first magnetized through one of many ways of magnetization. Once magnetized, a magnetic field is established within and near the material. Finely milled iron particles coated with a dye pigment are then applied to the specimen. These magnetic particles are attracted to magnetic flux leakage fields and will cluster to form an indication directly over the discontinuity.
Procedure
(Details of the procedure were not provided in the original text, only the heading was present.)
Advantages
- Can detect both surface and near-surface indications.
- Surface preparation is not as critical compared to other NDE methods. Most surface contaminants will not hinder detection of a discontinuity.
- A relatively fast method of examination.
- Indications are visible directly on the surface.
- Low-cost compared to many other NDE methods.
Limitations
- Non-ferrous materials, such as aluminum, magnesium, or most stainless steels, cannot be inspected.
- Examination of large parts may require use of equipment with special power requirements.
- May require removal of coating or plating to achieve desired sensitivity.
- Limited subsurface discontinuity detection capabilities.
- Post-demagnetization is often necessary.
- Alignment between magnetic flux and indications is important.
- Each part needs to be examined in two different directions.
- Only small sections or small parts can be examined at one time.
Applications
- Weld inspections, aircraft maintenance.
- Automotive industry.
- Railroad industry.
- Manufacturing processes.
Gibbs Phase Rule
Gibbs phase rule states that if the equilibrium in a heterogeneous system is not affected by gravity or by electrical and magnetic forces, the number of degrees of freedom is given by the equation:
F = C - P + 2
where:
- C is the number of chemical components,
- P is the number of phases.
Basically, it describes the mathematical relationship for determining the stability of phases present in the material at equilibrium conditions.